Page 50 - Numerical Analysis and Modelling in Geomechanics

COMPRESSED AIR TUNNELLING 31
of tunnelling or surface conditions. Furthermore, they do not predict the zone of
influence of the air leakage, which is crucial when tunnelling in urban areas.

Deficiencies of the current practice

Although there has been extensive use of this tunnelling method there is no
theoretically rigorous method for predicting how the air, water and soil interact.
For example, seepage of water into the tunnel can result in groundwater lowering
and settlement, over-pressuring can cause ground heave, high air pressures or
large quantities of air can fracture or disturb the soil and reduce its strength. Also,
the air can travel laterally to adjacent excavations. Records of the Dartford
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Tunnel refer to high air losses issuing 800 m from the face and a blow-out 100
m from the face. These situations are of immediate concern to those constructing
the tunnel, but the threat to deep excavations supported by temporary works,
dewatering projects, basements, cuttings, other tunnels and surface structures
must also be considered.
An appropriate air pressure must be applied in the tunnel to balance the
groundwater pressure and, due to leakage of air from the tunnel, a constant
recharge of air must be supplied to maintain that pressure. If air leakage
increases, a greater volume of air must be supplied or measures taken to reduce
the air losses. However, in its most simplistic form, compressed air as a means of
groundwater control is justified on the premise that if the air pressure in the
tunnel is equal to the pressure of the water in the ground, a state of equilibrium
should be created (see Figure 2.2).
Figure 2.2 shows the relative pressures for an air pressure set equal to the
water pressure at the tunnel invert level. The resulting over-pressure at the tunnel
crown, P , is the difference between the two pressures. As long as there is no
r
significant flow of water towards the tunnel face, the ground should achieve a
satisfactory effective strength for stability at air pressures less than the full head
of water.
As well as changing the effective stress conditions, there can be a secondary
positive effect of reducing ground settlement. The air pressure provides a
support to the tunnel face and walls.
However, it is common for layers of soil with different geological origins and
properties to be encountered in driving tunnels. This inhomogenity is
compounded by the flow of air through the ground, which may cause some
changes in the structure, state of stress, properties and the strength of the soil.
Also a region of the soil is dewatered by the air flow, so the properties of the soil
in this region will be different to those in the saturated soil. These conditions
result in a very complex geotechnical environment.