Dr S. N. Kiek has sent the following contribution—I read with great interest the paper by S. A. F. Murrell. In it he discussed relationships between faulting and shear parameters of rocks as they occur in nature. Measurements made on rock cores are only meaningful if the rock is homogenous and unjointed—a rare occurrence.
Some years ago I was engaged in a series of experiments in which an attempt was made to assess the shear strength of rockfill. The aim was to discover the angles of internal friction of large rock particles. Experimental difficulties precluded full scale tests on account of the limitations of the apparatus available. The largest size was a little over 100 mm. Many experiments were made using smaller particle sizes with the idea of finding the relationship between particle size and angle of friction. As the relationship is logarithmic, it is possible that some extrapolation to larger sizes is slightly justified, there being no other way of obtaining any estimate whatever. It appeared that probably a maximum is reached at about 30 mm. There is some considerable scatter between rock types and this represents a general average. Angle of friction for this size is about 43°, decreasing above and below it. At 100 mm, it is about 38°, the figure commonly used in design of rockfill zones in earth dams.
It was also found that the angle of friction varied very systematically with lateral pressure, although the actual relationship varied between different rock