Innovative slurry trench methods for the rehabilitation of small dams

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 are presented and the modified slurry trench methods used to rehabilitate the dams 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 water. If, however, the fluid is degradable, a 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 the rehabilitation of a small dam in Oregon with a bio-polymer trench used to create a new toe drain.

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, known as 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 much 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 sheet piles in a riverbed where conventional pile driving was impossible due to the presence of boulders and cobbles. 12 pp., 7 references.