Patrick Reynolds, Freelance Reporter
The toughest settlement control regime for rail tunnels under London is being set for the Crossrail TBM drives and stations excavations. Belt weighers, specifications on annular grout availability, project-wide geotechnical monitoring, and delegation of authority to Section Engineers for construction operations, are part the regime. These detailed measures are described in this latest article on the geotechnical features of the scheme, the earlier ones being a project overview , and the site investigation and geological studies undertaken. Future articles will expand on the project coverage, exploring different aspects in greater detail, and reporting on performance during construction.
Eight TBMs will excavate the project's total 20km of twin running tunnels
- Excavation of the twin running tunnels on Crossrail is anticipated to be reasonably straightforward but will have the toughest settlement control regime yet for tunnelling below the capital.
- The historical progression of maximum allowable volume losses on rail tunnel projects in London have reduced from 2% on JLE (the Jubilee Line Extension of London Underground) and CTRL (the Channel Tunnel Rail Link project) respectively, to 1.7% in the Bill to Parliament for Crosssrail. During detailed design for Crossrail, this has reduced further to an assumption of 1%.
- Analyses suggest that, for Crossrail, ground surface settlements may be up to a maximum of 15mm-20mm with volume losses up to a "moderately conservative" level of 1% - a threshold much tighter than those set for JLE and CTRL, although losses achieved on CTRL were much less than the mandatory limit now set for all Crossrail bores. The limit however will be set much lower at 0.5% volume loss in certain areas.
- The volume loss limits of 1% and 0.5%, respectively, will be specified in the tunnelling contracts.
Timeline for start of construction at Crossrail's main underground stations
- Crossrail planners are confident of achieving these minimised settlement and volume loss specifications through a combination of measures focused on TBM longevity and spoil weighing. In addition, there will be an integrated monitoring regime and measures throughout the entire tunnelling works that include having more annular grout available on standby, greater authority for Section Engineers, and a major IT system to support project management of the works.It is assumed that closed mode excavation will be used for the majority of the drives but each contractor will decide on open or closed mode operation depending on conditions and on the need to meet strict face loss limits.
- Aside from the TBM drives, tighter settlement controls will also be used for SCL (sprayed concrete lined) tunnels and excavations at, or near, existing underground stations and other key works. These will be monitored as part of the integrated settlement control regime.
- TBM specifications
A key factor in achieving TBM longevity, as required for settlement and volume loss control, is to have more power in the machines, possibly up to double that specified for previous TBM tunnel projects in London. The increased power should help to ensure the capacity to balance face pressure and support the ground without causing overpressure or weakening of the soil structure.
- Another important factor in delivering longevity is minimising the need for maintenance or cutterhead tool changes, especially for any major intervention. Crossrail however has the advantage of several station excavation contracts along the route that would afford opportunities for intervention work, unlike on the long CTRL headings that provided no intermediate access opportunities.
- Spoil weighing
Management and control of spoil extraction, and coordination with build rates of lining, is critical. Therefore, a key determinant of overall performance is to establish exactly how the excavation element is proceeding. A major effort has gone into this aspect of the works by establishing the specification of a system to continuously measure the weight of spoil being removed and for the data to be analysed, monitored and reported.
- The measurement system is to operate to a tolerance of +/- 5%, which is very small when looking at a maximum volume loss under the tunnelling contracts of 1%. To achieve this, the Crossrail planners have drawn on a wide range of industry experience, including the JLE and CTRL projects (where there was also the Lavender Street, Stratford, sinkhole incident), and in the knowledge that tunnel drives, in general, can be controlled to a very high degree.
Bond Street Station
- On Crossrail, the weight measurement system is to work on the fixed spoil transfer conveyor running along the backup of the TBM to the discharge onto the tunnel haulage system. An external consultant from the aggregates industry was employed to refine the specification for the system, primarily advising on the position of the weighing equipment and to help ensure regular quality assurance testing.
- Crossrail is being explicit in determining the spoil weighing specifications. Contractors will have to address these and work to ensure effective compliance.
- Crossrail will also monitor spoil on the transfer belt with video and laser scanning systems although it is noted that these are not as reliable weighing systems.
