Geotechnical parameters - Practical Engineering Geology - page 226

5.8.1 Theoretical effect on shear strength of
included boulders
Hencher (1983d) and Hencher et al. (1985) report on the back-
analysis of a landslide involving colluvium containing a high percen-
tage of boulders, in which an attempt was made to estimate dilation
angles on the basis of the coarse fraction percentage estimated in the

field and measurements taken from idealised drawings. These esti-
mated

field dilation angles were added to the strength for the matrix,
determined from laboratory testing. West et al. (1992) took this
further and identi u-

ence shear strength, based on physical modelling and back analysis of
slopes (these are illustrated in Figure 5.30). Factors envisaged included:
boulders preventing failure along an otherwise preferred failure path,
failure surface forced to deviate around a boulder, and a failure zone
incorporating the boulder. Triaxial tests reported by Lindquist &
Goodman (1994) similarly concluded that boulders increase the mass
strength. Additional review is provided by Irfan & Tang (1993).
Practical methods for addressing the strength of mixed soils and
rocks remain dif

ed several ways that included boulders might in

cult. One of the main problems is that such masses
can be highly heterogeneous and dif cult to characterise realistically.

The other is that whilst trends of increasing mass strength with percen-
tage of rock clasts and boulders are clear, general rules have not yet
been formulated. Further advances will probably be by numerical

Figure 5.30
Mechanisms of
failure through a
mixed rock and soil
slope. After West
et al. (1992).

(a)

(b)

(c)