Water Drawdown

the temporary lowering of the level or pressure of ground water when constructing foundation pits, mining mineral deposits, driving tunnels, or constructing subways. Water drawdown is achieved by various means depending on the depths of the workings being drained and on the permeability properties of the rock. Drawdown utilizing drill holes equipped with sucker-rod or centrifugal borehole pumps makes it possible to lower the water level to as deep as 300 m. Light well points permit drawdown to depths of 4-5 m when the length of the working member of the well points is up to 8 m. For drawdown to more than 5 m, well points are installed in several tiers. The best results using light well points are obtained in water-bearing sands of uniform composition and with seepage coefficients of 1-5 m per day. Drawdown by means of ejector pumps is possible up to a depth of 22 m. The pump consists of ejector well points, centrifugal pumps, and distributing and collector pipelines. Each well point has a water-jet lifter or ejector at the bottom of the installation. The ejector well points are recommended for use in water-bearing sands of uniform composition and structure that have a seepage coefficient of up to 0.1 m per day. A vacuum-concentric well drawdown device was developed in the USSR in 1967 for the complex conditions of draining alternating seams of water-bearing and water-resistant rock. The vacuum-concentric well device consists of an ejection well point with a filter jacket running the entire height of the water-bearing horizon. All water-bearing strata revealed by the well are drained by this device, and vacuuming is done by means of the ejector, which speeds the draining process. Open-drainage pumping equipment is used to pump ground water that flows into foundation pits at construction projects.

In cases where mining operations cut across an entire water-bearing stratum it is necessary to supplement draw-down with drainage.