Comparison of Five Different Methods for Determining Pile Bearing Capacity

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Comparison of Five Different Methods for
Determining Pile Bearing Capacity

• by
• Jim Long, Univ. of Illinois
• Wisconsin DOT
• February 6, 2009
• Madison, WI

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Agenda

• Discuss Objectives/Tasks of Project
• General Approach
• Specifics
• Prediction Methods Investigated
• Databases used for Assessment
• Interpretation of Data
• Assessment of Predictive methods
• Improved Method
• Implementation into LRFD

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Objective

• To quantify the ability of the five methods
  (Wisc-EN, FHWA-Gates, PDA, corrected Gates,
  WS-DOT) for predicting pile bearing capacity in a
  way that allows Wisconsin DOT to assess when or
  if it is appropriate to use each of the methods
  and to confidently estimate the
  reliability/safety and economy associated with
  each method.

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Tasks

• Task 1 - Literature Review
• Task 2 - Data Collection
• Collect pile information from the Marquette
  Interchange
• Collect pile information from other past projects
  of WisDOT
• Collect pile information from the PIs on
  Collection of pile load tests
• Catalog the character of the load test
  information
• Task 3 Analysis
• Quantify the ability of EN, Gates, and PDA to
  agree with capacity from static load tests
• Quantify the ability of EN, Gates, to agree with
  capacity from PDA, and quantify agreement between
  EN and Gates
• Identify limitations to the Gates method
• Develop an improved modified Gates
• Assess Washington State DOT method developed by
  Allen
• Identify efficiency and impact of using promising
  methods compared to EN formula
• Task 4 - Report Submission

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Studies Collected for DB1

• Flaate (1964)
• Olson and Flaate (1967)
• Fragaszy (1988, 1989)
• Paikowsky (1994)
• Davidson (1996)
• FHWA/Long (2001)
• NCHRP 507 and Allen(2005/2007)

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Database 1 (all cases SLTs)
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Results for 5 predictive methods based on DB1

• EN-Wisc
• FHWA-Gates
• FHWA-Gates (corr)
• PDA
• Washington DOT (Allen)

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Wisconsin - EN formula

• c 0.2 for Wisconsin
• Most states use built-in division by 6 to get
  allowable bearing by specifying H in ft, and s in
  inches. Study shows that the estimate ends up to
  be about a FS 3.1 wrt ultimate capacity.

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Methods Gates formula

• Gates modified by FHWA
• Gates modified in this Study

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Effect of Corrected Gates
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PDA

• Based on measurement of strain and velocity in
  the pile during driving
• Case method is applied details in Report
• There are different interpretation methods
  available and different damping values that can
  be applied makes the method more adaptable to
  local conditions, but also makes the method
  non-standard.
• Advantages can determine energy going into pile
• Disadvantage does not account for setup
  determines capacity at the time of driving

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Methods Wash DOT (Allen)

• where
• Feff Hammer efficiency factor
• 0.55 for Air/Steam all piles
• 0.47 for OED with steel piles
• 0.35 for CED with all piles
• 0.37 for OED with concrete or timber piles
• Nb Number of blows/in
• E hammer energy in ft-kips
• Qult Ultimate pile capacity (kips)

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Statistical Results QP/QM

• Mean COV Method
• 0.43 0.47 Wisc-EN
• 1.11 0.39 WSDOT
• 1.13 0.42 FHWA-Gates
• 0.73 0.40 PDA
• 1.20 0.40 FHWA-Gates for all piles lt750
  kips
• 1.02 0.36 corrected FHWA-Gates lt750
  kips

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Observations

• In terms of scatter
• corrected Gates (least scatter, limited to lt750k)
• WSDOT
• FHWA-Gates (lt750k), PDA
• EN (greatest scatter)
• Trend for Gates is to underpredict at higher
  capacity and overpredict at lower capacity
  address issue by restricting capacity lt 750k

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Database 2 DB2

• Two sets of Data Collected
• Wisc(JHL) 220 piles in which there are estimates
  of capacity from dynamic pile behavior
• Wisc (MI) Marquette Interchange collection of
  96 piles. Estimates can be made with all dynamic
  methods. PDA and CAPWAP results for BOR.
• few static load tests

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DB2
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PDA

• EOD results with PDA determine the capacity of
  pile at the time of driving
• BOR results determine capacity at beginning of
  restrike
• BOR better accommodates effects of setup
• CAPWAP for BOR provides even better estimate of
  pile capacity

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DB2 - Emphasis

• will be on data in which there are estimates of
  capacity based on CAPWAP (BOR)

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Summary Tables
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LRFD Resistance Factors

• two approaches (FOSM, FORM)
• Determines the resistance factor necessary for a
  target reliability (index)
• unknowns accounted for in both loads and
  resistance
• variables
• pred method (bias and cov)
• loads (bias and cov)
• target reliability (beta 2.0, 2.5, 3.0)
• based on NCHRP 507

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Resistance Factors - FOSM

• lR bias factor (which is the mean value of
  QM/QP ) for resistance
• COVQD coefficient of variation for the dead
  load
• COVQL coefficient of variation for the live
  load
• COVR coefficient of variation for the
  resistance
• bT target reliability index
• gD load factor for dead loads
• gL load factor for live loads
• QD/QL ratio of dead load to live load
• lQD, lQL bias factors for dead load and live
  load

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LRFD Resistance Factors
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LRFD Efficiency
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Since Report Submission

• We submitted report in June, 2008
• We have been continuing to work with IDOT
  reanalyzing and reviewing more data and methods
• If we look at the same data as we have for WSDOT,
  and Fit the tail of the distribution, we can
  justify higher resistance factors

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Effect of Fit to Tail

• For ? 2.33
• FORM
• Original
  Fit to
• Value
  Tail
• FHWA-Gates 0.42 0.46-0.50
• Corrected Gates 0.54 0.54-0.63
• WSDOT 0.46 0.56-0.59