oa Civil Engineer in South Africa - Applicability of the Geomechanics Classification to the Orange-Fish Tunnel rock masses

Volume 21, Issue 8
  • ISSN : 0009-7845



The applicability of the Geomechanics Classification to the rock masses of the 5.3 m diameter, 82 km long Orange-Fish Tunnel, and thus to the sedimentary strata of the Karoo Sequence in general (which covers more than one-half of South Africa), is considered to be of primary importance if it is to be used for future tunnelling projects in similar rock formations.

An account is given of the application of the Geomechanics Classification as a 'follow-up' study to a number of difficult mudrock tunnel sections where construction problems of various degrees of severity occurred in either unsupported or fully supported rock masses. The classification is also applied as a comparison to representative stable and potentially unstable test localities in different sedimentary rock types where no or only minor problems were encountered up to the time that the permanent concrete lining was placed.
The Geomechanics System appears to be generally suitable for the engineering classification of Karoo rocks, during both the design and construction stages, provided that the various classification parameters are readily available.
Additional classification parameters are, however, essential if the system is to be used as a guideline for the reliable assessment of the artificial support requirements in such rock types. These include the determination of rock durability (particularly of the mudrock material) and the routine monitoring of both the rate of deformation and the fractured condition of the tunnel rock mass. Such additional parameters are also necessary for the cross-checking of the classification predictions on a routine basis. These should further aid a more meaningful differentiation for rock masses which may show similar geomechanics rock mass ratings but a marked variation in actual geomechanical behaviour.

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