Back in the early 90’s I worked as a radio operator in the maritime office of the company that built the Second Severn Crossing. Our role was to cover the maritime operations on the river and co-ordinate site traffic with normal river users, act as an auxiliary coastguard and safety office.
I’ll always remember the day that we attempted to lay the first caisson on the river bed. The principle of construction was that a caisson (a hollow box of concrete weighing in at several thousand tonnes) was constructed on land and then transferred to two crawlers on loan from NASA. These would then roll down a slip way and on to a specially constructed barge called the “SAR III”. From here the box was taken out into the river and held under a platform, similar to a large oil rig, called the “LISA A”. The LISA A had an enormous twin crane that would lift the box off of the back of the barge and then when the SAR III had moved clear, lower it on to the sea bed at low tide so that it could be dropped in the exact spot. Once there it would be anchored to the river bed and filled with concrete to form a base for one of the bridge supports. In order that the SARIII remained in the correct position under the LISA A it was held in place using a dynamic positioning system that relied on a team of surveyors setting up laser reflectors on the LISA A and in the case of the first caisson, on shore.
Come the day of the main event, the press were on hand as well as some local dignitaries to witness the final part of this massive operation. Those on the sea wall were asked not to wear anything reflective to affect the operation of the laser system. It so happened that on the sea wall that day there was a member of the senior management who wasn’t exactly the most popular man in the world shall we say. He was wearing a life jacket with reflective patches on each side of his chest. (I know, you can see where this is going!!). The SAR III positioned itself under the LISA A and then switched to the dynamic laser system to hold it under the crane. The lasers searched for the reflectors and yep, one of them locked up onto the jacket of the senior manager. As he strutted off down the sea wall, several thousand tonnes of concrete and barge followed him down the river. The frame that captured and lifted the box was already over the box itself and so one of its legs left a good 10 foot scratch along the side of the box. For many years this was (and maybe still is) visible on the right hand side of the first caisson under the bridge on the English side.
Finally, on getting home after a long days work, I switched on the local evening news, only to see another member of the management explaining to camera that the “trial run” had gone perfectly and that the main attempt would happen the following day.
Suffice to say, it wasn’t a trial run, the senior manager returned to head office and wasn’t seen on site again and we had a good chuckle for many a day after.
