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Bulkhead Design for AMD Page 15 October 27-29, 1998 <br />The mean 28-day concrete compressive strength, f~~, from tests <br />on the typical 3000 psi bulkhead concrete mix should not be less <br />than 9200 psi (ACI 318-95, Section 5.3.2.2 and Merritt, 1983, p <br />5-5) This fir should conservatively yield the design concrete <br />strength of 3000 psi, or higher, under the most adverse working <br />conditions. This high mean strength is necessary because it is not <br />possible to obtain the ACI specified number of compression test <br />cylinder tests. The ACI specifies a minimum of 30 test sets, each <br />set being the average of two tests from a concrete batch, to <br />evaluate concrete mix strength (ACI 318-95, Sections 5.3.1.1 and <br />5.6.1.4 and.ACI 219-77, Section 4.1). It is not possible to adjust <br />the mix proportions to the specified 28-day compression test data <br />in the field because it rarely takes one day to completely fill a <br />bulkhead. Regardless, compression test cylinders of the concrete <br />placed in the bulkheads should be prepared to verify the strengt`; <br />of the concrete. Curing of test specimens near the downstream <br />bulkhead face is recommended because that is a more realistic <br />bulkhead environment. <br />Bypass and Sampling Pipe Considerations <br />The corrosion resistance of bulkhead pipe penetrations must be <br />considered with respect to bulkhead life. It would be best if <br />'~ there were no i e enetrations. However <br />p p p pipe penetrations are <br />necessary to pass mine drainage through the bulkhead during <br />construction. In addition, some means is necessary to permit <br />release of impounded water, if required it some time in the future. <br />It is also wise to be able to monitor water pressure behind the ' <br />bulkhead in order to determine the elevation of the mine pool and <br />any unanticipated impoundment loss preventing planned design head <br />being achieved. <br />The corrosion rates and the resulting probable life of piping <br />of various stainless steels and pipe diameters should be evaluated. <br />This analysis must use the site specific acid mine water <br />concentration and temperature. For example, the maximum measured <br />surface corrosion rate (Cr) for the Carpenter 20Cb3 stainless steel <br />pipe used at the Friday Louden Bulkhead was less than 0.005 in/yr <br />for continuous exposure to the maximum solution concentration at <br />the maximum pressure and temperature. The testing was performed by <br />the pipe manufacturer. <br />The design problem is to estimate when the bursting strengths <br />of the corroded stainless steel piping will drop below the maximum <br />hydraulic pressure ( ). The calculation method used for the Friday <br />Louden pipe penetrations is shown in Equations 26 and 27. <br />P - zst <br />D <br />t _ PD <br />1 - zs <br />(26) <br />- 15 - <br />