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bulkhead is to be low-pressure grouted to increase leakage resistance along <br /> the plug/rock contact. The length of the bulkhead is designed to prevent <br /> shear of the 3.000 psi concrete. The rebar is designed to resist any <br /> potential bending stresses that may develop. The simian depth of 127 ft <br /> below the surface is designed to prevent hydrofracturing. <br /> The probable effectiveness of the 14-ft long low-pressure grouted <br /> bulkhead designed for the Friday Lowden is indicated by a 6-ft thick ungrouted <br /> lightly reinforced concrete bulkhead previously installed. This initial <br /> bulkhead was installed 255 ft in from the Friday Lowden Tunnel portal, at a <br /> depth of 62 ft below the surface, and tested under 212 ft of water head. *The <br /> eventual plugging of three upper level portals in the mammoth Nine will <br /> probably resalt in the greater water head of 670 ft. The new low-pressure <br /> grouted bulbead is to be installed in competent rock 613 ft is from the <br /> Friday Lowden Tunnel portal, at a depth of approximately 150 ft below the <br /> surface. The new bulkhead is designed--tt-CuPport slightly pore than three <br /> (--)ties the hydraulic head applied to the original bulkhead. <br /> Introduction <br /> The design of concrete bulkheads to resist hydraulic pressure is mines is <br /> yell documented. first, it is necessary to have a long enough plyg to prevent <br /> leakage along the contact between the concrete and rock, around the outside of <br /> the bulkhead. Second, it is necessary to have a long enough plag to prevent <br /> shear failure either is the concrete or the rock. Third, the tensile bending <br /> stress that may develop at the downstream face of the plug mast either be kept <br /> below Americas Concrete Institute (Acl). allowable limits for plain concrete Or <br /> steel tensile reinforcement ant be added pear the downstream face. for <br /> thinner bulkheads, steel reinforcement for shrinkage and temperature stress <br /> crack control is advisable for both ends bf the bulkhead. near surface -- <br /> bulkheads designed to prevent acid mine drainase'have three additional <br /> requirements. The bulkhead oust be as close to the portal as possible to <br /> sinimize tunnel rehabilitation and bulk construction costs. The•depth of <br /> sear surface bulkheads must be sufficient to prevent formation breakdown at <br /> the design Wraulic preisure, i.e. hydrofracturing. Finally, data must be <br /> obtained for the rock and `rater quality to permit design. The piping through <br />