Questions and answers: fractures and hemispherical projection 1 07will maximize the range of RQD valuesa. How do the RQD values
computed with this threshold compare to the earlier values?
A7.6 (a) This question requires computation of fracture frequency in
different directions. The fundamental equation to use (see EM 1) is
h, = Cbl Ai I cos 0, I, where h, = the fracture frequency in a given scanline
direction, the hj are the individual set frequencies of the n sets, and the Qi
are the angles between the set normals and the scanline.
*I Horizontalborehole vehical
boreholeFirstly, we determine the angles between the normals to the three
fracture sets and the three directions required in the question. For this,
we can either use the hemispherical projection, or compute the angles
vectorially. To find the angle between two lines on the hemispherical
projection, pIot the points representing the two Iines, rotate the tracing
paper so that the two points lie on a great circle, and then read off the
angle where the small circles intersect the north-south line on the tracing
paper. The figure shows how this is done for all of the angles required.
For a vertical borehole
The table below shows the required angle, the calculation of hl cosel for
each set, and the sum of these contributions.
Set h Angle between set normal hl cos 61
and required direction
1 7.72 23 7.106
2 3.07 58 1.627
3 5.34 75 1.382
Sum 10.115The method of choosing the optimal value of the RQD threshold value for optimizing
the RQD sensitivity is given in Hamson J. P. (1999), Selection of the RQD threshold value
in RQD assessments. Inf. J. Rock Mech. Min. Sci., 36,5,673-685.