336.3R-93 Design and Construction of Drilled Piers · PDF fileDesign and Construction of Drilled Piers Reported by ACI Committee 336 ACI 336.3R-93 ... Covers the design and construction

ACI 336.3R-93(Reapproved 2006)

Design and Constructionof Drilled Piers

Reported by ACI Committee 336

Clyde N. Baker, Jr. M. T. Davisson Jim Lewis Jagdish S. Syal

Steven C. Ball John A. Focht, Jr. John F. Seidensticker Edward J. Ulrich

P. V. Banavalkar M. Gaynor Shyam N. Shukla Samuel S. White

Joseph A. Bohinsky William P. Hackney Bruce A. Suprenant John J. Zils

Joseph P. Colaco Fritz Kramrisch

Hugh S. LacyChair

ACI Committee Reports, Guides, and Commentaries areintended for guidance in planning, designing, executing, andinspecting construction. This document is intended for the useof individuals who are competent to evaluate the significanceand limitations of its content and recommendations and whowill accept responsibility for the application of the material itcontains. The American Concrete Institute disclaims any andall responsibility for the stated principles. The Institute shallnot be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contractdocuments. If items found in this document are desired by theArchitect/Engineer to be a part of the contract documents, theyshall be restated in mandatory language for incorporation bythe Architect/Engineer.

Covers the design and construction of foundation piers 30 in. (760 mm) indiameter or larger made by excavating a hole in the earth and then filling itwith concrete. Smaller-diameter piers have been used in noncollapsingsoils. The two-step design procedure includes: (1) determination of overallpier size, and (2) detailed design of concrete pier element itself. Emphasisis on the former, which involves interaction between soil and pier.Construction methods described include excavation, casing, placement ofconcrete and reinforcing steel, and installation by the slurry displacementmethod. Criteria for acceptance are presented along with recommendedprocedures for inspection and evaluation.

Keywords: axial loads; bearing capacity; bending; bending moments; caps(supports); concrete construction; deflection; excavation; foundations;lateral pressure; linings; loads (forces); moments; observation; piers; placing;quality control; reinforced concrete; slurry displacement method; soilmechanics; structural design; tolerances (mechanics); tremie concrete.

CONTENTSChapter 1—General, p. 336.3R-2

1.1—Scope1.2—Notation1.3—Limitations1.4—Definitions

336.3R

ACI 336.3R-93 supersedes ACI 336.3R-72 (Revised 1985) became effective May 1,1993.

Copyright © 1993, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic ormechanical device, printed, written, or oral, or recording for sound or visual reproductionor for use in any knowledge or retrieval system or device, unless permission in writingis obtained from the copyright proprietors.

Chapter 2—General considerations, p. 336.3R-52.1—General2.2—Factors to be considered2.3—Pier types2.4—Geotechnical considerations

Chapter 3—Design, p. 336.3R-83.1—Loads3.2—Loading conditions3.3—Strength design of piers3.4—Vertical load capacity3.5—Laterally loaded piers3.6—Piers socketed in rock3.7—Pier configuration

Chapter 4—Construction methods, p. 336.3R-194.1—Excavation and casing4.2—Placing reinforcement4.3—Dewatering, concreting, and removal of casing4.4—Slurry displacement method4.5—Safety

Chapter 5—Inspection and testing, p. 336.3R-235.1—Scope5.2—Geotechnical field representative5.3—Preliminary procedures

-1

336.3R-2 ACI COMMITTEE REPORT

5.4-Inspection procedures5.5-Concreting5.6-Exploration methods to determine soundness of

piers5.7-Reports5.8-Criteria for acceptance5.9-Corrective measures

Chapter 6-References, pg. 3363R-296.1-Recommended references6.2-Cited references

CHAPTER l-GENERAL

1.1-ScopeThis report deals with design and construction of

drilled pier foundations which are constructed by dig-ging, drilling or otherwise excavating a hole in the earthwhich is subsequently filled with plain or reinforcedconcrete. Engineers and constructors have used the termscaissons, foundation piers, bored piles, drilled shafts, sub-piers, and drilled piers interchangeably. Only the termdrilled pier will be used in this report.

Structural design and construction of drilled pier foun-dations are the primary objectives of this report. Yet geo-technical considerations are vital because variations inthe soil properties have a critical influence on design andconstruction. Therefore, relevant aspects of soil mechan-ics are also discussed herein. For the successful designand construction of the drilled pier foundation, it isnecessary that a reliable set of data on the supportingearth be obtained. For this task, combined attention andcooperation of the Geotechnical Engineer, StructuralEngineer and constructor is essential because limitationsof construction often govern the design.

This report is intended primarily for use in buildingconstruction, but the sections on construction methods,inspection and testing are equally applicable to bridgeand other construction.

1.2-NotationDimensioning method: F=force, L=length, and D=

dimensionless.Ab

= base area of pier, L2Ao

= surface area of pier shaft, L2B = foundation width, or width of beam column

element 4

Li= soil cohesion, FL-2

= diameter of pier shaft, Ldb

= diameter of bearing area, Ldp

= embedded length of pier, LD = net dead loads, FDg = gross dead load, F

4= depth of soil overburden, L

E%,= pier length, L= modulus of elasticity of concrete, FL-2

Eqe

= load effects of earthquake, F= height above ground of horizontal load, L

FFSFS1FS2HHgI, IcIcr

IeIg

ks

K1K2

KRKKKyLMMcrMg

Mmaxnnh

N

Pp-y

pqPtPanPupPULTqaqpQRR1

= compressive strength of concrete, FL-2

= average side resistance, FL-2

= modulus of rupture of concrete, FL-2

= unit load transfer from shaft to soil atdepth Z, FL-2

= vertical load deflection curve at an elementof pier, FL-1, L

= vertical load, F= factor of safety, D= Factor of safety for bearing resistance= Factor of safety for side resistance= length of pier above ground surface, L= horizontal shear at ground surface, F= moment of inertia of concrete, L4

= moment of inertia of the transformedcracked section of concrete, L4

= effective moment of inertia, L4

= moment of inertia of gross concrete sec-tion, L4

= modulus of horizontal soil beam reaction,FL-2

= constant, FL-3

= constant, dimension to be selected in eachindividual case so that the dimensions of ksbecomes L-3

= coefficient of rotational restraint, D= moment coefficient, D= soil reaction coefficient, D= passive pressure coefficient, D= deflection coefficient, D= live loads, F= bending moment, FL= cracking moment, FL= moment at ground surface, usually applied

to pier by superstructure, FL= maximum bending moment, FL= exponent, D= constant of horizontal modulus of subgrade

reaction, FL-3= number of blows required in standard pene-

tration test to drive a 2 in. (5 cm) samplingspoon 12 in. (30 cm) into the ground, usinga 140 lb (64 kg) weight dropping 30 in. (76cm), D

= soil reaction, FL-1

= lateral load deflection curve at an elementof pier, FL-1, L

= bearing forces acting at the base, F= total allowable pier resistance, F= anchorage resistance, F= uplift due to submergence, F= ultimate lateral load, F= allowable end bearing pressure, FL-2

= ultimate end bearing pressure, FL-2

= ultimate compressive capacity, F= used to denote R1 or R2= relative stiffness factor for constant ks (de-fined in Section 3.4.1), L

DRILLED PIERS 336.3R-3

R2 = relative stiffness factor for variable, ks (definedin Section 3.4.1), L

S = slope of elastic curve, DSn

= negative side resistance, F

2= positive side resistance, F

T”= undrained shear strength, FL-2

= relative stiffness factorV = shear, FW = wind load, FW = distributed load along pier length, FL-1

wb = deflection at base of pier, Lwb = movement of the shaft at depth z, Lx = distance along the pier, Ly = lateral deflection of pier, LYt = distance from centroidal axis of gross section,

neglecting reinforcement, to extreme fiber intension, L

z = vertical depth below ground surface, La = a factor for determining adhesion as a part of

the soil cohesion value, D

:= unit weight of soil, FL-3

= base of Napierian logarithms, D0 = angle of rotation, deg

:= ratio of reinforcement, D= capacity reduction factor, D

&I = angle of internal friction in soil, deg

1.3-LimitationsThis report is generally limited to piers of 30 in. (760

mm) or larger diameter, made by open constructionmethods, where water control inside the excavated holedoes not require pneumatic provisions. Smaller diameterpiers have been installed where soils are consistentlystable or casings are left in place. However, it is difficultto detect sidewall collapse in small diameter piers duringconcrete placement and casing extraction.

Piers installed by the use of hollow stem augers arenot part of this report. Rectangular piers on spread foot-ings in deep excavations or foundations constructed with-out excavations by methods such as mortar intrusion ormixed-in-place are also beyond the scope of this report.

1.4-DefinitionsArchitect-Engineer: The person who is responsible for

the esthetic and overall design of the structure andcarries out the responsibilities defined in this report.

Bearing stratum: The soil or rock stratum supportingthe load transferred to it by a drilled pier or similar deepfoundation unit.

Bearing type pier: A pier that receives its principalvertical support from a soil or rock layer at the bottomof the pier.

Bell: An enlargement at the bottom of the shaft forthe purpose of spreading the load over a larger area orfor the purpose of engaging additional soil mass for upliftloading conditions.

Cup: An upper termination of the shaft, usually placedseparately, for the purpose of correcting deviations from

desired shaft location, facilitating setting of anchor boltsor dowels within acceptable tolerances, or combining twoor more piers into a unit supporting a column.

Casing: Protective steel tube, usually of cylindricalshape, lowered into the excavated hole to protect work-men and inspectors entering the shaft from collapse orcave-in of the sidewalls, and/or for the purpose of ex-cluding soil and water from the excavation.

Combination bearing and side resistance type pier: Apier that receives a portion of its vertical support frombearing at the bottom and a portion from side resistancedeveloped along the shaft.

Construction Manager: The person, firm or corporationwith whom the Owner enters into an agreement to act inthe Owner’s behalf during construction.

Project documents: Documents covering the requiredwork and including the project drawings and project spe-cifications.

Project drawings: Part of the project documents; draw-ings which accompany contract specifications and com-plement the descriptive information for drilled pierconstruction work required or referred to in the contractspecifications.

Constructor: The person, firm, or corporation withwhom the Owner enters into an agreement for construc-tion of the work.

Project specifications:The specifications that arestipulated by Contract for a project and may employ ACI336.1 by reference and that serve as the instrument fordefining the mandatory and optional selections availableunder the specification.

Controlled slurry: Slurry that is made to conform to thespecified properties given in Table 1.

Design bearing pressure: The vertical load per unit areathat may be applied to the bearing stratum at the level ofthe pier bottom. Design bearing pressure is selected bythe Geotechnical Engineer on the basis of soil samples,tests, analysis, judgment, and experience; with dueregard for the character of the loads to be applied andthe settlements that can be tolerated.

Design vertical side resistance: The allowable verticalfrictional resistance in force per unit area that may beapplied on the shaft of a pier to resist vertical load.Design side resistance is selected by the GeotechnicalEngineer.

Drilledpier: Concrete cast-in-place foundation elementwith or without enlarged bearing area extending down-ward through weaker soils or water, or both, to a rock orsoil stratum capable of supporting the loads imposed onor within it. The drilled pier foundation has been re-ferred to as a drilled shaft, drilled caisson, or largediameter bored pile. The drilled pier foundation with anenlarged base may be referred to as a belled caisson,belled pier, or drilled-and-underreamed footing. Drilledpier foundations excavated and concreted with water orslurry in the hole have been known as slurry shafts, piersinstalled by wet hole methods, or piers installed by slurrydisplacement methods.

336.3R-4 ACI COMMlTTEE REPORT

Table l-Typical slurry properties

Item to be measured I Range of results at 68 F I Test method

Density prior to concreting (pcf)

a. Friction pierb. End bearing

March funnel viscosity, (sec.) prior toconcreting

85 max70 max

26 to 45

API 13BSection 1

API 13BSection 2

Marsh funnel and quart)

Sand content by volume, (%) beforeconcreting

a. Piers with design end bearingb. Piers with no design end bearing

pH, during excavation

4 max10 max*

8 to 12

API 13BSection 4

(Sand-screen set)

AP1 13BSection 6

(Paper test strips or glass- Electrode pH meter)

Sand in polymer slurry immediatelyprior to concreting

Density of polymer slurry

Viscosity of polymer slurry

1% max

63.5 pcf max

50 max

* Higher sand contents have been successfully used in some locations.

Flexible pier: A pier with a length to diameter ratiowhich will allow significant flexural deformations fromlateral loads; the theoretical point of fixity is within thepier shaft.

Geotechnical Engineer: An engineer with experience insoil mechanics and foundations who is designated tocarry out the responsibilities defined in this report.

Head: The top of the pier.Inspection: Visual observation of the construction,

equipment, and materials used therein, to permit theGeotechnical Engineer to render a professional opinionas to the Constructor’s conformance with the Geotech-nical Engineer’s recommendations or Contract Docu-ments. Inspection does not include supervision of con-struction nor direction of the constructor. Inspection mayrange from the down-hole observation of each pier bythe Geotechnical Engineer or the use of down-holecameras, to surface observations and testing.

Kelly bar: The stem of the drill used to advance thedrilled pier.

Owner: Party that contracts for approved work per-formed in accordance with the contract documents.

Permitted: Permitted by the Architect-Engineer.Pig: A disposable device inserted into a tremie or

pump pipe to separate the concrete from the pier exca-vation fluid inside the pipe.

Qualified: Qualified by training and by experience oncomparable projects.

Rabbit: Same as Pig.Rigid pier: A pier with a small depth-to-diameter ratio

which will have insignificant flexural deformations underlateral load. Lateral movements will be rotational typeinvolving the entire length of the pier.

Rock socketed pier: Pier supported by both sideresistance and end bearing within rock.

Side resistance type pier: A pier that receives itsprincipal vertical support from side resistance along theshaft.

Shaft: Drilled pier above bearing surface exclusive ofthe toe or bell, if any.

Side resistance: Soil or rock friction or adhesion devel-oped along the side surface of the pier.

Slurry: Drilling fluid that consists of water mixed withone or more of various solids, or polymers. See Table 1.

Slurry displacement method (SDM): Method of drillingand concreting, where controlled slurry is used to sta-bilize the hole. The slurry may be used (a) for the main-tenance of the stability of the unlined drilled pier hole;or, (b) to allow acceptable concrete placement whenwater seepage in a drilled pier hole is too severe topermit concreting in the dry.

Socket: Portion of pier within bearing stratum.Structural Engineer: An engineer contractually desig-

nated to carry out the structural design and other definedduties.

Submitted: Submitted to the Architect-Engineer forreview.

Testing agency: The firm retained to perform requiredtests on the contract construction materials to verify con-formance with specifications.


