ABSTRACT
Another practice that is normally followed is to base the allowable pressure on the unconfined compressive strength, qu' of the rock obtained in a laboratory on a rock sample. A factor of safety of 5 to 8 is used on qu to obtain qa' Past experience indicates that this method is satisfactory so long as the rocks in situ do not possess extensive cracks and joints. In such cases a higher factor of safety may have to be adopted.
