Recent Developments in Ground Improvement for Mitigation of Seismic Risk to Existing Embankment Dams

The majority of large embankment dams in the United States in operation today were constructed over half a century ago — before major earthquakes in 1964 jump-started geotechnical earthquake engineering and initiated systematic evaluations of the seismic stability of these earth structures and the soil deposits on which they are built. Many dams have required strengthening owing to increased levels of anticipated shaking or inadequate resistance of the embankment or foundation materials. Soil stabilization and ground improvement methods have included buttress fills, removal and replacement of weak foundation soils, deep dynamic compaction, vibro-compaction, vibro-replacement, explosive compaction, deep soil mixing, jet grouting, compaction grouting, and permeation grouting. In most cases the design strategy is to strengthen the dam to prevent vertical crest deformations that would exceed the freeboard and transverse cracking to depths greater than the remaining freeboard after the earthquake. Special attention must also be given to assuring that seepage paths cannot form along conduits or other discontinuities. In many cases stabilization to prevent excessive upstream deformations is costly and difficult, so in recent years there have been several cases where ground improvement and construction of stabilizing trenches and berms has been confined to the downstream slope and foundation area.