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Design of Micropiles for Slope Stabilization

J. Erik Loehr, Ph.D., P.E. U i University it off Mi Missourii ADSC Micropile Design and Construction Seminar Las Vegas, Nevada April 3-4, 2008

Outline

Background • Typical implementation • Construction sequence

Stability Analysis Issues Prediction of resistance for micropiles Comparison of predicted and measured resistance Summary and conclusions

2

1

In-situ reinforcement schemes Shotcrete

Soil Dowels

Fill

Soil N il Nails Stiff Clay Relic Shear Surface

Reticulated Micropiles

Firm Stratum 3

after Bruce and Jewell, 1986

Common implementation

anchor micropiles

4

2

Oso Creek Landslide Stabilization

5

Photo courtesy of John Wolosick

Stability analysis for reinforced slopes Potential Sliding Surface

Raxial

R

Rlat

Reinforcing Member 6

3

Stability analysis for reinforced slopes

Same methods of analysis used for reinforced slopes • Same assumptions p invoked • Same solution methods used

Only change is to include magnitude of known force(s) into equilibrium calculations • Force must be consistent with “breadth” considered in stability analyses (i.e. force/unit width)

Search for critical sliding surface can also become more challenging • Magnitude of resisting force changes with location • Numerous “local minima” frequently exist

7

Example

Micropiles battered at +/- 45 deg.

Fill F=1.00

weathered shale

8

4

Result

F=1.44 34 K

Fill 7K

weathered shale

11 K 37 K

9

Stresses on sliding surface 9000 8000

Effective Normal Stress (psf) increase in stress due to upslope p p ppile

Mobilized Shear Resistance (psf)

7000

Stress (psf)

6000 5000 4000

decrease in stress due to downslope pile

3000 2000 1000 0 -200

-150

-100

-50

0

50

X coordinate (ft) 10

5

Impact of reinforcement on stability

Reinforcement contributes to stability in two ways: • Direct resistance to sliding • Modifying normal stress on sliding surface

Both of these can be significant Relative magnitude of contributions depends on: • • • •

Orientation of reinforcement w.r.t. soil movement Type of reinforcement Depth of sliding Frictional resistance of soil

11

Prediction of micropile resistance

Potential sliding surface

12

6

The challenge

Micropiles are passive elements

Soil provides both load and resistance…load transfer is complex

Numerous limit states

Must consider compatibility of axial and lateral resistance

Must be able to mobilize resistance within tolerable deformations

13

Prediction of micropile resistance

Estimate profile of soil movement Resolve soil movement into axial and lateral components t Independently predict mobilization of axial and lateral resistance • Using “p-y” analyses for lateral load transfer g “t-z” analyses y for axial load transfer • Using

Select appropriate axial and lateral resistance with consideration given to compatibility and serviceability

14

7

Soil movement components +θ

−θ

Slope Surface

δlat. δaxial δsoil α−θ

α−θ

δaxial

δlat.

δsoil Sliding Surface 15

p-y analyses for lateral resistance Input Profile of Lateral Soil Movement

L-Pile Model

Lateral Component of moving soil

δ lat

Soil Lateral Resistance (p)

Pile Bending Stiffness (EI)

Sliding Surface Transition (Sliding) Zone

Stable Soil (no soil movement)

z 16

8

Lateral resistance from p-y analyses

Use “soil movement” option (L-Pile v4.0M or v5) For an assumed depth of sliding: 1. Apply displacements in soil above sliding surface 1 2. Determine response from p-y analyses 3. Mobilized resistance is shear force in micropile at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached • Shear force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth • NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE

17

Mobilization of lateral resistance 0.0

Pile Deformation (in) 1.0 2.0 3.0 4.0

0

Mobilized Bending Moment (kip-in) Mobilized Shear Force (kip) 5.0 -1500 -750 0 750 1500 -80 -40 0 40 80 0 0

10

10

10

20

20

20

30

30

30

40

40

50

50

d=0.1 in d=1.0 in d=3.0 in

Depth (ft)

clay

slide 40

rock 50

18

9

Mobilization of lateral resistance 50 45

Mobilized Shear Force (kip)

40 35 30 25 20 15 10 5 0 0.0

1.0

2.0

3.0 4.0 Total Slope Movement (in)

5.0

6.0

19

t-z analyses for axial resistance Input Profile of Axial Soil Movement

δ axial

Cap Bearing

Axial Component of moving soil

Soilil Shear S Sh Resistance (t)

Pile Axial Stiffness (EA)

Sliding Surface T Transition iti (Slidi (Sliding)) Z Zone Stable Soil (no soil movement)

Soil End Bearing (Q)

z

20

10

Axial resistance from t-z analyses For an assumed depth of sliding:

1. Apply displacements in soil above sliding surface 2. Determine response p from t-z analyses y 3. Mobilized resistance is axial force in shaft at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached • Axial force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth • NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE

21

Mobilization of axial resistance 0

20

40

Mobilized Axial Load (kip) 60 80 100

120

140

160

0

10

d 0.1 in d=0.1 d=0.3 in d=0.42 in d=0.5 in

clay

Depth (ft)

20

30

slide 40

rock 50

22

11

Mobilization of axial resistance 160

Mobilized Axial Force (kip)

140 120 100 80 60 40 20 0 0.00

0.25

0.50 0.75 1.00 Total Slope Movement (in)

1.25

1.50

23

Limit states for soil reinforcement

Soil 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

24

12

Repeat for other sliding depths…

Result is two resistance functions that describe resistance versus position along reinforcement

