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

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• aquiclude. The aquiclude then forms the bottom and the slung wall the sides of the containment. This narrow trench is then backfilled with impervious materials to form a permanent cutoff. When the backfill consists of a mixture of soil and bentonite, the con- struction work is called a soil- bentonite (SB) slurry cutoff wall. On projects where the materi- al excavated from the trench is suitable for use as backfill, the SB system can be most economical because of the minimum amount of backfill materials required. After the trench has been excavated under a bentonite slurry, more slurry is mixed with the soil adjacent to the trench. A bulldozer is used to work the material to a consistency similar to wet concrete. It is then pushed back into the trench so that the backfill slope displaces the bentonite slurry forward. Excavation and backfilling are phased to 10-6 E a X 10-7 } J m oC W IL W O H iZ 10-8 U LL W O V 10-8 make the operation continu- ous with relatively small quantities of new slurry re- quired to keep the trench full and to mix backfill. DESIGN The characteristics of SB slurry cutoff walls and their usefulness in both dewatering and pollution control may be evaluated by looking at the major design considerations of a slurry cutoff wall: permeability, strength, compressibility, compatibility, and durability. Permeability is usually the most critical design parameter. SB cutoff walls normally have a permeability less than 10 - 6 cm/sec and occasionally as low as 10 - 9 cm/sec. Improved impermeability of an SB wall is usually accomplished by increasing the natural day content or the bentonite con- tent. Early research on soil- bentonite indicated that the percentage of fines (material O o I I° 1 0 "\? °o D'App Silty F ° olonia and R ines COO yan, 1979 0 0 0 •• O \ • • ••• • •• % •o • (P o • • • • t• •o • • 1 0, \ • •• \ M •• • j \ D'Appolonia and Ryan, •• \ 1979 Clayey Fines •-CLAYEY FINES I I • O- SILTY FINES 10 20 30 40 50 60 70 80 90 100 0ib-200 SIEVE CONTENT passing the No. 200 sieve) correlated closely to permea- bility. Recent Geo-Con data has shown that the factors affecting permeability are more complex than originally thought and that the earlier correlation is unreliable. The overall effectiveness of the wall depends both on the SB backfill and on the filter cake which is formed as slurry es- capes into the surrounding soil. Although the filter cake (k < 10 - 9 cm/sec) is typically ignored for design purposes, it does contribute to the im- permeability of the "in situ" wall. The percentage of coarse- grained particles has the greatest effect on both strength and compressibility. As the percentage of coarse- grained particles increases, strength increases and com- pressibility decreases. While soil-bentonite backfill is semi- fluid when mixed, within a short time it takes a "set" and usually ends up at a strength and consistency similar to the native soil. In heavily traveled areas which must be used before the wall has had time to gain strength, it is usually advisable to pro- vide crossings over the SB wall. This will prevent damage to the wall and allow for unrestricted traffic. These crossings are usually con- structed of compacted clay, steel plates, geofabrics, and/or reinforced concrete, depending upon the application Under most conditions, the only strength requirement for SB walls is that they approxi- mately equal the strength of the surrounding soil. Gradual- ly, as the water content of the SB backfill comes to equilibri- um with the surrounding soil, this requirement is met. However, another considera- tion which affects both the strength and the stability of the installation concerns the amount of pressure that will build up behind the cutoff wall. In dewatering excava- tions the wall should be posi- tioned far enough behind the slope to ensure the stability of the excavation. In any permanent installation of a slung cutoff wall, the abil- ity of the wall to remain im- pervious to the underground environment is always an im- portant question. The materi- als involved are bentonite clay, mixing water, and soil. In situations which involve clean water, these materials are indefinitely stable and no reduction in permeability is experienced. However, if compatibility is in question because of the presence of certain pollutants, tests can be performed using the back- fill materials from the site and the actual groundwater. The bentonite mix can then be ad- justed to provide a satisfacto- rV solution for practically all cases. Typically, the durability of the SB wall is unaffected by changes in hydraulic condi- tions. In order to design the wall to resist piping, the gra- dation of the SB backfill can be evaluated by filter criteria. Usually, a well-graded backfill is preferred. Since the wall is buried, it is highly unlikely that the wall will dry out and crack. However, to prevent desiccation, the top of the wall may be capped with clay or extra SB backfill. CONCLUSION Slurry cutoff walls are gaining wide recognition for use in dewatering and pollution con- trol. They offer a cost effec- tive solution to many groundwater problems in new and remedial work. The econ- omy, convenience and posi- tive control of groundwater afforded by slurry cutoff walls is bringing them acceptance and application on an increas- ing number of projects. •