Geology Reference
In-Depth Information
of ground anchors: Hong Kong and UK
In the 1970s and 1980s, many ground anchors were installed to
stabilise cut slopes in Hong Kong, but this practice largely came to a
halt following failure of a number of anchors in the slope shown in
Figure 7.16.
The problem was recognised following the explosive fail-
ure of concrete anchor head covers as the bar anchors broke.
Investigation showed that whilst the steel anchor bars were protected
from chemical attack along most of their lengths, either by grease or by
cement grout, close to a steel coupler there was an air space and
inadequate protection, which allowed rusting and failure. Similar
deterioration has been observed in the UK on the A685, above the
M6 near Tebay, where rock bolts were installed, some with built-in
dynamometers to allow loads to be checked periodically (Edwards,
1971). The bolts, with lengths up to 9.15m, had
fixed anchor lengths
varying from 0.31 to 0.61m (epoxy resin). The rest of the length was
grouted with either cement or bitumastic, following a nominal loading
of 50 kN. Several of the bolts have now lost their anchor plates and
therefore are unable to carry their design loads due to deterioration of
the rock mass and general rusting of the assemblages
(Figure 7.17)
.
Designers and owners need to recognise long-termmaintenance require-
ments, including regular inspection and testing and the likelihood that
anchors will need to be replaced periodically. Edwards (1971) identi-
fied the need for corrosion protection and long-term maintenance but
this appears not to have been done. Even in more recent projects, and
despite stringent standards in force for design and construction,
including corrosion protection, anchors fail. The design of rock cut
slopes at Glyn Bends on the A5 in North Wales required 4,500m of
ground anchors stressed to between 400 kN and 600 kN (Green &
Hawkins, 2005). Within 10 years of construction, the new section of
road had to be closed after two anchor heads fell off and other anchors
failed on testing (NorthWales Geology Association, 2006). Anchorages
