Statistical and hydro-mechanical coupling analyses on groundwater drawdown and soil deformation caused by dewatering in a multi-aquifer-aquitard system

Abstract

Groundwater withdrawn in a multi-aquifer-aquitard system (MAAS) may cause vertical cross-flow supplement and result in groundwater drawdown and soil deformation in both aquifers and aquitards. This study investigates the responses of groundwater and deep soils to dewatering in the MAAS through statistical and numerical analyses. Field data of ten pumping tests conducted in the MAAS, in Shanghai urban area are collected, and a statistical analysis for groundwater drawdown and ground settlement is performed. Field measurements indicate that the drawdowns are distributed linearly along radial distance in logarithmic coordinates. However, the ground settlements are disproportional to the drawdowns and show a trilinear distribution along the radial direction. To examine the inconsistent distributions of the groundwater drawdown and the ground settlement, representative dewatering models are developed numerically and solved explicitly by the coupled Biot’s three-dimensional consolidation theory. Numerical results of dewatering in MAAS are compared to those obtained by a non-coupling formula. Moreover, effect of a micro-confined aquifer on drawdown and soil deformation is discussed. It is found that soil compression in each stratum is closely related to the decline of the pore pressure, whilst small expansions are observed in early dewatering stages. Due to the hydro-mechanical coupling effect, the ground surface settlement near the pumping well obtained by the coupled numerical method is significantly smaller than that by the non-coupling formula. Owing to horizontal and vertical cross-flow supplement, an additional aquifer embedded in aquitard above the pumping aquifer can relief both the groundwater drawdown and ground surface settlement significantly.