“Rock defects and loads on tunnel supports” by Karl Terzaghi (1946) was a landmark paper in tunneling literature and, for many years, it provided the basis for the rational design of tunnels, particularly those constructed in North America. There are still many valuable lessons to be learned from this work and it is recommended reading for anyone seriously interested in the practical aspects of tunnel design and construction.
The “tunnel supports” discussed by Terzaghi were primarily steel sets and these were designed to support the “rock load” due to the weight of the broken ground resulting from the excavation of the tunnel. This concept is illustrated in Fig. 1 and Terzaghi developed a set of guidelines for estimating the rock load for different geological conditions.