For many years now, the use of trenchless (No-Dig) technology has increased considerably across many areas of the utility and buried service industry. A significant proportion of this use has centred on applications in the water sector. However, the technology can also be successfully used by rail engineers.
There are numerous reasons for buried services to cross under railway tracks. For example, recent experience has shown just how difficult rail transportation can be when track drainage systems, rivers, streams, culverts and other under-rail crossings fail to work effectively. Cables, water pipes and sewers (other than surface water drainage) also need to cross under railway lines. The repair and maintenance of these systems and the installation of new crossings by traditional techniques can bring train speeds down to a crawl as works progress beneath operating rail lines.
A variety of trenchless technologies now offer increasingly effective solutions for under-track installations and repairs that minimise disruption to rail services by ensuring that tracks are not disturbed.
The types of works that have or can be utilised in under-track operations are:
» Surveying, identification and tracing/mapping of existing culverts, pipelines, ducting, cabling and cable ducts that run beneath rail lines;
» Renovation of existing pipes, culverts, ducts and their associated manholes/accesses using various lining/repair options;
» Construction of new rail crossing pipelines using guided or unguided auger boring, microtunnelling/pipejacking, horizontal directional drilling, pipe ramming or moling techniques;
» Replacement of old or damaged pipes with a new pipe of similar or larger size using pipebursting techniques.
It is a very wide topic, so this article will look at the options available for new installations.
Making an impact
For new service/utility installations there is a wide range of technologies that allow under- track works with minimal impact on train services.
At smaller diameters, there is the option to utilise moling techniques. These use a
reciprocating hammer unit to push through the soil installing a trailing pipe or duct as it advances. However, these units tend to be unguided and have limited range depending on ground conditions. As they operate by compressing the ground through which they pass around the outside of the hammer they also need to be used at a depth where this ground compression would not affect
the track above due to any heave created by the hammer’s passing.
Continuing on the impact hammer theme, another technique that uses hammer technology is pipe ramming. This utilises steel pipe as either a carrier or as the final pipeline. Ground conditions have to be suitable for the pipe to be pushed through.
The steel pipe is positioned at the start point of the new crossing and the impact hammer, normally a much larger and more powerful unit than those used
for moling, is installed at the rear of the pipe. When the hammer is activated, the steel pipe is driven into the ground along a preset route. Whilst the system is generally unguided, the fact that such installations tend to be used over relatively short distances means that any deviation from the target line is relatively small.
Providing the ground is relatively self-supporting, no loss of ground occurs as the pipe advances because the ‘spoil’ remains within the pipe until the pipe ram is complete. This means there should be little, if any, heave or subsidence in the ground surrounding the new pipe during the installation process and so little or no effect on the tracks above. The spoil is then removed from the pipe, making it ready for use.
An alternative technology that has been used to install rail crossings is horizontal directional drilling (HDD). One of the major advantages of this technique is that it is steerable so it can follow a pre-determined course from one side of the track to another. This steerability also means that the crossings can be designed to start at ground level and then pass at any required depth beneath the track and the track substructure without any interference with railway operation during the works.
This technique is used for the installation of plastic pipes as both operating mains and casing pipes. Ductile and steel pipe, if handled correctly, can also accommodate the curvatures normally associated with such bores.
HDD works are normally multi-stage operations. The first stage requires a small diameter pilot bore to be driven on the required line of the installation. The steering of the bore can either be by wireline monitoring, if no track access is available, or by using walkover antenna systems that track a transmitter sonde located close to the boring head underground. More recent developments in the latter system have meant that, over limited distances, the sonde/ receiver system can also now be used to monitor the pilot bore without requiring track access for plotting the bore location.
Once the pilot bore is completed, it is upsized (reamed) open to the required diameter for the final pipe, duct or cable being installed which is then pulled through the finished size bore.