Lateral Thinking for Innovative Sewer Replacement using Pipe Eating Technique

28 May

The project was completed for client Scottish Water, with Pipeline Drillers Ltd as the specialist contractor working for George Leslie. The project location was on Kingsknowe Crescent, Edinburgh, Scotland. Whilst at the planning stage various options for the completion of the works were considered including: Open Cut, Horizontal Directional Drilling (HDD), Standard Pipe Bursting and Auger Boring

The Open cut method was deemed not to be an option due to the location and route of the pipeline, which was over 6 m deep for part of its length and would have required the removal of many mature trees and the amount of spoil that would be created and the storage/disposal requirements for this spoil. Also there was potential for massive local disruption and environmental damage that such an operation would create. Overall, this meant that the cost of the open cut option was also highly prohibitive.

HDD was also deemed to be a non-starter due to limited site access and the limited site footprint at the installation site which meant that there was little space for an HDD rig and its support equipment. In addition HDD would not be accurate enough to install the gravity sewer to the gradient required.

Standard Pipe Bursting, whilst somewhat similar to the technique ultimately used, was not suitable because of the limited size of the equipment that could physically be used on site. No machine that could be transported and utilised on the site would have the capacity to achieve the necessary pull-back tonnage needed to complete the work. Given that an upsize from 300 mm to install a 710 mm o.d. PE pipe would be required, it was that this would not be possible even with the largest commercially available burster rig due to the extremely stiff ground conditions.

A very large auger rig would probably have accomplished the drive but with weights in excess of 3,000 kg and large excavations required, this option was also discounted.

Limiting Site Paramenters

The length of the section of pipe that needed to be replaced was 60 m, with the existing pipe running at depths between 3.5 m at one end and 6 m at the other. Ground conditions comprised very stiff clay with boulders. The terrain also banked up and down along the full pipe length with the surface being mostly wooded, which added significantly to the access difficulties. The woods are also utilised by the attendees of Special Needs School under supervised access. Therefore any works needing major surface access to the woods would have prevented use of the woods by the school for the duration of the project and for some time after to allow the grass and woodland to re-establish.

Access to the site was very difficult with one end of the run being limited by the close proximity of both a canal and a railway line whilst at the other end was the Special Needs School. Taking these factors into account it became obvious that only relatively small size construction equipment could be used to complete the works.

A solution to these obstacles required some significant lateral thinking. After careful consideration of the options and the restrictions of the site it was decided to utilise a very innovative combined auger boring rig and pipe burst rig assembly to complete a ‘pipe eating’ replacement technique.

At one side of the run, where the existing pipe ran at 3.5 m deep, a Perforator PBA150 auger boring system with 150 tonnes of thrust was to be used. At the reception end of the run there would be positioned a Hammerhead 100 tonne capacity HB100 pipe bursting rig, weighing just 1.3 tonnes, provided by Vp plc U Mole the sole UK agent for Earthtool, the US manufacturer of HammerHead equipment. The maximum size of excavator that could access the site was also a relatively small 14 tonne unit, which in the event and in the ground conditions sometimes struggled in sections to achieve its operational requirements.

Installation Works

The process of bursting the old pipe and replacing it with the new larger pipe in this case may be termed ‘Assisted Pipe Eating’. With both the auger boring unit and the bursting rig positioned in their respective 6 m long by 3.5 m wide operating pits, a series of ‘uplift’ pulling rods were passed through the existing pipe to connect the machines. The pipe eating head was attached to the lead pipe of the jacking rig which also had a cutting ring on the front edge to aid passage of this pipe into the ground. The jacked and pulled pipe was a 762 mm diameter steel pipe, which was to act as a permanent sleeve.

The pipe eating head that was pushed into the ground by the Perforator unit was specially designed with a mandrel which ran inside the existing pipe whilst breaking it up insitu. The shards of the old pipe were to be retained within the confines of the pipe eating head and transported down the sleeve pipe using the auger spiral of the Perforator rig.

With the new pipe and bursting head positioned, the pulling rods from the burster rig were attached to the front end of the pipe eating head mandrel. Working together to provide up to 250 tonnes of force at the pipe eating head, the Perforator unit jacked the assembly head forward whilst at the same time the burster rig pulled the assembly towards its operating pit.

Not only did this provide sufficient thrust to the bursting head to fracture the old pipe but also enough pressure to ensure the new pipe passed into the ground effectively. Simultaneously to the push/pull operation, the displaced earth and fragments of the broken pipe were transferred back down the permanent steel sleeve by the auger spiral and removed to the surface by skips from the jacking pit, preventing excessive compacting of earth at the upsizing head. The pulling force from the HB100 also ensured that the pipe eating head remained on the same line and level as that of the old pipe. The steel carrier pipe was extended at the end of each push/pull stage by welding on a new 3 m long section at the jacking rig end. During the pipe eating works, both machines had to work hard with instances throughout the operation where the HB100 burster rig operated at its maximum tonnage capacity.

Towards the end of the replacement run, as the pipe eating head approached the burster rig position, jacking was paused so that the thrust wall of the burster could be removed. The burster rig was disconnected and the final few meters of the installation was completed using the auger rig alone so that the pipe eating head and the lead edge of the sleeve pipe could be extended into the burster pit.

The replacement works were successfully completed and the remaining spoil and old, now broken, pipe cleaned from the steel carrier pipe. A 600 mm diameter twin wall PE sewer pipe was inserted through the sleeve pipe and connected at either end to the remainder of the sewer network at both new and existing manholes.

The duration of the works totalled 2 weeks with the length of the project being longer than might have been expected for this length of run due to the access and logistical difficulties of transporting the equipment, carrier pipe sections and the final product pipe to site and removal of all equipment once the works were complete.

One of the biggest timing difficulties came from the lack of storage space at the pipe jacking pit which meant that each individual 3 m steel pipe section had to be brought in from off-site to be available for installation in the jacking frame just as it was needed. The route through the woodland made this a difficult journey with each pipe section. Ultimately on completion of the installation both contractor and client were highly satisfied with the results and high quality of the final product and the local population barely knew the works had been undertaken.

The use of this innovative trenchless technique also meant that waste control was more easily managed with the only waste produced being that created to dig the operations pits and that removed from the sleeve pipe by the auger string. The trenchless option also meant that the carbon footprint of the works was also very much reduced as compared to potential alternative installation methods.

Peter Taylor, managing director of Pipeline Drillers Ltd said of the project: “This was not an easy project by any means. The logistics bordered on nightmarish with limits on equipment size, site storage, transportation and precise timing. However, our crew worked extremely well with George Leslie’s workforce to safely and efficiently complete what was something of a unique project under challenging circumstances in an excellent timeframe.”

For U Mole, Frank Gowdy, said: “Given the circumstances of the project U Mole was very pleased to be able to support such an inventive approach to the difficulties posed on site. We congratulate Peter Taylor and his team for the innovative use of the combined jacking and pipe bursting systems that brought success to a very challenging situation.”