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Sunnyside Gold Corp. Page 13 March 10, 1993 <br /> variability of concrete tensile strength. Reinforcing bars, with <br /> the higher, more consistent and dependable tensile strength of <br /> steel, permit the installation of a shorter bulkhead. A reinforced <br /> concrete bulkhead relies on the rebar to carry the flexural tensile <br /> stresses at the air-side of the deep-beam designed bulkhead. <br /> The reinforced concrete bulkheads were designed to support the <br /> factored dead and fluid loads (D and F) of the hydraulic pressure <br /> acting against the bulkheads. The ACI code requires that the dead <br /> and fluid loads be multiplied by 1.4 (ACI 318-89, Sections 9.2.1 <br /> and 9.2.5) . In addition, the design flexure strength of the <br /> reinforced concrete deep-beam structure was multiplied by a <br /> strength reduction factor of 0. 90 (ACI 318-89, Section 9.3 .2. 1) . <br /> The shear strength of the reinforced concrete deep-beam structure <br /> was multiplied by a strength reduction factor of 0.85 (ACI 318-89, <br /> Section 9. 3.2.3) . These factors represent actual factors of safety <br /> for design of 1.56 for flexure and 1. 65 for shear. Flexural <br /> loading of the bulkheads proved to be the critical criteria. This <br /> probably results from the design assumption that bulkhead loads are <br /> transferred to the tunnel walls solely in the horizontal direction. <br /> This in effect assumes that the roof and floor are not load <br /> carrying contacts. This is a conservative assumption, and the <br /> reinforcement bars are to be placed as a two-way mat, vertical as <br /> well as horizontal. A two-way path for load transfer will be <br /> provided but not included in the deep-beam strength calculations. <br /> Two shear analyses were performed for each of the bulkheads. <br /> The shear strength (V,) of the nominal 3000 psi design concrete mix <br /> (f'c) is presented in ACI 318-89, Section 11. 3 . 1. 1 as twice the <br /> square root of the design strength, 110 psi for 3000 psi concrete. <br /> One shear analysis was performed for the bulkhead completely within <br /> concrete and just inside the rock/concrete contact. The concrete <br /> strength controls because of the higher strength of the wall rock <br /> at all of the bulkhead locations, as presented in Appendix B. The <br /> strength of the weakest of eleven bulkhead wall rock samples tested <br /> was nearly 3-1/2 times stronger than the 3000 psi design concrete <br /> strength, whereas the strongest was almost 11-1/2 times stronger. <br /> The other shear analysis was performed for the deep-beam critical <br /> section within the bulkhead (ACI 318-89 , Section 11.8) . The <br /> critical section for shear is 0 . 15 times the bulkhead span (ACI <br /> 318-89, Section 11. 8 . 5) , 1. 95 feet in from the wall of the 13-foot <br /> wide American Tunnel, 1. 65 feet for the 11-foot wide F-Level Terry <br /> Tunnel, 1. 50 feet for the 10-foot wide F-Level Brenneman/Sunnyside <br /> Connection and 1. 35 feet for the 9-foot wide B-Level bulkhead <br /> between the Brenneman Mine and the Sunnyside Mine. The calculation <br /> procedure for the critical section shear strength is described in <br /> the 1989 ACI reinforced concrete code, Section 11.8. 7. <br /> Hitching, or notching, of the bulkheads into the tunnel walls <br /> has not been included in the plans and specifications. Notching <br /> was not included because the concrete strength is lower than the <br /> rock strength. Failure of a concrete filled notch in the walls <br /> would occur through the weaker concrete. Over-excavating within <br /> the bulkhead locations would necessitate installation of rebar <br /> shear reinforcing to replace the strength of the stronger rock <br />