Cross passages, connection tunnels, caverns and adits in soft ground have traditionally been constructed using a simple shield or timber heading with cast-in-place or bolted lining. These methods have increasingly been displaced by NATM or the sprayed concrete lining (SCL) method of tunnelling which was first developed in Austria in the 1960s, primarily for tunnels in rock. The technique exploits the inherent strength of the ground to allow rapid sequential excavation which is then supported by sprayed concrete to form a full ring before the next advance. This is particularly efficient for short tunnels or tunnels with complex geometry.

Both the traditional and SCL methods of tunnelling require the face and exposed ground to be stable during excavation until the lining is complete and reaches design strength, typically over a period of a few hours for each advance. This is relatively straightforward in cohesive soils, such as London Clay, but is potentially problematic where water-bearing granular soils or rock are exposed or near the open face. Any groundwater leakage through a sprayed concrete primary lining can also lead to difficulty in placing and completing membrane seals and associated secondary lining.

Measures to control groundwater during excavation are typically temporary works with the completed tunnels designed to be waterproof to avoid the need for ongoing pumping for the life of the structure. Alternatives to dewatering are physical exclusion methods including permeation grouting in fissured rock, jet grouting (which usually requires surface access) in soils, and artificial ground freezing.

The cost of these alternatives means that they are typically used only when dewatering is not viable due to identified unacceptable risks which might relate to off-site environmental concerns or settlement impact, or design risks such as potential for high flows or overbleed interface conditions.

Conventional surface dewatering techniques can be installed in advance of the face, avoiding both the artesian conditions and logistical challenges inherent in below-ground working. In-tunnel or in-shaft techniques avoid the need for surface access and can directly target any water-bearing horizons present (see figure 1). The most efficient approach can be a combination of surface and below-ground dewatering techniques.

For cross passages in an urban setting, surface access may not be available, in which case in-tunnel techniques may be the only option. Groundwater pressures will certainly need to be controlled and substantially eliminated in any water-bearing granular horizons exposed in the face. In addition, control measures may be required in any water-bearing granular horizons present within approximately a tunnel diameter of the face. The temporary works SCL tunnel designer needs to define the distance and target groundwater pressures required to provide face and ring stability during each advance.


This article is primarily concerned with open-face tunnels excavated in predominantly cohesive or low-permeability soils but where water-bearing granular or rock horizons are, or may be, present within or near the tunnel face. Surface access constraints, particularly in an urban setting, can restrict borehole investigations in the immediate vicinity of the heading and granular horizons may be intermittent. Good examples of these conditions are the Lambeth Group soils in the London Basin and the superficial glacial soils in Toronto, Canada.

Grouting and other common forms of physical cut-offs are not well suited to stabilising intermittent medium-to-low permeability silty/sandy soils where surface access is constrained or not available. Compressed air working or ground freezing could be options but often carry significant cost or programme penalties, or health and safety risks.

The setting outlined above presents several challenges for any dewatering strategy which are summarised in figure 1. Key points are:

  • Artesian conditions at tunnel horizon: conditions are said to be artesian where the groundwater pressure in a target horizon is above the well install level. In these conditions, inflow would occur as soon as the borehole reaches the target horizon and control measures are required to stabilise the bore, avoid ground loss and allow installation of the well and any filter.
  • Interface drainage: close well spacing is likely to be required to maximise drawdown where a sand/clay interface is present at invert or within the tunnel face.
  • Uncertainty: the combination of surface access constraints for borehole investigations and intermittent water-bearing granular horizons mean that it is often not feasible to define a detailed ground model in the immediate vicinity of the planned tunnel, so the dewatering strategy needs to include some investigation and monitoring, and be sufficiently robust and flexible to account for this uncertainty.

Where a main tunnel is driven by a closed-face TBM, it can be feasible to investigate ground and groundwater conditions at a planned cross passage or connection tunnel by drilling out through the grout holes in the TBM lining and installing transducers to monitor groundwater pressures where groundwater is encountered. Other in-tunnel investigation strategies include augur probe drilling, cone penetration testing (CPT) and natural gamma geophysical logging of cased boreholes.


