Remember concrete paved roads? You don’t see many of them now. I wonder why? Well, the continuous bump, bump, bump while riding along was enough to put anyone off, even the most hardy of travellers. I often wonder whether that road experience has stayed in our psyche and is one of the reasons why the idea of a paved, or, as we would prefer to say in railways, slab track has always met with scepticism and some resistance.
Enough of this psychology. Whatever the reasons were for not taking slab track seriously, attitudes have been slowly changing over the last decade or so. Up until now, the use of slab track on the UK rail network has been confined to a few tunnels with restricted clearances alongside a small, random list of sites with similar restrictions. Rail Engineer has written a number of articles on this subject, such as the trials in Asfordby Tunnel near Melton Mowbray (issue 116, June 2014) where PORR Slab Track Austria was employed.
Since then, the same system has been installed in Winchburgh Tunnel near Edinburgh, Queen St. Tunnel in Glasgow, and on Gospel Oak in London. It has had a full Product Approval from Network Rail since January 2017.
PORR is a construction company based in the Austrian capital and the oldest company listed on the Vienna Stock Exchange. Formed in 1869 by Arthur Porr, hence the name, it now employs more than 19,000 people and is involved in many major projects, across Austria, Germany, Eastern Europe, Scandinavia and the Middle East, as well as some railway work in the UK.
Since the late 1970s, PORR has been working in partnership with Austrian Railways (ÖBB) to develop a sustainable and cost-effective form of slab track. This partnership has been very successful because, from the outset, both parties were able to play an integral part in developing a sustainable and efficient design that both meets the client’s specification and can be constructed in an efficient and safe manner.
Both client and contractor’s aspirations have been met – an outcome that is often pursued but not always achieved. The system they jointly developed is now known as Slab Track Austria (STA).
The resulting PORR STA system has been extensively used on a major rail initiative in Germany. Back in 1991, shortly after reunification, the German government wanted to improve transport links between East and West Germany. It created a construction project, VDE 8, for a new 300km/h high-speed line between Berlin and Munich.
PORR was contracted to design and build three major sections of the railway route. Its patented slab track, STA, was installed over a total length of 320km, in tunnels, on bridges and in open sections. Operations started successfully in December 2015 on the VDE 8.2 section, from Erfurt to Leipzig and Halle. Since December 2017, the sections VDE 8.1.2, from Coburg to Illmenau, and VDE 8.1.3, from Bad Staffelstein to Coburg, have been in operation. Trains have been running on the STA slab track layout at speeds of 300km/h. Prior to commissioning, this slab track was tested at 330 km/h.
The oldest section of STA was actually installed back in 1989, so it has been around for some time. But what is so special about this slab-track design?
It is based on a cleverly designed slab of concrete – 5.2 metres long, 2.4 metres wide and 0.16 metres deep, weighing about five tonnes. However, a unique attribute of the STA is that the width of the slab can be reduced down to 2.1 metres where there are problems with clearances, often found in old Victorian tunnels on the UK network.
The slab itself is precast and can be made, either in a precast concrete factory or at an on-site facility established specially for the project. In either case, PORR is able to ensure that high levels of quality control are maintained at all times, the work is carried out in a safe environment minimising the activities on site and reducing the risk to its employees.
The reinforced concrete slab is not tensioned and the rail support seats are integrated into the design. There are two rectangular openings, each 0.64 metres by 0.92 metres, located in the centre of the slab.
However, one of the key elements of the slab design is the elastomeric layer, manufactured from recycled rubber, which covers the soffit of the slab and the sides of the two tapered openings. This elasticity, plus the elasticity of the rail pads, enables stresses created by train movement to be dissipated, absorbing vertical movement of up to 1.5mm. The elastomeric layer also helps to reduce vibration and structure-borne noise, thus offering protection to supporting structures and reducing the noise created by passing trains – an important feature in built-up areas and tunnels.
In each slab there are five small holes, approximately 50mm in diameter, one in each corner of the slab and one in the centre, designed to accommodate simple screw jacks. The jacks are used on-site to provide finite adjustment of the slab level before pouring self-compacting concrete through the two rectangular openings to secure the slab in its final location.
Maintaining track alignment
The self-compacting concrete requires no vibration, which further reduces the activities and risk on site and minimises the risk of any movement in the slab, thus ensuring track alignment is maintained during construction. The two rectangular openings, which are slightly tapered, are now full of reinforced concrete and the tapered shape of these openings helps the slab to resist lateral and vertical forces and works as an anchor to ensure the slab remains secure.
