Resultados de la encuesta

There is some merit in considering groundwater control for tunnels and shafts separately from methods used for surface excavations (although cut-and-cover tunnels are effectively surface excavations and can be dewatered accordingly). Even though the great majority of tunnels are constructed below groundwater level, traditionally most tunnels through water-bearing ground are constructed without dewatering by pumping from wells.

Dewatering is sometimes used for shaft construction, although many shafts are constructed as flooded or ‘wet’ caissons. This method involves the shaft lining being constructed at ground level and sunk (by jacking or kentledge) into the ground, while material is excavated by grab from within the flooded caisson. This avoids the need to lower groundwater levels, but carries its own set of risks. It can be difficult to control excavation levels when grabbing through considerable depths of water, and problems have occurred when due to over-excavation, or when boulders are present.

Ground treatment is often used to exclude groundwater from tunnels.

Methods used include jet or injection grouting and artificial ground freez-ing, carried out from within the tunnel or from the surface. Any residual seepage into the tunnel is dealt with by maintaining some sump pumping capacity at the tunnel face.

Because many tunnelling methods construct a relatively watertight lining immediately behind the working face, only a small area of tunnel face is exposed to water-bearing ground. This confined environment has allowed methods to be developed whereby groundwater is excluded by maintaining the tunnel face at a fluid pressure more or less equal to the groundwater pressure.

The traditional way of balancing groundwater pressure is by compressed air working (Fig. 5.7(a)). This method, in use since the nineteenth century,

uses pressurized air in the tunnel face and working area (for some distance from the face) requiring the miners to work in air pressures above atmos-pheric, with concomitant health risks. Physiological effects on workers exposed to compressed air mean that the method can only be used up to 3.5 bar of pressure above atmospheric (equivalent to 35 m below water level), and only then with very careful medical controls on working arrangements. Most compressed air working is carried out at pressures of Figure 5.7 Pressure balancing techniques used to exclude groundwater from tunnels.

(a) Compressed air, (b) earth pressure balance tunnel boring machine.

less than 0.75 bar above atmospheric (known as low pressure compressed air); costs rise considerably above 0.75 bar (high pressure compressed air) due to the additional medical constraints. Compressed air working can also be used for shaft construction, when an air deck and airlocks are used to seal the top of the shaft.

An alternative method of balancing ground and groundwater pressures at the tunnel face was developed in the late twentieth century – the earth pres-sure balance (EPB) tunnel boring machine (TBM). EPB machines are a form of full face TBM that cuts the soil from the tunnel face, and forms a fluid or ‘earth paste’ (consisting of soil cuttings, groundwater and conditioning agents such as polymer or bentonite muds) in a plenum chamber behind the face (Fig. 5.7(b)). By controlling the rate at which the fluid or paste is extruded from the plenum chamber as the TBM excavates and moves for-ward, the face can be supported by a balanced pressure. The TBM driver and miners work in a ‘free air’ (i.e. atmospheric) environment behind the plenum chamber, avoiding the health risks of compressed air working.

Cases do sometimes arise when groundwater control by pumping is used in tunnel construction. Hartwell (2001) describes several case histories.

Applications of groundwater lowering for tunnelling include:

i For construction of shafts to launch or receive TBMs, and for con-struction of ‘soft eyes’ in shaft linings.

ii The entry or exit of TBMs into or out of shafts, portals, outfalls and other structures.

iii For construction of tunnel enlargements, step plate junctions, cross pas-sages, adits or other connections where the tunnel lining has to be breached temporarily.

iv To lower groundwater levels to allow compressed air working at reduced pressures (ideally less than 1 bar), for example to allow access to the working head of a TBM for maintenance purposes. The dewatering wells should be located with care to avoid compressed air escaping through the ground to the wells.

v To reduce pore water pressures in fine-grained soils such as silts or very silty sands, to reduce the risk of the soils liquefying as a result of vibra-tion from the machinery in the TBM.

vi To lower groundwater levels to below invert (to effectively ‘dewater’

the tunnel) to allow open face tunnelling methods to be used in other-wise unstable ground. This is sometimes necessary when a TBM designed for relatively stable soils present over most of the tunnel length has to traverse a short section of alluvial or glacial soils which may be present in a buried channel or other geological feature.

vii For recovery of damaged or inundated TBMs.

viii To control groundwater velocities to allow use of ground treatment methods (such as artificial ground freezing or grouting) in problematic conditions.

Where dewatering is used for tunnel works, wells drilled from the surface can be used, provided that access is available above the tunnel, and that there are no intervening service pipes between ground level and the tunnel.

If surface access is not available, it may be possible to drill small diameter wells radially out from the tunnel or shaft (or both). This technique was used in the 1990s on the London Underground Jubilee Line Extension and also on the Storebaelt railway tunnel in Scandinavia (Doran et al. 1995). An example of well arrangement is shown in Fig. 5.8.

Drilling out through the existing tunnel lining is a challenging task, and runs the risk of destabilizing or inundating the tunnel; it is essential that such works are meticulously planned and executed, and supervized by experienced personnel. Drilling must normally be carried out through stuff-ing boxes and blow-out preventers secured and sealed to the tunnel linstuff-ing.

