1. Introduction

The route of the M5 Motorway passes to the west of Bristol, crossing the River Avon just upstream from the docks at Avonmouth. The line ultimately chosen west of the river gradually ascends the 300–400 ft (109–182 m) high Carboniferous Limestone hills of the Tickenham Ridge. The motorway passes over the crest of the hill through the Court Hill saddle at 243 ft (74·06 m) OD. Climbing the hillside the road is in a cutting with a maximum face of 100 ft (30·48 m).

At the western end of this ridge and towards the head of the Gordano Valley is a steep north facing scarp where the Carboniferous Limestone has been thrust northwards over the less resistant Pennant Series which in this area is dominantly sandstone. For a length of about 8000 ft (2438 m) between Wynhol Farm and Hill Lane the line follows the exposure of the thrust fault requiring the construction of retaining walls and other measures to revet the rock faces in both the Pennant Series and Carboniferous Limestone and to deal with cavities, fissures and problems arising from the jointing. Over the major part of this length the dip was into the hillside and the limestone was massive, but the faulting.

At the time of the preliminary feasibility soil survey in 1961, the alignment of the route was up to 1000 ft (305 m) south of the line ultimately adopted and it was not anticipated that this line sandstone was badly disturbed and brecciated

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