effective practices for geotechnical and foundation ......performance verification •base grouting...
TRANSCRIPT
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Effective practices for geotechnical and foundation engineering in
design-build
Dan Brown, Dan Brown & Assoc.
Steven Saye, Kiewit Infrastructure Engineers
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Key Topics
• Pre-bid geotechnical data
• Risks & differing site conditions
• Alternative Technical Concepts (ATC’s)
• Performance based specifications
• Case studies 2
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NCHRP 429 – Geotechnical Information Practices in Design-Build
Describes effective practices in geotechnical procurement, design, and quality management
• Qualifications & experience of D-B geotechnical team is key to quality
• Use over-the-shoulder reviews to expedite schedule
• Weight geotechnical factors appropriately to the importance for project success
• Use ATC’s to allow bidders to reduce risk 3
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Pre-bid geotechnical data
• Forms the basis of the D-B Team’s design, schedule and bid
• May be < 25% of the quantity of information required for final design
• D-B Team must address the limitations in the provided information at bid time
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Pre-bid geotechnical data
• Quality and timeliness of basic information is critical
– Stratigraphy and coverage
– Emphasis on basic soil and rock material properties
– Limitations of pre-bid foundation tests
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Stratigraphy and Coverage
• Need thorough program of borings or soundings for pre-bid estimating
– More routine borings with cpt soundings preferred over few borings with sophisticated testing
– Provide surveyed information for 3D modeling
– Televiewer data with rock coreholes
– Provide core samples for examination by bidders
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Example: CPT Soundings
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Example: ATV Logs of Rock Core Borings
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Need basic material properties
• SPT hammer energy measurements
• Groundwater levels
• Pavement section data
• Electronic files for CPTu, or GINT logs
• Borings to sufficient depth
• Lab tests on “Undisturbed” samples
• Follow Agency’s Geotechnical Manual 9
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Limitations of pre-bid load tests
• Expensive for Owner and may be of little value to bidder
• Tied to Owner’s concept
• Subject to low-bidders means and methods
• May yield more ?’s than answers
• May invite claims
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Timing of pre-bid geotechnical data
• Late data is of very limited usefulness!
• Causes confusion & inefficient design!
• Estimators throw money into the bid for last minute surprises!
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Importance of high-quality pre-bid geotechnical data
Typical Problems:
• No SPT hammer energy measurements
• No groundwater level data
• No pavement section data
• Geotechnical data input into Gint format but electronic files not provided to D-B Teams
• CPTu electronic files not provided
• Investigation does not extend deep enough 12
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Risks & differing site conditions (DSC)
• DSC provisions contribute to a fair and equitable allocation of risk on design-build projects
• Provides a means to address risks and reduce contingency costs
• Usually (but not always) included on public works D-B projects
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Risks and DSC
Agency should:
• Provide all available geotechnical information in a data report
• Describe background geology and known conditions
• Avoid exculpatory language and overly broad descriptions
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Risks and DSC
D-B team should:
• Document the basis for design and construction (escrow)
• Evaluate the impact of reasonable variations in ground conditions
• Develop contingency provisions in construction plans and schedule
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Risks and DSC: Geotechnical Baseline Reports (GBR)
• Defines basis for bid and allocates risks
• ASCE guidance in “Geotechnical Baseline Reports for Construction Suggested Guidelines” Essex (2007)
– The Technical Committee on Geotechnical Reports of the Underground Technology Research Council
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Differing site conditions and application of baseline geo reports
• Baseline does not eliminate DSC but more clearly defines when DSC occurs
• Example baselines: rock elevations, thickness of soft alluvium, groundwater elevations
• Approach to some high risk items may be prescriptively defined; for example landslide stabilization measures or contaminated soil
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Alternative Technical Concept (ATC)
• Fosters innovation from D-B process
• Potential to lower costs/improve schedule
• May represent unfamiliar technology to owner agency
• Potential challenge with performance verification
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Alternative Technical Concept (ATC)
Agency should:
• Maintain confidentiality
• Talk early and often. Talk one-on-one
• Require performance testing of concepts; demonstrate effectiveness prior to construction
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Alternative Technical Concept (ATC)
D-B team should:
• Consider risks and allocation
• Provide examples and experience
• Plan for performance testing of concepts; demonstrate effectiveness prior to construction
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Alternative Technical Concept (ATC)
Examples:
• Base grouting of drilled shafts to enhance capacity
• Reliance on long term pile setup
• Use of equipment that is not covered in specifications
• Innovative ground improvement techniques 21
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Performance based specifications
Owner describes the expected outcome without prescribing a specific approach or design
• Fosters innovation
– Allow the Contractor to select means and methods
• Reduces potential claims
• Requires performance verification
