Moving Mountains at Sebastopol
19 Sep 2008
Steep sided valley
If you were to catch a train from Cardiff Station to travel up the Rhymney Valley Line, you would pass a medieval castle and stop at a number of stations including Tir-Phil. Just beyond this station the single track runs along a narrow formation perched 30m above the Rhymney river. Above this track formation the steep slopes of the valley, now occupied by sheep and horses, extend upward for another 500m - which is not surprising, since we are close to the Brecon Beacon mountain range.
Frequent landslips
This impressive and very picturesque location is known as Sebastopol and it has been an area of concern for railway engineers since the line was opened in the 1850s. It is designated a landslide area and the scars and undulations on the hillside above and below the track prove it. The line has suffered two major landslips, one in 1906 and the second in 1932 and there has been a Temporary Speed Restriction (TSR) in place for the last 4 years.
Trains have had to reduce their speed from 50 mph to 10 mph. Just before last Christmas, the movement was so severe that a 5 mph TSR was imposed. A substantial amount of remedial work was carried out subsequently, including the installation of 145 Grundomat piles alongside the track formation.
150mm movement in one year
Network Rail have previously dewatered the embankment more than three years ago with Dean & Dyball as the appointed contractor. As part of the work, they installed a well equipped weather station at the site and also a rake of inclinometers. This enabled NR to monitor the local weather conditions and any emerging trends. It also enabled them to monitor any movement of the numerous slip planes that were active up and down the valley side.
It was established that track stability was being continually affected by a slip that had moved more than 150mm in the previous 12 months. This meant that the local Permanent Way maintenance team led by the Track Section Manager Gavin Evans continually had to visit the site to drop ballast and realign the track.
Finding a solution
The information that NR were now able to gather indicated that the situation would get worse. The option to close the line was unacceptable to Network Rail since it would be detrimental to the development of the area. There was no alternative other than to develop an engineering solution that would rectify the problem once and for all.
Design consultants Tony Gee & Partners were appointed by NR to produce a scheme that would stabilise the ground movement both above and below the track formation and enable the 50 mph line speed to be restored. The design team leader Austin Weltman used the valuable information gathered from the dewatering installation and the inclinometers to help develop a solution to the problem.
The solution chosen was to install 257 hollow steel tubes, 400mm in diameter and ranging from 14m to 22m in length. These tubes, placed in augered holes at 1.2m centres, will have a 75mm cover of concrete on the outside giving an overall diameter for the pile of 550mm. The track formation being piled is approximately 200m length.
A C40/50 grade reinforced concrete capping beam will provide a continuous wall alongside the track and at each pile location Dywidag Permanent Ground Anchors are being installed through the beam. These anchors will be between 27 and 37 metres long, inclined at 35 degrees and grouted into the ground underneath the track formation. In addition to this work, a five by five grid of 175 similar piles will be installed approximately 10m above track level, set back into the hillside. All the tubes are to be filled with grade 25/30 reinforced concrete.
Six week closure
To carry out this work, the track has to be removed and a six week closure of this part of the line imposed whilst the 257 piles adjacent to the track are installed. It was estimated that a further two weeks would be required to work within the Rules of the Route and a final two weeks is required to complete the work to the lower slope without the need for possessions.
Negotiations with the train operator Arriva Train Wales were concluded and it was agreed that the line between Rhymney and Tir-Phil would be closed from 25th July until 8th September and passengers would be transported between these stations by bus.
The Area General Manager for NR stated:- ’Ultimately, we do not want to compromise on passengers’ safety or risk suspending the line permanently. This project has been carefully planned to bring minimal disruption to current passenger services. We are grateful for the support from Arriva Train Wales, allowing us to close the line for a short period of time, so that the most critical part of the work can be carried out effectively.’
He emphasised that this was ’a highly complex project that will provide a robust solution. The challenge may be huge but nothing can compare to the benefits that this project will bring to passengers.’
A significant undertaking
NR has in place a Great Western Earthworks & Structures Framework Agreement with Carillion who are described as the ’Framework Contractors.’ The work at Sebastopol is within the scope of the arrangement and so Carillion was instructed to organise and prepare for this work to be carried out.
However, as you have probably already gathered, this is no run-of-the-mill task. First of all, the cost of the project is in the region of £8m. The site itself is perched on a steep, often saturated and very unstable valley side. The solution has to be permanent and the time constraints are challenging. To cap it all, Sebastopol it is claimed, has a reputation for rainfall levels that cannot be matched anywhere in Wales.
