Page 51 - Numerical Analysis and Modelling in Geomechanics

32 A.A.JAVADI

Figure 2.2 Illustration of the principle of pressure balance and over-pressure.

When shotcrete is used as tunnel lining, the permeability of shotcrete to air has
a significant effect on controlling the air losses. The permeability of shotcrete to
air is strongly governed by shrinkage cracks during the curing process which is a
function of its thickness, surface area, temperature and the construction process.
Laboratory experiments on small samples do not represent these aspects of the
behaviour of in-situ shotcrete. Therefore, at present, back analysis of field
measurement is probably the most reliable method to be used for this purpose
although complete and reliable field data on compressed air losses are rare.
At present, for estimation of air losses from a tunnel driven under compressed
air, a constant value is assumed for the permeability of shotcrete. As will be
shown later, the permeability of shotcrete varies significantly with time and this
variation has a significant effect on the air losses.

A numerical model for prediction of air losses
A numerical model has been developed to simulate the flow of air from a tunnel
face and perimeter walls. The numerical model has two components: one for
analysing the air flow from the tunnel face using the finite element method and
the other for the estimation of the air flow from the tunnel perimeter walls.

Analysis of air flow through tunnel face

The problem of the flow of air from the tunnel face through partially saturated
ground in compressed air tunnelling is a very complex boundary value problem