The engineering problems caused by swelling and slaking mudrocks in tunnel Construction are widely recognized, as is the need to test them to determine their relevant geomechanical properties. The tendency of the rock material to deteriorate with time is one of the main factors which influences the long-term behaviour of tunnel mudrock masses after exposure to the air. However, timely preventative measures can control the deterioration. Assessment of the rock durability is thus of considerable engineering importance.
This paper discusses the more important criteria which control the weathering and disintegration of the wide-spread Karoo Sequence mudrocks of South Africa (Lower Triassic age). Reference is also made to a rock durability classification which has proved successful in identifying and predicting the engineering behaviour of such rock types.
The engineering problems caused by swelling and slaking rocks are widely recognized, as is the need to test them to determine the type and extent of their Swelling behaviour and to measure this for Purposes of design (International Society for Rock Mechanics, 1989).
The process of swelling has been considered by several authors to be the causative mechanism of rock squeeze, although it is not by any means the sole factor related to such a phenomenon. Harper et al (1979) are of the opinion that it is necessary to evaluate swelling Potential in order to predict the engineering performance of an excavation In a potential rock squeeze environment.
Swelling has also been identified as a major controlling factor in the disintegration of mudrocks upon exposure to the air (Olivier, 1990).
The phenomenon of slaking, although often consequent upon swelling, is complex. It may involve causes and mechanisms other than those which are recognized as being reponsible for Swelling.
This paper deals with some of the more Important parameters which control the engineering behaviour of the wide-spread Karoo Sequence mudrocks of South Africa (Lower Triassic age). Reference is also made to a rock durability classification system in predicting the swelling and weathering behaviour of such rock types.
The author is of the opinion that a fundamental study of the time-dependent behaviour of the rock material is necessary for a meaningful assessment of the geomechanical properties of rock masses consisting of potentially-swelling and slaking mudrocks. Such a study should include observations on the weathering characteristics of the intact rock material, as well as the magnitude of the swelling and shrinkage strains which could develop after exposure to the air (Olivier, 1979 a).
In the case of Karoo mudrocks it was found that the tendency of the rock material to deteriorate with time was one of the main factors which influenced the long-term geomechanical behaviour of tunnel rock masses after exposure to the air (Fig. 1). Other criteria such as the occurrence of prominent near-horizontal bedding features and a low rock strength
(Figure in full paper)
The details of the relevant rock material characteristics have been discussed in a previous paper by the author (Olivier, 1990).
