In the past, numerous methods and devices have been developed for the trenchless laying of pipelines in the ground to enable sensitive areas on the surface to be crossed. Geological considerations, time and cost budgets are the crucial factors determining the choice of the most suitable laying technique. Underground pipeline laying poses many problems, such as how to work in a space-restricted area or circumvent possible obstacles both rapidly and cost-effectively.

The direct pipe method combines the advantages of the established laying methods of microtunnelling and horizontal directional drilling (HDD), thereby opening up potential new applications. One single, continuous working operation is sufficient for the trenchless laying of a pre-fabricated pipeline and the simultaneous creation of the necessary bore hole.

As with pipe jacking, earth excavation is undertaken by a microtunnelling machine. The machine is navigable and uses a flushing circuit to transport the excavated material to the surface. Modern and proven controlled pipe jacking techniques ensure accurate measurement of the current position along the intended route. The force required to feed the pipeline forward is exerted by a new type of feed device, the pipe thruster. The pressure necessary for the boring process is transferred along the pipeline to the cutterhead.

The pipe thruster was developed as an auxiliary tool for the pullback of the pipe in the HDD method. It was presented for the first time at the Hannover Fair in spring 2006. It embraces the prefabricated and laid out pipeline and pushes it into the ground at thrusts of 5m.

The requisite bore hole is excavated by a slurry microtunnelling machine, which is arranged at the front of the pipeline. In terms of its general function, the machine is very similar to a conventional microtunnelling machine, one difference however being its length. To ensure the requisite curve motion of the machine and subsequent pipeline in culverts, it features two to three back-up pipes. All of the individual back-up pipe connections feature articulated tensile couplings for optimum control of the machine. Another advantage is that in an emergency, the machine and pipeline can be extracted along with the pipe thruster.

Just like the microtunnelling method, prior to launching, the machine is positioned at the required access angle on a launch rail in front of the launch seal, which protects against ingress of groundwater and soil. The pipeline is mounted on rollers behind the launch pit and welded with the conical section of the machine at the rear.

The clamping unit of the pipe thruster embraces the pipeline and thrusts it into the ground along with the machine. The pipe thruster can be used for pipelines up to an outside diameter of 60in (1,524mm). The forces to be anchored depend on the pipeline access angle and the maximum thrust or retraction force to be applied. Horizontal forces applied can be absorbed by lateral support profiles mounted in theshaft while deep sheet pile or bore pile walls can be used for the vertical forces.

In the course of the direct pipe development process outlined below, the individual process components have been permanently improved and adapted to changing demands. For example, a launch rail is to be used in setting up the machine in the launch pit. It should be possible to save two to three days during setup using this hydraulic height- and angle-adjustable support.

The pilot project
With the aid of a HK500PT pipe thruster and an AVN1000 direct pipe boring machine, it was possible to successfully install a 464m steel pipeline under the Rhine in 2007. The steel pipe was intended to serve as a protective pipe for a water line and various protective cable pipes at a later stage. As the lack of space prevented the 48in (1,219mm) o.d. pipe from being installed in one piece, it was laid in sections of approximately 90m. The fact that the pipeline was pushed with only 80t into the small target pit in Worms Port, Germany, within 13 days shows that the friction arising during the direct pipe method is very low despite there being no lubrication applied along the entire length of the pipeline. This advantage has since been displayed in the form of relatively low thrust forces required within the framework of various other projects.

As the Worms project involved a bare, uncoated steel pipe, it remained to be seen whether coated product pipes such as gas or oil pipelines could also be clamped and thrust by the pipe thruster without any damage being incurred. In a lab-quality test in the Herrenknecht workshop in Schwanau, initial evidence was provided in collaboration with a German gas supplier. Tests with a PE coating (polyethylene) and a fibreglass coating (GfK on PE) indicated that no damage is incurred to the coating when full clamping force is applied by the clamping unit and maximum thrust by the two large hydraulic cylinders.

Ems-outfall project
The next step in the development of the method was to install a 280m brine outlet pipeline for the construction of the Jemgum natural gas storage facility at the Rysumer Nacken near Emden. A PE-coated 48in steel pipe, in which a DN900 fibreglass pipe was subsequently inserted, was installed for a German energy supplier.

During project planning, the direct trenchless advance of fibreglass pipes in the partially soft tidal flats of the Ems estuary was not regarded as possible. A casing pipe could not be laid out in the water on account of the currents prevailing, the setup required that advance was made from land. However, wing to the tight spatial conditions, the pipe could only be installed in sections of 36m, which would have posed a risk of the bore hole collapsing if the HDD method had been applied along with the welding associated with this process.

At advance speeds of up to 250mm/min, it was possible to install a 36m pipe within four hours. A coupling process generally involves one to two shifts.