25

Resistance functions (per member) 0

Axial Resisting Force (kip) 50 100 150 200

250

0

0

Lateral Resisting Force (kip) 50 100

150

0

10

10

Ultimate d

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J. Erik Loehr, Ph.D., P.E. U i University it off Mi Missourii ADSC Micropile Design and Construction Seminar Las Vegas, Nevada April 3-4, 2008

Outline

Background • Typical implementation • Construction sequence

Stability Analysis Issues Prediction of resistance for micropiles Comparison of predicted and measured resistance Summary and conclusions

2

1

In-situ reinforcement schemes Shotcrete

Soil Dowels

Fill

Soil N il Nails Stiff Clay Relic Shear Surface

Reticulated Micropiles

Firm Stratum 3

after Bruce and Jewell, 1986

Common implementation

anchor micropiles

4

2

Oso Creek Landslide Stabilization

5

Photo courtesy of John Wolosick

Stability analysis for reinforced slopes Potential Sliding Surface

Raxial

R

Rlat

Reinforcing Member 6

3

Stability analysis for reinforced slopes

Same methods of analysis used for reinforced slopes • Same assumptions p invoked • Same solution methods used

Only change is to include magnitude of known force(s) into equilibrium calculations • Force must be consistent with “breadth” considered in stability analyses (i.e. force/unit width)

Search for critical sliding surface can also become more challenging • Magnitude of resisting force changes with location • Numerous “local minima” frequently exist

7

Example

Micropiles battered at +/- 45 deg.

Fill F=1.00

weathered shale

8

4

Result

F=1.44 34 K

Fill 7K

weathered shale

11 K 37 K

9

Stresses on sliding surface 9000 8000

Effective Normal Stress (psf) increase in stress due to upslope p p ppile

Mobilized Shear Resistance (psf)

7000

Stress (psf)

6000 5000 4000

decrease in stress due to downslope pile

3000 2000 1000 0 -200

-150

-100

-50

0

50

X coordinate (ft) 10

5

Impact of reinforcement on stability

Reinforcement contributes to stability in two ways: • Direct resistance to sliding • Modifying normal stress on sliding surface

Both of these can be significant Relative magnitude of contributions depends on: • • • •

Orientation of reinforcement w.r.t. soil movement Type of reinforcement Depth of sliding Frictional resistance of soil

11

Prediction of micropile resistance

Potential sliding surface

12

6

The challenge

Micropiles are passive elements

Soil provides both load and resistance…load transfer is complex

Numerous limit states

Must consider compatibility of axial and lateral resistance

Must be able to mobilize resistance within tolerable deformations

13

Prediction of micropile resistance

Estimate profile of soil movement Resolve soil movement into axial and lateral components t Independently predict mobilization of axial and lateral resistance • Using “p-y” analyses for lateral load transfer g “t-z” analyses y for axial load transfer • Using

Select appropriate axial and lateral resistance with consideration given to compatibility and serviceability

14

7

Soil movement components +θ

−θ

Slope Surface

δlat. δaxial δsoil α−θ

α−θ

δaxial

δlat.

δsoil Sliding Surface 15

p-y analyses for lateral resistance Input Profile of Lateral Soil Movement

L-Pile Model

Lateral Component of moving soil

δ lat

Soil Lateral Resistance (p)

Pile Bending Stiffness (EI)

Sliding Surface Transition (Sliding) Zone

Stable Soil (no soil movement)

z 16

8

Lateral resistance from p-y analyses

Use “soil movement” option (L-Pile v4.0M or v5) For an assumed depth of sliding: 1. Apply displacements in soil above sliding surface 1 2. Determine response from p-y analyses 3. Mobilized resistance is shear force in micropile at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached • Shear force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth • NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE

17

Mobilization of lateral resistance 0.0

Pile Deformation (in) 1.0 2.0 3.0 4.0

0

Mobilized Bending Moment (kip-in) Mobilized Shear Force (kip) 5.0 -1500 -750 0 750 1500 -80 -40 0 40 80 0 0

10

10

10

20

20

20

30

30

30

40

40

50

50

d=0.1 in d=1.0 in d=3.0 in

Depth (ft)

clay

slide 40

rock 50

18

9

Mobilization of lateral resistance 50 45

Mobilized Shear Force (kip)

40 35 30 25 20 15 10 5 0 0.0

1.0

2.0

3.0 4.0 Total Slope Movement (in)

5.0

6.0

19

t-z analyses for axial resistance Input Profile of Axial Soil Movement

δ axial

Cap Bearing

Axial Component of moving soil

Soilil Shear S Sh Resistance (t)

Pile Axial Stiffness (EA)

Sliding Surface T Transition iti (Slidi (Sliding)) Z Zone Stable Soil (no soil movement)

Soil End Bearing (Q)

z

20

10

Axial resistance from t-z analyses For an assumed depth of sliding:

1. Apply displacements in soil above sliding surface 2. Determine response p from t-z analyses y 3. Mobilized resistance is axial force in shaft at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached • Axial force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth • NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE

21

Mobilization of axial resistance 0

20

40

Mobilized Axial Load (kip) 60 80 100

120

140

160

0

10

d 0.1 in d=0.1 d=0.3 in d=0.42 in d=0.5 in

clay

Depth (ft)

20

30

slide 40

rock 50

22

11

Mobilization of axial resistance 160

Mobilized Axial Force (kip)

140 120 100 80 60 40 20 0 0.00

0.25

0.50 0.75 1.00 Total Slope Movement (in)

1.25

1.50

23

Limit states for soil reinforcement

Soil 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

24

12

Repeat for other sliding depths…

Result is two resistance functions that describe resistance versus position along reinforcement

25

Resistance functions (per member) 0

Axial Resisting Force (kip) 50 100 150 200

250

0

0

Lateral Resisting Force (kip) 50 100

150

0

10

10

Ultimate d

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