water, control
Foundations often extend below ground-water level. In clay seepage may be negligible; but with such pervious foundation material as sand or gravel, it is impossible to excavate below water level without an inflow of material. This may produce instability in excavation slopes and settling of adjacent structures or cause a loss of bearing capacity in the foundation material. Drainage ditches sump pumps are used in such cases, but it is also recommended dewater the site with well points before excavation. Such well points space.3 to 8 ft. apart (depending on the permeability of the deposit) can usually lower the water tabl.
15 to 20 ft. The materials above the lowered water table can then be excavated effectively. Additional well points may be installed as excavation deepens, or wells larger diameter may be used, each with a submersible deep- pump. The lowering of the ground water table is important, as effective weight of the soil within the depth of the lowered water table increases. As a result that layer and the soil layers below it get an additional load and consolidation.
