Several classification systems have been developed during the last four decades to predict rock pressures for tunnel support design. Most of these systems are based on correlations of rock mass quality with capacity of supports used in corresponding tunnel sections. As such, these systems are likely to be conservative. It is, therefore, desirable to evaluate these classification systems by measuring actual rock loads on supports during the construction stages of tunnels. In view of the above, two tunnels located in the lower Himalayan region of India have been instrumented. Vertical and horizontal rock pressures estimated from different classification systems have been compared with the short-term observed values at four test-sections to support the use of the Q-system of Barton.


In den letzten vier Jahrzehnten wurden mehrere Klassifikationssysteme zur Vorhersage des Gebirgdrucks fuer Tunnelauskleidungen entwickelt. Die meisten dieser Systeme setzen die Gebirgsqualitat mit der Kapazitat der Tragglieder im betreffenden Tunnelabschnitt in Beziehung. Es ist anzunehmen, dass diese Systeme zu sehr auf der sicheren Seite liegen. Es ist daher wunschenswert, diese Klassifikationssyteme durch Messungen des tatsachlich vorhandenen Gebirgsdrucks auf die Tragglieder wahrend des Bauphasen des Tunnels zahlenmassig zu bestimmen. Unter Beruecksichtigung des oben Gesagten wurden in zwei Tunnel in der unteren Himalaya Region Indiens Messinatrumente eingebaut. Der nach den verschiedenen Klassifikati-onssystemen abgeschatzte vertikale und horizon tale Gebirgsdruck wurde mit Werten aus Kurz- Zeit-Beobachtunqen aus vier Testbereichen verglichen urndie Anwendbarkeit des Q-Systems nach Bartonzu stuetzen.


Durant ces quatre dernieres decades, plusieurs systemes de classification ont ete developppes pour prevoir des pressions rocheuses au moment du dimensionnement des soutenements des tunnels. La majorite de ces systemes s''appuie sur des correlations entre la qualite de la masse rocheuse et la capacite des soutenements utilises en comparant les sections des tunnels. De tels systemes paraissent etre conservateurs. II est donc souhaitable d''evaluer ces systemes de classification en mesurant les charges actuelles des roches sur les soutenements pendant les phases de construction des tunnels. C''est a cette fin que deux tunnels situes dans une region relativement basse de l''Himaya en Inde, ont ete equipes. Des pressions rocheuses verticales et horizon tales ont ete estimees a partir des differents systemes de classification et comparees aux valeurs observ~es pendant de courtes dur~es dans quatre sections d''essai afin de soutenir l''application du systeme "Q" de Barton.


Rock pressure has been catagorised as loosening, squeezing, swelling and creep pressures. Out of these, the present study is restricted to the loosening pressure only which is defined as the probable weight of the rock mass likely to fall out from the roof upon the supports. Basically, two approaches are prevailent to determine the loosening pressure for tunnel support design. In the first approach attempts have been made to correlate the description of rock mass with supports used. This is essentially an empirical practice and several classification systems have been developed along these lines. The second approach is based on instrumentation. This is called ''build as you go'' philosophy and is useful only during the construction stage. Classification systems are often used at the design stage. In view of the lack of adequate instrumentation it has not been possible so far to evaluate the various classification systems and the choice of a particular system has often been made on adhoc basis.

(Figure in full Paper)

An attempt has, therefore, been made in the present study to evaluate the various classification systems. Rock pressures predicted from these systems have been compared with the observed values in order to identify a sui table approach for tunnel support design under non-squeezing rock conditions. The case histories of Naneri- Bhali and Salal Hydel tunnels have been chosen for this purpose.

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