Care must be taken to avoid loosing ground into the tunnel during drilling and subsequent pumping – this is a particular risk in fine-grained uniformly graded soils such as silts and sands. Experience suggests that in such soils if groundwater heads are in excess of around 10 m above tunnel invert, suc-cessful well installation will be very difficult.

References

Bell, F. G. and Mitchell, J. K. (1986). Control of groundwater by exclusion.

Groundwater in Engineering Geology (Cripps, J. C., Bell, F. G. and Culshaw, Figure 5.8 Typical pattern of dewatering wells drilled out from tunnel for localized depressurization to allow cross-passage construction (after Doran, Hartwell, Roberti et al. 1995).

M. G., eds). Geological Society Engineering Geology Special Publication No. 3, London, pp 429–443.

Cashman, P. M. (1994). Discussion of Roberts and Preene (1994). Groundwater Problems in Urban Areas (Wilkinson, W. B., ed.). Thomas Telford, London, pp 446–450.

Cole, R. G., Carter, I. C. and Schofield, R. J. (1994). Staged construction at Benutan Dam assisted by vacuum eductor wells. Proceedings of the 18th International Conference on Large Dams, Durban, South Africa, 625–640.

Doran, S. R., Hartwell, D. J., Kofoed, N. and Warren, S. (1995). Storebælt Railway tunnel – Denmark: design of cross passage ground treatment. Proceedings of the 11th European Conference on Soil Mechanics and Foundation Engineering, Copenhagen, Denmark.

Doran, S. R., Hartwell, D. J., Roberti, P., Kofoed, N. and Warren, S. (1995).

Storebælt Railway tunnel – Denmark: implementation of cross passage ground treatment. Proceedings of the 11th European Conference on Soil Mechanics and Foundation Engineering, Copenhagen, Denmark.

Glossop, R. and Skempton, A. W. (1945). Particle-size in silts and sands. Journal of the Institution of Civil Engineers, 25, 81–105.

Greenwood, D. A. (1994). Engineering solutions to groundwater problems in urban areas. Groundwater Problems in Urban Areas (Wilkinson, W. B., ed.). Thomas Telford, London, pp 369–387.

Hartwell, D. J. (2001). Getting rid of the water. Tunnels & Tunnelling International, January, 40–42.

Preene, M. and Powrie, W. (1994). Construction dewatering in low permeability soils: some problems and solutions. Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, 107, 17–26.

Preene, M., Roberts, T. O. L., Powrie, W. and Dyer, M. R. (2000). Groundwater Control – Design and Practice. Construction Industry Research and Information Association, CIRIA Report C515, London.

Roberts, T. O. L., and Preene, M. (1994). Range of application of construction dewatering systems. Groundwater Problems in Urban Areas (Wilkinson, W. B., ed.). Thomas Telford, London, pp 415–423.

Somerville, S. H. (1986). Control of Groundwater for Temporary Works. Construction Industry Research and Information Association, CIRIA Report 113, London.

Terzaghi, K., Peck, R. B. and Mesri, G. (1996). Soil Mechanics in Engineering Practice, 3rd edition. Wiley, New York, p 305.

Further reading – exclusion methods

Steel sheet-piling

Williams, B. P. and Waite, D. (1993). The Design and Construction of Sheet-Piled Cofferdams. Construction Industry Research and Information Association, CIRIA Special Publication 95, London.

Vibrated beam walls

Privett, K. D., Matthews, S. C. and Hodges, R. A. (1996). Barriers, Liners and Cover Systems for Containment and Control of Land Contamination.

Construction Industry Research and Information Association, CIRIA Special Publication 124, London, pp 59–60.

Slurr y trench walls

Jefferis, S. A. (1993). In-ground barriers. Contaminated Land – Problems and Solutions (Cairney, T., ed.), Blackie, London, pp 111–140.

Structural concrete diaphragm walls and secant pile walls

Puller, M. (1996). Deep Excavations: A Practical Manual. Thomas Telford, London, pp 97–116.

Jet grouting

Essler, R. D. (1995). Applications of jet grouting in civil engineering. Engineering Geology of Construction (Eddleston, M., Walthall, S., Cripps, J. C. and Culshaw, M. G., eds). Geological Society Engineering Geology Special Publication No. 10, London, pp 85–93.

Lunardi, P. (1997). Ground improvement by means of jet grouting. Ground Improvement, 1, 65–85.

Mix-in-place columns

Blackwell, J. (1994). A case history of soil stabilisation using the mix-in-place tech-nique for the construction of deep manhole shafts at Rochdale. Grouting in the Ground (Bell, A. L., ed.). Thomas Telford, London, pp 497–509.

Greenwood, D. A. (1989). Sub-structure techniques for excavation support.

Economic Construction Techniques. Thomas Telford, London, pp 17–40.

Injection grouting

Bell, A. L., (ed.). (1994). Grouting in the Ground. Thomas Telford, London.

Little, A. L. (1975). Groundwater control by exclusion. Methods of Treatment of Unstable Ground (Bell, F. G., ed.). Butterworths, London, pp 37–68.

Ar tificial ground freezing

Harris, J. S. (1995). Ground Freezing in Practice. Thomas Telford, London.

Compressed air

Megaw, T. M. and Bartlett, J. V. (1981). Tunnels: Planning, Design, Construction, Volume 1. Ellis Horwood, Chicester, pp 125–156.

Site investigation for