35ft Mechanically Stabilized Earth Wall
The following problem for verification is reviewed in Pockoski M., Duncan, J.M. (2000). Comparison of computer programs for analysis of reinforced slopes, Report of a Study Sponsored by the Virginia Tech Center for Geotechnical Practice and Research.
The slope considered is vertical at 35 ft high, and is reinforced by a series of geogrid layers, 21ft in length. The safe design strength of the geogrid is 1,185 lb/ft. The soil profile consists of a homogenous sand with a granular backfill in the zone around the geogrid layers. The geometry and material parameters are shown below.
The reinforcement force is modeled using a geosynthetic with 100% area coverage and a tensile strength of 1,185 lb/ft. As the reviewed problem does not consider pullout resistance, the upper and lower interface angles are specified to be much stronger such that the tensile strength will always govern.
This example serves to illustrate the importance of thoroughly searching for the critical factor of safety and slip surface, and not relying solely on the auto search procedure. The slip surface and factor of safety calculated using Bishop's method and the auto search are shown below.
However, drawing a very broad search grid as shown below reveals a different situation.
The auto search has stumbled into one of many possible local minima that exist for the problem. It is important to supplement the auto search with search grids that capture a broad area of possible slip centers. With the search grid as above, we see that the critical slip surface now passes through the toe of the wall as shown below, and with a much larger radius. The calculated factor of safety is now 1.404, which agrees well with the published range of 1.41 to 1.5 using Spencer's method.
Download the MacSlope file with the analysis results for this verification example here