state of Practice- Micropile Structural& Geotechnical Design

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State of Practice: MicropileStructural and Geotechnical Design

Presented by:

Jonathan K. Bennett, PE, D.GE

Presented to:

DFI / ADSC Micropile Seminar, Salt Lake City, UTMarch 21, 2013

GEOTECHNICAL CONSTRUCTION: TECHNICAL TRAINING SERIES

Introduction

This Session’s Objectives:

• Explore the state of practice in terms of design.

We’ll Do That By Covering:

• A Quick Introduction to Micropiles in General

• Current Design Codes and Practice

• Design Example and Comparison

• Micropile Research Findings that Extend the State of Practice

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What is a Micropile?

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What is a Micropile?

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Said another way, A Micropile is a pile that…

• Is drilled and grouted,• Is 12 inches or less in diameter,• Is a replacement vs a displacement pile,• Is typically reinforced, and• May or may not have steel casing left in place

permanently.

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Micropile Installation

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• Micropiles are typically installed by drilling them into the ground using either cased or uncased construction and rotary, rotary percussive or down‐hole hammer drilling systems.

• Temporary or permanent casing can be utilized for installation of micropiles where support of the drilled hole sides is required (caving soils).

• Generally, the hole is drilled and cleaned, the reinforcing core inserted into the hole and then the hole is grouted from the bottom up using a tremiegrouting methods.

• Where rock drilling is required, rotary percussive or down hole hammer equipment is used for rapid hole advancement.

Micropile Features

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• Micropiles can be installed at angles and are able to resist both axial and lateral loads.

• Micropiles develop their axial capacity primarily through the bond between grout and soil or rock in the bonded zone of the pile. Because of this, micropiles provide both tension and compression resistance thus making them useful in a variety of applications.

• They are installed using mostly the same drilling and grouting equipment that is used for tiebacks and soil nailing.

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Micropile Features

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• Because of the installation methods used (DHH and rotary percussive drilling), micropiles can be used in soil and rock conditions where the use of conventional deep foundation systems are not a reasonable alternative (such as Karst topography, where modest obstructions are present, or in low‐headroom conditions).

Micropile Origins

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Dr. Fernando Lizzi (January 2, 1914 – August 28, 2003)is considered the father of micropile technology

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Micropile Origins

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Dr. Lizzi started to work for the company SACIF in 1947, but shortly afterwards was the first (and for some time, the only) civil engineer of the newly formed company, Fondedile, where he remained as Technical Director for nearly 50 years. During this time while, Italy specifically, and Europe generally, were being reconstructed, he developed the technology later named pali radice (root pile, micropile) for the restoration of damaged monuments and buildings at the Scuola Angiulli in Naples. The first

international application of micropiles was seen in Germany in 1952 for the underpinning of Krupp, in Essen‐Bochum and then the Kerkini Dam in Greece. The technique was later applied in hundreds of works by Fondedile in various countries. Pali radice have been used extensively in the restoration of monuments, e.g. Ponte Vecchio in Florence in 1966 and the stabilization of the Leaning Tower of

Burano in Venice. He died in Naples on August 28, 2003. [Wikipedia]

Micropile Origins

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Micropile Origins

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History and Increase in Usage

• 1950’s – Post WWII Europe – Dr. Fernando Lizzi – Root Piles (Palo Radice)

• 1970’s – US specialty contractors start to dabble with micropiles and gradually increase capacity

• 1990’s ‐ Rapid Emergence in US following FHWA Research• 1997 FHWA Micropile State of Practice Document• 2000 FHWA Micropile Guidelines• 2003 DFI Guide Specification• 2005 NHI / FHWA Micropile Reference Manual• 2006 IBC Micropile Section Adoption• 2007 AASHTO LRFD Design Specification Adoption• Increase in use since inception such that 2003 market estimated to be in excess of $300M in US alone.

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Types of Micropiles

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Types of Micropiles

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Typical “High Capacity Micropile”

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Types of Micropiles: Hollow Bar

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Hollow Bar MicropileAka “Injection Bore” or “Self Drilling Anchor”

Advantages:• High bond transfer values.• Can be installed in caving soils

without casing.


