by Martin Prene, WJ Engineering Resources
This paper was first published in GE’s September 1993 issue.
Excavations in fine soil are often unstable when carried out below the natural groundwater level. Stability can be improved by reducing pore water pressures around the excavation. Construction dewatering is a technique for locally reducing pore pressures on a temporary basis by pumping from an array of wells. However, conventional wellpoint and deepwell systems, commonly used for dewatering in coarse soils, are ineffective in fine soils, and vacuum well systems are normally necessary. In this paper, data are presented from several projects involving excavations in low permeability Eocene soils such as the Barton, Bracklesham and Bagshot beds, and methods of estimating steady state flow rate are discussed. Although the particular strata described are only present in the south-east of England, these case studies illustrate several important points which are applicable to fine soils in general.