The presentations given to the British Tunnelling Society on 16th March 2000 told of the design and construction of the three most complex interchanges on London Underground‘s new Jubilee Line Extension (JLE), focusing on the changes that had been made following the disastrous collapse of the Heathrow tube tunnel.
Peter South and Steve Parker were working for the Balfour Beatty-AMEC JV, responsible for JLE Contract 102, Waterloo to Green Park. John Wallis and David Sharrocks were working for the Costain-Taylor Woodrow (CTW) JV responsible for JLE Contract 104, London Bridge. Each contractor had chosen sprayed concrete linings as an integral part of the station and interchange construction process. The labyrinthine arrangement of shafts, connecting tunnels and escalators, many with awkward curves and junctions, lend themselves to sprayed concrete.
The action was placed against the background of the collapse at Heathrow and the halt of construction in full flow for reviews and redesign. The redesigns, which emerged in order to get construction underway again, were reported in context. Compensation grouting was discussed in its various applications and some of the more important temporary works designs were described.
BBA’s Waterloo
Peter South began with a description of the scope of the stations work for Contract 102. These included:
All the works had to be constructed without causing noticeable distress to any existing structures, buildings or underground facilities. The works were constructed in typical London geology of London Clay overlain by Terrace Gravels. Permeation grouting of the gravels was required where the clay cover was 6m or less or where structures were to be built through them. Compensation grouting was to be carried out under structures at risk of damage beyond the ‘slight’ category. All the Waterloo tunnels had been planned to be constructed using sprayed concrete, with the exception of the escalator tunnels in the Terrace Gravels and the upper levels of shafts.
The main tunnel access was from Jubilee Gardens site where a spur tunnel was constructed to bring all equipment into the tunnels and to enable mucking out.
The access tunnel was used to prove the sprayed concrete process and verify the design parameters. Settlement analysis showed that if a face loss of 1.5 per cent was experienced, then up to 200mm of settlement could be experienced in certain locations. From this came the scope of works for compensation grouting.
Construction work forged ahead. Then came the collapse at Heathrow. The JLE works were suspended for a full review of the design and the work constructed to date. The subsequent scrutiny of the installed work was very thorough. The extent of the redesign required was very wide ranging; not through the lack of integrity of the original design, but through a lack of confidence in approving such designs in the context of criticism surrounding events at Heathrow. The running tunnels were redesigned to include full circle lattice girders with top heading and bench construction. The face was to be domed and sprayed for every advance.
In addition, tunnels larger than running tunnels, such as platform tunnels, would have to be driven with a full-size pilot to act as a type of “ground-dowel”. SGI linings were substituted in the upper concourse, and escalator and travelator tunnels. The use of compensation grouting was kept to a minimum by careful driving. However, where it was used it caused some problems, notably in the upper concourse where the cast-iron lining distorted under the grout loads.
South was adamant that the sprayed concrete coped much better than the conventional rings with the uneven loading from grouting. An escalating number of props were needed to prevent the failure of the cast iron. When the limit was reached, compensation grouting was abandoned and the ground allowed to settle in a controlled way. Cross-passage work also had to be carried out using conventional methods which, South implied, were slower than the sprayed concrete techniques originally planned.
South concluded with the observation that the works at Waterloo were significantly changed as a result of the problems at Heathrow. He thought that the SGI linings used in the revised schemes had been thinned down due to the high strength of the material. This had led to reduced rigidity at the seat joints resulting in distortion at the joints under uneven loading. In contrast NATM sprayed concrete had been proved to be safe and competent, provided that the urge for production respected the engineering discipline.
Westminster Station tunnels
Steve Parker continued with a description of the tunnelling and temporary works at Westminster. The main works consisted of the 1.8km long running tunnels with the widenings to form the station, two step-plate junctions at Green Park, the plethora of cross passages and the settlement control temporary works.
The lead in work to the commencement of tunnelling required the compensation grouting scheme to be operational, the muck-away by river to be ready and the design by Wirth Howden of a shield for the station drives which mucked away in front of itself. The main running tunnels were excavated using twin Wirth Howden 360-deg 4.9m-diam. backhoe shields equipped with telescopic forepoling. The tunnels were constructed in 4.45m i.d. precast concrete linings 1.0m long which were bolted at the start and end of the drives, but otherwise were an expanded design. The best progress was 24m in a 12-hour shift with the expanded linings. At the station the running tunnel drives became pilots for the enlarged tunnels.
