Advanced Interpretation of Instrumented Micropile Load Tests - PowerPoint PPT Presentation

Advanced Interpretation of Instrumented Micropile Load Tests International Workshop on Micropiles, Toronto September 28, 2007 Terence P. Holman, Ph.D., P.E. Senior Engineer-Geotec, MORETRENCH Thomas J. Tuozzolo, P.E. Vice President, MORETRENCH

Introduction • Two case histories of strain gauge instrumented micropile load tests – Case History No. 1 – 167 Johnson Street – Case History No. 2 – Dublin Road Pump Station (DRPS) – All piles Type B pressure grouted with typically developed pressures of 345 kPa • Highlight aspects of pile mechanics – Degradation of secant pile modulus – Nonuniform load distribution – Generation of micropile tip resistance and shaft resistance

Case History No. 1-167 Johnson St • 40+ story residential high rise on mixed mat/spread footing foundations • Dense to v. dense sand and sand/gravel deposits • Excessive δ v beneath heavily loaded elevator core • Minimize δ v � incorporate δ v micropiles to create “piled raft” effect – Allow high δ and low F.S. • 2 strain gauge instrumented load tests – 14 gauges per pile Dense to v. dense m/c SAND (NYCBC 7-65)

Ground Conditions and Pile Design DL=1112 kN, TL=2224 kN SR-1 SR-2 0 0 m LB-1 URBAN FILL SG-1 SG-1 384 mm LB-2 Isolation 3 LB-3 Casing LB-4 Depth below Ground Surface (m) LB-5 273 mm 6 LB-6 Casing LB-7 LB-8 6.1 m LB-9 SG-2 SG-2 9 SAND, SM TO TR GRAVEL (SP) EB Strain Gauge 12 9.1 m SG-3 SG-3 15 12.2 m SG-4 18 13.7 m SG-4 15.2 m SG-5 21 SW Strain Gauge 17.8 m 24 SG-5 0 25 50 75 100 125 150 Uncorrected SPT N-value (blows/0.3 m)

Instrumentation • Spot-weldable gauges on bar (10 ea.) – Accuracy=15 µε , Sensitivity=0.4 µε • Embedment gauges in grout (4 ea.) – Accuracy=15 µε , Resolution=1.0 µε • Grout strength and unconfined modulus testing – E=13.5 to 14.5 GPa (Unconfined secant at ε =0.11% ) – f’ c =44.8 MPa (cylinders) to 62.1 MPa (cubes)

Test Pile Installation Left - Installation of 273 mm test element (pile) Top – Installation of 194 mm diameter reaction anchor, 1334 kN capacity

Test Pile Construction Left - Installation of 273 mm test element (pile) Top – Buried old foundation wall and obstructions

Load Testing Data 0 2 SR-1 0 Depth below Ground Surface (m) 4 Tip Isolation Casing 6 Geometry Change 8 Max. Test Tip Pile Casing 10 Load= 2669 kN Geometry Change 10 Pile Butt Settlement (mm) 12 Net Permanent 14 Settlement=8 mm P=556 kN 20 P=1112 kN P=1669 kN 16 P=2224 kN Post Failure (1771 kN) Residual 18 0 200 400 600 800 1000 Net Permanent 30 Measured Strain ( µε ) Settlement=29 mm 0 SR-2 2 Plunging Failure Depth below Ground Surface (m) at 2224 kN 40 4 Tip Isolation Casing 6 Geometry Change SR-1 (6.1 m Bond) 8 SR-2 (9.1 m Bond) 50 10 Tip Pile Casing Geometry Change 0 1000 2000 3000 12 Applied Pile Top Load (kN) 14 P=556 kN P=1112 kN 16 P=1669 kN P=2224 kN P=2669 kN 18 0 400 800 1200 1600 2000 Measured Strain ( µε )

Case History No. 2-DRPS • Ground loss, heave, PUMP WET WELL INLET CHAMBER CHAMBER and settlement (BUILT) (NOT BUILT) (NOT BUILT) around 3 pump station structures following excavation and pile driving SILT, SAND, AND CLAY • Complex ground FILL conditions – Excess head/high groundwater levels – Marine glauconitic SHEET PILES silty fine sand GLAUCONITIC F/M SAND deposits

Ground Conditions and Pile Design DL=534 kN, TL=1067 kN 0 m 0 B-1 (pre-failure) SILTY SAND (SM) B-2 (pre-failure) Isolation B-3 3 B-4 SAND, SILT, AND CLAY Casing Depth below Ground Surface (m) B-5 B-6 6 194mm OD Zone of Ground (SM, ML, CL) Casing SW Strain Loss/Disturbance 9 Gauges 1.5 m 12 13.6 m GLAUCONITIC F/M SAND 15 16.8 m 18 20.3 m 21 24 0 10 20 30 40 50 60 Uncorrected SPT N-value (blows/0.3 m)

