Studies On Blockiness Of Fractured Rock Mass

If the theoretical analysis or the engineering practice concern relates to the rock mass structure, both of them will treat the rock mass as the continuum or the isolated block system, however, there is essential difference between these two rock structures. For the fractured rock mass, the blockiness level is not only its basic characteristics but also the foundation for selecting the rock structures.The fracture is the decisive factor for the blockiness level in the rock mass, and the geometrical parameters of the fracture have different influence on the blockiness level. Simplistically, in the rock mass with three groups disc fracture structure orthogonally intersected by each other, that fracture diameter is at least 2 times of its spacing is the critical condition for the fractures cutting the trapped rock mass into small blocks.In the present paper, the block percentage (B) is defined as an index serving as the value scale for the blockiness level of the rock mass. In the case study, according to the grading standards for fracture persistence and spacing given by ISRM, 35 rock masses in different fracture diameter (D) and spacing (S) are selected and identified in GeneralBlock program. When analyze the identifying results, the grading curve which is employed to analyze the soil mass's grain-size distribution in soil mechanics is introduced into the block volume curve calculation for the rock mass, and combined with block percentage (B) to analyze the blockiness level better. Then B values of different rock mass models are calculated on the basis of the formula given in the paper. The fitting analysis on the relation between the B value of the rock mass and the D and S values of the fracture can get the function relation of them. It is a piecewise function: if D/S ratio is equal or greater than 2 , the function relation follows the negative exponential distribution with a constant; if D/S ratio is less than 2 , the B value is zero.The concept of representative elementary volume (REV) is induced from the model range selection for the rock mass model establishment. The research on the REV of the fractured rock mass also is a basic problem in the rock mass mechanics and rock mass structure, and B values can be employed to confirm the REV of the rock mass as an emerging method. The REV has different sensitivities to D and S of the fracture, so for the 35 fractured rock masses, the multiple of S rather than the insensitive factor to the REV is used to represent the REV. In the establishment and calculation of the model, deleting the small block induced by anthropogenic factor and repeatedly calculating can eliminate or reduce the error.Finally, this paper presents a new method for the rock mass classification: combine the B value and the division value calculated from block volume distribution curve, and classify the rock masses according to the division value. This new method classifies the rock masses into non-blockiness rock mass, slight-blockiness rock mass, moderate-blockiness rock mass and blockiness rock mass. Based on this standard, 35 rock masses are classified. The classification result indicates that almost all of them are non-blockiness and blockiness rock mass, no rock mass is slight-blockiness rock mass, 3 of them are moderate-blockiness rock mass.