Toe: The bottom of the pier.Various pulverized solids: Approved solids used to

make slurry including bentonite, attapulgite, and site clay.Wet hole method: Methods used when a pier extends

through a caving stratum. One method is by drilling anoversized hole through a caving stratum and insertingcasing, or by loosening the soils without excavating, or byusing approved slurry displacement methods to allow cas-ing placement. The casing is inserted into the hole afterthe caving soils are fully penetrated, and then it is seated.The loosened soils or slurry inside the casing are bailedor pumped out permitting the hole to continue to ad-vance by drilling dry.

CHAPTER 2-GENERAL CONSIDERATIONS

2.1-GeneralThe function of a pier foundation is to transfer axial

loads, lateral loads, torsional loads and bending momentsto the soil or rock surrounding and supporting it. To per-form this function, the pier interacts with the soil or rockaround and below it and with the superstructure above.The relationship of the pier to the earth is one of themost important variables in the pier design.

In the absence of a theory that can encompass all ofthe factors involved, simplifying assumptions must bemade. However, subtle aspects of construction oftengovern the design.

2.2-Factors to be consideredComputational results and expected behavior must be

evaluated on the basis of the following variables:2.2.1 Subsurface conditions-Soil stratification, ground

water conditions and the depth, thickness and nature ofthe rock, sand or other material constituting the bearingstratum influence the construction method and the foun-dation design. Specifically, the design bearing pressuredetermines the size of the bell or bottom area of theshaft. The properties of the materials above and in thevicinity of the bottom and the effect of disturbance dueto construction activity on the soil properties determinethe feasibility of constructing a bell without slurry.Permeability, groundwater and soil properties determinewhether the use of casing, slurry or dewatering will berequired; dictate the method of placing the concrete; andmay influence ground loss considerations. Shear strengthand deformation characteristics of the soil penetrated bythe shaft determine whether side resistance will be adesign factor. Side resistance may act to support super-structure loads or it may be a major applied downdragload on the shaft.

2.2.2 Site conditions-Available construction area, siteaccess, and headroom, as well as existing facilities to beprotected against settlement, ground loss, noise, or con-tamination, influence the choice of construction methodand thus the design. The effects of the design and the

construction methods used for new piers may includesubsidence which is caused, for example, by removingfine grained materials from the surrounding soil by waterflow due to dewatering or consolidation. These effects onadjacent and new structures must be evaluated.

2.3.3 Inspection and quality control-The validity ofsimplifying assumptions made on the basis of field ex-ploration obtained by borings or in-situ testing resultsshould be confirmed by observation by the GeotechnicalEngineer. Scope and method of observation, obtainabletolerances, and quality control influence the refinementto which the design can reasonably be carried. Con-versely, allowable tolerances may determine constructionmethods, scope of observation and quality control.

The design and installation of drilled piers are multi-phase tasks in which proper quality control and qualityassurance in construction is vital to the success of the as-installed pier. Without proper quality control and qualityassurance, the probability of a successful foundation isreduced. Even the highest quality structural element canhave its capacity as a load carrying member significantlyreduced due to installation details and the relationship ofthe installed concrete element to the surrounding soil.

The presence of the Geotechnical Engineer should berequired during the pier installation. The Geotechnicaland Structural Engineers, together, should develop thespecifications which should include clearly defined re-quirements for testing laboratory services and inspection.

2.2.4 Constraints-Construction and design are bothaffected by available construction expertise and equip-ment, available materials, and building code require-ments. The limitations of construction will often governthe design.

2.2.5 Design considerations-In conjunction with theconsiderations mentioned above, the designer must com-pute vertical and lateral loads and moment imposed onthe pier. The length and section properties of the pier,distribution of load on end bearing, lateral resistance,and side resistance are determined on the basis of loadsand subsurface conditions.

2.2.6 Laterally Loaded pier-The pier stiffness, EI,subgrade response and their interaction are important inthe analyses of laterally loaded piers. Soil response is theleast predictable variable. Pier deflection is often thelimiting factor in determining acceptable lateral loadsrather than failure load.

2.3-Pier types It is convenient to divide piers into types according to

the manner in which axial loads are transferred to thesoil or rock, and according to the response of the pier tolateral load. To which type or types a given pier may beassigned depends on the qualities of the soil and rockaround the shaft and at the bottom of the bell or pier,the character of the contact surface between pier and soilor rock, the relative stiffness factor and the embeddedlength of the pier.

2.3.1 Axially supportedpiers-With respect to axial load


Fp

support, there are three types of piers.2.3.1.1 Bearing type pier (See Fig. 2.3.1.1)-A

straight-sided or belled pier sunk through weaker soilsand terminating on a layer of satisfactory bearingcapacity is an example of a bearing type pier.

The bearing area may be increased by a bell at thebottom of the shaft. However, the soils in which the bellis constructed must have sufficient cohesion to permit theexcavated void to stay open until the concrete is placed.In caving soils, the bell may require grouting or instal-lation by slurry displacement methods. Alternatively, theshaft may be enlarged to eliminate the need for the bell,or extended into a material in which a bell can be exca-vated.

- COLUMN DOWELSOR ANCHOR BOLTS

SET TEMPLATEWHERE

z CAP

PIER REINFORCEMENT

\ EXTEND ASREQUIRED

I -

SELL

Fig. 2.3.1.1--Example of bearing type piers

2.3.1.2 Combination bearing and side resistance typepier (See Fig. 2.3.1.2)-A shaft extended (socketed) intoa bearing stratum in such a manner that a part of theaxial load is transferred to the sides of the pier and therest of the load is carried in end bearing.

STEEL-WIDE-FLANGECORE SECTION(OPTIONAL)

C A S I N G L E F T -IN HOLE(OPTIONAL)

+A,

COMPRESSIBLE

‘-AWN

ig. 2.3.1.2-Combination bearing and side resistance typeier

2.3.1.3 Side resistance type pier (See Fig. 2.3.1.3)-Apier built into a bearing stratum in such a manner thatthe load is carried by side resistance, because the endbearing is negligible or unreliable; for example, in caseswhere cleanup of the bottom of the hole is impractical.

WEAKROCK

b

Fig. 2.3.1.3--Side resistance type pier

2.3.2 Laterally loaded piers-On the basis of responseto lateral load, there are two pier types.

2.3.2.1 Rigid pier (Fig. 2.3.2.1)-A pier so short andstiff in relation to the surrounding soil that lateral deflec-tions are primarily due to rotation about a point alongthe length of the pier and/or to horizontal translation ofthe pier. The rotational resistance of a rigid pier is


governed in part by the load deformation characteristicsof the soil adjacent to and under the embedded portionof the pier, and also by the restraint, if any, provided bythe structure above.

F

DEFLECTED POSITION

Hg = SHEAR AT GROUND SURFACE

Mg = MOMENT AT GROUND LINE

Fig. 2.3.2.1--Rigid type pier [after Davisson (1969), withnotation altered]

2.3.2.2 FIexible pier (See Fig. 2.3.2.2)-A pier ofsufficient length and with flexural rigidity (ET) relative tothe surrounding soil such that lateral deflections are pri-marily due to flexure.

U, = SHEAR AT GROUND SURFACEMI = MOMENT AT GROUND SURFACE

Fig. 2.3.2.2--Flexible type pier [after Davisson (1969), withnotation altered]

2.4-Geotechnical considerationsIt is necessary for the designer to have an adequate

knowledge of the underground site conditions in order toselect a foundation system which is constructible, andeconomical. The subsurface exploration should be thor-ough, with enough samples and data to adequately esta-blish the soil properties within the zones of interest. Theinvestigation should consider the effect of geologic detailson foundation design and performance. Such considera-tions as collapsing soils, fill, shrinkage and swellingconditions, slope stability, rock cavities, potential rockcollapse, and weathering profiles should be evaluated asneeded. The Geotechnical Engineer should determinethe scope of investigation needed for pier design.

The scope of the investigation should include:2.4.1 Number of borings -A sufficient number of bor-

ings should be made to establish with reasonable certain-ty the subsurface stratification (profile), and the locationof the water table. Where the piers are to terminate inrock, the bedrock surface profile and character shouldalso be established with reasonable accuracy.

2.4.2 Depth of borings in soil deposits -Boring depthshould be adequate to investigate settlement of the bear-ing stratum below the pier. Where practical, at least oneboring should go into the bedrock.

2.4.3 Water table and dewatering-If water is encount-ered within the zone of pier penetration, the site explora-tion should obtain pertinent information so any necessarydewatering systems or required slurries may be specified.This should include, as a minimum, the water table eleva-tion(s) (there may be more than one), anticipated fluctu-ations, if any, and permeability data.

2.4.4 Piers to bedrock level-where piers are to besocketed into bedrock, probes or cores should be ex-tended into the bedrock a depth of at least twice thediameter of the bearing area below the base level, butnot less than 10 ft (3 m). This depth is necessary todetermine rock strength and condition (if fractured, etc.),and to ensure that the pier does not terminate on asuspended boulder. Cores are preferred when the piercapacity is high and the rock quality is critical toestablishing maximum pier capacity.

2.4.5 Soil strength-In cohesive soil, a sufficientnumber of undisturbed soil samples should be taken toobtain the unit weight and the soil strength parameters,and to obtain depth trends, since individual samples maybe erratic. In cohesionless soils, it is common practice toestimate the soil density and determine the allowable soilpressure based on the standard penetration test (SPT),cone penetration test (CPT), dilatometer, or pressuremeter.

2.4.6 Load tests-For large projects or in cases ofuncertainty, load tests are desirable. Reaction can be pro-vided by belled or socketed shafts. In addition, Osterberg


(1989) has developed a method using a jack seated ongrout at the base of the pier with flanges that fill the fullpier diameter. The pier is filled with concrete with thehydraulic pipe and telltales extending to the groundsurface. Both end bearing and side resistance are tested.The maximum test load is when failure is reached eitherin side resistance or in end bearing. If the maximum sideresistance is exceeded, the test can be extended furtherconventionally with a reaction frame and dead weights oranchor shafts to permit applying higher end bearing load.

Where the water level would not be a problem, it isalso possible to drill a full sized shaft and perform aplate load test at the shaft base level to establish thebearing capacity. This has the additional advantage ofallowing visual observation of the subsurface conditionsprior to design. Soil samples should be obtained andprobes performed in soils below the plate after loadtesting. Load tests may also be performed on small di-ameter, instrumented, drilled piers and the resultsextrapolated to larger diameter piers.

2.4.7 Lateral response-At present, the most completemethod for evaluating lateral response of piers is someform of a beam on an elastic foundation mathematicalmodel using a computer (Reese, 1977a; 1977b; 1984;1988; Penn. D.O.T.). The major variables are the sub-grade response and stiffness (,!?I>. Subgrade reaction maybe modeled as a linear spring or as an elastic-plasticmaterial using p-y data (Reese, 1977a; 1977b; 1984; 1988;Penn. D.O.T.). Since no unique method of modeling thesubgrade response is universally accepted, the Geotech-nical Engineer should develop the subgrade responsemodel based on the model for which the Engineer hashad the best local experience.

CHAPTER 3-DESIGN

3.1-LoadsThe design of piers consists of two steps:

a) Determination of pier size or overall concrete di-mensions

b) Design of the concrete pier element itself

In Step (a), which involves interaction between soiland pier, all loads should be service loads and all soilstresses at allowable values (see Section 3.2). The appliedservice loads do not include load factors.

In Step (b), the pier is designed by the strengthmethod. Normally, the service loads are used to calculatethe resulting moments, shears and axial forces which aremultiplied by the appropriate load factors for the variouscases of loading to structurally design the pier. In thecase of a non-linear p-y curve and/or variations of shaftaxial load (resulting from non-linear f-z curves for sidefriction), loads must be multiplied by the load factors.The soil pressures required to maintain equilibrium withthese factored loads are fictitious and serve no other pur-

pose than to obtain the moments, shears, and axial forcesnecessary for strength design of the concrete pier (seeSection 3.3). Where moments or eccentric loading condi-tions are involved, the fictitious soil pressures required toresist factored loadings may have distributions differentfrom those found for the service load conditions.

3.1.1 Axial loads-Axial loads may consist of the axialcomponents of:

D = dead loads from the supported structure andweight of the pier, less weight of material dis-placed by the pier (net weight of the pier)

Dg = dead loads from the supported structure andweight of the pier (gross weight of the pier).

L = live loads from the supported structure includ-ing impact loads, if any, reduced in accor-dance with the applicable building code

W,Eq = axial effects from wind or earthquake, respec-tively

Spl = positive side resistance, acting upward on thepier; normally caused by downward movementof the pier relative to the surrounding soil

SP2 = downward side resistance to resist upwardload, acting downward on the pier

Sn = negative side resistance, acting downward onthe pier; caused by settlement of the sur-rounding soil relative to the pier, normally anultimate value. It does not include a factor ofsafety.

Pq = bearing resistance acting at the basePup = uplift force due to submergence of the struc-

turePan = anchorage capacity from rock or soil anchors

See Section 1.2

3.1.2 Lateral loads and moments- Lateral loads arecaused by unbalanced earth pressures, thermal movementof the superstructure, wind and/or earthquake generatedforces. Moments may be generated by axial loads appliedwith eccentricity and by lateral loads, and may be in-duced by the superstructure through connections to thepier.

3.2--Loading conditionsThe forces interacting between the soil and the pier

are determined from the following combinations of load-ing, whichever produces the greater value for the itemunder investigation.

3.2.1 Axial loads-Maximum and minimum loadingconditions should be investigated for pertinent stages ofconstruction and for the completed structure.

3.2.1.1 Maximum loading--Excess weight of the pierfoundation over the weight of the excavated soil, negativeside resistance (down drag), and long-term redistributioneffects on side resistance should be considered. For ex-ample, an initial upward acting side resistance maylessen, disappear or reverse with time from downdrag.


a) Dead load, live load, side resistance, and uplift:When positive (upward acting) side resistance ispresent:

D + L - Pup < Pq/FS1 + SP1/FS2

When negative side resistance is present:

(3-1)

D + L - Pup < (Pq + SP1)/FS - Sn (3-2)

Eq. (3-l) or (3-2), whichever applies to the conditioninvestigated, should always be satisfied.

b) Dead load, live load, side resistance, uplift andwind or earthquake:When positive side resistance is present:

0.75 (D + L + W - Pup) < (Pq/FS1 + SP1/FS)(3-3)

When negative side resistance is present:

0.75 (D + L + W - Pup) < (Pq + Sp1)/FS-Sn)(3-4)

In Eq. (3-3) and (3-4) W should be entered at its max-imum downward acting value. Side friction resistance andend bearing developed at different displacements are de-pendent on soil properties. Side resistance is often devel-oped at low displacements of 0.1 to 0.4 in. (3 to 10 mm)while tip resistance is developed at large displacements(2 to 5 percent of pier diameter in cohesive soils andelastic parts of the resistance in granular soils) (Reeseand O’Neill, 1988). Factors of safety should be appliedseparately to these resistances when considering relativedisplacement. The value of Sn in equation 3-4 is some-times reduced due to pile strain from applied verticalloading, F.

