Civil Engineering Reference
In-Depth Information
anchors. If the trend of the anchors was
between the trends of the line of intersec-
tion and the bridge load (i.e.
α
T
=
The initial stabilization work involved selective
scaling and bolting of the face, but it was found
that this only provided an improvement for one
or two years before new rock falls occurred as the
rock weathered and blocks loosened on joint sur-
faces. Rock falls were a potential hazard because
the curved alignment and stopping distance of
as much as 2 km meant that trains could not be
brought to a halt if a rock fall was observed.
In order to provide long-term protection against
rock falls, it was decided to excavate the face to
create a ditch that was wide enough to contain
substantial falls from the new face. This work
involved a drilling and blasting operation to cut
back the face to a face angle of 75
◦
, and con-
structing a gabion wall along the outer edge of the
ditch that acted as an energy absorbing barrier to
contain rock falls (Wyllie and Wood, 1981).
The railway and highway were located on
benches cut into a rock slope above a river, and
there were steep rock faces above and below
the upper bench on which the railway was loc-
ated; a 30 m length of the track was supported
by a masonry retaining wall (Figure 14.15). The
035
◦
),
it was possible to reduce the anchor force
required to achieve the required factor of
safety to 8.75 MN.
•
It is noted that the discussion in this case
study only addressed the stability of the
wedge, and did not discuss the method
of attaching the tensioned bridge cables
to the rock wedge. Also, it is assumed
that all the external forces acted through
the center of gravity of the wedge so that
no moments were generated.
14.5 Case Study IV—Circular failure
analysis of excavation for rock fall
ditch
14.5.1 Site description
As the result of a series of rock falls from a rock
face above a railway, a program was undertaken
to improve stability conditions (Figure 14.15).
Tension crack
Center of rotation
Excavated face
Original slope
Ground water
surface
Gabion
Potential
sliding surface
Railroad
Retaining wall
Ditch width
Highway
River
Figure 14.15
Geometry of slope above railway in Case Study IV. Sketch shows dimensions of ditch after
excavation of slope, and shape of potential circular sliding surface through rock mass.
