An Overview of Trenchless Technologies in Current Use
The need for new installations of underground utility systems, as well as for the refurbishment of existing systems, has been increasing steadily as population growth and urban expansion have continued to increase. In many areas, the space beneath the ground is becoming as congested as that above it. Consequently, digging trenches for installation or refurbishment is now widely regarded as neither a practical nor an economical option and one that could even result in damage to existing underground structures. To replace this impractical approach, engineers have developed a number of trenchless technologies that can be used instead.
In practice, a trench can also have a negative impact on events above ground. For example, it can cause a disruption to the flow of vehicular and pedestrian traffic, making it necessary for both to undertake lengthy and time-consuming diversions. Just imagine the extent of the potential chaos if such a trench were to be dug across all six lanes of an urban motorway or, perhaps, even one of the runways at a major airport. Even when the task is almost complete, this does not just involve replacing and compacting the previously removed earth. Additional time and expense will be required to restore the working surfaces of these structures. By contrast, the use of appropriate trenchless technologies presents none of these difficulties.
Sometimes, it is not activities that are disrupted by trenching but the environment itself. While we all recognise the value of heritage sites and areas of natural beauty, these often form the most convenient route for an underground pipeline. This means that the requirements of the painstaking, detailed restoration of the worksite will add significantly to the overall project cost and duration. Clearly, these are all good reason to avoid digging a trench wherever possible, so it’s time to take a look at the available trenchless technologies and how they work. These trenchless technologies fall into three categories, depending upon the nature of the application. Thus, the options for new installations differ from those used to replace an old pipeline and those that are best for repairing a worn or damaged pipeline.
The methods used to make repairs without exposing pipelines involve inserting or creating a new lining inside the damaged pipe. The oldest method is known as slip lining and consists of inserting a slightly narrower pipe inside the damaged one and plastering the gap between them. A newer alternative uses a fibre or polyester liner filled with resin and inserted into the damaged pipe under pressure. Applying heat causes the resin to be cured in place in order to complete the repair. The use of thermoformed liners is a third option in which, once the liner is in position, steam is used to mould it so that it fits the contours of the host pipe precisely. There are two trenchless technologies which can be applied to pipe replacement. Both involve the destruction of the old pipe in situ and the simultaneous insertion of the replacement pipe. Pipe bursting is achieved by pulling a special bursting head through the old pipe to break it, replacing it with a new pipe attached to the tail-end of the bursting head. Pipe splitting works similarly but the bursting head uses blades to cut the old pipe longitudinally.
When laying new pipes, there are also two possible trenchless technologies of which the first is known as micro-tunnelling. This technique is best applied when installing small-bore high-density polyethylene (HDPE) pipes, especially in areas where there may be existing utilities. It combines jacking with the use of a remote-controlled boring machine to create a narrow tunnel. It can be used in soft or hard ground as well as for underwater installations. However, horizontal directional drilling (HDD) is, by far, the more versatile of the two trenchless technologies designed for new pipeline installations. The technique employs a specially designed drill train with a remote-controlled and steerable drill bit to create a pilot hole along the projected pipeline route.
The progress of the drill train can be monitored from above to ensure it remains on course. The operator can alter its course, should an obstruction be encountered. On arrival at the endpoint, attaching a reamer and reversing its direction allows the pilot hole to be enlarged to accommodate the pipe. Horizontal directional drilling offers a rapid, cost-effective, non-disruptive, and eco-friendly alternative to digging trenches.