City Council Minutes 1976 08 10 - Laserfiche WebLink

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COLO. TECH. CENTER DR. ROBINSON CONT'D known what strengths these rocks are and how much coal you can leave to still support the roof. In a coal mine they go all the way in as far as they were going to mine with a series of drifts and then they mine back out so that they can rob the pillars as they went, or minimize the strength of the pillars and let the ground cave behind them. Now they would do this in many areas to recover just as much coal as possible; main haulage ways, main access shafts they might leave pillars for support. We have marked on this map on discussions that came up at the LUC meeting and it is mentioned in there. They talk about the angle of rock. They talk about the fact that they not only want you to note all the area directly above that mine as hazardous and possibly failing, but also an area equivalent to a 300 angle of drop otherwise the failure into this opening typically in most mines is not disburted. It comes in at an angle; we go between the vertical and the line of failure below the angle of draw and they recommend a 300 angle. That is a standard number for Wyoming. It worked beautifully in Rock Springs. It has never been measured in Colorado, we have never been able to find it. Where we have done our studies in Colorado it is in mines less then 150 feet deep and the angle has always been vertical There is absolutely no evidence for the acceptance of a 300 angle of rock, but this is one of the recommendations based on regional experience. This is an example of where you would have subsidence, where coal has been mined out and you have a drop of this section; now this amount of drop can never exceed the amount of coal that is removed. One of the men that I am working with in Wyoming, before he came to run the Trona mine was operating a coal mine in England and he was mining 5 miles out underneath the North Sea. He had only 500 feet of rock between him and the North Sea. And they had a regular 3 to 5 foot of subsidence of the floor in the North Sea They knew it, mined it, and designed for it and they didn't get wet. This is not an art, it is an engineering science that has been well worked. One of the ways they mind, they don't take it all out and they leave rooms and pillars. This is just an example of that. These are the conditions 10.