Micropiles: Design Considerations &

Construction Aspects

Courtesy: Hayward Baker

Overview

• ADSC-IAFD• Historical Background• Micropiles Defined• Typical Applications• Design Considerations• Advantages and Limitations• Construction Aspects and Equipment• Load Testing• QC / QA

ADSC-IAFD

• Non-profit, international, trade association based in Irving, TX

• Represent anchored earth retention, drilled shaft, micropile construction/design industries

• Members include• Specialty subcontractors• Manufacturers & suppliers• Design engineers, academicians,

and government agencies

• Chapters - 9 in U.S., 2 in Canada, 1 in Central America

ADSC-IAFD

• Establish standards & specifications• Conduct design, construction and inspection seminars• Develop and disseminate technical data and literature• Conduct and fund practical and beneficial research• Provide a forum for technology transfer• Promote ethical practice• Interface with corresponding industries and agencies

• FHWA, ACOE, PTI, OSHA, DOTs, etc.

Mission, Vision and Goals

ADSC-IAFD

• Joint committee between ADSC-IAFD and DFI• Comprised of interested engineering professionals (aka,

industry competitors) dedicated to providing• Primary assistance in writing of applicable specifications

• Review, commentary and formal acceptance of design and construction/technological specifications

• A network of industry professionals to perform research necessary for advancement of Micropiling technologies

• Current design, construction, and testing publications, guidelines and, specifications (available in ADSC Technical Library)

Micropile Committee

Historical Background

• Dr. Fernando Lizzi (Italia) in 1950s – pali radice• 1950s – soil reinforcement mechanism for historical

structures (lightly loaded elements)• 1960s – gained acceptance and usage in Great

Britain and Germany• 1970s – introduced to U.S. and global markets• 1980s – gained acceptance in U.S.• 2000s – increasing (widespread) global use

High capacity steel and grout elementsSeries of proprietary efforts

Micropiles Defined

• Heavily reinforced, small diameter, drilled elements installed with neat cement grout

• Let’s dissect this:• Heavily Reinforced - typically reinforced with drill casing and/or

high strength bars

• Small diameter - limited to ≤12 inches (typ. 4 to 7 inches)

• Drilled - excludes driven piles and other foundation types

• Neat Cement Grout – grout does not contain aggregate (aggregate can be used in certain formations)

Classification

• Categorized based on design use & installation means• Used in almost any ground type• Transfers load to a more competent layer• Stabilize/reinforce a potential sliding mass

• Design Use• Case I: axially or laterally loaded elements• Case II: group of elements used for soil reinforcement and

stabilization (reticulated micropiles)

• Installation Process• Types A thru E

• Theoretically, any combination of “Design Use” and “Installation Process” is possible

Types andNotations

Design Use

• Support Structural Loads• Compression piles• Tension anchors• Seismic retrofit - for

lateral, vertical and torsional loads

• Excavation support• Good for restricted access• Eliminate mult.

mobilizations

Case IMicropiles

Typical Applications Case IMicropiles

Structural Support

Earth RetainingStructure

Foundations

Foundations forNew Structures

Underpinningof ExistingStructures

SeismicRetrofitting

ScourProtection

Repair/Replacement

of ExistingFoundations

Arresting/Prevention

of Movement

Upgrading ofFoundation

Capacity

Design Use

• Settlement Control• Underpinning

of structures• Ground

strengthening• In situ

Reinforcement• Slope

stabilization

Case IIMicropiles

Typical Applications Case IIMicropiles

In-SituReinforcement

Slope Stabilization

And EarthRetention

GroundStrengthening

SettlementReduction

StructuralStability

TYPE A(GravityGrouted)

TYPE B(Pressure grout through casing)

TYPE C(Gravity grout; one phase of

post-grouting)

TYPE D(Gravity grout; mult. phases ofpost-grouting)

TYPE E

Packer

Pressure Gauge

Installation Process Types andNotations

(Hollow bar drilling methods; grout used as flushing medium)

Design Considerations

• Structural Component• Code requirements (local, state or federal) – e.g. AASHTO• Design the reinforcing steel (casing / bar) and infill grout• Design according to ASD, LFD, or LRFD • Loading - axial, lateral, bending• Performance - deflections, group behavior, connection details

• Geotechnical Component• Design similar to conventional piling and anchor systems• Most critical component is grout-to-ground bond• Bond is affected by

• In situ conditions - geology, groundwater conditions

• Construction process - drilling operations, hole cleaning,

grouting, grout quality

Structural andGeotechnical Issues

Design Considerations

• Design is similar to drilled shafts and ground anchors• Interface shear strength (or ground-to-grout bond)

