Micropiles for Slope Stabilization

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

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

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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

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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

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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)

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

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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

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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

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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

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Prediction of micropile resistance

Potential sliding surface

12

6

The challenge „

Micropiles are passive elements

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Soil provides both load and resistance…load transfer is complex

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Numerous limit states

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Must consider compatibility of axial and lateral resistance

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Must be able to mobilize resistance within tolerable deformations

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Prediction of micropile resistance „ „

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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

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

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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

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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

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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

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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:

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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

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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

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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

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Limit states for soil reinforcement „

Soil failure • passive failure (lateral) above or below sliding surface • pullout failure (axial) above or below sliding surface

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Structural failure • flexural failure • shear failure • axial failure - compression - tension

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Serviceability limits

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Repeat for other sliding depths…

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

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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