11.1 Introduction - personal.its.ac.idpersonal.its.ac.id/files/material/4278-handayanu-oe-pondasi5.pdf · build pile foundations, ... structural safety. The following list identifies

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11.1 Introduction

Piles are structural members that are made of steel, concrete, or timber. They are used tobuild pile foundations, which are deep and which cost more than shallow foundations.(See Chapters 3, 4, and 5.) Despite the cost, the use of piles often is necessary to ensurestructural safety. The following list identifies some of the conditions that require pilefoundations (Vesic, 1977):

1. When one or more upper soil layers are highly compressible and too weak to sup-port the load transmitted by the superstructure, piles are used to transmit the load tounderlying bedrock or a stronger soil layer, as shown in Figure 11.1a. When bedrockis not encountered at a reasonable depth below the ground surface, piles are used totransmit the structural load to the soil gradually. The resistance to the applied struc-tural load is derived mainly from the frictional resistance developed at the soil–pileinterface. (See Figure 11.1b.)

2. When subjected to horizontal forces (see Figure 11.1c), pile foundations resist bybending, while still supporting the vertical load transmitted by the superstructure.This type of situation is generally encountered in the design and construction ofearth-retaining structures and foundations of tall structures that are subjected to highwind or to earthquake forces.

3. In many cases, expansive and collapsible soils may be present at the site of a pro-posed structure. These soils may extend to a great depth below the ground surface.Expansive soils swell and shrink as their moisture content increases and decreases,and the pressure of the swelling can be considerable. If shallow foundations areused in such circumstances, the structure may suffer considerable damage.However, pile foundations may be considered as an alternative when piles areextended beyond the active zone, which is where swelling and shrinking occur.(See Figure 11.1d)

Soils such as loess are collapsible in nature. When the moisture content of thesesoils increases, their structures may break down. A sudden decrease in the void ratio ofsoil induces large settlements of structures supported by shallow foundations. In such

535

Pile Foundations

(a)

Rock

(b)

(e) (f)

Zone oferosion

(d)

Stablesoil

(c)

Swellingsoil

Figure 11.1 Conditions that require the use of pile foundations

536 Chapter 11: Pile Foundations

cases, pile foundations may be used in which the piles are extended into stable soillayers beyond the zone where moisture will change.

4. The foundations of some structures, such as transmission towers, offshore platforms, and basement mats below the water table, are subjected to upliftingforces. Piles are sometimes used for these foundations to resist the uplifting force.(See Figure 11.1e.)

5. Bridge abutments and piers are usually constructed over pile foundations to avoidthe loss of bearing capacity that a shallow foundation might suffer because of soilerosion at the ground surface. (See Figure 11.1f.)

Although numerous investigations, both theoretical and experimental, have beenconducted in the past to predict the behavior and the load-bearing capacity of piles ingranular and cohesive soils, the mechanisms are not yet entirely understood and maynever be. The design and analysis of pile foundations may thus be considered somewhatof an art as a result of the uncertainties involved in working with some subsoil condi-tions. This chapter discusses the present state of the art.

11.2 Types of Piles and Their Structural Characteristics

Different types of piles are used in construction work, depending on the type of load to becarried, the subsoil conditions, and the location of the water table. Piles can be divided intothe following categories: (a) steel piles, (b) concrete piles, (c) wooden (timber) piles, and(d) composite piles.

Steel Piles

Steel piles generally are either pipe piles or rolled steel H-section piles. Pipe piles can bedriven into the ground with their ends open or closed. Wide-flange and I-section steelbeams can also be used as piles. However, H-section piles are usually preferred becausetheir web and flange thicknesses are equal. (In wide-flange and I-section beams, the webthicknesses are smaller than the thicknesses of the flange.) Table 11.1 gives the dimen-sions of some standard H-section steel piles used in the United States. Table 11.2 showsselected pipe sections frequency used for piling purposes. In many cases, the pipe pilesare filled with concrete after they have been driven.

The allowable structural capacity for steel piles is

(11.1)

where

cross-sectional area of the steelallowable stress of steel ( 0.33–0.5 fy)

Once the design load for a pile is fixed, one should determine, on the basis of geo-technical considerations, whether is within the allowable range as defined byEq. 11.1.

When necessary, steel piles are spliced by welding or by riveting. Figure 11.2ashows a typical splice by welding for an H-pile. A typical splice by welding for a pipepile is shown in Figure 11.2b. Figure 11.2c is a diagram of a splice of an H-pile by rivetsor bolts.

When hard driving conditions are expected, such as driving through dense gravel,shale, or soft rock, steel piles can be fitted with driving points or shoes. Figures 11.2d and11.2e are diagrams of two types of shoe used for pipe piles.

Steel piles may be subject to corrosion. For example, swamps, peats, and otherorganic soils are corrosive. Soils that have a pH greater than 7 are not so corrosive. To off-set the effect of corrosion, an additional thickness of steel (over the actual designedcross-sectional area) is generally recommended. In many circumstances factory-appliedepoxy coatings on piles work satisfactorily against corrosion. These coatings are not eas-ily damaged by pile driving. Concrete encasement of steel piles in most corrosive zonesalso protects against corrosion.

Following are some general facts about steel piles:

• Usual length: 15 m to 60 m• Usual load: 300 kN to 1200 kN

Q(design)

< fs 5 As 5

Qall 5 Asfs

11.2 Types of Piles and Their Structural Characteristics 537

• Advantages:a. Easy to handle with respect to cutoff and extension to the desired lengthb. Can stand high driving stressesc. Can penetrate hard layers such as dense gravel and soft rockd. High load-carrying capacity

• Disadvantages:a. Relatively costlyb. High level of noise during pile drivingc. Subject to corrosiond. H-piles may be damaged or deflected from the vertical during driving through

hard layers or past major obstructions


y

y

w

w

x x

d1

d2

Table 11.1 Common H-Pile Sections used in the United States

Moment of

Designation, Flange and webinertia

size (mm) Depth Section area thickness w Flange width ( )

weight (kg/m) (mm) ( ) (mm) (mm)

HP 204 6.84 11.3 207 49.4 16.8HP 254 10.8 14.4 260 123 42

246 8.0 10.6 256 87.5 24HP 312 15.9 17.5 312 271 89

308 14.1 15.49 310 237 77.5303 11.9 13.1 308 197 63.7299 10.0 11.05 306 164 62.9

HP 334 19.0 19.45 335 370 123329 16.5 16.9 333 314 104324 13.9 14.5 330 263 86319 11.3 11.7 328 210 69

HP 361 22.2 20.45 378 508 184356 19.4 17.91 376 437 158351 16.8 15.62 373 374 136346 13.8 12.82 371 303 1093 108

3 132

3 152

360 3 174

3 89

3 109

3 129

330 3 149

3 79

3 93

3 110

310 3 125

3 62

250 3 85

200 3 53

lyylxxm2 3 1023

m4 3 1026

d2d13


Table 11.2 Selected Pipe Pile Sections

Outside Wall Area ofdiameter thickness steel

(mm) (mm) (cm2)

219 3.17 21.5

4.78 32.1

5.56 37.3

7.92 52.7

254 4.78 37.5

5.56 43.6

6.35 49.4

305 4.78 44.9

5.56 52.3

6.35 59.7

406 4.78 60.3

5.56 70.1

6.35 79.8

457 5.56 80

6.35 90

7.92 112

508 5.56 88

6.35 100

7.92 125

610 6.35 121

7.92 150

9.53 179

12.70 238


Weld

(a)

Weld

(b) (c)

(d)

Weld

(e)

Weld

Figure 11.2 Steel piles: (a) splicing of H-pile by welding; (b) splicing of pipe pile by welding;(c) splicing of H-pile by rivets and bolts; (d) flat driving point of pipe pile; (e) conical drivingpoint of pipe pile

Concrete Piles

Concrete piles may be divided into two basic categories: (a) precast piles and (b) cast-in-situpiles. Precast piles can be prepared by using ordinary reinforcement, and they can be squareor octagonal in cross section. (See Figure 11.3.) Reinforcement is provided to enable thepile to resist the bending moment developed during pickup and transportation, the verti-cal load, and the bending moment caused by a lateral load. The piles are cast to desiredlengths and cured before being transported to the work sites.

Some general facts about concrete piles are as follows:

• Usual length: 10 m to 15 m • Usual load: 300 kN to 3000 kN• Advantages:

a. Can be subjected to hard drivingb. Corrosion resistantc. Can be easily combined with a concrete superstructure

2D

Square pile Octagonal pile

D D

Figure 11.3 Precast piles with ordinaryreinforcement

• Disadvantages:a. Difficult to achieve proper cutoffb. Difficult to transport

Precast piles can also be prestressed by the use of high-strength steel pre-stressingcables. The ultimate strength of these cables is about . During casting of thepiles, the cables are pretensioned to about , and concrete is pouredaround them. After curing, the cables are cut, producing a compressive force on the pilesection. Table 11.3 gives additional information about prestressed concrete piles withsquare and octagonal cross sections.

Some general facts about precast prestressed piles are as follows:

• Usual length: 10 m to 45 m• Maximum length: 60 m • Maximum load: 7500 kN to 8500 kN

The advantages and disadvantages are the same as those of precast piles.Cast-in-situ, or cast-in-place, piles are built by making a hole in the ground and then

filling it with concrete. Various types of cast-in-place concrete piles are currently used inconstruction, and most of them have been patented by their manufacturers. These piles maybe divided into two broad categories: (a) cased and (b) uncased. Both types may have apedestal at the bottom.

