UPDATE ON ISU DRILLED SHAFT

LRFD CALIBRATION STUDY

Jeramy C. Ashlock, Ph.D.

Richard L. Handy Associate Professor

jashlock@iastate.edu

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Ongoing Drilled Shaft Research Projects

1. Verification of LRFD Resistance Factors for Drilled Shafts Using Field Tests

PI: Sri Sritharan, Co-PI: Jeramy Ashlock, PhD Student: Philippe Kalmogo

Source: Iowa DOT and FHWA SPR

2. Cost-Effective Field Test Methods for LRFD Resistance Factors of Drilled Shafts

PI: Jeramy Ashlock, Co-PI: Sri Sritharan, PhD Student: Philippe Kalmogo

Source: MTC, Iowa DOT/IHRB, FHWA SPR and USDOT/OST-R

TAC Members:

Iowa DOT: Ahmad Abu-Hawash, Kyle Frame, Vanessa Goetz, Steve Megivern, Mike Nop, Gary Novey, Brian Worrel

FHWA: Chris Cromwell

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Outline

LRFD Calibration Project

Resistance factor calibration by other states

Current status of IA regional calibration

MTC Small Scale Drilled Shaft Load Test Project

Motivation

Preliminary field test results

Abaqus finite element modeling

LRFD Calibration Project

Project Tasks Task A: Expansion of the DSHAFT database

Task B: Site investigation and monitoring of tests

Task C: Verification of analysis procedures using new data

Task D: Verification and finalization of resistance factors

Task E: Products and technology transfer

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DSHAFT Database

Project website: http://sri.cce.iastate.edu/dshaft/

51 load test datasets from 11 states

O-cell and Statnamic tests

Construction methods include dry, wet and casing

Soil type includes clay, sand, mixed, IGM, and rock

28 data sets were usable in the preliminary calibration

5 additional usable data sets

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Drilled Shaft Load Test by State

IA:20

MN:3KS:5

MO:4

KY:1

IL:2

TN:2

NV:1

NE:1

SD:1

CO:7

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Challenges of O-Cell Load Test Results

O-cell capacity is typically reached before ultimate shaft or end bearing is fully mobilized

Typically only one of side resistance or end bearing reaches ultimate value, but not both

The reported equivalent top load-displacement curves do not go past 1-in or 5% diameter displacement criterion

Unit side shear vs. average shear zone displacement curves do not go beyond 1-in or 5% diameter displacement criterion

Result: extrapolation required

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Extrapolation Illustration8

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O-Cell Net Load (kips)

O-Cell Load-Displacement Curve

Upward Shaft Movement Downward Shaft Movement Hyperbolic Curve Fit

LRFD Calibration

Allen et al. (2005) TRC E-C079

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LRFD Calibration

Brown et al. (2010)

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LRFD Calibration

Commonly used Reliability Methods

FORM

FOSM

Monte Carlo

Modified version of FOSM used in Phase II project

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LRFD Calibration

Resistance bias (measured resistance/ultimate predicted resistance) required to use the equation on the previous slide

Measured resistance considered at 0.25 inch, 0.5 inch, 1 inch, and 5% of Shaft Diameter

Predicted ultimate resistance calculated using methods from O’Neill & Reese (1999) and Brown et al. (2010)

Soil Properties estimated from Bowles (1996)

α-method for cohesive soils

β-method for cohesionless soils

Horvath & Kenney (1979) and Kulhawy et al. (2005) for Rocks

Modified α-method for Cohesive IGM

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LRFD Calibration

Histograms of resistance bias

Assign a distribution type to the histogram: normal or lognormal?

