The paper presents the results of experimental laboratory studies focusing on the physics of failure. Different stages of failure of a rock mass around excavations are established, and assessment of its state is carried out. Conditions of stability of such a rock mass subject to the action of blast loads are proposed. Excavation stability is also shown to depend on duration of blast impulse. Analytical relationships are presented to assess the rates of rock block fall when destructing excavations with blast loads. The rate of rock block fall is found to depend on rock strength characteristics, blast impulse parameters and velocity of joint propagation. A brief analysis of problems connected with design of rock excavations for combined action of static and dynamic loads is done.
In the contemporary mining industry powerful explosions are practiced to raise the rate of the mineral resources mined, as well as that of boring, blasting and the work connected with breaking down and removal of a considerable volume of rock. Such explosions generate in the rock mass seismic blast waves, which affect the rock excavations and can cause their failure. Therefore, studying the process of rock excavation destruction and working out methods for determining the parameters of that process has become an urgent problem. The polarized-optical method based on physical modelling has certain advantages in studying wave propagation processes in a homogeneous rock mass (Khesin, 1975; Borulev et al., 1986; Sinitsin et al., 1985). In order to conduct experimental studies with physical models it is necessary to decide the questions of modelling, choosing model materials, experimental techniques and interpretation of the results obtained. Borulev et al. (1986) and Sinitsin et al. (1985) give criterial relations between the basic parameters of the failure process of the homogeneous isotropic rock mass. Their derivation is based on the statistical strength theory using Griffith's criterion for determining the moment when joint displacement starts. Using similarity criteria requires satisfying the relations established for the dimensions of joints, the density of their distribution in the rock mass, their propagation velocity, the specific surface energy in the natural and model material. It has been found that the process of failure in models of industrial glass is adequate (with allowance for the geometric scale) to that of the surrounding rock mass. The main result of the experimental laboratory studies is visual observation of failure dynamics in transparent models (Borulev et al. 1986; Sinitsin et al. 1985; Borulev et al., 1988). Formation of different failure zones in the surrounding rock mass determines the degree of its destruction, which is characterized by the failure coefficient m.. The value of the coefficient is¹ found in Table 1, where the index i denotes the degree of destruction. A procedure and a program for computing the parameters of the failure process of rock excavations induced by seismic blast waves have been developed on the basis of the investigation results. The mathematical base is the statistical strength theory and brittle fracture mechanics.