For earthquake resistance designs, 1.1Eq should besubstituted for W in Eq. (3-3) through (3-7), if the formeris greater.

In Eq. (3-l) through (3-4) uplift, Pup, should be en-tered at its lowest permanent value only.

3.2.1.2 Minimum loading--In Eq. (3-5) through (3-7), uplift Pup is entered at its maximum value. If:

0.9 Dg - 1.25 W - Pup > 0 (3-5)

no further investigation is needed. Otherwise:

Pup - 0.9 Dg < Sn + Pan/FS2

Pup - 0.9 Dg + 1.25 W < Sn + Pan/FS2

(3-6)

(3-7)

should both be satisfied. If sufficient side resistance isavailable, anchors to rock or soil, Pan will normally notbe necessary. In Eq. (3-5) and (3-7), W should be enteredat its maximum upward acting value.

3.2.2 Combined loadings-The effects of lateral loadsand moments are to be superimposed on the effects ofany simultaneously occurring axial loads in any of thecombinations listed in Section 3.2.1.

3.3--Strength design of piersFoundation piers embedded in soil of sufficient

strength to provide lateral support (Section 3.7.5) may beconstructed of plain or reinforced concrete. Design ofplain concrete piers is governed by the provisions in ACI318.1. Piers that cannot be designed using plain concretewith practical or desirable dimensions may be designedusing reinforced concrete in accordance with the pro-visions in ACI 318, Chapter 7, Section 7.10 and Chapter10, Sections 10.2, 10.3, 10.8.4, 10.9, and 10.15. In eithercase, the design may be based on the strength designmethod. Reinforced concrete may also be designed bythe alternate design method.

If the strength design method is used, all loadings (onthe left side of the equations in Section 3.2), whetheraxial, transverse, or moment, are to be multiplied by theappropriate load factors given below, and all reactions(on the right side of the equations) are evaluated fromthem. It is emphasized that these reactions have no rela-tionship whatsoever to ultimate soil values, but are onlyintended to balance the factored loadings (see Section3.1). The pier should also satisfy the compatibility re-quirements of soil reaction with upper estimates ofworking load. It is recommended that the strength designmethod be used for analysis regarding load capacity, butconcerning settlement and lateral motions, no loadfactors should be incorporated and only service loadsshould be used.

In the strength design method, the concrete sectionand reinforcing steel requirements may be determined byapplying load factors to computed shears and bendingmoments from working loads except for cases noted inSection 3.1.

If the alternate design method is used, all loadingsshould be service loads with unity load factors as allowedin Appendix B of ACI 318. Soil pressure for resistanceshould be allowable values that contain factors of safety.

3.3.1 Load factors for strength design-A load factor of1.4 should be used for dead load, D, uplift, Pup, andother loadings caused by liquid pressures on the structurewhere the maximum pressure can be well defined. Other-wise use a load factor of 1.7.

A load factor of 1.7 should be used for live load, L,wind load, W, earthquake forces of magnitude (1.1Eq),and other loading caused by lateral earth pressures onthe structure.

Structural effects of differential settlement, creep,shrinkage, and temperature changes should be includedwith the dead load, D, if they are significant. Evaluationshould be based on a realistic assessment of their occur-rence in service.

3.3.2 Strength reduction factors-Strength reductionfactors 4 are given in Section 9.3 of ACI 318.

336.3R10 ACI COMMITTEE REPORT

3.3.3 Pier reinforcing-Pier reinforcing is required toresist applied tensile forces or adequately transfer loadfrom the structure to pier.

3.4--Vertical loads capacity3.4.1 Capacity from soil or rock-The total ultimate

compressive and tensile capacities may be a combinationof end bearing and side friction. The maximum theoreti-cal ultimate capacity is expressed in the following equa-tion:

Q = SP1 + Pq

Where,Q = ultimate compressive capacitySP1 = ultimate side friction which may be taken as

the sum of friction on the shaft walls at givenelevations

Pq = ultimate end bearing

The designer should consider strain compatibility anddeflection in determining the factor of safety.

Factors of safety may vary from 1.5 to 5 for side fric-tion or end bearing, depending on the subsurface condi-tions, structural loads, and degree of confidence in thesubsurface parameters. The side friction and end bearingmay be described further by the following equations inconsistent units.

SP1 = foAo and Pq = qpAb

Where,fo = average unit side friction of a shaft elementAo = embedded surface areaqp = unit end bearing pressureAb = gross area of the shaft base (or bell)

The Geotechnical Engineer should estimates valuesfor fo and qp using the soil and/or rock properties andconstruction method. The values of fo, and qp vary widelyand are depth dependent. Determination o these valuesf may require iterative estimates of the allowable capacityof the drilled shaft foundation in collaboration with theStructural Engineer to satisfy both factor of safety andallowable settlement requirements. The total ultimatecapacity will be less than the maximum theoretical if theresidual resistance is less than peak side resistance sincepeak side resistance typically develops much faster thanmaximum end bearing resistance.

3.4.2 Estimate of pier settlement where unit loading andsoil properties are a design consideration--The soil com-pression properties should be determined to permit esti-mates of total and differential settlement. In-situ tests,such as cone penetrometer, pressuremeter or plate loadat pier subgrade, full scale load tests and laboratory testsof undisturbed pier subgrade soil are commonly used.Total pier settlement is the sum of pier base movementplus elastic pier shortening considering the effect of side

resistance.

3.5--Laterally loaded piers3.5.1 Lateral loads and moments-Drilled piers will be

subject to large lateral loads along the pier length incases when piers are used as retaining walls, walls toarrest slope movement, power pole foundations, oranchors. Also, when the earth pressures on the basementwalls are unequal or insufficient to resist the lateral loadsfrom the superstructure, the necessary resistance must beprovided by the foundations. This condition occurs whenthere is no basement, when the depth of the basementwalls below the ground surface is too shallow or whenthe lateral movements associated with the mobilization ofadequate earth pressures are too large to be tolerated.The piers will then be loaded with lateral forces at thetop, axial forces from overturning and, usually, momentsat the top.

The allowable pier head deflection in each design casemay be a few tenths of an inch or a few inches, depend-ing on the project requirements.

Piers that must sustain lateral load can be, and havebeen, designed successfully, by approximate methods.The allowable lateral load on a vertical pier can be ob-tained from a table of presumptive values found in somehandbooks, building codes, or from simplified solutionsthat assume a rigid pier and one soil type. However,these allowable loads may not be appropriate comparedto values that may be computed by the recommendedmethod herein and they provide no information on pierdeflection. Use of simplified solutions may be misleadingfor many drilled pier foundations.

3.5.1.1 Batter piers-- To avoid analyzing a pier forlateral loads, some designers assume, according to theapproach used for driven piles, that the lateral loads areresisted by the lateral component of axial loads taken bypiers installed on a batter. Most methods that are avail-able for the analysis of a pier group that includes batterpiers are approximate in that the movements of the pierhead under load are not considered. Battered piers indesign should be used with caution because constructoroften cannot properly construct the piers to the batterangle desired.

3.5.1.2 Beam on elastic foundation-Theory andexperience have shown that a more rational and a moresatisfactory solution of the lateral loaded pier design isobtained by using the method of soil-structure interactionwith the theory of a beam on an elastic foundation. Vari-able pier stiffness and multilayered soil systems are fun-damental parameters that can be addressed in the analy-sis using the beam on an elastic foundation theory.Because soil or subgrade response is the most criticalelement of the analysis, the Geotechnical Engineershould develop the soil response model. Although eitherthe Geotechnical or Structural Engineer may analyze thepier, analysis by the Geotechnical Engineer is recom-mended to minimize possible miscommunication or mis-interpretation of the soil response model.


3.5.2 Laterally loaded pier problem-The application ofa lateral load to the head of a pier causes lateraldeflection of the pier. The reactions that are generatedin the soil must be such that the equations of staticequilibrium are satisfied, and the reactions must beconsistent with the deflections. Also, because no pier iscompletely rigid, the amount of pier bending must beconsistent with the soil properties and the pier stiffness.

Thus, the problem of the laterally loaded pier is a“soil-structure-interaction” problem. The solution of theproblem requires that numerical relationships betweenpier deflection and soil reaction be known and that theserelationships be considered in obtaining the deflectedshape of the pier.

Technological advances have allowed the mathematicalproblem to be solved with relative ease, but the subgraderesponse characteristics are still uncertain in many cases.Strain gage measurements have made possible the deter-mination of soil response during the testing of full-scalepiers, and numerical solutions allow the deflected shape(lateral deflection) of a pier to be computed rapidly andaccurately even though the soil reaction against the pieris a non-linear function of pier deflection and of depthbelow the ground surface.

Although several methods (GAI Consultants, 1982;Borden & Gabr, 1987; Poulos & Davis, 1980) are avail-able for the analysis of drilled piers, the method reportedby Reese (1984) is shown in the following sections alongwith approximate methods that may be used for prelim-inaty analysis. The approximate methods are more suit-able in a single layer soil system.

3.5.3 Pier-soil interaction--The soil-structure inter-action problem can be illustrated by considering thebehavior of a strip footing, as shown in Fig. 3.5.3.1. Theassumption is usually made that the bearing stress isuniform across the base of the footing as shown in thefigure. However, under the stress distribution that isshown, the cantilever portion of the footing will deflectsuch that the downward movement at b is less than thedownward movement at a. The footing is probably stiffenough that the deflection of b with respect to a is small;however, the concept is established that the base of thefooting does not remain planar. Therefore, the bearingstress across the base of the footing conceptually shouldnot be uniform.

b

%‘*b

REAL ELASTIC MATERIAL(FRICTIONLESS BASE)

COHESIONLESS MATERIAL

Fig. 3.5.3.1--Soil structure interaction for a strip footing

Although the behavior of a strip footing involves soil-structure interaction, the potential economic advantageavailable by taking the curvature of the footing intoaccount is marginal. Fig. 3.5.3.2 shows a model of anaxially loaded pier with the soil replaced by a set ofmechanisms. The mechanisms show that the load transferin side resistance and in end bearing are nonlinearfunctions of the downward movement of the pier. A non-linear curve showing axial load versus pile-head move-ment can easily be obtained (Reese, 1984), if themechanisms can be described numerically.

NORMAL FORCE

I r1 11 ’11

'01 r

1

1 t1t1 r1 t

%wb

Fig. 3.5.3.2--Model of a pier under axial load [after Reeseand O’Neill (1988), with notation altered]

The two examples of soil-structure interaction givenserve to illustrate the kind of problem that must be

solved. A model for a laterally loaded pier is shown inFig. 3.5.3.3. A pier is shown with lateral loading at itstop. Again the soil has been replaced by a set of mechan-isms that conceptually define soil-response curves. Suchcurves give the soil resistance p (force per unit lengthalong the pier) as a function of pier deflection y. Themechanisms define bilinear curves as shown in Fig.


3.5.3.3, and it can be seen that the curves vary withposition along the pier. Therefore, p is a nonlinearfunction of both y and x . The p-y concept, though two-dimensional, is based on the synthesis of full scale pileand pier load tests and soil properties. Shear at the baseof the pier is neglected because the pier is consideredsufficiently long that lengths are assumed to extend belowthe theoretical depth of fixity. The determination of thep-y curves by the Geotechnical Engineer and the selec-tion of pier stiffness are the two most important con-siderations in the analysis of laterally loaded piers.

xFig. 3.5.3.3--Model of a pier under lateral loading showingconcept of bilineal soil response curves [after Reese andO’Neill (1988), with notation altered]

3.5.4 General methods of solution of an individual pier-Among the available methods, five are considered forthe solution of a single pier under lateral loads. Theseare: elastic method, curves and charts, static method,computer method with non-linear soil response using abeam on an elastic foundation, and non-dimensionalcurves. The elastic method has a limited application andlarge numbers of curves and charts would be needed inthe general case of the curves and charts method, sothese two methods are not discussed. The other threemethods are presented in appropriate detail.

The computer method using the beam on an elasticfoundation concept should be used in the design of piersunder lateral load. Although the method is easy toemploy with modern computers, the subgrade response

characteristics still remain complex and obscure. Aunique method with consistent parameters is not avail-able. Geotechnical experience and judgment are theprincipal elements of the analysis; hence, analysis by theGeotechnical Engineer is recommended.

The nondimensional and static methods have a placein the design process, but these methods are primarily forsmall diameter piles. These methods can be employed forpreliminary design or as a check of the computer outputin simple cases. The simplified methods are limited inthat multi-layered systems and complicated ground geo-metries cannot be considered. Frequently there is uncer-tainty regarding some of the parameters that enter designcomputations; for example, in the strength and deforma-tion characteristics of the supporting soil. The computermethod not only allows the Geotechnical Engineer to in-vestigate the influence of these uncertainties, since theresponse of the pier to small variations in parameters canbe readily seen, but also enhances the Engineer’sjudgment.

3.5.4.1 Preliminary design-For preliminary design,several methods of analysis can be used to evaluate thecapacity and deformation of laterally loaded piers. Themethod of Broms (1965) and Singh, et at. (1971) are pre-sented herein.

3.5.4.2.1 Ultimate capacity (Broms Method)-Theultimate lateral load capacity of a pier defines a loadingcondition in which a pier can fail with the developmentof a plastic hinge (long pier) or by unlimited deflection(short pier). The Broms method can be used to computethe ultimate lateral resistance of small piers in cohesiveand cohesionless soils as a function of the pier dimen-sion, type of loading and fixity at the head. In general,the capacity of short piers is covered by soil failure whilefor long piers, capacity is governed by structural failureof the pier. Deflections can be computed using thetheory of subgrade reaction (Tetzaghi, 1955) by assuminga linear relationship between load and deflection.

Piers in cohesive soil--For piers in cohesive soil, PULTmay be determined as the smaller of values obtainedfrom Fig. 3.5.4.1 in which the pier is considered eithershort or long. For piers in which the embedment lengthcontrols (i.e., long piers), the maximum bending momentis determined using the following relationships (Broms1964a):

l For free-head piers

MMAX = BPULT [(1.5+ (0.055PULT/SUB2) + (e/B)]

(3-10)

0 For fixed-head piers

MMAX = BPULT [ 0.75 + (0.028PULT/SUB2)] (3-11)

Piers in cohesionless soil (C)-For piers in cohesion-less soil, PULTT may be determined as the smaller ofvalues obtained from Fig. 3.5.4.2 in which the pier isconsidered either short or long. For piers in which the


embedment length controls (i.e., long piers), themaximum bending moment is determined using the fol-lowing relationships (Broms 1964b):

l For free-head piers

MMAX= PULT [e + 0.55(PULT/KPB)0.5] (3-12)

l For fixed-head piers

MMAX = 0.5PULT [e + 0.55(PULT/KPB)0.5] (3-13)

a) Pult related to yield moment

EMBEDMENT LENGTH. dp/d

b) Pult related to embedment length

Fig. 3.5.4.1--Ultimate lateral resistance of cohesive soils[after Broms (1964)]

a) Pult related to yield moment

b) Pult related to embedment length

Fig. 3.5.4.2-- Ultimate lateral resistance of cohesionless soils[after Broms (1964)]

3.5.4.3 Displacement analysis-Methods for eval-uating the displacement of laterally loaded piers forpreliminary design include subgrade reaction analyses(Reese 1984) for typical soil profiles and nondimensionalsolutions.

