Dewatering Principles
Construction dewatering involves removing groundwater from excavations to provide dry working conditions and prevent soil instability. The choice of dewatering method depends on soil permeability, excavation depth, groundwater depth, and project duration. The dewatering system must lower the groundwater table to at least 2 feet below the deepest excavation level to maintain stable working conditions.
Groundwater flow toward an excavation follows the principles of Darcy’s Law, which relates flow rate to hydraulic conductivity and hydraulic gradient. The radius of influence describes the distance from the excavation where groundwater levels are affected by dewatering. Pumping tests conducted before construction determine soil hydraulic conductivity and predict dewatering requirements.
Wellpoint Systems
Wellpoint systems are the most common dewatering method for shallow excavations. A wellpoint consists of a perforated pipe section with a screen and valve, installed by jetting into the ground. Wellpoints are installed at 3 to 10 foot intervals along the excavation perimeter, connected to a common header pipe and vacuum-assisted pump. The system can lower groundwater levels up to 15 feet in a single stage. soil compaction testing. excavation protective systems. groundwater control methods.
Multi-stage wellpoint systems are used for deeper excavations. Each stage lowers the water level incrementally, with pumps located at each stage level. The spacing between points, pumping rate, and number of stages are determined by soil conditions and excavation depth. Filter sand is placed around wellpoints to prevent fine soil particles from entering the system.
Deep Well Systems
Deep wells are used for excavations deeper than 15 feet or where wellpoints are not feasible. A deep well consists of a drilled borehole with a casing, screen section, and submersible pump. Well diameters range from 6 to 24 inches, with pump capacities from 10 to 500 gallons per minute. The wells are spaced around the excavation perimeter based on soil permeability and required drawdown.
The design of deep well systems requires knowledge of aquifer characteristics. The radius of influence determines well spacing, with typical spacings of 30 to 100 feet. Multiple wells create overlapping cones of depression that lower the water table across the entire excavation area. Continuous pumping may be required for the duration of construction activities below the water table.