It is likely that there are is something in the region of one million masonry arch bridges in use throughout the world today, principally carrying road and rail. European railways alone account for 200,000 bridges. These bridges form a vital asset and their replacement cost is almost incalculable. Despite being ancient in form, masonry arches are notoriously difficult to accurately assess. At all limit states their behaviour is complex, deriving their overall behaviour from the interaction of individual parts, blocks, bricks, mortar and fill.
In 2009 Network Rail’s Framework team instructed Carillion to carry out site investigations and develop designs on two masonry arch viaducts on the Glasgow and South Western (GSW) rail route at Crawick and Enterkin. This route is the main alternative to the West Coast Main Line connecting Glasgow to the South and carries a high percentage of freight and local passenger traffic.
Both structures suffer from similar defects – namely longitudinal cracks at the arch barrels behind the spandrel walls. However, in addition, Enterkin suffers from deformation of the parapet wall. The outcome of the design was to be a fully developed reinforced U deck with through ties to stabilise the viaducts and extend their service life.
Historic documents and drawings provided enough detail on the make-up of the structures. Internal spandrel walls backing on to the arch barrels support stone slabs for the deck. The original structural voids have been filled with concrete.
All arches on the structures have longitudinal cracks behind spandrel walls. The cracks go from springer to springer on the outside of the bend on the line carrying most freight.
Design development work began at Crawick in May 2009. It was established right at the start that given the make-up of the structures, the Cintec system would be ideally suited. The instruction to start developing the design for Enterkin followed in October. Real-time monitoring at Crawick revealed that the cracks were widening and the programme was brought forward. The implementation of ties was started in January 2010 and completed by April 2010. The installation of the Enterkin ties was started in May 2010 and is ongoing.
It was important to establish in a short time what was underlying the ballast and confirm that the void filling had taken place, so a number of site investigation techniques were used, including:
- Coring work
- Excavation of ballast in confined location
- Laser scan of the Enterkin parapets
- Radar scan at Enterkin
- Panda probe testing
The Permanent Way team quickly established from trial pitting that there was an excess of ballast on the structures. Over the years this has increased the loading and it is felt that this has to some degree contributed towards the current problems. The case was made to Network Rail for lowering the track, which was duly agreed, and the track is to be lowered by 300mm at Crawick and about 250mm on the up line at Enterkin.How the Cintec anchor works
The Cintec system comprises a steel section in a mesh fabric sleeve, into which a specially developed cementitious grout is injected under pressure. The flexible sleeve of woven polyester restrains the flow and expands to about twice its normal diameter, moulding itself into the shape and spaces within the walls, providing a mechanical bond.
The two viaducts have both similarities and differences: Both have the Cintec anchor system tying the structure back together to form a single mass; both have in situ slabs. However, Crawick uses the existing haunch whilst Enterkin uses L-sections and its parapet is tied back.The ongoing work
To date all ties are completed at Crawick and the initial indications from the remote monitoring show complete stabilisation of the arches. Enterkin results are ongoing. Work has started and is ongoing with a plan for completion approximately two weeks before commencement of the main works.
The project has used several site TruViewinvestigation techniques – both traditional and new, which have all found merit. All work has been done by rope access and has been carried out without any disruption to trains. A close relationship was established with the suppliers in developing the design and the company has gained valuable experience to stand it in good stead for tendering on future viaducts requiring similar repairs.
For further information, please contact:
Cintec International Ltd
Tel: 01633 246 614