Subgrade reaction analysis-The displacement of lat-erally loaded piers is based on the beam-on-elastic-sub-grade theory using simplifying assumptions regarding soilstress-strain behavior. The method of Singh, et al. (1971)can be used to compute the lateral capacity, displacementand maximum moment of piers in cohesive and cohesion-less soils as a function of pier dimensions, type of loadingand fixity of the head. The method is applicable providedthe ratio of pier length (Dp) to the Relative StiffnessFactor (T) is greater than 5.

The lateral load capacity and displacement may be de-termined using Fig. 3.5.4.3 through 3.5.4.6. The value ofT is determined using Fig. 3.5.4.3 or the following rela-tionship (Singh, et al. 1971):

T = (EcIc/nh)1/5 (3-14)

The values of EcIc for common shaft dimensions (as-


suming E c = 3 x 106 psi) (0.02069 x 106 MPa) are pre-sented along the base of Fig. 3.5.4.3 through 3.5.4.6.

20 30 5070 I00 200 3 0 0 500

E,T,X IO? lIb.-irr2)

Fig. 3.5.4.3--Relative stiffness factor [modified after Singh, et al (1971)]

0.03 cZ=a 0.02

Ef: 0.012H 0 . 0 0 7

0.003lo 20 30 so 70 loo 2 0 0 3 0 0 so0

4 12X IO? (Ib.-hz)

Fig. 3.5.4.4--Deflection for free-headed shafts subjected to1 kip lateral load [modified after Singh, et al (1971)]

Q a003

a002

--ii I Tiwa001 . t I. (

1O 2 0 3 0 s o 7 0 loo 2 0 0 3..00 ma.__2&x lo? ( lb-irk21

Fig. 3.5.4.5--Deflection of fixed-headed shaft subjected1 kip lateral load [modified after Singh, et al (1971)]

to

0 . 0 1

0 . 0 0 7

A

:; a.001

ii

o.m2 r-7 7-m I-l --I T

IO 20 30 so 70 100 200 300 so04 X,X I O ? lIb.-in.2l

Fig. 3.5.4.6--Deflection of a free-headed shaft subjected to1 kip lateral moment [modified after Singh, et al (1971)]

If T > 5 ft (1.5 m), use Fig. 3.5.4.4, 3.5.4.5, or 3.5.4.6,depending on the type of loading (i.e., force or moment)and head fixity (i.e., free or fixed), to determine thelateral deflection for a unit load or moment. For pierswith a fixed-head head, the maximum moment (Mmax) fora 1 kip (4500 N) horizontal load applied to the top of thepier should be determined as follows (Singh, et al. 1971):

MMAX==-0.92T (k-ft) @ z = 0 (3-15)

and,

MMAX ==+0.26T (k-ft) @ z = 2.15T (3-16)

For piers with a free head, the maximum bendingmoment should be taken as the larger of the following(Singh, et al. 1971):

MMAX = 0.98M + 0.45PT (k-ft) @ z = 0.5T (ft)(3-17)

and,

MMAX = 0.85M + 0.73PT (k-ft) @ z = 1.0T (ft)(3-18)

Finite difference method with nonlinear soil response--Preliminary design of laterally loaded drilled piers maybe based on the results of computer methods with non-linear soil response as reported by Reese (1984). The


nonlinear flexural rigidity effects of the pier may beincorporated into the analysis to consider the compositeproperties of the pier.

Nondimensional solutions-Preliminary design of lat-erally loaded drilled piers may be based on the results ofanalyses using nondimensional solutions as reported byReese (1984).

3.5.4.4 Final design-Although several methods areavailable (GAI Consultants, 1982; Poulos & Davis, 1980;Borden & Gabr, 1987), the analysis and design methodreported by (Reese, 1984) is presented herein. Thedesign should ensure that construction methods anddesign assumptions used in the analyses are consistent.

3.5.4.5 Computer design procedure-The computerdesign procedure considers the soil-structure interactionproblem using relationships (p-y curves) to define theground reaction (p) versus pier deflection (y) along thelength of the pier. The use of p-y curves requires bothstatic equilibrium and compatibility of reaction betweenthe pier and ground, with pier deflections consistent withthe stiffness of the pier and ground. Drilled piers areclassified as either long (flexible) or short (rigid) and aseither fixed against rotation or free to rotate at theground surface. The extent of head fixity depends on therelative stiffness between the pier(s) and cap (if present).

The analysis is first performed using a stiffness basedon the concrete modulus of elasticity and a gross momentof inertia. Then the analysis is refined using reduced pierstiffness values based on the composite section, loadingconditions, and code requirements.

3.5.4.6 Response of pier and soil to lateral loads andmoments-- Friction along the bottom of the cap shouldbe disregarded for design purposes because the slightestsoil consolidation beneath the cap eliminates it unlessspecial measures are taken to ensure continued lateralsoil shear resistance. The passive pressure against the capshould also be disregarded wherever excavation for repairor alteration of underground installations will render itineffective. Passive soil pressures mobilized against pierand pier cap can be effective in resisting lateral loads,provided the displacements to mobilize them can betolerated.

The solution of the theory of a beam on elastic foun-dation following the procedures in Reese (1984) of thepier under lateral load must meet two general conditions.The equations of equilibrium must be satisfied and de-flections and deformations must be consistent and com-patible. These two requirements are fulfilled by findinga solution to the following differential equation:

,liZ D’Y +F D’Ycz dr2-p-w=o (3-19)

where

F = axial load on the pier

y = lateral deflection of the pier at a point x alongthe length of the pier

p = soil reaction per unit lengthEcI = flexural rigidityW = distributed load along the length of the pier

Other beam formulae which are useful in the analysisare:

EZd’y=V’ dx’

EZdZy=M= dx2

and

dy s-=dx

(3-20)

(3-22)

where

V = shearM = bending moment of the pierS = slope of the elastic curve

The soil response is modeled as p-y data where p isdefined as force per unit length along the pier (Reese1984). The method for solving the governing equationsand p-y curves are available in the computer programreported by Reese (1984).

3.5.4.7 Scour-- For building structures and bridgeslocated in rivers or bays, the potential for loss of lateralcapacity due to scour should be considered in the design.Refer to Richardson (1991) for general guidelines andmethods to estimate and design bridge structure founda-tions to resist scour.

3.5.4.8 Cyclic loading-- The effects of cyclic loadingon the load-deformation behavior of laterally loadeddrilled piers should be considered in the design. Theeffects of cyclic lateral loading are most pronounced forfree-headed piers in stiff cohesive soils. Cyclic loading inloose granular soils also causes reduced resistance tolateral loading but the effect is much less pronouncedthan in clays (Reese 1984). In general, cyclic loading hasthe effect of progressively increasing deflections of piersin clays due to strain softening.

3.5.4.9 Group action-- Drilled piers in a group areconsidered to act individually when the center-to-center(CTC) spacing perpendicular to the direction of theapplied load is greater than 3d and when the spacing par-allel to the direction of the applied load is greater thanor equal to 8d. When the pier layout does not conform


to these spacings, the effects of shaft interaction shouldbe considered in the design. For the case of closely-spaced drilled piers in a group, the interaction behavioris typically accounted for indirectly using empiricalprocedures proposed by Reese (1984) and Poulos andDavis (1980). One procedure assumes a reduction in thecoefficient of lateral subgrade reaction for a pier in agroup from that of a single pier using the following ratios(Reese 1984):

CTC Shaft Spacing Ratio of Lateral(Parallel to direction Resistance of Shaft inof applied load) Group to Single Shaft(1)

8d 1.00

6d 0.70

4d 0.40

3d 0.25

(1) The reduction does not apply to the lead pier. Itdoes apply to all piles in the shadow of the lead shaft.

The amount of pier deflection should also be con-sidered when evaluating interference. For other spacing,interpolation may be used (Davisson, 1969; Prekash,1962).

3.5.4.10 Combined axial and lateral loading-Theground-line deflections and maximum moments of drilledpiers subject to lateral loads increase with increasing axialload. The effects of combined axial and lateral loadingare most pronounced for free head, short, small diameterpiers in loose or soft ground conditions. Combined load-ings can be accounted for in computer analyses (Reese1984).

3.5.4.11 Sloping ground-- For drilled piers whichextend through or below sloping ground, the potential foradditional lateral loading should be considered duringdesign. Placement of drilled piers above or on slopes willreduce the lateral capacity and increase displacementcompared to similarly sized and loaded shafts constructedin level ground. One method of analysis for piers instable slopes is given by Borden and Gabr (1987). Addi-tional consideration should be given for piers in slopeshaving low factors of safety (marginally-stable slopes) orshowing ground creep and for piers extending throughfills overlying soft soils bearing into more competentunderlying soil or rock formations. Lateral loading fromslope movement can be large and can fail piers in bothbending and shear.

The Geotechnical Engineer is required to give addi-tional consideration to the slope condition and its effectson both soil reaction and lateral load. Lateral load froman unstable slope may be modeled by negative p-y datato consider the 3-dimensional loading effects on the pier.

3.5.4.12 Allowable lateral displacements-- Allowable

lateral displacements for drilled piers should be devel-oped by the Structural Engineer and should be consistentwith the function and type of structure, the anticipatedservice life, and the consequences of unacceptable dis-placements on the performance of the structure.

3.5.4.9 Pier stiffness-- The flexural behavior of adrilled pier subjected to bending is dependent on its flex-ural stiffness, EI. The value of EI is the product of thepier material modulus of elasticity and the moment of in-ertia of the cross section about the axis of bending. Stiff-ness, EI, is essentially constant for the level of loading towhich a structural-steel member is subjected, but both Eand I vary as the stress conditions change for a rein-forced-concrete member (Reese 1984). For concrete, thevalue of E varies because of nonlinearity in stress-strainrelationships, and the value of I is reduced because theconcrete in the tension zone becomes ineffective due tocracking. The tensile weakness of concrete and theensuing cracking is the major factor contributing to thenonlinear behavior of reinforced concrete (Wang andReese, 1987).

In most cases, the EI-value of a reinforced-concretepier is assumed to be constant for simplicity in theanalysis although this assumption is not rational.Analyses by Wang and Reese (1987) have shown that themagnitude of error associated with this assumption canbe large.

3.5.4.9.1 Crack mechanism-- Cracks form whenthe flexural stress due to bending exceeds the tensilestrength of concrete. Immediately after the formation ofthe first crack, the stresses in the concrete near thecracking zone are redistributed. As loading continues,additional cracks open up on occasion, but in general theinitial cracks penetrate more deeply with an increase inload.

Many variables affect the development and character-istics of cracks. The major ones, percentage of reinforce-ment, bond characteristics and tensile strength of con-crete where cracks occur at random, and the location andspacing of cracks, are subjected to considerable variation.Studies have shown that the crack spacing and crackwidth follow a normal distribution and are influenced byeach other.

3.5.4.9.2 ACI 318 recommendations-- The stif-fening effect due to tension in the concrete must beconsidered in making a realistic prediction of short-termdeflections of reinforced-concrete beams and, likewise,piers. The American Concrete Institute has developed anapproximate method of computing stiffness taking intoaccount the effect of cracking (ACI 318). The method issummarized below.

= fJ=7. J)yP

(3-24)(3-25)


and

F’= specified compressive strength of concrete, psi

Lr= modulus of rupture of concrete, psi= moment of inertia of the transformed cracked

section of concrete4 = effective moment of inertia for computation

of deflection4 = moment of inertia of gross concrete section

about centroidal axis, neglecting reinforce-ment

4 = modulus of elasticity of concrete4 = maximum moment in member at stage deflec-

tion is computedMcr = cracking momentr, = distance from centroidal axis of gross section,

neglecting reinforcement, to extreme fiber intension

The effective moment of inertia, Ie, described in Eq.3-23 provides a transition between the upper and lowerbounds of Ig and Icr as a function of the level of crackingin the form of Mcr/Ma.

The flexural stiffness computed directly from thetransformed cracked-section, in terms of the EI at theinstant of cracking, has been recognized to be unrealisticin use. With the modification recommended by ACI 318,the flexural stiffness first shows a constant range ofstiffness for an uncracked section. After the cracks areinitiated by a higher moment, a rather smooth transitionof EI in terms of progressive cracking is represented.When a section is fully cracked due to tensile stresses, EIreaches a minimum value that can be calculated by thetransformed cracked-section method.

3.5.4.9.3 Recommended stiffness-- Pier flexuralstiffness (EI) should be calculated as an upper boundusing the gross concrete section as a first approximation.If the applied moments and lateral loads are of sufficientmagnitude to cause cracking when acting with concurrentaxial loads, then the flexural stiffness, using the effectivemoment of inertia, should be reduced as provided in ACI318. Use of a flexural stiffness between the gross sectionand cracked section values, in accordance with analysissimplified by the computer program, PMEIX (Reese1984) should provide a more realistic pier analysis modeland deflection prediction.

3.5.5 Factor of safety-- Two principal types of failureassociated with soil-structure interaction are identified: asoil failure and a pier structural failure. The soil failureis characterized by excessive deflection of the pier underlateral load and is usually associated with short piers. Thefactor of safety against lateral-load failure of a short piercan be increased significantly by increasing the pier pene-tration. A pier structural failure occurs when the bendingmoment becomes greater than the bending resistance ofthe pier.

The factor of safety should consider the kind of load-ings applied to the foundation, soil properties, and thestructure’s significance. Generally, four types of lateral

loadings on piers are recognized: short-term static,repeated, sustained, and dynamic. The associated soilresponses are different for each loading type. In dealingwith seismic loading, the soils should be tested to deter-mine their susceptibility to collapse or liquidity, thereduction of strength under rapid cyclic loading andwhere possible the p-y characteristics under rapid oscil-lation. These data may be used in the dynamic analysisof the structure as a whole to calculate the transverseloads on the piers. The dynamic analysis of a pier-sup-ported structure involves considerations beyond the scopeof this document.

With regard to sustained loadings, such as those whichoccur due to earth pressures, p-y curves for short-termloading can be used. No significant pier deflections willoccur with time if the supporting soil is granular. Somelong term deflections will occur if the pier is in stiff clay.Consolidation will occur in soft clay and the designermust use available theory and judgment to estimate theadditional deflection and bending moment that will oc-cur. Obviously, it is not possible to establish a specificprocedure for dealing with sustained loading because ofthe multitude of parameters that are involved, includingthe consolidation characteristics of the particular clay orsilt. The selection of a factor of safety, if sustainedloading occurs, must involve the use of considerablejudgment by the Geotechnical Engineer.

