Civil Engineering Reference
In-Depth Information
UDEC (Version 3.20)
500
Legend
Boundary plot
User defined grid value
Displacement contour
interval = 0.5
400
300
0.0
0.5
1.0
1.5
2.0
2.5
200
100
0
-100
Horizontal axis (m)
0
100
200
300
400
500
600
Figure 10.5
Rock mass failure mode for slope determined with UDEC.
Brown (1997). These initial properties are then
modified, as necessary, through the calibration
process.
Failure modes involve mainly shearing through
the rock mass. For homogeneous slopes where
the slide surface is often approximately circular,
intersecting the toe of the slope and becom-
ing nearly vertical near the ground surface. The
failure mode for the parameters listed earlier is
shown in Figure 10.5. The calculated safety factor
is 1.64.
The following is a comparison of a slope
stability analysis carried out using limit equilib-
rium circular failure analysis (Bishop method)
and numerical stability analysis. In Chapter 8, the
stability of a benched slope in strong, but closely
fractured, sandstone including a water table and
tension crack is described (see Figure 8.19). The
rock mass is classified as a Hoek-Brown material
with strength parameters:
σ
c
=
150 MPa
Disturbance factor,
D
=
0.7
The tensile strength is estimated to be 0.012 MPa.
For the Bishop's analysis method, the Mohr-
Coulomb strength is estimated by fitting a straight
line to the curved Hoek-Brown failure envelope
at the normal stress level estimated from the slope
geometry. Using this procedure, friction angle
and cohesion were
43
◦
φ
=
=
c
0.145 MPa
The mass density of the rock mass and water
were 2550 kg
/
m
3
(25.0 kN
/
m
3
) and 1000 kg
/
m
3
(9.81 kN
/
m
3
) respectively. The phreatic surface
is located as shown in Figure 8.19. Based upon
these parameters, the Bishop method produces a
location for the circular slide surface and tension
crack, as shown in Figure 8.19, and a factor of
safety of 1.39.
m
i
=
0.13
GSI
=
20
