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

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

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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

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SLIDE 1

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

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SLIDE 2

2

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

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SLIDE 3

3

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

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SLIDE 4

4

Soil Movement Components

− θ + θ

Slope Surface

θ θ

Sliding Surface

δaxial

lat.

δ δ

soil lat.

δ δaxial

soil

δ

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SLIDE 5

5

t-z analyses for axial resistance

Transition (Sliding) Zone z

axial

δ Input Profile of Axial Soil Movement Pile Axial Stiffness (EA) Soil Shear Resistance (t) Soil End Bearing (Q) Cap Bearing Sliding Surface Axial Component

  • f moving soil

Stable Soil (no soil movement)

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SLIDE 6

6

Mobilization of Axial Resistance

clay rock slide

10 20 30 40 50 20 40 60 80 100 120 140 160 Mobilized Axial Load (kip) Depth (ft) d=0.1 in d=0.3 in d=0.42 in d=0.5 in Upslope Micropile Sliding Depth = 33-ft

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7

Mobilization of Axial Resistance

50 100 150 200 250 300 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) Mobilized Axial Force (kip) 10-ft 33-ft 40-ft Upslope Micropile

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SLIDE 8

8

Axial Resistance Function

clay rock

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

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SLIDE 9

9

p-y analyses for lateral resistance

Transition (Sliding) Zone z

lat

δ L-Pile Model Input Profile of Lateral Soil Movement Pile Bending Stiffness (EI) Soil Lateral Resistance (p) Sliding Surface Lateral Component

  • f moving soil

Stable Soil (no soil movement)

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10

Mobilization of Lateral Resistance

clay rock slide

10 20 30 40 50 0.0 1.0 2.0 3.0 4.0 5.0 Pile Deformation (in) Depth (ft) 10 20 30 40 50

  • 1500
  • 750

750 1500 Mobilized Bending Moment (kip-in) 10 20 30 40 50

  • 80
  • 40

40 80 Mobilized Shear Force (kip) d=0.1 in d=1.0 in d=3.0 in

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11

Mobilization of Lateral Resistance

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

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SLIDE 12

12

Lateral Resistance Function

clay rock

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

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13

Example Problem

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SLIDE 14

14

Example – with Micropiles

Micropiles battered at +/- 45º

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SLIDE 15

15

Example Problem

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16

Stresses on sliding surface

1000 2000 3000 4000 5000 6000 7000 8000 9000

  • 200
  • 150
  • 100
  • 50

50 X coordinate (ft) Stress (psf) Effective Normal Stress (psf) Mobilized Shear Resistance (psf) decrease in stress due to downslope pile increase in stress due to upslope pile

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SLIDE 17

17

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”

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18

Modified p-y curves

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 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) Lateral Load Intensity, p (kip/in) p mob = 2.0 p mob = 1.0 p mob = 0.5 p mob = 0.25 p mob = 0.05 Soft Clay Model s u = 2,000 psf ε 50 = 0.02 z = 30-ft

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19

Modified t-z curves

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 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) Axial Load Intensity, t (kip/ft) α = 2.0 α = 1.0 α = 0.5 α = 0.3 α = 0.1 Soft Clay Model s u = 2,000 psf z ult = 0.06-in = 0.01*d

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SLIDE 20

20

Littleville Alabama Case

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Mobilized Bending Moments – Littleville

10 20 30 40 50

  • 40
  • 20

20 40 Bending Moment (in-kips) Depth (ft)

predicted measured (2+70U) measured (1+70U)

δ tot = 0.39-in upslope p mod = 0.2 10 20 30 40 50

  • 40
  • 20

20 40 Bending Moment (in-kips) Depth (ft)

predicted measured (2+70U) measured (1+70U)

downslope p mod = 0.2 δ tot = 0.31-in

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Mobilized Axial Resistance – Littleville

10 20 30 40 50

  • 60
  • 40
  • 20

20 40 60 Axial Load T, kip (+=tension) Depth, z (ft.)

predicted measured (2+70U) measured (1+70U)

δ tot = 0.24-in downslope α = 0.3 z ult = 0.06-in 10 20 30 40 50

  • 60
  • 40
  • 20

20 40 60 Axial Load T, kip (+=tension) Depth, z (ft.)

predicted measured (2+70U) measured (1+70U)

δ tot = 0.34-in upslope α = 0.3 z ult = 0.06-in

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23

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???
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24

Predicted Mobilization – Littleville

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 5 10 15 20 Total Slope Movement (in) Mobilized Shear Force (kip)

stiff clay model API sand model alternate calibration points

Upslope Micropile Slide Depth = 33-ft

  • 120
  • 100
  • 80
  • 60
  • 40
  • 20

20 40 60 80 100 120 0.0 1.0 2.0 3.0 4.0 Total Slope Movement (in) Mobilized Axial Force (kip) prediction A prediction A* calibration points Upslope Micropile Sliding Depth = 33-ft

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Large-scale Model Tests

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Large-scale Model Tests

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5 10 15 20 25 30

  • 300 -200 -100

100 200 300 Induced Bending Moment (lb-in) Position Along Pile (in. from bottom) 44 (2.8) LPile (2.8) 5 10 15 20 25 30

  • 1000
  • 500

500 1000 Induced Axial Load (lb) Position Along Pile (in. from bottom) 44 (2.8) t-z (2.8) T C

Model vs. measurement – no cap

Test 2-A, Member 3 (downslope), S/D=10

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Model vs. measurement – with cap

5 10 15 20 25 30 35 40 45

  • 1000
  • 500

500 1000 Induced Axial Load (lb) Position Along Pile (in. from bottom) 44 (1.9) t-z (1.9) T C 5 10 15 20 25 30 35 40 45

  • 1500
  • 500

500 1500 Induced Bending Moment (lb-in) Position Along Pile (in. from bottom) 44 (1.9) LPile (1.9)

Test 3-A, Member 2 (upslope), S/D=10

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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!!!

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Acknowledgements

ADSC/DFI Micropile Committee ADSC Industry Advancement Fund National Science Foundation

Grant CMS0092164

Many students…