The factor of safety for the design of a laterallyloaded pier is principally influenced by the confidence ofthe Geotechnical Engineer in the values of soil propertiesthat are selected for design. The Geotechnical Engineermust formulate judgments on soil properties that arebased on interpolations, extrapolations, and geologicalcontinuity. Uncertainties arise because of the smallvolume of soil that is sampled and because of unavoid-able sample disturbance and other errors in soil testing.A simple numerical value for the factor of safety is notappropriate.

3.5.6 Design organization-- A comment is desirableabout the design team that is responsible for the designof piers under lateral loading. One possibility is that theGeotechnical Engineer could provide soil-response curvesto the Structural Engineer who would do the computa-tions of bending moment and deflection. An alternaterecommended arrangement is to allow the GeotechnicalEngineer to also perform the lateral load analysis andhave the Structural Engineer furnish the applied loads. Inthis latter case, the Structural Engineer should review thelateral load analysis to assure that the pier design andestimated response are compatible with the design of thesuperstructure.

3.6-- Piers socketed in rockThis type of pier is socketed into rock to a depth of

one to six times the diameter of the pier for the purposeof developing high service loading capacity. The drilledpier consists of a heavy wall permanent casing fitted witha cutting shoe and seated into the top of rock. A steel


core or heavy reinforcing cage is encased in the concretewhich extends into the rock socket. A column cap is de-signed to transfer loads from the superstructure to one ormore rock socketed piers. The pier is designed to supportall load in the rock.

3.6.1 Load transfer to rock-- Test loading on highquality rock has demonstrated that, when the depth ofsocket exceeds twice the diameter, service loads areusually fully supported in resistance on the sides of therock socket (Horvath & Kenney, 1979; Koutsoftas, 1981)prior to mobilizing the available end bearing capacity.One of the following design assumptions is usually used:

a) The capacity is estimated based on end bearingonly including the effects of embedment withinthe rock

b) Capacity is derived solely from side resistanceparticularly if it is difficult to thoroughly cleanthe bottom of the drilled pier

c) The drilled pier capacity is estimated using bothend bearing and side resistance

The allowable end bearing is estimated by careful ex-amination of site specific rock cores, compression testson rock core and using local code limits or local practice.Pressuremeter tests and compressive strength tests onrock cores, adjusted to account for discontinuities in therock mass, are also used to estimate end bearing capacity(Canadian, 1985). Rock coring or rock drilling below thesocket in each drilled pier is necessary where rock qualityis variable.

The allowable unit shear within the rock socket is alsodetermined by examination of rock core and relating rockquality and type to local code limits. The compressivestrength of the rock and concrete have been used to esti-mate unit socket shear (Canadian, 1985). The unit socketshear that can be developed is also dependent on theroughness of the socket side walls. Grooves are some-times made in the side walls to enhance shear. Correla-tions between a roughness factor and the ratio of unitsocket shear to compressive strength of the weaker of theconcrete or rock are available (Canadian, 1985). Churndrill methods are sometimes used in hard rock in lieu ofrotary drilling to provide higher socket shear. Shearstresses, however, cannot exceed the allowable bondstress between the concrete and the sides of the socket,200 psi. (1.4 MPa) for Building Code of NYC, 1990;Mass. St. Building Code, 1988 or as given by local codeunless confirmed by load test.

Observation of the rock socket is desirable todetermine rock quality and roughness of the side wall.Observation methods include diver surveys, entering adewatered hole, surface examination with a drop light,probing in shallow holes and underwater televisionsurvey.

3.6.2 Settlement of piers-- Settlement of piers foundedon rock generally does not significantly exceed the elasticcompression shortening of the structural member. Settle-

ment is the result of open joints or seams of compres-sible material in the rock. Settlement can also result frominadequate removal of soft or weak material from thebottom of the rock socket. Pressuremeter tests or plateload tests may be used as an aid in estimating drilled piersettlement in soft rock or hard soil. Elastic solutions forestimating caisson settlement have been made, and areincluded in the Canadian Foundation Engineering Man-ual, 1985.

3.6.3 Structural design of rock socketed piers-- The pieris often designed as a composite column using the proce-dures outlined in Section 3.3. The steel core can be anH-section representing as much as 5 to 25 percent of thetotal pier area. Depending on local code requirements,the casing is sometimes used as a structural member insoils that are non-corrosive. Concrete usually has a com-pressive strength of 3,000 to 6,000 psi (20 to 41 MPa) at28 days. High strengths may be considered where theyare readily available. Where the rock socket is below thegroundwater table and the hole cannot be pumped dry,current practice is to place the concrete using tremiemethods for the full height of the pier.

3.7-- Pier configuration3.7.1 Bells-- The sides of the bell should slope at an

angle of not less than 45 deg with the horizontal for piersinstalled in the dry. Based on research and experiencethe following limitations for allowable end bearing arerecommended as a function of bell slope angle:

Maximum Allowable Min. Bell AngleBearing Pressure from Horizontal

6 tsf (600 KPa)6 - 10 tsf (600 - 1000 KPa)

10 - 15 tsf (1000 - 1500 KPa)greater than 15 tsf (1500 KPa)

45o

50o

55o

60o

The thickness at the edge of the bell should be at least6 in. (152 mm) (See Fig. 2.3.1.1). A 3-in. edge of the bellis sometimes used for piers with bearing pressures lessthan 6 tsf. The diameter of the bell should not exceedthree times the diameter of the shaft. The constructorshould be required to provide a positive means ofdemonstrating that the belled section has not caved inand is clean of soil whenever the bell angle is less than60 deg.

3.7.2 Caps-- Where used, the depth of the cap shouldbe sufficient to accommodate development of the verticalreinforcement from the shaft and the dowels or anchorbolts for the column.

3.7.3 Permanent steel Casing-- Permanent steel casing,if used for maximum confinement effects or as a structur-al member, should have a minimum thickness of 0.0075of the diameter of the pier shaft, unless the soil andwater pressure expected before or experienced duringexcavation, and the maximum anticipated degree of outof roundness requires greater thickness. The critical


buckling stress varies inversely with the cube of theradius (Broms, 1964b). Poor welds and damage due toplacement or handling also affect the ability of the casingto withstand external pressure. The casing should not beincluded in calculations of area or moment of inertia ofthe pier section, except for full length, continuous,noncorrugated casing.

3.7.4 Joints-- Construction joints in the shaft should beavoided if possible. When a construction joint becomesunavoidable, the surface of the concrete should be rough-ened and, in unreinforced piers, vertical dowels of suf-ficient length to develop the bars below and above thejoint should be set in the plastic concrete. The cross-sectional area of the dowels should be not less than 0.01times the gross area of the section in the shaft. Wherevertical reinforcing steel is already present, the dowelsneed only bring the percentage of steel up to 0.01.Where vertical reinforcement equals or exceeds p 0.01no dowels are required. There will normally be a con-struction joint between the shaft and the cap. In highseismic risk territory, the cap and the upper part of thepier may be subjected to moments of magnitude match-ing the ultimate moment capacity of the supportedcolumn and high shear loads. The combination of thetwo may constitute a severe splitting condition, requiringcareful attention to the arrangement of splices in verticalreinforcing steel at the two joints (pier to cap, and cap tocolumn), and to the provision of adequate spiral or hoopreinforcing in the cap and affected part of the pier.

3.7.5 Unbraced piers-- Soil having a standard pene-tration value N > 1 or undrained shear strength greaterthan 100 psf (5 KPa) may be considered to provide lat-eral support and effectively prevent buckling of piers atservice loads. Piers extending above the soil surface orpenetrating caverns or large voids and standing in air,water, or material not capable of providing lateral sup-port, should be designed as columns. Effective columnlength should be estimated based on end fixity. It can beapproximated when the top is pin connected by using theclear unsupported length plus two shaft diameters.

3.7.6 Cyclic loading-- Cyclic loading tends to reducethe lateral subgrade modulus, which may be reduced toapproximately 30 percent of the initial value. Thecombination of group action and cyclic loading mayreduce it to as little as 10 percent of the initial ks value(Davisson, 1969). The stress level, expressed as a per-centage of the stress at maximum deflection, should beconsidered. Small stress levels may result in negligiblereduction in subgrade modulus due to cyclic loading. Theactual reduction should be determined by the Geotech-nical Engineer, using tests where feasible.

3.7.7 Interconnecting ties-- Design for earthquake andother lateral loading may require individual piers or piercaps to be interconnected by ties. While the UniformBuilding Code (UBC 1985) reference recommends thateach tie should be capable of carrying in tension orcompression equal to 10 percent of the larger pierloading, ties may also be required to resist substantial

bending moments. The bending moments will redistributehorizontal loading on the piers, resulting in a need for amore rigorous analysis. Ties are also used to developpassive pressure in the soil under wind loads, and toreduce differential settlements.

3.7.8 Concrete protection for reinforcement, steelcolumns stubs or cores-- Where permanent casing is used,the protective concrete thickness should be no less than3 in. (76 mm), or 3 times (5 times preferred) the maxi-mum size of the coarse aggregate, whichever is larger.Larger clearance between reinforcing and the inside edgeof casing should be considered where the casing is to beremoved particularly when the reinforcing cage andcasing are long. This will reduce the chance that duringremoval the casing will bind against the reinforcing cage

CHAPTER 4--CONSTRUCTION METHODS

4.1--Excavation and casing4.1.1-- Construction methods should be used which in-

sure that the pier hole is properly located and plumb,that the soil adjacent to the hole is not unduly disturbedand that a clean hole of specified dimensions is providedfor its entire length. Each pier should be founded into oron the designated bearing stratum. Concreting should becontinuous and have the strength and minimum dimen-sions specified.

4.1.2-- Excavation may be performed by hand, augerdrill, bucket drill, clamshell, or any other type ofequipment or combination which will obtain the specifiedresults. Overdrilling should be avoided. Where a goodbearing stratum of limited thickness overlies poorermaterial or a water bearing stratum under significanthead, drilling to excessive depths may result in costlyconstruction extras or produce an inadequate foundation,due to reduced pier bearing capacity.

4.1.2.1 During the excavation of the shaft, theconstructor should make frequent checks on the plumb-ness of the shaft. Rough checks can be made by placinga carpenter’s level flush to the Kelly bar. Some causes ofexcessive deviations from plumb are:

a) Failure to initially position and then hold thedrill rig and auger on the design center of theshaft. Sometimes it is necessary to rotate the drillrig or move from the hole during excavation.Care should be taken to reposition and replumbthe auger prior to resuming the drilling.

b) When the auger encounters obstructions such asboulders, old foundations or rubble fill, the augermay tend to veer off and slant the hole.

c) If the drill rig itself is situated on soft ground,uneven settlements may cause the Kelly bar toveer out of alignment.


d) The additional force or torque applied whendrilling in very dense soils may change the Kellybar alignment.

4.1.3-- In hard, firm soils with little or no groundwaterseepage, casing may not be required except for safety.Loose casings, used solely for the purpose of protectingworkmen and observers, should be of at least ‘/4 in. (6mm) thick steel. It is normally removed from the holewhen observation is completed. Under other circum-stances, depending on the method of installation, theground and water conditions and the surrounding facil-ities, a tight casing, slurry, or other means may berequired to retain the earth. Where subsidence must beprevented or kept to a minimum, retaining of the earthshould be required in the Contract Documents.

4.1.4-- Belling of piers, where specified, may beperformed by machine or by hand. Where the nature ofthe soil being belled dictates, or if excessive groundwateris present, special procedures may be required. Someprocedures used successfully in the past are described inBaker (1986).

4.1.5-- When penetration into rock is specified, suchpenetration should be obtained by approved methodssuch as drilling, coring, chipping and chopping. Blastingshould not be permitted in confined areas where suchblasting may cause damage to casing, or affect the sur-rounding soil and property.

4.1.6-- When piers are to be founded on rock, theshaft should be cut into the rock and the bottom steppedor leveled so that the bearing is obtained on surfacessloping not more than 10 degrees.

4.1.7-- When straight shaft piers are designed forbearing in shales or other hard formations, all or aportion of the bearing load may be transferred to theformation by side resistance developed between the for-mation and lower part of the pier. It is the practice, insome localities, to cut a series of groove or key rings intothe sides of the pier hole near the bottom (See Fig.2.3.1.3), but under other circumstances shear rings arenot required to develop side resistance. Grooving isaccomplished by cutters attached to the boring equip-ment.

4.1.8-A drilled pier should not be excavated too closeto another drilled pier in which concrete has been freshlyplaced and has had insufficient time to set. The minimumdistance to prevent a blow-in or collapse of fluid concretefrom one hole to the other depends upon the soil proper-ties, pier geometry, and setting time for the concrete, andshould be determined by the Geotechnical Engineer.

4.2-- Placing reinforcement4.2.1-- Steel reinforcement, steel stubs or core sections

should be accurately placed and adequately supported inthe correct locations. Should the method of pier con-struction employed require removal of casing, careshould be exercised to ensure that the reinforcement orother embedded metal is not disturbed or exposed to sur-

rounding soil during the removal process. Spacers,capable of sliding on the casing, should be securelyattached to the reinforcement.

4.2.2-- The clear spacing between vertical reinforcingbars should be at least 3 times (preferrably 5 times) thesize of the maximum coarse aggregate or 3 times the bardiameter whichever is larger.

4.2.3-- Vertical splices of reinforcing bars arepermitted and should conform to the requirements ofACI 318. Generally, no more than 50 percent of the barsshould be lapped spliced at one location.

4.3-Dewatering, concreting and removal of casing4.3.1-- Casing should be used to effect a seal and cut

off water infiltration into the pier when the casing can beinstalled into an impervious stratum. Where such a sealagainst groundwater is not possible, a dewatering systemshould be installed which will permit proper excavation,observation and concreting of the pier. Should the de-watering system employed contemplate pumping insidethe pier, extreme caution should be used to make surethat the unbalanced water head that is created will notcause a “blow” (bottom heave or quick condition) anddisturb the proposed bearing stratum or surroundingfacilities.

