To prevent significant groundwater seepage into the excavation and to ensure stability of the excavation base or side slopes, it may be necessary to lower groundwater levels in advance of excavation. The temporary dewatering system is plumbed through a sediment tank with weirs so the turbidity meets Local Agency requirements before discharge into the municipality’s storm water or sewer. Condon-Johnson provides construction dewatering services using various methods, including Deep Wells, Well Points, and Eductors.
Deep well dewatering systems are the best choice for highly permeable aquifers when large volumes of water must be controlled.
Deep well dewatering uses drilled wells and submersible pumps to lower the groundwater table in the area surrounding the deep well. Pumping water from the deep well creates a cone of groundwater drawdown centered at the well. Typically these deep wells are installed around the outside of an excavation to lower the water level and allow an excavation to continue in dry, safe conditions.
Deep dewatering wells are typically installed using a top-drive rotary or percussive drilling method. After drilling the hole to depth and verifying connection with the target aquifer, a well screen is installed and surrounded by filter sand. The remainder of the annulus outside the well casing is backfilled with bentonite to form a seal. Then an electric submersible pump is lowered into the well and used to pump the groundwater to the header pipe and onto the discharge point for the system.
Well point dewatering is a versatile technology used for shallow excavations or aquifers where the water level needs to be lowered no more than 15 to 20 feet. Well point dewatering consists of a number of well points which are typically installed by drilling or flushing. If the excavation is shallow, the well points can be installed vertically from the ground surface. Alternately, they can be installed through the front face of a shoring excavation at a slight downward angle to deal with water encountered near the bottom of an excavation. The well points are then connected to a header pipe and a vacuum pump is used to create a vacuum in the header pipe that draws water up out of the ground.
Dewatering well points can be installed using the same equipment used for the installation of tiebacks which make them a very cost effective option where excavations encounter shallow water bearing layers that need to be dewatered.
An eductor dewatering system uses the same type of vacuum pressure at the well screen to pull down the ground water as the well point system. Instead of relying on a vacuum pump that is limited to a maximum lift of 2x-feet, the eductor system utilizes water pumped through a venturi valve at the bottom of any depth well to pull the water out of the formation. This pressure difference creates a vacuum that draws groundwater into the well where it combines with the circulating water and is carried through the return pipe to the central pump location.
The biggest advantage of this dewatering system is that there is no depth limit to the eductor well, whereas a dewatering point relying on a vacuum pump is effectively limited to 20 feet. Eductor well systems can be a very effective system to use in low permeability silty sands, silts, and clays where it is important to control the pore water pressure.