Mud Mountain Dam Concrete Cutoff Wall--A Case History

Abstract

Mud Mountain Dam is located on the White River in western Washington. The embankment dam is composed of a central impervious core and transition zones between the core and upstream and downstream rockfill shells. The dam extends across a narrow rock gorge with steep volcanic rock walls more than 275 ft high. Construction records, together with recent subsurface investigations, revealed that the 425-ft high embankment had defects in its internal core. Loose zones and voids had developed as a result of settlement and arching of core material within the narrow, steep rock canyon. The investigations confirmed that the core of the dam was progressively deteriorating due to seepage, differential settlement, and flushing from reservoir fluctuations. During 1989 and 1990, Soletanche Inc., under contract to the Seattle District, U. S. Army Corps of Engineers, constructed a concrete cutoff wall through the core of the dam and penetrated the rock foundation to prevent further erosion of the embankment core and to control seepage. During cutoff wall excavation, in the deep canyon section, the embankment core material experienced severe hydraulic fracturing under very nominal slurry pressure. A modification was made to the contract to close the cracks and compact the saturated core material by remedial grouting. Soletanche accomplished the job by means of an unprecedented recompression grouting program. Most of the 807.5-ft long cutoff wall is located 10 ft upstream of the dam centerline. Depth varies from 10 ft to 402.6 ft. Over 300,000 cf of dam core, 40,000 cu ft of mudflow overburden and 33,000 cu ft of rock were excavated for placement of the concrete cutoff wall.