Engineering Rock Mechanics

Foundation instabilify 305

Figure 17.19 Contours of radial stress induced by line loading inclined at an

arbitrary angle to the half-space surface (from Goodman, 1989).

be valid. If the rock is laminated and inter-layer slip is possible (as has been
discussed previously in connection with the @j theory), then the rock may
not be able to sustain the compressive and shear stresses either, even at
very low magnitudes of applied loads. An idea of the regions in which
inter-layer slip could occur can be obtained by applying the #j theory to a
contour of constant radial stress computed using the Boussinesq solution,
as developed by Goodman (1989) and as shown in Fig. 17.20.
The shape of these contours of radial stress, commonly known as 'bulbs
of pressure', is then seen to be affected by the occurrence of inter-layer slip,
with the result that the applied foundation load affects a greater depth of
rock than in the case of a CHILE material. The modified contour is only

1 _\a B I I! / \\\h I I Bulb for isotropic of pressure rock

I I

B

Parallel

to AA

-.

'L of uressure

for the layered

rock

Figure 17.20 Modification of radial stress contours due to inter-layer slip (from

Goodman, 1989).