Abstract Description: The chain-style trencher has proven to be a valuable tool for challenging trenching applications including cutoff wall installations in dams and levees. One of the primary uses of the trencher method in dam and levee applications uses in-situ mixing to create seepage barriers through earthen embankment. A trencher is a vertical, rotary chain mixing blade fixed to a track-mounted chassis which is agile, able to make turns as well as ascend and descend grades. This is typically 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. 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.
Seepage barriers can be constructed with 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 to flow without causing erosion issues to the earthen embankment.
As with all technologies, the trencher method does have some limitations. This paper will review recent projects and provide technical considerations for design, installation, and performance of trencher, conventional slurry excavation, and soil mixing methods. As authors who have experience with all three methods, the goal of this paper is to provide information and suggestions to allow for well-informed decision on method selection.
Learning Objectives:
Learn about and compare different methods of seepage barrier construction in dams and levees.
Demonstrate different types of seepage barriers and their application to dams and levees.
Review recent projects and provide technical considerations of design of the different methods.