Cased piles are made by driving a steel casing into the ground with the help of amandrel placed inside the casing. When the pile reaches the proper depth the mandrel iswithdrawn and the casing is filled with concrete. Figures 11.4a, 11.4b, 11.4c, and 11.4dshow some examples of cased piles without a pedestal. Figure 11.4e shows a cased pilewith a pedestal. The pedestal is an expanded concrete bulb that is formed by dropping ahammer on fresh concrete.

Some general facts about cased cast-in-place piles are as follows:

• Usual length: 5 m to 15 m • Maximum length: 30 m to 40 m • Usual load: 200 kN to 500 kN • Approximate maximum load: 800 kN

900 to 1300 MN>m21800 MN>m2



• Advantages:a. Relatively cheapb. Allow for inspection before pouring concretec. Easy to extend

• Disadvantages:a. Difficult to splice after concretingb. Thin casings may be damaged during driving

• Allowable load:

(11.2)

where

of cross section of steelof cross section of concrete

stress of steelstress of concrete fc 5 allowable

fs 5 allowable

Ac 5 area

As 5 area

Qall 5 Asfs 1 Acfc

D

Prestressedstrand

Wirespiral Prestressed

strand

Wirespiral

D

Table 11.3 Typical Prestressed Concrete Pile in Use

Design bearing

capacity (kN)

MinimumStrength

Area of crossNumber of strands

effective Sectionof concrete

Pile D section Perimeter 12.7-mm 11.1-mm prestress modulus(MN/m2)

shapea (mm) (cm2) (mm) diameter diameter force (kN) (m3 10 ) 34.5 41.4

S 254 645 1016 4 4 312 2.737 556 778O 254 536 838 4 4 258 1.786 462 555S 305 929 1219 5 6 449 4.719 801 962O 305 768 1016 4 5 369 3.097 662 795S 356 1265 1422 6 8 610 7.489 1091 1310O 356 1045 1168 5 7 503 4.916 901 1082S 406 1652 1626 8 11 796 11.192 1425 1710O 406 1368 1346 7 9 658 7.341 1180 1416S 457 2090 1829 10 13 1010 15.928 1803 2163O 457 1729 1524 8 11 836 10.455 1491 1790S 508 2581 2032 12 16 1245 21.844 2226 2672O 508 2136 1677 10 14 1032 14.355 1842 2239S 559 3123 2235 15 20 1508 29.087 2694 3232O 559 2587 1854 12 16 1250 19.107 2231 2678S 610 3658 2438 18 23 1793 37.756 3155 3786O 610 3078 2032 15 19 1486 34.794 2655 3186

aS 5 square section; O 5 octagonal section

233


Franki UncasedPedestal Pile

Maximum usuallength: 30 m–40 m

(g)

Western UncasedPile without

Pedestal


(f)

Franki CasedPedestal Pile

Straight steel pilecasing


(e)

Seamless Pile orArmco Pile

Thin metal casing


(d)

Monotube orUnion Metal Pile

Thin, fluted, taperedsteel casing drivenwithout mandrel

Maximum usuallength: 40 m

RaymondStep-Taper Pile

Corrugated thincylindrical casing

Maximum usuallength: 30 m

WesternCased Pile

Thin metal casing


(b)(a) (c)

Figure 11.4 Cast-in-place concrete piles


Figures 11.4f and 11.4g are two types of uncased pile, one with a pedestal and theother without. The uncased piles are made by first driving the casing to the desired depth andthen filling it with fresh concrete. The casing is then gradually withdrawn.

Following are some general facts about uncased cast-in-place concrete piles:

• Usual length: 5 m to 15 m• Maximum length: 30 m to 40 m• Usual load: 300 kN to 500 kN• Approximate maximum load: 700 kN• Advantages:

a. Initially economicalb. Can be finished at any elevation

• Disadvantages:a. Voids may be created if concrete is placed rapidlyb. Difficult to splice after concretingc. In soft soils, the sides of the hole may cave in, squeezing the concrete

• Allowable load:

(11.3)

where

of cross section of concretestress of concrete

Timber Piles

Timber piles are tree trunks that have had their branches and bark carefully trimmed off.The maximum length of most timber piles is 10 to 20 m. To qualify for use as a pile, thetimber should be straight, sound, and without any defects. The American Society of CivilEngineers’ Manual of Practice, No. 17 (1959), divided timber piles into three classes:

1. Class A piles carry heavy loads. The minimum diameter of the butt should be 356 mm.2. Class B piles are used to carry medium loads. The minimum butt diameter should be

305 to 330 mm.3. Class C piles are used in temporary construction work. They can be used perma-

nently for structures when the entire pile is below the water table. The minimumbutt diameter should be 305 mm.

In any case, a pile tip should not have a diameter less than 150 mm.Timber piles cannot withstand hard driving stress; therefore, the pile capacity is gen-

erally limited. Steel shoes may be used to avoid damage at the pile tip (bottom). The topsof timber piles may also be damaged during the driving operation. The crushing of thewooden fibers caused by the impact of the hammer is referred to as brooming. To avoiddamage to the top of the pile, a metal band or a cap may be used.

Splicing of timber piles should be avoided, particularly when they are expected tocarry a tensile load or a lateral load. However, if splicing is necessary, it can be done byusing pipe sleeves (see Figure 11.5a) or metal straps and bolts (see Figure 11.5b). Thelength of the sleeve should be at least five times the diameter of the pile. The butting endsshould be cut square so that full contact can be maintained. The spliced portions should becarefully trimmed so that they fit tightly to the inside of the pipe sleeve. In the case ofmetal straps and bolts, the butting ends should also be cut square. The sides of the splicedportion should be trimmed plane for putting the straps on.

fc 5 allowable

Ac 5 area

Qall 5 Acfc

Metalsleeve

Endscutsquare

Metalstrap

Metalstrap

(a) (b)

Ends cutsquare

Figure 11.5 Splicing of timber piles: (a) use of pipesleeves; (b) use of metalstraps and bolts


Timber piles can stay undamaged indefinitely if they are surrounded by saturatedsoil. However, in a marine environment, timber piles are subject to attack by various organ-isms and can be damaged extensively in a few months. When located above the watertable, the piles are subject to attack by insects. The life of the piles may be increased bytreating them with preservatives such as creosote.

The allowable load-carrying capacity of wooden piles is

(11.4)

where

area of cross section of the pilestress on the timber

The following allowable stresses are for pressure-treated round timber piles made fromPacific Coast Douglas fir and Southern pine used in hydraulic structures (ASCE, 1993):

Pacific Coast Douglas Fir

• Compression parallel to grain:• Bending:• Horizontal shear:• Compression perpendicular to grain:

Southern Pine

• Compression parallel to grain:• Bending: 11.4 MN>m2

5.7 MN>m2

1.31 MN>m20.66 MN>m2

11.7 MN>m26.04 MN>m2

fw 5 allowable

Ap 5 average

Qall 5 Apfw

• Horizontal shear:• Compression perpendicular to grain:

The usual length of wooden piles is 5 m to 15 m. The maximum length is about 30 mto 40 m (100 ft to 130 ft). The usual load carried by wooden piles is 300 kN to 500 kN.

Composite Piles

The upper and lower portions of composite piles are made of different materials. Forexample, composite piles may be made of steel and concrete or timber and concrete.Steel-and-concrete piles consist of a lower portion of steel and an upper portion of cast-in-place concrete. This type of pile is used when the length of the pile required foradequate bearing exceeds the capacity of simple cast-in-place concrete piles. Timber-and-concrete piles usually consist of a lower portion of timber pile below the permanentwater table and an upper portion of concrete. In any case, forming proper joints betweentwo dissimilar materials is difficult, and for that reason, composite piles are notwidely used.

11.3 Estimating Pile Length

Selecting the type of pile to be used and estimating its necessary length are fairly difficulttasks that require good judgment. In addition to being broken down into the classificationgiven in Section 11.2, piles can be divided into three major categories, depending on theirlengths and the mechanisms of load transfer to the soil: (a) point bearing piles, (b) frictionpiles, and (c) compaction piles.

Point Bearing Piles

If soil-boring records establish the presence of bedrock or rocklike material at a site within areasonable depth, piles can be extended to the rock surface. (See Figure 11.6a.) In this case,the ultimate capacity of the piles depends entirely on the load-bearing capacity of the under-lying material; thus, the piles are called point bearing piles. In most of these cases, the neces-sary length of the pile can be fairly well established.

If, instead of bedrock, a fairly compact and hard stratum of soil is encountered at areasonable depth, piles can be extended a few meters into the hard stratum. (See Figure11.6b.) Piles with pedestals can be constructed on the bed of the hard stratum, and the ulti-mate pile load may be expressed as

(11.5)

where

carried at the pile pointload carried by skin friction developed at the side of the pile (caused by shearingresistance between the soil and the pile)

If is very small,

(11.6)

In this case, the required pile length may be estimated accurately if proper subsoil explo-ration records are available.

