Chinchilla., page-27

Hi rip,

The goof balls at the super pit (stupid pit) sometime miss old underground workings, they have a tendency to run their trucks over them fully loaded until they work it out, Ive seen less than 20m of coverage support these massive dump trucks. That's with a 100m plus drop directly under the 20m of cover as well. Point is its not hard to work out what you need from drilling. The drill brings up solid cores of rock. You put them in a tray, orientate them and log what type of rock and where. From these cores you can make a detailed map of the ground conditions, and accurately predict what you need as cover. If you couldn't work out if 200m of cover will be competent and not allow gas to escape then the geologists and geotechnical engineers are a complete sham as a profession. This would be highly unlikely as its not that hard to do.
Just for a example 20m of cover is considered plenty in most underground mine settings where people are working below. And temp is not really a issue, its more to do with the structures in the ground and type of material, possibly chuck in hydrology study as well, is the water dynamic, static and how much, water can be a big factor as well.
If you look at something called the Young's Modulus of the different rock properties you will find even softer rocks have decent support characteristic. And you can predict easily how rock, clay, sediment, sands will react to different stresses. 1000m or 200m of cover over UCG is not important, its doing the required Geotech engineer work so as to understand if the Geology is suitable for UCG or not.

71