Conference preview
As the government/industry LINK Construction Maintenance and Refurbishment research project ,’Subsidence damage to buildings: prediction, protect and repair’, was drawing to an end, it was decided that three more actions were needed to make the most of the resources put into it by London Underground Limited, the Engineering Physical Sciences Research Council and DETR, many industry organisations through CIRIA, the research team at Imperial College and those employed by the Jubilee Line Extension (JLE) project. These were:
The latter two purposes led to the international conference, ‘Response of buildings to excavation-induced ground movements’, to be held at Imperial College on 17-18 July 2001. A summary of the themes of the six technical sessions to the conference.
The Conference Advisory Committee agreed that the conference should be based on discussion rather than the presentation of the 50 or so submitted papers; rapporteurs at each technical session will give an overview before introducing the topics for themed discussion. Papers from 14 countries cover a wide range of experience on how the effects of tunnelling can be predicted and their potential consequences mitigated.
As will be seen from the case studies, there are positive messages about the current capabilities – of tunnelling, compensation grouting, of instruments and management processes of monitoring.
Conference programme
The chief engineer of London Underground (LU), Keith Beattie will formally open the conference on 17 July after an introduction by Jim Moriarty of LU, chairman of the Conference Advisory Committee helping CIRIA with planning.
Professor Robert Mair will give the keynote lecture and the closing address will be given by Professor John Burland.
For information about the conference see the website: www.ciria.org.uk/conferencesground.htm, or contact Charles Perkin at CIRIA on +44 (0)20-7654 9014, or atcharles.perkin@ciria.org.uk
Accompanying exhibitors include Fugro, Gibb, Instrumentation Testing and Monitoring, Keller UK, London Underground, Sir Robert McAlpine, Thomas Telford, and Wilde and Partners.
Case histories
The book ‘Building response to tunnelling: case studies from construction of the Jubilee Line Extension’ (Edited by J B Burland, J R Standing and F M Jardine, and published by Thomas Telford Ltd) gathers together a large number of well documented case histories of ground and building response to construction of the JLE. Even within the restraints of the book, the material is a source of valuable field data for future practical and theoretical work.
The database set up to contain the construction and monitoring records is a valuable resource and has already proved to be a very useful outcome.
Some of the key lessons from this research are:
1) For a given tunnel configuration, volume loss is the fundamental parameter controlling ground movements, yet it cannot be easily or accurately predicted. It depends significantly on the tunnelling method and the skill and care of the operator. Consideration should therefore be given to specifying limits on volume loss as a contractual requirement. It is easily measured by selected control sites along the route and there is little ambiguity about the interpretation of the results.
2) The simple Gaussian subsidence curve gives a reasonable representation of the form of greenfield site transverse settlement troughs for single tunnels. The research has presented further evidence that significant interaction between tunnels can lead to forms of settlement trough which differ from that given by simple superposition.
3) A number of detailed case records show that building stiffness can substantially reduce the greenfield site relative deflections. However, there is evidence to show that load-bearing walls in the state of hogging can be more flexible than those in sagging. There is now overwhelming evidence that, for buildings founded on rafts and strip footings, little or no transfer of the horizontal strains in the underlying ground takes place up into the building. Further measurements are required for buildings on isolated pad footings and piles.
4) Very little damage has been observed or reported for those buildings studied by the research team and almost all of it was in Category 2 (slight) or less. A careful review of the reported damage revealed that it tended to occur at connections between adjacent structures particularly where there are differences in foundation type or ground treatment.
5) Numerous case histories show that transitory distortions in a building in the form of relative deflections or twist can be much larger than the final deformations. This is particularly so for tunnels that pass longitudinally or obliquely beneath.
6) A number of case histories have been assembled demonstrating the successful application of compensation grouting for protection against subsidence damage (Figure 3). The two main forms are concurrent grouting, carried out contemporaneously with excavation, and corrective grouting to reverse significant settlements. Consideration must also be given to the relative movements between adjacent buildings and to connecting structures such as brick vaults and services.
7) Control of urban tunnelling projects will depend to an increasing extent on reliable and precise methods of monitoring movements. Consideration needs to be given to the best methods of procuring and managing these vital operations. Automated methods of making total station and precision level measurements are now available.
Related Files
Profiles of settlement of a low-rise façade
Surface settlements at Southwark Park
Profiles of the façade