Qs < Qp

Qs

Qs 5 Qp 5 load

Qu 5 Qp 1 Qs

1.41 MN>m20.62 MN>m2


Rock

Weaksoil

Qu

Qp

Qu � Qp

(a)

LWeaksoil

Strongsoillayer

Qu

Qs

Qp

Qu � Qp

(b)

L

Lb

Lb � depth of penetrationinto bearing stratum

Weaksoil

Qu

Qs

Qp

Qu � Qs

(c)

L

Figure 11.6 (a) and (b) Point bearing piles; (c) friction piles

11.3 Estimating Pile Length 547

Friction Piles

When no layer of rock or rocklike material is present at a reasonable depth at a site, pointbearing piles become very long and uneconomical. In this type of subsoil, piles are dri-ven through the softer material to specified depths. (See Figure 11.6c.) The ultimate loadof the piles may be expressed by Eq. (11.5). However, if the value of is relativelysmall, then

(11.7)

These piles are called friction piles, because most of their resistance is derivedfrom skin friction. However, the term friction pile, although used often in theliterature, is a misnomer: In clayey soils, the resistance to applied load is alsocaused by adhesion.

The lengths of friction piles depend on the shear strength of the soil, the appliedload, and the pile size. To determine the necessary lengths of these piles, an engineerneeds a good understanding of soil–pile interaction, good judgment, and experience.Theoretical procedures for calculating the load-bearing capacity of piles are presentedlater in the chapter.

Compaction Piles

Under certain circumstances, piles are driven in granular soils to achieve proper com-paction of soil close to the ground surface. These piles are called compaction piles. Thelengths of compaction piles depend on factors such as (a) the relative density of the soilbefore compaction, (b) the desired relative density of the soil after compaction, and(c) the required depth of compaction. These piles are generally short; however, somefield tests are necessary to determine a reasonable length.

Qu < Qs

Qp


11.4 Installation of Piles

Most piles are driven into the ground by means of hammers or vibratory drivers. In specialcircumstances, piles can also be inserted by jetting or partial augering. The types of hammerused for pile driving include (a) the drop hammer, (b) the single-acting air or steam hammer,(c) the double-acting and differential air or steam hammer, and (d) the diesel hammer. In thedriving operation, a cap is attached to the top of the pile. A cushion may be used between thepile and the cap. The cushion has the effect of reducing the impact force and spreading it overa longer time; however, the use of the cushion is optional. A hammer cushion is placed onthe pile cap. The hammer drops on the cushion.

Figure 11.7 illustrates various hammers. A drop hammer (see Figure 11.7a) is raisedby a winch and allowed to drop from a certain height H. It is the oldest type of hammerused for pile driving. The main disadvantage of the drop hammer is its slow rate of blows.The principle of the single-acting air or steam hammer is shown in Figure 11.7b. The strik-ing part, or ram, is raised by air or steam pressure and then drops by gravity. Figure 11.7cshows the operation of the double-acting and differential air or steam hammer. Air or steamis used both to raise the ram and to push it downward, thereby increasing the impact veloc-ity of the ram. The diesel hammer (see Figure 11.7d) consists essentially of a ram, an anvilblock, and a fuel-injection system. First the ram is raised and fuel is injected near the anvil.Then the ram is released. When the ram drops, it compresses the air–fuel mixture, whichignites. This action, in effect, pushes the pile downward and raises the ram. Diesel ham-mers work well under hard driving conditions. In soft soils, the downward movement ofthe pile is rather large, and the upward movement of the ram is small. This differential may

Figure 11.7 Pile-driving equipment: (a) drop hammer; (b) single-acting air or steam hammer

Exhaust

Intake

Cylinder

(b)

Hammercushion

Pilecushion

Pile

Pilecap

Ram

(a)

Ram

Hammercushion

Pilecushion

Pile

Pilecap

11.4 Installation of Piles 549

(e)

Oscillator

Staticweight

Pile

Clamp

(f)

ExhaustandIntake

ExhaustandIntake

Cylinder

(c)

Hammercushion

Pilecushion

Pile

Pilecap

Ram

(d)

Pile cushion

Anvil

Hammercushion

Pile

Pile cap

Ram

Figure 11.7 (continued) Pile-driving equipment: (c) double-acting and differential air or steamhammer; (d) diesel hammer; (e) vibratory pile driver; (f) photograph of a vibratory pile driver(Courtesy of Michael W. O’Neill, University of Houston)

not be sufficient to ignite the air–fuel system, so the ram may have to be lifted manually.Table 11.4 provides some examples of commercially available pile-driving hammers.

The principles of operation of a vibratory pile driver are shown in Figure 11.7e. Thisdriver consists essentially of two counterrotating weights. The horizontal components of thecentrifugal force generated as a result of rotating masses cancel each other. As a result, asinusoidal dynamic vertical force is produced on the pile and helps drive the pile downward.

Figure 11.7f is a photograph of a vibratory pile driver. Figure 11.8 shows a pile-drivingoperation in the field.

Jetting is a technique that is sometimes used in pile driving when the pile needs topenetrate a thin layer of hard soil (such as sand and gravel) overlying a layer of softer soil.In this technique, water is discharged at the pile point by means of a pipe 50 to 75 mmin diameter to wash and loosen the sand and gravel.

Piles driven at an angle to the vertical, typically 14 to are referred to as batterpiles. Batter piles are used in group piles when higher lateral load-bearing capacity isrequired. Piles also may be advanced by partial augering, with power augers (see Chapter 2)used to predrill holes part of the way. The piles can then be inserted into the holes and dri-ven to the desired depth.

Piles may be divided into two categories based on the nature of their placement:displacement piles and nondisplacement piles. Driven piles are displacement piles,because they move some soil laterally; hence, there is a tendency for densification of soilsurrounding them. Concrete piles and closed-ended pipe piles are high-displacementpiles. However, steel H-piles displace less soil laterally during driving, so they are low-displacement piles. In contrast, bored piles are nondisplacement piles because their place-ment causes very little change in the state of stress in the soil.

20°,


Table 11.4 Examples of Commercially Available Pile-Driving Hammers

Maker of Model Rated energy Ram weighthammer† No. Hammer type kN-m Blows/min kN

V 400C Single acting 153.9 100 177.9M S-20 81.3 60 89.0M S-8 35.3 53 35.6M S-5 22.0 60 22.2R 5/O 77.1 44 77.8R 2/O 44.1 50 44.5

V 200C Double acting 68.1 98 89.0V 140C or 48.8 103 62.3V 80C differential 33.1 111 35.6V 65C 26.0 117 28.9R 150C 66.1 95–105 66.7

V 4N100 Diesel 58.8 50–60 23.5V IN100 33.4 50–60 13.3M DE40 43.4 48 17.8M DE30 30.4 48 12.5

†V—Vulcan Iron Works, FloridaM—McKiernan-Terry, New JerseyR—Raymond International, Inc., Texas

Figure 11.8 A pile-driving operation in the field (Courtesy of E. C. Shin, Universityof Incheon, Korea)

11.5 Load Transfer Mechanism 551

11.5 Load Transfer Mechanism

The load transfer mechanism from a pile to the soil is complicated. To understand it, con-sider a pile of length L, as shown in Figure 11.9a. The load on the pile is graduallyincreased from zero to at the ground surface. Part of this load will be resisted byQ(z50)

the side friction developed along the shaft, and part by the soil below the tip of the pile,Now, how are and related to the total load? If measurements are made to obtain

the load carried by the pile shaft, at any depth z, the nature of the variation found willbe like that shown in curve 1 of Figure 11.9b. The frictional resistance per unit area at anydepth z may be determined as

(11.8)f(z) 5DQ(z)

(p) (Dz)

Q(z) ,

Q2Q1Q2 .

Q1 ,


Qu

21

Q(z � 0)Q(z � 0)

Q(z)

�Q(z)

�z

Q1

Q2

(a) (b)

(e)

ZoneII

ZoneII

Zone I

Pile tip

Q

L

z

Qu

Qs

Qp

(d)

L

Qp Qs

Q2

Unitfrictionalresistance

z � 0

z � L

(c)

f(z) � �Q(z)

p � �z

Figure 11.9 Load transfer mechanism for piles

11.5 Load Transfer Mechanism 553

where of the cross section of the pile. Figure 11.9c shows the variation ofwith depth.

If the load Q at the ground surface is gradually increased, maximum frictional resis-tance along the pile shaft will be fully mobilized when the relative displacement betweenthe soil and the pile is about 5 to 10 mm, irrespective of the pile size and length L. However,the maximum point resistance will not be mobilized until the tip of the pile hasmoved about 10 to 25% of the pile width (or diameter). (The lower limit applies to drivenpiles and the upper limit to bored piles). At ultimate load (Figure 11.9d and curve 2 inFigure 11.9b), Thus,

and

The preceding explanation indicates that (or the unit skin friction, f, along the pileshaft) is developed at a much smaller pile displacement compared with the pointresistance, In order to demonstrate this point, let us consider the results of a pile loadtest conducted in the field by Mansur and Hunter (1970). The details of the pile and subsoilconditions are as follow:

Type of pile: Steel pile with 406 mm outside diameter with 8.15 mm wall thickness

Type of subsoil: SandLength of pile embedment: 16.8 m

Figure 11.10a shows the load test results, which is a plot of load at the top of the pileversus settlement(s). Figure 11.10b shows the plot of the load carried by the pile

shaft at any depth. It was reported by Mansur and Hunter (1970) that, for this test,at failure

and

Now, let us consider the load distribution in Figure 11.10b when the pile settlement(s) isabout 2.5 mm. For this condition,

Hence, at ,

and

Thus, it is obvious that the skin friction is mobilized faster at low settlement levelsas compared to the point load.

Q1

Qs5

574

1185 (100) 5 48.4%

Q2

Qp5

93

416 (100) 5 22.4%

s 5 2.5 mmQ1 < 574 kNQ2 < 93 kNQ(z50) < 667 kN

Qs < 1185 kN

Qp < 416 kN

Qu < 1601 kN

3Q(z)43Q(z50)4

Qp .

