Page 74 - Numerical Analysis and Modelling in Geomechanics

COMPRESSED AIR TUNNELLING 55

Figure 2.19 Reduction in permeability of cement paste at early age (reconstructed from
20
Illston ).

Inspection of Figures 2.7 and 2.11 shows that after a tunnel length of about
300 m, when the tunnel face moved in the clay layer with a very low
permeability, although the tunnel length increased, the air loss from the tunnel
walls and the total losses decreased with time. This happened while the
perimeter area of the tunnel in the permeable ground remained constant, so the
usual engineering assumption of constant permeability of the shotcrete lining
would lead to constant air losses. As this was not the case, the explanation of the
decrease in the actual air losses is that the time-dependency of the permeability of
shotcrete as it cures plays a key role in controlling the amount of air losses.
The results of the experimental study show that the application of air pressure
increases the shear strength of the soil. As the air pressure in the tunnel
increases, the water content of the soil decreases (the soil becomes dryer) and the
shear strength of the soil increases. By increasing the air pressure, the strength of
the soil increases to a peak value beyond which further increase in the air