One of the principal criteria in designing a hydraulic structure or evaluating stability of natural slopes, man-made slopes, underground workings etc. is the rock mass strength. The evaluation of this strength for the rock mass having displaced joint systems or other discontinua which make it blocky in structure is a difficult task necessitating large scale in-situ tests. The rock mass strength values depend on a number of factors such as the strength of rock blocks (determined by testing small specimens), strength of their contact interlock, scale factor, separate block difference in strength and deformability, shape of blocks and orientation of joints in respect of the principal stress vector.

In most cases the strength characteristics of jointed rock are defined by applying static load to large rock mass specimens (pillars of different size, pillars with cast-on-top concrete). The tests usually consist in breaking a prism with the aid of hydraulic jacks. The prism cut out in the 3oock mass Treasures about 0.5 ×0.5×0.8 m or 1.0×1.0×2,0 m3 (the later size is less common and applies to the weak rock only). The grave disadvantage of this method resides in the fact that the rock mass natural structure is inevitably disturbed in the process of cutting out the prism, which usually leads to an underestimation of the rock mass strength. Besides, all static in-situ tests are very labour-consuming which inhibits conducting them on mass scale and makes it difficult to acquire enough information for performing statistical analysis.

There also exists a number of empirical formulae for defining the rock mass strength proceeding from the relationship between the strength of a small specimenand that of the entire rock mass commonly known as the strength reduction factor (Protodyakonov 1967, Fisenko 1976, Bieniawski 1968 etc.). However, all empirical formulae yield only rough strength data and in most cases are believed to be no alternative to the in-situ test.

As may be seen from the experimental results (Johnson 1963, Freiberg & Fadeev 1968) the use of the explosive energy as a crushing load at the in-situ tests holds much promise, since it appears to be rather simple, enables to produce practically unlimited forces and test large rock masses without pressure (prior to loading) disturbance of their natural structure. The essence of the proposed method known as "cratering" consists in estimating the strength of the rock mass from the dimensions of the inclined extended charge crater.

After detonation of an explosive charge placed in the rock close to its surface the blasting crater develops. It is assumed (Johnson 1963, Oberbeck 1971) that explosive energy is basically consumed for crushing a certain volume of the rock mass (i.e. for breaking the constraints that make for the rock strength), for blowing the crushed material out of the crater (which depends on the weight of its volume and for heat losses.

When an inclined extended charge is used for making a crater in the jointed rock, close to the blast-hole mouth a tangible

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