d) Dewatering is the process of draining an excavated area that is flooded with

Question

d) Dewatering is the process of draining an excavated area that is flooded with rain water or ground water before construction can start. It is carried out where there is an accumulation of ground water at the site where water or sewer lines have to be laid, or excavation for foundations and basements has to be carried out. Explain in detail these major factors that affect the choice of dewatering system: i. ii. The Type of Excavation The Geological and the soil conditions ICLO3-PLO1:C2] (10 marks

Explanation / Answer

1. Type of Excavation

A conventional well point system is found economical and safe when dewatering has to be conducted in a ground area where the water table level is at a lower depth. A lowering of the water table level to a depth of 20 or 30 feet require a deep well system or jet-educators to conduct the dewatering.

For excavations that are surrounded by cofferdams, either well point system or a deep well system or both in combination can be employed for dewatering.

A deep well system or a jet-eductor well point system is the best choice for dewatering that requires penetration into a field previous soil or rock. These are mostly implemented for the construction of deep shafts or tunnels or caissons. The choice between the two is based on the soil formation in the area and the rate of pumping that is desired.

Other related factors include:

2. Geological Conditions and soil conditions

The type of dewatering system and drainage system for an area is best dictated based on the soil and the geological conditions of the area. A conventional well system or well point system can be used if the soil below the water table of the area is deep, free-draining sand and more or less homogeneous in nature.

If the soil is highly stratified and there exists an impervious layer of rock, shale or clay then well point systems that are installed closely in space can be employed. Wherever the soil is in need of the relief of artesian pressure, deep wells or jet educators well points in few numbers can be installed.

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