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6.1.2 Classification by Rock Load Factor (Terzaghi 1946)
Based in extensive experiences in steel arch supported rail tunnels in the Alps, Terzaghi (1946) classified rock mass by mean of Rock Load Factor. The rock mass is classified into 9 classes from hard and intact rock to blocky, and to squeezing rock. The concept used in this classification system is to estimate the rock load to be carried by the steel arches installed to support a tunnel, as illustrated in Figure 6.1.2a. The classification is presented by Table 6.1.2a.
Figure 6.1.2a Terzaghi’s rock load concept. For obtaining the support pressure (p) from the rock load factor (Hp), Terzaghi suggested the equation below, p = Hp γ H where γ is the unit weight of the rock mass, H is the tunnel depth or o r thickness of the overburden.
Attempts have been made to link Rock Load Factor classification to RQD. As suggested by Deere (1970), Class I is corresponding to RQD 95-100%, Class II to RQD 90-99%, Class III to RQD 85-95%, and Class IV to RQD 75-85%. Singh and Goel (1999) gave the following comments to the Rock Load Factor classificat ion: (a) It provides reasonable support pressure estimates for small tunnels with diameter up to 6 metres. (b) It gives over-estimates for large tunnels with diameter above 6 metres. (c) The estimated support pressure has a wide range for squeezing and swelling rock conditions for a meaningful application.
Table 6.1.2a Rock class and rock load factor classification by Terzaghi for steel arch supported tunnels