Load Testing Data 0 13 Plunging Failure SG-1 Level at 933 kN 14 Depth below Ground Surface (m) 10 Pile Butt Settlement (mm) 15 16 20 SG-2 Level 17 Net Permanent 30 Settlement=26 mm 18 P=267 kN 19 P=534 kN 40 P=801 kN P=934 kN 20 Post Failure SG-3 Level Full Unload 21 50 0 200 400 600 800 1000 0 200 400 600 800 1000 Measured Strain ( µε ) Applied Pile Top Load (kN)

Analysis and Interpretation • Nonlinear σ−ε behavior of composite pile section • Calculated load distribution along bond length • Deformation-based generation of micropile tip resistance and bond resistance

Composite Micropile Behavior 28 Estimated Secant Modulus (GPa) • Interpretation of load DRPS-Bond Zone distribution 26 – P= ε A p E p 24 • Composite pile has E sec (MPa)= -0.0063 ε + 27.46 22 complex σ−ε behavior • Secant modulus of 20 0 200 400 600 800 1000 composite pile Measured Strain ( µε ) 70 Estimated Secant Modulus (GPa) degrades with SR-1 Cased Zone Secant Modulus Approx. Secant Modulus (Field Data) 60 increasing strain Bond Zone Secant Modulus 50 – Linear degradation 40 model invoked E sec (GPa)= -0.0081 ε + 42.89 (Cased) 30 • Calculate E sec as f( ε ) 20 E sec (GPa)= -0.0104 ε + 26.32 (Bond) 10 0 200 400 600 800 1000 1200 Measured Strain ( µε )

Load Distribution 0 0 13 2 2 14 Depth below Ground Surface (m) Depth below Ground Surface (m) Depth below Ground Surface (m) 4 4 15 6 6 16 8 8 17 10 10 18 SR-1 SR-2 DRPS 12 12 P=556 kN 19 P=267 kN 14 14 P=1112 kN P=556 kN P=534 kN P=1669 kN P=1112 kN P=801 kN P=2224 kN 20 P=1669 kN 16 16 P=934 kN Post Failure P=2224 kN (1771 kN) Post Failure P=2669 kN Full Unload Residual 18 18 21 0 400 800 1200 1600 2000 2400 0 500 1000 1500 2000 2500 3000 0 200 400 600 800 1000 Interpreted Load (kN) Interpreted Load (kN) Interpreted Load (kN) • Non-constant mobilized bond stress for piles with short bond length (SR-1 and DRPS) – Approaches constant value near failure – 16-23 kN/m for SR-1, 25-28.5 kN/m for SR-2, 8.5-12.1 kN/m for DRPS • Significant ultimate tip resistance for SR-1 and DRPS – 19-25% of total ultimate capacity (300-700 kN)

Generation of Tip Resistance • Total pile deformation 800 Mobilized Tip Load (kN) δ = δ + δ + δ 600 c b t ∫ + = ( δ δ ) ε dz c b 400 L • Tip resistance mobilizes nonlinearly for piles with 200 SR-1 SR-2 short bond length DRPS 0 – Initial yield at settlement ratio 0 10 20 30 40 of 0.01 to 0.02 Calc. Pile Tip Settlement (mm) – Limiting values at settlement 1.2 ratio of 0.08 to 0.10 Normalized Tip Load Q t /Q tmax 1 • Small tip resistance 0.8 developed for SR-2 0.6 – No failure condition – Denser soils at pile tip 0.4 SR-1 • Trends similar to larger 0.2 SR-2 DRPS deep foundations 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Normalized Tip Settlement δ t /D b

Generation of Bond Resistance 300 Average Bond Shear Stress (kPa) 200 100 SR-1 SR-2 DRPS 0 0 10 20 30 40 Calc. Shaft Compression and Tip Settlement (mm) • Develops with compression of bond zone and tip displacement ( δ b + δ t ) • 6 to 8 mm of deformation required to initiate failure for short bond length piles ( ≈ 0.1% L b ) • Ultimate τ reached between 10 and 20 mm ( ≈ 0.2% L b ) • No failure for SR-2 with long bond length

Summary and Conclusions • Strain gauges can point out changes in pile geometry • Composite, nonlinear nature of micropiles complicates stress-strain response • Resistance distribution is nonuniform along bond length • Significant micropile tip resistance may be mobilized for shorter bond length piles • Instrument for better understanding!!

Summary and Conclusions • Implications for analysis and design – Structural assessment of micropile response should account for real σ−ε behavior (i.e. nonlinear material behavior) – For controllable design scenarios micropile tip resistance could be considered – Short bond lengths for micropiles should be used cautiously due to the relatively small bond movement/compression required to reach ultimate capacity