Pumping or Dewatering is often required in excavations or trenches where groundwater (or water table) exists at a level above the bottom of the excavation. Ground water can be found in any soil type but is commonly found ‘perched’ in pockets of well drained soil (sand and gravel dominant soils) mixed in among poorly draining soils (silt and clay dominant soils). It is also typically found when excavations are required at depths below the natural groundwater elevations in a given area. The water table in a certain area can fluctuate based on the surrounding conditions and weather, and most predominantly the season. Also, dewatering may be required when old or abandoned storm water pipes are accidentally broken open during excavation. Lastly, dewatering will be necessary if open trenches or excavations are exposed to rainfall.
Dewatering is typically a responsibility of the contractor written into the specifications or contract, which may at times seem unfair as underground conditions are typically considered an unknown, even when geotechnical investigations are done. Such geotechnical investigation soil borings only target certain areas and usually state explicitly that samples taken are not indicitave of the conditions on the entire project area.
Water is removed from excavation primarily by pumping. Small amounts of water can be removed with a basic small pump or ‘trash pump’. Larger amounts of water will require larger commercial pumps and pumping systems.
Excavations with high water table or other water issues requiring a dry hole for an extended period of time may utilize a Wellpoint system that draws down the ground water elevation around the perimeter of the excavation low enough for the work to be completed.