Effect of jointing on rock mass modulus

I would just very quickly like to take up the work that Maurice and Chappell have done in considering the effect of jointing on mass modulus. In the case of some work involving mudstone, generally pressuremeter testing has been carried out in boreholes for each socketed pile, but for various reasons, in some cases we had no pressuremeter data and we found joint frequencies in the order of 20 or more joints per metre. We had to make some assessment as to what effect those joints were having on the mass modulus. In order to obtain some feel for that, we took 467 pressuremeter tests and broke those up into groups: according to the angle of the jointing less than 45° or greater than 45° to the core axis, and also according to the type of jointing, i.e., whether it was tight and rough or whether it was slickensided. After considering the different groups of each type we found that we couldn’t really obtain any clear difference. This was a bit disturbing but then we reasoned that there was very small change in the mass modulus anyway so perhaps the effects of angle of the joints or the type of joints was being overridden by the general scatter and the generally small effect that we have had. I have a sketch here of the curve that we adopted for design purposes to overcome this problem and we can see that if the joint frequency is up to about 20 joints per metre the mass modulus is reduced only by about 20% and the results are plotted here as Curve 1, which is quite different to the curves 2 and 4 which concern much harder rocks. Curve 4 is from Deere’s work, and curve 2 is from some work in much stronger Melbourne mudstone. Curve 3 has been developed by others for Sydney Sandstone. The mudstone which provided the basis for Curve 1 concerns ratios of intact rock modulus to joint material modulus of about 4 to 20 and if you look at Brian Chappell’s curves on Page 32 of the proceedings Vol 1, you will find that for those ratios the modulus reduction factors are indeed around 0.8 or 0.9 so that there is I believe some very good field evidence to support the general concept of the very simple one layered model.