Articles & Case Studies

COOPERATION – THE KEY TO SUCCESS ON A CHALLENGING AUGER BORE FOR CROSSRAIL

Posted: Wednesday 4th March 2015

Crossrail is the long discussed and now under construction rail route that will link existing Network Rail services from Reading and Heathrow in the west, and Shenfield and Abbey Wood in the east. This is to be achieved by the construction of some 42 km of tunnels and 10 new underground stations. The main train tunnel boring operation is currently about 90% completed, with just 2 of the 8 TBMs used for the tunnelling work still having to complete their drives. On the rest of the project work is proceeding apace to complete the track laying and station builds.

With so much of the major large diameter works completed it would be easy to think that the difficult bit was almost done. But, like so many aspects in life and not least major project construction works, the devil is often in the detail. That is to say in many instances it can be smaller, apparently less impressive parts of the work that can bring significant challenges of their own when trying to meet expected completion dates.

PLUMSTEAD TO NORTH WOOLWICH

On the Hochtief/Murphy section of the Crossrail project, which comprises the Plumstead to North Woolwich section (known as Drive H) in Southeast London, the two main tunnelling machines, Sophia and Mary, have completed their 2.9 km drives. The works to complete the First Stage Concrete (FSC) track bed laying and track installation (which is being completed by a follow-on contractor) are well underway with an expected handover sometime in mid-2015.

As part of the completion operations, Hochtief/Murphy has needed to construct a number of drainage installations within the tunnel network. The installations all required the use of trenchless techniques to complete the works and both microtunnelling and auger boring systems have been utilised at one time or another.

One particular situation highlights very effectively the challenges that have had to be overcome to complete these bores. These particular drives comprised both 37 m and 62 m long installations to connect the nadir sumps (the lowest point in the tunnel section) to the main drainage rising main in both the northern (trains running eastbound) and southern (trains running westbound) running tunnels between North Woolwich and Woolwich Arsenal. The nadir sumps were located directly under the centre of the River Thames.

The original plan called for the installation of a 200 mm diameter drainage pipe to take water from each sump to a pumping station located in the confines of a crossover tunnel between the two main running tunnels. The challenges to these relatively short installations were not small however. First the ground conditions comprised chalk with significant and very hard flint inclusions. Second, the location beneath the river meant that the ground water pressure was expected to be around 4.5 bar. Third, the launch shaft for the bores was to be located within the confines of the crossover tunnel which only allowed around 3 m of manoeuvring room. Finally, the shaft position under the centre of the river meant that the logistics of transporting large items of equipment several hundred metres into the main tunnel whilst other finishing works on those tunnels continued were very difficult. Any installation technique therefore had to be able to meet all of these circumstances and still complete the bores successfully.

The reception side of the bores in the respective sumps were specially constructed concrete abutments that were designed with a soft eye into which the bore would hole. This soft eye was close to the main tunnel invert and this left little room for inaccuracy in the line and level of the drive.

After considering both microtunnelling and guided auger boring options for the various smaller diameter bores needed, including discussions with various machine suppliers and contractors, it became clear that in several instances the geology, location and equipment and material logistics meant that the guided auger boring option was the most favourable. This was because for the auger bore less equipment needed to be transported, the back-up equipment required as compared to full microtunnelling was much less and there was no requirement for the transport of slurry materials or slurry handling systems into the tunnel network. The guided auger bore system itself was also smaller and less difficult to manoeuvre into the confined working area of the launch shafts.

Even once this decision was made a further difficulty arose in that the ground conditions meant that only a specialist cutter head would have any realistic chance of completing the bores through the potentially heavily flinted ground. To this end, Hochtief/Murphy approached Oldham-based machine manufacturer Tunnel Engineering Services (UK) Ltd (TES) to investigate the option of using an Akkerman Guided Boring Machine in association with the company’s special hard ground cutter head known as the Eliminator, a process that had been used previously on earlier but much shorter drives.

PLANNING CHANGES

Discussions with TES highlighted the fact that the Eliminator would only operate to a minimum diameter of 300 mm, as opposed to the original 200 mm planned. This meant that Crossrail had to be consulted to ensure that there would be no problem if the drainage pipelines were increased in size to accommodate this requirement. After examining the proposal this changed was agreed by Crossrail.

Whilst this was advantageous to the main tunnel construction schedules, another potential problem was that the jacking pipes proposed for the drives were Naylor Denlok 300 mm diameter clay jacking pipes. Prior to the earlier shorter drives Naylor had pointed out that the standard joint seal used with the DN300 pipes is an NC Range EPDM seal and thrust plate that is encapsulated into stainless steel sleeve. This seal is however only tested to 2.4 bar operating pressure which would not be high enough to withstand the ground water pressures for the nadir sumps project.

Despite this Naylor did indicate that it also offered an NS type seal, which is normally only used on pipe of over DN300 in full microtunnelling applications. Naylor further indicated that it could provide this type of seal for use on the DN300 pipe for the auger bored drives on the nadir sump drives. The NS type seal has been tested by Naylor to 4.5 bar, sufficient for the drive in hand. By accepting this pipe joint arrangement Hochtief/Murphy was able to keep the advantages of using the clay pipe whilst achieving the water-tightness required for the drives.

Furthermore, to ensure the long-term viability of the drainage pipes under the constant water pressure of the ground below the river bed, Hochtief/Murphy decided that once installed the new nadir sump drainage pipes would need to be grouted around the bore annulus.

