Groundwater Control in Complex Ground Conditions

Abstract

AbstractTwo fifteen metre diameter ten metre deep underground tanks were constructed immediately alongside each other by sinking bolted segmental caissons and casting the secondary lining in situ. Preliminary site investigation indicated difficult ground conditions with a high risk of base failure for excavation in the dry. Wet construction of the base slab would avoid base instability but not the problem of hydrostatic uplift of the partially completed structures.Further investigation revealed a complex sequence of interbedded sands, silts and clays of alluvial and glacial origins. The granular horizons comprised a small number of thicker continuous bands, one occurring immediately below the proposed formation levels for the tank floor slabs, and several thinner irregular bands grading laterally into silts and clays.Piezometers in the more critical granular horizons indicated artesian and sub-artesian groundwater levels. Temporarily disconnecting at ground level the piezometer showing artesian conditions and measuring the resulting water flow indicated a granular body of restricted size or limited recharge. The absence of any effect on other piezometers offered confirmation of isolated granular bodies.Despite obvious difficulties in predicting the effectiveness of a groundwater lowering installation the significant advantages of constructing in the dry over wet construction mitigated in favour of dewatering. Sixteen deep pumped wells were installed around the circumference of the tanks to form a figure of eight pattern for the wells. To reduce the risk of a base failure due to artesian groundwater in any lobes of granular material encroaching within the plan excavation but not intersected by the pumped wells, four internal relief wells per tank were installed.Successful construction was achieved, although a minor interruption to the pumping clearly demonstrated the prudence of the internal relief well installation.The paper comments on some aspects of BS 5930 in relation to the construction described.