Soil stability under earthquakes: a sensitivity analysis

The slope stability behaviour of cohesive and cohesionless soil slopes was evaluated under earthquakes with different frequencies and amplitudes (0.01 to 1.0 g). The study focused on the computation of slope instability thresholds at different slope heights (5, 10 and 15 m) and inclinations. This parametric analysis was performed with a nonlinear finite element method (FEM) in plane strain using the Mohr Coulomb constitutive model and accounting for stiffness and strength increase with depth.
The steepest slope for each soil type was defined as the slope at its marginal stability with a safety factor under static conditions between 1.01-1.06. All the acceleration histories were sinusoidal functions at frequencies of 1, 2 and 4 Hz. The amplitude of these accelerations was gradually increased until the point they initiated instabilities in the slopes. These accelerations were denoted by critical peak accelerations (PGAc). This study demonstrates that lower peak accelerations (PGA) are needed to trigger instabilities at steeper slopes. Also, the highest frequency used in this parametric study (4 Hz) has higher PGAc, and in most cases the lowest frequency (1 Hz), which is close to the natural frequencies of the site (0.6 to 1 Hz) experience the lowest PGAc. The strength of the materials also governs the sliding thresholds, being higher for the stronger ones.