RBS#
The “Rock Block Strength”, RBS, was presented by Laubscher & Jakubec (2001) [1].
Laubscher defines block strength not just in terms of UCS of intact rock, but rather as:
“The ability of the rock blocks (bounded by discontinuities) to resist failure or degradation, including the effect of geological weaknesses.”
In this context, veins and infill materials, even if small in volume, act as planes of weakness within the blocks themselves, not just at block boundaries.
Therefore, the RBS is the strength of the joint bound primary block of rock adjusted for:
sample size
any non-continuous fractures and veins within the block.
To obtain the rock block strength (RBS) from the “corrected” IRS, various factors are applied dependent upon whether the rock blocks are homogeneous or contain fractures and/or veins.
If the rock block does not contain fractures or veins then the rock block strength is calculated by:
If internal fractures and veins are present, a further adjustment is made based on the number of veins per metre and the Moh’s hardness number of the vein infilling, using the chart given in Figure 1 [1]:
Figure 1: Adjustment factor for RBS as a function of the Moh’s hardness of the fillings and the frequency of the veins within the rock block.#
Only Moh’s hardness values up to 5 are used in the procedure, values greater than 5 are not likely to be significant [1]. Open fractures and veins are allocated a value of 1.
The figure transformation is the equivalent of using:
Finally, RBS is calculated by:
From the guidelines of Read & Stacey: [2]
As an example of the adjustment factor required for a block containing a number of gypsum veins [1], a block with an IRS value of 100 MPa contains an average 8 veins of gypsum per metre. The Moh’s hardness of gypsum is 2. The ratio of the vein frequency to fill hardness is thus 4, which provides an adjustment factor on the Y-axis of Figure 1 of 0.75. The BS value is thus 60 MPa (0.8 × 0.75 × 100).