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Types of Micropiles

FHWA Design Application Classifications

• Case 1 – Micropile is loaded directly and that load is resisted directly by the micropile and its reinforcement (normal foundation micropile).

• Case 2 – Micropile elements circumscribe and internally reinforce the soil so as to theoretically make a reinforced soil composite that resists external loads (reticulated micropile structure).

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Types of Micropiles

FHWA Construction Type Classifications

• Type A ‐ Gravity Grouted

• Type B – Pressure Grouted Through Casing

• Type C – Single Global Post Grout

• Type D – Multiple Repeatable Post Grout

• Type E* ‐ Hollow Bar

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Micropile Materials

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• Pipe Casing (typically mill secondary oilfield casing)

• Solid or Hollow Reinforcing Bars

• Neat Cement Grout

+

=

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Micropile Materials: Casing

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Typically, 80 ksi Mill Secondary Oilfield Tubular is the national norm for micropile casing.

Micropile Materials: Core Steel

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Core steel can be solid or hollow bars and is typically ASTM A615 grades 75, 80 or 95 or ASTM A722 Grade 150.

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Micropile Materials: Grout

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Grout used for micropiles is typically a neat water – cement mix that may or may not contain plasticizing admixtures for flowability.

Micropile Installation Equipment

Use essentially the same or similar drilling and grouting equipment used for installation of drilled and grouted ground anchors. There is

a wide range of sizes and configurations for micropile drills.

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Micropile Applications

• Foundation Piles

· Foundation Support through Difficult Subsurface Conditions

• Foundation Underpinning / Retrofit

• Slope Stabilization

• Earth Retention (A‐Frame & Reticulated Structures)

• Vertical Soil Reinforcement – Micropiles for Settlement Control in Soft Soils

• Ground Source Heating / Cooling – Energy Piles

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Micropile Applications

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Difficult Subsurface Conditions

Karst is a distinctive topography in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually limestone, dolomite, or marble).

This geologic process, occurring over long periods of time, results in unusual surface and subsurface features ranging from sinkholes, vertical shafts, disappearing streams, and springs, to complex underground drainage systems and caves.

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Karst Features

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Karst Features: Pinnacled Limestone

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Karst Features: Pinnacled Limestone

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Karst Map

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Micropile Feasibility

Micropiles are most cost effective when one or more of the following conditions exist:

• Difficult subsurface conditions, e.g. soils with boulders, or debris, existing foundations, high groundwater, etc.

• Restricted access or limited overhead clearance.

• Subsurface voids (karst).

• Vibrations and noise must be minimized.

• Settlement must be minimized.

• Relatively high unit loads are required (50k – 1000k) and other drilling methods are ineffective.

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Micropile Design

Fundamentally, Micropile design is the process of properly matching micropile components and overall configuration to the loads required.

In the interest of time and for simplicity, we will be examining design from the perspective of

structural and geotechnical design for resisting axial loads only.

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Load Transfer Mechanism

• For micropiles, the axial load is resisted primarily by the grout‐to‐soil or grout‐to‐rock bond capacity in the bonded zone of the pile. This allows resistance to both tension and compression forces.

• End bearing is not typically considered except in the case of a casing only micropile with a minimal rock socket. In that case, we rely on the confinement condition of the rock socket to provide resistance far in excess of what would typically be considered based on published bearing capacities.

Micropile Design

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Load Transfer – Fully Bonded

Micropile Design

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Load Transfer – Socketed Casing

Micropile Design

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Load Transfer – Cased w/ Reinforcing

Micropile Design

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Micropile Design

Basic Considerations for Micropile Design

• Determination of Axial and Lateral Loading Conditions

• Micropile Structural Design· Cased Section· Uncased Section

• Geotechnical Design Capacity• Pile to Foundation Connection• Deformations / Serviceability• Verification of Assumptions through Testing and QC

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Micropile Design

Micropile Design Guides and Specifications

• 1997 FHWA Micropile State of Practice

• 2000 FHWA Micropile Guidelines

• 2003 DFI Guide Specification

• 2005 NHI/FHWA Micropile Reference Manual

• 2006 IBC Micropile Section (2009 Rev)

• 2007 AASHTO LRFD Design Specification (2010 Rev)

