Behaviour of blast-induced damaged zone around underground excavations in hard rock mass

Abstract: The presence of blast-induced damaged zone around an excavation boundary has been an important concern in rock construction. It is generally believed that the presence of this zone can pose problems related to stability and flow and consequently impair the performance and functionality of the excavation. In fact, the immediate consequences of this zone are usually conceived in terms of safety and cost. Hence, some organizations have put in place guidelines for controlling the amount of damage induced by blasting. Since shotcrete or sprayed concrete is a widely used surface rock support, its performance depends primarily on the competence of the damaged zone. However, in some instances the use of shotcrete may unnecessary, but the lack of knowledge of the competency of the damage zone means, it is better not to take chances. It is therefore necessary to increase the knowledge and the understanding of the competency and behaviour of the damaged zone in order to predict the performance and functionality of a rock tunnel. To gain an understanding of the damaged or disturbed zone in general, significant efforts have been made over the last few decades in a broader area; the excavation disturbed zone. These efforts mainly focused on the characterization and classification of the damaged zone. Quantification of this zone has also been done in terms of mechanical, hydraulic and physical parameters, particularly to delineate the extent of the damaged zone. The characterization, classification and quantification of damaged zone were purpose specific and therefore, the definitions for damage zone are different and varying. In this thesis the damaged zone is defined as the zone where the rock has been significantly damaged such that the mechanical properties have been affected and that these changes are measurable by any state of the art measurement techniques. This definition also applies to the blast-induced damage zone. To be able to assess the significance of the blast-induced damage zone and its influence on the performance of an excavation, the mechanical behaviour of this zone must be understood. This thesis is therefore aimed in that direction and thus the objective. Several issues were investigated including; effects and consequences of blast-induced damage zone, most likely failure mechanisms, mechanical parameter sensitivity and their impact on the behaviour of the damaged zone, numerical modelling approach for damaged zone and indicators for failure from a continuum model, etc. A literature review and industrial questionnaire gave the direction for the investigations. Field and numerical methods were employed in the investigations. The results of these investigations are published in a series of papers that make up the thesis. In brief, the main results and conclusions can be summarized as follows: The blast-induced damage zone has been largely defined in terms of its extent and lacks a definition based on its inherent competency parameters, which are directly related to the stability of an excavation. The blast-induced damage zone thickness varies in most practical cases between 0.1 and 1.0 m, with an average ranging from 0.3 m to 0.5 m depending on whether perimeter blasting techniques are used or not. The reduction in the Young's modulus varies anywhere between 10 to 90 % of the undamaged rock value. In a field investigation reported in this thesis the Young's modulus of the damaged rock was found to vary between 50 and 90 % of the value for he undamaged rock mass. The thickness of the damaged zone was between 0.5 and 1.0 m. From the numerical study, the presence of the blast-induced damaged zone did affect the behaviour of the stability quantities, namely; deformation and induced boundary stresses. However, it cannot be concluded if the effects are significant enough to cause problems around an underground excavation in the hard rock mass type studied. The inherent properties of the damaged zone that affected its behaviour, identified in order of their significance are; the deformation modulus, followed by tensile strength and compressive strength. External factors such as the state of the in-situ stresses are seen to significantly influence the behaviour of the damaged zone. With the rock mass type and the in-situ stress regimes used in this thesis, the main failure mechanism within the damaged zone at shallow excavations is tension, while in deep excavations it is compression, as evaluated from the numerical analyses. The influence of the blast-induced damaged zone on failure is evident in shallow excavations but not in deep excavations. This was observed from the coupled continuum-discontinuum models. The study on the shotcrete-rock interface showed that the bond strength of the interface is important for the shear strength. Average bond strength of 0. 5 MPa was determined for interfaces with surfaces roughness with JRC values of 1-3, and 1.4 MPa for those with JRC values of 9-13. These values were determined for low normal load conditions ( MPa) which is often the case when shotcrete is used with rockbolt for rock support. The average adhesion or tensile strength of the interface was determined to be 0.56 MPa.