Report on Long Term Settlement

Transcription

1 Thames Tideway Tunnel Thames Water Utilities Limited Application for Development Consent Application Reference Number: WWO10001 Report on Long Term Settlement Doc Ref: Folder 90 September 2013 DCO-DT-000-ZZZZZ

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3 Thames Tideway Tunnel Report on long-term settlement List of contents Page number 1 Report on long-term settlement Introduction Background Relevant tunnelling projects in London Thames Tideway Tunnel strategy in respect of LTS... 3 i

4 Report on long-term settlement 1 Report on long-term settlement 1.1 Introduction Long-term settlements (LTS) due to excavations in London Clay and other geological deposits subject to consolidation processes have been recognised as a potential problem to overlying structures for a number of years. However, the problem has been largely discounted because there has been little or no evidence of damage to structures that could be reasonably attributed to works in London Because the issue has not presented a problem, few measurements of LTS have been taken. However, during and after the construction of the Jubilee Line Extension, measurements were taken and the LTS quantified. In particular, measurements at St James s Park indicated that the LTS in London Clay might be substantial, although no damage has been reported For the other geological materials to be excavated during construction of the Thames Tideway Tunnel project (the project ), principally the Chalk, Thanet Sands and the Lambeth Group that will be encountered in the eastern reaches, no LTS impacts on structures are expected or have been previously encountered This report has been prepared to consider LTS due to consolidation in the context of the project and to explain why it is unlikely that it will have any significant impacts on structures overlying the tunnels. 1.2 Background There is no doubt that LTS due to tunnelling works and other deep excavations has occurred all over London and in many other parts of the world for many years but that their structural impacts, if any, have gone unnoticed. This may also be because the movements tend to be slow and it is likely that the normal cycle of repair and redecoration of properties deals with any superficial cracking. Furthermore, the natural seasonal drying and shrinkage of clays due to variations in rainfall and moisture content is known to causes movement that may disguise that caused by one-off excavation It should also be appreciated that normal environmental effects such as variations in temperature and humidity lead to the minor cracking commonly observed in most structures. The impact of vibration due to the use of structures and heavy road and rail traffic may also have similar long-term impacts Changes in groundwater conditions are also common in the London area. For instance, these may be due to the recharging of the deep aquifer due to the long-term changes in water abstraction rates, or variations in seasonal rainfall. Particularly relevant to structures close to the River Thames and other connected watercourses would be the constant 1

5 Report on long-term settlement variation in groundwater level due to the tides, although these would not be expected to have significant effects in the deep clays While LTS does occur, it should be noted that settlement of itself is not necessarily damaging to structures. Very large settlements do occur (as in some mining areas for example) but little or no damage is apparent to overlying structures. The critical factor that determines structural damage is the rate of change of slope, which in turn is the rate of change of settlement of the ground (sometimes described in terms of deflection ratio ), which induces strain in a structure. For some structures the horizontal component of ground movement could also be of importance Such evidence that is available indicates that LTS due to tunnelling tends to widen the trough of settlement above the tunnel (the short-term impact) and may increase the curvature within the trough Tunnelling-induced settlement curvatures caused by consolidation processes have not been widely investigated and the processes themselves are highly dependent on the variability of the ground permeability, existing pore water pressures and the geometry of the strata. Uncertainties in defining the ground conditions make it particularly difficult to make reliable predictions of the LTS even at specific locations. 1.3 Relevant tunnelling projects in London Traditionally tunnelling impact assessments in the UK and throughout the world have concentrated on the short-term settlements caused by tunnelling and have not considered LTS to be a significant problem. There have been many recent tunnelling projects in the clays beneath London (e.g. Jubilee Line Extension, Channel Tunnel Rail Link, cable and water tunnels) and none of these projects found it necessary to carry out specific assessments of LTS impacts. For Crossrail, some generic studies have been carried out but these did not result in changes to impact-assessed categories or mitigation works. Also, a few specific studies were carried out on particular structures at the request of their owners Thames Water is currently (2013) building the Lee Tunnel between Abbey Mills Pumping Station and Beckton Sewage Treatment Works. This tunnel runs for much of its length below critical and old infrastructure the Northern Outfall Sewer. Settlement data is being collected and may be a source of useful information in the future although it is expected that monitoring will not continue beyond the completion of construction As the exploitation of underground space beneath cities has become essential, tunnelling technologies have developed rapidly; tunnel construction methods have improved in recent years. In particular the need to keep ground movement to an absolute minimum has driven the advancement of tunnel boring machines (TBMs) that provide continuous support to the ground (earth pressure balance machines and slurry shield machines). They have been shown to greatly reduce ground movement impacts on overlying structures The improvements in TBM performance have been paralleled by improvements in tunnel linings. Greater precision in casting the concrete 2