Check that the assumption of a normal or lognormal is indeed appropriate: normal plots, statistical tests such as Anderson Darling

Estimate the mean and standard deviation of the distribution

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LRFD Calibration

n 25

mean log 0.508

Standard Deviation log 0.859

COV log 1.690

mean 2.350

Standard Deviation 2.119

COV 0.902

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LRFD Calibration

n 43

mean log -0.035

Standard Deviation log 0.566

COV log 16.061

mean 1.119

Standard Deviation 0.644

COV 0.575

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LRFD Calibration

n 43

mean log 0.068

Standard Deviation log 0.448

COV log 6.572

mean 1.170

Standard Deviation 0.484

COV 0.413

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LRFD Calibration

n 23

mean log 0.605

Standard Deviation log 1.034

COV log 1.709

mean 3.157

Standard Deviation 4.434

COV 1.405

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LRFD Calibration by Others

Adapted after Liang et al. (2009)

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LRFD Calibration by Others

Kansas DOT (IGM)

Louisiana DOT (Mixed soils)Adapted after Yang et al. (2010)

Adapted after Abu-Farsakh et al. (2012)

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LRFD Calibration by Others

Missouri DOT

Strong Rock Cohesive Soils

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Current Study vs. AASHTO

Segmental Approach I (1 layer for each zone between gauges)1 inch Disp NCHRP 507 NCHRP 343 AASHTO FHWA

λ COV φ φ/λ λ COV φ φ/λ λ COV φ φ/λ

α-Method 1.89 0.61 0.38 0.20 - - - - - - - - 0.45 0.45

O'Neill & Reese (1999) β-Method 0.94 0.36 0.39 0.41 - - - - - - - - 0.55 N/A

Brown et. al (2010) β-Method 1.05 0.36 0.43 0.41 - - - - - - - - N/A 0.55

Horvath & Kenney (1979) 2.41 0.51 0.64 0.26 - - - - - - - - 0.55 N/A

Kulhawy et al. (2005) 1.14 0.58 0.25 0.22 - - - - - - - - N/A 0.55

Modified α-Method 3.31 0.79 0.41 0.12 - - - - - - - - 0.60 0.60

.05B inch Disp NCHRP 507 NCHRP 343 AASHTO FHWA

λ COV φ φ/λ λ COV φ φ/λ λ COV φ φ/λ

α-Method 1.96 0.63 0.36 0.19 0.97 - 0.30 0.31 - - 0.60 - 0.45 0.45

O'Neill & Reese (1999) β-Method 1.18 0.47 0.35 0.30 1.29 - 0.40 0.31 - - - - 0.55 N/A

Brown et. al (2010) β-Method 1.29 0.44 0.42 0.33 - - - - - - - - N/A 0.55

Horvath & Kenney (1979) 2.36 0.48 0.69 0.29 - - - - - - 0.55 - 0.55 N/A

Kulhawy et al. (2005) 1.28 0.60 0.26 0.20 - - - - - - - - N/A 0.55

Modified α-Method 3.66 0.71 0.55 0.15 1.70 - 0.75 0.44 - - - - 0.60 0.60

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Current Study vs. AASHTO

Segmental Approach II (layers of same material type grouped together)

1 inch Disp NCHRP 507 NCHRP 343 AASHTO FHWA

λ COV φ φ/λ λ COV φ φ/λ λ COV φ φ/λ

α-Method 1.77 0.51 0.47 0.26 - - - - - - - - 0.45 0.45

O'Neill & Reese (1999) β-Method 0.94 0.29 0.48 0.51 - - - - - - - - 0.55 N/A

Brown et. al (2010) β-Method 1.08 0.35 0.46 0.43 - - - - - - - - N/A 0.55

Horvath & Kenney (1979) 2.04 0.31 0.98 0.48 - - - - - - - - 0.55 N/A

Kulhawy et al. (2005) 1.06 0.38 0.41 0.39 - - - - - - - - N/A 0.55

Modified α-Method 2.71 0.57 0.61 0.22 - - - - - - - - 0.60 0.60

.05B inch Disp NCHRP 507 NCHRP 343 AASHTO FHWA

λ COV φ φ/λ λ COV φ φ/λ λ COV φ φ/λ

α-Method 1.87 0.56 0.42 0.23 0.97 - 0.30 0.31 - - 0.60 - 0.45 0.45

O'Neill & Reese (1999) β-Method 1.16 0.25 0.65 0.56 1.29 - 0.40 0.31 - - - - 0.55 N/A

