DEWATERING | TECHNICAL
two characteristics of the interaction between aquifers and excavations which are frequently misunderstood and which between them often dictate the strategy and control the viability of dewatering for tunnels and shafts. Marginal conditions for open face tunnelling often
comprise predominantly cohesive soils with one or more water bearing granular horizons which may have an upper confining layer or a lower low permeability horizon or both. Figure 1 illustrates a range of hydrogeological settings
with a planned tunnel in close proximity to a sand aquifer which is either unconfined, confined or a thin confined aquifer. In each case the interface between the low permeability horizon and the aquifer can be above the crown, within the face or below the invert. These scenarios illustrate several issues as follows. ● For Settings A, F, G, and I, the water-bearing horizon does not intersect the tunnel. The tunnel temporary works designer will need to check whether there is a requirement to control pore pressures in the water bearing granular layers during the tunnel works. If there is a need, it would be necessary to specify the target pore pressures for any horizon in the vicinity of the tunnel.
● For the remaining Settings, B, C, D, E, and H, the aquifer intersects the tunnel, and full reduction in pore pressure to the base of the tunnel or base of the water-bearing horizon will be required to maintain stability. For three of the settings, B, C, and D, the crown of the tunnel is in non-cohesive soil, and even if dewatered additional support measures, such as roof spiles, may be needed to support the crown during each advance.
● It is generally relatively straightforward to control groundwater levels by dewatering where there is a reasonable thickness (>1m or 2m) of the aquifer below the tunnel invert, as in Scenarios C, D, E, F, and I. This is because wells can be installed downwards from the surface or tunnel to target the aquifer with a target drawdown either to below invert, Scenarios C, D, and E, or perhaps above invert for Scenarios F and I where the cohesive soils present below invert are able to act as a plug and resist some uplift pressure.
● It is less straightforward to control groundwater levels where there is a sand/clay horizon, referred to as interface conditions, within the face or in very close proximity, Scenarios B and H. This is because the target drawdown level is the base of the aquifer, which generally requires close well spacing to reduce inevitable ‘overbleed’ of groundwater at the interface to tolerable conditions.
● For Scenarios A and G, the water-bearing horizon is above the crown. Whilst it may be feasible to control pore pressures it is important to appreciate that generally some residual groundwater will remain above the sand/clay interface.
Figure 2 shows the exposed face of a cross passage
excavation with London Clay above axis level and Harwich Formation/Lambeth Group Sands below axis, which extend to approximately 3m below invert level. This is a clay/sand interface, Figure 1 Scenario E, which is relatively straightforward to manage, in this case with an array of 15 surface deepwells readily achieving the 23m of drawdown required. Geotechnical engineers generally grasp the need
to assess the permeability of a target aquifer horizon and its significance in the design development of a groundwater control strategy. Some indications of the permeability of a granular horizon can be derived from sample descriptions, particle size distribution testing, natural gamma radiation measurements or in-situ variable head tests. Often less well understood is the significance of the aquifer boundary conditions, which can usefully be considered as the recharge or sources of inflow (and sometimes outflow) to an aquifer. For an unconfined aquifer, this is likely to be a combination of precipitation and leakage flow from streams, rivers, estuaries, ponds/lakes, or adjacent aquifers. Recharge to a confined aquifer is likely to be more
remote, perhaps from where the aquifer becomes unconfined. Confined aquifers – particularly thin,
Above, figure 2: Silvertown Tunnel cross passage CP7 face with London Clay/Harwich Formation - Lambeth Group sands interface SOURCE: NEIL MOSS, GALL ZEIDLER CONSULTANTS, AND RIVERLINX CJV
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