• Forthcoming ADSC / DFI Micropile Specification

• Forthcoming AASHTO LRFD Construction Specification

• Forthcoming NHI/FHWA Reference Manual Revions (LRFD)

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Micropile Design

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Micropile Design

Current Design Approaches for Micropiles

• Service Load Design

– Federal Highway Administration Manuals

– International Building Code

– Most Local Building Codes

• LRFD Design

– AASHTO LRFD Bridge Design Specifications

– Forthcoming FHWA / NHI Manual

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Micropile Design

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Micropile Design SLD vs LRFD

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Service Load or Working Load Design

Service Load ≤ Ultimate Load / FS

Allowable Stress or Working Stress Design

Actual Stress ≤ Yield or Ultimate Stress / FS


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Load and Resistance Factor Design (LRFD) utilizes various Load Factors with magnitudes based on type of load to account for variability in loading and various Resistance Factors of varying magnitudes based on material or resistance type to account for variability in

resistance.

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(FHWA, 1997)

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LOAD COMBINATIONSBuilding codes specify different load combinations for ASD and LRFD due to the difference in the way loads are considered in the two different methods. The

combinations below are from ASCE 7 and the 2010 IBC.

ASD Load Combinations LRFD Load Combinations

D+F 1.4(D+F)

D+H+F+L+T 1.2(D+F+T)+1.6(L+H)+0.5(Lr or S or R)

D+H+F+(Lr or S or R) 1.2D+1.6(Lr or S or R)+(L or 0.8W)

D+H+F+0.75(L+T)+0.75(Lr or S or R) 1.2D+1.6W+L+0.5(Lr or S or R)

D+H+F+(W or 0.7E) 1.2D+1.0E+L+0.2S

D+H+F+0.75(W or 0.7E)+0.75L+0.75(Lr or S or R)

0.9D+1.6W+1.6H

0.6D+W+H 0.9D+1.0E+1.6H

0.6D+0.7E+H


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It is difficult to directly compare SLD results and LRFD results because in LRFD, the factored loads

used in computing required resistance vary based on how much of different types of load are present because load factors are different for different types of load. Otherwise, the

relationship between SLD and LRFD would be the simple relationship:

Load Factor / Resistance Factor = Factor of Safety

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Micropile Structural Design

Basic Considerations for Micropile Structural Design

• Cased Length Analysis / Design

• Uncased Length Analysis / Design

• Design of Components in those zones

• You might include pile cap connection design in the structural design category.

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• Compression Strength (Ultimate)• Puc = fc’ Ag + Fy As

• Compression Strength (Allowable)• Pac = A fc’ Ag + B Fy Ac + C Fy Ab

• Tension Strength (Ultimate)• Put = Fy As

• Tension Strength (Allowable)• Pat = D Fy As

• Where A, B, C and D are reduction factors which express the allowable stresses as a percentage of ultimate stress. The magnitude of these reduction factors varies depending on which design code you are using.

• The core assumption with regard to the above compressive strength formulas is that the pile is sufficiently supported along its length by soil or rock such that buckling cannot occur. Most soils will provide a level of support that is sufficient to preclude outright buckling. However, the stiffness of the overburden soils can effect the actual pile capacity. This is not taken into account in the formulas.

Micropile Structural Design ‐ FHWA

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• Compression Strength (Allowable)

• Pac = 0.40 fc’ Ag + 0.47 Fy As

• Tension Strength (Allowable)

• Pat = 0.55 Fy Ab

• Maximum Test Load (Allowable)

• Ptc = 0.8 (fc’ Ag + Fy As)

• Ptt = 0.8 Fy Ab for ASTM A615 material

• Ptt = 0.8 Fu Ab for ASTM A722 material

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Micropile Structural Design ‐ IBC

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• Compression Loading

• Pac = 0.33 fc’ Ag + 0.40 Fy As

• Tension Loading

• Pat = 0.60 Fy Ab (same as PTI)

• Steel yield stress limited to 80 ksi.

• Steel reinforcement must carry at least 40% of the load.

