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• •k . . .�y Wig... • •r-..� <br /> Garrett and Campbell Pitt indicated that high-pres sure grouting of the <br /> rock adjacent to a bulkhead will result in a considerable increase in the <br /> allowable pressure gradient across the plug. However, high-pressure grouting <br /> is not an option for near surface plugging of old mine tunnels. Near surface <br /> high-pressure grouting will result in hydrofracing of the rock around the <br /> tunnel. <br /> The Society of mining Engineer's (SHE) Mining Engineering Handbook (1973) <br /> recommends. in section 26.7.4e 40 to 2S ft of plug length for each 1000 Psi-Of <br /> hydraulic head$ i.e. pressure gradients from 25 to 40 psi/ft. The reco&wnded <br /> concrete/rock grout pressure is 'a few hundred psi'. In practice, the <br /> grouting pressure oust be kept below the formation breakdown pressure to <br /> prevent hydrofracing. This limitation is particularly important for new <br /> surface bulkheads is order to prevent acid water comnwnication to the ground <br /> surface. <br /> The Dean uniaxial compressive strength of the igneous rock adjacent to <br /> the Friday Lowden bulkheads was measured at 1Se570 psi. Ws relatively <br /> !� massively jointed rock would be considered good rock. The only permeability <br /> iA this rock is along fractures. it would appear reasonable that the initial <br /> ungrouted bulkhead in the Friday Lowden Tunnel was functioning gear capacity <br /> at the 1S.3 psi/ft pressure gradient and could not be relied upon to withstand <br /> the planned•ultimate pressure gradient of 48.4 psilft. under the maMMUR <br /> planned head of 670 fte without excessive leakage. <br /> The length required for an ungrouted bulkhead for the Friday Lowden <br /> Tunnel is 19 ft for the 1S.3 psi/ft in the case of reproducing the original <br /> wWouted bulkhead to support the 670-ft head. A bulkhead length of S8 ft is <br /> required in the case of S psi/ft for an ungroated plug and 7 ft for a low- <br /> pressure grouted conCrete/rock contact following the Garrett ad Campbell Pit <br /> recommendations. A bulkhead length between 7 and 12 ft is required following <br /> the SHE recanoendations. The owner required that the bulkhead meet the <br /> requirement of as additional factor of safety of 2. It was therefore decided <br /> to design a 14-ft long bulkhead to resist the hydraulic pressure anticipated <br /> for the Friday Lowden. <br /> • -5- <br />