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Performance based specifications
• Geotechnical Investigations
– Set the minimum requirements for extent of exploration
– All the D-B Team to determine the type of investigation (borings, in situ tests, laboratory study)
– Reference the State Manual of Instruction
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Performance based specifications
• Foundation load testing – verification of both design and construction methods
– O-cell or Statnamic on drilled shafts
– Static and/or Dynamic testing on driven piles
• QC/QA and integrity testing for production
– Crosshole sonic or other
– Dynamic monitoring 24
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Performance based specifications
• Embankment settlements
– UDOT example: 4 inches allowable settlement in 5 years (after project completion) w/ < ¼ inch differential within 12 ft horizontal distance
• Liquefaction mitigation
• Vibration monitoring
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Performance based specifications
Agency should:
• Set realistic expectations; overly stringent performance requirements cost time and money that will be reflected in the bid cost
• Clearly define performance measurement requirements
• Use best value selection for optional items 26
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Performance based specifications
D-B team should:
• Plan to achieve performance requirements with robust contingency plans
• Budget adequately to satisfy performance measurement requirements with qualified subconsultants
• Approach best value selection for optional items strategically 27
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Selected Case Histories
• Honolulu Transit, Hawaii
• Biloxi Bay Bridge, Mississippi
• Hastings Bridge, Minnesota
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Honolulu Transit Project
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Complex Geology – GBR Provided
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Basalt Characteristics
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Cobbles & Boulders in Alluvium
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GBR for Honolulu Transit • Owner’s defined purpose “to help define the
contractual ground rules of the bidding process in regards to allocation and definition of geotechnical risks”
• “baselines are developed which will serve as a contractual basis for comparison with actual conditions encountered during foundation construction”
• “The Contractor may rely on these baselines in preparing his Price Proposal”
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Farrington guideway case Honolulu, HI
• Key components to the baseline report
– Geologic setting
– Site and subsurface conditions
– Previous construction experience
– Baselines for stratigraphy and ground characterization
– Design considerations
– Construction considerations 35
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Farrington guideway case Honolulu, HI
• Baselines developed for:
– Application of baselines to the contract price
– Geologic profile (fill, recent alluvium, coraline deposits, older alluvium, basalt, cobbles and boulders, clinker zones and voids)
– Groundwater
– Ground characterization
– Contamination 36
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Farrington guideway case Honolulu, HI
• Construction considerations:
– Anticipated ground behavior
– Groundwater control and management
– Ground support (casing) requirements
– Obstructions
– Rock excavation
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Performance Verification
• Base grouting ATC used in gravel zone within alluvium – load tests verify performance
• Load tests with different geologic conditions, construction methodology
• Testing for Basalt with clinker zones below tip
• Testing for Tuff with alluvium below tip
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Base Grouting Operations
Oscillator Drilling Equipment
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Keys for Honolulu Transit
• Robust pre-bid design to accommodate wide range of conditions
• GBR to limit risks to bidders
• ATC approach to mitigate deep soft alluvium
• Extensive performance verification with method shafts O-cell tests prior to production shafts
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Case History – Biloxi Bay Bridge
• US 90 Bridge over Biloxi Bay destroyed by Hurricane Katrina on August 29, 2005.
• Design-Build contract awarded June 7, 2006
• Project cost: $338.6 million
• Schedule date open to traffic: March 28, 2008
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Biloxi After Katrina
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Test Pile Program • 21 test and indicator piles, 19 on main bridge
• Verify drivability, hammer, cushions, etc.
• Multiple restrikes w/ PDA/CAPWAP
• Static, statnamic, lateral load tests
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Keys for Biloxi • Established long term setup with test piles
– Avoid restrikes, reduce schedule risk
• Axial resistance w/ limited reliance on end bearing
– Avoid variable pile lengths
• Hammer sufficient to drive to tip w/ minimal risk of refusal or damage
• Owner buy-in on approach to testing 44
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October 26, 2007 Biloxi Bay Bridge Opens Two Weeks Ahead of
Schedule
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Example: Hastings Bridge, MN
• Best value selection process
–price / technical score
• Key Issues:
–Verification of high capacity pipe piles
–North abutment settlement issue
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Hastings, MN Steel Pipe Piles
• 42” dia x 1” x 150’
• PDA used to monitor driving stress so pile can be driven hard
• Statnamic axial for verification 47
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Hastings, MN Steel Pipe Piles
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Pile Supported Embankment
Pile raft thru soft clay at north embankment 49
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Photo from the Star-Tribune
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Summary
• Need high quality, timely pre-bid geotechnical data
• Risks & differing site conditions should be addressed appropriately
• Use ATC’s to obtain value from D-B
• Use performance based specifications with accountability 52