Work progressing well
The Scheme Project Manager is Daniel Tipper. He has gained a considerable amount of experience working on a variety of projects for NR over the last few years, but he says that this is the most interesting and complex project he has done to date. However, he is kept busy, having a number of other projects on the go as well as this one. (Oh, and by the way, he has an excellent supply of waterproof clothing!)
He is responsible for ensuring that all goes smoothly and that Carillion deliver what is required. He is working closely with Lee Newcombe who is the Site Agent for Carillion and at the time of writing this article, the site is well established, an effective plan is in place, all the key items of plant are on site, piles are being installed, everyone is in a very positive frame of mind - and of course it is pouring with rain.
Temporary platform
A substantial haul road has been constructed by Carillion made up of 9000t of stone. It snakes down the hillside from the site access point to the track formation. At 7m wide, it is able to accommodate the largest items of plant including a substantial 100t crawler, lattice girder jib, crane and a slightly smaller but still substantial 50t telescopic crawler crane. Both of these cranes are now fully employed on site. A stone platform has been constructed to enable the cranes to service the site and a row of piles has been driven along the face of the platform to ensure it remains stable during the work.
Innovation
There is now a substantial stock pile of the steel tubes on site. They are provided by Corus from their works in Hartlepool. The quantity required and their length has caused some concern since the maximum length of pile that can be delivered to site is 18m. As a consequence, Carillion are using Phil Rees Welding a specialist welding subcontractor, to weld additional lengths to the tubes, as required. Phil Rees has also introduced a specially designed vehicle that is able to transport the extra long tubes from the site access down to the point of installation. Lee Newcombe has described this piece of plant as being a godsend and a good example of innovative thinking on behalf of their subcontractor. The vehicle is already in demand on other sites in the country which is good news for Phil.
Another example of innovative thinking and an item of plant that is proving to be worth its weight in gold, is the Mariner 55 all wheel drive, concrete mixer hired from Utranazz. Conventional concrete mixer vehicles are not able to negotiate the haul road gradient during wet weather so this vehicle is used to transfer the concrete to the work face. The equipment is built on a heavy dumper chassis and it is able to go more or less anywhere. Lee’s only complaint is that he could have done with a bigger one.
Good progress
So, at the beginning of the blockade all the track was removed by road/rail vehicles and stacked either end of the site. New rail has now been off loaded ready to be brought onto the site. All this work and the track reinstatement is being carried out by Gavin Evan’s track maintenance team. The two crawler cranes are in place on the newly established platform.
The Grundomat piles are being extracted like rotten teeth and hauled from site. The formation is being levelled and two 39T SOILMEC augers are drilling through the clay and mudstone down to the required level. Ritchies, the piling subcontractor, initially intended to use six smaller auger machines but an initial trial exposed their inability to cope with these ground conditions and the plan was quickly revised.
Geologist on site
Alan Marr of Tony Gee & Partners is the Resident Geologist on site. Part of his job is to monitor the performance of each rig as it drills down and ensure that each drill reaches the required level forming into rock sockets. Close attention to this detail enables the cranes to vibrate the steel tubes into position ready to receive the reinforcement cage and concrete fill. The process is working well despite the rain and so long as the plant on site continues to perform to expectations, and there is a lot of it, the project will be completed on time.
Other options
Was there a cheaper alternative solution? Other options were considered, such as the possible installation of a substantial counterweight below track level. However, this would have required the river to be diverted which in turn, would involve disrupting an adjacent, existing coal slag heap. This could have introduced all kinds of new engineering problems. A second alternative solution was to build a substantial culvert over the river to support a berm but this was quickly discounted on environmental grounds.
Beautiful valley
Neither of the alternative options would necessarily solve the problem and there is no scope here for half measures. The scheme chosen is not only designed to arrest the slip plane that directly effects the stability of the track formation but it is also designed to address the movement caused by the associated slip planes further up the valley side.
Hopefully, it will ensure that there will be a rail service for the Rhymney Valley community for the foreseeable future and it should also ensure that this beautiful valley, scarred by years of mining activity, will continue to revert back to its original state of natural beauty. I think that has to be progress by anyone’s standards and of course NR will get rid of one of their longest standing TSRs!
Copyright Colin Carr.