The machine tunnelled through sand, silt and in some cases clay. Unanticipated old banking reinforcements made of wood and water stones were also driven through. Obstacles crushed by the mixed-soil cutterhead and cone crusher were pumped to the surface through the slurry line. As the invert line ended in an insertion structure in the Ems, the machine was to be recovered in there. The required accuracy of only a few centimetres was easily achieved by the surveying system deployed.

Installation of gas pipelines
The next phase of development was the direct installation of product pipes. This was preceded by another lab-quality load test conducted on the pipeline coating. The test showed the pipe thruster would not cause any damage to the polypropylene (PP) coating and the green light was given to the Dutch gas supplier.

Following successful testing, a total of five projects were realised in the Netherlands in 2010. Crossings of between 360 and 540m in length bypassing obstacles such as archaeological sites, smaller rivers and a railway line formed part of the approximately 500km long North- South Route in 48in (1.22m) intended for transporting gas throughout the Netherlands in the future. This first-time installation of gas pipelines in the Netherlands represented a key milestone in the progress of the method.

The most unusual of these five projects was the 540m crossing of the very deep and busy Hartelkanaal in Rotterdam’s Europoort in summer 2010. What made this project so different was the required course depth of 30m under the ground surface and the ensuing very steep access and exit angles of 10 degrees and 12 degrees, respectively (approximately three to five degrees had been the standard to date). A second slurry pump, located within the pipeline, was needed to overcome the altitude. The excavation through sand and silt was completed in September 2010. The entire pipeline was installed in 10 sections of 54m each over a period of two weeks.

The NSTT (Netherlands Society for Trenchless Technology) awarded the client and construction company the 2010 No-Dig Award for successful realisation of the project using this alternative installation method. This No-Dig Award was the second of its kind with the first award presented in Moscow in 2008 by the ISTT (International Society for Trenchless Technology). The innovative process was also nominated for the Hermes Award at the Hannover Fair in 2008. And the method received the International Pipeline and Offshore Contractors Association New Technologies Award in San Francisco in 2009.

Another milestone was achieved during the last two of the five Dutch projects (both over 500m long) involving first-time crossing of a railway embankment using the method. The overburden under the railway tracks on the Zwolle-Almelo line totalled 15m. The 48in (1.22m) gas pipeline was thrust in one single piece and inserted into the target pit together with the machine with only 150t of thrust force. Advance rates of up to 124m were achieved in a shift lasting approximately ten hours.

US debut
The method celebrated its successful premiere in the US in August 2010. Unlike the 48in (1.22m) pipelines already installed in Germany and the Netherlands, the three gas pipeline crossings realised in Florida only involved outside diameters of 30in and 36in (762mm and 914mm). The drive lengths of 119 to 226m were shorter than trialled in Europe. Considering the lack of space, the microtunnelling machines used for these very small diameters cannot be fitted with a hydraulic power pack which means that the drive length is currently limited to approximately 250m for pipeline diameters which are smaller than 40in (1,016mm) o.d.

One particular feature of the first American direct pipe project is its alignment. Unlike previous alignments, the pipeline under Highway 70 not only had to be installed with a vertical curve (R = 914m) but with a horizontal one (R = 1,828m) too. The navigation system featuring a gyroscope and electronic water leveling system kept the machine on the specified space curve. After three days of tunnelling, in three day shifts of 12 hours each, the site team had installed the 215m gas pipeline in one go. The HK500PT pipe thruster used required average thrust forces of 15t and a maximum thrust of 28t.

In early 2011, the direct pipe boring machine, including a 36in (0.9m) pipeline, was extracted for the first time using the pipe thruster in the third project performed in the US. After tunnelling through 102m, the machine was recovered to the surface with the aid of the pipe thruster together with the pipeline, the cutterhead replaced and reinserted into the bore hole—all within a single day. During the pullback process, the bore hole was filled with bentonite to prevent it from collapsing. The cutterhead required changing after encountering an unexpected rock formation that could not be passed through without disc cutters. The remaining 124m were then installed over a period of three days.

Outlook
The direct pipe method allows for fast installation speeds. This has made the process a technically practical alternative to HDD and microtunnelling. The improved installation reliability in difficult soil—compared to HDD—as well as the economic advantage over conventional pipe jacking translate into considerable competitiveness on the part of this method.

The fact that in the past it has always been possible to overcome invariably new challenges is already an indication that the limits of the process will shift increasingly from one year to the next making areas of application ever more obvious—something which was difficult to assess in the early days. It will therefore remain exciting for observers and especially for those wishing to use the method.


The team celebrates a successful breakthrough in Florida Crossing the Hartelkanaal in Rotterdam, summer 2010 A rendering of the pipe thruster The pipe thruster on display at Messe Hanover – where it was first launched by Herrenknecht