No one dared question whether jacked box tunnelling might not be a genuine form of tunnelling. Indeed, as the presentation unfolded, the challenges, problems and successful solutions described became more and more dramatic and impressive.
Doug Allenby, chief engineer of Edmund Nuttall, began by recalling the paper he gave to the BTS a year ago, entitled ‘Twenty five years of progress in pipejacking’. In that presentation he discussed pipe jacking up to 2.5m diameter, with a UK record of 1000m jacked on the London Water Ring Main using an installed jacking shaft thrust of 1200 tonnes. The microtunnelling examples were confined to diameters of less than 900mm diameter, where lengths of 300mm were regularly achieved with a thrust of 400 tonnes.
At this presentation, Allenby returned to talk about jacking, but this time on a grand scale, with examples up to a total installed jacking capacity of 45 000 tonnes, the largest jacked box tunnel to date.
He continued by describing the relationship between Edmund Nuttall and John Ropkins Ltd. They had formed an agreement to work together for the promotion, marketing and execution of jacked box tunnelling throughout the UK and worldwide within the HBG Group, Nuttall’s parent company. To date , they had combined to install five such tunnels successfully and are currently associated with the installation of a further three.
The main components of a jacked box tunnel were illustrated by slides showing an installation below a busy railway line at Didcot in the UK. This box was 30m long, 5.9m wide and 3.6m high, and was designed for pedestrians and bicycles. Allenby explained that a jacked box often starts from a greenfield site, with a jacking pit below the natural ground level in order to enable a reaction against the ground itself to take place when the box is jacked.
Within the jacking pit, a jacking base is cast to form an accurate base for box structure construction and to provide a jacking reaction, with the thrust being transmitted into the ground. The box structure is then formed using traditional reinforced concrete casting techniques. Allenby stressed the need for a very high order of accuracy of casting. Each shield is designed to suit the project’s specific ground conditions, the intended excavation method and the provision of the required face loading to safeguard the overlying infrastructure (e.g. railway lines). He summarised the benefits of jacked box tunnelling as follows:
John Ropkins then proceeded to discuss the engineering principles and development of the anti-drag system. He recollected that he had last presented a paper at the ICE as long ago as 1977, on the way in which pipe jacking had been applied to the installation of underpasses and bridge sub-structures and abutments. In recent years, further advances, particularly in terms of the control of ground disturbance and box alignment, had been made in what is now termed jacked box tunnelling.
Ropkins contrasted the concerns associated with the jacking of several box sections having intermediate jacking stations with the benefits arising from having a long, accurately cast, single jacked box. He explained the arrangements for the control of ground drag by means of the ‘wire rope anti drag system’ (ADS) installed on the top and bottom surfaces of the box. This proprietary system consists of an array of closely spaced wire ropes which are initially stored within the box, next pass through slots provided in the shield and are then anchored to the jacking base. The ropes are gradually pulled out from within the box as it advances.
The top ADS forms a stationary separation layer between the overlying ground and the moving box, preventing the overlying ground from being dragged forward. The bottom ADS similarly separates the underlying ground from the moving box, preventing the underlying ground from being dragged and compacted, and producing the effect of ‘tracks’ for the box to slide on – similar to a tracked excavator. The wire rope ADS has the following characteristics:
Ropkins noted that an ADS is not normally required on the sides of the box because ground pressures here are generally quite low. Grease is employed to lubricate the top and bottom surfaces of the box to reduce the frictional drag and thus the jacking load. A bentonite slurry is sometimes used to lubricate the box sides.
He proceeded to cover the following topics in detail:
Ropkins emphasised that, although field tests had been undertaken, there was no substitute for measuring and monitoring on live projects in order to establish expected performance on a new project. During tunnelling, the following aspects of the work are carefully monitored and adjustments made accordingly to the jacking operation:
He explained how these engineering principles enable very large boxes to be installed with the development of very little ground surface movement. When tunnelling under live railway tracks it has been customary to impose a speed restriction, but with continuous fettling of the ballast, there may be situations where speed restrictions should be less severe or not warranted at all. Low pressure injection grouting is normally carried out on completion of tunnelling.
