REMEDIATION OPTIONS FOR AN EMBANKMENT DAM ON A LIQUEFIABLE FOUNDATION IN EASTERN KANSAS

A major embankment dam, approximately 140 feet high and over one mile long, is located in a zone of moderate seismicity in Eastern Kansas. Seismic and geotechnical investigations established that a maximum credible earthquake with moment magnitude 6.6 occurring 20 km away on the Humboldt Fault zone could induce liquefaction of the foundation soil under the lower portions of both upstream and downstream slopes. Numerous seismic retrofit solutions were studied under the Dam Safety Assurance Program of the Corps of Engineers, including the extreme options of “no action” and “replace embankment”. A preliminary evaluation of all identified alternatives was done to facilitate the selection of the best rehabilitation method. An initial screening of the potential remediation alternatives was based on the following acceptance criteria: (1) Safety requirement (acceptable factor of safety and deformations for post-earthquake condition); (2) Economic requirement (the annualized cost of modification should not exceed the annual project benefit); (3) Maintain project purpose (recreation, water supply, fish and wildlife, flood control, water quality, and navigation); and (4) Technical feasibility (feasibility under standard construction procedures, verifiability of results, safety during construction, etc.). Stabilization of the foundation soil under both the upstream and the downstream slopes was selected as the most efficient option. Jet grouting through pre-drilled holes from a platform built on the slope was the preferred technology for the upstream treatment. Temporary removal of the lower portion of the slope and improvement of the liquefiable zone of the foundation soil by soil mixing was considered the best solution for the downstream side. The preliminary design was intended to ensure a factor of safety in excess of 1.2 for post-earthquake stability with liquefaction expected to be induced by the maximum credible earthquake.