Built from 1992 to 1996, the 5.1km SSC (Second Severn Crossing) provides a critical link between England and South Wales across the Severn estuary. It consists of three major segments: two 2.1km concrete approach viaducts and a 950m central cable-stayed bridge, including a 450m span over the main navigational channel.
The SSC exists in a harsh environment, with a tidal range of 14m giving rise to very strong tidal currents. The bridge also crosses over Brunel’s Severn rail tunnel near the Avon (English) side. Consequently accurate monitoring of the bridge is vital. The approach viaducts are supported by 20 pairs of piers on the Welsh side and 21 pairs on the English side. The settlement of these piers is the subject of biennial monitoring via the bridge maintenance contractors.
The health and safety precautions for this work were stringent, as it involved working at heights of up to 50m, which could be over mudflats or strong tidal streams depending on the state of the tide (Measurements are taken from the top of each pier to it’s adjacent piers).
Access to the top of each pier entailed climbing through an access hatch and down a ladder from inside the hollow deck bridge structures. Working-at-height procedures were followed, with all personnel at risk of falling being fully harnessed and equipped with fall-arrest gear.
The monitoring was based on two simultaneous runs of trigonometric heighting, one along the northern line of piers and one along the southern line of piers. The assumption is made that the structure is only settling in the z-axis, so the trig heighting is based on fixed distances. There were two survey nails on each pier – a point for the instrument and a point for the measuring staff. The survey team consisted of four people: a pair of instrument operators and a pair of staff holders, each with top cover within the viaduct.
Multiple rounds of vertical angles were observed from the instruments to both staves on the preceding pair of piers. The staves then moved to the instrument piers, where the instrument observed the height of its plane of collimation. The staves then moved to the foresight piers where the vertical angle was again observed. This procedure continued over the bridge until all piers had been heighted to tolerance – each run reinforced the other and allowed the accuracy to be known.