Centrifuge modeling of deep excavations and their interaction with adjacent buildings

Major cities in the world are experiencing a rapid growth in population while becoming increasingly overcrowded and congested. In recent years, this has created a huge demand for underground infrastructure, which often involves the design of major mass transit tunnel systems; these tunnel systems (underground tunnels and metro stations) are becoming increasingly necessary to construct in very close proximity to existing buildings. The prediction of excavation-induced deformations therefore becomes a key issue in the planning and design process for these schemes. However, current design approaches are conservative and often lead to unnecessary concern and expenditure in the design and provision of protective measures. A better understanding of the mechanisms involved in the excavation-soil-structure interaction could reduce costs and help avoid potential problems. A series of small-scale model tests was carried out in the geotechnical centrifuge at Cambridge University to investigate the interaction between excavations and model buildings. Excavations (simulated by adopting a novel two-fluid technique) in a ‘free-field’ were also undertaken to assess the difference between free-field ground movements and those affected by a stiff model building. A detailed description of the centrifuge models and test procedures is presented in this paper, followed by the presentation of test results that demonstrate the effect of the stiffness of the model building on the excavation induced-displacements.The authors would also like to acknowledge the financial support provided by the Cambridge Commonwealth Trust and the Gates Cambridge Trust.This is a preprint of an article published in Geotechnical Testing Journal. The final version is available from http://www.astm.org/DIGITAL_LIBRARY/JOURNALS/GEOTECH/PAGES/GTJ20120209.htm

Interaction between surface structures and tunnelling in sand: Centrifuge and computational modelling

Tunnelling in urban areas requires a careful estimation of the consequence of soil settlements on existing buildings. In this paper the interaction between the excavation of a tunnel in sand and surface structures is investigated. A two dimensional finite element model is presented and validated through comparison with centrifuge test results, both with and without structures. The model is then used to perform a sensitivity study on the effect of building weight on soil movements and structural deformations. The results of the validation indicate that assuming a no-tension interface between the soil and the structure is essential to capture the soil-structure interaction that was experimentally observed. The parametric analyses show that the relation between the building stiffness and the tunnelling-induced deformations depends on the building weight.Financial support was provided by the Engineering and Physical Sciences Research Council of the United Kingdom, under grant reference number EP/K018221/1.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.tust.2015.07.01

Case studies of circular shaft construction in London

At present, there are few well-documented case studies of circular shaft construction, making it difficult for designers to estimate reliable ground movements arising from such construction. This paper describes field observations of ground surface settlement assembled during the construction of 27 circular shafts built for three major tunnelling projects in London: Crossrail, National Grid's London Power Tunnels project and Transport for London's Northern line extension. Two categories of shaft construction were identified: support before excavation (SBE) and excavation before support (EBS). For the SBE category, the shaft was first supported by pre-installed walls followed by excavation of the soil between the pre-installed walls. For the EBS category, the ground was progressively excavated in sections followed by construction of the shaft lining. Interpretation of the field observations showed the importance of the shaft construction method on ground movements. Settlements were much more significant for EBS shaft construction than for SBE shaft excavation, although settlement arising from the installation of pre-installed walls or dewatering operations should not be overlooked. Normalised charts are presented to help the industry make estimates of settlements due to circular shaft construction in London, with due consideration for different shaft geometries and construction methods. </jats:p

Pile-soil interaction and settlement effects induced by deep excavations

Deep excavations may cause settlement and damage to adjacent buildings, even if they are found on piles. The corresponding pile deformations are determined by axial and lateral effects. This paper describes an analytical model relating axial pile deformation to the vertical soil displacement resulting from the deep excavation and also suggests ways to determine the pile response to lateral displacements. The axial pile-soil interaction is clearly different for end-bearing and friction piles. Common generalizations that end-bearing piles settle the same as the soil settlement at the base level and friction piles with the ground surface settlement present lower and upper bounds, which are only valid for certain idealized cases. The settlement of piles with a large component of shaft friction is determined mainly by the actual load on the pile relative to the pile ultimate capacity. The lateral pile response is governed mainly by the relative stiffness of the pile to the soil. The proposed model was validated with measurements of the North South Line project in Amsterdam.This paper is based on the first author’s Ph.D. study at Cambridge University in cooperation with the Netherlands Centre of Underground Construction

Apparatus for centrifuge modelling of twin-tunnel construction

In urban areas it is common for pairs of tunnels to be used as a method for building rapid transit systems. Driven by an increasing population and demand for services, tunnels are more widespread in their use than at any previous time. Construction of any form of tunnel causes ground movements which have the potential to damage existing surface and sub-surface structures. Modern tunnelling practice aims to reduce these movements to a minimum but there is still a requirement for accurate assessments of possible damage to structures resulting from settlements. For tunnels driven in clay, superposition of settlement predictions made by considering a single tunnel is an accepted method used to estimate movements around pairs of tunnels. Previous research, particularly numerical studies, has indicated that this may not necessarily be sufficient. In this paper a series of centrifuge model tests designed to investigate settlements related to twin-tunnel construction are described. The development of the experimental apparatus for sequential twin-tunnel construction with variable centre-to-centre spacing and volume loss is described in detail

