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Bulkhead Design for AMD Page 6 October 27-29, 1998 <br />was estimated as approximately 9.5 feet, in this case. Formation <br />grouting beyond the potential blast damage zone was not undertaken <br />because the specific fracture flow channels were not known. After <br />water impoundment, groundwater inflow increased at a faulted tunnel <br />section about 100 feet downstream from the bulkhead. <br />Garrett and Campbell Pitt indicated that high-pressure <br />grouting of the rock adjacent to a bulkhead will result in a <br />considerable increase in the allowable pressure gradient across the <br />plug. However, high-pressure grouting is not an option for near <br />surface plugging of old mine tunnels. Near surface high-pressure <br />grouting could result in hydrofracturing of the rock around the <br />tunnel. <br />The length (L) of a low-pressure grouted bulkhead necessary <br />to meet the 90 psi/ft hydraulic pressure gradient criteria <br />necessitates the calculation of the maximum pressure head (p), as <br />follows: <br />HyW <br />P= iaa (1) <br />H - design water head <br />yw - water density <br />The required bulkhead length with low pressure grouting is: <br />L = no ft (2) <br />Perimeter Shear Strength Design <br />Bulkhead design to resist shear stresses resulting from water <br />impoundment involves evaluating concrete and rock shear strength <br />along the perimeter of the tunnel and shear in the concrete at the <br />critical section, as defined by the American Concrete Institute <br />(ACI 318-95, Sections 11.8.1 and 11.8.5). Critical section shear <br />includes the reinforcing bars, if present, in the designated <br />section and, therefore, cannot be evaluated until the bulkhead <br />reinforcing steel is tentatively selected. <br />The first requirement for evaluating bulkhead shear strength <br />at the perimeter of the tunnel involves testing the rock to see <br />whether the rock is stronger than the design concrete shear <br />strength. Typically, the shear strength, cohesion, of the rock <br />exceeds the design shear strength of the concrete. The measured <br />compressive strength of the intact latite porphyry that is adjacent <br />to the Ransom Tunnel bulkhead design example in Appendix A ranges <br />from 10,260 psi to 35,570 psi and the estimated shear strength, <br />cohesion, from approximately 2,500 psi to 8,900 psi. The concrete <br />design shear strength (fs), for the 3,000 psi concrete compressive <br />- 6 - <br />