Environmental Engineering Reference
In-Depth Information
Table 15.7.
Approximate stress reduction factors to account for valley shape (Hunter 2003; Hunter and
Fell 2002).
Average
Stress reduction factor (embankment location)
W
r
/H ratio
abutment
(river width
slope angle
Base
Mid to low
Mid
Upper
to height)
(degrees)
(0 to 20%)
(20 to 40%)
(40 to 65%)
(65% to crest)
0.2
10 to 20
0.93
0.95
0.97
1.0
20 to 30
0.88
0.92
0.96
0.98
30 to 40
0.82
0.88
0.94
0.97
40 to 50
0.74
0.83
0.91
0.96
50 to 60
0.66
0.76
0.86
0.94
60 to 70
0.57
0.69
0.82
0.92
0.5
25
1.0
1.0
1.0
1.0
25 to 40
0.93
0.95
0.97
1.0
40 to 50
0.91
0.92
0.95
0.05-1.0
50 to 60
0.87
0.88
0.93
0.05-1.0
60 to 70
0.83
0.85
0.90
0.05-1.0
1.0
All slopes
0.95-1.0
0.95-1.0
1.0
1.0
others, with the other factors such as particle size masking the 3D effects. If critical, the
effects should be modelled using 2D and preferably 3D numerical analyses, but ensuring
the modelling allows for the dam to be “built” in layers, since to “build” it numerically in
one stage leads to an over-estimation of the 3D effects. For approximate analysis the
stresses in the dam can be calculated using Table 15.7.
15.2.5
Selection of side slopes and analysis of slope stability
When the CFRD is constructed of hard, free draining rockfill, the upstream and down-
stream slopes are fixed at 1.3 H to 1 V or 1.4 H to 1 V, which corresponds roughly to the
angle of repose of loose dumped rockfill, and prevents raveling of the faces.
When gravel is used for the dam “rockfill” zones, flatter slopes are needed to prevent
raveling of the face. Usually 1.5 H:1 V has been adopted in these cases although 1. 6 H:1 V
has been used.
When weak rockfill has been used flatter slopes have again been adopted, e.g. 1.5 H:1 V
for Mangrove Creek Dam (MacKenzie and McDonald, 1985). If foundation strengths
dictate, flatter slopes may also be required, e.g. 2.2 H to 1 V was used for Winneke Dam
(Casinader and Watt, 1985). Cooke (1999) confirms these values.
Haul roads may be needed on downstream slopes, or defined berms may be incorpo-
rated in the face. In these cases steeper slopes between the “berm” located by the
haul road may be used, e.g. 1.25 H:1 V was used for Foz do Areia Dam (Pinto et al.,
1985).
In some cases, the upper 10 m to 15 m of the dam may be steepened to as much as
1.25 H to 1 V to provide the camber of the crest (Fitzpatrick et al., 1985).
As pointed out by Sherard and Cooke (1987) and Fitzpatrick et al. (1985), the stability
of the slopes in the dam are not usually analysed. This is in recognition of the fact that
CFRDs have no pore pressures in the rockfill and will remain stable under static loads
when constructed to the slopes described above. However, if analysis is to be carried out,
knowledge of the shear strength properties is required. The determination of rock prop-
erties is discussed in Section 6.8.
The stability of CFRD under earthquake loading is discussed in Chapter 12.
