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Bulkhead Design for AMD Page 7 October 27-29, 1998 <br />strength (f~) is 110 psi, specified by the American Concrete <br />Institute as follows: <br />fs= 2 f~ =2 3000 = 110 psi (ACI 318-95, Sec 11.3.1.1) <br />(3) <br />Obviously, the concrete is the critical design component for <br />perimeter bulkhead shear at the Ransom Tunnel. This not always the <br />case as was the case for the best ground in the Chandler Tunnel at <br />the Summitville Mine, as shown on Figure 3. <br />When concrete design shear strength (fs) is less than the rock <br />cohesion (c,1, the bulkhead length (L) needed to support the <br />maximum perimeter shear stress from the application of the maximum <br />pressure head (p), for a rectangular tunnel cross section with a <br />height of (h) and width of (Q) is: <br />phP <br />When rock cohesion (cr) is less than the concrete design shear <br />strength (fs), rock cohesion replaces concrete design shear <br />strength in Equation 4. <br />Plain Concrete Deep Beam Bendincl Stress Design <br />The American Concrete Institute's "Building Code Requirements <br />for Reinforced Concrete (ACI 318-95)" are recommended for design <br />because the bulkheads are analogous to reinforced deep-beam <br />concrete structures and because of the inherent conservatism of the <br />code. It is difficult to obtain good adhesion between a concrete <br />bulkhead and the roof and floor of a tunnel. The difficulty lies <br />in completely cleaning of the floor and keeping it clear of mud and <br />rock until the concrete is poured and in completely filling all the <br />voids in the roof, even with low-pressure grouting. The deep-beam <br />bulkhead should be conservatively assumed to act only one-way, <br />between the walls (ribsides) of the tunnel. However, two-way <br />reinforcing steel should be provided in bulkhead design to transfer <br />some load to the tunnel roof and floor despite the difficulty in <br />achieving intimate contact with the roof and removing all the loose <br />rock from the floor. The one-way design assumption in effect <br />produces a potential factor of safety of two, provided the more <br />difficult roof and floor contacts between the bulkhead concrete and <br />the rock are actually achieved by the recommended low-pressure <br />contact grouting. <br />The recommended deep-beam bending analysis is based on a <br />uniformly-loaded beam supported by the tunnel walls. This <br />conservative design approach can be further justified by the <br />- 7 - <br />