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Bulkhead Design for AMD Page 17 October 27-29, 1998 <br />SUMMARY <br />Near surface concrete tunnel bulkheads have successfully <br />impounded water, as indicated in Appendix B. Bulkheads can be <br />safely designed to impound acid mine water by considering <br />1) the leakage potential along the concrete/rock <br />contact, <br />2) the shear stress developed in the concrete and the <br />rock, <br />3) the bending moment resistance of the bulkhead, <br />9) the hydrofracturing potential, <br />5) the corrosion rates for the piping and concrete and <br />6) the earthquake for the bulkhead area. <br />Of the 22 bulkheads that I have worked on, five did not <br />successfully retain water. Three of those bulkheads were <br />unsuccessful because of construction deficiencies and two because <br />of unanticipated geologic conditions. One of the construction <br />problems was repaired and one of the geologically deficient <br />bulkheads redesigned and rebuilt to conform with the geology <br />encountered. <br />Low-pressure grouting of the concrete/rock contact is <br />recommended to increase the hydraulic gradient resistance between <br />the plug and the rock and to decrease the required length of the <br />bulkhead. The shear strength of the concrete and the rock must <br />exceed the perimeter shear stress developed by the maximum head. <br />The bending stress at the downstream face must either be kept below <br />allowable plain concrete design tensile strength or steel tensile <br />reinforcement must he placed near the down stream face to support <br />the potential tensile stresses. Tha bulkhead must be installed at <br />a depth sufficient to prevent hydrofracing the formation and the <br />loss of acid water to the formation joint system. Chemical attack <br />of the bulkhead concrete must be resisted and the corrosion rate of <br />the piping through the bulkhead must be balanced by sufficiently <br />thick pipe walls to provide for the required minimum bulkhead life. <br />The site specific earthquake loading hazard should be evaluated. <br />- 17 - <br />