Dewatering Considerations

The pulling and insertion shafts should be dry during installation to avoid disturbing the sub-grade in the shaft. Therefore, if rain is expected or the pipe invert is slightly below the GWT in clay soil, installation of a dewatering sump pump at one corner of the shaft is necessary. Ditches crossing the shaft, sloped towards the sump, lined with filter fabric, and filled with gravel may be needed to direct the water towards the sump. If the pipe invert is significantly below the GWT in sandy or silty soils, a more elaborate dewatering system is recommended, such as a well point system, deep wells, or a large sump pump system. As the water level is drawn down, soil particles travel with the water towards the dewatering system, undermining utilities and structures. As is the case with every dewatering system, the contractor should take all necessary measures to prevent the migration of the soil particles from underneath nearby buildings and utilities. The discharge flow volume in this case is expected to be large; therefore, a suitable discharge in compliance with the EPA requirements is necessary If a sump pump is used, preliminary treatment of flow to reduce the sediments may be needed before discharging into water streams.