A new working stress method for prediction of reinforcement loads in geosynthetic walls

Proper estimation of soil reinforcement loads and strains is key to accurate internal stability design of reinforced soil structures. Current design methodologies use limit equilibrium concepts to estimate reinforcement loads for internal stability design of geosynthetic and steel reinforced soil walls. For geosynthetic walls, however, it appears that these methods are excessively conservative based on the performance of geosynthetic walls to date. This paper presents a new method, called the K-stiffness method, that is shown to give more accurate estimates of reinforcement loads, thereby reducing reinforcement quantities and improving the economy of geosynthetic walls. The paper is focused on the new method as it applies to geosynthetic walls constructed with granular (noncohesive, relatively low silt content) backfill soils. A database of 11 full-scale geosynthetic walls was used to develop the new design methodology based on working stress principles. The method considers the stiffness of the various wall components and their influence on reinforcement loads. Results of simple statistical analyses show that the current American Association of State Highway and Transportation Officials (AASHTO) Simplified Method results in an average ratio of measured to predicted loads (bias) of 0.45, with a coefficient of variation (COV) of 91%, whereas the proposed method results in an average bias of 0.99 and a COV of 36%. A principle objective of the method is to design the wall reinforcement so that the soil within the wall backfill is prevented from reaching a state of failure, consistent with the notion of working stress conditions. This concept represents a new approach for internal stability design of geosynthetic-reinforced soil walls because prevention of soil failure as a limit state is considered in addition to the current practice of preventing reinforcement rupture.