4.3.2-- Infiltration of groundwater, from a source at ornear the bottom, at a rate of less than % in. (6 mm) riseper minute at the bottom of the pier should be consid-ered a dry pier and concrete may be placed by buckets,chuting, tremie pipe, or elephant trunks in a mannerwhich minimizes aggregate segregation; however, thetotal height of water in the bottom of the pier should notexceed 2 in. (51 mm) at the time that sufficient concretehas been placed to balance the water head. It is also per-missible to allow free fall of concrete as long as it can bedirected vertically on the centerline of the shaft, and itdoes not hit the sides of the shaft or the reinforcementcage. If a leak occurs some distance above the bottom,an appreciable amount of water could enter the pier inthe time needed to fill the pier up to the point of leak.Casing through the permeable stratum is recommendedfor this case. If infiltration of groundwater exceeds a riseof ‘/4 in. (6 mm) per minute, the pier should be consid-ered a wet pier, and concrete should be placed by anapproved tremie method such as gravity flow through awater tight vertical pipe or by concrete pumping. (ACI304R, Chapter 8, Concrete Placed Under Water). Beforeplacing concrete by the tremie method, the pier shouldbe filled with water to the natural water level so that thewater head inside and outside the pier are balanced. Inall cases where concrete is placed by an approved tremiemethod, casing or slurry should be used to retain thesides of the excavation during the concreting process.When concrete is to be placed by tremie methods, a re-design of the concrete mix is required to permit greaterslump with no decrease in the specified design strength.It is recommended that every reasonable effort be madeto obtain a dry hole. If these efforts fail, or if economy


dictates, it may be necessary to tremie the pier.4.3.2.1 Compaction of concrete-- The free

unobstructed fall of concrete with a minimum slump of4.0 in. (102 mm) produces adequate compaction up tothe top 5 ft (1.5 m) of depth. Vibration of the concreteis required only for this upper depth. Segregated con-crete and laitance should be removed before proceedingwith construction of the cap. Discharge of concretethrough a hopper with a short downpipe carefully cen-tered on the pier shaft is recommended for free, un-obstructed fall. The presence of a reinforcing cage in apier of small diameter may require dropping the concretethrough a long downpipe. This could then require agreater depth of top vibration, or proportioning of theconcrete mix for a higher slump and smaller maximumaggregate size similar to tremie concrete.

4.3.3-- If ground conditions are such that the casingmay be removed during the concreting of piers, theequipment and procedures used should ensure that theconcrete will not be disturbed, pulled apart or pinchedoff by earth movement. It is of extreme importance toestablish that this can be accomplished prior to theremoval of the casing and to check concrete level duringremoval. The level of the concrete should always bemaintained a minimum of 5 ft (1.5 m) above the bottomof the casing during the placing of concrete, butsometimes a much higher head is required, dependingupon the water head outside the casing. Proper con-sideration must be given to the proportioning of the mix,including cement factor, slump, and admixtures, in theplacing of concrete and the pulling of casings.

4.3.4-- Casings should be maintained in good shapeand free from old concrete on the inside surfaces thatwould tend to make pulling difficult. When ground move-ments are suspected, frequent measurements, by theConstructor of the shaft diameter should be made in atleast two perpendicular horizontal directions, and atreasonably spaced elevations to assure that the specifiedminimum shaft diameter is being maintained and thatout-of-roundness does not indicate imminent collapse.Casing should be of sufficient length to extend pastcaving soil. The casing diameter should closely match thediameter of the hole when removal of casing is intended.

4.3.5-- When piers penetrate very unstable strata, suchas peat, and casing is required to maintain the shape ofpier through these layers, then the casing should not beremoved.

4.3.6-- The theoretical volume of concrete required tofill the pier should be computed. If the actual volume(estimated by delivery tickets, for example) installed isappreciably less than the theoretical volume, the pier mayhave experienced pinching, collapse of side walls, orcontamination of tremie concrete. If a pier defect issuspected then options include investigation of the as-installed pier or immediate reinstallation before theconcrete sets up. Rejection of an unacceptable pier willrequire installation of one or more replacement piers atlocations that will facilitate load transfer from the

structure above.4.3.7-- The concrete in a pier should, if at all possible,

be placed in one continuous operation. If a constructionjoint is unavoidable, it should be treated in accordancewith Section 3.7.4. If the casing is to be removed, and thejoint was unintended, it may be necessary to cut thecasing at the joint and leave the portion below the jointin place permanently.

4.4--Slurry displacement method4.4.1 Excavation-- Methods and equipment used for

shaft excavation should leave the side of the hole and thebottom free of loose material which would prevent inti-mate contact of the concrete with firm, undisturbed soilor rock. The auger should be raised and lowered at aslow enough rate so the slurry does not swirl or causessuction on the side walls as the tool is withdrawn. Swirlof the slurry can cause scouring. Suction can cause cave-ins. Reclean the hole if more than 6 in. (152 mm) ofcuttings have settled in the bottom of piers designed withno end bearing.

All spoil and excavated materials should be kept awayfrom each open shaft excavation to avoid contaminationof the excavation after final cleanout.

4.4.2 Installation method-- Where drilled piers are tobe installed below the groundwater level and in caving orsloughing soils, a casing or slurry should be used tostabilize the excavation. The slurry level in the excavationshould be maintained at a minimum of 5 ft (1.5 m) abovethe static groundwater level and above any unstablezones a vertical distance that will prevent caving orsloughing of those zones into the excavation. Theconstructor should be required to demonstrate that stableconditions are being maintained. This can be accom-plished by taping and sounding the bottom conditionwhen no drilling is occurring.

Where the purpose of the slurry is only to maintain anopen hole until a casing is placed (wethole method), theinitial drilling fluid may be water unless experience hasshown that a slurry is required. If no casing is planned,then control of the slurry is much more critical. In somesoil profiles, it may be possible to use only water, but thisis uncommon. Increasing the slurry viscosity by mixingvarious mineral solids or chemical polymers with thewater will be required if water does not stabilize the pierexcavation and lift drill cuttings.

The slurry should be mixed in mud tanks on-site orarrive at the site premixed; combining or mixing slurry inthe shaft should not be permitted in the slurry dis-placement method but is permissible in the casingmethod. Polymer slurry may be mixed in the shaft.

The slurry should consist of water or a stable colloidalsuspension of various pulverized solids or polymersthoroughly mixed with water so that appropriate prop-erties are maintained. Attapulgite and bentonite shouldmeet API Specification 13A. The type of various solidsused will depend on the subsurface conditions and thecharacteristics of the mixing water. A test report from the


supplier giving the physical and chemical properties ofthe mud should be supplied to the GeotechnicalEngineer at the start of the work.

The slurry should be mixed, stored, and transportedusing equipment normally used on drilled pier projects.The water used to mix the slurry should be clean, freshwater, of a quality approved by the Geotechnical En-gineer. Any physical or chemical treatment of the wateror slurry that is considered necessary to have the slurrymeet the specification is the responsibility of the drilledpier constructor.

The slurry should meet the specifications given inTable 1. Water may be used in place of mixed slurry, butit must provide pier excavation stabilization and besubject to the approval of the Geotechnical Engineer.Slurry testing should be performed and recorded forquality control. All field test equipment should be pro-vided by the drilled pier constructor. The frequency oftesting should be a minimum of two sets of tests perwork shift, the first test being done at the beginning ofthe shift. Field conditions and the requirements of thedrilled pier constructor and the Geotechnical Engineermay make more frequent testing necessary, such asmultiple tests per pier, to assure an acceptable slurry.The drilled pier constructor should have available at thesite, a slurry sampler capable of obtaining slurry samplesat any depth within the drilled shaft excavation if re-quested by the Geotechnical Engineer.

The constructor should use such drilling tools andexcavation procedures that excessive negative fluidpressure in the excavations is prevented. At the com-pletion of excavation, the drilled shaft bottom should becleaned with an air lift system or a cleanout bucketequipped with a one-way flap gate that prevents spoils inthe bucket from re-entering the shafts, or other suitabletools. The hole slurry should meet the specifications priorto concreting. If cleaning, recirculating, desanding, orreplacing the slurry is necessary, the contractor should beprepared to do so.

Concreting the drilled piers should be completed thesame day that the excavation is complete. If this is notpossible, the excavation should be redrilled, cleaned, andslurry tested before concreting. The theoretical concretevolume may be plotted graphically for each pier size andcompared with the actual volume of concrete placed aftereach truck load (or at regular intervals if pumped) andnoted on the graph.

For piers with end bearing, the slurry sand contentshould be limited to 4 percent; otherwise the sand con-tent should be limited to 10 percent, but sand contents to25 percent have been used successfully in some locations.The slurry density should not exceed 75 pcf (1.2 mg/m2)for end bearing piers. The closer restrictions in the caseof end bearing piers are necessary to facilitate feeling thebottom with a sounding rod or weighted tape.

Slurry should be sampled and tested from the mudtank and from samples recovered within 1 ft (0.3 m) fromthe bottom of the drilled pier.

The bottom of the drilled shaft should be checked toconfirm that drill cuttings and hole sides are not fallingto the bottom. “Sounding” a tremie pipe or air lift pipe,or “feeling” with a weighted line are acceptable methodsfor checking bottom build-up of material. The permis-sible amount of drill cuttings at the bottom of the piershould be less than 6 in. (152 mm) in depth without re-cleaning the hole or modifying the slurry for piersdesigned without end bearing.

4.4.3 Reinforcing steel-- Reinforcing steel should beplaced in a shaft excavation after the Geotechnical En-gineer has completed his observations and has approvedplacement.

Following approval of the excavation, the reinforcingsteel should be centered in the hole at the correct posi-tion and elevation using spacers as necessary.

Reinforcing steel should not touch the sidewall of theexcavation and should be completely encased in concreteusing appropriate spacers as necessary. Minimum con-crete cover of 3 in. (76 mm) should be maintained andthe minimum clear spacing between reinforcing steelshould be 4.0 in. (102 mm) between horizontal rein-forcement and 3 times (5 times preferable) the size ofthe maximum coarse aggregate between vertical rein-forcement (a minimum of 3 in. [76 mm]).

4.4.4 Concrete-- All concrete should satisfy thefollowing requirements:

a) Concrete used in the slurry displacement methodshould have a slump of 7 - 9 in. (178 - 229 mm).

b) Maximum aggregate size should be % in. (19mm).

c) Adequate retarder to produce a 5 in. (127 mm)or greater slump after 4 hours. The type of re-tarder should be approved by the Engineer.

Placement of concrete should be made following ap-proval of the excavation by the Geotechnical Engineerand placement of the reinforcing steel.

Pier excavations should be clean before the start ofconcrete placement. An air lift or a clean out bucketequipped with a one-direction flow gate should be usedto clean the bottoms of each shaft, depending on theContract requirements.

If the design includes end bearing, the bottoms shouldbe sounded again with the weighted tape after the steelplacement and just prior to concrete placement. All con-crete must be placed within the time limit during whichthe excavation remains clean and stable and the concreteremains fluid. Placement should not begin until adequateconcrete supply is assured.

4.4.5 Concreting methods-- Concrete may be placedby tremie methods or by pumping. In either case, aplugged, capped, or “rabbit” plugged tremie should beinserted and seated in the excavation. The tremie shouldextend to the bottom of the shaft prior to the com-mencement of concrete placement and care should betaken to ensure that all slurry suspension is expelled from


the pipe during the initial charging process if the “rabbit”plug approach is used. The tremie pipe should be em-bedded in fresh concrete a minimum of 3 to 5 ft andmaintained at that depth throughout concreting toprevent entry of slurry into the pipe. The concrete shouldbe placed while keeping the tremie tip embedded in theconcrete. The first portion of concrete flow should bewasted by overflow at the top of the pier as it is usuallycontaminated with mud. Rapid raising or lowering of thetremie should not be allowed. Equipment and proceduresshould be described in writing to the Engineer for priorapproval.

In the capped tremie pipe approach, the tremie pipeor pump line should have a seal, consisting of a bottomplate or approved equal, that seals the bottom of thepipe until the pipe reaches the hole bottom and enoughconcrete has been placed to seal off water flow into thetremie pipe. The use of a disposable “rabbit” or “pig”inserted in the pipe to separate the concrete from theslurry is acceptable.

In the “rabbit” plug approach, the open tremie pipeshould be set on the bottom, the “rabbit” plug inserted atthe top and then concrete placed, pushing the “rabbit”plug ahead separating the concrete from the slurry.When the pipe is fully charged, the pipe should be liftedoff the bottom only enough to start the concrete flowing.

During tremie placement, the bottom of the tremiepipe should not be lifted above the concrete level. If theseal is lost, the pipe should be withdrawn, the seal re-placed, and the tremie operation restarted using the“capped” tremie approach.

Aluminum pipe or equipment should not be used forplacing concrete.

Exposed concrete should be protected against damageand should be cured and protected to prevent moistureloss and temperature extremes in accordance with ACI301.

4.5--SafetyThe following safety provisions should be regarded as

a minimum. Government regulations, OccupationalSafety and Health Administration (OSHA) may imposestricter measures.

4.6.1 Gas in hole-- All personnel involved in the exca-vation of a pier hole should be alert to toxic andexplosive gases which may be released into the hole. Gasmasks, gas detectors, adequate first aid equipment, andblowers to force fresh air to the bottom of the holeshould be readily available on the job site to assistpersonnel in emergencies. If gas is encountered or anti-cipated, absolutely no personnel entry will be permitteduntil the shaft has been properly vented and tested.

4.6.2 The use of full body safety harness and safetyrope is recommended for any person entering a pierhole. Protective steel casing to retain soil in the shaftwalls should be employed when any person is in the shaftor working at the bottom. A protective cage (diving bell)may sometimes be used in lieu of full length casing.

4.6.3 The top of any pier hole should be covered whenexcavation work is discontinued or finished and the holeleft open for any reason. The cover should be substantialand strong enough to prevent any person from fallinginto the hole.

CHAPTER 5--INSPECTION AND TESTING

5.1--ScopeThe purpose of inspection and testing is to see that

the pier is constructed in accordance with the designassumptions and specifications, within acceptable tol-erances or, if excessive deviations occur, to furnishinformation needed for corrective measures. It involves,but it is not limited to, checking the location, excavation,plumbness of the shaft, and bell and shaft dimensions ifapplicable; determining the proper depth of excavationand the strength of bearing stratum; and observing thatproper materials and concrete placement procedureshave been used.

5.1.1 Common causes for faulty piers-- Some of themost frequent conditions that lead to faulty piers are:

b)c)

d)

e)

f)

g)

h)i)

j)

k)l)m)n)0)

a) Development of voids in a concrete shaft due toimproper pulling of casing and the use ofconcrete having too little slumpConcrete placed in seepage water in the pierSide cave-in of soil, resulting in contaminatedconcreteImproper location or plumbness of pier, or im-proper reinforcementSurface cave-in of soil resulting in contaminatedconcrete or soil-filled voidsImproperly placed tremie concrete resulting insegregation or intermixing concrete with slurry orwaterSqueeze of shaft or bell excavation prior to con-creting or during concretingCasing collapseExcess water or contaminated concrete at coldjoints resulting in a reduction in concrete qualityMigration of water, resulting in dilution of theconcrete mixture and a reduction in concretequalityPoor concrete delivered to siteInadequate bell sizesInadequate bearing materialIncomplete slurry displacement during concretetremie placementSand settling out of slurry during concreteplacement, resulting in stiffening the concrete(slump loss) and folding over of concretetrapping the sand sediments

5.2--Geotechnical field representativeThe Geotechnical Engineer should be responsible for

drilled pier installation inspection. The field repre-


sentative should be on the staff of the GeotechnicalEngineer and should be qualified in pier constructionwork, and have sufficient technical educationalbackground to interpret observations correctly and tocommunicate what is seen. Continuous full-time obser-vation of each pier installation is recommended.

