SLOPE STABILIZATION USING

DRIVEN PILES

Mohamed Ashour, Ph.D., P.E.

University of Alabama, Huntsville

Southeastern Transportation Geotechnical

Engineering Conference (STGEC)

October 5, 2010

Fig. 12 Pile Rows for Slope Stabilization (Thomson et al. 2005)

Slope Stabilizing Piles/Shafts Effectively Act as Shear

Dowels across the Slip Plane

Piles in rows

Failure surface

Piles extending

Into stable soil

Sliding

soil

mass

Piles in rows

Failure surface

Piles extending

Into stable soil

Sliding

soil

mass

Current Practice

Sliding Granular

Soil Mass

Stable soil

Sliding Surface

Driving Force

(Passive pressure)

Driving Force ( Passive pressure)Sliding Cohesive

Soil Mass

Soil-Pile Resistance Stable soil

Soil-Pile Resistance

Stable soil

P-y curve

CHALLENGES:

• Characterization and Evaluation of the Mobilized

Lateral Pressure Induced by the Moving Soil

Mass on the Pile

• Interaction between Stabilizing Piles

and Soil Arching Effect

• Soil Flow-around Failure

Soil-Pile Resistance

Stable soil

Pile Cross Section Sliding Soil Mass

Flow-Around Soil with Pile

A. SIMPLE WEDGE FAILURE

Wedge Width

Wed

ge H

eig

ht

Hw

Bedrock

Hr

Water

Pile Head

A

B

x

N

T

B. Pre-existing Failure Surface

Roadway slope

Targeted failure surface

Driven Piles for

slope stabilization

Roadway

Shear stresses along

failure surface

C. Anticipated Failure Surface

Pile

Pile head

load Po

Successive mobilized

wedges

m

m

Mobilized zones as

assessed experimentally

Horizontal and Vertical Growth in the Soil Passive Wedge

Pile

The SW model is based on

• Stress-strain and strength

behavior of the soil as assessed

in the triaxial test,

• Soil effective stress analysis

• Plane strain problem

• Beam on Elastic Foundation

0 20 40 60

Ground Surface Displacement, mm

0

1

2

3

4

5

6

PD/P

Ra

nkin

e

0 20 40 60 80

Soil Driving Pressure, pD, kN/m

2

1

0

De

pth

, m

Sliding Surface

Yo=

6.5

mm

18 m

m

24 m

m

31 m

m

40 m

m

Sta

bil

i zin

g P

ile

Progressive Driving Force Induced

by Sliding Soil Mass

0 2 4 6 8

Pile Deflection, y, in.

30

25

20

15

10

5

0

De

pth

fro

m G

rou

nd

Su

rfa

ce

, ft

No Tie-Back

with Tie-Back

Tie-Back Location

3 ft5 ft

-40 0 40 80 120 160 200

Pile Deflection, y, in.

30

25

20

15

10

5

0

De

pth

fro

m G

rou

nd

Su

rfa

ce

, ft

No Tie-Back

with Tie-Back

TIE-BACK IN PSSLOPE

Slope Stability

Limit Equilibrium Analysis

Pile

W 14 x 211

Mp = 1625 kip-ft

Desired FS of Supported Slope = 1.3

-2 0 2 4 6 840

35

30

25

20

15

10

5

0

Dep

th( m

)

-2 0 2 4 6 8Deflection, y (in)

0 400 80040

35

30

25

20

15

10

5

0

De

pth

(ft )

0 400 800Moment (Kip-ft)

-80 0 8040

35

30

25

20

15

10

5

0

Dep

th(f

t )

-80 0 80Shear (Kips)

Pile Spacing4 ft6 ft8 ft

Po

-1000 0 1000 2000 300040

35

30

25

20

15

10

5

0

De

pth

(ft )

-1000 0 1000 2000 3000Line Load, p (lb/in)

Pile Spacing4 ft6 ft8 ft

Pile Spacing4 ft6 ft8 ft

Pile Spacing4 ft6 ft8 ft

C-Phi SoilCohesion = 300 psfFriction angle= 34 deg

failure surface failure surface

(ft)

10 FT

Problem 1

Problem 2

Pv

Ph

M1

Y

Z Z

Ph

Fill

Pile Head

W

Problem 3

Additional Problems

-50 0 50 100 150 200

Moment, kips-ft

40

36

32

28

24

20

16

12

8

4

0

De

pth

, ft

Pile 4

Pile 5

PSSLOPE

-1 0 1 2 3Displacement, in

40

35

30

25

20

15

10

5

0

De

pth

, ft

Pille 4

Pile 5

PSSLOPE

ROCKRock

Tygart Lake Test Site, WV

(After Richardson 2005)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Displacement: in

33

30

27

24

21

18

15

12

9

6

3

0

De

pth

be

low

gro

un

d le

ve

l: f

t

Average slope displacement, day 1345

Average pile displacement, day 1345

PSSLOPE - Uncracked Section

PSSLOPE - Cracked Section

Rockfill

Embankment fill

Intact weatheredand unweatheredWeald Clay

-50 0 50 100 150 200

Bending moment: kip-ft

33

30

27

24

21

18

15

12

9

6

3

0

De

pth

: ft

PSSLOPE Uncracked Section

PSSLOPE - Cracked Section

Measured (Spline smoothing)

Embankment Profile, UK

(Smethurst and Powerie 2007)

Unit weight, Friction angle, Effective cohesion

: Ib/ft3

: degrees : Ib/ft3

Weald Clay embankment fill 121 25 20.9

Softened Weald Clay

embankment fill

121

19

20.9

Weathered Weald Clay 121 25 20.9

Weald Clay 127 30 104.4

Rockfill 121 35 0

Soil type c

SUMMARY:

The current analysis/program provides the following:

• Limit equilibrium analysis for existing or anticipated

failure surface

• Evaluation of the progressive driving pressure of

sliding mass as a function of soil-pile displacement

with varying safety factors

•Implementation of tie-back as an elastic support

• Consideration of the flow-around failure of soil which

limits the soil mass interaction with the pile

• The effect of pile properties and spacing

• LRFD recommendations