A dewatering well is designed to remove groundwater from a pit, excavation or any construction activities impacted by high water levels such as a slope or trench.
Efficiently designing a well is critical to the success of a dewatering project. The design impacts the number of wells required and prevents inoperable wells. A well consists of a well screen, and blank well casing surrounded by a gravel/filter pack.
Geotechnical reports/borings must be reviewed to determine how deep the well will be drilled and where the well screen should be placed. Hydraulic conductivity data provides information on the anticipated flow rate from the water bearing formations. The screen and filter pack design should be based upon data from the formation Sieve Analysis.
Filter packs maximize the flow of water into the well while minimizing sediment infiltration. Filter design is based upon the sieve analysis of the native formation. Uniform and rounded sands are best, with the filter thickness being 2-8 inches.
Several methods of filter pack design are utilized, which all use the native formation grain size to determine the grain size range of the filter pack. This helps ensure that the native formation will not pass through the selected filter pack and will also allow the required amount of water to pass through into the well.
Well screens are designed to hold back the filter pack. Therefore, slot sizes need to be large enough to allow approximately 10% of the filter to pass at a reasonable entrance velocity. The length and diameter of the screen are based on pump size, transmitting capacity required, and targeted aquifer zone. Screen types to be considered are PVC (Mill Slot), Wire Wrap (Continuous Slot), or Bridge Slot.
Water quality is essential for the design and maintenance of wells. The type of screen and casing utilized will depend on the water quality. Typical materials used in the construction of wells are PVC and Stainless Steel. Some constituents in the water (Iron, Manganese) may cause fouling of the well over time, requiring it to need rehabilitation.
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