Monitoring the Deflections of the Suspension Cables of the Severn Suspension Bridge by GNSS

Some 106 bridges and tunnels exist to cross the River Severn in the UK. There are two large bridges that cross the Severn towards the estuary’s entrance, linking Bristol and South Wales. These are the M48 Severn Bridge and the M4 Second Severn Crossing. The Severn Bridge is a 1,600m long suspension bridge, with a main span length of 988m, and the M4 Second Severn Crossing has a total length of 5km, with the longest span being a 482.8m section of a cable stayed bridge.
A series of field surveys were carried out in 2010, to monitor the magnitude and frequencies of the M48 Severn Bridge’s movements. This was carried out through attaching 9 dual frequency survey grade GNSS receivers on the bridge itself, being positioned relative to two reference stations located adjacent to the structure. The bridge’s GNSS receivers were attached to the tops of the four towers, as well as directly onto the suspension cables. Traditionally, this type of work has been carried out by attaching the receivers to the bridge deck or parapet, but this is thought to be a novel data gathering exercise.
Overall, some 3 days of raw code and carrier phase GNSS data were gathered, at rates of 10Hz and 20Hz. Initial processing has been conducted, as well as an outline analysis of the results. This paper describes the survey, and presents a selection of the key results.