Readers might recall that the early part of 2015 was extremely wet, even by UK standards. As a consequence of this extreme, inclement weather, there were a number of locations on the railway network that suffered badly from flooding and/or instability of railway cuttings and embankments.
One of the many articles published by Rail Engineer during this wet weather period was on the 350,000 tonne landslip that extended more than 150 metres through a cutting leading up to a tunnel portal at Harbury (issue 127, May 2015).
As a consequence of the landslip, the line between Leamington Spa and Banbury was closed for six weeks causing major disruption to passengers travelling between Oxford and Birmingham. Network Rail called on the skills and expertise of its emergency earthworks contractor, J Murphy and sons, to stabilise the embankment and restore the track formation in order to allow the 50 freight and 80 passenger trains that use the route every day to recommence their services.
Although the weather conditions continued to be unsympathetic to the challenges facing the engineers, the route was restored in less than six weeks – much to the relief of Network Rail and the travelling public.
A more sophisticated monitoring system than previously used was installed on the embankment slopes and around the tunnel portal. This included 80 wireless slope sensors and wall sensors for the tunnel portal. These sensors were designed to detect any future microscopic movement and the information produced was made available, and continues to be made available, to Network Rail around the clock.
Throughout this period of work, more than 320,000 tonnes of earth has been removed from the slip area using a fleet of Moxy dump trucks and excavators provided by various plant suppliers. The aim was to move the toe of the embankment 30 metres away from the tracks, thus removing the fault line and re-profiling the embankment. A further 150,000 tonnes of earth has also been removed from further along the cutting to stabilise the embankment.
A more permanent solution needed
Before re-opening the route to trains, design details were signed off, revised slope angles achieved, dewatering levels met, signalling tested, track alignment surveyed and structural checks of the tunnel portal carried out. However, it was recognised that a more permanent solution was still needed.
To this end, Murphy has remained on site as the principal contractor. The design services of Tony Gee and Partners have been engaged to provide a more permanent solution, focussing primarily on the Harbury tunnel portals.
In addition, Murphy has acquired the services of Force Contracting Services Ltd. This company started out in 2007, primarily as a fencing contractor, and managing director Ben Layden has grown the business, which now works in a number of industry sectors throughout the UK. Based in Nottinghamshire close to the M1, Force is in a good position to respond rapidly to a client’s needs, whether they be fencing, de-vegetation, civil engineering or construction. It employs more than 50 staff and has only recently entered into the railway sector over the last three years.
The bulk of the work currently underway is situated around the tunnel portals where there is still concern about potential movement and the level of instability that still exists. Under the guidance of project manager Jim Broe. the work that Force is carrying out for Murphy involves the installation of approximately 300 soil nails above the tunnel entrance and around the portals. Access is challenging and the slopes are steep. A specific level of expertise is required to gain access to the site, position the drilling rigs and carry out the challenging task of soil nailing on such a steep embankment.
Therefore, Force is using a specialist team to carry out this work. This involves using a team, highly trained in rope access procedures, to abseil down the slope, position the drilling rigs and then drill a 100mm diameter core, approximately six metres long, into the tunnel portal embankment.
A 32mm diameter reinforcement bar is inserted into the bore, which is then back filled with grout and finally plated. Offsetting the anchors helps to lock the ground by creating friction between the soil and the grout, ensuring that the embankment remains stable. Finally, a geotextile wire mesh is being fixed to the anchors to cover the whole embankment and provide further stability.
To complement this work, Force has constructed a new drainage channel around the top of the tunnel portals to capture water flowing off the embankments. In addition, a Kee-Klamp handrail has been installed to follow the profile of the portal and offer safe access for inspection and clearing vegetation in the future.
To further improve access to the tunnel portal areas, Kwik-Step access steps are being introduced that will start on firm ground at the top of the portal embankments and finish at the bottom of the embankment over the crown of the tunnel, a task that requires considerable care and attention given the location.
The major slip did considerable damage to the wing walls of the tunnel portals and it was decided that stability would be ensured by nailing and shotcreting the embankments that were previously supported by those portals. This would not only provide stability but would also negate the need to rebuild the wing walls concerned.
Finally, the brickwork around the tunnel entrance had been repaired by previous generations who decided that, instead of renewing the brickwork, they would spray concrete over the weathered brickwork to protect it. Unfortunately, this protection has now become life expired so Force has been instructed by Murphy to remove the crumbling material and expose the sound brickwork behind, which would then enable specialists Gunform to shotcrete the area during a key possession planned in August.
All the above work is well underway and nearing completion, with the August possession being the final key milestone. The intention is for all the work to be finished in September by which time all the contractors should feel proud of what they have achieved following on from one eminent railway engineer, I K Brunel, who built this railway.
The railway first came to the village of Harbury in 1847 as part of the construction of the main Oxford to Birmingham GWR line. The cutting is located to the north of the village and at the time it was considered to be a significant engineering feat. At over 34 metres deep, the cutting was the largest man-made cutting in the world that was dug entirely by hand through blue lias clay. It was completed in 1852.
Today, Murphy, Force and others are helping to ensure that this railway will last for several more generations.