Innovative solutions in unusual project to stabilise historic viaduct

About the project

Enable Infrastructure (formerly BCM Construction) were subcontracted to install temporary ground support within the arches of a Victorian viaduct on the East Coast Main Line. As the viaduct was slowly deteriorating, this work was essential so that Network Rail could keep the ECML open as a safe and reliable railway for passengers. 

The project presented several distinctive features, including a number of challenges. It was part of Story Contracting’s CP6 Structures Framework on London North East awarded by Network Rail. 

The five-span masonry arch structure north of Hatfield, Hertfordshire, was originally built in the 1850s and had been widened at some later point. The structure had been stabilised previously with pattress plates and ties, with some of the arches also having been loose-filled with soil to provide support.  

Because of the unusual way in which the structure had been widened, it was necessary to install the ground support from the top down to prevent future movement in the backfill behind the wingwalls (which were being stabilising by piling carried out by another contractor). 

The works involved installing a wall in three rows, with each row being underpinned as the excavation progressed downwards, so as to leave the whole height supported. Once this was installed, the outer spans were infilled with a combination of concrete, foam concrete and grout to provide permanent support to the temporary wall by surrounding it with cementitious material. The exposed face was faced with matching brickwork. 

As one element of the concrete infill, Enable Infrastructure had to install 880 dowels, each 800mm long and grouted into a 35mm diameter hole. It was during this process that we discovered a void that was not shown on any of the drawings.  

Another complication was the restricted access to parts of the site, because of the close proximity of the viaduct to the River Lea. 

After devising a number of innovative solutions to meet the various challenges safely and efficiently, we completed the work successfully for Story Contracting.  

The challenge

CHALLENGE
Owing to the number of holes required for the dowels, the use of traditional handheld tools would have involved exposure of workers to hand-arm vibration syndrome (HAVS) and required closely monitored short trigger times to maintain compliance and prevent this serious health hazard. This threatened to be a huge issue throughout the project. 

SOLUTION
Instead of using handheld tools, we deployed a Positioner Actuator Manipulator (PAM). This innovative machine can be operated by a single operator, with the arm taking both the weight of the tool and almost all of the vibration – a fact which is emphatically demonstrated by its eight-hour trigger time. The further advantage of the PAM is that without the downtime and fatigue that result from using handheld tools, productivity is significantly higher. 

CHALLENGE
Span 1 of the viaduct was located between the principal site road and the River Lea. The river was dammed to allow access during most of the project, but the Environment Agency licence stipulated that the dam had to be removed in mid-September, leaving no safe way to access the site. This risked serious delays in completing the project.  

SOLUTION
Working with Story Contracting, Enable Infrastructure mitigated the delays by installing a new trackside pedestrian walkway. We also installed steel-concrete pipes across the river, which were double-skimmed to mitigate concrete spillage. 

CHALLENGE
Discovery of the previously unknown void during installation of the dowels.  

SOLUTION
We undertook in-depth surveys to ascertain the extent and nature of the void and then devised a solution involving infilling it so that the installation of the dowel bars could go ahead.