ANALYSIS OF ROCK SLOPE STABILITY UNDER EARTHQUAKE LOADING

Earthquake effects on rock slope stability were evaluated using pseudostatic forces and Newmark displacement. The pseudostatic method is the more conservative as it applies during the entire earthquake shaking, whereas the Newmark displacement method develops only when ground acceleration exceeds a critical value. For this seismic rock slope study, discontinuity data, measured on an I-90 rock cut near Wallace, Idaho, were analyzed using seismic data obtained from the USGS. The natural slope orientation is about 45/189. Discontinuity properties are: mean dip/dip direction, 75/220 for bedding; 85/215 for foliation, 65/310 for joint set 1, 75/145 for joint set 2, 50/345 for joint set 3; 66/220 and 83/218 for two faults. Back-calculated strength parameters are Phi=25 to 40°, and cohesion=zero. Historically, earthquakes of MMI V to VI have occurred within about 30 miles of Wallace. For the analysis, a maximum acceleration, 0.05g, was selected based on acceleration attenuation relationships. As a result, the stereographic projection shows that plane failure and toppling are unlikely but the intersections of bedding and joints form wedges of concern. Limit equilibrium analysis with intersection of 50/240 and 65/135 (Wedge 1) results in FS=1.14 when dry and FS=1.03 under seismic conditions. Intersection of 75/300 and 65/155 (Wedge 2) results in FS=1.90 when dry and FS=1.72 under seismic conditions. Realizing FS=1.03 for earthquake conditions, slope failure along Wedge 1 is only marginally stable. From displacement analysis, yield acceleration is 0.07g for Wedge 1 and 0.33g for Wedge 2, both larger than the maximum acceleration. This shows rock slope displacement is unlikely and it's necessary just to stabilize bench scale wedges. Considering both methods, Wedge 1 is only marginally stable under earthquake loading.