UPDATE ON ISU DRILLED SHAFT LRFD CALIBRATION STUDY. Jeramy C. Ashlock, Ph.D. Richard L. Handy Associate Professor

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1 UPDATE ON ISU DRILLED SHAFT LRFD CALIBRATION STUDY Jeramy C. Ashlock, Ph.D. Richard L. Handy Associate Professor

2 Ongoing Drilled Shaft Research Projects 2 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

3 Outline 3 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

4 LRFD Calibration Project 4 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

5 DSHAFT Database 5 Project website: 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

6 Drilled Shaft Load Test by State 6 CO:7 SD:1 NE:1 NV:1 TN:2 IA:20 IL:2 KY:1 MO:4 KS:5 MN:3

7 Challenges of O-Cell Load Test Results 7 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

8 Displacement (in) Extrapolation Illustration 8 O-Cell Load-Displacement Curve O-Cell Net Load (kips) Upward Shaft Movement Downward Shaft Movement Hyperbolic Curve Fit

9 LRFD Calibration 9 Allen et al. (2005) TRC E-C079

10 LRFD Calibration 10 Brown et al. (2010)

11 LRFD Calibration 11 Commonly used Reliability Methods FORM FOSM Monte Carlo Modified version of FOSM used in Phase II project

12 LRFD Calibration 12 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

13 LRFD Calibration 13 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

14 LRFD Calibration 14 n 25 mean log Standard Deviation log COV log mean Standard Deviation COV 0.902

15 LRFD Calibration 15 n 43 mean log Standard Deviation log COV log mean Standard Deviation COV 0.575

16 LRFD Calibration 16 n 43 mean log Standard Deviation log COV log mean Standard Deviation COV 0.413

17 LRFD Calibration 17 n 23 mean log Standard Deviation log COV log mean Standard Deviation COV 1.405

18 LRFD Calibration by Others 18 Adapted after Liang et al. (2009)

19 LRFD Calibration by Others 19 Kansas DOT (IGM) Adapted after Yang et al. (2010) Louisiana DOT (Mixed soils) Adapted after Abu-Farsakh et al. (2012)

20 20 LRFD Calibration by Others Missouri DOT Strong Rock Cohesive Soils

21 Current Study vs. AASHTO 21 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 O'Neill & Reese (1999) β-method N/A Brown et. al (2010) β-method N/A 0.55 Horvath & Kenney (1979) N/A Kulhawy et al. (2005) N/A 0.55 Modified α-method B 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 O'Neill & Reese (1999) β-method N/A Brown et. al (2010) β-method N/A 0.55 Horvath & Kenney (1979) N/A Kulhawy et al. (2005) N/A 0.55 Modified α-method

22 Current Study vs. AASHTO 22 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 O'Neill & Reese (1999) β-method N/A Brown et. al (2010) β-method N/A 0.55 Horvath & Kenney (1979) N/A Kulhawy et al. (2005) N/A 0.55 Modified α-method B 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 O'Neill & Reese (1999) β-method N/A Brown et. al (2010) β-method N/A 0.55 Horvath & Kenney (1979) N/A Kulhawy et al. (2005) N/A 0.55 Modified α-method

23 Selecting Final Resistance Factors 23 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.

24 Selecting Final Resistance Factors 24 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

25 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)

26 MTC Reduced-Scale Drilled Shaft Project 26 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

27 Reduced Scale Drilled Shaft Top Down Load Test 27 Project: Drilled Shaft Axial Load Test Program, Design No. 916, and 1016 Council Bluffs Interchange System, Pottawattamie County, IA

28 Overview 28 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).

29 29 5 ft Diameter O-Cell Test Shaft

30 ft Diameter Top Down Test Shaft

31 ft Diameter Top Down Test Shaft

32 32 2 ft Diameter Top Down Test Shaft

33 Reaction Frame 33 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)

34 Instrumentation Plan kip load cell String Potentiometers (2) Wood Reference Beams (2) String Potentiometers (2) 400 kip actuator & pump Barcode staff for digital survey level LVDTs with magnetic mounts (2) 28 strain gauges installed ea. shaft ( and ) Angles bonded or anchored to shaft Excavate 12 in. deep around shaft Compression telltale pipes & rods

35 35 Completed Shafts

36 36 Hydrostone

37 37 Telltale Rods

38 38 String Pots and LVDTs

39 39 Hydraulic Pump

40 40 Data Acquisition

41 41 During Test

42 42 Timelapse 18 Shaft

43 43 Timelapse 24 Shaft

44 44 Load Test Results

45 45 Unit Skin Friction Curves

46 Top DIsplacement (in) Abaqus Finite Element Model of Test 46 Current model not fully functional Load-Displacement Curve Load (kips) in Shaft 24 in Shaft Abaqus Simulation

47 47 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

48 work in progress Thank You! Questions and Comments are Welcome