Considerations for monitoring of deep circular excavations

Understanding the magnitude and distribution of ground movements associated with deep shaft construction is a key factor in designing efficient damage prevention/mitigation measures. Therefore, a large-scale monitoring scheme was implemented at Thames Water's 68 m deep Abbey Mills Shaft F in East London, UK, constructed as part of the Lee Tunnel Project. The scheme comprised inclinometers and extensometers which were installed in the diaphragm walls and in boreholes around the shaft to measure deflections and ground movements. However, interpreting the measurements from inclinometers can be a challenging task, as it is often not feasible to extend the boreholes into ground unaffected by movements. The paper describes in detail how the data are corrected. The corrected data showed very small wall deflections of less than 4 mm at the final shaft excavation depth. Similarly, very small ground movements were measured around the shaft. Empirical ground settlement prediction methods derived from different shaft construction methods significantly overestimate settlements for a diaphragm wall shaft. The results of this study will help to inform future projects, such as the forthcoming 25 km long Thames Tideway Tunnel with its 18 deep shafts being constructed adjacent to existing infrastructure. Engineering and Physical Sciences Research CouncilThis is the author accepted manuscript. The final version is available from ICE Publishing via http://dx.doi.org/10.1680/jgeen.15.0006

Twin-tunnelling-induced ground movements in clay

Modern tunnelling methods aim to reduce ground movements arising from the construction process. In clay strata the usual method of construction is by tunnel boring machine, which allows close control of the tunnelling process; however, any movements have the potential to cause damage to existing structures at, and below, the ground surface. The construction of underground rail systems often comprises two tunnels running in opposite directions. Common practice for assessing construction-generated movements around these tunnels is to make predictions based upon individual tunnel construction and utilise superposition to generate a total deformation profile. This approach does not take into account the strain- or stress-dependent effects between tunnel constructions. A delay may result in unanticipated ground movements generated by the construction of the second tunnel. The effect of this delay on the ground movements arising between the first and the second tunnel construction process was investigated in a series of plane strain centrifuge tests. The ground movements at and below the surface were monitored and were assessed against superposition-based predictions for surface settlement with the outcomes highlighting some inconsistencies. A procedure for predicting both surface and subsurface vertical settlement profiles in the plane transverse to the advancing tunnels in clay is suggested

Centrifuge modelling of tunnelling with forepoling

Geotechnical centrifuge modelling provides a means by which geotechnical events and processes can be better understood. In particular, the technique has proved invaluable when investigating collapse mechanisms in small scale models that can be related to full scale events. A series of eight plane strain centrifuge model tests investigating the effect of inserting inclusions around the annulus of a single tunnel in over consolidated clay has been conducted using the geotechnical centrifuge at City University London. The model used a compressed air supported circular cavity to simulate the tunnel. Stiff resin inclusions embedded around its periphery were used to represent closely spaced forepoles forming grout umbrella arches. Image processing was used to obtain patterns of displacements at the subsurface and displacement transducers measured vertical settlement at the ground surface level. The investigation focused on how different arrangements of forepoling affected tunnel stability. The influence of forepoling on normally accepted plastic collapse mechanisms is discussed. An optimisation of the forepoling layout is uggested in accordance with the findings

The monitoring of an existing cast iron tunnel with distributed fibre optic sensing (DFOS)

Instrumentation is vital to tunnelling projects for validation of design assumptions, monitoring of trigger levels during construction and also serving as a feedback loop to understand the deformation mechanisms of the particular problem which could be used to improve future designs. It can also be used for long term monitoring of the tunnel for maintenance purposes. Distributed fibre optic sensing (DFOS) systems based on brillouin optical time domain reflectometry are able to provide continuous and distributed strain measurements to be taken along the entire length of, for example, an existing cast iron tunnel where the fibre optic cable length is fully attached. This enables engineers to understand the stresses and strains that develop within the lining caused by external influences; which in this case, the construction of a new tunnel directly underneath it, rather than relying on discrete point measurements of displacement from conventional methods of monitoring. Nonetheless, proper installation of DFOS is of paramount importance to obtain high quality data. This paper aims to provide some practical guidance on the planning and installation of DFOS and presents a brief case study on the monitoring of London’s Royal Mail tunnel during the construction of the large Crossrail platform tunnel, located directly below it at Liverpool Street Station. It demonstrates the potential benefits of using such systems in complex tunnelling scenarios.The authors would like to gratefully note the contributions in the form of logistical and technical support from Crossrail (particularly Mike Black, Stephen Roberts, Alison Norrish and Chris Dulake), Royal Mail (particularly Mitch Harris), ARUP (particularly Mike Devriendt), CH2 M Hill (particularly Peter Wright). This project would not have been possible without the financial support from Laing O’Rourke for the first author’s PhD studentship as well as the continuous support from the UK Engineering and Physical Sciences Research Council (EPSRC) and Innovate UK through their funding of the Centre for Smart Infrastructure and Construction (CSIC) at Cambridge.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s13349-015-0109-

Monitoring the effects of tunnelling under an existing tunnel-fibre optics

Underground tunnel networks are at the heart of United Kingdom's infrastructure, carrying more than 1,100 million passengers each year along its 249 miles long network via 11 underground lines, serving 270 stations.With a complex existing underground rail network already in place; it is inevitable that new tunnels will be constructed within close proximity to existing tunnels. At London Liverpool Street Station, the new eastbound Crossrail platformtunnel was constructed underneath the existing Royal MailTunnel at a parallel alignment over a length of over 100m with a clear distance of less than 2m separating the two. Fibre optic strain sensing system based on Brillouin Optical Time Domain Reflectometry (BOTDR) was installed to measure continuous strain profiles of the cast iron linings of Royal Mail Tunnel. This has provided valuable insights to the deformation mechanisms both during the pilot and final tunnel enlargement. © 2014 Korean Geotechnical Society