INNOVATIVE SLURRY TRENCH METHODS FOR THE REHABILITATION OF SMALL DAMS

Steven R. Day
Christopher R. Ryan
Gary Fisk

The rehabilitation of small dams often requires the reduction or control of groundwater seepage through or under earthen dams. The slurry trench method is well known for creating impermeable groundwater barriers and has been used for decades to create economical and positive cutoff walls in the core or foundation soils beneath dams and dikes of many types and sizes. This paper examines three innovative modifications of the slurry trench method and their applications in the rehabilitation of three small dams. Case histories of each dam and slurry trench method are described.

The slurry trench technique uses an engineered fluid for support of trench walls. Usually the fluid is bentonite slurry that coats the trench walls and permanently blocks the free flow of permeable backfill, such as sand or gravel can be backfilled in the trench to create a permeable zone such as a toe drain or chimney drain. This method is known as the bio-polymer trench drain method and has been used for more than a decade in the waste remediation industry to collect contaminated groundwater, and more recently in the rehabilitation of dams. A case study is presented of a small dam in Oregon that was rehabilitated with a toe drain constructed by the bio-polymer trench method.

Most slurry walls are backfilled with a mixture of soil and bentonite that provides an impermeable, but non-structural barrier. In some cases, a material with moderate structural strength is desirable, especially, for small dams under loads imposed by earthquakes or higher operating levels. A mixture of soil, cement and bentonite (SCB) has recently seen increasing acceptance. SCB is stronger and more impermeable than cement-bentonite grout, but flexible enough to allow for deformation, and usually less costly. A case study is presented of a small dam in Pennsylvania that used SCB for the core of the dam.

The slurry trench method can be used as an economical excavation method for removing boulders and obstacles with minimal excavation and maximum economy. A case study is presented of a small diversion dam in New Mexico where the slurry trench method was used to install steel sheetpiles in a riverbed where conventional pile driving was impossible due to the presence of boulders and cobbles.