5.3--Preliminary proceduresPrior to design, a subsoil study should be made by

registered Geotechnical Engineers using experiencedpersonnel to perform borings, sampling and field testing.For this study no unreasonable restraints on the amountof soil exploration should be imposed by the Owner,Architect or Structural Engineer.

In addition to determining the location, strength andcompressibility of bearing materials at boring locations,it is desirable to anticipate problems that may be encoun-tered during construction. It is sometimes necessary todrill exploration holes at each pier location. This may bethe case when the piers will pass through fills containingconcrete and other obstructions, or where bearing depthsor bearing materials are quite variable. In some cases,large exploratory auger holes or test piers are required toassist in the evaluation of actual conditions to beencountered.

Subsurface boring and test data and reports should bemade available to the constructor before bidding. Negoti-ated contracts with qualified foundation constructors arerecommended. If more than one constructor is consid-ered qualified by the Geotechnical Engineer, then com-petitive bids may be obtained. Before a contract issigned, it is important that there should be agreementbetween the Geotechnical Engineer, the StructuralEngineer or Architect, and the constructor as to foun-dation design and problems which may be encounteredduring construction. The constructor should submit aplan for foundation construction work for review by theGeotechnical Engineer and the Structural Engineer orArchitect, and any differences of opinion should beresolved. A preconstruction meeting to review theprocedure is important.

Provisions for handling unforeseen conditions shouldbe available, and methods for such conditions should beagreed upon and become part of the contract.

5.4- Inspection proceduresAdequate inspection requires downhole observations

and tests on each end bearing pier hole wherever prac-tical (see Drilled Shaft Inspectors Manual). Observationfrom the ground surface may be required because of un-safe conditions or slurry placement. Procedures forobservation from the top are described in 5.4.3.9.

Side resistance piers do not normally require down-hole observation unless shear rings are specified.

5.4.1 Location-- The horizontal deviation of the actualpier center at the top (cutoff) from the design centershould be determined with instruments and recorded. Ifa protective casing is used, the top must be checked with

a plumb bob from staked reference lines, and not bereferenced to the surface casing.

5.4.2 Plumb deviation-- Plumb deviation refers to thehorizontal deviation (from the vertical) of the pier and isalso know as bottom plumbness. The horizontal deviationof the lower end of the shaft or bell should be referencedto the vertical plumb line of the pier design center at thetop.

If for some reason the pier cannot be entered, theplumb deviation can be estimated by moving the plumbline carefully from the design center at the top to theedge of the shaft at the bottom in all four directions andmeasuring the distance from the center. Care should beexercised for this type of measurement, since the plumbbob suspended from the long line will tend to move ex-cessively, or may adhere to the pier walls at the bottom.

5.4.3 Removal of obstructions-- The geotechnical repre-sentative should check that all obstacles which could bedetrimental to the proper construction of the piers areremoved and logged.

5.4.3.1 Casing-- Where casing thickness, length,diameter or other properties have been specified, thegeotechnical representative should see that these require-ments are satisfied.

5.4.3.2 Loss of ground--Ground subsidence canoccur from surface caving, squeezing of clays into thepier excavation, flow of saturated silts and sands into thehole and from the removal of soil during pumpingoperations.

The Geotechnical field representative should checkthe water discharge from the pumps periodically to deter-mine the percent of fines carried in the water. Sedimenttanks with overflow weirs are useful, and frequentlynecessary, for this purpose. Soil movements which couldcause damaging loss of ground should immediately bebrought to the attention of the constructor and theArchitect or Engineer by the Geotechnical field repre-sentative. Difficulties in inserting and removing thedrilling tools in the shaft excavation provide an indicationof squeezing of soils. Volume comparison of theoreticalversus actual concrete quantities can also be helpful.

5.4.3.3 Control of ground water-- Water in the pierexcavation can occur from loss of seal around the tem-porary casing (or surface inflow if no top casing is used),seepage from a granular saturated layer in the shaft orbell, or inflow from the bottom of the excavation. TheGeotechnical field representative should observe thewater control methods and report on any inadequaciesnoted. For proper observation of bottom clean-up,testing of bearing material and proper placement ofconcrete, the water level in the bottom of the hole shouldbe held to a depth not exceeding 2 in. (51 mm). Wherethe top flow cannot be adequately controlled, specialprocedures will be required (see Section 4.3.2). Thespecial procedures adopted must have the specificapproval of the Architect or Structural Engineer and theGeotechnical Engineer. A representative coring of con-crete placed by special techniques in or through water or


slurry is often required to check the soundness andcontinuity of the concrete. Sonic and gamma loggingtechniques used in pre-placed access tubes have alsoproven useful for this purpose.

When use of a slurry precludes physical observation ofend bearing pier bottoms, definitive evidence of reachingthe specified bearing material must be obtained by theuse of a remote testing device, borings, special samplingand sounding techniques, or other means.

5.4.3 .4 Depths of pier-- The geotechnicalrepresentative must determine when the bearing materialhas been reached and when the depth of the pier isadequate. This determination can be made by appropri-ate tests on samples of the bearing material and correla-tion with the original design boring logs.

5.4.3.5 Belled piers-- Bell bottoms should bereasonably flat. Downhole observation is advisable tocheck the bell shape, dimensions, and concentricity, andverify that the bell roof and shaft walls are stable. TheGeotechnical field representative should make actualmeasurements to see that the bell meets the specificationrequirements. When shaft and bell entry is not consid-ered safe, special remote or indirect measuring pro-cedures are required as described in 5.4.3.9 and inreferences.

5.4.3.6 Cleanout-- Good cleanout of soft, loose,disturbed soil in the bottom of an end bearing straightshaft or belled pier should be obtained. The amount ofspoil should cover not more than 10 percent of thebearing area to a maximum depth of 2 in. (51 mm).Downhole observation is necessary to see that the spe-cified limitations are being met. It should be noted thatsome buckets tend to ride up on spoil as they cut the bellwalls and build-up of spoil can occur. If satisfactorybottom cleanup cannot be accomplished by the drillingequipment, hand clean-up by workmen is necessary.

In some unfavorable soil and ground water conditions,caving of bell roofs and soil movement into the bottomfrom seepage water can occur after initial cleanup. Forthese reasons, a final observation should be made justprior to the start of concrete placement. Any additionaldisturbed material should be removed at this time to thesame quality of cleanup as that recommended above.

If it is considered unsafe by the contractor to sendworkmen into the hole because of water seepage andrunning silt or an unstable bell roof condition, thenspecial measures are required these normally would bediscussed with the Geotechnical Engineer and owner, andconsidered extra cost and paid for as an extra. They mayinclude casing off the problem area and belling below, ifsoil conditions permit, enlarging the bell under slurry,tremie placing a cement grout to fill the bell, allowingthe grout to set up over night and then redrilling the bellpartly in the grout and partly below so that the remaininggrout collar provides support for the unsuitable soil. TheGeotechnical field representative must carefully observethese special procedures to see that the design foun-dation criteria is accomplished.

In certain situations, where the bell undercut is small,direct construction of the design bell under controlledslurry (see Section 4.4.1.2 and 4.4.1.5) may be permis-sible. However, in this case bottom cleaning with an airlift and sounding of the bottom to check the bottom con-dition prior to concreting are required.

5.4.3.7 Bearing material-- In hard soil and soft rockformations, close observation and testing of the bearingmaterial is important, and it requires experienced judg-ment. Upon reaching the bearing stratum and prior tobelling and cleaning, a strength test or a penetrometertest should be made on the bottom material. One ormore probe holes may be drilled below the design levelwhere water or gas problems would not result therefrom,and if useful information is expected. The use of probeholes and the information to be gained from them shouldbe discussed at the preconstruction meeting prior tocommencement of shaft excavation.

In clay soils, hardpan, or soft shales, tests should bemade for shear strength in order to check that the re-quired bearing capacity is met. Care should be exercisedto make sure that the sampling and testing is performedon undisturbed material which is representative of thematerials over the bearing area. It is good practice tomake several tests over large bottom areas to establishthe degree of uniformity of material and the represen-tativeness of the original test samples.

When piers are placed on or in formations of expan-sive clay or shale, the loads on the bearing material mustbe sufficient to resist uplift as determined by appropriatetests, or the pier bearing level must be situated at suf-ficient depth to preclude that moisture content at thatlevel will vary after construction. Surface sources of watermust be avoided through proper drainage systems, toprevent free water from percolating along the perimeterof the pier. Otherwise the pier must be designed to resistuplift. The magnitude of uplift forces as well as theability of the pier shaft and/or bell to resist the upliftshould be determined by the Geotechnical Engineer.

Where the bearing material is granular and containsless clay, triaxial shear tests may be performed as part ofthe original exploration program to confirm strength pro-perties and to avoid field delays. Also, in-situ pres-suremeter tests, split-barrel penetration tests, DutchCone Penetration Test or other specified suitable testsmay provide meaningful data. Where the properties andthe range of variability of a bearing stratum have beenadequately determined during the geotechnical explor-ation, visual corroboration of the bearing stratum may besufficient. This is more cost effective than time con-suming laboratory or in-situ tests.

In some areas where cavernous rock conditions arepresent, a borehole at each drilled pier location may beneeded in advance of pier installation.

Drilling of small diameter probe holes into the bearingstrata to provide information on the adequacy or unifor-mity of the bearing stratum with depth should be donewith caution in clay or hardpan when water-bearing sand


and silt layers or lenses are suspected within or below thebearing stratum. If such layers are penetrated, seriousconstruction problems may result from infiltration andthe washing of silt or sand into the pier. Probe holes arerecommended only where soft layers are suspectedunderneath the bearing layer or when it is felt necessaryto supplement original boring investigations. When probeholes indicate soft material below the bearing stratum,the pier should be extended deeper or undisturbed sam-ples of the soft soils should be secured, appropriatelaboratory tests made, and the results analyzed by theGeotechnical Engineer to determine the allowablebearing capacity of the pier, and the bell enlargedappropriately.

The need for probe holes should be minimized byperforming more extensive exploration in advance ofconstruction.

5.4.3.8 Bearing material under slurry-- If constructionof the pier is performed under controlled slurry so thatphysical bottom observation is not practical, the bottomcan be checked by the geotechnical representative byputting his hand on the airlift pipe and “feeling” thebottom as the drill operator raises the pipe 6 in. (152mm) off the bottom and then lets it drop. A sharp im-pact is felt when it hits the bottom if adequately cleaned.The geotechnical representative should also check thebottom with a wire attached sounding rod. A % in. (19mm) diameter bar about 3 ft (0.9 m) in length makes agood “sounding rod.” If desired, samples of the bottomcan be obtained by attaching special split-barrel samplingtubes either to the drill rig Kelly bar or to the airlift pipeand then dropping them on the bottom with sufficientforce to penetrate the bearing material.

5.4.3.9 Bearing material hard rock formations-- It isoften difficult to determine in advance the necessarydepth of excavation into the rock without a boring ateach pier. The geotechnical representative should makea serious effort to observe that the pier is founded on asound formation and not on boulders. Soft, weatheredand broken rock should be removed to hard solid forma-tions unless the design is based on support directly onthe weathered rock. In some cases weathered rock mayextend tens or hundreds of feet deep. Where piers aresocketed into the rock and part of the load is carried byside resistance (see Section 3.6), the properties of therock assumed in the design need to be known by the geo-technical representative so that the adequacy of the rockcan be checked in the field. Sound competent rock isnormally required for the entire depth of the socket aswell as for the bottom bearing material, except whereengineering analysis indicates that this is not necessary.

Inspection holes are recommended in order to deter-mine if seams of clay or other soft material exist in therock close enough to the bearing level to affect thedesign of the pier. From one to three probe holes arerecommended, depending upon the size of the pier, thedesign bearing pressure, and the anticipated quality ofrock. Additional probe holes may be made to determine

the extent and thickness of seams before a decision ismade to remove the rock down through the unacceptablelayers. The probe holes can be drilled with an air drilland should extend into the rock a minimum depth equalto the shaft diameter. The speed of air drilling should benoted by the inspector and the sides of the drill holechecked with a feeler rod, a borescope or TV camera todetermine the location and size of cracks and clay seams.Tolerable settlement usually governs the permissible sizeand location of cracks and clay seams.

5.4.3.10 Bearing material under water or slurry --Hard fomtation-- If construction of the pier is performedwhile under water or controlled slurry so that bottomobservation and probing is not practical, a probe holeshould be made in the rock at each pier location prior topier construction in order to assess rock quality anddetermine socket length and bearing elevation. “Sound-ing” of the bottom can be performed as indicated in5.4.3.7.1.

5.4.3.11 Pier inspections-- The constructor shouldcheck each pier hole for methane and carbon dioxide gasbefore entering. If gas exceeds acceptable limits, it mustbe purged before any personnel enter, or else gas masksmust be used. No smoking or welding should be permit-ted until the absence of gas is determined. If purgingproves impractical so that inspection of the bottom is notpossible, determine the bell size by observing the positionof previously calibrated marking on the Kelly or bellbucket when the bell bucket is fully extended duringbelling. If there is any question about the adequacy ofthe mechanical clean up at the bottom, the bell can beoversized and any remaining spoil back-bladed to theoversize area at the edge of the bell.

5.4.3.12 Safety precautions-- All Occupational Safetyand Health Administration (OSHA) safety regulationsshould be followed for personnel entering a shaft forobservation or cleaning or any other purpose.

5.5--ConcretingThe following monitoring should be followed:5.5.1 Observe if the reinforcing steel is clean and

properly placed, and whether bar sizes and lengths are asdesigned.

5.5.2 Observe the condition of the pier bottom justprior to placing of concrete. If sediment has accumulatedon the bottom, reclean and re-check prior to concreting.

5.5.3 Visually observe to the extent practical thecondition of the shaft walls or steel casing which will bein contact with the fresh concrete, and note the positionof the water level behind the casing. The concrete shouldbe placed immediately after these observations.

5.5.4 Visually observe to the extent practical themethod of placing concrete in the pier to see that thereis no segregation or contamination of the materials whensuch methods as free fall through a hopper, tremie pipe,or back chute are used.

5.5.5 Perform tests on fresh concrete such as slump,air content and wet unit weight when required, and cast


test specimens. It is recommended, in addition to a set ofcylinders for each specific batch of concrete yardageplaced (typically 50 yard to 100 yards or day’s pour, ifless), one test cylinder be taken from every truckload andtested at seven days to provide indicator information todetect potential problems.

5.5.6 Observe that concrete is placed continuouslywithout interruption or long delays, and that a sufficienthead of concrete is maintained inside the casing tobalance the head of water outside the casing. This willprevent inflow of water and soil quick condition into thefresh concrete as the casing is withdrawn. Compare thetheoretical volume with the actual volume of concreteplaced (see Section 4.3.6).

