Such water may come from precipitation or it may come from ground-water seepage originating from any of a number of sources, such as surface water percolating through the soil, underground streams, perched water moving over impervious soil strata, or adjacent permanently sat-urated soil areas where the excavationextends below the water table.
Some shallow excavations in relatively dry soil conditions may remain free of standing water without any intervention. But most excavations require some form of dewatering, or extraction of water from the excavation or surrounding soil. The most common method of dewatering is to remove water by pumping as it accumulates in pits, called sumps, created at low points in the excavation. Where the volume of ground-water flowing into the excavation is great, or with certain types of soils, particularly sands and silt, that may be softened by constant seepage, it may be necessary to keep ground-water from entering the excavation at all. This can be done either by pumping water from the surround-ing soil to depress the water table below the level of the bottom of the excavation or by erecting a watertight barrier, such as a slurry wall, around the excavation (Figure 2.26).
so that cannot be drained by a suction pump stationed at ground level, two rings of well points may be required, the inner ring being driven to a deeper level than the outer ring, or a single ring of deep wells with submersible force pumps may have to be installed.
|Figure 2.27 Well point dewatering. Closely spaced vertical well points connect |
to the larger-diameter header pipe.
An excavation is kept dry despite the close proximity of a large body of water. Two dewatering pumps are visible in the foreground. A pair of header pipes and numerous well points can be seen surrounding the excavation.
In some cases, well points may not be practical: they may have insufficient capacity to ensure that an ex-
cavation remains dry; restrictions on the disposal of groundwater may limit their use; reliability due to power outages may be a concern; or lowering of the water table may have serious adverse effects on neighboring buildings by causing consolidation and settling of soil under their foundations or by exposing untreated wood foundation piles, previously protected by total immersion in water, to decay. In these cases, a watertight barrier wall may be used as an alternative (Figure 2.26). Slurry walls and soil mixed walls (pages 40–45) can make excellent watertight barriers. Sheet piling can also work, but it tends to leak at the joints. Soil freezing is also possible. In this method, an array of vertical pipes similar to well points is used to continuously circulate coolant at temperatures low enough to freeze the soil around an excavation area, resulting in a temporary but reliable barrier to groundwater. Watertight barriers must resist the hy-drostatic pressure of the surrounding water, which increases with depth, so for deeper excavations, a system of bracing or tiebacks is required. A watertight barrier also works only if it reaches into a stratum of impermeable soil such as clay.
Otherwise, water can flow beneath the barrier and rise up into the excavation.