Design Dilemma: Use peak or residual strength of soil

Abstract

Most design methods for geosynthetic reinforced soil structures are based on limit equilibrium analysis. The required strength and length of the reinforcement is calculated based on the shear strength of the soils through which potential slip surfaces are likely to pass. Many design guidelines require a free-draining compacted backfill. Such soil exhibits strain softening behavior and thus there is a question of whether peak or residual shear strength should be used in the limit equilibrium analysis. The end result of this selection may have significant economic consequences especially when the foundation soil is competent. This paper recognizes the potential for the development of progressive failure. To produce safe and economical structures, a hybrid approach is proposed for design analysis. The location of the critical slip surfaces is determined based on peak strength as observed in laboratory soil element testing as well as centrifugal models of reinforced slopes. Accounting for the possibility that soil strength along these surfaces will degrade to its residual plastic value, the limit equilibrium analysis is repeated to determine the required long-term reinforcement strength. A simplified analytical design methodology is presented. Parametric studies show that the hybrid approach allows for marginal reduction in strength of reinforcement as compared to the `pure’ residual strength approach. However, the required length using the hybrid approach may decrease significantly depending on the slope and soil properties. Since the economics of geosynthetics are more sensitive to its area or length than to its strength in many instances, the presented approach may have significant design implications.