8.2.1 Open sumps or ditches

The open sump method is to collect the groundwater seeping onto the excavation bottom in an open sump，placed in the excavation bottom，by gravity or other natural means and pump the collected water out.If the excavation area is very large or the base is a long narrow shape，several sumps may be placed along the longer side or simply use a long narrow sump，which is called an open ditch.Both the open sump and the ditch are gravity draining methods，which can be the least expensive dewatering method if conditions are favorable（Powers et al.，2007）.In practice，the former is more widely used than the latter.

As shown in Figure 8.1，open sumps are usually placed near the retaining wall at the lowest ground or excavation surface.The depth of an open sump is generally 0.6⁃1 m.

Figure 8.1 Dewatering method：the open sump method

The open sump method is the most common and economical method of dewatering.However，its application is confined to permeable layers such as sandy and gravelly soils.Since the bottom of the open sump is lower than the excavation bottom，it will shorten the seepage path along which groundwater from outside seeps into the excavation zone and as a result the exit gradient on the sump bottom will be larger than that on the excavation surface.This fact may bring about sand boiling on the sump bottom，which has to be prevented.