Articles & Case Studies


Posted: Tuesday 28th July 2009

Mosley Moling Ltd of Stevenage, Hertfordshire recently undertook a pilot auger bore crossing of the M11 motorway near Chigwell, North London for the installation of a new 610 mm diameter surface water outfall pipe.

The client for the project was ODA Logistics Centre with PJ Carey acting as the main contractor for the works.

The work was required as part of the local area infrastructure redevelopment requirements as well as improving the local surface drainage facilities.


As the required pipeline ran beneath the busy M11 where it leaves the Greater London area, there was little effective alternative but to use a trenchless solution for the installation. Any thought of utilising traditional open cut was set aside because of the major disruption this would have caused to traffic flows on the motorway.

After completing ground investigation at the site using test pits and bore holes on either side of the motorway, the ground conditions were found to comprise largely stiff clays with potential inclusions of clay stone and boulders which made it an ideal site for the use of the pilot auger bore pipe installation technique. Not only were the ground conditions perfect for this type of equipment, but the pilot auger method also provided a solution which would minimise the potential for heave or settlement of the road surface beneath which the pipeline would be installed. The only potential problem was that should clay stone or boulders be encountered during the bore the machine chosen for the work would need to be able to apply the necessary forces to cut through them. This is why the selection of a steel jacking was made as it allowed a higher jacking force application should it be needed. In the event the bore passed through only the stiff clay.

The laser guidance system of the pilot auger method also meant that a very high degree of accuracy could be achieved on both line and level, which was another major requirement of the gravity driven surface water drainage pipeline.

Having been subcontracted by Carey’s to complete the bored installation Mosley Moling chose a Perforator® PBA150 pilot auger boring unit for the work. The unit would be required to complete the crossing in a single drive of some 65 m length to install the 610 mm diameter steel pipe which would form the final product pipe. Part of the reasoning behind choosing this machine was its ability to work from a relatively small shaft to the similarly small reception shaft which was being designed as a manhole shaft for the operating pipe once completed. Using a system that could work to and from small shafts meant that works on other sections of the pipeline could proceed uninterrupted by the boring works.


The PBA 150 pilot guided auger boring machine (also known as a horizontal thrust boring machine) is one of family of six PBA units offered by Perforator®. The most popular range includes the PBA 40, PBA 85V, PBA 150 and PBA 155 systems. The thrust capacity of the machines is 40, 85, 150 and 155 t respectively.

The machines are designed for the steered and non-steered trenchless installation of underground service pipes. They can be efficiently adapted to all current methods by the use of different drilling tools and the application of appropriate equipment to the prevailing soil classes. The units are accurately steerable using the Optic Electronic Navigation System (OEN) which is a laser based system used during the pilot bore stage of the operation.

The PBA 150 unit on the Chigwell project is a well-proven, powerful and robust system that will install pipeline of diameters from 219 mm up to 900 mm. Several different pipe types can be installed using the machine over bore lengths of up to 100 m.

The unit is, like the family of PBAs, driven by a diesel power pack which is located outside of the launch of the launch shaft. Launch shafts can be either circular or rectangular with the capacity to offer a thrust wall to the rear of the machine to facilitate a reaction face for the jacking forces created during operation.

The maximum torque provides by the PBA 150 is 18,000 Nm, with maximum torque capacity of the PBA 40, 85V, 155 being 6,000, 16,000 and 36,000 Nm respectively.

Where long pipe lengths need to jacked in one push, extension frames are available of between 1,000 mm and 12,000 mm which fit to the front of the machine in the start shaft to handle the extra pipe length.


On the Chigwell site, the installation works were completed very quickly once the pilot auger machine was on site and installed with the pilot bore taking just one day to complete. The reaming of the pilot bore to size and the steel pipe installation took in total 10 days to complete. This was in part due to the fact that the steel pipe sections were installed in lengths of 2 m per push with each section needing to be full seam welded to the previously installed length before further installation could proceed. The pipe installation work commenced on 24 February 2009 with completion 11 days later.

The reliability of the Perforator unit allied with the skills of the Mosley crew ensured that the installation was completed without problems on either the pilot bore or the reaming and pipe installation operations. The site set up works including shaft sinking of the 4.3 x 3.5 m rectangular concrete lined launch shaft with thrust wall to rear, the 1.2 m diameter concrete ring reception shaft and subsequent site reinstatement was completed by PJ Carey. The site location also meant that no special arrangements were required in terms of limiting local residential or business disruption.

Quantity Surveyor for Careys, Paul Orbell, commented: “Mosley carried out the work quickly and efficiently and we were very impressed with the extremely high accuracy achieved by the Auger Boring system. Reliability in this kind of application is paramount and that is what was delivered.”

For Mosley Moling, Luke Mosley, managing director said: “We opted for the Perforator PBA 150 unit because we knew it would give us the performance we needed on the short important motorway crossing. We were familiar with the Perforator system from previous use by our company and have a high regard not only the machines themselves but also for the support and back up that is available from the supplier. The unit performed well on site and we have had very positive feedback on the project from our client.”

Tel: +44 (0)1142 693258.
Fax: +44 (0)1142 889380.

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