Scott County, IA: Soil-Bentonite Slurry Wall, Lost Grove Lake Dam And Spillway
Geo-Con was contracted to install a soil-bentonite cutoff wall. The cutoff wall was designed to cut off seepage under a new earthen dam that is being constructed to create the new Lost Grove Lake. An outlet pipe was designed to penetrate the slurry wall. Geo-Con proposed and constructed a soil-cement-bentonite section to allow excavation into the slurry wall without sloughing.
The length of this cutoff wall is 770 linear feet with depths ranging from 31 to 44 feet. The cutoff wall was installed using a CAT 385 excavator with a long reach attachment and a 5-foot wide heavy-duty trenching bucket. This bucket is wider than what is typically used on a slurry wall, and was custom built to meet the specification on this project of a 5-foot wide cutoff wall. Soil-bentonite backfill was created using the excavated trench spoils and borrow material from other areas of the project. Borrow material was used to control the gradation of the mixture to comply with the specification.
A section of soil-cement-bentonite wall was installed to facilitate the placement of the primary spillway outlet piping by the General Contractor. The excavation for the installation of the piping required cutting approximately 8 feet into the cutoff wall. Geo-Con proposed and installed the SCB section to give the cutoff wall backfill low strength instead of the effective zero strength of the soil-bentonite backfill. The SCB section was installed by isolating a 100 foot long section of the slurry filled trench using a steel H-beam to act as in "end-stop" for the backfill.
The significant challenge on this project was the key-in to bedrock. The slurry trench was excavated through a weathered limestone rock until excavation refusal was encountered in the competent bedrock. Geo-Con was able to achieve adequate key-in to ensure a seal at the bedrock interface.