Behavior Of Closely Jointed Rock

It frequently happens in engineering and mining situations that "bad" rock is encountered which consists of rock broken up into blocks a few inches or less in diameter by a network of intersecting joints. These joints may be open, or filled by veins of solid material, or covered with weathering products. Essentially, the material consists of an aggregate of small interlocking blocks. In such circumstances, major joint systems usually can be recognised and studied, but there are probably many minor planes of weakness which may cause the blocks to be of almost random shape. It is extremely difficult to determine the properties of such materials. Measurements on small specimens give the properties of the constituent rock and not the jointed material. At the other extreme, the material has occasionally been tested as loose aggregate in large triaxial pots, but this gives no information about the strengthening effect of the interlocking of the small blocks of rock. One simple approach, which has been much used, is to regard the rock as randomly jointed on a small scale and to apply soil mechanics theory to it. In this case, the normal and shear stress at failure are related by [ ] where the coefficient of friction p and the cohesion c will be those of the individual joints. Even in very bad rock, it is usually possible to drill core in small diameters of 1 to 2 in. which will contain a number of joints, both open and filled. The testing of such joints provides one simple method of determining some of the properties of closely jointed rock and will be discussed in detail in the next section. Calculations based on Eg. 1 with values of c and µ obtained in this way may be expected to give a lower limit for the strength of structures in rock.