Qs

Q2 5 Qp

Q1 5 Qs

Q(z50) 5 Qu .

Q2 5 Qp

f(z)

p 5 perimeter


At ultimate load, the failure surface in the soil at the pile tip (a bearing capacity fail-ure caused by ) is like that shown in Figure 11.9e. Note that pile foundations are deepfoundations and that the soil fails mostly in a punching mode, as illustrated previously inFigures 3.1c and 3.3. That is, a triangular zone, I, is developed at the pile tip, which is pusheddownward without producing any other visible slip surface. In dense sands and stiff clayeysoils, a radial shear zone, II, may partially develop. Hence, the load displacement curves ofpiles will resemble those shown in Figure 3.1c.

11.6 Equations for Estimating Pile Capacity

The ultimate load-carrying capacity of a pile is given by the equation

(11.9)

where

capacity of the pile pointfrictional resistance (skin friction) derived from the soil–pile interface (seeFigure 11.11)

Numerous published studies cover the determination of the values of and Excellentreviews of many of these investigations have been provided by Vesic (1977), Meyerhof (1976),and Coyle and Castello (1981). These studies afford an insight into the problem of determiningthe ultimate pile capacity.

Qs .Qp

Qs 5 Qp 5 load-carrying

Qu 5 Qp 1 Qs

Qu

Qp

00 400 800 1200 1600 2000 2200

Load at the top of pile, Q(z = 0) (kN)

Settl

emen

t, s

(mm

)

Dep

th, z

(m

)

s = 2.5 mm

s = 11 mm

Q(z = 0) (kN)

5

10

15

20

25

30

35

0 400 800 1200 1600 2000

3

6

9

12

15

16.8(a) (b)

s = 5 mm

Figure 11.10 Load test results on a pipe pile in sand (Based on Mansur and Hunter, 1970)

11.6 Equations for Estimating Pile Capacity 555

q�

Qu

Qs

Qp

D

(a)

(b) Open-Ended Pipe Pile Section

(Note: Ap � area of steel � soil plug)

(c) H-Pile Section

Steel

Soil plug

Steel

Soil plug

D

d2

d1

L � Lb

Lb � length of embedment inbearing stratum

L � length of embedment

Figure 11.11 Ultimate load-carrying capacity of pile

Point Bearing Capacity,

The ultimate bearing capacity of shallow foundations was discussed in Chapter 3.According to Terzaghi’s equations,

and

Similarly, the general bearing capacity equation for shallow foundations was given inChapter 3 (for vertical loading) as

Hence, in general, the ultimate load-bearing capacity may be expressed as

(11.10)

where and are the bearing capacity factors that include the necessary shapeand depth factors.

Pile foundations are deep. However, the ultimate resistance per unit area developed atthe pile tip, may be expressed by an equation similar in form to Eq. (11.10), although thevalues of and will change. The notation used in this chapter for the width of apile is D. Hence, substituting D for B in Eq. (11.10) gives

(11.11)qu 5 qp 5 crNc* 1 qNq* 1 gDNg*

Ng*Nc* , Nq* ,

qp ,

Ng*Nc* , Nq* ,

qu 5 crNc* 1 qNq* 1 gBNg*

qu 5 crNcFcsFcd 1 qNqFqsFqd 1 12gBNgFgsFgd

qu 5 1.3crNc 1 qNq 1 0.3gBNg (for shallow circular foundations)

qu 5 1.3crNc 1 qNq 1 0.4gBNg (for shallow square foundations)

Qp


Because the width D of a pile is relatively small, the term may be dropped fromthe right side of the preceding equation without introducing a serious error; thus,we have

(11.12)

Note that the term q has been replaced by in Eq. (11.12), to signify effective verticalstress. Thus, the point bearing of piles is

(11.13)

where

of pile tipof the soil supporting the pile tip

point resistancevertical stress at the level of the pile tip

bearing capacity factors

Frictional Resistance, Qs

The frictional, or skin, resistance of a pile may be written as

(11.14)

where

of the pile sectionpile length over which p and f are taken to be constant

friction resistance at any depth z

The various methods for estimating and are discussed in the next several sec-tions. It needs to be reemphasized that, in the field, for full mobilization of the point re-sistance the pile tip must go through a displacement of 10 to 25% of the pilewidth (or diameter).

Allowable Load, Qall

After the total ultimate load-carrying capacity of a pile has been determined by summingthe point bearing capacity and the frictional (or skin) resistance, a reasonable factor ofsafety should be used to obtain the total allowable load for each pile, or

where

Qall 5 allowable load-carrying capacity for each pileFS 5 factor of safety

The factor of safety generally used ranges from 2.5 to 4, depending on the uncertaintiessurrounding the calculation of ultimate load.

Qall 5Qu

FS

(Qp),

QsQp

f 5 unit

DL 5 incremental

p 5 perimeter

Qs 5 S p DLf

Nc* , Nq* 5 the

qr 5 effective

qp 5 unit

cr 5 cohesion

Ap 5 area

Qp 5 Apqp 5 Ap(crNc* 1 qrNq*)

qr

qp 5 crNc* 1 qrNq*

gDNg*

Unit pointresistance,qp

(Lb /D)cr

qp � ql

L/D � Lb /D

Figure 11.12 Nature of variationof unit point resistance in a homogeneous sand

11.7 Meyerhof’s Method for Estimating Qp 557

Soil friction angle, ��(deg)

N*q

N* q

01

600

400

200

1008060

40

20

1086

4

2

8001000

10 20 30 4540

Figure 11.13 Variation of the maximum valuesof with soil friction angle (FromMeyerhof, G. G. (1976). “Bearing Capacity andSettlement of Pile Foundations,” Journal of theGeotechnical Engineering Division, AmericanSociety of Civil Engineers, Vol. 102, No. GT3,pp. 197–228. With permission from ASCE.)

frNq*

11.7 Meyerhof’s Method for Estimating

Sand

The point bearing capacity, of a pile in sand generally increases with the depth of embed-ment in the bearing stratum and reaches a maximum value at an embedment ratio of

Note that in a homogeneous soil is equal to the actual embedmentlength of the pile, L. However, where a pile has penetrated into a bearing stratum,Beyond the critical embedment ratio, the value of remains constant That is, as shown in Figure 11.12 for the case of a homogeneous soil,

For piles in sand, and Eq. (11.13) simpifies to

(11.15)

The variation of with soil friction angle is shown in Figure 11.13. The interpolatedvalues of for various friction angles are also given in Table 11.5. However, shouldnot exceed the limiting value that is,

(11.16)Qp 5 ApqrNq* < Apql

Apql ;

QpNq*

frNq*

Qp 5 Apqp 5 ApqrN*q

cr 5 0,

L 5 Lb .

(qp 5 ql).qp(Lb>D)cr ,

Lb , L.

LbLb>D 5 (Lb>D)cr .

qp ,

Qp


The limiting point resistance is

(11.17)

where

pressure ( )soil friction angle of the bearing stratum

A good example of the concept of the critical embedment ratio can be found from thefield load tests on a pile in sand at the Ogeechee River site reported by Vesic (1970). Thepile tested was a steel pile with a diameter of 457 mm. Table 11.6 shows the ultimate resis-tance at various depths. Figure 11.14 shows the plot of qp with depth obtained from the fieldtests along with the range of standard penetration resistance at the site. From the figure, thefollowing observations can be made.

1. There is a limiting value of qp. For the tests under consideration, it is about 12,000 kN/m2.2. The ( )cr value is about 16 to 18.L>D

fr 5 effective5100 kN>m2pa 5 atmospheric

ql 5 0.5 paNq* tan fr

Table 11.5 Interpolated Values ofBased on Meyerhof’s Theory

Soil friction angle, (deg)

20 12.421 13.822 15.523 17.924 21.425 26.026 29.527 34.028 39.729 46.530 56.731 68.232 81.033 96.034 115.035 143.036 168.037 194.038 231.039 276.040 346.041 420.042 525.043 650.044 780.045 930.0

Nq*f

Nq*

11.7 Meyerhof’s Method for Estimating Qp 559

Table 11.6 Ultimate Point Resistance, qp, of Test Pile at the OgeecheeRiver Site As reported by Vesic (1970)

Pile diameter, Depth of embedment, qp

D (m) L (m) ( )

0.457 3.02 6.61 3,3040.457 6.12 13.39 9,3650.457 8.87 19.4 11,4720.457 12.0 26.26 11,5870.457 15.00 32.82 13,971

kN,m2L ,D

Figure 11.14 Vesic’s pile test(1970) result—variation of qp andN60 with depth

Range ofN60 at site

Pile pointresistance

Pile point resistance, qp(kN/m2)

0

6

4

2

8

10

12

14

0

10 20 30 40 50

0 4000 8000 12,000 16,000 20,000

N60

Dep

th (

m)

3. The average N60 value is about 30 for Using Eq. (11.37), thelimiting point resistance is 4pa N60 5 (4)(100)(30) 5 12,000 kN/m2. This value isgenerally consistent with the field observation.

Clay ( )

For piles in saturated clays under undrained conditions the net ultimate load canbe given as

(11.18)

where cohesion of the soil below the tip of the pile.cu 5 undrained

Qp < Nc*cuAp 5 9cuAp

(f 5 0),

f 5 0

L>D > (L>D)cr.


