Environmental Engineering Reference
In-Depth Information
16.3.2.3
Shear strength of infilled joints and seams showing evidence of
previous displacement
BC Hydro recommend that the shear strength of previously displaced features (e.g.
sheared or crushed seams) is determined from:
tan(
i
(16.7)
n
r
where
r
effective residual friction angle of the infill, sheared or crushed material;
i
average roughness angle which is reduced from the measured value to account for the
lack of intimate contact of the walls of the feature as detailed below.
The residual frictional strength of the feature is generally determined by shear tests on
undisturbed samples measured at sufficient displacement to ensure that the residual
strength of the feature has been reached. For thick gouge (crushed rock) zones or, if undis-
turbed samples cannot be obtained, remoulded, normally consolidated samples of the
material are tested for their residual strength. Gouge is often layered so care should be
exercised to ensure that the weakest layer of material is selected for testing. Sufficient con-
solidation time must be allowed and rates of shearing must be sufficiently slow to allow
pore pressure dissipation in accordance with accepted principles of soil mechanics. BC
Hydro (1995) suggest the residual frictional component of shear strength for design be
determined from the mean of the residual strength values if consistent results are obtained
and a sufficient number of tests are done (about ten or more). However the authors' expe-
rience is that it is difficult to obtain true residual strengths and it is recommended more
conservative values be adopted, e.g. the lower quartile or lower bound of test results.
To determine the roughness angle of previously sheared features, field measurements
are obtained of the mean amplitude of asperities of the upper and lower surfaces of
the shear, the mean inclination of asperity surfaces and the mean thickness of infilling. The
value of roughness (
i
) will be less than the measured inclination of asperities because the
walls of the feature are not in contact.
Figure 16.7
indicates the reduced values of rough-
ness for features with various ratios of seam thickness to asperity amplitude (from the
USBR, based on Ladanyi and Archambault, 1977).
In many cases there may be a lower strength discontinuity (such as a thin graphitic
shear) located along one of the contacts of the rock wall and seam material. BC Hydro
(1995) recommend such features should be noted during the geological data gathering
process and tested separately from the bulk of the seam material, to allow the strength of
the weakest element to be determined.
16.3.2.4
Shear strength of thick infilled joints, seams or extremely weathered beds
with no previous displacement
As indicated by BC Hydro (1995), such features would normally be excavated from the
foundation of concrete dams. If they do exist, they suggest using:
n
tan(
)
(16.8)
where
effective friction angle of the infilling or seam material, based on tests of
remoulded samples. Any cohesion component c
, indicated by testing, is usually very
small and should be ignored.
16.3.2.5
Shear strength of jointed rock masses with no persistent discontinuities
Careful mapping of the foundation of most concrete dams will show that there are kine-
matically feasible failure mechanisms which are controlled by discontinuities (bedding,
