A Comparison of Trencher Method vs. Slurry Trenching and Soil Mixing for Seepage Barriers in Dams

The chain-style trencher has proven to be a valuable tool for challenging trenching applications including cutoff wall installations. One of the primary uses of the trencher method in dam and levee applications is in-situ mixing to create seepage barriers within earthen embankments. A trencher is a vertical, rotary chain mixing blade fixed to a track-mounted chassis. In-situ mixing approaches can be beneficial to dam and levee projects as cutoff walls can be constructed on the upstream, crest or downstream sides, or combination thereof, on the embankment. Possible seepage barriers constructed with the Chain-Mixing-Method (CMM), also known as the trencher method, includes soil-bentonite or soil-cement-bentonite. In addition to seepage barriers, soil-mixing techniques can be used for stabilization and gravity buttresses. Furthermore, the trenchers can also be outfitted with backfill delivery attachments to simultaneously install groundwater relief trenches. These relief trenches are typically installed along the toe of the slope by emplacing a granular backfill to provide a planned pathway for seepage without causing erosion issues to the earthen embankment. The trencher can provide several benefits for these applications relative to conventional slurry trenching and soil mixing, such as operating from the dam crest (minimal width) without the need for mixing ponds or plants due to in-situ technology. Often the embankment does not need to be degraded simply to make construction platform for the seepage barrier. As with all technologies, the trencher method does have limitations. This paper will review recent projects and provide technical considerations for design, installation, and performance of trenchers, conventional slurry excavation, and soil mixing methods. This paper provides information and suggestions to better inform the selection of an appropriate method.