11.8 Vesic’s Method for Estimating

Sand

Vesic (1977) proposed a method for estimating the pile point bearing capacity based on thetheory of expansion of cavities. According to this theory, on the basis of effective stressparameters, we may write

(11.19)

where

effective normal ground stress at the level of the pile point

(11.20)

(11.21)

and

capacity factor

Note that Eq. (11.19) is a modification of Eq. (11.15) with

(11.22)

According to Vesic’s theory,

(11.23)

where rigidity index for the soil. However,

(11.24)

where

(11.25)

modulus of elasticity of soilPoisson’s ratio of soilshear modulus of soilaverage volumatic strain in the plastic zone below the pile point

The general ranges of for various soils are

In order to estimate [Eq. (11.25)] and hence [Eq. (11.24)], the following approxima-tions may be used (Chen and Kulhawy, 1994)

IrrIr

Silt : 50 to 75Sand(relative density 5 50% to 80%): 75 to 150

Ir

D 5 Gs 5 ms 5 Es 5

Ir 5 rigidity index 5Es

2(1 1 ms) qr tan fr5

Gs

qr tan fr

Irr 5Ir

1 1 IrD

Irr 5 reduced

Ns* 5 f(Irr)

Ns* 53Nq*

(1 1 2Ko)

Ns* 5 bearing

Ko 5 earth pressure coefficient at rest 5 1 2 sin fr

5 ¢1 1 2Ko

3≤qr

sro 5 mean

Qp 5 Apqp 5 Ap sorNs*

Qp

11.8 Vesic’s Method for Estimating Qp 561

(11.26)

where

(11.27)

(11.28)

On the basis of cone penetration tests in the field, Baldi et al. (1981) gave the fol-lowing correlations for

(for mechanical cone penetration) (11.29)

and

(for electric cone penetration) (11.30)

For the definition of see Eq. (2.41). Table 11.7 gives the values of for variousvalues of and

Clay

In saturated clay ( condition), the net ultimate point bearing capacity of a pile canbe approximated as

(11.31)

where cohesionAccording to the expansion of cavity theory of Vesic (1977),

(11.32)

The variations of with for condition are given in Table 11.8.Now, referring to Eq. (11.24) for saturated clay with no volume change, .

Hence,

(11.33)Irr 5 Ir

D 5 0f 5 0IrrNc*

Nc* 5 4

3 (ln Irr 1 1) 1

p

21 1

cu 5 undrained

Qp 5 Apqp 5 ApcuNc*

f 5 0

(f 5 0)

fr.Irr

Ns*Fr ,

Ir 5170

Fr(%)

Ir 5300

Fr(%)

Ir :

D 5 0.005a1 2 fr 2 25

20 b

qrpa

ms 5 0.1 1 0.3a fr 2 25

20 b (for 25° # fr # 45°)

m 5 c100 to 200(loose soil)

200 to 500(medium dense soil)

500 to 1000(dense soil)

pa 5 atmospheric pressure ( < 100 kN>m2)

Es

pa 5 m

562

Tab

le 1

1.7

Bea

ring

Cap

acity

Fac

tors

B

ased

on

the

The

ory

of E

xpan

sion

of

Cav

ities I rr

10

20

40

60

80

100

200

300

400

500

2512

.12

15.9

520

.98

24.6

427

.61

30.1

639

.70

46.6

152

.24

57.0

626

13.1

817

.47

23.1

527

.30

30.6

933

.60

44.5

352

.51

59.0

264

.62

2714

.33

19.1

225

.52

30.2

134

.06

37.3

749

.88

59.0

566

.56

73.0

428

15.5

720

.91

28.1

033

.40

37.7

541

.51

55.7

766

.29

74.9

382

.40

2916

.90

22.8

530

.90

36.8

741

.79

46.0

562

.27

74.3

084

.21

92.8

030

18.2

424

.95

33.9

540

.66

46.2

151

.02

69.4

383

.14

94.4

810

4.33

3119

.88

27.2

237

.27

44.7

951

.03

56.4

677

.31

92.9

010

5.84

117.

1132

21.5

529

.68

40.8

849

.30

56.3

062

.41

85.9

610

3.66

118.

3913

1.24

3323

.34

32.3

444

.80

54.2

062

.05

68.9

295

.46

115.

5113

2.24

146.

8734

25.2

835

.21

49.0

559

.54

68.3

376

.02

105.

9012

8.55

147.

5116

4.12

3527

.36

38.3

253

.67

65.3

675

.17

83.7

811

7.33

142.

8916

4.33

183.

1636

29.6

041

.68

58.6

871

.69

82.6

292

.24

129.

8715

8.65

182.

8520

4.14

3732

.02

45.3

164

.13

78.5

790

.75

101.

4814

3.61

175.

9520

3.23

227.

2638

34.6

349

.24

70.0

386

.05

99.6

011

1.56

158.

6519

4.94

225.

6225

2.71

3937

.44

53.5

076

.45

94.2

010

9.24

122.

5417

5.11

215.

7825

0.23

280.

7140

40.4

758

.10

83.4

010

3.05

119.

7413

4.52

193.

1323

8.62

277.

2631

1.50

4143

.74

63.0

790

.96

112.

6813

1.18

147.

5921

2.84

263.

6730

6.94

345.

3442

47.2

768

.46

99.1

612

3.16

143.

6416

1.83

234.

4029

1.13

339.

5238

2.53

4351

.08

74.3

010

8.08

134.

5615

7.21

177.

3625

7.99

321.

2237

5.28

423.

3944

55.2

080

.62

117.

7614

6.97

172.

0019

4.31

283.

8035

4.20

414.

5146

8.28

4559

.66

87.4

812

8.28

160.

4818

8.12

212.

7931

2.03

390.

3545

7.57

517.

58

From

“D

esig

n of

Pile

Fou

ndat

ions

,”by

A. S

. Ves

ic. S

YN

TH

ESI

S O

F H

IGH

WA

Y P

RA

CT

ICE

by

AM

ER

ICA

N A

SSO

CIA

TIO

N O

F ST

AT

E H

IGH

WA

Y A

ND

TR

AN

SPO

RT.

Cop

yrig

ht 1

969

by T

RA

NSP

OR

TAT

ION

RE

SEA

RC

H B

OA

RD

. Rep

rodu

ced

with

per

mis

sion

of

TR

AN

SPO

RTA

TIO

N R

ESE

AR

CH

BO

AR

D in

the

form

at T

extb

ook

via

Cop

yrig

ht C

lear

ance

Cen

ter.

f9

Ns*

11.9 Coyle and Castello’s Method for Estimating Qp in Sand 563

For ,

(11.34)

O’ Neill and Reese (1999) suggested the following approximate relationships for Ir

and the undrained cohesion, cu.

Ir 5Es

3cu

f 5 0

Table 11.8 Variation of with forCondition based on Vesic’s Theory

10 6.9720 7.9040 8.8260 9.3680 9.75100 10.04200 10.97300 11.51400 11.89500 12.19

Nc*Irr

f 5 0IrrNc*

0.24 500.48 150

�0.96 250�300

Note: pa � atmospheric pressure< 100 kN>m2.

Ir

cu

pa

The preceding values can be approximated as

(11.35)

11.9 Coyle and Castello’s Method for Estimating in Sand

Coyle and Castello (1981) analyzed 24 large-scale field load tests of driven piles in sand.On the basis of the test results, they suggested that, in sand,

(11.36)where

vertical stress at the pile tipcapacity factor

Figure 11.15 shows the variation of with and the soil friction angle fr.L>DNq*

Nq* 5 bearing

qr 5 effective

Qp 5 qrNq*Ap

Qp

Ir 5 347acu

pab 2 33 # 300


Example 11.1

Consider a 15-m long concrete pile with a cross section of fully em-bedded in sand. For the sand, given: unit weight, ; and soil friction angle,

. Estimate the ultimate point with each of the following:

a. Meyerhof’s methodb. Vesic’s methodc. The method of Coyle and Castellod. based on the results of parts a, b, and c, adopt a value for

Solution

Part aFrom Eqs. (11.16) and (11.17),

For , the value of (Table 11.5). Also,

. Thus,

Again,

Hence, .

