Recent Developments in the Axial, Lateral and Torsional Response of Drilled Shaft Foundations

Transcription

1 Recent Developments in the Axial, Lateral and Torsional Response of Drilled Shaft Foundations Armin W. Stuedlein, PhD P.E. Associate Professor Geotechnical Engineering

2 Presentation Outline Motivation and global objectives Project 1: Axial and Lateral Response of Shafts w/ High Strength Bar and Steel Casing Project 2: Torsional Response of Shafts Axial and Lateral Response Experimental field test program Salient aspects of integrity test results Performance of Shafts to loading Torsional Response Experimental field test program Performance of Shafts to Loading Preview of forthcoming numerical model Summary and conclusions

3 Motivation and Global Objectives: Project 1 Investigation of High Strength Bar and Steel Casing Research objectives driven by ODOT Bridge Design Group, with addition of ADSC WCC member firm suggestions Main Concern: Seismic. Increased seismic loads necessarily require increased steel reinforcement using current design specifications The density of steel in the rebar cage can cause difficulty during concreting, leading to voids and loss of cover Objectives: Evaluate the use of high strength (80 ksi) steel Evaluate use of permanent steel casing in design for flexure, lateral load transfer Evaluate use of hollow bar as dual purpose elements (structural, CSL access), compare to TIP Thermal Wires

4 Motivation and Global Objectives: Project 1 Investigation of High Strength Bar and Steel Casing Axial Load Transfer: Develop t-z and q-z curves for Willamette Valley soils Compare t-z and q-z curves between uncased and cased shafts Compare t-z and q-z curves to FHWA recommendations Lateral Load Transfer: Develop p-y curves for Willamette Valley soils Compare p-y curves between uncased and cased shafts, and against typical p-y curves available in commercial software Compare development and location of plastic hinge across experimental shafts

5 Motivation and Global Objectives: Project 2 Torsional Response of Drilled Shaft Foundations Research objectives driven by ODOT Traffic Structures and Bridge Design Groups Improve their understanding of torsional load transfer For DOTs, torsional loading typically due to: Wind loading on cantilevered signs Seismic loading of skewed bridges and flyovers Concerned about the lack of design guidance: Very little full-scale test data exists NO full-scale load transfer data exist (prior to this work) No significant assessment of the accuracy of torsional load transfer models exist No reliable tools / software for modeling load transfer available

6 [ Experimental Field Test Program ]

7 Experimental Field Test Program Test Shafts and Experimental Variables Four shafts considered: MIR: the baseline shaft, constructed with 2% internal steel (60 ksi) HSIR: constructed with 1.5% internal steel (80 ksi) CIR: cased shaft, 37 OD with 0.5 wall; with 2% internal steel (60 ksi) CNIR: cased shaft, 37 OD with 0.5 wall; with 0.15% internal steel (60 ksi); drilled with 37 auger Shaft Drill Diameter (in) Casing Wall Thickness (in) Actual Volume of Concrete (yd 3 ) Internal and External Steel (%) MIR 36 N/a HSIR 36 N/a CIR CNIR MIR HSIR

8 Experimental Field Test Program HSIR: high strength with hollow bar Compare opening size in reinforcement cage (HSIR in foreground, MIR upper left corner). Hollow bar capped at base of shaft. 7

9 Subsurface Conditions West, A CPT-3 CPT-4 Uncased Shafts CPT-1 CPT-5 Cased Shafts CPT-2 East, A' 0 Atterberg Limits (%) Stiff to very stiff, silty CLAY to clayey SILT with a thin SAND lens ?? Stiff sandy SILT and medium dense silty SAND with gravel Depth (m) 15 Stiff to very stiff, silty CLAY to clayey SILT Depth (ft) q t (tsf) q t (tsf) q t (tsf) q t (tsf) q t (tsf) Distance 70 (ft) 80

10 Construction: June

11 Completed Shafts 10-month Aging prior to Axial Loading MIR CNIR CIR HSIR 10

12 [ Integrity Tests Results ]

13 Integrity Tests and Results Crosshole Sonic Logging (CSL) Tests Advantages: Widely accepted, standardized Relatively quick, cheap High resolution data Disadvantages: Provides indicated in straight-line path of wave (only) Need to wait 7 to 10 days, then window closes after ~21 to 28 days No indication of concrete cover thickness / quality CSL through hollow bar: Theoretically should provide no disadvantage Threads should improve bond at concrete/steel interface

