SLURRY TRENCH CONSTRUCTION OF A VERTICAL FOUNDATION FILTER AT WASCO DAM
Gary Fisk, P.E.
Jack Gagliardi, P.E.
Steven Day

A modified slurry trench method was used on a small dam remediation project to quickly and economically excavate a vertical filter trench through foundation soil across the toe of Wasco Dam and up the 3:1 slopes on one groin of the dam. Generally, the slurry excavation method is used for cutoff trenches to create a barrier to seepage. This modified slurry excavation method uses a synthetic biopolymer slurry to form a temporary seepage barrier that was chemically degraded after sand placement allowing the filter to regain its permeability. The proximity to a pristine stream in a popular recreation area precluded the use of other degradable slurries that relied on biocides additives.

Excavation of the slurry trench and backfilling with filter sand was accomplished in one week using a small crew with two hydraulic excavators and a portable slurry mixing plant. After the filter trench was completed and backfilled, a second-stage gravel drain and a perforated toe-drain pipe were installed in a 4-foot-deep trench downstream of the filter trench.

This project showed the economic feasibility, constructability and benefits of using slurry construction on other small dams where foundation seepage problems require remediation. This paper highlights the non-structural and structural remediation alternatives considered, compares the benefits and costs of slurry construction to mass excavation, compares guar gum slurry to polyacrylamide biopolymer slurry, shows the applicability of slurry trench excavation for placing a two-stage filter, and discusses pertinent construction issues.