Although weakness zones normally occur along only 1 - 15 % of a tunnel, they form an important feature in rock tunnelling since they can be vital for the safe completion of a project. Low stability and short stand-up time, sometimes associated with water ingress are the main challenges that may arise in such rock masses. The main types of weakness zones and how they can be traced during field investigations are shortly described. It is important to know the pattern of the regional and larger weakness zones in an area for evaluation of the optimum tunnel alignment. Risk sharing contract provisions where the possible variations in the rock mass conditions are described, afford the freedom to choose the best excavation and rock support method, which often results in large cost savings. Some examples of methods used in Norway for tunnelling through weakness zones of very low stand-up time are illustrated.
Weakness zones play an important part in hard rock tunnelling. They not only cause increased tunnelling and project cost, but in many cases are of vital importance for the safe Completion of a project, and can at times cause substantial delays.
A weakness zone is a layer, zone or vein where the rock mass properties are significantly poorer than in the surrounding materials.
An understanding of the geological development and the occurence of such weakness zones features is important in the planning and construction of a project. Close cooperation between the engineering geologists, the design engineers and the contractor can often give large savings of costs and time.
Weakness zones, as all materials in the Earth's crust, are formed as a result of particular geological processes. Knowledge of the geological history of a region is necessary in understanding the mode of occurrence and extent of the zones.
There are three main groups of weakness zone formations:
tectonic fracture zones
layers or lenses of weak rocks
altered zones of 1) and 2)
The many types of rock formation and varied tectonic histories cause a corresponding variety in occurence and structures of the weaknezz zones. The number and the intensity of the orogenetic and faulting periods that the bedrock in an area has been subjected to, are perhaps the most important factors for the development of weakness zones.
An example of this is the difference in the geology between Sweden and Norway shown in figure 1. Most of Sweden is made up of old basement rocks which were faulted mainly in Precambrian time, while the Norwegian bedrock has been exposed, not only to the Precambrian orogenies, but also to faulting in the Caledonian (in the Devonian period) and the Alpine orogeny. During the latter orogeny, faulting occurred along the Norwegian coast in connection with the westward continental drift of Greenland. Figure 1 shows also that the Norwegian - bedrocks do not only consist of Precambrian igneous and highly metamorphic rocks.