The principles of the Norwegian Method of Tunnelling NMT have now been used for about 40 years including the design and construction of tunnels for a wide variety of purposes and under very different rock mass conditions. This has involved the use of the Q-system to describe rock masses and to give recommendations of permanent rock support.

During all these years, unlined pressure tunnel projects have also been built in weak rock masses with the presence of swelling minerals, slaking, and water sensitive rocks. These rock mechanical scenarios have demanded that the Q-system is supplemented by other techniques, namely, rock testing, rock mass testing, monitoring and numerical calculations, to come up with suitable and long-term rock support designs.

In this paper, unlined pressure tunnels from several power plants built in rock masses with presence of swelling clay and slaking rock, and under the NMT philosophy, are analysed in an attempt to define the impact of such rock properties in the final rock support design. The study describes the site investigations done to assemble proper and reliable geo-mechanical models and to describe the interaction between the rock around the opening and the unlined tunnel itself, supported with reinforced ribs of shotcrete RRS.

The analyses done have considered comparisons between geology types, mapped Q-values, tested swelling and slaking properties, theoretical rock support according to the Q-system, and final permanent rock support design. These back-analyses have not only served to determine the impact of the swelling/slaking behavior on the rock support design, but also to formulate correlations that could contribute to the improvement in the use of the Q-system in future projects with similar rock mass conditions.

1 Introduction

Relevant experience has been gained from the recent construction of four hydropower plants built under the NMT principles in rock masses containing swelling clay and slaking propensity. The first scheme is Moglicë HPP (Albania), partly excavated through deformed flysch sequences, while the other three plants (-of confidential name) are in South America and built in either volcano-sedimentary rocks or metasedimentary units. The 4 schemes sum up 25 km of conventional tunnelling with permanent rock support consisting mainly in the form of reinforced shotcrete ribs (RRS) plus anchored concrete inverts, or cast concrete liners (CCA) at locations with high swelling activity. This paper gives a description of the site investigations done to derive reliable input to the permanent rock support design. It also presents an analysis of the effect of slaking on the design of permanent rock support to derive correlations that improve the understanding and use of the Q-system (Barton et al. 1974) in slaking -prone environments.

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