4  Slope Stabilization With EPS Geofoam

4.4         Effect of Using Geofoam in Increasing the Factor of Safety

Slopes with circular critical surfaces were found to have the same factor of safety and identical slip surfaces using either FLAC or GeoSlope. This is examined for both cases, with foam and without foam. GeoSlope and FLAC gave the same factor of safety and the same starting point of the circular slip surface for slopes (not having geofoam) as given by Taylor charts. 

4.4.1        Cohesionless Soil

Failure surfaces for homogenous cohesionless soils are generally shallow (McCarthy, 1998) and forms near surface as shown in figure 4-4. The limit equilibrium analysis gave essentially the same critical surfaces as the finite difference analysis. The factor of safety and failure surface does not change unless geofoam can be configured to replace soil in the shallow failure zone.

Geofoam does not contribute to addressing the stabilization problem represented in figure 4-5. The failure zones develop in the area between the foam and the slope face.

Figure 4‑1 Shear Strain Contours in a Cohesionless Soil Slope

Figure 4‑2 Shear Strain Contours in a Geofoam-Stabilized Cohesionless Soil Slope

4.4.2        Cohesive Soil

The slip surface for a general slope of a homogeneous cohesive soil is either a deep-seated circle or a toe circle (McCarthy, 1998). In both cases, a large amount of soil next to the slope edge acts in the driving force. Figure 4-6 shows the failure zones for a clay soil obtained using FLAC. The soil profile of this cross section is a cohesive soil over stiffer or hard soil. For such cases, the failure surface almost touches the stiff soil interface. Approximately the same failure surface develops with GeoSlope analysis.

            By using geofoam in the cross section, the surface with a minimum factor of safety is deeper and below foam blocks. The surface is no longer circular. The limiting equilibrium search must be made general to include non-circular surfaces. Results from FLAC analysis indicate that the factor of safety increases by using geofoam and that the failure zones surround the foam as shown in figure 4-7.

Figure 4‑1 Shear Strain Contours in a Cohesive Soil Slope

Figure 4‑2 Shear Strain Contours in a Geofoam-Stabilized Cohesive Soil Slope

4.4.3        Increasing The Factor of Safety

One way to increase the factor of safety of a slope is to decrease the inclination by either flattering or benching. This would result in reduction of usable space at the crest. Geofoam stabilization however maintains the inclination of the original slope and while increasing the factor of safety. In some cases, the slope can even be made steeper with improvement in factor of safety, as shown in figure 4-8. The more the amount of foam that is added, the larger the failure area becomes resulting in a higher factor of safety. From the figure it can be seen that the increase in the factor of safety is large up to a certain width value. Increasing the width further will result in a slight increase in the factor of safety depending on the slope value.  The distribution of a certain amount of foam will affect the factor of safety as shown in figure 4-9. The closer is the shape of the foam fill to the slip surface shape the higher is the factor of safety. This holds as long as the foam does not encroach upon the area of the stabilizing force.

Figure 4‑1 Effect of EPS Width on the Factor of Safety for various Slopes

Figure 4‑2 Effect of EPS Width on the Factor of Safety for Different EPS Configurations

Figures 4-10 shows the variation of the factor of safety for both static and dynamic conditions, with the width and the depth of the foam fill. From the figure it can be seen that the increase in the factor of safety is large up to a certain width value for the one layer. Increasing the width further will result in slight increase in the factor of safety. At a certain width the factor of safety tends to be constant where the failure likely to occur in the slope side of the foam. Figure 4-11 shows that the factor of safety for the case of foam is higher than that for the case of no foam. The purpose of presenting figures 4-8 to 4-11 is to show the benefit of using geofoam to increase the factor of safety.

Figure 4‑3 Effect of EPS Width on the Factor of Safety for Two Thicknesses

Figure 4‑4 Effect of Using Geofoam on the Factor of Safety for Different Horizontal Accelerations