• Based on presumptive bond strength values (e.g., in FHWA) or based on experience

• Allowable stresses in grout and steel are straight forward• Challenges

• Interaction and transition between different cross sections• Strain compatibility

• Between various steel materials (rebar and casing) and cementitious grout

Axial Loading – Compressive or Tensile

Design Considerations

• Assume concentric axial loading

• Assume fully composite cross section• Pc,ult=f(Pcasing+Pbar+Pgrout)• Pb,ult=f(Pbar+Pgrout)

• Assume full load transfer to top of bond zone/rock socket• Conservative• Controls design

Structural Design – Axial Loading

Design Considerations

• Lateral Strength = f(soil, casing/bar size, rotational restraint, casing threads)

• In Soil ≤ ±20 kips; in Rock ≤ ±130 kip (maybe more)

• Critical Zone: top 5-10 ft (maximum stresses)• Analysis - Computer programs available

Structural Design – Lateral Loading

• Perform p-y analysis• Consider P-Δ effects• Consider soil nonlinearity• Perform push-over analysis

• If lateral response is critical, perform load tests to develop p-y curves

Courtesy: Schnabel Engineering

Design Considerations

• Issue for lateral load-generated bending moments• FHWA/ASD approach (ignores grout)

• Simplified Method (Richards & Rothbauer, 2004)

Structural Design – Combined Axial and Bending Loading

fa = operative axial stressFa = allowable axial stressfb = operative bending stressFb = allowable bending stressFe

’= Euler buckling stress=(p2E)/(2.12(kL/r)2)

Pc = max. axial compression load on pilePc,allow = allowable compression loadMmax = max. bending moment in pileMallow = allowable bending moment in casing

𝑓 𝑎𝐹𝑎

+𝑓 𝑏

(1− 𝑓 𝑎𝐹𝑒′ )𝐹 𝑏

≤1.0

𝑃𝑐

𝑃𝑐 ,𝑎𝑙𝑙𝑜𝑤+𝑀𝑚𝑎𝑥

𝑀𝑎𝑙𝑙𝑜𝑤≤1.0

Design Considerations

• Computation of Axial Deflection(or elastic shortening)

elastic = PL / AE

• L (length)• In competent soil = length above bond

length + ½ bond length• In rock = length above bond length

• AE (axial stiffness) considers• In compression = Steel and concrete• In tension = Steel only• Note:

Structural Design – Deflections (Performance)

P

L

Design Considerations

SOFT ORWEAK LAYER

LAYER WHEREBOND ZONE ISFORMED

SOFT ORWEAK LAYER

LAYER WHEREBOND ZONE ISFORMED

Structural and Geotechnical Design – Group Effects

• For loading and settlement analyses, consider group effects similar to other conventional deep foundation systems (e.g., drilled shafts and driven piles)

Design Considerations

• Structural Design Issues• Load transfer (axial and shear) – micropiles to footings

• Shear transfer - from grout to concrete• Bearing stresses at top of micropile - Bearing plate needed?• Punching shear or pullout – esp. at corners of pile cap• Adequate pile cap depth for shear?

Structural Design – Pile Cap Connections

Bearing Plate

Stiffener

Design Considerations

• Connection strength research (Gómez and Cadden, 2006)

Structural Design – Pile Cap Connections

Friction induced at the top of theinsert due to flexural stresses

Poisson Effect

Dilation Effect

Design Considerations

• Micropile - a composite element (casing, bar, grout)

• Concept - have the composite pile’s materials share a common strain level at failure (ef)• For unconfined concrete: ef = 0.3% (assume same for grout)

• For steel (bar and casing), to have ef = 0.3%: Fy,max = (ef)(Es) = (0.003)*(29,000 ksi) = 87 ksi

• Cannot use steel with Fy > 87 ksi!• Precludes use of Gr. 150 bars

• BUT - grout within micropiles is confined

Structural Design – Strain Compatibility

Design Considerations

• ADSC-IAFD and Industry Advancement Fund Research • “Grout Confinement Influence on Strain Compatibility in

Micropiles” (FMSM Engineers, 2006)

• In rock: micropile is passively confined• Allows Fult of bar to develop

• In soil: micropile is actively confined• Allows large steel stress to mobilize

• Stress-strain (s-e) relationship of confined grout is nonlinear (bilinear)

• Axial load continues to increase beyond 0.3%

Structural Design – Strain Compatibility

Construction Aspects

• Solid Bar Micropiles• Drill the borehole (with / without casing)• Install the reinforcing elements into drilled borehole