Micropile Structural Design Comparison

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FHWA Design CriteriaCompression: Pa = 0.40fc’Ag + 0.47FyAb

Tension: Pa = 0.55FyAb

DFI / IBC Design CriteriaCompression: Pa = 0.33fc’Ag + 0.40FyAb

Tension: Pa = 0.60FyAb (same as PTI)

Imposed LimitationsFHWA Compression: Fy = 87 ksi max (strain compatibility )DFI Compression: Fy = 87 ksi max (strain compatibility )IBC Compression: Fy = 80 ksi max

IBC Compression: 0.40FyAb >= 0.40Pa

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Micropile Structural Design ‐ LRFD

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Note that in this format, the product of load factors and mean load effects are combined as opposed to combining load effects alone. This

differs from traditional Working Stress or Service Load analysis where the load effects alone are

combined without load factors.


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10.9.3.10.2 ‐ Axial Compressive Resistance

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10.9.3.10.2a ‐ Cased Length


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10.9.3.10.2b ‐ Uncased Length

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10.9.3.10.3 ‐ Axial Tension Resistance


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Section 10.5 – Limit States and Resistance Factors

10.5.5 – Resistance Factors

10.5.5.2.5 – MicropilesResistance factors shall be selected from Table 10.5.5.2.5‐1 based on the method used for determining the nominal axial pile resistance. If the resistance factors provided in Table 10.5.5.2.5‐1 are to be applied to piles in potentially creeping soils, highly plastic soils, weak rock, or other marginal ground type, the resistance factor values in the Table should be reduced by 20 percent to reflect greater design uncertainty.

The resistance factors in Table 10.5.5.2.5‐1 were calibrated by fitting to ASD procedures tempered with engineering judgment. The resistance factors in Table 10.5.5.2.5.‐2 for structural resistance were calibrated

by fitting to ASD procedures and are equal to or slightly more conservative than corresponding resistance factors from Section 5 of

the AASHTO LRFD Specifications for reinforced concrete column design.

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Micropile Structural Design – LRFD Comparison

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Structural Design – Comparison

Compression Case

FHWA ASD

Pac = 0.40 fc’ Ag + 0.47 fy As

IBC ASD

Pac = 0.33 fc’ Ag + 0.40 fy As

AASHTO LRFD EQUIVALENT ASD FORMULA

Pac = 0.36 fc’ Ag + 0.425 fy As (LFavg = 1.5)

Pac = 0.38 fc’ Ag + 0.45 fy As (LFavg = 1.42)


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Structural Design – Comparison

Tension Case

FHWA ASD

Pat = 0.55 fy Ab

IBC ASD

Pat = 0.60 fy Ab

AASHTO LRFD EQUIVALENT ASD FORMULA

Pat = 0.533 fy Ab (LFavg = 1.5)

Pat = 0.563 fy Ab (LFavg = 1.42)

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Micropile Information (Given)

Casing Size: 7” OD X 0.500”Casing Strength: N80 Mill Secondary

Fy = 80 ksi minimum

Core Size: #18 Full LengthCore Strength: ASTM A615 Gr 80

Fy = 80 ksi

Grout Strength: fc’ = 4000 psi

Cased Length: 40.00’

Rock Type: Limestone

Socket Diameter: 7.5” = 0.625’

Plunge Length: 1.00’


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Basic Cross Section Properties

#18 Bar Core, 7”OD X 0.500” Casing, 7.5” Socket Diameter

CASED SECTION

Abar = 4.00 in2 (#18)

Acasing = 3.1416(ro2‐ri

2) = 10.21 in2

Agrout = 3.1416(3)2‐4.00 = 24.27 in2

UNCASED SECTION

Abar = 4.00 in2 (#18)

Agrout = 3.1416(3.75)2‐4.00 = 40.18 in2

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Compression Structural Design– Cased Length


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Compression Structural Design ‐ Uncased Length

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Tension Structural Design


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Tension Structural Design

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Structural Design ‐ Comparison

Compression Allowable Service Load – CasedLength

Compression Allowable Service Load – Uncased Length

Tension Allowable Service Load

FHWA ASD 573 k 215 k 176 k

IBC ASD 487 k 181 k 192 k

AASHTO LRFD (LFavg=1.50)

518 k 194 k 171 k

AASHTO LRFD (LFavg=1.42)

547 k 205 k 180 k


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440

460

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500

520

540

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FHWA ASD IBC ASD AASHTO LRFD (LF = 1.50) AASHTO LRFD (LF = 1.42)

Axial Load

(kips)

Compression Allowable Service Load Cased Length

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Axial Load

(kips)

Compression Allowable Service Load Uncased Length


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Axial Load

(kips)

Tension Allowable Service Load

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Micropile / Foundation Connection

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Micropile Geotechnical Design

• For design purposes, micropiles are usually assumed to transfer their load to the ground through grout‐to‐ground skin friction, without any contribution from end bearing (FHWA, 1997).

