Method for Prediction of Micropile Resistance for Slope - PowerPoint PPT Presentation

Method for Prediction of Micropile Resistance for Slope Stabilization J. Erik Loehr, Ph.D., P.E. University of Missouri Dan A. Brown, Ph.D., P.E. Auburn University 2007 International Workshop on Micropiles Toronto, Ontario September 27, 2007

Limit States for Soil Reinforcement � Geotechnical failure • passive failure (lateral) above or below sliding surface • pullout failure (axial) above or below sliding surface � Structural failure • flexural failure • shear failure • axial failure - compression - tension � Serviceability limits 2

Proposed Approach � Estimate/assume profile of soil movement � Resolve soil movement into axial and lateral components � Predict mobilization of axial and lateral resistance • Using “t-z” analyses for axial load transfer • Using “p-y” analyses for lateral load transfer � Select appropriate axial and lateral resistance with consideration given to movement required to mobilize resistance 3

Soil Movement Components + θ − θ Slope Surface θ δ δ axial δ δ axial lat. lat. δ δ soil soil θ Sliding Surface 4

t-z analyses for axial resistance Input Profile of Axial Soil Movement δ axial Cap Bearing Soil Shear Axial Component Resistance ( t ) of moving soil Pile Axial Stiffness ( EA ) Sliding Surface Transition (Sliding) Zone Stable Soil (no soil movement) Soil End z Bearing ( Q ) 5

Mobilization of Axial Resistance Mobilized Axial Load (kip) 0 20 40 60 80 100 120 140 160 0 d=0.1 in Upslope Micropile d=0.3 in Sliding Depth = 33-ft 10 d=0.42 in clay d=0.5 in 20 Depth (ft) 30 slide 40 rock 50 6

Mobilization of Axial Resistance 300 Upslope Micropile 250 Mobilized Axial Force (kip) 200 10-ft 33-ft 150 40-ft 100 50 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Total Slope Movement (in) 7

Axial Resistance Function Axial Resisting Force (kip) 0 20 40 60 80 100 120 140 160 180 200 0 Upslope Micropile 10 clay 20 Sliding Depth (ft) 30 40 rock 50 8

p-y analyses for lateral resistance Input Profile of Lateral Soil Movement L-Pile Model δ lat Soil Lateral Resistance ( p ) Lateral Component of moving soil Pile Bending Stiffness ( EI ) Sliding Surface Transition (Sliding) Zone Stable Soil (no soil movement) z 9

Mobilization of Lateral Resistance Pile Deformation (in) Mobilized Bending Moment (kip-in) Mobilized Shear Force (kip) 0.0 1.0 2.0 3.0 4.0 5.0 -1500 -750 0 750 1500 -80 -40 0 40 80 0 0 0 d=0.1 in d=1.0 in 10 10 10 d=3.0 in clay 20 20 20 Depth (ft) 30 30 30 slide 40 40 40 rock 50 50 50 10

Mobilization of Lateral Resistance 1600 160 Upslope Micropile Upslope Micropile 1400 140 Mobilized Bending Moment (in-kip) z=10-ft 1200 120 Mobilized Shear Force (kip) z=33-ft z=45-ft 1000 100 800 80 600 60 z=10-ft 400 40 z=33-ft z=45-ft 200 20 0 0 0.0 2.0 4.0 6.0 8.0 10.0 0.0 2.0 4.0 6.0 8.0 10.0 Total Slope Movement (in) Total Slope Movement (in) 11

Lateral Resistance Function Lateral Resisting Force (kip) 0 20 40 60 80 100 120 140 160 0 Upslope Micropile Ultimate d<1-in 10 clay 20 Sliding Depth (ft) 30 40 rock 50 12

Example Problem 13

Example – with Micropiles Micropiles battered at +/- 45º 14

Example Problem 15

Stresses on sliding surface 9000 Effective Normal Stress (psf) 8000 increase in stress due Mobilized Shear Resistance (psf) to upslope pile 7000 6000 Stress (psf) 5000 decrease in stress due to downslope pile 4000 3000 2000 1000 0 -200 -150 -100 -50 0 50 X coordinate (ft) 16

Comparison with measured resistance � Compared resistance predicted using proposed method with measured values • Mobilized axial resistance • Mobilized bending moments � Used measured values for: • Soil strength • Pore water pressures • Soil deformations � Developed “best match” using “p-modifiers” and “t-modifiers” 17

