Micropiles for Slope Stabilization

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Design of Micropiles for Slope

Stabilization

J. Erik Loehr, Ph.D., P.E. U i it f Mi i University of Missouri

ADSC Micropile Design and Construction Seminar Las Vegas, Nevada

April 3-4, 2008

Outline

_Background

• Typical implementation

Construction sequence

• Construction sequence

Stability Analysis Issues

Prediction of resistance for micropiles

Comparison of predicted and measured

resistance

2

(2)

In-situ reinforcement schemes

Fill Soil Dowels Soil N il Shotcrete Stiff Clay Relic Shear Surface Nails 3

after Bruce and Jewell, 1986

Reticulated Micropiles Firm Stratum

Common implementation

anchor micropiles 4

(3)

Oso Creek Landslide Stabilization

5

Photo courtesy of John Wolosick

Stability analysis for reinforced slopes

Potential Sliding Surface

R

lat

R

axial

R

6

Reinforcing Member

(4)

Stability analysis for reinforced slopes

Same methods of analysis used for reinforced

slopes

• Same assumptions invokedp

• 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

7

Search for critical sliding surface can also become

more challenging

• Magnitude of resisting force changes with location

• Numerous “local minima” frequently exist

Example

Micropiles battered F=1.00 Fill weathered shale Micropiles battered at +/- 45 deg. 8

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Result

Fill weathered shale 11 K 37 K 7 K 34 K F=1.44 9

Stresses on sliding surface

7000 8000 9000

Effective Normal Stress (psf)

Mobilized Shear Resistance (psf) increase in stress due to upslope pile 3000 4000 5000 6000 7000 Str es s (p sf )

decrease in stress due to downslope pile p p p 10 0 1000 2000 -200 -150 -100 -50 0 50 X coordinate (ft)

(6)

Impact of reinforcement on stability

Reinforcement contributes to stability in two

ways:

• Direct resistance to sliding

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

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Orientation of reinforcement w.r.t. soil movement

• Type of reinforcement

• Depth of sliding

• Frictional resistance of soil

Prediction of micropile resistance

Potential sliding surface

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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

13

lateral resistance

Must be able to mobilize resistance within

tolerable deformations

Prediction of micropile resistance

_{Estimate profile of soil movement}

Resolve soil movement into axial and lateral

t

components

Independently predict mobilization of axial

and lateral resistance

• Using “p-y” analyses for lateral load transfer

• Using “t-z” analyses for axial load transfer

14

g

y

Select appropriate axial and lateral resistance

with consideration given to compatibility and

serviceability

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Soil movement components

− θ + θ Slope Surface

δ

lat. lat.

δ

_axial soil

δ

α−θ α−θ 15 Sliding Surface

δ

_axial

δ

_soil soil

p-y analyses for lateral resistance

lat

δ

L-Pile Model

Input Profile of Lateral Soil Movement

Pile Bending Stiffness (EI) Soil Lateral Resistance (p) Sliding Surface Lateral Component of moving soil 16

Transition (Sliding) Zone

z

Stable Soil (no soil movement)

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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. Apply displacements in soil above sliding surface

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

17

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

Mobilization of lateral resistance

0

0.0 1.0 2.0 3.0 4.0 5.0 Pile Deformation (in)

0

-1500 -750 0 750 1500 Mobilized Bending Moment (kip-in)

0

-80 -40 0 40 80 Mobilized Shear Force (kip)

d=0.1 in clay slide 10 20 30 D ept h ( ft) 10 20 30 10 20 30 d=1.0 in d=3.0 in 18 rock 40 50 40 50 40 50

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Mobilization of lateral resistance

40 45 50 15 20 25 30 35 40 Mobi liz ed S hea r F or ce (k ip) 19 0 5 10 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Total Slope Movement (in)

t-z analyses for axial resistance

ax ial

δ

Input Profile of Axial Soil Movement

S il Sh Cap Bearing T iti (Slidi ) Z Pile Axial Stiffness (EA) Soil Shear Resistance (t) Sliding Surface Axial Component of moving soil 20

Transition (Sliding) Zone

z

Soil End Bearing (Q)

Stable Soil (no soil movement)

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Axial resistance from t-z analyses

