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

Abstract

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.Key words: geosynthetics, reinforcement, walls, loads, strains, design, K-stiffness method.

Evaluation of K-Stiffness Method for Vertical Geosynthetic Reinforced Granular Soil Walls in Japan

“…The improved accuracy of the K-stiŠness method was demonstrated by Allen et al (2003) by showing that for the same 11 wall case histories, the average ratio (bias) of the measured to predicted maximum load in a reinforcement layer was 0.99 and the coe‹cient of variation (COV) was reduced to 36z.…”

“…They demonstrated that the current AASHTO Simpliˆed Method (AASHTO, 2002) used in North America is excessively conservative for internal stability design of geosynthetic reinforced soil walls constructed with granular backˆll. For example, based on 11 walls, the average ratio (bias) of the measured to predicted maximum reinforcement load in a wall was 0.45 and the coe‹cient of variation (COV) was 91z (Allen et al, 2003;Bathurst et al, 2005). This means that, on average, reinforcement loads were over-predicted by a factor of two and the spread in bias values with respect to the mean bias value was also about a factor of two at ±1 standard deviation.…”

“…To overcome these deˆciencies Allen et al (2003) proposed an empirical design method (K-stiŠness Method) developed through back-ˆtting (calibration) to reinforcement loads deduced from the same database of full-scale walls mentioned above. The prediction of the maximum reinforcement load in a layer is expressed as:…”

“…The eŠects of the in‰uence factors that appear in this equation are noted later in the paper as each parameter is introduced. Further details of the development of the original K-stiŠness Method can be found in the papers by Allen et al (2003) and Bathurst et al (2005). The implementation of the original KstiŠness Method can be found in the WSDOT (2005) guidance document.…”

“…Adopting the terminology introduced by Allen et al (2003), these walls correspond to a facing batter angle of v＝0. However, a wide range of facing types and geometry were used in their construction.…”