Part bFrom Eq. (11.19),

Qp 5 1014 kN

Ap(0.5paNq* tan fr) 5 (0.45 3 0.45) 3(0.5) (100) (143) ( tan 35) 4 < 1014 kN

ApqrNq* 5 (0.45 3 0.45) (255) (143) < 7384kN

5 255kN>m2

qr 5 gL 5 (17) (15)Nq* < 143fr 5 35°

Qp 5 ApqrNq* # Ap(0.5paNq* tan fr)

Qp

Qpfr 5 35°g 5 17kN>m3

0.45 m 3 0.45 m

�� 30°

Bearing capacity factor, N*q

70

30

20

10

40

50

60

010 20 40 60 80 100 200

Em

bedm

ent r

atio

, L/D

32°

34°

36°

38°40°

Figure 11.15 Variation of with (Redrawn after Coyle and Costello, 1981)

L>DNq*

From Eq. (11.26),

Assume (medium sand). So,

From Eq. (11.27),

From Eq. (11.28),

From Eq. (11.25),

From Eq. (11.24),

From Table 11.7, for and , the value of . Hence,

Part cFrom Eq. (11.36),

For and , the value of is about 48 (Figure 11.15). Thus,

Part dIt appears that obtained from the method of Coyle and Castello is too large. Thus,the average of the results from parts a and b is

Qp

Qp 5 qrNq*Ap 5 (15 3 17) (48) (0.45 3 0.45) < 2479 kN

Nq*L>D 5 33.3fr 5 35°

L

D 5

15

0.45 5 33.3

Qp 5 qrNq*Ap

Qp 5 ApsorNs* 5 (0.45 3 0.45) (139.96) (55) < 1559 kN

Ns* < 55Irr 5 41.2fr 5 35°

Irr 5 Ir

1 1 IrD 5

56

1 1 (56) (0.0064) 5 41.2

Ir 5 Es

2(1 1 ms)qr tan fr 5

25,000

(2) (1 1 0.25) (17 3 15) ( tan 35) 5 56

D 5 0.005a1 2 fr 2 25

20 b a

qrpa

b 5 0.005a1 2 35 2 25

20 b a

17 3 15

100 b 5 0.0064

ms 5 0.1 1 0.3a fr 2 25

20 b 5 0.1 1 0.3a

35 2 25

20 b 5 0.25

Es 5 (250) (100) 5 25,000 kN>m2

m < 250

Es

pa5 m

5 139.96kN>m2

sor 5 c1 1 2(1 2 sin fr)

3dqr 5 a

1 1 2(1 2 sin 35)

3 b (17 3 15)

Qp 5 ApsorNs*

11.9 Coyle and Castello’s Method for Estimating Qp in Sand 565


Use . ■

Example 11.2

Consider a pipe pile (flat driving point—see Figure 11.2d) having an outside diameterof 406 mm. The embedded length of the pile in layered saturated clay is 30 m.The following are the details of the subsoil:

Depth from Saturated unit

ground surface weight,

(m)

0–5 18 305–10 18 30

10–30 19.6 100

The groundwater table is located at a depth of 5 m from the ground surface. Estimateby using

a. Meyerhof’s methodb. Vesic’s method

Solution

Part aFrom Eq. (11.18),

The tip of the pile is resting on a clay with . So,


From Eq. (11.35),

So we use .From Table 11.8 for , the value of . Thus,

Qp 5 ApcuNc* 5 c ap

4b a

406

1000b

2

d (100) (11.51) 5 149.0 kN

Nc* 5 11.51Irr 5 300Irr 5 300

Ir 5 Irr 5 347acu

pab 2 33 5 347a

100

100 b 2 33 5 314

Qp 5 ApcuNc*

Qp 5 (9) (100) c ap

4b a

406

1000 b

2

d 5 116.5 kN

cu 5 100 kN>m2

Qp 5 9cuAp

Qp

cu(kN>m2)g(kN>m3)

Qp 5 1250 kN

1014 1 1559

2 5 1286.5 kN

Note: The average value of is

■

11.10 Correlations for Calculating with SPT and CPT Results

On the basis of field observations, Meyerhof (1976) also suggested that the ultimate pointresistance in a homogeneous granular soil may be obtained from standardpenetration numbers as

(11.37)

where

the average value of the standard penetration number near the pile point (about10D above and 4D below the pile point)

Briaud et al. (1985) suggested the following correlation for in granular soil withthe standard penetration resistance .

(11.38)

Meyerhof (1956) also suggested that

(11.39)

where penetration resistance.

Example 11.3

Consider a concrete pile that is in cross section in sand. The pile is15.2 m long. The following are the variations of with depth.

Depth below ground surface (m)

1.5 83.0 104.5 96.0 127.5 149.0 18

10.5 1112.0 1713.5 2015.0 2816.5 2918.0 3219.5 3021.0 27

N60

N60

0.305 m 3 0.305 m

qc 5 cone

qp < qc(in granular soil)

qp 5 19.7pa(N60)0.36

N60

qp

pa 5 atmospheric pressure ( < 100 kN>m2 or 2000 lb>ft2)

N60 5

qp 5 0.4paN60 L

D # 4paN60

(L 5 Lb)qp

Qp

116.5 1 149.0

2 < 133 kN

Qp

11.10 Correlations for Calculating Qp with SPT and CPT Results 567


a. Estimate using Eq. (11.37).b. Estimate using Eq. (11.38).

Solution

Part aThe tip of the pile is 15.2 m below the ground surface. For the pile, . Theaverage of 10D above and about 5D below the pile tip is

From Eq. (11.37)

Thus,


■

11.11 Frictional Resistance (Qs) in Sand

According to Eq. (11.14), the frictional resistance

The unit frictional resistance, f, is hard to estimate. In making an estimation of f, severalimportant factors must be kept in mind:

1. The nature of the pile installation. For driven piles in sand, the vibration causedduring pile driving helps densify the soil around the pile. The zone of sand densi-fication may be as much as 2.5 times the pile diameter, in the sand surroundingthe pile.

2. It has been observed that the nature of variation of f in the field is approximatelyas shown in Figure 11.16. The unit skin friction increases with depth more or

Qs 5 Sp DLf

5 575.4 kN

Qp 5 Apqp 5 Ap319.7pa(N60)0.364 5 (0.305 3 0.305) 3(19.7) (100) (24)0.364

Qp 5 893 kN

Ap(4paN60) 5 (0.305 3 0.305) 3(4) (100) (24) 4 5 893 kN

Ap c0.4paN60aL

Db d 5 (0.305 3 0.305) c (0.4) (100) (24) a

15.2

0.305b d 5 4450.6 kN

Qp 5 Ap(qp) 5 Ap c0.4paN60a L

D b d # Ap(4paN60)

N60 5 17 1 20 1 28 1 29

4 5 23.5 < 24

N60

D 5 0.305 m

Qp

Qp

less linearly to a depth of and remains constant thereafter. The magnitude ofthe critical depth may be 15 to 20 pile diameters. A conservative estimatewould be

(11.40)

3. At similar depths, the unit skin friction in loose sand is higher for a high-displacement pile, compared with a low-displacement pile.

4. At similar depths, bored, or jetted, piles will have a lower unit skin friction com-pared with driven piles.

Taking into account the preceding factors, we can give the following approximaterelationship for f (see Figure 11.16):

For to

(11.41)

and for to L,

(11.42)

In these equations,

earth pressure coefficientvertical stress at the depth under considerationfriction angle

In reality, the magnitude of K varies with depth; it is approximately equal to theRankine passive earth pressure coefficient, at the top of the pile and may be less thanKp ,

dr 5 soil-pile sor 5 effective K 5 effective

f 5 fz5Lr

z 5 Lr

f 5 Ksor tan dr

Lr,z 5 0

Lr < 15D

LrLr

D

z

� L

(a) (b)Depth

f

L

L�

Unitfrictionalresistance, f

K��o

Figure 11.16 Unit frictional resistance for piles in sand

11.11 Frictional Resistance (Qs) in Sand 569


the at-rest pressure coefficient, at a greater depth. Based on presently availableresults, the following average values of K are recommended for use in Eq. (11.41):

Pile type K

Bored or jettedLow-displacement driven to High-displacement driven to

The values of from various investigations appear to be in the range from to

Based on load test results in the field, Mansur and Hunter (1970) reported the fol-lowing average values of K.

Coyle and Castello (1981), in conjunction with the material presented in Section11.9, proposed that

(11.43)

where

average effective overburden pressuresoil–pile friction angle

The lateral earth pressure coefficient K, which was determined from field observations, isshown in Figure 11.17. Thus, if that figure is used,

(11.44)

Correlation with Standard Penetration Test Results

Meyerhof (1976) indicated that the average unit frictional resistance, for high-displacement driven piles may be obtained from average standard penetration resis-tance values as

(11.45)

where

value of standard penetration resistanceatmospheric pressure

For low-displacement driven piles

(11.46)fav 5 0.01pa(N60)

(<100 kN>m2 or 2000 lb>ft2) pa 5 (N60) 5 average

fav 5 0.02pa(N60)

fav ,

Qs 5 Ksor tan(0.8fr)pL

5 0.8fr dr 5 sor 5

Qs 5 favpL 5 (Ksor tan dr)pL

Precast concrete pilesccK 5 1.5

Steel pipe pilesccK 5 1.26

H-pilesccK 5 1.65

0.8fr.0.5frdr

1.8Ko 5 1.8(1 2 sin fr)<Ko 5 1 2 sin fr1.4Ko 5 1.4(1 2 sin fr)<Ko 5 1 2 sin fr

<Ko 5 1 2 sin fr

Ko ,

Briaud et al. (1985) suggested that

(11.47)

Thus,

(11.48)

Correlation with Cone Penetration Test Results

Nottingham and Schmertmann (1975) and Schmertmann (1978) provided correlations forestimating using the frictional resistance obtained during cone penetration tests.According to this method

(11.49)

The variations of with for electric cone and mechanical cone penetrometers areshown in Figures 11.18 and 11.19, respectively. We have

(11.50)Qs 5 Sp(DL)f 5 Sp(DL)arfc

z>Dar

f 5 arfc

(fc)Qs

Qs 5 pLfav

fav < 0.224pa(N60)0.29

��

Earth pressure coefficient, K

30°

31°

32°

33°

34°35°

36°

36

15

10

5

20

25

30

35

00.15 0.2 1.0 2 5

Em

bedm

ent r

atio

, L/D

Figure 11.17 Variation of Kwith (Redrawn after Coyleand Castello, 1981)

L>D



Example 11.4

Refer to the pile described in Example 11.3. Estimate the magnitude of for the pile.

a. Use Eq. (11.45).b. Use Eq. (11.47).

