February 2005
	Analyzing Geofabrics with SLOPE/W

Last month we were at the Geo-Frontiers 2005 Conference in Austin, Texas. The conference was a combination of the Geo-Institute 2005 Congress and the Geosynthetics 2005 Conference, and consequently many of the participants were from the geofabrics industries. A question frequently asked at our exhibition booth was, “Can GEO-SLOPE’s software be used to analyze the stability of fabric-reinforced retaining walls?” The answer is yes. The following is a brief illustration of what can be done.

The diagram shows a retaining wall with four geogrid layers. The backfill is granular, and the pullout resistance is therefore governed by the frictional characteristics between the backfill and the geogrid, the overburden stress and the length of embedment behind the critical slip surface. For this illustrative example, the friction angle was taken to be 24 degrees, with a soil-fabric interface factor of 1.6 and a safety factor of 1.5. The geogrid was taken for design purposes to have an ultimate or tear strength of 80 kN and an allowable tensile capacity of 40 kN (i.e., a safety factor of 2.0).

The red zone in the diagram is a zone where all the trial slip surfaces have a Factor of Safety between 1.30 and 1.35; twenty-five trial slip surfaces fall in this relatively narrow band, suggesting that potential slippage of the retained soil wedge is within a shear zone rather than one distinct slip surface.

The red box along the geogrid behind the critical slip surface is a graphical representation of the required embedment. For the upper layer, the red box is equal to the geogrid length behind the critical slip surface, indicating that the pullout resistance governs in this case. For the other three layers, the red box is shorter than the available geogrid length behind the critical slip surface, and consequently the design tensile capacity governs for these layers. Also note that the required embedment diminishes with depth which reflects the increase in overburden stress on the geogrid.

SLOPE/W accommodates various options for including the effect of geofabric reinforcement in a stability analysis. These include the following:

• The frictional characteristics between the fabric and the soil can be a constant per unit length or a variable depending on the overburden.
• The force orientation represented by the geofabric can vary between the fabric axial direction and the inclination of the slip surface.
• The force can be applied at the point where the slip surface intersects the fabric or it can be evenly distributed along the fabric between the slip surface and the wall.
• The forces representing the geofabric can optionally also be a function of the SLOPE/W computed factor of safety; that is, the safety factors applied to the reinforcement are the same as for the stability of the retained soil wedge.

This example considers only potential toe instability. It is often the design governing mode of instability, but other possible modes of failure such as a base or foundation failure also need to be considered separately as part of another SLOPE/W analysis.

John Krahn, Ph.D., P.Eng.
President and CEO, GEO-SLOPE International Ltd.

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