Dewatering


Dewatering

Dewatering (pronounced de-water-ing) is the removal of water from solid material or soil by wet classification, centrifugation, filtration, or similar solid-liquid separation processes, such as removal of residual liquid from a filter cake by a filter press as part of various industrial processes.

Construction dewatering, unwatering, or water control are common terms used to describe removal or draining groundwater or surface water from a riverbed, construction site, caisson, or mine shaft, by pumping or evaporation. On a construction site, this dewatering may be implemented before subsurface excavation for foundations, shoring, or cellar space to lower the water table. This frequently involves the use of submersible "dewatering" pumps, centrifugal ("trash") pumps, eductors, or application of vacuum to well points.

Contents

Dewatering by deep wells

A deep well typically consists of a borehole fitted with a slotted liner and an electric submersible pump. As water is pumped from a deep well, a hydraulic gradient is formed and water flows into the well forming a cone of depression around the well in which there is little or no water remaining in the pore spaces of the surrounding soil. Deep wells work best in soils with a permeability of k=1x10−3m/s to 1x10−5m/s; the amount of drawdown that a well can achieve is limited only by the size of the pump.[1]

Deep wells can be installed in a ring around an excavation to lower the water level and maintain a safe, dry site. Several equations can be used to design deep well dewatering systems, however many of these are based on empirical data and occasionally fail. Practice and experience, along with a firm understanding of the underlying principles of dewatering, are the best tools for designing a successful system.[2] Some dewatering situations "are so common that they can be designed almost by rule of thumb".[3]

Deep wells are also used for aquifer testing and for groundwater drainage by wells.[4]

Dewatering by wellpoints

Wellpoints are small-diameter (about 50 mm) tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering pump. Wellpoints are typically installed at close centers in a line along or around the edge of an excavation. As a vacuum is limited to 0 bar, the height to which water can be drawn is limited to about 6 meters (in practice).[5] Wellpoints can be installed in stages, with the first reducing the water level by up to five meters, and a second stage, installed at a lower level, lowering it further.The water trickling between the deep wells may be collected by a single row of well point at the toe. This method ensures a much thicker width free from seepage forces.

Control of pore pressures

Roberts, T.O.L.; Roscoe, H., Powrie, W. and Butcher, D.J.E. (2007). "Controlling clay pore pressures for cut-and-cover tunneling". Geotechnical Engineering 160 (4): 227–236. doi:10.1680/geng.2007.160.4.227. ISSN 1353-2618.  </ref>

Tectonic dewatering

See also

References

  1. ^ CIRIA515 Groundwater control - design and practice. Spon. London. 2000.
  2. ^ The design of groundwater control systems using the observational method. TOL Roberts and M Preene. Geotechnique 44, No. 4, 727-734, December 1994.
  3. ^ On the analysis of dewatering systems. JK White. Proceedings of the Xth International Conference of Soil Mechanics and Foundation Engineering, June 1981.
  4. ^ ILRI, 2000, Subsurface drainage by (tube)wells: Well spacing equations for fully an partially penetrating wells in uniform or layered aquifers with or without anisotropy and entrance resistance, 9 pp. Principles used in the "WellDrain" model. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line: [1] . Free download "WellDrain" software from web page : [2] , or from : [3]
  5. ^ The adaptable wellpoint. JK White. Water Services, May 1982.

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Further reading


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