Evaluation of Active Earth Pressure in Unsaturated Retaining Structures in Presence of Tension Cracks

Cracks are often found at the crest of a slope in backfill soil behind retaining structures prior to their failure. The presence of cracks increases lateral earth pressure incurred in retaining walls by reducing the soil shear resistance. This paper employs an analytical solution for determining the tension crack depth (TCD) in unsaturated backfills and quantifying the effect on active earth pressure (AEP). Tension cracks occur at a depth where normal stress transitions from compression to tension. The maximum TCD is determined by evaluating the normal stress distribution along the critical log spiral failure surface using a limit equilibrium effective stress analysis. A parametric study is performed to investigate the effects of cohesion, angle of friction, air entry suction, and flow rates on TCD and AEP. Results suggest that tension cracks significantly increase AEP. TCD is found to be dependent on soil properties. Fine-grained soils with lower air entry suctions could generate deeper cracks. Increase in soil cohesion increased TCD while soil friction angle had minimal effect on tension crack formation. Finally, deeper cracks were developed during evaporation when higher suction developed within the soil layer. The proposed solution can be used in forensic studies of failed structures to determine the depth and strength of required reinforcement layers to be placed in marginal backfills to prevent the formation of cracks or rehabilitate the existing cracks.