818-6/AFM 88-32 and NAVFAC DM 7.3
b. Slurry walls. A cutoff to prevent or minimize seepage into an excavation can also be formed by digging a narrow trench around the area to be excavated and backfilling it with an impervious soil. Such a trench can be constructed in almost any soil, either above or below the water table, by keeping the trench filled with a bentonite mud slurry and backfilling it with a suitable impervious soil. Generally, the trench is backfilled with a well-graded clayey sand gravel mixed with bentonite slurry. Details regarding design and construction of a slurry cutoff wall are given in paragraphs 4-9g(2) and 5-55.
c. Concrete walls. Techniques have been developed for constructing concrete cutoff walls by overlapping cylinders and also as continuous walls excavated and
concreted in sections. These walls can be reinforced and are sometimes incorporated as a permanent part of a structure.
d. Steel sheet piling. The effectiveness of sheet piling driven around an excavation to reduce seepage depends upon the perviousness of the soil, the tightness of the interlocks, and the length of the seepage path. Some seepage through the interlocks should be expected. When constructing small structures in open water, it may be desirable to drive steel sheet piling around the structure, excavate the soil underwater, and then tremie in a concrete seal. The concrete tremie seal must withstand uplift pressures, or pressure relief measures must be used. In restricted areas, it may be necessary to use a combination of sheeting and bracing with wells or wellpoints installed just inside or outside of the sheeting. Sheet piling is not very effective in blocking seepage where boulders or other hard ob structions may be encountered because of driving out of interlock.
e. Freezing. Seepage into a excavation or shaft can be prevented by freezing the surrounding soil. However, freezing is expensive and requires expert design, installation, and operation. If the soil around the excavation is not completely frozen, seepage can cause rap-
id enlargement of a fault (unfrozen zone) with consequent serious trouble, which is difficult to remedy.
2-9. Summary of groundwater control methods. A brief summary of groundwater control methods discussed in this section is given in table2-l.