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JLE opens the casebook

CIRIA CONFERENCE: CIRIA's international conference, Response of buildings to excavation-induced ground movements, will be held at Imperial College on 17 and 18 July. Conference advisory committee chairman Jim Moriarty of London Underground and Fin Jardine

This conference stems from the London Underground Limited Jubilee Line Extension project in London.

During the massive project, which involved extensive tunnelling beneath historic buildings including landmarks such as the Ritz Hotel, the Royal Automobile Club and the Big Ben clock tower, LUL made extensive efforts to observe and record movements to learn as much as possible for future schemes.

A major collaborative research project, led by CIRIA, was initiated between LUL and Imperial College, London. It was titled the LINK CMR (construction, maintenance and refurbishment) project, Subsidence damage to buildings:

prediction, protection and repair.

Funding came from industry, the Engineering & Physical Sciences Research Council and the Department of the Environment, Transport and the Regions (DETR).

Results of research and case studies from construction of the Jubilee Line Extension (JLE) should help in more economical provision of future cross-London rapid transit systems.

It is particularly important that these case records should reassure owners of properties near proposed new tunnels and stations, because they demonstrate the remarkable control and precision that can be applied to the protection of buildings otherwise at risk.

The conference has three principal objectives which were decided by the conference advisory committee:

To pass on lessons gained from the case studies and research associated with construction of the JLE;

To learn about other technical developments since the JLE was completed;

To provide a forum for the discussion of these advances and their implications for practice.

The conference will be opened by LUL chief engineer Keith Beattie and the opening and closing lectures will be given by Professor Robert Mair of Cambridge University and Professor John Burland of Imperial College.

The event will feature new UK and international research and the latest developments in the methods of monitoring, damage prediction and protection.

More than 50 papers by experts from 14 countries, most of which contain case study information, will be covered in six technical sessions.These discuss various aspects of the subject and will include technical discussions led by a rapporteur.

A book presenting the research results and case studies of buildings and greenfield sites and their response to the JLE tunnelling will also be published in the near future (see box below left).

Session 1: Prediction of damage to buildings and other structures from tunnelling Chairman: Professor Ed Cording (University of Illinois at Urbana Champaign) Rapporteur: Chris Pound (Mott MacDonald) Papers for this discussion session include case histories from Denmark, Russia, Germany, Holland and UK.

They involve shield and earth pressure balance machine (EPBM) tunnelling and the effects of other construction activities on existing tunnels.

The discussion topics that may be considered here are likely to recur in later sessions and could include the following:

How do different tunnelling methods and the quality of workmanship affect volume loss?

How does the presence of a building affect ground movements (and how is this influenced by the type of building and its foundation)?

What inputs are needed to make realistic predictions and how are they to be obtained?

For what situations would empirical predictive methods and advanced numerical methods of prediction be appropriate?

How should predictions be made if there are no case histories specific to the locality and method?

Are we predicting the 'right'things, and do we know what the 'right'things are?

How are predictions to be used after they have been made, given that very often the contract arrangements and the construction methods change?

Session 2: Effectiveness and viability of protective measures, particularly compensation grouting Chairman: Professor Harry Poulos (Coffey Geosciences) Rapporteur: Rab Fernie (Cementation Foundations Skanska) When the JLE was being planned, there was little experience of compensation grouting and very few documented case histories.There are now many thorough case study records, some given as papers here, others as chapters in the book (see box on facing page) giving the results of the LINK research project.

These confirm the control that be achieved by flexible but complex and heavily monitored procedures.

Other methods, both traditional and novel, were also used to protect buildings along the JLE route from settlement damage, but it is compensation grouting that tends to take centre stage.

The papers include case histories from USA, Russia, Spain, Belgium (featuring work carried out beneath Antwerp's central station) and proposals for Hong Kong, as well as the London examples.

Other techniques described in the papers include underpinning, pipejacked roof support, face dowels, jet grouting, ground freezing and soil extraction. The discussion topics could well include the following:

Criteria for choice of appropriate protective measures Cost-benefit analysis of the protective measures in relation to tunnelling method (and advance rates) and costs of repair Process questions, which for compensation grouting include matters of technique (eg pastes, pressures, efficiencies, quantities) and questions of safety and control (proximity to existing structures and the new works, exclusion zones around tunnel face and frequency of interventions to counter settlement) Whether there could be a conflict between safety of the tunnel and building protection, particularly in respect of intunnel and external monitoring How to minimise the effects of the often extensive enabling works, such as access shafts and tube-a-manchette (TAM) installations The differing requirements for grouting beneath heavily loaded foundations and lightly loaded zones.