Brown et. al (2010) β-Method 1.31 0.29 0.66 0.50 - - - - - - - - N/A 0.55

Horvath & Kenney (1979) 2.76 0.56 0.62 0.23 - - - - - - 0.55 0.55 N/A

Kulhawy et al. (2005) 1.53 0.58 0.33 0.21 - - - - - - - - N/A 0.55

Modified α-Method 3.72 0.58 0.80 0.21 1.70 - 0.75 0.44 - - - - 0.60 0.60

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Selecting Final Resistance Factors

Brown et al. (2010):

Comentary in the AASHTO LRFD Specifications (AASHTO, 2007) describes the resistance factors as:

“developed using either statistical analysis of drilled shaft load tests combined with reliability theory (Paikowsky et al., 2004), fitting to Allowable Stress Design (ASD), or both. When the two approaches resulted in a significantly different resistance factor, engineering judgment was used to establish the final resistance factor, considering the quality and quantity of the available data used in the calibration”.

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Selecting Final Resistance Factors

Resistance factors from reliability theory analysis may not always be higher than code recommendations

Allen (2005): “If the adequacy of the input data is questionable, the final load and resistance factor combination selected should be more heavily weighted toward a level of safety that is consistent with past successful design practice, using the reliability theory results to gain insight as to whether or not past practice is conservative or non-conservative”

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COST-EFFECTIVE FIELD TEST

METHODS FOR LRFD RESISTANCE

FACTORS OF DRILLED SHAFTS

Project Team:

Jeramy Ashlock (PI)

Sri Sritharan (Co-PI)

Philippe Kalmogo (PhD Student)

MTC Reduced-Scale Drilled Shaft Project

Objective: Demonstrate that top–down load tests on smaller diameter shafts can be used to evaluate the unit skin friction more economically and provide actual top-load displacement curves

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Reduced Scale Drilled Shaft Top Down Load Test

Project: Drilled Shaft Axial Load Test Program, Design No. 916, and 1016 Council Bluffs Interchange System, Pottawattamie County, IA

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Overview

Project: Drilled Shaft Axial Load Test Program, Design No. 916, and 1016 Council Bluffs Interchange System, Pottawattamie County, IA

Soil type around test shaft : clay and sand

Full scale test shaft

Diameter: 5 ft

Embedment length: 95 ft

Estimated nominal resistance: 4,000 kips

Two reduced scale test shafts

Diameters: 1.5 ft, 2 ft

Estimated nominal resistance:

350 kips for 2 ft shaft using Loadtest’s measured unit side shear of 0.3 ksf in top 5 ft

(capacity is 387 kips (188 kips factored) using theoretical unit side resistance of 1.23 ksf).

262 kips for 1.5 ft shaft (298 kips using side resistance (136 factored) of 1.23 ksf in top 5

ft).

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5 ft Diameter O-Cell Test Shaft29

1.5 ft Diameter Top Down Test Shaft30

1.5 ft Diameter Top Down Test Shaft31

2 ft Diameter Top Down Test Shaft32

Reaction Frame

Four HP 12x74 reaction piles, total length: 98 ft = 95 ft embedded + 3 ft above ground. Capacity estimated using Iowa DOT BDM =120 kips (φ = 0.5)

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Instrumentation Plan

Wood Reference Beams (2)String Potentiometers (2)

Excavate 12 in. deep around shaft

Angles bonded or anchored to shaft

String Potentiometers (2)

LVDTs with magnetic mounts (2)

Compression telltale pipes & rods

400 kip load cell

400 kip actuator & pump Barcode staff for digital survey level

28 strain gauges installed ea. shaft

(24×120 and 4×350 )

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Completed Shafts35

Hydrostone36

Telltale Rods37

String Pots and LVDTs38

Hydraulic Pump39

Data Acquisition40

During Test41

Timelapse – 18” Shaft42

Timelapse – 24” Shaft43

Load Test Results

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Unit Skin Friction Curves

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Abaqus Finite Element Model of Test

Current model not fully functional

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Load-Displacement Curve

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24 in Shaft

Abaqus Simulation

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Upcoming Project Tasks

Complete finite element modelling of the shafts and analysis of the load test data

Perform tension tests of shorter shafts at Spangler Geotechnical Laboratory

With input from TAC finalize selection of the resistance factors and verify them in upcoming tests

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Thank You!

Questions and Comments are Welcome

…work in progress…