- Consultation and reference is also underway with other ongoing tunnelling projects including current construction of Barcelona's metro Line 9, in Spain. Crossrail said there is a good settlement control system in place on the project, employing belt weighing, data logging of different parameters, and telemetry. Crossrail is, in fact, looking to establish more systematic contacts with other major transport tunnelling projects in the world to share information and experiences.
- Crossrail design work also includes specification of a complementary excavation control system for the tunnelling works. This is being established by the Arup-Atkins JV under its C122 design contract.
Tottenham Court Road underground complex
- Further settlement control measures
A large volume of data will be generated by the tunnelling and excavation works, and therefore, part of the control and management challenge addressed by Crossrail is to ensure their rapid assessment and central production of meaningful criteria to support prompt decision-making for the client. Contractors may also have their own IT systems but these must support and reference the central system. This is being undertaken on a project-wide basis. The toughened-up rules giving authority for directing the timing, sequence and duration of tunnelling works are also being applied project-wide.
- Instrumentation and monitoring
Data gathering and its centralised management are being implemented by the client through the Underground Construction Information Management System (UCIMS). All parties will feed their data in a uniform method into UCIMS, which will analyse the figures and generate a range of dedicated reports for different groups. These include the client and the various contributing parties, or stakeholders, including, the contractors, London Underground, Network Rail, Docklands Light Rail, Thames Water, National Grid Gas, as well as property owners, local authorities and other statutory agencies.
- UCIMS is to provide real-time data and key information on construction progress and monitoring. Every party would be able to access an overview report while each stakeholder would see more detailed information relating solely to its assets.
- The UCIMS database is a major part of the Instrumentation and Monitoring Contract (C701), which was recently awarded to Instrumentation, Testing and Monitoring Ltd (ITM), a UK firm that has worked on projects including CTRL.
- A second part of the contract is to undertake subsurface infrastructure monitoring.
- Contractors on the project may also have their own instrumentation and monitoring systems and databases, but the data must be supplied promptly – automatically if possible – to the client’s UCIMS database. While the client's central operations base, at its head offices in Canary Wharf, will hold the UCIMS system for the entire project, staff will also be based in up to 25 local control rooms that will also accommodate representatives of the contractor and others.
- Authority of Section Engineers
The experience on CTRL was an important part of the general development of UCIMS but it also showed the value of using control rooms and of establishing authority over production activity, such as by assigning dedicated
Liverpool Street Station
- Section Engineers to monitor and control production from the outset of tunnelling. Data and print-outs after each ring-build cycle enabled comparisons with previous cycles and to establish TBM behaviour.
- In allocating responsibilities, Section Engineers had sole authority for, and control over, the systems to start the conveyor belts. Crews could not begin production in the tunnel. This was an important part of matching volume extracted to ring build and help avoid missing any aberrations in spoil volumes before continuing the drive.
- In a briefing earlier this year, TunnelTalk was told by Neil Moss, now former Acting Head of Tunnels, Portals and Shafts for Crossrail, and Mike Black, the Geotechnical Manager, told TunnelTalk, that dedicated Section Engineers will be assigned to control rooms on Crossrail to analyse data and from that information operate the belt with sole authority. This is a key part of the learning from past experience that is being taken forward into the tunnelling specifications for Crossrail they said.
- Grout and bentonite
Looking back to other useful experiences on CTRL, Crossrail also plans to adopt two further measures with respect to annular grouting operations. the use of a two-part thixotropic annular grout system, and having extra grout available in the tunnel on standby at all times.
- The two-part thixotropic grout is weak and gels rapidly in the annulus surrounding the precast concrete rings, unlike common cement-based grout which goes off too slowly.
- Crossrail sees the spoil weigh system as being crucial to accurate preparation and control of grout supplies in advance of building each 1.5m long ring of seven segments and a key. However, to mitigate any delay or problem in the sequencing of grout and segment delivery, the client has specified that extra grout should always be available in the tunnel. On CTRL, some experience had segment and grout supply trains carrying about 20% extra grout. On Crossrail, following risk analysis, about twice the grout needed is to be made available on standby in the tunnel on a continuous basis.
- Crossrail is also planning to use injection bentonite to help support the annulus around the TBM shield, which should help eliminate the proportion of total volume loss that is attributable to the passing of the shield itself.
