Case studies of construction dewatering in fine grained Eocene soils : Preene, M Ground EngngV26, N7, Sept 1993, P23–27

Abstract

This paper presents several case studies of construction dewatering, involving excavations in low-permeability Eocene soils such as the Barton, Bracklesham and Bagshot beds. It discusses methods of estimating steady state flow rate. Although the strata described are present only in south east England, the case studies illustrate several important points that apply to fine soils in general. For fine soils, the principal drainage mechanism is consolidation of saturated soil, not replacement of pore water by air. Although the soils' moisture content can be reduced by only a few percent, excavations can be made considerably more stable by controlling pore pressures. This is illustrated by applying the theory of effective stress. It was found that particle size distribution (PSD) tests overestimated permeability by up to two orders of magnitude, compared with values back-calculated from recorded flow rates. The maximum in situ test result was usually a better estimate. For the case studies, permeability values, back- calculated from the performance of construction dewatering systems, were usually between 0.000001 and 0.00004 m/s. One case study showed that close sources of recharge can increase flow rate by an order of magnitude. Here, the Eocene soils may be overlaid by permeable gravels, so that recharge from gravels may be affected by minor geological features that must be assessed carefully in design.