Discussion on Limit Equilibrium Analysis Models and Modes for Reinforced Soil Slopes and Walls
Reinforced soil structures, retaining walls and steep slopes, are normally designed by limit equilibrium methods. In this case, three "independent" analyses are commonly performed - internal, external and local stability analysis - in order to define a typical cross section for the structure. Beside this failure modi the so-called "compound" mode can occur: the failure surface crosses both the reinforced fill and the unreinforced backfill. Unfortunately, this mode is being often forgotten by design engineers and owners, which can lead to failure. Comparative calculations are presented using common geotechnical limit-equilibrium-based procedures, showing the risks of inappropriate "cheap" design "forgetting" the "compound" mode. Additionally, some other "risky" issues are discussed.
The so called "compound" mode of failure of reinforced slopes and walls has always to be checked, because very often it controls the design. Classic methods based on cylindrical (e.g. Bishop) and polygonal (e.g. Janbu) failure surfaces can be used. Neglecting the compound mode results in a "cheaper", but risky solution. The results of the limit equilibrium analyses presented regarding this issue are confirmed by more sophisticated numerical FEM- (not shown) or FLAC-analyses (e.g. Leshchinsky & Vulova 2001). Last note: In internal analysis an absurd may occur: for a configuration with longer reinforcement layers, lower FOS may be reached due to the larger area for searching for failure surfaces. It is definitely not realistic!