Dewatering
During construction, excavations must be kept free of standing water. Such water may come from precipitation or it may come from groundwater 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 saturated soil areas where the excavation extends 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 groundwater 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 groundFigure 2.25 Rock anchors are similar to tiebacks but are used to hold jointed rock formations in place around an excavation.
Figure 2.26 Two methods of keeping an excavation dry, viewed in cross-section. The water sucked from good points depresses the water table in the immediate vicinity to a level below the bottom of the excavation. Watertight barrier walls work only if their bottom edges
are inserted into an impermeable stratum that prevents water from working its way under the walls. water from entering the excavation
at all.
This can be done either by pumping water from the surrounding 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). Well, points are commonly used to depress the water table. These are vertical sections of pipe with screened openings at the bottom that keep out soil particles while allowing water to enter.
Closely spaced well points are driven into the soil around the entire perimeter of the excavation. These are connected to horizontal header pipes leading to pumps that continually draw water from the system and discharge it away from the building site. Once pumping has drawn down the water from the table in the area of the excavation, work can continue “in the dry” (Figures 2.27 and 2.28).
For excavations deeper than the 20 feet (6 m) or so that cannot be drained by a suction pump stationed at ground level, two rings of good points may be required, the inner ring is 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.
In some cases, well points may not be practical: they may have insufficient capacity to ensure that an excavation 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, the 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, the waterproof barrier can be used as an alternative (Figure 2.26). Slurry walls and mud mixed walls (pp. 40-45) can make excellent water-resistant barriers. Sheet piling may also work, but it can leak into the joints. Soil freezing is also possible. In this method, a series of vertical pipes, such as fine points, are used to continuously circulate the coolant at low temperatures to freeze the soil around the excavation area, resulting in a temporary but reliable barrier to groundwater.
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Watertight barriers must resist the hydrostatic 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.