Instability investigation of cantilevered seacliffs

Wave action is a fundamental mechanism in seacliff erosion, whereby wave undercutting creates an unstable cantilevered seacliff profile and can lead to large catastrophic cliff failures, thus threatening coastal infrastructure. This study investigated the instability of two such failures that occurred in Solana Beach, California, by combining terrestrial LIDAR scanning, cantilever beam theory and finite element analysis. Each landslide was detected by evaluating the surface change between subsequent high resolution digital terrain models derived from terrestrial LIDAR data. The dimensions of failed cantilever masses were determined using the surface change measurements and then incorporated into failure stress analysis. Superimposing stress distributions computed from elastic cantilever beam theory and finite element modeling provided a method to back-calculate the maximum developed tensile and shear stresses along each failure plane. The results of the stress superposition revealed that the bending stresses caused by the cantilevered load contributed the majority of stress leading to collapse. Both shear and tensile failure modes were investigated as potential cliff failure mechanisms by using a comparison of the back-calculated failure stresses to material strengths found in laboratory testing. Based on the results of this research, the tensile strength of the cliff material was exceeded at both locations, thus causing the cliffs to collapse in tension. Copyright © 2008 John Wiley & Sons, Ltd.