Water softening-induced retrogressive landslides: Experiments and theoretical calculations of back scarp inclination

This study investigates the inclinations of back scarps in water softening-induced retrogressive landslides and proposes a new theoretical method based on a two-surface failure mechanism to calculate these inclinations. A test setup was developed to reproduce water softening-induced retrogressive landslides by segmentally injecting water into the bottom of the sliding zone from downstream to upstream positions. The inclinations of multiple back scarps were measured for three model slopes with varying programs of water injection. Then, a new method, called the two-surface failure analysis method, was presented to calculate the inclinations of the back scarps. Finally, the practical applicability of the theoretical method was tested with real landslides. Multiple sliding masses in the water softening-induced retrogressive landslides were successfully reproduced in the tests, and each sliding mass exhibited two failure surfaces, a first gentle basal sliding surface and a subsequent steep back scarp; the inclinations of all the back scarps were within the range of 60–77°. Moreover, the relative errors between the calculated and measured inclinations ranged from 4.35% to 9.09% for the three model slopes. The relative errors between the calculated and actual inclinations ranged from 1.49% to 11.11% for the real landslide. These small differences indicate that the theoretical method is reliable. The theoretical method may be used to aid in the design of more targeted monitoring or geological exploration and coring.