304 Foundation and slope instability mechanisms1 Boo, 90"The maximal radius for any given contour is always co-linear with the
applied force. This means that for the case where the applied forces are
not normal to the surface, the contours rotate as shown below.
line of zero
radial stress,/' /' ,_-- .... ---...
1%-,1,1' zone of
radial stress(c) The #j theory utilizes the fact that slip occurs on a fracture if the
applied stress subtends an angle greater than the friction angle to the
fracture normal (see A16.6). If we superimpose fracture orientations on a
diagram of radial stress contours, then we can identify where the radial
stress is acting at such an angle that slip is likely to occur. Once slip does
occur, then the stress field is deformed so that it extends deeper into the
rock mass. This is shown diagrammatically below following Goodman
(1989) '.
3Goodman R. E. (1989) Introduction to Rock Mechanics. Wiley, Chichester, 2nd edn.,
562pp.