Influence of Joint Dip Angle on Size and the Number of Key Blocks in the Tunnel

The numerical approach to probabilistic key block analysis is used to examine the effect of joint dip angle on the sizes and numbers of key blocks in the 25-feet square tunnel. The three joint sets with identical angles between them are modeled on a rock mass through which the tunnel excavated. A series of tests are performed by varying the identical dip angles of the three joint sets in the increments of 22.5 degrees under two different spacing distributions. Results of probabilistic key block analysis indicate that joint dip angle has a profound influence on the key block size and the number of key block formed. The steeply dipping joints generate a greater number of large key blocks and create a larger key block size than do the shallowly dipping joints for both spacing distributions. With this knowledge, the range of sizes and numbers of key blocks, the essential elements for support design in a tunnel, can be predicted.