During the course of the project design various options were suggested as to how to install the necessary grout pipes during the pipe pushing operation of the clay pipes. Ultimately it was decided to place the grout pipe at the top of the jacked pipe to ensure they did not snag the pipes during the push-in and remained free for the grouting works post installation. One final concern was that of the grout pressure for the annulus filling. However, it was calculated that the grout pressures required would not exceed the capacity of the pipe joint seals, so the project could proceed.

THE BORES

Once the plan for the bores had been developed and agreed by all parties concerned the hard bit started, the bores themselves itself. Hochtief/Murphy decided to hire the Akkerman Guided Boring Systems with a GBM 308A jacking frame and Eliminator set direct from TES and hire in extra labour force from B&W Tunnelling to undertake the bore, adding additional experience to the HMJV team for this type of machine. To ensure that the tunnelling crew was up-to-speed on both the Akkerman and the Eliminator equipment, TES provided an operator who in turn trained a further operator on site, both of whom drove the machine on opposite shifts for the duration of both the drives.

The Akkerman GBM 308A jacking frame is designed to work out of minimum 2.5 m diameter shaft. The unit assembly comprises the jacking frame, thrust and pullback drive, shaft wall adapters and operator platform. It is operated by a three speed hydraulic operator controlled motor. Jacking forces of up to 100 t are possible with this jacking frame.

The spoil extraction is achieved using an auger chain within the jacked casing. The Eliminator, used with hollow augers, allows the system to be steered using the optical theodolite-based guidance system which observes a battery powered illuminated LED target to maintain line and grade accuracy, located behind the cutter head. The target’s position is monitored by the operator via a digital monitor mounted to the jacking frame and steering corrections are made when necessary. The system has a design accuracy of 6 mm at up to 90 m length of bore.

The Eliminator cutter head used on the nadir sump drives is a lead pilot tool for use in non-displaceable ground conditions. The unit is equipped with carbide bullet bits to break out hard ground and has three independent steering control devices to maintain line and grade. The cutter head is also equipped with jetting nozzles to increase cutting performance and ease spoil removal. Unlike the more conventional pilot auger boring system the Eliminator acts as the pilot drive reducing the pilot auger process from a three-stage to a two-stage process. The Eliminator requires 80 hp of hydraulic input horsepower to operate effectively and has an outside diameter of 406 mm which provides the bore for the 300 mm i.d. pipe. Whilst the jacking frame thrust capacity is up to 100 t the Eliminator is designed to operate at a minimum 20 t face pressure to allow the carbide bullet bits to operate effectively.

Accessing the launch shaft was a challenge in itself. To get machinery and materials into the shaft first a crane gantry had to be erected from the southern running tunnel into the cross passage tunnel. This left little space for much else. Then a power generator for the shaft pumps and the power pack for the Akkerman GBM were placed in the north running tunnel next to the cross passage entry to keep the power feeds as short as possible.

With accessibility for materials being easier from the eastern end of the tunnel network, all supplies were lowered into the tunnel at the Woolwich Arsenal station construction site. Everything then had to be transported to the nadir sump site using forklift units with limited lifting capacity. This meant breaking down the Akkerman jacking frame into its smallest component parts before reassembling in the launch shaft.

Once the machine was in place, the Eliminator was brought to the launch shaft and set up with the steel jacking casings for the first pilot drive. The unit was launched into the ground for the shorter 37 m bore using an entrance seal at the face which was designed to limit ground water inflows during the bore.

As the drive progressed new casing sections were added to advance the Eliminator through the chalk/flint ground. Some 10 m short of the holing position on the 37 m drive the machine was withdrawn to investigate a potential void which it was though had caused a misalignment on the drive. The machine was sent back to TES for investigation where it was established that water had infiltrated the electronics, these were replaced and the unit was resealed. Unusually this gave the operator a chance to see the Eliminator bore open for inspection. HMJV carried out a CCTV survey of the open bore which confirmed that the line/level were good with no voids but that some significant, longitudinal open fissures could be seen. Once it had been determined that the bore could continue with a minor modification to Eliminator/jacking casing joint to counter any void that might be in the ground, the drive was completed successfully.

The Naylor Denlok pipes were then brought to the launch shaft from the Woolwich Arsenal station site. Due to the presence of the grouting tubes the pipe jacking operation had to be undertaken with great care so as not to snag or damage the grout pipes.

Whilst progress of the drives was affected and some delays occurring largely due the massive water inflow at times (water flows were estimated at in excess of 30 to 40 l/sec into the relatively small 3 m diameter shaft that was used as the launch shaft in this instance) and the some very hard flint inclusions in the difficult ground, the Eliminator cutter head ultimately handled the bore well. For example the second 62 m long pilot bore operation was completed after some 48 hours of actual boring time. Once the Eliminator had holed through the product pipe jacking operation took a further 24 hours to complete.

Commenting on the auger bore operation Frank O’Connor, Tunnelling Construction Manager for the Hochtief/Murphy joint venture said: “This was nothing if not a challenging installation and as with all such construction projects, this one had its ups and downs. But the level of cooperation between all those concerned in developing the methodology and the professional drive from all of them to provide a viable and effective solution to some very difficult engineering problems, meant that ultimately we have a successful installation that will provide the drainage necessary to keep Crossrail trains running efficiently for very many years to come.”




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December 2018

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