• This assumption results in a pile that is for the most part geotechnically equivalent in tension and compression.

• Suggested bond values can be found in the FHWA Manuals as well as in the PTI Recommendations for Prestressed Rock and Soil Anchors.

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Allowable Geotechnical Capacity ‐ FHWA

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• IBC Code does not offer specific guidance for bond values for geotechnical design of micropiles.

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Summary of Typical Grout to Ground Bond Values for Preliminary Micropile Design

Soil / Rock Description Typical Range of Grout-to-Ground Nominal Strength

Type A Type B Type C Type D

English (psi) SI (kPa) English (psi) SI (kPa) English (psi) SI (kPa) English (psi) SI (kPa)

min max avg min max avg min max avg min max avg min max avg min max avg min max avg min max avg

Silt and Clay (some sand) 5.1 10.2 7.6 35 70 52.5 5.1 13.8 9.4 35 95 65 7.3 17.4 12.3 50 120 85 7.3 21.0 14.1 50 145 97.5

soft, medium plastic

Silt and Clay (some sand) 7.3 17.4 12.3 50 120 85 10.2 27.6 18.9 70 190 130 13.8 27.6 20.7 95 190 142.5 13.8 27.6 20.7 95 190 142.5

stiff, dense to very dense

Sand (some silt) 10.2 21.0 15.6 70 145 107.5 10.2 27.6 18.9 70 190 130 13.8 27.6 20.7 95 190 142.5 13.8 34.8 24.3 95 240 167.5

fine, loose-medium dense

Sand (some silt, gravel) 13.8 31.2 22.5 95 215 155 17.4 52.2 34.8 120 360 240 21.0 52.2 36.6 145 360 252.5 21.0 55.8 38.4 145 385 265

fine-coarse, med-very dense

Gravel (some sand) 13.8 38.4 26.1 95 265 180 17.4 52.2 34.8 120 360 240 21.0 52.2 36.6 145 360 252.5 21.0 55.8 38.4 145 385 265

medium-very dense

Glacial Till (silt, sand, gravel) 13.8 27.6 20.7 95 190 142.5 13.8 45.0 29.4 95 310 202.5 17.4 45.0 31.2 120 310 215 17.4 48.6 33.0 120 335 227.5

medium-very dense, cemented

Soft Shales 29.7 79.8 54.8 205 550 377.5 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

fresh-moderate fracturing

little to no weathering

Slates and Hard Shales 74.7 200.2 137.4 515 1380 947.5 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --



Limestone 150.1 300.2 225.2 1035 2070 1553 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --



Sandstone 75.4 250.2 162.8 520 1725 1123 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --



Granite and Basalt 200.2 609.2 404.7 1380 4200 2790 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --



Type A - Gravity grout only.

Type B - Pressure grouted through the casing during casing withdrawal.

Type C - Primary grout placed under gravity head, then one phase of secondary "global" pressure grouting.

Type D - Primary grout placed under gravity head, then one or more phases of secondary "global" pressure grouting.


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Micropile Geotechnical Design ‐ LRFD

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Section 10.9.3 – Strength Limit State Design

10.9.3.5 – Nominal Axial Compression Resistance of a Single Micropile

Micropiles shall be designed to resist failure of the bonded length in soil and rock, or for

micropiles bearing on rock, failure of the rock at the micropile tip.

