Abstract

Special care has always been taken to ensure the general safety of the Clifton Suspension Bridge and to maintain its conditions. The bridge is nearly 110 years old and carries more traffic now than ever before. Its preservation as a monument to the engineering genius of Isambard Kingdom Brunei is vital.

Over the years concern has been expressed as to whether the limestone rock upon which the bridge is founded on the Leigh Woods side of the gorge was as massive and sound as that on the Clifton side—although it must be emphasized that no manifestation of subsidence or foundation displacement has ever been discovered. This concern was sharpened a few years ago when certain buildings in the Bristol area were thought to be at risk due to the finding of solution cavities and thick clay-filled joints in the limestone in their locality.

The massive character of the rock on the Clifton side is plain to see in a near vertical cliff 250 ft high but on the Leigh Woods side of the Gorge the surface is at a slope averaging about 40 degrees to the horizontal. Although outcrops are large, much of the slope is covered by soil debris and vegetation.

An investigation into the strata underlying the Leigh Woods abutment was therefore commenced by putting down 4 drillholes, one at each corner of the abutment forming a square grid with approximately 95-ft sides. From these boreholes an electrical survey was carried out by passing very low frequency electric currents between electrodes lowered into the holes. Resistances were measured between the sides and diagonals of the square at increments of depth. Great importance was attached to the coring of the drill holes and a very high degree of recovery was achieved. The cores obtained were of hard, light and dark grey crystalline Carboniferous Limestone cut through with thin irregular calcite veins. Very thin bands of mudstone were also found. One band was 1 in thick but all others were thinner and, as no positive correlation between cores could be obtained, the bands were judged to be lenticular. The bands penetrated lay parallel to the dip which was confirmed as being approximately 30 degrees in an upstream direction. The overall impression gained from the four drillholes was of a very massive, sound rock formation. No cavities were penetrated except for some very small calcite filled vughs.

The electrical survey detected an anomaly close to the surface and another, less well defined, at greater depth. The former seemed likely to be in a fairly central position immediately beneath the abutment. A fifth drillhole was therefore put down through the structure of the abutment into the underlying rock. This hole was again carefully cored and revealed that the abutment itself was hearted by well grouted sandstone plums. Below the abutment a breccia filled fissure was penetrated. This Triassic filling consisted of irregular blocks of limestone, sandstone and siltstone with a calcareous cement. Although a hard material and quite suitable as a foundation, it was considered a sufficient explanation for the upper anomaly.

With the lower anomaly being so ill defined it would have been difficult and costly to investigate it by drilling further holes which would have had to be either horizontal or inclined. Many explanations for the anomaly were possible—it could be due to further breccia filling, to a comparatively more massive limestone or, more unlikely, to a cavity—but even the last explanation was considered acceptable bearing in mind its probable location some 75 ft below the abutment.

The installation of monitoring devices in the drillholes to detect movement over a period had been allowed for but, in view of the generally satisfactory outcome of the investigations, this further work was not considered necessary.

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