LESSONS LEARNED FROM THE BHUJ, INDIA, EARTHQUAKE FOR THE SEISMIC SAFETY OF DAMS BUILT ON ALLUVIUM

The Bhuj, India, earthquake of 26 January 2001 was M7.9 and affected an area with dozens of dams, both earth and masonry, all constructed on alluvial deposits. Their behavior shows

l. The level of ground shaking is the most critical factor. For free field motions under 0.2 g, an alluvial foundation is safe. For higher accelerations, attention to the foundation is needed.

2. The principal hazard is from liquefaction in unconsolidated and water saturated granular foundation soils.

3. N values of 20 or less, occurring to depths of about 20 m, indicate susceptibility to liquefaction.

4. Curtain walls and core trenches do not inhibit liquefaction.

5. When susceptible layers are found, they must be treated or replaced under the full area of the dam.

6. The first step to assure seismic safety is a proper geological-seismological evaluation that provides maximum credible earthquake(s) and corresponding ground motions.