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Section 10.9.3.5 – Nominal Axial Compression Resistance of a Single Micropile

The factored resistance of a micropile, RR, shall be taken as:


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Section 10.9.3.5 – Nominal Axial Compression Resistance of a Single Micropile

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Micropile Testing – LRFD Verification

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Section 10.9.3 – Strength Limit State Design

10.9.3.5 – Nominal Axial Compression Resistance of a Single Micropile

10.9.3.5.4 – Micropile Load Test

The load test shall follow the procedures specified in ASTM D1143 for compression and ASTM D3689 for tension. The

loading procedure should follow the Quick Load Test Method, unless detailed longer‐term load settlement data is needed, in which case the standard loading procedure should be used. Unless specified otherwise by the Engineer, the pile axial

(nominal) resistance shall be determined from the test data using the Davisson Method as presented in Article 10.7.3.8.2.

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Micropile Structural Design – LRFD Limitations

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• Load Combinations and Load Factors in Section 3 (Table 3.4.1‐1) were developed specifically for bridges and may not be applicable to other structures.

• Current Resistance Factors are calibrated based on fitting to ASD, not on reliability theory. Therefore does not truly reflect reliability based design at this time except in format.

• No Strength Limit State Checks for lateral loads. Not enough consensus exists in terms of design methodology for LRFD.

• Includes strain compatibility related stress limitations which have been shown to be erroneous for reinforcing in a confined condition.

• Davisson is the criteria for determining the Resistance of a micropile. Davisson is considered by many to be overly conservative and inappropriate for micropiles.

Micropile Testing

• It is typical for any substantial micropile project to include some sort of testing program.

• Generally based on ASTM D1143 Quick Test.• The older FHWA specifications prescribed testing to 2.5 X

Service Design Load.• Newer FHWA publications recommend 2.0 X DL. 2.0 DL is

appropriate in most cases. Test to 2.0 DL for best economy.• Tension testing is generally considered to be conservative

compared to compression testing because it neglects any end bearing and is often more economical for checking capacity. However tension test results will not give representative movement results for compression case.

• Compression testing requires anchors to hold down testing apparatus adding to cost but gives representative results for compression loading.

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Micropile Testing

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Micropile Testing

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Micropile New Frontiers

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ADSC / DFI Joint Micropile Committee

The Micropile Committee is a joint committee comprised of members from both the ADSC‐IAFD and the Deep Foundations Institute (DFI), and is comprised of interested engineering

professionals dedicated to providing:

• primary assistance in the writing of applicable specifications • review, commentary and formal acceptance of design and

construction specifications • a network of industry professionals to perform research

necessary for the advancement of Micropile technologies


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Committee Objectives

• Have four (4) committee meetings per year to conduct the business of the Committee,

• Sponsor and execute one (1) to two (2) industry educational seminars each year,

• Canvas the committee membership to investigate future research activities and needs that may be suitable to participate in or recommend to the ADSC IASC (Industry Advancement Steering Committee) or DFI Committee Project Fund for sponsorship.

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Specifications

• DFI / ADSC Guide to Drafting a Specification for Micropiles• AASHTO LRFD Bridge Design Specification – Micropiles• AASHTO LRFD Bridge Construction Specifications – Micropiles• Input on Development and Maintenance of IBC Micropiles Section• Currently updating DFI / ADSC Micropile Guide Specification• Will provide input on New FHWA / NHI Micropile Manual


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Research

• Micropile Strain Compatibility Testing• Micropile Bearing Plates: Are They Necessary• Position Paper on the Use of Mill Secondary Casing • Reticulated Micropile State of Practice

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Micropile Strain Compatibility Testing

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Micropile Bearing Plates: Are They Necessary

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Position Paper on the Use of Mill Secondary Casing

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Reticulated Micropile State of Practice

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Questions?

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What’s a Micropile?

Questions?

General Q & A

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Questions?

For more information on micropiles:

• Upcoming DFI / ADSC Micropile Seminars – Annual or Semi‐Annual

• DFI Micropile Committee Q&A Website (www.dfi.org)

• My personal blog on Micropile Design and Construction www.micropile.org

• MD&C on Facebook www.facebook.com/Micropiles

• FHWA Geotechnical Engineering Library

• Contact me and I will schedule a time to come to your office and provide specific micropile training tailored to your needs.

Jon Bennett – [email protected]

(724) 443‐1533 x54107 Office / (304) 707‐4840 Mobile

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Questions?

THANK YOU!for Your Time and Attention

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Documents

state of Practice- Micropile Structural& Geotechnical Design