City Council Minutes 1976 06 01 - Laserfiche WebLink

Home Browse Search

COLO. TECH. CENTER CaNT'n When we compile these maps we know what the elevation of this coal is. It ranges in here from around 260 to 280 feet. Most of these shafts give us elevations where they mined the coal. So, we drilled down just using a standard rotary bit, as used in water well drilling, down to possibly 40 feet above where we think the mine opening is. From here we go to the coring technique and we take out a solid core, or we hope a solid core, we drill by using a diamond bit and a core barrel and it takes out a solid core of rock. We drill that way all the way down through a mine opening and hope- fully into the floor. This gives us physical evidence of what that rock looks like in the ground, so that we can evaluate what has happened in that mine. You take out 4 - 7 to 10 feet of coal and find out what has happened. What caved in, or is there a big opening there? The roof can completely crumble and form a lot of rocks and fall into the opening and just from a chimney, which may have a lot of rock above it to subside or form subsidence at the surface. The blocks may interlock so that they can support the roof itself. The roof can bend just like a flexible board, a piece of plywood. the space that was occupied by the coal can be distributed up between all the little layers in the mine to some level above the mine. Or, it may go to the surface and you may have subsidence. In other cases, not in this particular mine, we have even drilled and found the opening. The opening was filled with water. We can look at the pillars. If we drill into a pillar and we have solid rock all the way down, drill through the coal if it is even and solid then we can assume that the coal and pillar are in sound shape. In other cases, such as this one, we just finished today, the pillar is completely crushed. It has just been mashed, just as though you sat on it, and yet we can identify the coal, we can identify the interval, but we know that that coal pillar has failed under the load of the rock from the face of the opening in trying to support the pillar. Based on this evidence, the mine map, the drilling, and I must say experience drilling several of these holes in this country, we have come to a conclusion as to what the probability would be of failure in any area. -10-