Qs

Steelpile

Timberpile

Schmertmann (1978);Nottingham and Schmertmann (1975)

Concretepile

L /D

��

00

0.5

1.0

1.5

2.0

10 20 30 40

Figure 11.19 Variation of with embedment ratio for piles in sand: mechanical cone penetrometer

ar

Steelpile

Timber pile

Schmertmann (1978);Nottingham and Schmertmann (1975)

Concrete pile

L /D

��

00

1.0

2.0

3.0

10 20 30 40

Figure 11.18 Variation of with embedment ratio for pile in sand: electric cone penetrometerar

c. Considering the results in Example 11.3, determine the allowable load-carryingcapacity of the pile based on Meyerhof’s method and Briaud’s method. Use afactor of safety, .

Solution

The average value for the sand for the top 15.2 m is

Part aFrom Eq. (11.45),


Part c

Meyerhof’s method:

Briaud’s method:

So the allowable pile capacity may be taken to be about 490 kN. ■

Example 11.5

Refer to Example 11.1. For the pile, estimate the frictional resistance

a. Based on Eqs. (11.41) and (11.42). Use and .b. Based on Eq. (11.44).c. Using the results of Part d of Example 11.1, estimate the allowable bearing

capacity of the pile. Use .

Solution

Part aFrom Eq. (11.40), . Refer to Eq. (11.41):At

At sor 5 (6.75) (17) 5 114.75 kN>m2z 5 6.75 m:

f 5 0sor 5 0z 5 0:Lr 5 15D 5 (15) (0.45) 5 6.75m

FS 5 3

dr 5 0.8frK 5 1.3

Qs

Qall 5 Qp 1 Qs

FS 5

575.4 1 911.1

3 5 495.5 kN

Qall 5 Qp 1 Qs

FS 5

893 1 556.2

3 5 483 kN

Qs 5 pLfav 5 (4 3 0.305) (15.2) (49.13) 5 911.1 kN

fav 5 0.224pa(N60)0.29 5 (0.224) (100) (15)0.29 5 49.13 kN>m2

Qs 5 pLfav 5 (4 3 0.305) (15.2) (30) 5 556.2 kN

fav 5 0.02pa(N60) 5 (0.02) (100) (15) 5 30 kN>m2

N60 5 8 1 10 1 9 1 12 1 14 1 18 1 11 1 17 1 20 1 28

10 5 14.7 < 15

N60

FS 5 3



So

Thus,


From Figure 11.17,

Part cThe average value of from parts a and b is

From part d of Example 11.1, . Thus,

■

Example 11.6

Consider an 18-m long concrete pile (cross section: ) fully embed-ded in a sand layer. For the sand layer, the following is an approximation of the conepenetration resistance (mechanical cone) and the frictional resistance with depth.Estimate the allowable load that the pile can carry. Use .

Depth from

ground surface (m)

0–5 3040 735–15 4560 102

15–25 9500 226

fc (kN>m2)qc (kN>m2)

FS 5 3fcqc

0.305 m 3 0.305 m

Qall 5 Qp 1 Qs

FS 5

1250 1 2020

3 5 1090 kN

Qp 5 1250 kN

Qs(average) 5 1659 1 2380

2 5 2019.5 < 2020 kN 2 USE

Qs

Qs 5 (1.3) (127.5) tan3(0.8 3 35) 4 (4 3 0.45) (15) 5 2380 kN

K 5 0.93

L

D5

15

0.455 33.3; fr 5 35°

sor 5(15) (17)

25 127.5 kN>m2

Qs 5 Ksor tan (0.8fr)pL

5 481.75 1 1177.61 5 1659.36 kN < 1659 kN

5 a 0 1 79.3

2 b (4 3 0.45) (6.75) 1 (79.3) (4 3 0.45) (15 2 6.75)

Qs 5 (fz50 1 fz56.75m)

2 pLr 1 fz56.75m p(L 2 Lr)

f 5 Ksor tan d 5 (1.3) (114.75) 3tan (0.8 3 35) 4 5 79.3 kN>m3

11.12 Frictional (Skin) Resistance in Clay 575

Solution

From Eq. (11.39),

At the pile tip (i.e., at a depth of 18 m), . Thus,

To determine , the following table can be prepared. (Note: .)

Depth from

ground surface (m) �L (m) fc (kN/m2) (Figure 11.19) p�L� fc (kN)

0–5 5 73 0.44 195.95–15 10 102 0.44 547.5

15–18 3 226 0.44 363.95

Hence,

■

11.12 Frictional (Skin) Resistance in Clay

Estimating the frictional (or skin) resistance of piles in clay is almost as difficult a task asestimating that in sand (see Section 11.11), due to the presence of several variables that can-not easily be quantified. Several methods for obtaining the unit frictional resistance of pilesare described in the literature. We examine some of them next.

Method

This method, proposed by Vijayvergiya and Focht (1972), is based on the assumption thatthe displacement of soil caused by pile driving results in a passive lateral pressure at anydepth and that the average unit skin resistance is

(11.51)

where

effective vertical stress for the entire embedment lengthundrained shear strength (f 5 0) cu 5 mean

sor 5 mean

fav 5 l(sor 1 2cu)

l

Qall 5Qu

FS5

1991.05

35 663.68 < 664 kN

Qu 5 Qp 1 Qs 5 883.7 1 1107.35 5 1991.05 kN

Qs 5 1107.35 kN

9

a9

L>D 5 18>0.305 5 59Qs

Qp 5 Apqc 5 (0.305 3 0.305) (9500) 5 883.7 kN

qc < 9500 kN>m2

qp < qc

Qu 5 Qp 1 Qs


The value of changes with the depth of penetration of the pile. (See Table 11.9.) Thus, the to-tal frictional resistance may be calculated as

Care should be taken in obtaining the values of and in layered soil. Figure 11.20 helpsexplain the reason. Figure 11.20a shows a pile penetrating three layers of clay. According toFigure 11.20b, the mean value of is Similarly, Figure 11.20cshows the plot of the variation of effective stress with depth. The mean effective stress is

(11.52)

where of the vertical effective stress diagrams.A1 , A2 , A3 , c 5 areas

sor 5A1 1 A2 1 A3 1 c

L

(cu(1)L1 1 cu(2)L2 1 c)>L.cu

cusor

Qs 5 pLfav

l

(a)

Depth(b)

Depth(c)

L

L1

L2

cu(1)

L3

Area � A1

Undrainedcohesion, cu

Verticaleffectivestress, ��o

cu(2)

cu(3)

Area � A2

Area � A3

Figure 11.20 Application of method in layered soill

Table 11.9 Variation of with pile embedmentlength, L

Embedmentlength, L (m)

0 0.55 0.336

10 0.24515 0.20020 0.17325 0.15030 0.13635 0.13240 0.12750 0.11860 0.11370 0.11080 0.11090 0.110

l

l

11.12 Frictional (Skin) Resistance in Clay 577

Method

According to the method, the unit skin resistance in clayey soils can be represented bythe equation

(11.53)

where adhesion factor. The approximate variation of the value of isshown in Table 11.10. It is important to realize that the values of given in Table 11.10may vary somewhat, since is actually a function of vertical effective stress and theundrained cohesion. Sladen (1992) has shown that

(11.54)

where

vertical effective stressfor bored piles and for driven piles

The ultimate side resistance can thus be given as

(11.55)Qs 5 Sfp DL 5 S acup DL

$ 0.5C < 0.4 to 0.5sor 5 average

a 5 Ca sorcu

b0.45

aa

aa 5 empirical

f 5 acu

a

a

Table 11.10 Variation of (interpo-lated values based on Terzaghi, Peckand Mesri, 1996)

1.000.2 0.920.3 0.820.4 0.740.6 0.620.8 0.541.0 0.481.2 0.421.4 0.401.6 0.381.8 0.362.0 0.352.4 0.342.8 0.34

Note:< 100 kN>m2

pa 5 atmospheric pressure

# 0.1

a

cu

pa

a


Method

When piles are driven into saturated clays, the pore water pressure in the soil around the pilesincreases. The excess pore water pressure in normally consolidated clays may be four to sixtimes However, within a month or so, this pressure gradually dissipates. Hence, the unitfrictional resistance for the pile can be determined on the basis of the effective stress parame-ters of the clay in a remolded state Thus, at any depth,

(11.56)

where

effective stress(11.57)

friction angle of remolded claypressure coefficient

Conservatively, the magnitude of K is the earth pressure coefficient at rest, or

(11.58)

and

(11.59)

where ratio.Combining Eqs. (11.56), (11.57), (11.58), and (11.59), for normally consolidated

clays yields

(11.60)

and for overconsolidated clays,

(11.61)

With the value of f determined, the total frictional resistance may be evaluated as

Correlation with Cone Penetration Test Results

Nottingham and Schmertmann (1975) and Schmertmann (1978) found the correlation forunit skin friction in clay (with ) to be

(11.62)

The variation of with the frictional resistance is shown in Figure 11.21. Thus,

(11.63)Qs 5 Sfp(DL) 5 Sarfcp(DL)

fcar

f 5 arfc

f 5 0

Qs 5 Sfp DL

f 5 (1 2 sin fRr )tan fRr"OCR sor

f 5 (1 2 sin fRr ) tan fRr sor

OCR 5 overconsolidation

K 5 (1 2 sin fRr )"OCR (for overconsolidated clays)

K 5 1 2 sin fRr (for normally consolidated clays)

K 5 earth fRr 5 drained

b 5 K tan fRr sor 5 vertical

f 5 bsor

(cr 5 0).

