Trenchless technology refers to a variety of techniques used to install, repair or replace underground infrastructure while minimising surface disruption. Trenchless methods can:

• Lessen environmental impact – trees and root systems are usually unaffected, sensitive areas are not dug up, there are fewer spoil heaps so the likelihood of sediment washing into drains and waterways decreases, and ultimately, there is less spoil to be removed.
• Minimise social impact – traffic delays are reduced, and where road traffic is impacted, it is generally for a shorter period of time than with open-cut installation.
• Offer a sustainable choice – trenchless technology has greener credentials than open-cut methods. The reduction in both excavation and traffic disruption translates to lower CO₂ emissions.

There are also consequential benefits and savings, largely linked to the decreased social impact of a trenchless installation. These include the time that would have been wasted by road users stuck in traffic jams (particularly in urban areas), the running cost of cars sitting in those traffic jams, reinstatement costs, and the loss/reduction of business due to limited access to business locations.

The use of trenchless technology in the installation of infrastructure is ever increasing; what was once used only occasionally has now become commonplace. As a result, contractors in the industry are highly experienced and well resourced, constantly investing in research and new equipment to ensure the industry continues to grow and improve.

Techniques

Below are three major techniques commonly used in the oil and gas industry, and in pipeline installation.

HDD

Horizontal directional drilling (HDD) techniques are used for the steerable installation of new pipelines, ducts and cables. A pilot hole is drilled along the required path to provide steering information, and the bore is then back-reamed (in a single or multi-stage operation, depending on the ground conditions and project requirements) to a larger diameter to accommodate the product pipe.

During the final ‘pullback’ stage, the product pipe is attached to the reamer by means of a swivel connector, and is pulled into the enlarged bore as the drill string is withdrawn. A load monitoring device may be used to prevent overstress of the product pipe.

In earlier years, HDD was used mainly for the installation of pressure pipes and cable ducts, where precise gradients are not usually critical. In recent years, drilling machines and guidance systems have offered improved accuracy, and it is expected that the technique will become increasingly popular for gravity pipelines.

Equipment capabilities have also improved recently, both in the power and diameter of installation available, and in the wider range of ground conditions that can be bored.

Microtunnelling and pipe jacking

Microtunnelling and pipe jacking are essentially from the same family of pipeline installation techniques and can be used for installations from about 150 mm diameter upwards.

The term microtunnelling applies to remotely controlled, steerable, evacuation tunnelling methods for pipelines. Microtunnelling is well suited to situations where a pipeline has to conform to rigid line and level criteria.

The most critical factor in any microtunnelling project is the geology. Extensive ground investigation should be carried out to determine the soil characteristics along the proposed alignment. Modern technology has enabled this method to be applied to a wide range of ground conditions from waterlogged sands and gravels, through soft or stiff, dry or waterlogged clays and mudstones, to solid rock.

Microtunnelling was first introduced into Australia in the mid to late 1980s. Since this time, microtunnelling has been extensively used for the installation of new pipeline infrastructure, particularly for the water industry.

A pipe jack is defined as a system of directly installing pipes behind a shield machine by hydraulic jacking from a drive shaft, such that the pipes form a continuous string in the ground. The pipes, which are specially designed to withstand the jacking forces likely to be encountered during installation, form the final pipeline once excavation is complete.

An essential feature of pipes for both microtunnelling and pipe jacking is that the entire joint is contained within the normal pipe wall thickness.

Pipe length varies according to the microtunnelling system used, the pipe diameter, and constraints of space. Typical pipe segment lengths usually range from 1–2.5 m, although lengths of 0.75 m are available for small diameters and longer pipes are sometimes available.

Pipe jacking and microtunnelling are both commonly used for mainline or trunk pipelines.

Pipe ramming

Pipe ramming is a non-steerable system of forming a bore by driving a steel casing– usually open-ended – using a percussive hammer from a drive pit. Soil is then removed by augering, jetting with water or compressed air. In appropriate ground conditions a closed casing can be used.

Pipe ramming is most often used to install new pipelines or casings into which new utilities will be installed. Installation distances have increased dramatically in recent years from approximately 50 m to up to 100 m. Steel pipe is used for the casing, and once installed, it can be used as a pipeline or as ducting for most types of pipe or cable.

Typically, this method can install diameters of 100–1,500 mm. However, bores of up to 3,700 mm diameter have been installed in suitable ground conditions, using impact ramming hammers of up to 800 mm diameter generating ramming forces of up to 40,500 Newton metres.

A typical ramming operation requires the establishment of a solid base at the launch of the installation. The first length of steel pipe is positioned on guide rails set to the line of the bore, a cutting edge is formed or fitted to the lead end of the pipe, and the ramming hammer is attached to the rear of the pipe. The ramming hammer forces the steel pipe into the ground along the dictated line and when one pipe has been driven the hammer is stopped and removed, and the next length of steel pipe is welded in place. The cycle is repeated until the leading edge of the first pipe arrives at the reception end or shaft.

The main issue associated with pipe ramming is the fact that there is usually no means of monitoring the direction of the pipe during a bore, so it is vital to establish a clear bore path. Overall, the principles of pipe ramming are relatively simple, and the technique can offer cost-effective solutions to relatively short length installation projects.

Conclusion

Across Australia, the trenchless technology industry is continuing to expand. Increasing urban build-up and environmental concerns have provided the stimulus for more trenchless projects, while the increasing sophistication of the technology and companies involved has allowed the industry to delivery.

The potential uses for trenchless technology are diverse. Where installation or rehabilitation crosses a local road, under an airport strip, a river, or past existing infrastructure, trenchless technology is an economic choice.