Ropkins ended with descriptions of four projects in increasing order of complexity, aspects of which were illustrated by slides. They were located at Didcot, Thurrock, Silver Street in north London and Lewisham in south London all in the UK.
Allenby returned to describe two further jacked box projects which had reached new heights of impressiveness. The first was the 50m long x 23m wide x 9.5m high culvert which was jacked through a 12m high embankment supporting the main London-Bristol railway at Dorney, near Maidenhead, UK. This is currently the longest box structure to have been jacked in the world, and is also believed to be a ‘first’ as a structure jacked through frozen ground. It forms a vital link in a 11km long flood alleviation channel for the River Thames.
The intended method of construction required:
The grouting proved problematic. Allenby explained that a trial was planned but time constraints required the trial to proceed at the same time as grouting the embankment itself had started. The trials demonstrated that the security of the embankment could not be guaranteed by grouting. This led to the decision to freeze the embankment.
At Dorney the drilling and installation of vertical freeze pipes was not allowed, so an array of horizontal freeze pipes was installed along the culvert alignment between the south side headwall and the north side reception cofferdam. By careful design of the freeze hole configuration, the headwall and reception cofferdam were incorporated into the block of frozen ground, cutting off the surrounding water. A total of 180 freeze and instrumentation holes of 90mm were drilled. Drill steerage problems were encountered, primarily due to the granular nature of the ground and the substantial lengths of the holes, necessitating a number of additional holes in areas where the freeze hole spacing was considered excessive.
In order to minimise ground heave under the railway tracks, and allow the dissipation of pore water pressure, the freeze was started from the bottom upwards. It took three months to freeze the ground, using a freeze plant of four chilling units producing a total cooling capacity of 720 000 Kcal/h at -30°C. Freezing the sands and gravels improved their strength and stability dramatically, enabling the use of an open face shield having four full height compartments.
To prevent the ADS becoming frozen into the ground, arrays of heating cables were cast into the culvert floor just above the steel soffit plate; the roof, sides and floor were overcut; the ADS was injected with a low temperature resistant grease; and the side wall overcuts were injected with a bulk low temperature resistant bentonite lubricant.
Allenby gave many more technical details. Among the most impressive were descriptions of the purpose built Webster 2000 lower roadheader and 120kW upper articulated transverse cutting unit, and the round the clock 28 day jacking period, with a maximum recorded jacking thrust of 6000 tonnes. With careful track reballasting during tunnelling, a fully operational railway was maintained at all times.
The presentation’s finale was even more impressive. Allenby described the Nuttall Ropkins input to the $400m contract awarded to the Slattery, Interbeton, J.F.White, Perini JV for the construction of three jacked box road tunnels in Boston, US. Details are shown in Table 1.
These physical dimensions, combined with jacking thrust and contract value, make this the world’s largest jacked box tunnelling contract to date. Allenby explained, with slides, how each tunnel has to pass under 11 mainline and commuter rail tracks, with the longest tunnel passing through the foundation of a bascule bridge over the Fort Point Channel. The ground is totally reclaimed and, in addition to the bridge foundations, there was particular concern as to how to deal with obstructions in the form of a timber pile supported, unreinforced concrete depressed trackway and the existing granite blockwork sea wall, again timber supported.
The decision was taken to freeze the ground along each tunnel alignment to stabilise the mixed ground and to ‘lock’ the obstructions into the ground mass. After detailed discussions with the railway owners, track access was negotiated for drilling and installation of 2000 vertical freeze holes for the three tunnels.
His final slides showing the enormous Ramp D cofferdam and construction of the boxes were accompanied by a remarkable array of construction details and dimensions which conveyed the enormous size and complexity of this project. Ramp D is actually a two-part box with an interjack station, since the cofferdam would have otherwise been unable to resist the full jacking load – another ‘first’ for jacked box tunnelling.
Allenby’s predictions for the future were:
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