Issue 27

X. Ran et alii, Frattura ed Integrità Strutturale, 27 (2014) 74-82; DOI: 10.3221/IGF-ESIS.27.09 79 - Equivalent plastic strain. The Fig.5 shows the influence of damage to equivalent plastic strain. The curves are plastic strain when existed damage or no, and existed damage but not consider the change of elastic modulus or permeability or cohesion. From the diagram we can see that the range of plastic area is always 0.4 w r which not changed in different conditions. But the maximum value of plastic strain will be affected by different conditions, especially the change of elastic modulus. For example, the maximum plastic strain will change from 0.04 to 0.15 if exists damage but not consider the change of elastic modulus. Figure 5 : The influence of rock damage on plastic strain. - Stress state: 20 days after drilling a borehole, the hoop stress (   ) and radial stress ( r  ) in the direction of the minimum ground stress is indicated in Fig. 6(a). Radial stress on the wall is reduced rapidly owing to excavation. 0.4 w r far from the wall, there is the maximum hoop stress and the maximum difference between hoop stress and radial stress. So combined with Fig.3 we can find that there is shear damage in the range of 0.4 w r from the wall. Now, the wall rock is also stable through damaged. If the pressure of drilling fluid or pore pressure changed, however, the shear damage will changed correspondingly. Once the value of shear damage reached to its limit ( max D  ), the rock collapsed. Therefore, it is necessity to analyze the damage evolution and the distribution of stress in damaged area. (a) (b) Figure 6. The distribution of hoop-radial stresses: (a) m c =12 MPa; (b) m c =50 MPa. If the rock strength is very large (assume m c =50 MPa), rock damage is not occur (Fig. 6(b)). There are only elastic zone and protolith zone in the wall rock. The maximum difference between hoop stress and radial stress is occurred on the wall. This means rock shear damage is appeared firstly on the wall, and then extended along the direction of the minimum ground stress. The rock collapse will be moved in a similar way. - Displacement. Fig. 7 shows the rock displacement along the direction of the maximum and the minimum ground stress. The points in the direction of the maximum ground stress all moved toward the centre of borehole. The maximum displacement is 3.06 mm at the point on the wall, and it reduces gradually away from the borehole. The points in the direction of the minimum ground stress all moved deviate from the centre of borehole. The maximum displacement is 1.24 mm at the point 0.4 w r far from the wall.