TECHNICAL | DEWATERING


DEWATERING FOR OPEN FACE TUNNELS: INTERFACES AND BOUNDARY CONDITIONS


Dr Toby Roberts of WJ Group discusses how consideration of interfaces and boundary conditions inform development of conceptual models to characterise interactions between tunnels and aquifers, and so give focus to dewatering plans along with geotechnical and hydrogeological investigations


Open face techniques such as the sequential excavation method (SEM) combined with sprayed concrete lining (SCL) allow rapid tunnel or shaft advancement in stiff cohesive clay soils. In these conditions the exposed face typically remains stable for a few hours while the sprayed concrete is applied and reaches design strength before the next advance. Where a heading interacts with an aquifer resulting in


Unconfined aquifier


water bearing granular soils in the face there will be an obvious risk of groundwater inflow leading to ground loss and face instability. The benefits of open face tunnelling in terms


of speed and flexibility are such that there can be significant advantage in controlling groundwater levels sufficiently to allow their application in more marginal conditions. Interfaces and boundary conditions are


Confined aquifier Thin confined aquifier


Interface above crown


A D G


Right, figure 1:


Hydrogeological settings for an open face tunnel


Interface in


face aquifier B E H


cohesive soil


Interface below invert


C


F


I


10 | June 2023


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