Looking at the headlines in the local papers, it seems that the town of Borehamwood was rudely awakened one Saturday night by Network Rail doing some piling on the railway. This was necessary to replace some overhead electrification portals and masts and was part of planned works which will continue at weekends until June 2014.
Neighbours had been informed, but perhaps Network Rail underestimated how far sound travels at night. An apology was issued with an assurance that more would be done to explain the work to residents.
However, this situation does illustrate an interesting problem. With the railways becoming ever busier, more work has to be done at night. Some of that work is inherently noisy, with piling being perhaps one of the worst offenders. Various options exist, with some being more effective than others in particular ground conditions and some being noisier than others. So can efficiency be traded off against noise? And what options are there for the railway engineer with a foundation problem?
The logical place to turn to for answers is the Federation of Piling Specialists. The Rail Engineer asked chairman Jim De Waele for his analysis of the options open to railway engineers.
Piling and rail applications
Construction projects in the railway environment, perhaps more than any other, are particularly challenging and bring with them a multitude of demands and considerations, many of which are applicable to piling. Issues such as noise, space, time constraints and when works can be undertaken, which for rail is typically weekends and / or overnight, all impact on the project and that is without including rail’s own unique health and safety considerations, structural requirements and the demands and issues specific to the local geology.
Whilst the geology cannot be altered, the Federation of Piling Specialists (FPS) always recommends engaging with the proposed piling contractor from the outset. Their experience, help and advice can positively impact the success of the project. They can also provide valuable technical input regarding the type of piling techniques available, their pros and cons and help address some of the issues typically encountered.
Whilst a detailed technical overview of all available piling techniques is beyond the scope of this article, some of the more common methods and their applications to railway schemes are presented in the table.
A wide variety
However, railway projects can vary widely from Overhead Line Electrification (OLE), signalling masts and gantries through to stations, tunnels, and bridges. OLE forms the main part of recent rail schemes to which piling applications have been applied.
In urban areas, noise is typically the biggest environmental concern, and few would argue that a hammer driven through the night is going to win rail or the piling contractor any friends. Of course, there are various non-percussive techniques that that can be considered to address this and so involving the piling contractor at design stage is critical in making sure the right solution is implemented.
When considering the preferred method of piling, there are a number of considerations that need to be accounted for, marrying the design requirements with the local conditions along with the machines and methods available to the project team. This means that the piling technique can only be optimised with the involvement of the principal contractor, the piling specialist and the designer. However, experience suggests that often this team is formed too late in the construction cycle to allow the optimum solution to be considered.
The fact is that there is no “one size fits all” technique, all piling techniques applied to rail applications are a trade-off to some degree and schemes need to be planned accordingly.
Matching method to application
Over the years, various piling types have come to the fore. When installing piles for OLE masts, the driven pile is now the most common. Nowadays this is commonly a 610mm tube and can be installed from a road rail vehicle (RRV) such as a Mega or Giga Railer. There are also some “high output” trains on the network and dedicated rail mounted piling rigs that also are able to install these piles.
Where noise or vibration restrictions are in place then similar piles can be “spun” in. However, these foundations are normally 457mm or 355mm in diameter recognising that the power of the equipment is inevitably restricted.
All these steel piles can be used in conjunction with tie rods or soil nails to provide additional lateral capacity when necessary. Being made of steel there is no curing time and with the fitting of an adjustable head plate, the OLE mast can be connected on top of the pile immediately after the pile is set. On a similar theme, H piles have also been used in the past, being either vibrated or driven into the ground.
For signals and light weight structures screw piles are often selected. These again can be installed using RRVs, readily adapted with a rotary motor.
In and around stations, where concrete can be easily delivered to the site, bored micropiles are likely to be cost-effective and there is a range of piling rigs that can be sourced to suit the access conditions.
Whatever method is chosen and applied, it is going to be a compromise of often conflicting demands, be that local geology, plant, access or possible noise pollution issues and health and safety. This does not mean to say that any methodology is set in stone. Working with the piling contractor from the outset will allow all factors to be taken into account, which instead of arriving at a compromise solution will instead allow an optimal solution to be delivered that meets everyone’s needs and with the minimal of disruption to all.