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