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Bulkhead Design for AMD Page 5 October 27-29, 1998 <br />ungrouted 6-ft thick bulkhead in the Friday Louden Tunnel was 15,3 <br />psi/ft and did not leak when subjected to the measured 212 ft of <br />head. Chekan (1985) analyzed Garrett and Campbell Pitt's pressure <br />gradient data and produced a graphical version of their data. <br />Figure 2 presents a modified version of the data that indicates <br />that an ungrouted plug should be able to withstand a pressure <br />gradient of approximately 21.3 psi/ft at a factor of safety of one. <br />They also recommended a minimum factor of safety of 9 in good rock, <br />yielding a recommended maximum pressure gradient of just over 5 <br />psi/ft for average field conditions. Garrett and Campbell Pitt <br />(1961) reported unacceptable leakage along the concrete-rock <br />contact at 9.8 psi/ft when their ungrouted experimental bulkhead <br />was pressurized to 75 psi. Obviously, the effectiveness of <br />bulkhead concrete filling can vary widely, at least with respect to <br />construction practice. It would not be realistic to attempt to <br />build an ungrouted acid mine drainage bulkhead. <br />Garrett and Campbell Pitt indicated that pressure grouting of <br />the concrete-rock contact of their experimental bulkhead would <br />permit pressure gradients of 163 psi/ft without obvious leakage. <br />Applying a factor of safety of four produces a design pressure <br />gradient of over 40 psi/ft when the concrete-rock contact was <br />grouted. The indicated benefit from pressure grouting the <br />concrete-rock interface is an eight fold decrease in bulkhead <br />length required to prevent unacceptable leakage. <br />What constitutes "unacceptable" leakage is a function of the <br />bulkhead. The South African mining experience, reported by Garrett <br />and Campbell Pitt. (1961), indicates acceptable long .term leakage <br />along the concrete-rock contact and through the rock immediately <br />around the bulkhead ranges from 3 gpm to 13 gpm and that 17 gpm was <br />acceptable for short term leakage. Coogan and Kintzer (1987) <br />indicate that 33 gpm leakage was not acceptable for a hydro tunnel <br />but was acceptable when reduced to 11 gpm. <br />The leakage requirement for acid mine drainage bulkheads is <br />generally more restrictive. In every case the goal is to reduce <br />the flow to occasional drips at the bulkhead face. One contract <br />specification is to limit the quantity of inflow at or within a <br />specified distance from the downstream bulkhead face. The 1250 <br />Bulkhead in the Reynolds Adit had such a requirement. The Reynolds <br />Adit is in a weak, fractured and faulted rock formation. Before <br />construction several tunnel sections were dripping measurable <br />acidic groundwater. Limited formation grouting around the tunnel <br />at the bulkhead location was employed before bulkhead construction. <br />The purpose of the limited 6-foot radial formation grouting was to <br />lower the permeability of the blast damage zone immediately <br />adjacent to the tunnel walls. Obert and Duvall (1953) reported <br />rock damage 48 hole radii from spherical explosive charges. <br />Petykoph et al (1961) reported rock damage from 66 to 72 radii from <br />cylindrical explosive charges. Since tunnel blasting always <br />involves cylindrical charges the thickness of the blast damage zone <br />- 5 - <br />