TECHNICAL | DEWATERING
Barrier A boundary
Fine sand
Right, figure 3:
Confined aquifer boundary conditions for an open face tunnel
Distant B boundary
Fine sand
Proximate C boundary
Fine sand Gravel channel
confined aquifers – can sometimes have very
remote or limited sources of recharge which can significantly simplify the required extent and capacity of a groundwater control scheme. This is illustrated in Figure 3 which shows a sand horizon interacting with a tunnel heading. The sand horizon is the same level and thickness for each case but with different boundary conditions. Figure 3A shows an aquifer of limited lateral extent
so has barrier boundaries with minimal recharge; once drawn down inflows will be minimal and recovery of pore pressures when pumping is stopped will be slow. Figure 3B shows an extensive sand horizon with a
distant source of recharge which will have persistent inflow and faster recovery than 3A. In Figure 3C the sand horizon links to a thick gravel
horizon providing a close and potentially significant source of recharge. In this case inflows will be greater and it may be challenging to achieve acceptably stable conditions at the interface. It is important to appreciate that boundary conditions
cannot be assessed from a geotechnical investigation comprising logged boreholes, soils samples, laboratory testing, piezometer monitoring, and in situ variable head tests or packer tests.
12 | June 2023
Aquifer boundary conditions can only be assessed
from a hydrogeological investigation, which includes an appropriately designed pumping test comprising one or more test wells and an array of piezometers. The piezometer response to pumping on the wells is controlled both by the aquifer parameters, primarily the permeability, but also, providing the pumping stresses the aquifer sufficiently, by the boundary conditions. Had the cross passage in Figure 2 been a few metres
lower in the geological sequence then we might have been faced with the potentially more challenging Figure 1 Scenario H with a clay/sand interface in the face. A possible strategy for this situation would be to scale back the surface well scheme and introduce in-tunnel wellpoints targeting the sand/clay interface each side of the cross passage. Note that the presence of the TBM drives at each end of the cross passage may act as a cut-off boundary for the aquifer between, curbing inflows and simplifying the dewatering arrangements. In summary interfaces and boundary conditions are
relatively simple concepts which can help develop the conceptual model, characterise the interaction between a tunnel or shaft and aquifer, and provide focus for a dewatering strategy and associated geotechnical and hydrogeological investigation.
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