Field measurements of passive earth forces in steep, shallow, landslide-prone areas

Passive earth resistance plays an important role in slope stability analyses for predicting shallow landslide susceptibility. Three-dimensional models estimate the contribution of this factor to slope stability using geotechnical theories designed for retaining structures and add it to the resistive forces. Systematic investigations have not been conducted to quantify this resistance in soils experiencing compression during the triggering of shallow landslides. This study presents field-scale experimental data of passive earth force for cohesive and frictional clayey gravel evaluated at different combinations of soil depths and slopes. The experimental setup included a specialized device composed of a steel structure and a stiff plate that moved toward a mass of soil. In both dynamic and quasi-static states, force-displacement curves and maximum compression resistance were determined for several water content conditions induced by a rainfall simulator. The maximum dynamic force ranged from 8.49 kN to 31.67 kN for soil depths ranging between 0.36 m and 0.50 m, whereas the quasi-static force corresponded to 60% of the dynamic force. Furthermore, rainfall generated an additional decrease of compression resistance compared to that measured in the field. A comparison of measured data with theoretical models of passive earth force indicated that Rankine’s solution provided the best estimate, whereas the logarithmic spiral approach significantly overestimated passive earth force by up to 70%. Therefore, the correct choice of geotechnical formulation or the direct use of field measurements to estimate passive earth force may significantly improve the accuracy of 3-D limit equilibrium models for assessing slope stability over natural landscapes.