Modified p-y curves 1.6 Soft Clay Model 1.4 s u = 2,000 psf ε 50 = 0.02 1.2 Lateral Load Intensity, p (kip/in) z = 30-ft p mob = 2.0 1.0 0.8 p mob = 1.0 0.6 0.4 p mob = 0.5 0.2 p mob = 0.25 p mob = 0.05 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Lateral Deflection, y (in) 18

Modified t-z curves 10.0 Soft Clay Model 9.0 s u = 2,000 psf z ult = 0.06-in = 0.01* d 8.0 Axial Load Intensity, t (kip/ft) 7.0 α = 2.0 6.0 5.0 4.0 α = 1.0 3.0 2.0 α = 0.5 α = 0.3 1.0 α = 0.1 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Axial Deflection, z (in) 19

Littleville Alabama Case 20

Mobilized Bending Moments – Littleville Bending Moment (in-kips) Bending Moment (in-kips) -40 -20 0 20 40 -40 -20 0 20 40 0 0 predicted predicted measured (2+70U) measured (2+70U) 10 10 measured (1+70U) measured (1+70U) downslope upslope p mod = 0.2 p mod = 0.2 20 20 Depth (ft) Depth (ft) 30 30 40 40 50 50 δ tot = 0.31-in δ tot = 0.39-in 21

Mobilized Axial Resistance – Littleville Axial Load T, kip (+=tension) Axial Load T, kip (+=tension) -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 0 0 10 10 20 20 upslope Depth, z (ft.) Depth, z (ft.) α = 0.3 z ult = 0.06-in 30 30 downslope α = 0.3 z ult = 0.06-in 40 40 predicted measured (2+70U) predicted measured (1+70U) measured (2+70U) 50 50 δ tot = 0.34-in δ tot = 0.24-in measured (1+70U) 22

Summary of evaluations � Comparison of measured and predicted forces reasonable � BUT…must use modified p-y and t-z models � Possible reasons: • Drained vs. undrained loading • Group and/or scale effects • Softening of pile-soil interface • Pile inclination • Error/bias in measurements: - Shear strength parameters - Soil movement • Others??? 23

Predicted Mobilization – Littleville 120 18.0 Upslope Micropile Upslope Micropile 100 16.0 Slide Depth = 33-ft Sliding Depth = 33-ft 80 14.0 60 Mobilized Shear Force (kip) Mobilized Axial Force (kip) 40 12.0 20 10.0 0 8.0 -20 -40 6.0 -60 stiff clay model prediction A 4.0 API sand model -80 prediction A* alternate 2.0 -100 calibration points calibration points -120 0.0 0.0 1.0 2.0 3.0 4.0 0 5 10 15 20 Total Slope Movement (in) Total Slope Movement (in) 24

Large-scale Model Tests 25

Large-scale Model Tests 26

Model vs. measurement – no cap 30 30 Position Along Pile (in. from bottom) Position Along Pile (in. from bottom) 44 (2.8) 25 25 44 (2.8) LPile (2.8) t-z (2.8) 20 20 15 15 10 10 5 5 0 0 -300 -200 -100 0 100 200 300 -1000 -500 0 500 1000 Induced Axial Load (lb) T Induced Bending Moment (lb-in) C 27 Test 2-A, Member 3 (downslope), S/D =10

Model vs. measurement – with cap 45 45 40 40 Position Along Pile (in. from bottom) 44 (1.9) Position Along Pile (in. from bottom) 44 (1.9) LPile (1.9) 35 35 t-z (1.9) 30 30 25 25 20 20 15 15 10 10 5 5 0 0 -1500 -500 500 1500 -1000 -500 0 500 1000 Induced Axial Load (lb) C T Induced Bending Moment (lb-in) 28 Test 3-A, Member 2 (upslope), S/D =10

Conclusions � Proposed uncoupled method suitable for predicting micropile resistance when cap influence is limited � Use of modified p-y and t-z models required � When cap interaction is significant, uncoupled method does not accurately predict response � Full axial resistance frequently mobilized at relatively small soil movements � Full lateral resistance frequently not mobilized without substantially greater soil movements � Additional data needed!!! 29

Acknowledgements � ADSC/DFI Micropile Committee � ADSC Industry Advancement Fund � National Science Foundation Grant CMS0092164 � Many students… 30