The Great Western main line (GWML) is one of the oldest and busiest in the country, linking London with the Midlands, the South West and West and most of South Wales. Engineered by Isambard Kingdom Brunel, it was originally founded in 1833 and ran its first trains in 1838.
Now, with freight and passenger traffic continuing to grow rapidly, the line is undergoing a £2.8 billion process of upgrade and electrification to allow the introduction of faster cleaner and greener rolling stock which will provide a 20% increase in passenger capacity.
However, none of this expansion and improvement seems to come without its share of challenges to overcome. Not least of which is the re-engineering of many of the original overbridges, some dating back more than 150 years.
One such challenge is at Hanneys Bridge near Grove, Oxfordshire. Here, BAM Nuttall was faced with the challenge of raising the vertical clearance of a single span overbridge to accommodate overhead line equipment (OLE) as part of the electrification of the Wootton Bassett to Reading section.
Hanneys Bridge carries an unsealed public byway across the line, providing north-south access to a sewerage works from the village of Grove, immediately south of the line. Built in the 1870s, the bridge required partial demolition and reconstruction to create sufficient OLE clearance.
BAM Nuttall site engineer Cathal Nee described the works: “The increase in elevation of the bridge deck, from four metres to 5.25 metres from the top of the rail head to the soffit, meant that the approach ramps at each side of the structure also had to be raised to meet the new bridge deck level. To achieve Eurocode compliant design criteria and accommodate the required 40 tonne vehicular loadings, the old ramps had to be cut down and completely replaced with new structures.”
Ground surveys identified predominantly soft surrounding soils, highlighting the need for major reconstruction. In light of these findings, and because of close previous working relationships and extensive experience in such conditions, BAM brought in geotechnical specialists Maccaferri to propose a value engineered solution based on the construction of new reinforced soil approach ramps.
On the plus side, the surveys revealed that the original Victorian brick abutments to the bridge were still in excellent condition and would only require stabilisation and relatively minor reinforcement to increase their height and bearing width to accommodate the new raised deck.
As the line had to remain operational throughout the reconstruction, the original bridge deck was removed and replaced over the Christmas period 2015. Two new cill beams were installed on the raised and deepened abutments and the new deck was craned into position.
Engineering a better solution
The solution proposed by Tony Gee and Partners, with detailed design work undertaken by Maccaferri, required the complete removal of the original ramps and the construction of a pair of replacement ramp structures. As there was no land-take, the design was to be undertaken within the footprint of the existing embankments. A number of options were considered but a reinforced soil solution was adopted, using the Maccaferri Green Terramesh system over a geogrid-reinforced load transfer platform.
Maccaferri engineer Nico Brusa takes up the story: “The stability of an embankment constructed on soft soil such as at Hanneys Bridge is governed mostly by the shear resistance of the foundation and is an issue of bearing capacity.”
According to BS8006:2010, reinforcement may be placed at foundation level to prevent shear failure both in the embankment fill and in the foundation soil. BAM excavated the subgrade beneath the exiting ramps to a depth of up to 400mm and Paralink, a high strength and stiff reinforcement layer, was then introduced to improve embankment stability. This reinforcement provided the additional strength needed to achieve the equilibrium state, increasing the safety factor against catastrophic failure.
Nico continued: “With the stiff geosynthetics reinforcement at the base of the embankment, the resulting stress condition will be ‘vertical and inward’ rather than ‘vertical and outward’, as would be the case for an unreinforced embankment.”
Stability analysis carried out using Mac.St.A.R.S 4.0 (Maccaferri Stability Analysis of Reinforced Soil and walls) and MacBars (Maccaferri software for Basal Reinforcement) indicated that the use of bonded geogrids made of straps comprising a core of high tenacity polyester tendons encased in a durable sheath of low- density polyethylene (LDPE) would provide sufficient support for the embankment to ensure that stability is enhanced to an acceptable factor of safety.