For an assumed depth of sliding:

1. Apply displacements in soil above sliding surface

2. Determine response from t-z analysesp 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

21

• NOTE: MUST ALSO CONSIDER DEFORMATIONS

REQUIRED TO MOBILIZE RESISTANCE

Mobilization of axial resistance

0

0 20 40 60 80 100 120 140 160 Mobilized Axial Load (kip)

d=0.1 in clay slide 10 20 30 D ept h ( ft) d 0.1 in d=0.3 in d=0.42 in d=0.5 in 22 rock 40 50

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Mobilization of axial resistance

140 160 40 60 80 100 120 Mobil iz ed A xi al For ce ( kip) 23 0 20 40 0.00 0.25 0.50 0.75 1.00 1.25 1.50 Total Slope Movement (in)

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

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• axial failure

-

compression

-

tension

Serviceability limits

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Repeat for other sliding depths…

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Result is two resistance functions that

describe resistance versus position along

reinforcement

Resistance functions (per member)

0

0 50 100 150 200 250 Axial Resisting Force (kip)

0

0 50 100 150

Lateral Resisting Force (kip)

Ultimate clay 10 20 30 Sl idin g D epth (f t) 10 20 30 Sl idin g D epth (f t) d<1-in 26 rock 40 50 40 50

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Input for stability analyses (per lineal foot)

0

0 10 20 30 40 50 Axial Resisting Force (kip/ft)

0

0 5 10 15 20 25 Lateral Resisting Force (kip/ft)

spacing = 6-ft clay 10 20 30 Sl idin g D epth (f t) 10 20 30 Sl id ing D ep th ( ft) 27 rock 40 50 40 50

Member resistance divided by member spacing

Comparison w/ measured values

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Mobilized bending moments – Littleville

0

-40 -20 0 20 40 Bending Moment (in-kips)

predicted

0

-40 -20 0 20 40 Bending Moment (in-kips)

predicted 10 20 30 D ept h ( ft) measured (2+70U) measured (1+70U) downslope pmod = 0.2 10 20 30 D ept h ( ft) measured (2+70U) measured (1+70U) upslope pmod = 0.2 29 40 50 _δ tot = 0.31-in 40 50 _δ tot = 0.39-in

Mobilized axial resistance – Littleville

0

-60 -40 -20 0 20 40 60 Axial Load T, kip (+=tension)

0

-60 -40 -20 0 20 40 60 Axial Load T, kip (+=tension)

10 20 30 Dep th, z (ft .) downslope α = 0.3 10 20 30 De pt h, z (f t.) upslope α = 0.3 zult = 0.06-in 30 40 50 predicted measured (2+70U) measured (1+70U) δtot = 0.24-in α 0.3 zult = 0.06-in 40 50 predicted measured (2+70U) measured (1+70U) δtot = 0.34-in

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Large-scale model tests

31 25 30 b ot to m ) 44 (2.8) LPile (2.8) 25 30 b ott o m ) 44 (2.8)

Model vs. measurement – no cap

10 15 20 io n Alon g P ile (i n. fr om b 10 15 20 on Alo ng Pil e ( in. fr om b _{t-z (2.8)} 32 0 5 -300 -200 -100 0 100 200 300 Induced Bending Moment (lb-in)

Po sit i 0 5 -1000 -500 0 500 1000 Induced Axial Load (lb)

Posi

ti

T C

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35 40 45 o tt om ) 44 (1.9) t-z (1.9) 35 40 45 b ot to m ) 44 (1.9) LPile (1.9)

Model vs. measurement – with cap

10 15 20 25 30 o n Al ong Pil e ( in. f ro m b o ( ) 10 15 20 25 30 io n Alon g P ile (i n. fr om b 33 0 5 10 -1000 -500 0 500 1000 Induced Axial Load (lb)

Pos iti o T C 0 5 10 -1500 -500 500 1500 Induced Bending Moment (lb-in)

Po

sit

i

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

Summary and Conclusions

Prediction of resistance for reinforcement requires

consideration of soil-structure interaction

• Cannot predict resistance based on structural capacity alone!!!

Both axial and lateral components of resistance can

substantially influence stability

• Relative contribution depends on pile orientation and pile/soil characteristics