Abstract

For many years, blocks of rock have occasionally fallen from the slope of the Avon Gorge on to the A4 road (the Portway), which runs along the eastern side of the Gorge. Indeed, there is no reason to believe this was not a hazard long before Brunei designed the impressive suspension bridge over the river.

The photograph opposite shows the general area at the eastern side of the Gorge the bridge tower being almost directly at the top of a sub-vertical rock cliff face, about 75 m high. The photograph also shows a number of fractures, aligned almost parallel to the rock face. It does not show the partially dissolved fracture, about 300 mm wide, which can be seen from the Portway in the cliff immediately behind the eastern tower.

In the late 1970s, the owners of the cliff face and the highway authority both became increasingly conscious of the danger of falling blocks and instigated a study to determine most effective and economically viable course of action. Although it was considered that bolting and meshing could provide a solution, it was felt that long-term security and environmental requirements would best be served by the construction of a canopy.

Figure 1 shows the fractured nature of the rock when it was inspected after the face had been cleaned of all vegetation. The sub-parallel tectonic-stress induced fractures are clear, as is the broken state of the limestone beds.

Figures 2 and 3 show the detailed state of the rock when inspected.

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