HomeAdvertising featureHolding back the tide at Cobb Valley culvert

Holding back the tide at Cobb Valley culvert

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The railway network includes some stunning structures, built by the foremost engineers of the day.

The list includes the Royal Albert Bridge between Devon and Cornwall (IK Brunel, 1859), the Royal Border Bridge in Northumberland (Robert Stephenson, 1850), Forth Bridge (Sir John Fowler and Sir Benjamin Baker, 1890), Glenfinnan Viaduct, Inverness-shire (Robert McAlpine & Sons, 1901), Ribblehead Viaduct on the Settle-Carlisle line in North Yorkshire (John Sydney Crossley, 1875) and the Twemlow Viaduct in Holmes Chapel, Cheshire (George W Buck, 1841).

There are even some modern ones. The Medway Viaduct (HS1, 2003), and Loughor Viaduct in South Wales (Carillion and Tony Gee & Partners, 2013) are good examples.

In a list published in 2013, Network Rail identified 28,451 bridges in its portfolio, excluding footbridges and culverts. It commented: “A bridge is defined as a structure with a span of ≥1800mm.”

Small and unseen

But what of the ones under 1800mm? Often overlooked, and certainly not iconic, these are the drains and culverts that run under the railway all over the country – around 30,000 of them, just on Network Rail infrastructure. They have to be maintained, they may well have water running through them either constantly or in times of flood, and any problems with them may well go unnoticed until it is too late.

Like most marine structures, culverts are simple, but can become damaged and dangerous with water erosion over time. A good example is the Cobb Valley Culvert in Anglesey, North Wales. This runs directly under the A5 highway and North Wales Coast railway between Valley and Holyhead, adjacent to the Cymyron Strait. With water flowing through the structure at approximately four metres per second (nine mph), the high flows and poor conditions prevented any safe means of access to carry out inspections and repair works.

Kaymac Marine & Civil Engineering was commissioned by Network Rail to control the powerful flow of water through the structure so that a structural survey and any remedial works could be carried out safely. However, with the Irish Sea to the north and the inland tidal lagoon to the south of the structure, there is constant flow, making this a challenging operation if not controlled correctly.

Water flows through the structure on both rising and falling tides, with slack water lasting only a very short time – around 10-15 minutes – so this was no easy job for the Kaymac team as the works had to take place at night during blockades of the railway line and within the short window at slack water.

Tidal challenge

To control the water flow, Kaymac designed a steel guide and sluice gate system that would hold the water back and prevent any flow through the structure. Due to the volume of water that flows through the structure during the tidal cycle, the sluice could only be lowered and raised at certain states of the tide lest it became jammed in the steel guides or, potentially, damaged during the operation.

Although the sluice could be lowered and raised using pneumatic winches or manual chain pulls, Kaymac used a crane positioned on the A5 carriageway to ensure that the sluice did not become obstructed through possible un-balanced lifting as a result of using two separate winch systems.

Working against the clock during a Saturday night possession, the specially designed and fabricated steel guides were installed and a sluice gate lifted into position, ready for operation at slack water, using a 25-tonne crane positioned within the road closure on the A5. Slack water was at approximately 02:00 and there was a 10-15 minute window to lower the gate into place, which was successfully achieved.

A stop log system was then installed at the seaward side of the structure at relevant states of the tide throughout the week using a 10-tonne mobile crane working within the temporary traffic management set up on the A5. Following installation of the stop logs and the final sealing, the sluice gate was removed using two six-tonne air hoists during another Saturday night possession; a road closure was in place and a 25-tonne crane on site as a precautionary measure.

The whole structure became totally accessible and, after having been inspected safely by divers, re-pointing and fracture repairs were carried out.

On completion of the repair works, the sluice gate was re-fitted, allowing safe removal of the stop logs. The sluice gate was then raised at slack water, allowing free tidal flow through the structure. The upper steel guides permitted the steel sluice gate to be raised and then fixed into the open position until Network Rail requires further maintenance or inspection work to be carried out in the future.

In this instance, although working on a rail culvert, the Kaymac team called upon its expertise in marine engineering to assist. The company regularly inspects lock gates, installs cofferdams and carries out scour protection work, an activity which is also in demand around rail bridges over rivers and estuaries.

With facilities at Swansea, Bristol and in Kent, Kaymac is well placed to assist with all types of ‘wet’ work and specialises in innovation above and below the water line.

Read more: Swanage Railway Reconnected




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