Impact of Cohesion on Seismic Design of Geosynthetic-Reinforced Earth Structures

This paper calculates the thrust of lateral earth pressures exerted by unstable slopes comprised of c-phi soil and subjected to seismic (pseudostatic) loading conditions. Although the proposed method can be used for seismic stability analysis of geosynthetic-reinforced earth structures (GRESs), the formulation and results are also applicable to many other relevant earth retention systems. To study the impact of cohesion, the authors develop formulations to determine the seismic active earth pressure coefficient for c-phi soils resulting in a closed-form solution. The pseudostatic formulation considers the effect of tensile cracks, backslope inclination, and batter while assuming a log spiral failure surface. Two formulas are presented. One is for the conventional inclination of thrust. The second formulation considers a more feasible inclination of the thrust when it is likely to act against facing units with large setbacks constructed for large batter walls. The authors perform parametric studies and investigate the effects of relevant parameters. The results are compared to those obtained from an alternative limit equilibrium method and a rigorous upper-bound solution in limit analysis of plasticity. The proposed method is used to illustrate the impact of cohesion on the seismic design of GRESs. The formulas are incorporated into a spreadsheet, thus providing a tool to assess the thrust for c-f soils. However, given the high impact of cohesion combined with the significant uncertainties related to cohesion in partially saturated soils during the lifespan of the structure, cohesion's design value warrants careful consideration. Consequently, cohesion should be used in design only when its value during the lifespan of the structure can be assessed with confidence or when provisions taken to prevent potential surface water infiltration will sustain a verified value of apparent cohesion. (C) 2014 American Society of Civil Engineers.