Page 45 - Numerical Analysis and Modelling in Geomechanics

Chapter 2
Numerical and experimental study of

compressed air tunnelling
A.A.Javadi

Introduction
Compressed air as a means of dewatering the ground and assisting the
construction of tunnels has been used for over a hundred years and has been
successfully applied to a wide range of geological and construction
circumstances. The use of compressed air, particularly with shotcrete lining, has
proved to be, in many instances, an economic and effective measure in
dewatering the ground, minimising the settlement and stabilising the tunnel face.
Despite having been used for over a century, the adoption of a tunnel pressure
and the required volume of air to maintain that pressure, are currently based on
judgement and empirical formulae. The risks associated with leakage of air from
tunnels are not addressed by these methods. Such risks include surface
settlement, uplift of structures, tunnel collapse and blow-out. A principal reason
for these shortfalls is that the path and zone of influence of air leakage and the
distribution of the air pressure in the ground cannot be predicted. Furthermore,
the effects of the compressed air flow on the state of stress, strength and
settlement of the ground have not been understood. In view of the paucity of
scientific guidance for predicting these, a numerical model has been developed to
simulate the flow of air through soils and to study the consequences of using
compressed air in soils. In what follows, the details of the numerical model, its
validation and a numerical/ experimental procedure which has been established
to study the effects of the air flow on the shear strength of the ground will be
presented.

Compressed air tunnelling
When driving tunnels under compressed air, an appropriate air pressure is
applied in the tunnel to balance the groundwater and stabilise the face. The air
displaces the groundwater, desaturates the ground and flows through the ground
to the surface. The volume of air flowing through the ground must be
compensated by a continuous supply of air from air compressors. An accurate
estimation of the compressed air losses is essential at the planning stage to