1993-04-27_REVISION - M1977378 (2)

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L.R. Perino f Page 11 __.jrii 27, 1993 The Division requests (p 7) consideration of the pore pressure that will be present on fractures adjacent to the bulkhead after closure, as follows, "The shear strength analysis of the wall rock should further consider the likelihood that water will be flowing through the fracture system once the hydraulic head builds up behind the bulkheads." The pore pressure developed by water being forced through the rock around a bulkhead will of necessity decrease from the mine pool pressure to atmospheric pressure, i.e. hydraulic pore pressure will decrease from the waterside to the airside of the bulkhead. Garrett and Campbell-Pitt (1961, p 1291) attempted to locate any instance of spalling of the rock immediately downstream of a high pressure bulkhead and failed. They assumed that the pressure is able to dissipate at a rate sufficient to prevent a hydraulic pressure induced blowout. Similarly, they state, "there is again no evidence of the free faces of plugs having spalled under pore water pressure. They speculated (1961, p 1287) that the concrete bulkhead compresses under the applied hydraulic pressure spread laterally in accordance with Poisson's ratio, maintaining contact with the outward movement of the radially pressurized tunnel walls. They demonstrated that this was true by inserting tap points for water in the rock-concrete contact close to the face. They state, "It is apparent that the concrete does, in fact, retain contact with the rock since the rock/concrete can be made watertight by reasonable grouting". If the highest pressure water present at the waterside of a bulkhead can be prevented from penetrating the interface between the rock and the concrete and, thereby, reduce the effective stress at that critical location it is unlikely to enter other, less adversely oriented, fractures and reduce their resistance to shearing. Loading the bulkhead must produce a significant radial stress, as calculated previously, that must be overcome for water to enter the fracture system in the rock adjacent to the bulkhead. In addition, any radial hydraulic pressure within the natural or blast induced rock fracture system is unlikely to be present on only one side of a bulkhead. Any hydraulic radial pore pressure will be resisted by the elastic compressibility of the bulkhead concrete. Finally, the statement is made in item 8, "Either way, the potential for failure through the rock mass needs to be quantified." Failure of a bulkhead during filling of the mine pool requires a failure path. The most likely path is along the rock-concrete interface. However, no bulkhead failures have been reported as the result of movement along the rock-concrete interface. The shear strength across the interface has obviously exceeded the shear stress along the interface. The same must be true for the critical section concrete punching shear strength of the concrete. The description of the only documented structural distress of a bulkhead (Garrett & Campbell-Pitt, 1961, p 1289) appears to be a description of excessive bending of a reinforced