Our client needed to install a signal cable across a 23m wide section of the river Avon in Dorset. The signal cable will be a vital component in the water level monitoring and flood forecasting system.
From here to there – where the new cable needed to go
But how do you get a cable across a river which is over 3m deep – and how do you protect it and ensure that it stays in place?
Working with the team at Commercial and Specialised Diving Ltd, a plan was devised to use a 63mm twinwall duct, laid as a single run across the bed of the river.
2m lengths of heavy equal angle steel were prepared and galvanised. These would be laid over the duct and fixed to it. The steel would offer impact protection to the duct and also act as ballast to hold it in place on the river bed.
The banks at this location are piled vertical walls. On the right hand bank the duct was laid in to an existing but redundant stilling tube and on the left bank the duct was fixed into the web of the existing sheet piling.
The underwater work was carried out by a diver equipped with surface supplied equipment. This is where the air is supplied to him from the surface through a hose known as an umbilical. The air supply is constantly monitored by another member of the team who can also communicate with the diver through the umbilical. A second diver remained on the bank fully suited up, ready to provide assistance if required or even a rescue should a problem occur. Two further members of the team worked from a support boat.
The first job for the diver was to clear the weed from the river bed along the route of the duct. With this done the ducting, complete with draw cord was floated across the river and loosely secured on the opposite bank. The duct was then pushed up the stilling tube and secured at a pre-cut access port.
The first section of steel was then roped down to the diver from the bank. The diver then laid the steel over the duct, the steel and duct were then fixed together.
The next sections of steel were loaded on to the support boat and carried in to the channel, where they were roped down to the diver one at a time and laid and secured in to position.
At the far bank the duct was fixed in to the web of one of the sheet piles.
Lots of kit required
The cable was then securely tied to the draw cord and pulled through the duct from the far bank. Once the cable had emerged from the duct the draw cord was removed and the cable coiled up for later connection.
Made it – job done
Once all of the works in the water were complete the diver was retrieved from the river and given a well earned hot cup of tea – he had been in the water for over an hour.