GEO-SLOPE International Ltd, Calgary, Alberta, Canada
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SEEP/W generated pore-water pressures in SLOPE/W
stability analysis
1
Introduction
The objective of this illustration is to observe how to use finite element pore-water pressure results in a
stability analysis. Including or deliberately ignoring negative pore-water pressures can be critical to
understanding and interpreting a slope stability analysis. In particular, the objectives of this illustration
are to:
Set up and solve a steady-state finite element SEEP/W simulation. In CONTOUR, show the
positive pressure heads that develop.
Set up a slope stability problem in SLOPE/W based on the SEEP/W finite element mesh and
computed pore-water pressure; determine the critical slip surface and the factor of safety using
the SEEP/W pore-water pressure; and graph the pore-water pressure and strength along the slip
surface.
Repeat the analysis, but remove the advanced parameters from the soil property information.
Graph the pore-water pressure and strength along the slip surface and note how the negative
pore-water pressures have been ignored.
2
Feature highlights
GeoStudio feature highlights include:
Unit flux boundary conditions
Potential multiple seepage faces
Integrating SEEP/W and SLOPE/W
Effect of suction on stability
3
Geometry and boundary conditions
The seepage portion of the analysis is illustrated in Figure 1. In general, the slope is comprised of multiple
layers with a finer, lower permeability layer located half way up the slope face. Note that more coarse soil
soils in region 1 and 3 have the same hydraulic properties (Ksat 1x10-3 m/day). In addition, a steady-state
infiltration rate of q = 3.0x10-5 m/day is applied along the top, with a pressure equals zero condition on the
downstream surface. A potential seepage review has been applied along the face of the slope.
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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Infiltration 3.0e-5 m/day
20
Ksat 1e-3 m/day
15
Seepage face
Elevation
Ksat 1e-5 m/day
10
Pressure zero
Ksat 1e-3 m/day
5
0
0
5
10
15
20
25
30
35
40
45
50
55
Distance
Figure 1 Seepage problem definition
4
Material properties
The hydraulic functions are illustrated in Figure 2. These functions are represented by 2 data points, and
adequately show a drop in conductivity as suction increases (or as soil dries out). A 2-point function is not
realistic for all cases, but it is more than adequate to illustrate saturated / unsaturated flow.
Figure 2 Simplified hydraulic conductivity functions
The soil property information for the SLOPE/W portion of the analysis is given in Table 4.
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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Table 4 Slope soil information
Layer
Unit Weight
Phi
Cohesion
Unit Weight
above WT
Phi B
Sand
18
25
5
18
15
Silty Clay
19
20
5
19
15
5
Results and discussion
Three analyses are set up in this file. There is a steady state seepage file and two “child” SLOPE/W
analyses. The two SLOPE/W analyses both point to their “parent” seepage analysis for detailed porewater pressure information. The difference in the two SLOPE/W files is that, in one, the effects of added
strength due to “suction” is included. Both SLOPE/W models are GLE method with Auto Location of the
slip surface and optimization enabled.
Figure 4 shows the SEEP/W computed perched water table for the steady state solution. It is clear that
there is a perched water table within the embankment. Infiltration along the top is at a rate high enough to
build up a perched zone of positive water pressure within the lower conductivity silty clay band. At the
right side of this band, a singular point of seepage water has developed on the slope.
Figure 4 shows the computed slip surface and factor of safety for the case where suction effects are
included in the analysis. SLOPE/W was able to read the seepage results directly from SEEP/W in order to
compute the actual pore-water pressures at the base of each slice. Figure 5 shows the actual pore-water
pressures applied on each slice. Notice how the pore pressures on the slices change from negative to
positive to negative and back to positive, as the slice number increases from left to right. It would not
have been possible to accurately establish this type of pore-water pressure condition without the use of a
rigorous saturate-unsaturated seepage flow model.
Figure 6 shows the contributing strength components applied to the first SLOPE/W analysis. The
cohesion was fixed as a material property and is constant at 5 kPa. The frictional component depends on
the slice base normal force and the suction component depends on the pore-water pressure as it varies
across the slope. It is interesting to look at the suction strength contribution and compare it with the
seepage pore-water pressures. There is no suction strength where the pore-water pressures are positive.
Infiltration 3.0e-5 m/day
20
Ksat 1e-3 m/day
15
Seepage face
Elevation
Ksat 1e-5 m/day
10
Pressure zero
Ksat 1e-3 m/day
5
0
0
5
10
15
20
25
30
35
40
45
50
Distance
Figure 3 Computed water tables. Note perched zone and seepage face
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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1.141
20
Sand: Including Phi B
15
Seepage face
Elevation
Silty Clay: Including Phi B
10
Pressure zero
Sand: Including Phi B
5
0
0
5
10
15
20
25
30
35
40
45
50
Distance
Figure 4 Slip surface including suction effects
Pore-W ater Pressure (kPa) vs. Distance (m)
20
Pore-Water Pressure (kPa)
15
10
5
0
-5
-10
-15
-20
0
5
10
15
20
25
30
Distance (m)
Figure 5 SEEP/W pore pressures at base of slices in SLOPE/W
Figure 7 and Figure 8 show results of the same SLOPE/W model but with suction strength effects
neglected in the analysis. The overall factor of safety is reduced from 1.141 to 1.077.
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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Figure 6 Contributing strengths over slip surface (with suction effects)
1.077
20
Sand: Excluding PhiB
15
Seepage face
Elevation
Silty Clay: Excluding PhiB
10
Pressure zero
Sand: Excluding PhiB
5
0
0
5
10
15
20
25
30
35
40
45
50
Distance
Figure 7 Slip surface with no suction effects
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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Figure 8 Contributing strengths over slip surface (no suction effects)
SEEP/W Example File: Seepage and stability.doc (pdf) (gsz)
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