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 Mission, Vision and Goals 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. ADSC-IAFD Micropile Committee 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)
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 elements Series of proprietary efforts Micropiles Defined Heavily reinforced, small diameter, drilled elements installed with neat cement grout Lets 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 Types and Notations 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 Design Use Case I Micropiles 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 I Micropiles Typical Applications Structural Support Earth Retaining Structure Foundations Foundations for
New Structures Scour Protection Underpinning of Existing Structures Repair/ Replacement of Existing Foundations Seismic Retrofitting Arresting/
Prevention of Movement Upgrading of Foundation Capacity Design Use Case II Micropiles Settlement Control Underpinning of structures Ground strengthening
In situ Reinforcement Slope stabilization Case II Micropiles Typical Applications In-Situ Reinforcement Slope Stabilization And Earth Retention
Ground Strengthening Settlement Reduction Structural Stability Types and Notations Installation Process Pressure Gauge Packer
TYPE A TYPE B TYPE C TYPE D TYPE E (Gravity Grouted) (Pressure grout through casing) (Gravity grout; one phase of
post-grouting) (Gravity grout; mult. phases of post-grouting) (Hollow bar drilling methods; grout used as flushing medium) Design Considerations Structural and Geotechnical Issues 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 Design Considerations Axial Loading Compressive or Tensile 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 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 Structural Design Lateral Loading 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
Perform p-y analysis Consider P- 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 Structural Design Combined Axial and Bending Loading Issue for lateral load-generated bending moments FHWA/ASD approach (ignores grout)
+ ( 1 fa = operative axial stress
) 1.0 F = allowable axial stress a fb = operative bending stress Fb = allowable bending stress Fe= Euler buckling stress=(p2E)/(2.12(kL/r)2) Simplified Method (Richards & Rothbauer, 2004) + 1.0 , Pc = max. axial compression load on pile
Pc,allow = allowable compression load Mmax = max. bending moment in pile Mallow = allowable bending moment in casing Design Considerations Structural Design Deflections (Performance) Computation of Axial Deflection (or elastic shortening) elastic = PL / AE P L (length) In competent soil = length above bond
length + bond length In rock = length above bond length AE (axial stiffness) considers In compression = Steel and concrete In tension = Steel only Note: L Design Considerations SOFT OR WEAK LAYER LAYER WHERE BOND ZONE IS FORMED
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 Pile Cap Connections 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? Bearing Plate Stiffener Design Considerations Structural Design Pile Cap Connections Connection strength research (Gmez and Cadden, 2006) Friction induced at the top of the insert due to flexural stresses
Poisson Effect Dilation Effect Design Considerations Structural Design Strain Compatibility Micropile - a composite element (casing, bar, grout) Concept - have the composite piles 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 Design Considerations Structural Design Strain Compatibility 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%
Construction Aspects Simplified General Procedure 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) 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 Rotary only Drifter, rotation/percussion Double Head Systems Sonic Head
Drill Rigs Types of Drilling Construction Equipment Drill pipe (casing), augers Drill and casing bits Under-reaming and ring bits Percussion tooling Air and grout swivels Tooling Soil and Rock Drilling
Tooling Soil and Rock Drilling Construction Equipment 1. Single Tube Advancement (End of Casing Flush) Legend 2. Rotary Duplex 3.
6. Hollow-Stem Auger Casing Rod Crown Shoe Bit Construction Equipment Grout Mixers Colloidal Mixers Paddle Mixers
Grout Pumps Single / Double Piston Screw pump Grout Mixers and Operation Compression, Tension, and Lateral Load Testing Load Testing Compression Load Test Tension Load Test
Lateral Load Test Deformation Instrumentation 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 Pre-Construction Tasks and Concerns
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 QC / QA Program Micropile Specification Prescriptive vs. Performance Contractor Qualification Prequalification On-site pre-production test program Definition of responsibility
Owner / owners representative Contractor Engineer Inspector Pre-Construction - Contracting 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 Field Inspection of Micropile Installation
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 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? www.adsc-iafd.com
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