[PPT]Micropiles: · Web viewADSC-IAFD Non-profit, international, trade association based in...
Transcript of [PPT]Micropiles: · Web viewADSC-IAFD Non-profit, international, trade association based in...
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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