ABSTRACT:

Rock Mass Rating and the Q-system were employed for the classification of rock mass in the periphery of a shaft sunk in igneous intrusive formations. The rock classification data obtained by two geological mapping teams are compared in this experimental study. It was found that there were differences in the rock quality assessment obtained by the two teams. The magnitude of differences was greater for Q values when compared with those of the Q' values. The study also highlights the overall effects of the active stress component in Q-system and the discontinuity orientation in RMR. The results presented provide factual data for interpretation by engineers and researchers. Attention to the reproducibility of rock classification may also be useful in contractual considerations.

1. INTRODUCTION

The reproducibility of rock classification has received little attention in literature. Rock classification provides the basis for selection of a support system and geotechnical analysis in mining and civil engineering projects. It may also be used in project contracts to determine a rate for excavation cost. Therefore, the accuracy of geological mapping and rock mass classification has impacts on the design and cost of projects. This study stems from a comparison of geological mappings during a shaft-sinking project. Two geotechnical engineering teams, assigned by the project design engineer (Engineer) and the contractor (Contractor) conducted the geological mapping. The rock mass classification data for a selected study zone on the shaft periphery surface, as shown in Figure 1, are analyzed for this study.

2. TERMINOLOGY

The following terms are given for clarity as generally they lack a universal definition and have some philosophical meanings.

2.1. Reproducibility

Reproducibility is the ability of an experiment to be accurately reproduced or replicated. It is the variation in the average of rock classification results made by different geologists mapping the same rock exposure area.

2.2. Repeatability

Repeatability is the degree to which a geological mapper duplicates his/her rock mass classification for a given rock exposure area. It is an overall measure of the credibility and quality of his/her mapping.

2.3. Accuracy

Accuracy is defined as the closeness with which a measurement agrees with the standard.

3. THE PROJECT

The 180 m long shaft is part of a water conveyance project in Canada. The shaft was sunk from ground elevation through 30 m of overburden materials into igneous bedrock. The overall rate of construction advancement in the shaft was 35 mm per hour. A drill and blast method was used for the shaft excavation in 1.7 m advances. In each advance, the excavated periphery was scaled and mapped by two different teams of mappers. The shaft wall was then supported by welded wire mesh and the application of shotcrete. Rock bolts were then installed at 1.5 m spacing, followed by drilling for the next round of excavation. The shaft dimensions and ground profile are illustrated in Figure 1. When the shaft reached its final depth, a cavern was then excavated to launch Tunnel Boring Machines for boring two horizontal tunnels from the bottom of the shaft.

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