REVISING TERZAGHI'S TUNNEL ROCK LOAD COEFFICIENTS

Field experience and measured rock loads have shown that the massive steel ribs required by Terzaghi's coefficients are overly heavy, conservative and costly. The steel ribs used in Europe in association with the NATM shotcrete and rock bolts are very slender and lightweight. The risk associated with revising Terzaghi's 1946 coefficients and providing more slender, lightweight steel ribs in tunnels is clearly not significant. Using the revised curve (which changes only rock classes 4, 5 and 6 and does not apply to the special cases of squeezing or swelling ground) costs might be reduced on USA tunnels to approach costs of tunnels now being built in Europe and Japan. For the covering abstract of the symposium see IRRD 284392. (TRRL)