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<br /> <br />Letter to Anne Beierle -3- October 2. 1996 <br />is the Division's opinion that 1471 psi should drive the design for <br />the bulkhead. <br />(b) It is stated in Section 2.3.1 of the TR-006 submittal that a <br />bulkhead length of 22 feet would be adequate and would provide a <br />safety factor of 1.25. There are several criteria that must be <br />considered in determining the required bulkhead length, and bulkhead <br />length is a primary consideration when estimating bulkhead costs. <br />The following criteria have been evaluated by the Division, and the <br />results demonstrate that a bulkhead substantially longer than 22 <br />feet may be required: <br />(i) Concrete shear on tunnel perimeter: <br />f6=2~=2 2500=100psi <br />(ACI 318-89, sec. 11.3.1.1) <br />L=1471(15.75)(17.s)/[2(ls.7s+17.5)100] <br />L=61ft. <br />where: fe=concrete shear strength (psi) <br />f~=concrete compressive strength (psi) <br />p=pressure head (psi) <br />h=tunnel height (feet) <br />1 =tunnel width (feet) <br />L=bulkhead length (feet) <br />(ii) Hydraulic pressure gradient: <br />The pressure gradient across a bulkhead is the hydraulic <br />pressure in psi divided by the length of the bulkhead in <br />feet. With low pressure grouting of the concrete-rock <br />contact, the allowable gradient is 41 psi per foots. <br />Without grouting the haulage tunnel bulkhead project would <br />be untenable. <br />L = 1471 psi / 41 psi per ft. = 36 ft. <br />From the foregoing analysis, the bulkhead may need to be as long 61 <br />feet, and may need to be even longer if it is not reinforced for <br />deep-beam bending stress. Design for stability under earthquake <br />loading may also increase the required length for the bulkhead. <br />1 Garrett, W.S. & L.T. Campbell-Pitt, 1961, Design and construction <br />of underground bulkheads and water barriers: 7th Commonwealth Mining & Met <br />Cong, S African Inst Min & Met, v3, p 1283-1301 <br />