Session 3: Management of the monitoring process Chairman: Professor Michele Jamiolkowski (Politecnico di Torino) Rapporteur: John Dunnicliff (geotechnical instrumentation consultant) There are many reasons for monitoring the performance of new and the response of existing structures, for example safety, protection against damage, process control, quality control and research.

All of these are implicit in the case studies of papers to the conference and, indeed, in the huge amount of monitoring undertaken for the JLE.

There are two golden rules for instrumentation (outlined by J Dunnicliff and AJ Powderham, Recommendations for Procurement of Geotechnical Instruments and Field Instrumentation Services).

First, the purpose of geotechnical instrumentation and monitoring is to assist with answering specific questions about soil/structure interaction.

If there are no questions, there should be no instrumentation.

Second, to provide the best basis for securing reliable and high quality data, and hence for securing best value, the people who have the greatest interest in the answers to the questions should have a major role in obtaining the data.

Technological developments, whether of measurement systems and instrumentation or of the recording and processing of data, have created both opportunities and risks.

They can lead to greater understanding of the engineering processes: they can create 'information overload'even distrust.

Measurements are not in themselves sufficient for decision; taken for one reason, they are unlikely to be adequate indices of another purpose.

But in a complex construction process, such as urban tunnelling, many types of monitoring are essential.

For the benefits to be fully realised, a great deal of attention has to be given to management for all of the monitoring processes.

Virtually all the conference papers, being case studies, by definition use the results of monitoring.Some deal with the new and proposed measurement systems, others look to the integration of the construction works with ongoing, real-time observations to inform control of the construction process.

It is an exciting time for development in the application of instrumentation in construction.

There is a considerable body of recorded experience - based on measurement, not anecdote - made possible by the emphasis on monitoring, but it is always necessary to check whether the right things are being measured at the right time for the right reasons.

This discussion session is likely to include lively debate about what is required from monitoring and how best to obtain it through design, procurement and management and questions such as:

How to procure instrumentation and monitoring services for different purposes How are interpretations of monitoring results brought into the site decision-making process?

Are the people charged with responsibility for monitoring listened to?

What are the implications of huge amounts of data?

Is there a conscious stage of ensuring monitoring results are understood, ie a review established as an early trigger limit?

Are the right things being measured? or is it that engineers tend to measure what they can measure in the hope that it will be an appropriate indicator?

With construction methods becoming increasingly instrumented (eg machine parameters on EPBMs), can feedback loops be developed to link construction process controls to direct observations of settlement (volume loss)?

Session 4: Case studies from the Jubilee Line Extension Chairman: Ivan Chudleigh (Infraco) Rapporteur: Jan Hellings (Maunsell Group) The format of this session will differ from the other technical sessions for two reasons.

First, it is focused on JLE examples and, second, it will comprise three presentations.

Jamie Standing of Cambridge University will present results of a study by the LINK project research team of the response of a building to construction of a step-plate junction near Green Park.

David Harris of Geo- technical Consulting Group will then describe the preventive works and monitoring of St Stephen's Tower at the Palace of Westminster that houses Big Ben.

The rapporteur, Jan Hellings, will give an overview of the other JLE-based papers, which describe the effects on buildings and surface settlements at different sites along the route, and compensation grouting and building monitoring on Contract 102 - Westminster and Waterloo stations, tunnels from Green Park to Waterloo.

These papers add to the coverage of the other technical sessions, but points specific to the JLE works include:

There was almost no significant damage, even with settlements as much as 70mm in places.Where slight damage was seen it was often at junctions with other buildings.

Precise control was achieved by compensation grouting, notably for the Big Ben clock tower, but for many other structures as well. The figure (right) represents the tilt of the Clock Tower and the control achieved.

Some of severest relative deflections can be the transitory ones, particularly for tunnels that pass longitudinally or obliquely beneath a building, but the control given by compensation grouting allows the possibility of modifying the deflected profile of a building during the course of tunnelling.

