Page 68 - Numerical Analysis and Modelling in Geomechanics

COMPRESSED AIR TUNNELLING 49

Table 2.5 Applied stresses and stress state variables for specimen 1 (test series 1).

Figure 2.12 Change in shear strength with increasing air pressure for specimen 1 (test
series 1).

and stress state variables associated with the five stages of loading for which the
air pressure was increased in steps. After consolidation, when there was no
further volume change detected by the back pressure controller, the specimen
was loaded under a constant strain rate of 0.042 mm/h. Readings were taken of
strain and deviator stress until the deviator stress passed the peak point. The peak
point was recognised as the point after which the deviator stress versus axial
strain curve became nearly horizontal (Figure 2.12). After that, the deviator
stress was reduced to zero in a few steps while recording the changes in axial
strain at the end of each step. Then the stresses associated with the next stage of
loading were applied and the consolidation and shear tests were repeated with the
new set of stresses. The procedure was repeated for each successive stage of
loading according to Table 2.5. At the end of each stage of loading, a part of the
axial strain was recovered while an irrecoverable part remained in the sample.