• Casing (if not same as drill casing)• Reinforcement steel (with proper corrosion protection)• Centralizers

• Fill the borehole with cement grout• Typically neat cement grout; no sand added

• Hollow Bar Micropiles• Drill and grout simultaneously (typ. a more fluid grout used)• After depth is reached, flush hole with structural grout

(replacing grout used for drilling)

Simplified GeneralProcedure

Construction Aspects Simplified General Procedure- Solid Bar Micropiles

Advantages

• High-performance

• High capacity - design loads up to 500+ tons

• Good for various loading• Tension, compression, lateral, combined

• Applicable for wide range of ground conditions

• Adaptable for varying height requirements• Used in open headroom and restricted access

• Low noise and vibration – due to drilling operation

• Can penetrate obstacles

Advantages

Limitations

• Lateral capacity limitations for vertical micropiles• High slenderness ratio (length/diameter)

• May not be appropriate for seismic retrofit (vertical micropiles)

• Limited experience in their use for slope stabilization• Not cost effective vs. conventional piling systems in open

headroom conditions• High lineal cost relative to conventional piling systems• Requires good QC / QA

• Especially with grouting

• Requires specialized equipment

Construction Equipment Drill Rigs, Tooling,and Grouting

Construction Equipment Drill Rigs -Types of Drilling

• Rotary only• Drifter, rotation/percussion• Double Head Systems• Sonic Head

Construction Equipment

• Drill pipe (casing), augers• Drill and casing bits• Under-reaming and ring bits• Percussion tooling• Air and grout swivels

Tooling – Soil andRock Drilling

Construction Equipment

Legend Percussion (Casing)

Percussion (Rod)

Rotation (Casing)

Rotation (Rod)

Flush Casing

CrownShoe

Rod

Bit

1.Single Tube

Advancement(End of Casing

Flush)

2.Rotary Duplex

3.Rotary PercussiveConcentric Duplex

4.Rotary PercussiveEccentric Duplex

5.“Double Head” Duplex 6.

Hollow-StemAuger

Tooling – Soil andRock Drilling

Construction Equipment Grout Mixersand Operation

• Grout Mixers• Colloidal Mixers• Paddle Mixers

• Grout Pumps• Single / Double Piston• Screw pump

Load Testing Compression, Tension, and Lateral Load Testing

CompressionLoad Test

LateralLoad Test

TensionLoad Test

DeformationInstrumentation

Quality Control / Quality Assurance

• Specific areas to concentrate to ensure a well-run QC/QA program• Pre-construction meeting(s)

• Field Inspection

• Load testing program

• Reporting and documentation

QC / QA Program

• Meeting(s) – some may be same person/company• Engineer, Micropile Design Engineer, Prime Contractor,

Micropile Specialty Contractor, Excavation Contractor, Geotechnical Instrumentation Specialist, Inspection Firm

• Discussion Topics • Project requirements• Construction procedures• Contract documents and layout• Reporting procedures and requirements• Installation schedule • Other concerns

Pre-ConstructionTasks and Concerns

QC / QA Program Pre-Construction- Contracting

• Micropile Specification• Prescriptive vs. Performance

• Contractor Qualification• Prequalification• On-site pre-production “test” program

• Definition of responsibility• Owner / owner’s representative• Contractor• Engineer• Inspector

QC / QA Program Pre-Construction -Owner Responsibility

• Geotechnical reports and data• Work restrictions, site and environmental limitations• Overall scope of work• Level of corrosion protection• Testing criteria and in-service performance criteria• Method of measurement and payment• Requirements for QA/QC and verification• Construction techniques that are not acceptable since

they may adversely impact the structure and/or the subsurface conditions

QC / QA Program Pre-Construction -Contractor Responsibility

• Details of all construction steps

• Gaining access (physically) to every pile location

• Setting up of load test frames

• Handling of spoils

• Construction records

QC / QA Program Pre-Construction -Project Specific Responsibility

• Easements, utility locations• Micropile type, design, and layout

• Connection design and details

• Corrosion protection details• Micropile testing procedures and requirements• Instrumentation requirements• Reports on load testing• Construction schedule

• Sequencing and coordination of work

QC / QA Program

• Every pile is a data point• Observe and document

• Drilling, installation of reinforcing, and grouting• Inspection should be performed by micropile designer

• Timeliness• Collect, document (including photographs), prepare, review and

deliver required reporting documentation

Field Inspection ofMicropile Installation

QC / QA Program Field Inspection -Typical Micropile Log

From Table 8-2 (FHWA, 2005)

QC / QA Program Field Inspection -Typical Micropile Log

Thank you for your attention!

Questions?

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