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the ground is heavily altered, the weakest shear strength is in the concrete. Based on the rock visible directly in front of the bulkhead it appears that the altered rock was mostly removed from the perimeter. Per ACI 318, the design shear strength of concrete is twice the square root of the compressive strength. Assuming a conservative compressive strength of 2000 psi, the design shear strength is 89 psi or 12,880 psf. Multiplying the perimeter of the bulkhead face (19.4 ft) by the length of 3 ft results in an area of 58.2 sf. Multiplying that by 12,880 psf results in a design shear load capacity of 749,602 pounds. The load on the bulkhead is a function of the water pressure on the face area. With a face area of 22 sf and a maximum water pressure of 45 psi (6480 psf), the driving force is 142,560 pounds. Dividing 749,602 by 142,560 gives a factor of safety of 5.3. Hence, shear is not a likely failure mode under any loading condition. Structural failure occurs when the concrete plug itself fails due to deep beam bending or shear failure through the reinforced concrete due to water pressure or earthquake induced water hammer. It can be avoided by making the bulkhead long enough and adding rebar reinforcement at both faces. The Times Mine Bulkhead structural strength can be calculated per ACI 318 deep beam analysis, as described in Abel 1998. This analysis compares the tensile bending stresses on air side face to the allowable concrete tensile strength. The rather involved calculations were done on a spreadsheet and the results are summarized below. For the proposed operational, current, design, and maximum operational conditions, a three-foot-thick unreinforced concrete bulkhead is acceptable. For the maximum load condition, rebar consisting of at least number 5 bars 12 inches on center each way would be required. Based on statements by CMC, rebar was used in the construction but the size and spacing are not known (no documentation available). Based on this, a structural failure is unlikely yet possible. Please note that these calculations do not take earthquake loading or water hammer effects into account. Should those be requested, D&A will have to research appropriate earthquake loads for the site. Structural Failure Condition Head(ft) Bulkhead Minimum Required Factor of Safety Length ft Bulkhead Length ft Operation 4.5 3 0.9 3.3 Current 15.2 3 1.6 1.9 Design 17.6 3 1.7 1.8 Max Op 22.1 3 2.0 1.5 Maximum 102.6 3 4.2 0.7 Concrete degradation occurs when the acidic mine waters chemically break down the concrete. It can be avoided by using sulfate resistant cement (ASTM C-150 Type V), pozzolans like flyash and microsilica, and possibly permeability reducing admixtures in the mix. It is also common to place lime in the tunnel upstream of the bulkhead to neutralize the acidic mine waters in the vicinity of the bulkhead. This technique is effective in the short term but may provide only limited beneficial use in the long term. For the Times Mine Bulkhead, there is no information on the concrete mix design used. Hence, one must assume that it contained standard Type-I cement. The quality of the water behind the -4 -