2019-01-04_REVISION - M1994117 (3)

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CMC Response: The Times Mine bulkhead is a single concrete bulkhead seal. This type of concrete seal is sometimes referred to as a parallel plug. The bulkhead is situated in a very narrow portion of the Times Mine cross-cut. It is three feet thick and spans the full width and height of the mine. It is estimated that there is at least twenty-four (24) feet of solid Boulder Creek Granite above the top of the concrete bulkhead. It was constructed in a location in the Times Mine adit where the Boulder Creek Granite is very solid, stable and competent. This location is also free of joints and fractures, and no timbers were required to hold the ground in this part of the mine. There was no need to create keyways to anchor the bulkhead, because of the very competent nature of the wallrock in this mine. Pressure grouting impervious Boulder Creek Granite would not have been feasible or necessary to prevent water seepage. There are no water pathways around the bulkhead, and no impounded water has ever been observed leaking around the edges of the bulkhead. A high strength, acid resistant concrete mixture was used to construct the bulkhead. The concrete that was used to construct the Times Mine bulkhead was mixed underground by hand and placed in wooden forms manually. There was no water present on the floor of the mine when the first concrete was placed into the lowermost wooden forms. The concrete was installed in successive horizontal layers, with the no delay greater than ten or fifteen minutes between mixing and placing the concrete in the wooden forms during the construction of the entire bulkhead. Steel reinforcing rods (rebar) were installed vertically and horizontally in the concrete layers to provide additional strength to the bulkhead. This process took more than five days to complete, which insured that each layer of the concrete was cold before another layer was added to the bulkhead forms. The concrete was worked into the roof cavity in the last wooden forms to ensure that there was a good seal around the back (roof) of the mine bulkhead. Any small, but noticeable voids around the outer edges of the outside of the bulkhead were also filled with concrete. It was installed and in place for more than six months before any water was pumped behind the bulkhead to make certain that the concrete bulkhead was completely cured. There were no large faults or fractures detected by Russell R. McLellan, Mining Geologist, when he mapped the Times Mine workings in 1947. The very narrow vein that the cross-cut was driven on was not developed for more than one-hundred and twenty-five (125) feet, so there are no old stopes or raises near or above the location of the bulkhead. When the Times Mine was reopened in 1987, the mine workings were examined by a number of people associated with the project. Every geologic and mine feature mapped by Russell R. McLellan was in place. No water inflows were observed, and the mine workings were essentially dry. The underground mine workings were re-surveyed to establish the location for the drill hole that would be used to pump water from the Times Mine. Several weeks were spent hauling all of the old track and any loose wooden timbers out of the mine workings before the concrete bulkhead was installed to ensure that no wooden debris would interfere with the extraction of water from behind the Times Mine bulkhead. 6