ABSTRACT
The Blocsidis card has the function of recording essential field measurements on the card and carrying out a preliminary analysis in the field.
Similar to the Wedge card, this card uses a rotating equal angle stereonet to plot the a stereogram of the three principal joint set great circles based on their dip and azimuth values measured using a geological/ rock mechanics compass (Breithaupt and Fribourg are popular makes). The stereo-plot is further extended to determine the required angles for the formulae to allow a distribution curve to be made of the block sizes in the rock mass. The product of the cosines of the three angles ø, θ and ψ, cosψ.cosø.cosθ have to be either calculated or can be determined from two sets of Cartesian graphs on the reverse side of the card (in Wang et al. (1990) α is used instead of ψ as α, β and ε are preferably used on the card as the dip of joint sets A, B and C). The remaining coefficients (also provided) are then used to obtain the percentages of block size allowing a distribution curve to be made in a fashion similar to a sieve analysis for soils. The prototype Blocsidis card is shown in Figure 3 with a plot of the joint sets from the Wang et al. paper plotted over the reference stereonet. The stereo-net disc has been rotated to a position to enable measurement of the angle 90◦-ψ. Often students, engineers and, not too infrequently, even engineering geologists are not familiar with the use of stereonets for determining angles in three-dimensional space. Up to now one was left guessing, especially as an engineer, how the plots were made and used! A more recent update of the CIRIA-CUR special report on Coastal Engineering left out the method altogether; one can only speculate
that the simple stereogram showing only the reference circle and the three great circles with the intersection angles could not be understood and so was left out in the more recent edition.