5.5.7 Observe the level of the concrete during theinitial casing pull. If the concrete is observed to rise,immediately stop operations and make necessary correc-tions in the procedures and investigate damage caused bya potential discontinuity in the concrete.

5.5.8 The Geotechnical field representative should beconstantly alert for evidence that soil is included in theconcrete by any mechanism.

5.5.8.1 Soil cave-ins when placing concrete in uncasedholes of questionable stability-- Close observation of thehole during concrete placement is necessary to hopefullydetect a cave-in. During winter months, when there is anexcess of vapor from the hot concrete, detection is moredifficult. When a cave-in takes place, the pier must becleaned out, cased, and the concrete placed again. It isalways possible for a soil cave-in to go undetected.Placing in uncased holes should be avoided except insoils of reliable stability, such as cohesive soils ofadequate strength. The constructor is responsible forconstructing a continuous pier.

5.5.8.2 Surface soil cave-in over top of casing-- Thiscan occur only when the casing does not protrude suffi-ciently above ground surface or when there is a spoil piletoo close to the pier location.

5.5.8.3 Side soil cave-in as casing is pulled andconcrete placed-- This results either from inadequate headof concrete in the casing at the time of pulling to balancethe forces tending to cause soil cave-in, or, it can occurif the concrete becomes “hung up” in the casing either byarching due to low slump or because of a fast set. Thesuction created as the casing and concrete are pulledtends to cause an influx of soil and water beneath thecasing. The Geotechnical field representative, as well asthe Constructor, should confirm that these conditions donot occur, or that they are corrected if they do occur.This can require redrilling the hole using proper pro-cedures after deciding what went wrong.

5.5.9 When concrete is placed underwater either bytremie procedures or by pumping, the following observa-tions should be carefully noted:

a) Water level in the pier is static prior toconcreting

b) Adequate initial separation of concrete and

c)

d)

e)

f)

water. Check the dimensions of the pig to seethat the fit is close. If a plate is attached tobottom of pipe, check the release mechanismPlacement of tremie or pump pipe on the bottomof the pierPipe is fully charged with concrete before liftingpipe off bottomMonitor the level of concrete in the pier with aweighted tape, and observe position of tremie orpump pipe. Continue monitoring the relativeposition of concrete and pipe to see that pipe isalways in concreteCompare calculated theoretical concretevolumeswith actual placed volumes after placement

5.5.9.1 If concrete is placed under slurry withoutcasing, and a specially designed slurry has been specifiedfor a side resistance pier design, measure viscosity, den-sity and sand content of slurry to see that it meets speci-fications, in addition to procedures outlined in 5.5.9.

5.6--Methods of exploration to determine soundness ofpiers

Critical soil or ground water conditions, or ques-tionable construction methods may raise doubt about theintegrity of certain piers on a project. The most fre-quently used method for determining the soundness of apier is core drilling with associated probings on the sidesof the core hole, TV inspection, borescope inspection,and sonic testing between two core holes. Sonic devicesand techniques without core holes, such as surfacereflection techniques or buried sonic devices in thebottom of the pier have also been used (Terzaghi &Peck, 1967). However, sonic testing from the surface isunlikely to detect unsatisfactory bottom conditions.

For concrete placement underwater or with slurryusing appropriate tremie or concrete pumping proce-dures, a satisfactory and relatively convenient method forchecking concrete integrity and quality after placement isto use either sonic testing or gamma logging techniquesin preplaced access tubes extended to the base of thepier (Baker, 1986).

5.7--ReportsDaily Geotechnical field representative’s reports,

signed by the Geotechnical field representative anddrilled shaft constructor should be summarized promptlyfor the Architect, Structural Engineer and GeneralContractor. These reports should cover the followingitems when applicable:

a) Accurate location and dimensions of pier holesbored

b) Accurate top and bottom elevations; The degreeof accuracy necessary should have been previ-ously specified by the Structural and the Geo-technical Engineer)

c) Type of shaft-excavating methods used


o

e)

f)

g)

h)

i)

j)

k)

l)

m)

n)

0)

p)

q)

r)

s)

Description of materials encountered duringexcavationDescription of groundwater conditions encoun-teredDescription, location and dimensions of ob-structions encountered and whether removal wasattainedDescription of temporary or permanent casingplaced, including purpose, length, walI thickness,and anchorage or seal obtained, if anyDescription of any soil and water movement, belland wall stability, loss of ground, methods forcontrol, and pumping requirementsData secured for all shaft, bell and shear ringmeasurementsDescription of cleanout methods and adequacyof initial cleanoutElevation at which bearing material encountered.Description of bearing material, probe holesmade, method of probing, rate of drilling in rock,samples taken, tests made and conclusionsreached with regard to adequacy of bearingmaterial. (If inspection probing and testing isdone independently by the Geotechnical En-gineer, the conclusions of the Geotechnical En-gineer should be included.)Description of adequacy of cleanout just prior toconcrete placementRecord of depth of water in hole and rate ofwater infiltration prior to concrete placementRecord of reinforcing steel inspection for posi-tion and adequacyMethod of concrete placement and removal ofcasing, if any. Record head of concrete duringremoval of casing. Record elevation of concretewhen vibration startedRecord of any difficulties encountered. Thisshould include possible soil inclusion, possiblevoids, possible shaft squeeze-in and possiblecasing collapseCondition of concrete delivered to site includingrecord of slump, unit weight, air content, andother tests and cylinders made for compressiontestingRecord of any deviations from the specificationsand decisions required

5.8--Criteria for acceptanceThe following minimum criteria for acceptance of

completed piers should be included in the specifications.Additional or closer tolerances may be specified by theArchitect or Structural Engineer in the contract docu-ments. Any variations should be brought to the attentionf the Architect, Structural Engineer, and Geotechnical

Engineer prior to acceptance.5.8.1 Location and plumbness-- Unless compensated

for in the structural design, permissible construction

tolerances for plumbness should be in accordance withACI 336.1. The tolerance on location at the top of thepier should be not more than 4 percent of the pierdiameter or 3 in. (76 mm) in any direction, whichever isless. Tolerances smaller than 2 in. (51 mm) are difficultto obtain.

5.8.2 Shafts, bells, and shear-- Measurements ofshafts, bells and shear rings should be made by the Geo-technical field representative. The area of the shaft andbell bottom should not be less than 98 percent of thatspecified. In no case should the bell roof slope at anangle less than 45 degrees with the horizontal and thedesign edge should be at least 6 in. (152 mm) thick. Thebell roof slope should be a straight line or be concavedownward. In no case should it be concave upward bymore than 3 or 6 in. (76 or 152 mm) thick per Section3.7.1.

5.8.3 Cleanout-- All loose material and spoil should becleaned out of shafts and bells prior to placing concrete.In the case of end bearing piers the volume of loosematerial and spoil should never exceed an amount thatwould cover 10 percent of the area to a depth of 2 in.(51 mm).

5.8.4 Reinforcement-- Refer to ACI 117.5.8.5 Pier Acceptibility for Concrete Placement-- A pier

should not be considered acceptable for concrete place-ment until the inspection procedures described in Section5.4 have been accomplished as applicable.

5.9--Corrective measuresShould allowable tolerances be exceeded or should

concrete be undersize or understrength, correctivemeasures described in Baker, et al (1971) may be used.There is no simple economical method to conclusivelydetermine if an as-installed pier is defective althoughseveral methods are available that may be sufficientlyreliable for certain low stress situations (Davis, 1991). Ifnecessary, suspect piers can be load tested by either con-ventional static means or by large strain dynamic testing(Baker, et al, 1992). In the case of a shaft being off loca-tion or out of plumb, if the out of tolerance deviation isdiscovered prior to concreting, it may be possible tocorrect most economically by increasing structural rein-forcement of the pier and/or adding structural lateralrestraint. This approach requires analysis by the Struc-tural Engineer. If the out of tolerance is too severe tomake this approach practical, it may be necessary tobackfill the hole with a leanmix grout and to carefullyredrill, periodically checking and correcting for plumbtolerance. For this case, the strength of the grout at thetime of redrilling the hole should be designed to bereasonably comparable to the strength and hardness ofthe ground being drilled. Alternatively, it may be possiblethat the shaft can be redrilled sufficiently oversize that adesign size shaft of the correct size and plumbness fallswithin the overall dimensions of the new overize asdrilled shaft.


CHAPTER 6--REFERENCES

6.1--Recommended references

American Concrete Institute117 Standard Specification for Tolerances for

Concrete Construction and Materials301 Specifications for Structural Concrete for

Buildings318 Building Code Requirements for Reinforced

Concrete318.1 Building Code Requirements for Structural Plain

Concrete336.1 Standard Specification for the Construction of

Drilled Piers364R Guide for Measuring, Mixing, Transporting, and

Placing Concrete

American Petroleum Institute13A Specification for Drilling-Fluid Materials13B Recommended Practice Standard Procedure for

Field Testing Oil-Based Drilling Fluids

Deep Foundat ions Ins t i tu te and the (ADSC)International Association of Foundation Drilling

Drilled Shaft Inspector’s Manual-Prepared by the JointCaisson-Drilled Shaft Committee, 1989

The above publications may be obtained from the follow-ing organizations:

American Concrete InstituteP.O. Box 19150Detroit, MI 48219

American Petroleum Institute1220 L Street NorthwestWashington, D.C. 20005

Deep Foundations InstituteBox 281Sparta, NJ 07871

6.2--Cited referencesBaker, Clyde N., “Pier Design and Construction in

Water Bearing Soils,” Proceedings of the InternationalConference on Deep Foundations, Beijing, China, 1986,sponsored by The Deep Foundations Institute, Sparta,New Jersey.

Baker, Clyde N., Drumright, E.E., Briaud, J.L.,Mensah-Dwumah, F., Parikh, G., Drilled Shafts for BridgeFoundations, U.S. Depart. of Transp. Fed. Hwy. Admin.,1992, McLean, VA.

Baker, Clyde N., and Khan, Fazlur, “Caisson Con-struction Problems and Correction in Chicago,” Proceed-ings, ASCE, V. 97, SM2, Feb. 1971, pp. 417-440.

Borden, R.H. and M.A. Gabr, Analysis of CompactPole-Type Footings; Lt base: Computer Program forLaterally Loaded pier Analysis Including Base and SlopeEffects, 1987, Department of Civil Engineering, NorthCarolina State University at Raleigh, report prepared forthe North Carolina Department of Transportation andFederal Highway Administration, p. 215.

Bronx, Bengt B., “Lateral Resistance of Piles inCohesive Soils,” Proceedings, American Society of CivilEngineers, V. 90, No. SM2, March, 1964a, pp. 27-63.

Broms, Bengt B., “Lateral Resistance of Piles inCohesionless Soils,” Proceedings, American Society ofCivil Engineer, V. 90, No. SM3, May, 1964b, pp. 123-156.

Broms, Bengt B., “Design of Laterally Loaded Piles,”Proceedings, American Society of Civil Engineers, V. 91,No. SM3, 1965.

Building Code of the City of New York, August, 1990,Section 27-713 (C26-1109.8) “Caisson Piles.”

Canadian Foundation Engineering Manual, SecondEdition, 1985, The Canadian Geotechnical Society, Part3, Section 20.1, “Deep Foundations on Rock.”

Davis, Allen G., “The Development of Small StrainIntegrity Testing at Drilled Shafts: A Review,” Trans-portation Research Board, Washington, D.C., Jan., 1991.

Davisson, M.T., “Design of Deep Foundations for TallBuildings Under Lateral Load,” University of IllinoisBulletin, V. 66, No. 78, Feb. 14, 1969, pp. 157-174.

GAI Consultants, Inc., Laterally Loaded Drilled PierResearch, Volume I: Design Methodology, EPRI, 1982.

Horvath, R.G. and Kenney, T.C., “Shaft Resistance ofRock-Socketed Drilled Piers,” ASCE Convention, Oct.,1979.

Koutsoftas, D.C., “Caissons Socketed in Sound MicaSchist,” Journal of the Geotechnical Engineering Division,ASCE, V. 107, No. GT6, June, 1981.

Massachusetts State Building Code, Fourth Edition,August, 1988, The Commonwealth of Massachusetts, Sec-tion 739, “Concrete-Filled Pipe with Steel Core Caissons.”

OSHA Safety and Health Standards--ConstructionIndustry Part 1926, U.S. Department of Labor, Occu-pational Safety and Health Administration, Washington,D.C.

Osterberg, J. “New Device for Load Testing DrivenPiles and Drilled Shafts Separates Friction and EndBearing,” Proceedings of the International Conference onPiling and Deep Foundations, London, May, 1989.

Pennsylvania Dept. of Transportation Drilled ShaftDesign Manual.

Poulos, H.G. and E.H. Davis, Pile Foundation Analysisand Design, John Wiley & Sons, New York, NY, 1980, p.397.

Prakash, S., “Behavior of Pile Groups Subjected toLateral Load,” PhD Thesis, University of Illinois, 1962.

Reese, L.C., “Laterally Loaded Piers: ProgramDocumentation,” Journal of Geot. Engr. Div., ASCE, V.103, No. GT4, 1977a.

Reese, L.C. and Wright, S.J., “Drilled Shaft Designand Construction Guidelines Manual,” V. I & II, U.S.


Department of Transportation, 1977b.Reese, L. C., Handbook for Design of Piles and Drilled

Shafts Under Lateral Load, U.S. Dept. of Trans., FederalHighway Admin., Office of Implementation, 1984,McLean, Va., July, 1984.

Reese, L.C. & O’Neill, M.W. “Drilled Shafts:Construction Procedures and Design Methods,” U.S.Dept. of Transp. Fed. Hwy. Admin., 1988, McLean, Va.,Publication No. FHWA-HI-88-042.

Richardson, E.V., Harrison, L.J., David, S.R.,Evaluating Scout at Bridges, U.S. Department of Tramp.,Federal Highway Administration, Office of Research andDevelopment, 1991, McLean, VA, Publication No.FHWA-IP-90-017.

Singh, A., R.E.-W. Hu, and R.D. Cousineau, “LateralLoad Capacity of Piles in Sand and Normally Consoli-

dated Clay,” Civil Engineering, ASCE, V. 41, No. 8, 1971,pp. 52-54.

Terzaghi, K., “Evaluation of the Modulus of SubgradeReaction,” Geotechnique, V. V, No. 4, 1955, pp. 297-326.

Terzaghi, Karl, and Peck, Ralph B., “Soil Mechanics inEngineering Practice,”John Wiley and Sons, Inc., NewYork, 1967, 751 pp.

Unifom Building Code, International Conference ofBuilding Officials, Whittier, CA, 1985.

Wang, S.T. & Reese, L.C., "The Effect of Non-LinearFlexural Rigidity on the Behavior of Concrete PilesUnder Lateral Loading,” Texas Section, ASCE, 1987,Houston.

ACI 336.3R-93 was submitted to letter ballot of the committee and was approvedaccording to Institute balloting procedures.