In the context of a need to better engineer the theory and practice of shotcrete lining for underground support, the British Tunnelling Society recently held a half-day seminar focusing on recent soft-ground experience: ‘Lessons to be learned from recent shotcrete tunnelling in London Clay’.
Leading UK tunnelling engineers suggested ways in which the lessons learned could be combined with better design engineering principles and quality control to minimise risk and improve confidence.
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Heathrow
The Heathrow Express and its effects on other projects were at the forefront of participants’ minds. David Powell and Chris Pound of Mott MacDonald highlighted the key design detail and construction issues of the London Heathrow Airport central terminal area collapse, putting them into the context of ‘robust’ design principles. They considered whether the design changes made following the incident were necessary or whether the main problems were of construction practice and monitoring. Factors examined included ground behaviour, settlement, the predicted and actual lining loads, the construction sequence to be used at junctions, and the sequencing of multiple excavations.
In a similar vein, Guy Lance of W S Atkins reviewed shotcrete construction since the incident, and the reports and papers published for industry guidance. He questioned how the information gained had been used in the development of tunnelling projects, and whether current construction techniques and supervisory methods are providing the levels of quality, performance, production rates and safety now demanded.
Implementation
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By GlobalDataSeveral speakers examined the implementation of designs drawing on major projects in London Clay such as new designs for Heathrow Express tunnels, stations on the London Underground Jubilee Line Extension (JLE), and construction of the Heathrow Baggage Tunnel. Prof Robert Mair of the Geotechnical Consulting Group reviewed the design of the JLE Waterloo Station concourse and platform areas. Here the two 9.1m od platform tunnels are linked with an 11.8m od concourse tunnel between them. A key design issue in the construction programme was whether or not the secondary reinforced concrete lining would be required prior to construction of the adjacent concourse tunnel which was only 1.5m away. An additional factor to be considered was the use of compensation grouting.
The design was checked using finite element analysis to assess the deformation and stresses induced in the sprayed concrete lining. Prof Mair described the analysis results and consequent decisions on design and construction sequence.
Also on the subject of Waterloo, Bob Frew of Maunsell described the utilisation of sprayed concrete lining for permanent support of the station crossover to meet programme requirements rather than design issues. The original design for permanent lining consisted of a conventional step-plate layout with an in situ reinforced concrete lining cast within reinforced shotcrete temporary support.
The permanent lining was revised to a hybrid in situ reinforced concrete invert and sidewalls combined with a shotcreted crown. Frew addressed the practical difficulties of changing the design and construction philosophies at a late stage.
Work at Heathrow Airport was not only a matter of ‘rescue’ but of re-examining the design. John Cadei of Maunsell covered this reassessment for the Heathrow Express tunnels and the appropriate conceptual design criteria. He considered a range of factors affecting temporary and permanent lining for shafts and junctions as well as tunnels.
Monitoring
Dr Myles O’Reilly sounded a cautionary note or two about reliance on modern design procedures at the expense of independent checking of calculations and the ‘real world’ of monitoring actual movements by instrumentation. He said that, despite the use of finite element analysis, settlement calculations should still be checked independently.
Ross Dimmock of MBT (Materials Building Technologies), one of the major suppliers for shotcrete materials and equipment, employed examples to stress the importance of correct specification, equipment and skill in the application of sprayed concrete for tunnel support lining.
Chris Clayton of Southampton University concentrated on the monitoring and instrumentation of shotcreted linings and ground behaviour. He pointed out that the mass of data recoverable from the wide range of instrumentation now available can be overwhelming. In common with most projects, the Heathrow Express had extensive and varied instrumentation installed. He compared the efficacy and reliability of various types of instrumentation, and in particular those which are best suited to monitoring the construction of sprayed concrete lined tunnels. Southampton University has developed a model of ground behaviour around an advancing tunnel.
Complex design
On the subject of implementation of construction, there were two presentations on the JLE and the Heathrow Baggage Tunnel. David Sharrocks and Bob Allen, now both of London Bridge Associates, presented some lessons and observations gained from tunnelling the London Clay for the London Bridge Station. In a very complex design, sprayed concrete lining was proposed for use in combination with compensation grouting to provide a flexible and effective solution to some of the design and construction difficulties. A successful trial of compensation grouting led to method acceptance but, after the Heathrow Express collapse, Health and Safety Executive interest gave rise to intensive investigations into the process. This led to modifications to the London Bridge plans.
Similarly, the construction of the Heathrow Transfer Baggage System tunnel was also stalled by the Heathrow Express incident, as described by Colin Eddie of contractor Miller Civil Engineering. The scheme involved driving for 1400m in London Clay at a 4.5m id with segmental lining with two large transfer chambers constructed using sprayed concrete for both primary and secondary linings. The Heathrow Express collapse occurred as construction on the first chamber was due to start, and consequently a submission was made to the Health and Safety Executive for approval to proceed. The presentation covered the results of comprehensive ground monitoring.
Finally, Duncan Southcott of Allianz Cornhill Engineering explained the principles that insurance companies work on in deciding whether a construction procedure such as NATM is ‘OK’. He cited the Heathrow event as an example of the need for construction insurance, allowing clients to transfer risk to insurers. Losses of the size of those from Heathrow seriously affect the ability of insurers to make a return, and so they take an interest in construction methods and the associated level of risk. Southcott pointed out that, although sprayed concrete lining at Heathrow was a catastrophic loss, recent work has been completed successfully.
Southcott explained that insurers are looking for the application of good risk assessment and management procedures throughout the project process. They are keen to be involved from an early stage so they can share their knowledge of past losses and try and prevent a recurrence, as well as adding value to contractors’ risk management.
Conclusion
The seminar successfully highlighted some important issues about how shotcrete lining should be employed within proper engineering principles of design, application and quality control. Some lessons, particularly in project management and compliance with structural details, are well established but there remain other areas of concern. These include limitation of instrumentation to what will actually be useful, rather than to provide comfort and assurance for clients.
On the other hand, notice must be taken of actual observations rather than what the design says should happen. The new procedures in 3-D finite element analysis, as well as existing 2-D work, should improve the predictive function in more closely matching designs to actual conditions, but many emphasised the need for efficient monitoring procedures.
