Recent applications of the Q-system of rockmass classification are given. It is shown that Four potential storage sites with different rockmass conditions may have different optimal cavern dimensions. Support costs may increase disproportionately if dimensions are chosen that are smaller or larger than the theoretical optimum of 18 to 24 metres span. The Q-system is also used for mapping rockmass conditions during tunnel and cavern construction, to aid in the choice of permanent support. Examples include 25 m2 and 167 m2 headrace tunnels, and an underground sewage treatment plant constructed in 1 km of caverns, 16 m in span. Mapping of associated collector and outlet tunnels is also illustrated. The former is being excavated by full-face tunnel boring machines. Finally it is shown how the Q-value can give a preliminary estimate of the in situ deformation modulus, and the range of likely deformations.

INTRODUCTION

Estimates of support are required at three stages in a project: for the feasibility studies, for the detailed planning, and finally during excavation itself. In view of the potential economic importance of support costs it is preferable that the support estimates are as accurate as possible for all three stages. The accuracy will depend partly on the effectiveness of the geological investigations, and partly on the ability to extrapolate past experiences of support performance to new rockmass environments. Underground excavations are constructed with some confidence primarily because of all their successful predecessors. A practical method of extrapolating past experiences of support performance to new rockmass environments is the Q-system (Barton, Lien and Lunde, 1975). Several years experience by a number of users have shown it to be a useful aid in making design decisions. It has been used during feasibility and detailed planning work, and particularly during construction. Here it provides a logical system for quantitative geological mapping of tunnels and large excavations, and is helpful in indicating suitable permanent support. Several recent examples will be given later in this paper.

THE Q-SYSTEM OF ROCKMASS CLASSIFICATION

The Q-system is essentially a weighting process in which the positive and negative aspects of a rockmass are assessed quantitatively by evaluating six factors, i.e. number of joint sets, joint roughness, type of clay fillings, water inflow, stress levels etc. A store of experience is searched to find the most appropriate support measures, taking into account the rockmass quality (Q), the excavation dimensions, and the safety requirements (purpose of excavation, ESR). The rockmass descriptions and ratings for each of the six parameters are given in Table 1. Additional notes on the use of the classification system are given elsewhere. It is important to observe that the values of Jr and Ja relate to the joint set or discontinuity most likely to allow failure to initiate. The important influence of orientation relative to the tunnel axis is implicit.

ROCKMASS REQUIREMENTS FOR PERMANENTLY UNSUPPORTED EXCAVATIONS

A very interesting area of application for the Q-system is the recognition of rockmass characteristics required for safe operation of permanently unsupported openings.

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