Ejector dewatering systems are employed to control pore pressures and to lower groundwater levels to provide stable working conditions in excavations. As can be seen from the Roberts diagram, they are particularly suited to operating in fine soil conditions.
Ejector systems are able to extract groundwater and generate a high vacuum at the base of wells up to 50 m deep and of as little as 50 mm diameter. Vacuum drainage can provide dramatic improvement in the stability of silty fine sands and laminated silts and clays by the control of excess pore pressures. Ejector wells have been successfully installed in raking boreholes to dewater beneath inaccessible areas such as railway lines and canals. Ejector systems have a number of operational advantages over other dewatering techniques: they are flexible in level and layout, stable in operation and able to run dry without damage.
Supply pumps at ground level feed high-pressure water to each ejector well head via a supply main. The supply flow passes down the well and through a nozzle and venturi in the ejector. The flow of water through the nozzle generates a vacuum in the well and draws in groundwater. The supply flow and extracted groundwater mix, return to the surface and feed back to the pumping station via a return main. The return flow is used to prime the supply pumps and the excess water extracted is discharged by overflow from the priming tank. A single pumping station can be used to operate up to about 75 ejector wells installed in an appropriate array around the works.
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