In any underground construction project, accounting for groundwater is a must. If groundwater is present, dewatering applications must be initiated to lower the groundwater table and ensure a dry and safe working environment.
The art of dewatering is a special talent that comes with experience, and it is important to choose a qualified dewatering contractor with knowledge of dewatering and soil stabilization techniques to achieve the task. By selecting an experienced dewatering contractor who is familiar with the dewatering equipment being employed, the guesswork is taken out of the equation and adequate information can be gathered and assessed to ensure proper application.
That information includes:
- Nature of the soil
- Groundwater hydrology
- Size and depth of the excavation
- Proposed methods of excavation and ground support
- Proximity of existing structures, their depth and type of foundation
- Design and function of the structure being built
Once all information is gathered and the jobsite has been surveyed, a dewatering plan is developed. Wellpoint dewatering is considered one of the oldest methods of practical pre-drainage and is the process used to temporarily remove groundwater from the soil in a localized area to accomplish the construction of a pipeline, foundation, or any other below-ground structure, or to perform soil remediation. It is the recommended method for dewatering an excavation up to 18 ft deep for a single stage.
Wellpoint systems are known for their ability to dewater water-bearing soils such as sand and gravel and are used extensively in work like soil stabilization, pressure relief for dams and levees, and for water supply for municipalities and industrial plants. In construction work, a wellpoint system is usually a series of properly sized wellpoints, surrounding or paralleling the area to be dewatered, and connected to a header pipe by risers and swing hoses. Depending on the volume of water being handled, more than one wellpoint system may be needed.
Thompson Pump and Mfg. Co., one of the nation’s leading manufacturers of heavy-duty, diesel-powered bypass and dewatering pumps, recommends two types of wellpoint pumps – rotary and piston. With rotary wellpoint pumps, a vacuum is created by the rotation of a pair of rotors inside the pump housing. As the rotors mesh a cavity is formed at the inlet of the pump drawing liquid into the housing. As the rotors rotate, the liquid is carried in the pockets formed between the rotors and housing where the meshing of the rotors compresses the liquid and forces it out of the discharge port.
The double-acting piston pumps convert rotational energy applied to the pulley to linear motion to a set of pistons via two conically milled gears which are mounted on either side of the crankshaft. A vacuum is created inside the cylinders from the motion of the pistons. On the piston upstroke, the suction valve opens and the delivery valve closes allowing the water to be drawn into the cylinder. On the down stroke, the force of the piston causes the suction valve to close and the delivery valve to open forcing the fluid in the cylinder out through the discharge.
Additional types of dewatering systems include:
Jet Ejector System
- Used to dewater or stabilize soils with low permeability that can be drained by gravity flow (some silts, sandy silts)
- Usually for deep excavations where small flows are required
- Supply water down a pipe to create a vacuum and then pump up the ejected water and groundwater
- Not limited by depth, when conditions are right
Vertical Sand Drains
- Used to conduct water from an upper stratum to a lower more pervious stratum
- Not effective in highly pervious soils
- Used to dewater or stabilize soils that can be drained by gravity flow
- Usually for large, deep excavations where large flows are required
- Used to stop or minimize seepage into an excavation
- Requires installation down to an impervious formation