Page 77 - Numerical Analysis and Modelling in Geomechanics

58 A.A.JAVADI
Parameters A and B in the proposed equation (2.8) are related to a specific
make of shotcrete used in the Feldmoching tunnel in Germany. However, the
proposed procedure is generic and can be used for any tunnel driven under
compressed air with shotcrete as a temporary or permanent lining.
In this study, an existing theoretical relationship between the permeability of
soil to air and water has been used in the numerical model. This relationship does
not address all the parameters affecting the air permeability of partially saturated
soils. This needs further investigation. A study on the air permeability of
partially saturated soils is the subject of current research. However, again, the
presented procedure is generic and should an improved relationship emerge, it
can easily be incorporated in the numerical procedure.
The change in the shear strength of the soil in compressed air tunnelling has
been studied qualitatively and quantitatively. A method has been presented to
quantify the change in shear strength of the soil due to the flow of the
compressed air through the ground. The method is based on the integration of the
results of the testing procedure and the numerical analysis described above. The
numerical model can predict the change in the pore-air pressure in the ground
due to the air flow through the ground, from which the change in the state of
stress in the soil (represented by the stress state variables) can be calculated. A
series of tests can be carried out on the soil samples representative of the in situ
soil to study the shear strength behaviour of the ground qualitatively. The
equation for the failure envelope of the soil, which describes the change in the shear
strength of the soil with the change in the stress state of the soil, can be obtained
from results of the tests. By integration of the results of the numerical model with
the laboratory test results within the established procedure, the effects of the
change in the internal air pressure in the tunnel on the shear strength of the soil
can be estimated. This is particularly useful if a laboratory testing programme is
required for the investigation of a site for tunnelling under compressed air.
The results of the tests indicate that the application of internal air pressure
during the excavation of a tunnel, besides being an internal support to the
excavation, leads to an increase in the strength of the soil. However, increasing
the air pressure beyond a certain limit could lead to a decrease in soil strength
and cause a stability problem for the excavation. This is consistent with
tunnelling practice and is commonly referred to as a blow-out.
The method proposed in this study can be used to provide a more realistic
estimation of air losses during compressed air tunnelling. This is critical
information for planning and design of underground construction when
compressed air is considered as a method of controlling groundwater. The
proposed method can also be used to assess the risk of tunnel collapse and blow-
out. It improves current understanding of the interaction between this tunnelling
method and the ground.