There remains a need to improve the terminology of grouting.

There is now a substantial body of experience that has been recorded and examined in case studies which will benefit future tunnelling and infrastructure works in London.

Session 5: Prediction of damage to buildings from open excavations Chairman: Amar Bhogal (Institution of Civil Engineers) Rapporteur: Kelvin Higgins (Geotechnical Consulting Group)Several of the conference papers concern movements induced by open excavations on sites in Ireland, Taiwan , India and the UK.

The ground conditions of these case studies include soft and stiff clays and glacial till.

Most of the papers examine the effects on different types of building, but one paper is about the effect on existing tunnels.

The types of subject that a re likely to be discussed in this sessions could include:

What methods (empirical or numerical modelling) a re appropriate for predicting ground movements and their effects on nearby buildings, services, and tunnels?

Given the time-dependent behaviour of clays, is it readily possible to offset the settlement of adjacent structures by the heave that develops around and at the base of the excavation?

How much effect do associated construction activities such as the installation of diaphragm walls have, and how are inte rim stages of excavation, that can be more onerous for buildings, taken into account?

What are appropriate preventative measures to stop movements, particularly in relation to global (ground mass ) and local (wall) movements?

Examples of trigger limits of movements in diffe rent situations and how these relate to consequences if limits are exceeded.

Comparison of ope rational and actual stiffness of propping, wall and ground.

Session 6: Effects of building stiffness, different configurations and time

Chairman: Sam Thorburn (consultant) Rapporteur: Giulia Viggiani (Universita di Roma - Tor Vergata)

The session papers come from Italy. Germany, USA, Spain, and UK. Their subjects cover 2D and 3D numerical modelling, 1g and centrifuge modelling and case studies of buildings affected by tunnelling.

In addition to the key questions implied in the session title and the range of ground types in the case histories, the papers deal with the effects of works preliminary to tunnelling, the estimation of building stiffness, methods of modelling masonry, the effects on subsurface structures, assessing past movements of buildings, and whether and how tunnel-induced ground movements continue into the long term.

As the last technical session of the conference, this discussion session will benefit from contributions on these inter-related topics in the preceding sessions.

Specific questions that may be raised could include:

How to assess building stiffness for different types of structure, modes of ground displacement and geometry, and whether it changes.

How, over the long term, do tunnel-induced settlements develop, and are they a problem?

How to take into account different alignments and geometry of buildings and tunnels, and any interactions between them because of the timing of tunnel drives.

What are the effects in the longer term of the grout and TAMs remaining in the ground?

How are tensile strains to be considered? What are the appropriate unit lengths by which critical horizontal strains are determined for different circumstances?

How well do engineers understand the condition of historic buildings and what they have previously experienced?

Recorded information

Building response to tunnelling: case studies from construction of the Jubilee Line Extension, London This two-volume book will present the research results and case studies of buildings and greenfield sites and their response to the JLE tunnelling.

The first volume presents overall findings from the research case studies and information common to the series of building and greenfield site case histories detailed in Volume 2. Individual chapters were written by experts who participated in the research and by Imperial College and LUL-based researchers.

The JLE project between Green Park and Canada Water stations is described, along with the methods of settlement prediction and building damage assessment and the design of the research. These are followed by accounts of the geology and historical development of the sites of the case studies. Separate chapters describe the tunnelling methods and the protective measures.

The research methodology of measurement types, their precision and data handling also includes formally made best-practice predictions. Full results of the before-the-event predictions of surface and atdepth ground movements at two greenfield sites and the settlements of four buildings are also presented.

The concluding chapter of Volume 1, by Imperial College's Professor John Burland, presents findings from the research.

Volume 2 contains the 27 case studies in their geographical sequence along the project route, from Green Park in the west to London Bridge on London Clay, and then on the Lambeth Group beds eastward to Canada Water station. These include two instrumented greenfield sites.

The case study buildings include some of international importance, such as the Big Ben clock tower, prestigious public and commercial buildings in central London and residential buildings in Southwark, including two tower blocks. Important case studies are those where compensation grouting was used.

The book is edited by John Burland and Fin Jardine and will be published by CIRIA and Thomas Telford.

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