Civil Engineering Reference
In-Depth Information
14.4.5 Seismicity
face, and the plunge coincided with the sag angle
of the catenary created by the sag in the cables.
The rock anchors were installed in the upper
surface of the bench and extended through the
bedding plane into stable rock to apply normal
and shear (up-dip) forces to the bedding plane.
The seismic coefficient for the site was 0.1. The
stability analysis used the pseudo-static method in
which the product of the seismic coefficient, the
gravity acceleration and the weight of the wedge
was assumed to produce a horizontal force acting
out of the slope along the line of intersection of
the wedge.
14.4.7 Stability analysis
The stability of the abutment was analyzed
using the comprehensive wedge analysis proced-
ure described in Appendix III, and the computer
program SWEDGE version 4.01 by Rocscience
(2001). The input data required for this ana-
lysis comprised the shape and dimensions of the
wedge, the rock properties and the external forces
acting on the wedge. Values of these input para-
meters, and the calculated results, are listed on
the next page.
14.4.6 External forces
The external forces acting on the wedge com-
prised water forces on planes 1, 2 and 5, the seis-
mic force, the bridge load and the rock anchors.
Figure 14.14 shows the external forces in plan
and section views.
The water forces were the product of the areas
of planes 1 and 2 and the water pressure distri-
bution. The seismic force was the product of the
horizontal seismic coefficient and the weight of
the wedge. The analysis procedure was to run the
stability analysis to determine the weight of the
wedge (volume multiplied by rock unit weight),
from which the seismic force was calculated.
For the bridge, the structural load on the abut-
ment due to the tensioned cables had a magnitude
of 30 MN, and trend and plunge values of 210
◦
and 15
◦
, respectively. The trend coincided with the
bridge axis that was not at right angles to the rock
(i)
Wedge shape and dimensions
The shape of the wedge was defined by
five surfaces with orientations as shown in
Figure 14.13.
22
◦
/
270
◦
(a)
Plane 1 (bedding):
80
◦
/
150
◦
(b)
Plane 2 (fault F1):
(c)
Plane 3
(upper slope):
02
◦
/
230
◦
U
1
(a)
(b)
N
k
h
W
T
Q
U
1
T
U
2
k
h
W
Q
Legend
k
h
W
—horizontal seismic force = 14.1MN
T
—tension force in anchor = 10.5MN
Q
—tension in bridge cables = 30.0MN
U
1
—water force on plane 1 = 19.4MN
Figure 14.14
Sketch showing magnitude
and orientation of external forces on wedge:
(a) section view along line of intersection;
(b) plan view.
U
2
—water force on plane 2 = 6.5MN
W
—weight of wedge = 140.6MN
W
