SEISMIC RISKS TO THE SEEPAGE CUTOFF WALL AT CLEARWATER DAM

摘要

The seepage cutoff wall being built at Clearwater Dam is subject to significant risks from earthquake-induced liquefaction and embankment deformations. Located near the New Madrid seismic zone in southeastern Missouri, Clearwater Dam is a zoned embankment dam built on a deposit of saturated, gravelly soils underlain by karstic bedrock. To mitigate seepage through the foundation, the Corps of Engineers is constructing a seepage cutoff wall along the full length of the dam. Design of the cutoff wall required consideration of seismic hazards, with a focus on the Operating Basis Earthquake (OBE). Corps policy allows for substantial damage to the wall during a Maximum Credible Earthquake (MCE), because the usual freeboard is large and protects against the loss of the pool. To assess liquefaction hazards in the gravelly foundation deposit, short-interval SPT and oversized samplers were employed. Dynamic deformation analyses, carried out using an explicit finite difference numerical model, showed that modest displacements could be expected in the absence of liquefaction. A structural analysis of the deformed cutoff wall demonstrated the advantages of constructing the cutoff with a low-strength, plastic concrete having a stiffness that matches the embankment soils.