Page 46 - Numerical Analysis and Modelling in Geomechanics

COMPRESSED AIR TUNNELLING 27
determine the plant capacity to supply a sufficient volume of air to maintain a
safe pressure whilst allowing for leakage from the tunnel. A realistic estimation
of air losses can reduce the cost of air supply requirements and preparation for
possible changes in air losses with tunnel advance, physical conditions and time.

Numerical modelling

A numerical model has been developed to simulate the flow of air through soils
in compressed air tunnelling. The model is based on finite element analysis of
the flow of air through the ground in compressed air tunnelling. The numerical
model can predict the zone of influence of the air flow, the distribution of the
pore-air pressure in the ground, the direction and velocity of the air flow, the
deformed shape and position of the groundwater profile and the air losses from
the tunnel face and walls. Field data from a tunnel in Germany has been used to
verify and calibrate the numerical model.
Furthermore, a relationship has been established to describe the variation of
the air permeability of shotcrete tunnel lining with time and the technique of
parameter identification has been used to determine the parameters of this
relationship. A genetic algorithm has been used in the optimisation procedure. It
has been shown that the time-dependency of permeability of shotcrete plays a
key role in controlling the air losses in driving tunnels under compressed air with
shotcrete as a temporary or permanent lining and this time-dependency should be
taken into account in design.

Effect of the air flow on shear strength of the ground
A procedure has been established to predict the changes in the shear strength of
the ground due to the flow of air. The procedure is based on integration of the
results of the numerical model and some concepts and theories of unsaturated
soil mechanics.
A programme of multistage triaxial testing was carried out in a modified
triaxial apparatus to study the effects of the flow of air on the shear strength of
the partially saturated ground. The results show the way in which the compressed
air flow helps to increase the strength and stability of the ground. The results of
the tests have been used to define a shear strength envelope, which can be used
to predict the change in the shear strength of the ground due to the flow of
compressed air and changes in the air pressure.

Factors affecting air losses
The total amount of air lost from a tunnel is the sum of three main sources:

(1) Air loss from the tunnel face
(2) Air loss from tunnel perimeter walls