- Research projects
There are two research projects underway on Crossrail related to ground movement and its potential effects on third
Canary Wharf Station
- party tunnels and utilities due to excavations for, and construction of, shafts and boxes as well as junctions in tunnels.
- The projects are being performed to produce extra data to help the planning, design, control and management of such works in future. They are being undertaken by:
- • The Department of Civil Engineering at the University of Cambridge, which is investigating ground movement (surface and underground) resulting from deep excavations for retaining walls, boxes and shafts.
The research, which will focus on generating data through monitoring live properties during construction of Crossrail, will not be used as part of the design or construction processes for the project.
• Imperial College London is researching the behaviour of cast iron bolted linings, such as those that line much of the capital's existing Underground network, and their response to nearby excavation. Working to original material specifications, the research will use specially cast, half-scale linings due to the practical limits of the testing equipment. The laboratory experiments will be used to help review past approaches to improve understanding and planning for future mitigation measures.
- Separately, Crossrail has been involved in a CIRIA project to produce guidance on how to deal with risks posed to construction works from unexploded ordnance. Given London's history, this has been a real cause for concern on past underground projects and is a concern for many cities in Europe and elsewhere around the world. The report has been published as CIRIA Report C681.
- Crossrail Geotechnical Series - 1 - Preparing the ground for Crossrail - TunnelTalk, Aug 2010
Crossrail Geotechnical Series - 2 - Site investigation and geological expectations - TunnelTalk, Aug 2010
Monitoring contract for Crossrail - TunnelTalk, June 2010
Crossrail management mobilized - TunnelTalk, May 2009
- Crossrail PowerPoint presentation, BTS Conference, September 2009, London
Four papers presented at the Fifth Symposium on Geotechnical Aspects of Underground Construction in Soft Ground held in 2005 by Technical Committee 28 (TC28 - Underground Construction in Soft Ground) of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). Proceedings published in 2006 by Taylor & Francis Group.
- Settlement due to tunnelling on the CTRL London Tunnels, Bowers KH & Moss NA
- The effect of new tunnel construction under existing metro tunnels; Moss NA, Bowers KH;
- Remote real-time monitoring of tunnelling-induced settlement using image analysis; Take WA, White DJ, Bowers KH & Moss NA
- The effects of tunnelling on piled structures on the CTRL; Jacobsz SW, Bowers KH & Moss NA, Zanardo G.
- Closed-Face Tunnelling Machines and Ground Stability, BTS report, Thomas Telfor
- Underground Construction Monitoring Guide, produced by BTS and to be published soon by Thomas Telford
- CIRIA REPORT C681: Unexploded ordnance (UXO) A guide for the construction industry
Allowable volume loss calculations September 2010
When I worked in the UK I always wondered how to get along with the historical progression of allowable volume losses. According to the British Standard some companies told me that they do some calculations about volume losses in every phase of design, let's say C then D, E, and by F, and for most design-build criteria as well, they go from 3.5% to 2.5% then 2% and arrive later at 1.5% or 1% in Stage F. Good practice would be to use the same allowable volume loss criteria during all stages of design and invoice the client accordingly. Instead of using the actual allowable volume loss from the beginning, the actual rate they will use for the final design turns up in stage F. In this way the design hangs on a rate that for some tunnel shapes will not be accepted in design stage C because the maximum allowable volume loss is too high! If they would use the actual 1% rate from the start it would be fine.
Further this calculation is very conservative for the actual volume of the tunnel shape x percentage of the actual progress, or let us say the indicator time. How long will the excavation and support go on? Also, the method of excavation, whether by TBM or NATM (SCL), is not indicated in this calculation. So it is just a total guess and will never reach true accuracy.
Tunnelling in London has been going on since Brunel's Thames Tunnel in 1825 and it is only now, in the last few years, that there is a real start to calculate this issue. By now we should be able to know what is feasible or what is not doable. With regard to applying the reduced assumption of 1% face loss on Crossrail, has someone just woken up and started thinking about this? Let us hope for better news from the island.
Add your comment
- Thank you for taking the time to share your thoughts and comments. You share in the wider tunnelling community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language professional.