Traditional surface dewatering techniques are summarised in table 1. The suction depth constraints for wellpoints mean they are typically not applicable to surface installations for tunnel works, leaving deep wells for medium-to-high permeability soils and ejector wells for lower permeability soils. Ejectors can develop a vacuum if the well liner is sealed which can enhance drainage of fine soils.

In practice, surface dewatering techniques are often preferred where access is available to avoid the logistical complications of in-tunnel wellpoint installation and pumping in an advancing tunnel. In an urban setting, surface access constraints, particularly for plumbing, monitoring and power cables, plus the inefficiency of targeting a thin intermittent granular horizon with surface wells, may favour in-tunnel techniques. Often a combined approach with surface dewatering, particularly at shafts where surface access should be available, and in-tunnel wellpoints, perhaps from a TBM tunnel, can be the most efficient dewatering strategy. The surface scheme provides for some initial drawdown allowing the in-tunnel wellpoints to be installed under a reduced groundwater head. The possibility of close spacing and longer screens in the target stratum for inclined in-tunnel installations allows efficient targeting of water-bearing horizons and sand/ clay interface conditions.


The typically shallow drawdown, requirement for close spacing and low individual well yields favour the use of wellpoints, which rely on vacuum pumps, for in-tunnel works (see figure 2).

Installation of wellpoints from a TBM tunnel into pressurised granular soils requires the use of an insert grouted into the lining fitted with a blowout preventor to control ground loss during installation. Lost-bit drilling is then used to drill the casing to the target depth, and the well screen and liner are installed as the casing is withdrawn (see figure 3).

Experience has shown that the quality of installations is improved as groundwater pressures are reduced such that bore control measures can be downgraded. For this reason, wellpoints are connected to the pumping system immediately following installation so that subsequent installations can benefit from the drawdown achieved. The wellpoint pumps may be mounted on a frame or platform to maintain access along the main drive (see figure 2), or if levels permit, can be located at a nearby shaft or cavern. Ejector systems can be used in place of wellpoints if the drawdown below the pump platform level exceeds about 5m-6m – which is the practical drawdown limit for a wellpoint system in fine soils. Discharge is generally directed to a tank or sump in the tunnel or shaft, and then pumped to surface for disposal to a water course or sewer.

Depressurisation of confined granular horizons present above or below the face can sometimes be achieved using passive relief wells. These are wells installed into the target stratum which are allowed to drain under gravity into the tunnel without pumping. In practice, allowing even small volumes of groundwater to discharge to the invert of a tunnel under construction is almost always bad practice – this can easily be avoided by using a wellpoint system to collect the water from each well/wellpoint and pump it away from the tunnel invert.

The grouted insert arrangement for installing wellpoints through a tunnel lining also allows the installations to be sealed and back grouted on completion of the works. If required, the sealing plug can be recessed to achieve a flush surface finish with the tunnel lining.


The ideal dewatering well array is a ring of wells around the structures where drawdown is required. For a laterally extensive aquifer which extends down to some metres below the target drawdown level, a relatively wide well spacing, say 10m-30m, may be appropriate. Providing access is available, surface wells can be a good option in this situation.

When targeting thin or intermittent horizons, particularly where a sand/clay interface may be exposed in the face, a closer spacing of 1m-3m may be required, and in-tunnel wellpoints are likely to be appropriate to achieve the necessary close spacing. Note also that groundwater control targets typically only apply to the unclosed SCL ring and associated face which advance as the tunnel works progress. Also, a large SCL cavern, greater than 10m, is excavated sequentially and can have 10 or more individual headings (see figure 4). This means that construction of an extensive complex structure, such as an underground station, may have multiple open faces across the works, each with its own local advancing drawdown targets.

The implications of this are that the optimum strategy is dependent on,

  • The hydrogeological ground model and importantly any uncertainties in the nature or sequence of strata,
  • The geometry of the tunnelling works, and
  • The sequence of tunnelling works which may have some flexibility.

