The Chelmer and Blackwater Navigation, is a canal that spans 14 miles from Chelmsford in Essex to the sea lock at Heybridge near Maldon. The canal, which opened in 1797, was once a trade route, and is now used mainly for leisure purposes during the summer months.
Waterways that cross the canal are diverted underneath it by culverts that are known locally as chunkers. The chunker at Heybridge carries the flow of the Langford ditch and, like the canal, is more than 200 years old. It suffered a partial collapse in 2005 when the old elm timbers gave way, causing part of the canal to leak into the drainage channel. Some 230 properties upstream, some of which had been built in the flood plain, were put at risk of flooding.
Temporary measures such as sandbags and additional pumps failed to solve the underlying problems and in 2009, Royal HaskoningDHV was asked to undertake the detailed design of a new 35m long culvert, provide site supervision, and commercial project management of the construction.
Chris Fosbeary, Royal HaskoningDHV’s project manager, recalls the scene: “We needed to pile across both sides of the canal bed to secure it, and then excavate the chunker. We diverted the stream into the canal and pumped out the water, but it was still pretty wet down there. The chunker was replaced with a new culvert and we constructed headwalls at both the intake and outfall ends.
“There were a number of constraints relating to the project which included the grounds of a converted mill, now a Grade 2 listed building, were on one side of the chunker, so we could only get vehicular access from the other side. On the other side of the chunker services which could not be diverted including a medium pressure gas main were present. As we were working in a residential environment and due to the issues with services, we were careful to keep noise and vibration to a minimum. For example, the steel piles were pressed into the ground using a hydraulic press system, instead of driving them with an impact hammer as you would normally do.
“Working in a conservation area brought additional constraints, such as the need to obtain consent to remove trees that were obstructing the work.
“We refined and outline design previously produced by another consulting engineer to make cost savings and meet environmental objectives. We stripped out a lot of concrete from the original design and used steel sheet piles instead, which reduced the carbon footprint significantly as were able to use recycled piles and was also more cost effective.
“As designers we have to design out as many hazards as is reasonably practicable, and using pre-fabricated products instead of casting concrete on site reduced the health, safety and environmental risks significantly.”
Royal HaskoningDHV has considerable experience in this area and in 2010 developed a culvert design guide with the Construction Industry Research and Information Association (CIRIA), focusing on the whole-life asset management of culverts and their assessment as part of a catchment system.
Other skills also played a part in the work. Our Environment experts carried out an environmental appraisal and prepared an action plan. Our specialist contaminated land team were called in to investigate a patch of oil contamination in the ground water to ensure it did not impact on the work. Ground investigation was carried out to determine the nature and properties of the underlying soil. The Infrastructure and Buildings team carried out structural surveys of nearby buildings.
Work began in December 2009 and had to be completed by April 2010 to allow the canal to reopen for the summer season. If the work had not been completed on time, the next available opportunity to work on the site would have been in October 2010. There was no slack in the programme and in quick succession the initial surveys and studies were completed, the designs were prepared and refined, and the construction work was completed by external contractors. It was important to award the construction contract directly to one chosen contractor to avoid the time consuming process of competitive tendering, and working with the client’s Procurement team, Royal HaskoningDHV was instrumental in enabling this to happen.
Chris Fosbeary praises the contribution of all involved: “We worked well with Environment Agency’s supply chain. The client was very receptive and was closely involved throughout the design and construction.”
The project was completed on time and cost 40 per cent less than the original budget. The flood risk has been reduced by using a 1.2 metre pipe which increases the capacity of the new chunker and improves its ability to carry water away from the area. The new design will enable the Environment Agency to implement a wider flood alleviation scheme, and the Chelmer and Blackwater Navigation is open to the public and more secure than ever before.