14 Integrity Tests and Results Crosshole Sonic Logging (CSL) Tests Comparison of the p-wave signal received indicate clear difference in signal quality Signal through hollow bar is clear, undamped, regular Signal through PVC is erratic, muddled Observations reflected in the waterfall plots 1000 Crosshole Signal (millivolts) CIR - PVC Tubes HSIR - Hollow Bar Time (microseconds)

15 Integrity Tests and Results Thermal Integrity Profiling (TIP) Thermal Wires Advantages: Data captured in 12 to 48 hours following concreting Processing time short High resolution measurements on one-foot intervals Can produce diameter profile critical for instrumented shafts and interpreting measured strains Less cage congestion no access tubes Disadvantages: Care is necessary when handling cage wires may be pinched or cut Care is necessary when torching cage stabilizers Shorter history with use Fewer specialists available with necessary expertise Information at base of shaft??

16 Integrity Tests and Results Thermal Integrity Profiling (TIP) Thermal Wires Shaft MIR shown here as example Time to reach peak heat of hydration: 46 hours Shaft diameter consistently larger than 36 auger dia. Cage is slightly off-center, as indicated by Wires 3 and 4 (slightly cooler) Off-centering ~3/4 max, but concrete cover > 3

17 Integrity Tests and Results Thermal Integrity Profiling (TIP) Thermal Wires Near surface temperature differences attributed to form used above ground Temperature variations in uncased shafts very similar, but indicates definitive differences One inch smaller radius for HSIR Temperature and inferred shaft radius for CIR indicates gap between soil and shaft Implications for load transfer Embedment Depth (ft) Mean Temperature ( o F) Steel Sonotube casing Drilled Shaft Ground Surface MIR HSIR CIR (a) Inferred Shaft Radius (inches) Reinforcement Cage Ground Surface (b)

18 [ ] Overview of Axial and Lateral Loading Test Results

19 Axial Loading Tests 18

20 19 Axial Loading Tests: Response at Shaft Head Uncased shafts: nearly identical initial response, High-strength Shaft performed better despite having slightly smaller diameter 1400 kips at 0.15 Cased Shafts: significantly less capacity than uncased Shaft CNIR was drilled with 37 auger, same as casing OD; Shaft CIR used 36 auger - substantial effect Shaft Head Displacement (in) Shaft Head Displacement (in) Shaft Head Displacement (in) Applied Load, Q (kips = 1,000 lbf) Applied Load, Q (kips = 1,000 lbf) Uncased Shafts MIR HSIR Applied Load, Q (kips = 1,000 lbf) Cased Shafts CIR CNIR MIR HSIR CIR CNIR

21 20 Axial Loading Tests: Load Transfer Significant bending incurred in all four tests Load transfer attempted to be corrected for load transfer Cased shafts perform in a very stiff, rigid manner Effect of construction sequence and gap between casing and borehole apparent TIP can be used to identify need for post-grouting remediation MIR CIR HSIR CNIR

22 Axial Loading Tests: t-z response 21 UNCASED SHAFTS Mostly hardening-type response Auger-induced MIR HSIR belling at midshaft produced largest response CASED SHAFTS Much lower average shaft resistance CIR CIR (smaller auger) produced peak and softening CNIR Cased Shafts: shallow t-z curves affected by bending and gapping

23 Lateral Loading Tests: Preliminary Results MIR HSIR CIR CNIR 22

24 Lateral Loading Tests: Preliminary Results Initial response similar between cased and uncased shafts Plastic hinge developed at about 200 kips for uncased shafts Cased shafts perform well to 8, no indication of hinge CNIR stiffer than CIR, surprising Shear Force (kip) MIR HSIR CIR CNIR Applied Lateral Displacement (in)

25 Depth (ft) Displacement (in) Lateral Loading Tests: Preliminary Results Lateral deflection profiles near-finalized p-y curves not yet developed, will be generated using strain gages and lateral deflection profiles Cased shafts are significantly stiffer, HSIR stiffer than the baseline shaft MIR; CNIR stiffer than CIR Ground Surface 45.8 kips 71.4 kips 105 kips 186 kips 229 kips 231 kips Displacement (in) Displacement (in) Definitive Hinge at 3.5D Ground Surface 45.8 kips 71.4 kips 105 kips 186 kips 229 kips 231 kips Ground Surface 48.9 kips 72.9 kips 108 kips 190 kips 245 kips 403 kips Displacement (in) Ground Surface HSIR MIR CIR CNIR Definitive Hinge at 3.5D Point of bending at 8.5D Also at 8.5D 48.9 kips 72.9 kips 108 kips 190 kips 245 kips 403 kips

