On the Factor of Safety in Reinforced Steep Slopes

US national design guidelines distinguish between slopes and walls using the face angle as an arbitrary criterion. Angles less than 70° render reinforced slopes whereas steeper angles render reinforced walls. While the expected performance of reinforced walls and slopes may be different, the basic stability analyses should be the same. However, because of historical reasons, the evolution of design for either type of structure followed a different path. As a result, the outcome of analyzing a particular reinforced soil structure will be different when slope stability analysis or wall analysis are employed. This disagreement exists even near 70° where the two structures are basically classified as either walls or slopes. In fact, current slope stability approach yields much more conservative designs as compared with wall analysis. This is contrary to performance expectations that are normally stricter for reinforced walls. This paper suggests a basic step linking the two ‘types' of structures. Historical evolution of designs produced a non-rational difference in the definition of the factor of safety used in geosynthetic-reinforced walls and slopes. It is suggested to use the same definition; this definition has proven to yield safe wall structures for three decades. Parametric studies indicate that use of the ‘new' factor of safety in slopes may result in significant reduction of required strength and length. That is, strength may be reduced by a factor of two or more while length may be reduced by 30% or much more.