Compensation grouting was used in many locations to protect buildings, structures, sewers and gas mains. As can be imagined lengthy negotiations were needed to agree the positions of grouting shafts in the vicinity of the Houses of Parliament. Permeation grouting was also needed in the gravels overlying the London clay, to improve the ground strength and allow construction of the east station platform tunnel.
Access for this permeation grouting was impossible, so an alternative was designed. This solution comprised seven 1047mm diam. pipes jacked 165m to form a canopy. These were filled with concrete and provided an umbrella protecting the underlying tunnels from the effects of compensation grouting. The shield erection chambers were then constructed. Due to difficult geometry, the eastbound tunnel shield had to be erected at a lower than optimum level to allow working space; requiring it to be jacked up almost a metre when ready to start work.
Pilot tunnel dismantling for station
In order to construct the station enlargements, the expanded linings had to be dismantled and the running tunnel enlarged to 7.0m i.d., lined with SGI. The instability resulting from the key removal during dismantling was countered by using a special shield.
The pilot tunnel was strapped for 30m ahead of the shield. One of the keys was then removed using a hydraulic jack, followed by the progressive dismantling of the rest of the ring by hoist. Twenty minutes was the average time to dismantle a ring. Further excavation was carried out by backhoe with manual trimming where required.
As the shield was driven back towards the working shaft at Jubilee Gardens, muck had to be transported forward of the shield using an arrangement of conveyors. To minimise settlement, work was carried out in a controlled sequence including temporary face support using five breasting plates. The excavation for one ring took 45-60 min. Dismantling could be carried out concurrently with the enlargement mining. The accompanying compensation grouting was done in blocks ahead of and behind the shield. Excavation could be carried out once the front block was complete, and shoving could commence once the back block was done.
In spite of all this work and its inherent risks to historic buildings, Big Ben settled only 3mm more than predicted and with a differential settlement across the foundations of only 5mm.
London Bridge – Northern Line
John Wallis changed the subject to Contract 104, London Bridge Station. This section comprised, not only the construction of the new Jubilee Line, but also the upgrading of the Northern Line and the total modernisation of the London Underground-Railtrack Interchange. In addition a new ticket hall was to be constructed under Borough High Street which would link both the Northern and Jubilee Line Stations. Wallis described the same scenario as South. Sprayed concrete work was also suspended here.
Work on the Northern Line had been progressing well in sprayed concrete. As a result of design reviews, the Client instructed the reversion to traditional design using cast iron linings. A 3.66-m pilot tunnel was driven. This was then enlarged for the Northern Line Station using SGI linings.
Additional site investigation revealed uncertain cover to the Northern Line tunnel under the River Thames. Therefore the northern step plate junction and its approach were constructed using compressed air, with direct access from the existing tunnel. This resulted in a closure of the Northern Line for four months. The concurrent Jubilee Line and Railtrack interchange works were described by David Sharrocks.
London Bridge – all the rest
Sharrocks described the scope of the redesign precipitated by the Heathrow Collapse and the concessions made in order to gain approval for restarting work and optimising the construction programme. Sprayed concrete was used to form a sub-invert pilot tunnel along each station tunnel.
These were used to form in situ reinforced concrete invert sections for the subsequent 8.5m dia. NATM enlargement. This eliminated any reservations about weaknesses in sprayed concrete invert sections. The pilot also enabled advantageous reduction of face area and potential geometric problems. A secondary lining was required, which was in fact cast iron. This was a utilitarian solution as the iron had already been ordered as a contingency measure. A second noteworthy change came as an instruction to construct the concourse in 10m-dia. cast iron. It became imperative to use NATM to minimise the programme delays. The concourse had to be built in sprayed concrete and then lined with cast iron. Sharrocks credited the expert input from Dr Sauer Co for the design and review processes.
Sharrocks ended with a tribute to all those who had been involved in the London Bridge work, particularly David Wise, Billy Wright, Gordon Linfoot, John Trench and John Hilleard who had variously fallen victims to cancer or heart disease during the life of the project.
To conclude we reflect on the words of Peter South, “Following the success of the NATM process on this project the merits of its use should be examined …let us not throw away the advantages of NATM on the back of one unfortunate incident, but let us learn from the mistakes of the past and blend the old with the new to the best advantage.”