26 [ ] Project 2: Torsional Response of Drilled Shafts

27 Motivation for Work In the Pacific Northwest, wind speeds are not trivial December October 1934: 1962: 2014: Peak gusts of km/hr In Kansas, Max wind gust speed recorded July 11, 1993: 162 km/hr, not trivial 26

28 Experimental Program: Subsurface Conditions Southwest, A CPT-3 CPT-2 CPT-1 Northeast, A' B Distance (m) EDS TDS B TDSFB 1 2 stiff to very stiff, Silty Clay to Clayey Silt Depth (m) 3 4 5?????? Sand to Silty Sand? Does the silty sand layer (Dr = 75% and ~1m thick) contribute significant torsional resistance? 6 medium dense, Sand to Silty Sand q q t (MPa) SPT-N t (MPa) Water Content and Atterberg q t (MPa)

29 Experimental Program: Test Shafts Design signal pole and pole structure: SM3 (ODOT Standard Dwgs. TM651, TM653) Design axial load: 4.75 kn Shear: 10.5 kn Moment: kn-m Torque: kn-m ODOT design procedure: Using Broms (1964), in consideration of lateral resistance (only), choose shaft length that provides FS = 2.15 Torsion not considered directly (??!!) 28

30 Experimental Program: Loading Tests 29

31 [ ] Quasi-Static Loading 30

32 Experimental Program: Quasi-Static Applied Rotation at Head vs. Developed Torque TDSFB TDS Torque, Torque, T (kn-m) T (kn-m) TDS TDSFB 50 TDSFB: Measured 50 TDS: Measured TDS: Extrapolated Rotation, θ θ (deg)

33 Experimental Program: Quasi-Static Torsional shear strain profiles: with depth Diameter, D (m) (b) TDS TDSFB D Diameter, D (m) D 1 Depth (m) Unreliable Gages Absolute Shear Strain, ε 45 (microstrain) Absolute Shear Strain, ε 45 (microstrain)

34 Experimental Program: Quasi-Static Torsional Load Transfer Observed over Tributary Areas: with Depth TDS -1 0 Torque (kn-m) TDSFB Ground Surface 1 Depth (m) 2 3 CPT 4 Unreliable Gages q t (MPa)

35 34 Experimental Program: Quasi-Static Torsional Load Transfer Observed over Tributary Areas: with Rotation θ/τ 1.6E-3 1.2E-3 8.0E-4 4.0E-4 0.0E+0 Unit Shaft Resistance, τ (kpa) Hyperbolic Model Fitting (TDS) r s Omitted θ/r s = 1.82E -2 θ E -4 R² = a b θ/r s = 1.08E -4 θ E-4 R² = Observed from 3.1 to 4.1 m Observed from 2.1 to 3.1 m Observed from 3.1 to 4.1 m Observed from 2.1 to 3.1 m Hyperbolic Fit (a) a b (b) Unit Shaft Resistance (kpa) Unit Shaft Resistance, τ (kpa) Unit Shaft Resistance (kpa) Unit Shaft Resistance, τ (kpa) Rotation, θ (deg) ) 5 E-4 ) to 0.18 m 0.18 to 1.1 m 1.1 to 2.1 m 2.1 to 3.1 m 3.1 to 4.1 m Extrapolated 0 to 0.18 m 0.18 to 1.1 m 1.1 to 2.1 m 2.1 to 3.1 m 3.1 to 4.0 m Shaft in SAND Shaft in Clayey SILT All of shaft in Clayey SILT (c) TDS (d) TDSFB Rotation, θ (deg)

36 Skipping the summary but thanks to colleagues and the Sponsors! [ ] Colleagues Prof. Andre Barbosa Qiang Li, OSU PhD Student 35

37 SPONSORS

38 References Li, Q., Stuedlein, A.W. and Barbosa, A.R. (2017) "Torsional Load Transfer of Drilled Shaft Foundations," Journal of Geotechnical and Geoenvironmental Engineering, Vol. online at: Stuedlein, A.W., Li, Q., Zammataro, J., Belardo, D., Hertlein, B., and Marinucci, A. (2016) "Comparison of Non-Destructive Integrity Tests on Experimental Drilled Shafts," Proceedings, 41st Annual Meeting of the Deep Foundations Institute, New York, NY. 10 pp.