The material used, Paralink, is manufactured by Linear Composites, a Yorkshire-based Maccaferri subsidiary, and is used worldwide for the construction of embankments over soft soils, over piles and for those constructed over areas where voids are present.
According to Maccaferri, they are amongst the most tried and tested geogrids in the world offering 120- year design life and high performance.
The new reinforced soil approach ramps were constructed using the Green Terramesh system and installed by BAM Nuttall under the guidance of Maccaferri. Green Terramesh is a modular formwork system designed to produce a steeply sloping vegetated face which will quickly blend in with the surrounding rural landscape. It is specifically designed for use in reinforced soil construction, supporting steep sloping embankments in road, rail, housing and commercial developments.
This system is a one-piece, factory-made unit, which includes an erosion control mat and factory-fitted brackets to create a steep slope face. The system is ready to install on site without any additional accessories.
After the foundation has been prepared, the system is placed in position empty, and securely fastened to adjacent units using C-rings along all edges, to form a continuously connected, monolithic structural unit. The erosion mat, pre-installed immediately behind the sloping faces of the unit controls erosion and promotes rapid vegetation establishment. A wedge of topsoil is placed behind and in contact with the blanket to provide a moisture and nutrient reservoir, essential for successful vegetation.
No external support, shuttering or accessories are required when installing Green Terramesh, which considerably increases the speed of the installation.
Courses of Green Terramesh units were placed back-to- back, between 10 and 14 metres apart at the base of the structure, to form the opposing faces of the 60 metre long approach ramps, with a maximum height of six metres. As installation progressed, Maccaferri Paragrid, biaxial-array geosynthetic straps, were introduced within the reinforced embankment to provide stability to the structure.
The backfill used to construct the ramps was a well- graded granular material in compliance with specification for highway works. The material, however, exhibited a sulphate content five times higher than normally allowed for 6I/6J material.
Kesternich testing to ISO 6998, undertaken on the durability of the polymeric-coated woven wire mesh on the Green Terramesh units, adequately demonstrated the high level of resistance to sulphate attack of the products. Also, the Paralink and Paragrid will remain stable in high pH environments, as declared on the BBA Certification, with a durability up to 120 years.
The geometry at the interface with the bridge abutments and the new ramp structure was highly complex, requiring significant use of 3D CAD modelling to determine the configuration of the various reinforced soil elements used. This geometrical complexity arose from the re-use of the existing abutments and installation of a narrower bridge deck.
Sam Doe, design engineer for Tony Gee and Partners, explained: “To overcome this complex interface geometry, the Green Terramesh system was adapted so it could be used to form an 85 degree slope rather than the more usual 70 degree. To allow for this, and to avoid compaction works taking place over the railway, the Green Terramesh units at the interface were backfilled with lean-mix concrete. This allowed for the tops of the abutments, which were exposed due to the installation of a narrower bridge deck, to be utilised in the design and allowed for an aesthetically pleasing brickwork cladding to be specified.”
With an increase in elevation of road level at the bridge of some two metres, it was necessary to similarly increase the level of the approach embankments while maintaining an adequate factor of safety of the abutments. This issue was addressed by constructing reinforced soil walls of Maccaferri Terrawall immediately behind the abutments, so that the forces exerted by the new structure onto the abutments were sufficiently low.
With consideration of long-term strain development within the geogrids, a detailed construction methodology was developed to minimise post-construction strains in the reinforced soil structures and minimise stresses transferred to the abutments.
Commenting on the successful implementation of the reconstruction work, Sam Doe said: “By combining reinforced soil walls, slopes and basal reinforcement, a highly adaptable and elegant design solution can be developed. The reinforced soil elements can be combined seamlessly to overcome the many challenges faced on a project such as Hanneys Crossing.”
Reconstruction of the Hanneys Bridge approach ramps began in late March 2016 and was due for completion during mid-May. Throughout the works, the GWML remained open to traffic, testament to the close working relationship between the design, supply and construction partners.