Effect of Tension Crack Formation on Active Earth Pressure Encountered in Unsaturated Retaining Wall Backfills

Despite the potentially significant influence of tension cracks on earth pressures, almost all of the existing methods either do not account for cracks or treat them in a simplistic manner when dealing with unsaturated soils. The main objective of this study is to quantify the influence of tension crack formation on active earth pressures (AEP) in unsaturated backfills. A limit equilibrium-based framework for the determination of the tension crack depth (TCD) and AEP in unsaturated backfills is proposed. An effective stress approach is employed along with logarithmic spiral failure geometry to assess the influence of the interdependence between normal stress distributions, suction stress, and soil tensile strength on the formation of tension cracks under steady flow conditions. The maximum TCD is explicitly determined by evaluating a normal stress distribution and associated tensile stresses mobilized along the critical failure surface in retained backfill, with or without tensile strength. A parametric study is performed on a hypothetical marginal backfill to investigate the impact of a tension crack on AEP with and without considering the soil tensile strength under different flow rates. The results indicate that the formation of tension cracks will significantly increase AEP, while considering a nonzero tensile strength as a function of the matric suction will decrease the TCD and AEP. Upon comparison, it is shown that classic earth pressure theories may significantly underestimate or overestimate TCD in unsaturated backfills. The proposed approach presents a useful tool for forensic studies of failed structures and determining the depth of cracks in existing retaining earth structures that are to be rehabilitated. (c) 2020 American Society of Civil Engineers.