The concrete plug must be strong enough to support the hydrostatic pressure of mine water to seal the pit mouth of abandoned mines. In previous report, the test result of model plugs which were installed in hard rock was reported, and suggested that the conical plug is strong enough to support the pressure. In this report, the result of elastic F.E.M. analysis using the elastic constants obtained in previous report is reported. Furthermore, test results of model plugs installed in soft rock are reported. In order to explain these test results, elasto-plastic F.E.M. analysis considering tensile crack are performed.
The results of analysis obtained are as follows;
1. A remarkable tensile stress concentration is generated at the corner part of the rear end face of the plug.
2. Caused by this stress concentration, tensile crack is initiated and propagates to the free surface of rock, forming a conical fracture surface.
3. In the case of permitting the pressurized fluid to intrude along this fracture surface (experimental condition of previous report), stress field varies extremely, and elastic analysis become no more adequate to explain the fracture of the plug.
4. In the case of inhibiting the intrusion (experimental condition of this report), the formation of conical fracture conspicuously reduces the supportability of rock to the pressure.
5. Thus, the plug must be designed to reduce this stress concentration. One example of plug shape which can reduce stress concentration is also shown.