cu .

b

11.13 Point Bearing Capacity of Piles Resting on Rock 579

Steel piles

Nottingham and Schmertmann (1975);Schmertmann (1978)

Concrete andtimber piles

��

00

0.25

0.5

0.75

1.0

1.25

1.5

0.5 1.0 1.5 2.0fcpa

Figure 11.21 Variation of with for piles in clay ( pressure)<100 kN>m2

pa 5 atmosphericfc>paar

11.13 Point Bearing Capacity of Piles Resting on Rock

Sometimes piles are driven to an underlying layer of rock. In such cases, the engineer mustevaluate the bearing capacity of the rock. The ultimate unit point resistance in rock(Goodman, 1980) is approximately

(11.64)

where

compression strength of rockangle of friction

The unconfined compression strength of rock can be determined by laboratory tests onrock specimens collected during field investigation. However, extreme caution shouldbe used in obtaining the proper value of because laboratory specimens usually aresmall in diameter. As the diameter of the specimen increases, the unconfined compres-sion strength decreases—a phenomenon referred to as the scale effect. For specimenslarger than about 1 m in diameter, the value of remains approximately constant.There appears to be a fourfold to fivefold reduction of the magnitude of in thisprocess. The scale effect in rock is caused primarily by randomly distributed large and

qu

qu

qu ,

fr 5 drained

qu 5 unconfined

Nf 5 tan2(45 1 fr>2)

qp 5 qu(Nf 1 1)


small fractures and also by progressive ruptures along the slip lines. Hence, we alwaysrecommend that

(11.65)

Table 11.11 lists some representative values of (laboratory) unconfined compressionstrengths of rock. Representative values of the rock friction angle are given in Table 11.12.

A factor of safety of at least 3 should be used to determine the allowable pointbearing capacity of piles. Thus,

(11.66)Qp(all) 53qu(design)(Nf 1 1) 4Ap

FS

fr

qu(design) 5qu(lab)

5

Table 11.11 Typical Unconfined Compressive Strength of Rocks

qu

Type of rock MN m2

Sandstone 70–140Limestone 105–210Shale 35–70Granite 140–210Marble 60–70

,

Example 11.7

Refer to the pile in saturated clay shown in Figure 11.22. For the pile,

a. Calculate the skin resistance by (1) the method, (2) the method, and(3) the method. For the method, use for all clay layers. The top10 m of clay is normally consolidated. The bottom clay layer has an

. (Note: diameter of pile � 406 mm)b. Using the results of Example 11.2, estimate the allowable pile capacity .

Use .FS 5 4(Qall)

OCR 5 2

fRr 5 30°bbla(Qs)

Table 11.12 Typical Values of Angle of Friction of Rocks

Type of rock Angle of friction, (deg)

Sandstone 27–45Limestone 30–40Shale 10–20Granite 40–50Marble 25–30

f9

fr

11.13 Point Bearing Capacity of Piles Resting on Rock 581

Solution

Part a(1) From Eq. (11.55),

[Note: ] Now the following table can be prepared.

Depth cu �cup

(m) (m) (kN/m2) (Table 11.10) (kN)

0–5 5 30 0.82 156.835–10 5 30 0.82 156.83

10–30 20 100 0.48 1224.0

(2) From Eq. 11.51, Now, the average value of is

To obtain the average value of the diagram for vertical effective stress variation withdepth is plotted in Figure 11.22b. From Eq. (11.52),

sor 5A1 1 A2 1 A3

L5

225 1 552.38 1 4577

305 178.48 kN>m2

sor ,

cu(1)(10) 1 cu(2)(20)

305

(30) (10) 1 (100) (20)

305 76.7 kN>m2

cufav 5 l(sor 1 2cu).

Qs < 1538 kN

DLaDL

p 5 p(0.406) 5 1.275m

Qs 5 SacupDL

Groundwatertable

A1 � 225

A2 � 552.38

A3 � 4577

z

(a) (b)30 m 326.75

130.9510 m

5 m90

��o (kN/m2)

5 mSaturated clay

Clay5 m

20 m

cu(1)�

� 30 kN/m2

� 18 kN/m3

cu(1)�

� 30 kN/m2

� 18 kN/m3

Claycu(2)�sat

� 100 kN/m2

� 19.6 kN/m3

Figure 11.22 Estimation of the load bearing capacity of a driven-pipe pile


From Table 11.9, the magnitude of is 0.136. So

Hence,

(3) The top layer of clay (10 m) is normally consolidated, and Forfrom Eq. (11.60), we have

Similarly, for

For from Eq. (11.61),

For

So,

Part b

From Example 11.2,

Again, the values of from the method and method are close. So,

■Qall 5 Qu

FS 5

133 1 1632.5

4 5 441.4 kN < 441 kN

Qs < 1538 1 1727

2 5 1632.5 kN

laQs

Qp < 116.5 1 149

2 < 133 kN

Qu 5 Qp 1 Qs

5 (p) (0.406) 3(13) (5) 1 (31.9) (5) 1 (93.43) (20) 4 5 2670 kN

Qs 5 p3fav(1)(5) 1 fav(2)(5) 1 fav(3)(20) 4

fav(3) 5 (1 2 sin 30°) (tan 30°)"2¢130.95 1 326.75

2≤ 5 93.43 kN>m2

OCR 5 2,

fav 5 (1 2 sin frR)tan frR"OCR sor

z 5 10–30 m

fav(2) 5 (1 2 sin 30°) (tan 30°) ¢90 1 130.95

2≤ 5 31.9 kN>m2

z 5 5–10 m.

5 (1 2 sin 30°) (tan 30°) ¢0 1 90

2≤ 5 13.0 kN>m2

fav(1) 5 (1 2 sin fRr ) tan fRr sor

z 5 0–5 m,fRr 5 30°.

Qs 5 pLfav 5 p(0.406) (30) (45.14) 5 1727 kN

fav 5 0.1363178.48 1 (2) (76.7) 4 5 45.14 kN>m2

l

11.14 Pile Load Tests 583

Example 11.8

A concrete pile in cross section is driven to a depth of 20 m belowthe ground surface in a saturated clay soil. A summary of the variation of frictional re-sistance obtained from a cone penetration test is as follows:

Depth Friction resistance, (m) fc (kg/cm2)

0–6 0.356–12 0.56

12–20 0.72

Estimate the frictional resistance for the pile.

Solution

We can prepare the following table:

Depth fc L fcp( L)(m) (kN/m2) (Figure 11.21) (m) [Eq. (11.63)] (kN)

0–6 34.34 0.84 6 211.56–12 54.94 0.71 6 285.5

12–20 70.63 0.63 8 434.2

(Note: )

Thus,

■

11.14 Pile Load Tests

In most large projects, a specific number of load tests must be conducted on piles. Theprimary reason is the unreliability of prediction methods. The vertical and lateral load-bearing capacity of a pile can be tested in the field. Figure 11.23a shows a schematicdiagram of the pile load arrangement for testing axial compression in the field. The loadis applied to the pile by a hydraulic jack. Step loads are applied to the pile, and sufficienttime is allowed to elapse after each load so that a small amount of settlement occurs. Thesettlement of the pile is measured by dial gauges. The amount of load to be applied foreach step will vary, depending on local building codes. Most building codes require thateach step load be about one-fourth of the proposed working load. The load test should becarried out to at least a total load of two times the proposed working load. After the desiredpile load is reached, the pile is gradually unloaded.

Qs 5 g arfcp(DL) 5 931 kN

p 5 (4) (0.305) 5 1.22 m

Da9Da9

Qs

fc

305 mm 3 305 mm

11

11.1 Introduction

Piles are structural members that are made of steel, concrete, or timber. They are used tobuild pile foundations, which are deep and which cost more than shallow foundations.(See Chapters 3, 4, and 5.) Despite the cost, the use of piles often is necessary to ensurestructural safety. The following list identifies some of the conditions that require pilefoundations (Vesic, 1977):

1. When one or more upper soil layers are highly compressible and too weak to sup-port the load transmitted by the superstructure, piles are used to transmit the load tounderlying bedrock or a stronger soil layer, as shown in Figure 11.1a. When bedrockis not encountered at a reasonable depth below the ground surface, piles are used totransmit the structural load to the soil gradually. The resistance to the applied struc-tural load is derived mainly from the frictional resistance developed at the soil–pileinterface. (See Figure 11.1b.)

2. When subjected to horizontal forces (see Figure 11.1c), pile foundations resist bybending, while still supporting the vertical load transmitted by the superstructure.This type of situation is generally encountered in the design and construction ofearth-retaining structures and foundations of tall structures that are subjected to highwind or to earthquake forces.

3. In many cases, expansive and collapsible soils may be present at the site of a pro-posed structure. These soils may extend to a great depth below the ground surface.Expansive soils swell and shrink as their moisture content increases and decreases,and the pressure of the swelling can be considerable. If shallow foundations areused in such circumstances, the structure may suffer considerable damage.However, pile foundations may be considered as an alternative when piles areextended beyond the active zone, which is where swelling and shrinking occur.(See Figure 11.1d)

Soils such as loess are collapsible in nature. When the moisture content of thesesoils increases, their structures may break down. A sudden decrease in the void ratio ofsoil induces large settlements of structures supported by shallow foundations. In such

535

Pile Foundations

Documents

11.1 Introduction - personal.its.ac.idpersonal.its.ac.id/files/material/4278-handayanu-oe-pondasi5.pdf · build pile foundations, ... structural safety. The following list identifies