TAILIEUCHUNG - ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 3

84 Tunnelling in weak rocks Influence of shape of the opening Some empirical approaches listed in Table have been developed for flat roof and some for arched roof. In case of an underground opening with flat roof, the support pressure is generally found to vary with the width or size of the opening, whereas in arched roof the support pressure is found to be independent of tunnel size (Table ). RSR-system of Wickham et al. (1972) is an exception in this regard, probably because the system, being conservative, was not backed by actual field measurements for caverns. The mechanics. | 84 Tunnelling in weak rocks Influence of shape of the opening Some empirical approaches listed in Table have been developed for flat roof and some for arched roof. In case of an underground opening with flat roof the support pressure is generally found to vary with the width or size of the opening whereas in arched roof the support pressure is found to be independent of tunnel size Table . RSR-system of Wickham et al. 1972 is an exception in this regard probably because the system being conservative was not backed by actual field measurements for caverns. The mechanics suggests that the normal forces and therefore the support pressure will be more in case of a rectangular opening with flat roof by virtue of the detached rock block in the tension zone which is free to fall. Influence of rock mass type The support pressure is directly proportional to the size of the tunnel opening in the case of weak or poor rock masses whereas in good rock masses the situation is reverse Table . Hence it can be inferred that the applicability of an approach developed for weak or poor rock masses has a doubtful application in good rock masses. Influence of in situ stresses Rock mass number N does not consider in situ stresses which govern the squeezing or rock burst conditions. Instead the height of overburden is accounted for in equations and for estimation of support pressures. Thus in situ stresses are taken into account indirectly. Goel et al. 1995a have evaluated the approaches of Barton et al. 1974 and Singh et al. 1992 using the measured tunnel support pressures from 25 tunnel sections. They found that the approach of Barton et al. is unsafe in squeezing ground conditions and the reliability of the approaches of Singh et al. 1992 and that of Barton et al. depend upon the rating of Barton s stress reduction factor SRF . It has also been found that the approach of Singh et al. is unsafe for larger tunnels B 9 m in squeezing ground .

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