Dewatering is associated with excavations where there is a need to work in dryer soils without being affected by the water table or moisture at a project site. A decision to perform dewatering at an excavation site is determined by the elevation of the bottom of an excavation, the height of the water table, the permeability and saturation of the soils, the degree of stabilization required along the edges of the excavation, and the duration and nature of the work being performed. The type of equipment and pumps selected for a dewatering effort are also determined by these factors.
As a general rule, the greater the soil permeability, the more likely it is that dewatering will be required, if the bottom of an excavation is lower than the water table and dry conditions are desired. Fine clay soils are very cohesive and impermeable and it is feasible that for shorter periods of time, excavations in these soils will not require dewatering equipment, since water seepage will be slow and limited. Conversely, course sands in an environment with a high water table would require dewatering if work in dry conditions were desired.
There are different types of dewatering equipment available, including construction utility pumps, well points, and deep well pumps. If dewatering is required, utility pumps, often referred to as “trash pumps”, are placed at the bottom of the excavation in a sump, or low elevation point. A series of prepared ditches within the excavation may be used to direct water to the sump. The pump intake hose is usually screened and filtered to prevent fine soil particles from being pumped out of the excavation along with the water. This is an important step as it avoids altering the soil properties near the pump intake. The removal of fine particles can result in settlement and side slope cave-ins during longer dewatering operations. Filters employed may be as simple as straw bales, gravel, or filter fabric. A discharge hose then carries the water from the bottom of the excavation to the surface elevation for disposal.
Well points are a series of individual pipes, commonly made of polyvinylchloride (PVC) , with screened cuts at their lower ends. They are placed in the soil in a line and function as a system along a desired length of an excavation. The well point pipes are then connected with a header pipe, or manifold, at the surface, and connected to a pump and discharge hose. The water table’s top boundary develops a scalloped appearance along the line of placement, as each well removes water from its point of influence. Well points work best in sandy soils. They are placed relatively close to the excavation site, and therefore a concern is the seepage pressure of the water being removed, which can remove fine soil particles and destabilize the excavation. Well point intakes are screened to mitigate this potential.
Deeper single wells and pumps can be placed to draw down the water table from a single point further below an excavation site, better ensuring that there will be no loss of fine soil particles. However, deeper and larger well systems and pumps are often more costly, as well as more expensive to place and operate, and appropriate soil characteristics across a wider area must exist.