6 Report on long-term settlement segments and the development of sophisticated gaskets between them has led to highly impermeable structures. Reducing the relative permeability of the lining and the surrounding ground reduces the impact of the tunnel acting as a drain the process that leads to consolidation settlement These improvements have been evidenced by measurements on the recently-constructed Channel Tunnel Rail Link tunnels beneath East London into St Pancras station. Extensive measurements were taken at a site in Dagenham and maximum long-term settlements of about 16mm were essentially complete within a year (Standing and Selemetas to be published). This was attributed by the authors to the A2 unit of the London Clay and the relatively impermeable grouted and gasketted tunnel lining In his 2008 Rankine Lecture, Professor Robert Mair considered LTS at some length. His conclusions were as follows: Although the magnitude of long-term settlement can be appreciable, the settlement profiles are generally very wide, with consequent curvature and differential settlements being generally small: hence the potential damage to buildings and services caused by long term settlements is likely to be of little consequence in most cases. 1.4 Thames Tideway Tunnel strategy in respect of LTS The TTT strategy follows that currently employed by Crossrail. Crossrail comprises two large tunnels (each similar in size to the project s main tunnel) running beneath many sensitive parts of London and Crossrail has not routinely carried out specific structural assessments for LTS. However, the interests of asset owners overlying the tunnels have been addressed by assurances and undertakings that settlement monitoring will be carried out until significant ground settlement has abated and that, where required, further assessment and mitigation will be carried out prior to the works For the Thames Tideway Tunnel, the assessors responsible for the structural impact assessment programme during preliminary design stages were required to identify any structures that are susceptible to damage from long-term consolidation settlement. Currently no such structures have been identified but, where underlying soils may be susceptible to LTS, it is anticipated that this will be considered further during the detailed design stage Where asset owners raise particular concerns, TTT may consider the structures on an individual basis Specifically, Section of the project s Settlement Information Paper (17 December 2012) states: Monitoring would continue after the TTT [Thames Tideway Tunnel project] works until ground movements attributable to the TTT project have ceased or the rate of settlement is less than or equal to 2mm per annum. These criteria mean that the risk of any further ground movements arising 3

7 Report on long-term settlement from the construction of the TTT project are so small that they pose no risk of detrimental impact to third party infrastructure and buildings In conclusion, the Thames Tideway Tunnel s approach to minimise settlement would be to employ in tunnel measures to limit ground movements to acceptable levels. These will include: a. Specifying high performance TBMs closed face TBMs for all main and long connection tunnel drives and slurry type TBMs for the eastern tunnel drives in Chalk b. installing instrumentation and monitoring to ensure the tunnel structures and ground movements behave as predicted c. using measures such as increased face pressures for the TBM in appropriate ground conditions d. specifying high standards of workmanship and construction management for the tunnelling works e. preparing design specifications to ensure the acceptability of the design and in particular tunnel and shaft lining systems f. potentially using ground improvement methods, from within the tunnel or from ground level, to improve the engineering properties of the ground and reduce ground movements The Thames Tideway Tunnel s commitment to minimising the impact on third-party infrastructure and the Settlement Information Paper are embedded in the Code of Construction Practice and secured by draft Development Consent Order requirement PW6. Reference: Mair, R. J. (2008). Tunnelling and Geotechnics: Rankine Lecture Ge otechnique 58, No. 9,

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