An example of an in-tunnel dewatering scheme for a cross passage is shown in figure 5. A water-bearing granular horizon has been identified at invert which is targeted with an array of downwards wellpoints installed from one of the TBM drives. Groundwater pressure monitoring can be undertaken in the invert of the second TBM tunnel. The dewatering strategy would specify the initial wellpoint array with procedures for monitoring and toolbox measures, comprising supplementary wellpoint installations, which would only be installed if required. Probe drilling, shown in red, would then be carried out to prove stable conditions in advance of the cross-passage excavation. A longer cross passage, greater than about 15m, might require wellpoints to be installed from both TBM tunnels at each end of the cross passage. Where the location or extent of the granular horizon is uncertain, then preliminary probe drilling through the TBM lining grout-holes combined with a larger probing array would be required. These initial probe holes could be completed as wellpoints if water-bearing granular horizons are encountered.

A more complex dewatering strategy was required for two 10m diameter by 36m-long TBM launch adits from a drive shaft. The shaft was constructed with deep diaphragm walls to form a cut-off and so required minimal dewatering. The adits were constructed using SCL techniques as top heading, bench and invert with a water-bearing granular horizon at invert with 25m excess head. The initial dewatering array comprised a combination of surface wells and inclined wellpoints installed from the shaft through the diaphragm wall with the aim of facilitating construction of the first 12m of tunnel. Once the first 12m of the adit was complete, an array of wellpoints was installed from bench level to target the granular horizon to each side of adit. This allowed the top heading to be completed and the bench as far as 24m where an additional array of wellpoints was installed. The final wellpoint array was then installed once the bench was completed to the full length of the adit. This closed up the spacing ahead of the face to facilitate completion of the invert to the end of the adit.

For each phase of the works, the aim was to achieve appropriately spaced wells/wellpoints around the advancing face with closer spacing needed for the bench and invert which was sufficiently deep to approach the sand/clay interface. In-tunnel dewatering for single tunnels can lead to inefficient use of resources if the dewatering installation team is standing while the SCL team is advancing the tunnel and vice versa. With two adits under construction, the SCL tunnelling and dewatering installation/probing teams were able to exchange faces as the works advanced, minimising down time and maximising efficient use of resources.

In each case, the strategies outlined above included toolbox options for supplementary wellpoints and the potential to alter the tunnel construction sequence in response to groundwater level monitoring data. Contingency plans can also include options to dispense with wellpoints or a less efficient sequence of working if the monitoring data indicates that these are not required.

The dewatering strategy for a larger structure or cavern, where tunnelling is taking place on multiple faces (see figures 4 and 6) is more complex but may also present greater opportunity to exploit the sequence and programme of the works to advantage. Examples of strategies that have been used include:

  • Use of surface wells, particularly at shafts, to achieve initial drawdown to reduce artesian pressures to facilitate more closely spaced shaft or in-tunnel wellpoints as in the adit example above.
  • Installation of wells or wellpoints from a top heading, which has no or a lesser drawdown requirement than the bench or bottom heading, as in the adit example above.
  • Installation of wells or wellpoints from one tunnel or from a side-wall drift to facilitate construction of an adjacent tunnel or drift. Installation of wellpoints from the TBM tunnel for the cross-passage strategy above is just one example of this.
  • Pilot tunnels are often ‘blind’, meaning there is no access other than through the enlargement face which is very constrained and not available during an advance until the ring is complete and at design strength. At the same time, pilot tunnels can be the ideal location for wells or wellpoints and pumping equipment to facilitate the enlargement tunnel works. Service connections for power supply, discharge or monitoring cables can be installed to surface or to an adjacent tunnel to facilitate remote operation of a dewatering system installed in a blind tunnel.
  • Temporary facilities tunnels can be constructed in advance of the main works tunnel at higher level, such that dewatering is not required for construction. The facilities tunnel can then be used for well installation and to house pumping plant during construction of the main tunnels works below. This is a rather more extreme example of dewatering from a top heading to a facilitate construction of the bottom heading.
  • Existing historical, perhaps redundant, tunnels can sometimes be exploited for access for well or wellpoint installation and housing of pumping plant.

The options above are all strategies for exploiting the hydrogeological ground model, geometry of the works and programme to achieve a base case dewatering scheme for each phase of the works with sufficient options and flexibility to meet the risks identified. It is apparent that the sequence and programme for the tunnelling works should be closely coordinated with the dewatering strategy.