Seismic Slope Stability Assessment in a Moderate Seismicity Region, Hong Kong

Abstract eng:
The current code of practice for building design in Hong Kong does not require any seismic considerations and generally there is no requirement to consider seismic ground motion in the design of slopes. However, the Chinese Code for seismic design of buildings, GB 50011 – 2001, categorises Hong Kong as being in Zone VII+ with a 10% in 50 year peak ground acceleration of 15%g. Conventional earthquake design for a slope is usually based on a pseudo static slope stability calculation. To limit the slope movement in earthquake, it is recommended that the critical acceleration of slope (Ac) shall be no less than half of design peak ground acceleration. The critical acceleration of slope is the ground acceleration that gives a FoS against failure of unity. This paper presents an estimation of the earthquake induced displacement of typical slopes subjected to various levels of seismic ground motions using the finite difference package FLAC with the dynamic option. The input earthquake time histories are based on the scenario events obtained from the de-aggregation of recent seismic hazard study in Hong Kong. These scenario events, in terms of earthquake magnitude and distance, are those that contribute the most to the seismic ground motion at each seismic hazard level. The current design practice for new slopes in Hong Kong requires a minimum FoS against failure of 1.4 for slopes with a high consequence to life. This study shows that for such a slope subjected to a rare earthquake event (2,475 year return period), the earthquake-induced resultant displacement is less than 30mm. For an existing slope with a minimum FoS of 1.2, the induced displacement could exceed 100mm under such an earthquake. The movement is concentrated on the shallow slope surface. However, this may still threaten the integrity of a particular hazardous installation. In such circumstances, it is recommended to consider seismic loading specifically for essential and hazardous facilities above or below the existing slopes with a FoS against failure of less than 1.2.

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