Influence of large-scale inhomogeneities on a construction dewatering system in chalk

Large-scale anisotropy and inhomogeneities resulting from the presence of fissures, fractures and high-permeability zones can have a major impact on local groundwater flows, and affect significantly the performance of a construction dewatering system. A groundwater model for the HS1 Channel tunnel rail link Thames tunnel southern approach excavation is used to show that the observed performance of the construction dewatering system cannot adequately be explained unless such features are incorporated. While experience suggested that such features were likely to be present in this geological setting, at the design stage there was significant uncertainty regarding their scale. Groundwater flow modelling enabled the location and extent of the inhomogeneities present to be deduced in some detail. Features such as highly permeable zones might be identified by more thorough site investigation, including the more widespread use of pumping tests. However, the influence of anisotropy is harder to establish, primarily because its effect is most significant once a horizontal flow barrier (for example, a retaining wall) is in place, which is generally not the case during site investigation.