Each slab is separated from the next one by a 40mm joint, allowing for expansion or any form of deformation that could be caused by creep, shrinkage or temperature changes. These gaps can also serve as drainage outlets for surface water as well as providing a pathway for any cables that need to cross the track.
There is a specification for the construction of transitions from slab track to ballasted track that involves the use of synthetic resin to strengthen the ballast by varying degrees from totally interlocked ballast to free-flowing.
With regard to transitions from straight to curved track, STA offers two options. The first allows for conventionally shaped slabs to be placed in position, with adjustments made using the screw jacks, whilst the second option allows for the slab to be specially cast in the factory with the transition built-in. Both options have been used very successfully, the choice is usually determined by client preference.
Flexibility in design
Because of the flexibility available when working in a factory environment, slabs have been designed to accommodate switch and crossing layouts. Also, concrete guide rails can be adapted into the design, which has proved popular on the slabs that have been designed for the Doha metro system in the capital of Qatar. PORR is installing STA slabs across the 162km Doha network, with work expected to be completed later this year.
Slab design can be adjusted to accommodate most requirements, including one that will be welcomed by the infrastructure maintainer – dedicated track access points. Additional units designed to sit level with the rail head can be incorporated into the design to enable road/rail vehicles, trolleys and emergency vehicles to gain easy access without damaging the infrastructure as well as providing protection against derailment.
Whole life cost
The design life of an STA slab is 60 years, although detailed research suggests that this could be extended to as much as 80 years. The cost of construction is quite low, given that the process uses low-mass concrete and is fairly straightforward.
Cost analysis research suggests that the savings made from the reduced maintenance required for STA track will equate to a payback of within 15 to 20 years when compared to ballasted track systems. The opportunity for significant savings, as well as increased network availability due to the reduced maintenance requirement, has to mean that this system is a serious contender for any new railway route, one of which, of course, is HS2.
However, inevitably things will go wrong. A derailment caused by a ‘hot box’ could damage a significant length of track and there is always the possibility of sub-base settlement. These are always understandable points raised against the use of slab track. So how would the STA design cope with these problems when they arise?
The answer is really quite simple. If the problem is local settlement, then the existing slab would be released by drilling out the concrete fill in the tapered openings. The screw jacks would then be used to raise the slab into the correct position, after which it would be bedded-in by pouring fresh self-compacting concrete through the openings into the newly created void. Adequate strength is achieved within 24 hours.
Smaller adjustments can be made through the rail fastening system, without the need to alter the level of the slab.
With regard to derailments causing damage to the slab, the expectation is that, in most cases, this will be limited to the haunches around the rail base plates, leaving the slab itself intact. Replacing the plates themselves is quite straightforward and quick to do. Any repairs to the surrounding concrete seating and slab would be carried out at the same time, allowing trains to run at a reduced speed dependent on the severity of the damage.
As a last resort, slabs could be replaced, but research carried out by PORR and ÖBB suggests that this would only be needed in exceptional circumstances.
Benefits of early engagement
As already intimated, PORR is obviously keen to be involved in the development of HS2 and the experience gained from the construction of the aforementioned German high-speed route highlights the benefit of early contractor involvement and an integrated design and construction process. To understand, at an early stage, the type of track required enables the design team to consider its effect on the design of viaducts, tunnels and embankments.
The HS2 team will have to make some difficult decisions, and soon. It appears that it has already decided that the line throughout should have a concrete sub-base, which clearly indicates that slab track is a serious option. PORR, like its competitors, has already lined up partners for the precast construction of the slabs and for the many other aspects related to the construction of a new railway. It is a very complex process, however the potential rewards are significant.
Clearly, in collaboration with the client ÖBB, PORR has developed a slab track product that has been proven suitable for a high-speed railway at speeds in excess of 300km/h. The cost/benefit analysis is very persuasive and the slab design is kept simple, yet has subtle appealing differentiators. The quality of the precast unit can be assured, and the process of installation on-site is a highly sophisticated operation, developed over decades with highly trained teams well-versed in educating and supervising a local workforce.
As has already been proven, the speeds that HS2 require are attainable and the PORR STA system offers a high quality, durable and sustainable solution.
It is a very interesting time for the UK rail industry. Many key decisions are going to be made over the coming months and certainly PORR, with its STA system, must be a very serious contender.
Watch this space!
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