Civil Engineering Reference
In-Depth Information
(a)
(b)
Figure 4.3
Shallow circular failure in closely jointed rock mass (closely jointed, slightly weathered basalt,
Island of Oahu, HI).
(a)
(b)
Figure 4.4
Circular failure in residual soil and weathered rock (weathered basalt, Island of Oahu, HI).
crack opening along the sub-vertical, orthogonal
joint set. Under these circumstances, the shear
strength used in stability analysis is that of the
bedding surfaces. Section 4.2 of this chapter dis-
cusses the strength properties of discontinuity
surfaces, and Section 4.3 describes methods of
measuring the friction angle in the laboratory.
Figure 4.3 shows a slope cut in a slightly
weathered, medium weak basalt containing low
persistence, closely spaced joints that occur in a
wide variety of orientations. Because the joints are
discontinuous, no single joint controls stability of
the slope. However, if a sliding surface was to
develop in this slope it would follow a stepped
path, which would partly lie along joint surfaces
and partly pass through intact rock. The shear
strength of this complex sliding surface cannot be
determined analytically, so a set of empirical equa-
tions has been developed from which the cohesion
and friction angle can be calculated with respect
to the degree of fracturing and the rock strength.
This procedure is described in Section 4.5.
Figure 4.4 shows a slope cut in a weathered
rock in which the degree of weathering varies
