Environmental Engineering Reference
In-Depth Information
Table 16.3.
Rock discontinuity shear strength parameters (Adapted from BC Hydro 1995).
Discontinuity type
Typical feature
Strength parameters (Note 1)
Clean discontinuity (No previous
Clean joint, bedding surface
f(
b
,
i
);
c
0
displacement)s
Thick infilled or extremely
Infilled seam, joint, or extremely
f(
) of infill or extremely
weathered seam (No previous
weathered bed
weathered material (2);
c
0
displacement)
Discontinuity with previous
Sheared zone or seam, bedding
f(
r
) of shear surface
displacement
surface shear, crushed seam
and/or sheared or crushed
material, and f(i) of wall rock;
c
0
Multiple discontinuity
Highly jointed rock mass
m
b
, s, a,
ci
Notes: (1) Strength parameters as defined in Section 16.2:
f
function of
effective cohesion (at zero normal stress)
b
effective basic friction angle (for wet surfaces)
r
effective residual friction angle
i
average roughness angle
m
b
, s, a Hoek-Brown criterion (1995) parameters
ci
uniaxial compressive strength of intact rock
(2) Test to be carried out on remoulded samples;
c
to be based on peak strength under drained
conditions;
c
to be neglected.
BC Hydro (1995) recommend the methods given in Table 16.3. The descriptive terms
for discontinuities (defects) used by BC Hydro have been modified here to conform with
the definitions in Chapter 2, Section 2.3 and Figure 2.1.
16.3.2.2
Shear strength of clean discontinuities
The shear strength of clean, rough (or smooth) discontinuities should be assessed using
the approach of Patton (1966), who recommended use of the form of Equations 16.5 or
16.6.
(16.5)
tan(
i
)
if
n
b
n
ns
or
c
tan
if
(16.6)
a
n
b
n
ns
where
n
n
u
effective normal stress;
ns
effective normal stress at which the
strength of asperities is exceeded; u
internal water pressure (pore pressure) within the
discontinuity;
b
basic friction angle (for wet surfaces) of the discontinuity wall rock;
n
effective normal stress across discontinuity;
i
average roughness angle; c
a
effec-
tive apparent cohesion.
Figure 16.5
shows the application of the equations.
The basic friction angle (
b
) is derived from shear tests on sawn and lightly ground sur-
faces of the discontinuity (Barton, 1982; Barton and Bandis, 1991) or alternatively by
measuring both vertical and horizontal displacements of a discontinuity specimen during
laboratory testing and calculating the roughness angle i, which is then subtracted from the
measured friction angle to determine
b
. An alternative method is by tilt testing on rock
