2008-12-05_PERMIT FILE - M2008082 (4)

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• • SOIL S+eE;Nt TOvNITE SWRRi WALLS Technical Brief 0 Geo-Con- Geotechnical Construction Corporate One-Bldg. II Suite 400 4075 Monroeville Blvd. Monroeville, PA 15146 (412) 856-7700 Regional Offices: CA (916) 858-0480 FL (813 626-0751 TX (940 383-1400 NJ (609) 232-7662 T-SW-01-85 --------- - -- - Soil Bentonite Slurry Walls Soil-Bentonite slurry walls are subsurface, non- structural walls that act as barriers to the lateral flaw of groundwater and water-bome pollutants. So&-Bentonite (SB) cutoff walls are constructed using the slurry trench technique and are composed primarily of soil and bentonite, a natural day mineral. The principal advan- tages of SB cutoff walls are the low permeability of the wall and their general suitability for both new and remedial appli- cations. APPLICATIONS The soil-Bentonite slurry trench technique has been in use in the United States since the 1940's. The early applica- tions of soil-Bentonite walls were for dewatering large ex- cavations and as hydraulic barriers in dams and dikes. Recently, there has been a growing number of applica- tions of SB slurry walls for pollution control, particularly on projects where a positive leachate cutoff is required. Recent advances in the capa- bility of excavating equipment and refinements in technique have brought the cost of slur- ry walls down. Slurry walls are now economically com- petitive on projects where compacted clay cutoffs, leachate collectors, sheeting, or well points would have previously been used. Some typical applications are to: • Seal dams and dikes • Contain sanitary and hazardous waste landfills • Oewater structural exca- vations • Hydraulically isolate lagoons and holding ponds • Enclose oil and chemical tank farms • Intercept seepage from slopes • Contain oil spills SCHEMATIC SECTION THROUGH SB SLURRY CUT-OFF Backhoe Keys Trench Into Gay Layer Backfill - Placed Here XG.W.L. --_Sl l L - -- urry eve Backfill Bentonite Slurry Sloughs Forward Slurry walls are particularly well suited for remedial appli- cations. Usually, SB walls can be constructed without disturbing the function or operation of existing facilities. CONSTRUCTION The construction sequence of the cutoff wall is the same whether it is to be used to cutoff groundwater or pollut- ed leachates. The major characteristic of slung cutoff wall construction is the use of bentonite-water slurry which allows excavation without the use of other lateral support. Slurry cutoff walls are built by excavating a narrow trench (2-4 ft. wide) while pumping in the slurry and maintaining its level at or near the top of the trench during the excavation process. Usually, the trench is keyed into an underlying R r,