216 research outputs found
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Twin-tunnelling-induced changes to clay stiffness
With urban tunnel construction growing worldwide, the ability to accurately predict the ground and structural response to tunnelling and the associated risks is now more important than ever before. Engineers are expected to consider all aspects of tunnel engineering in order to safeguard existing infrastructure, by employing field monitoring, physical modelling and numerical analysis in developing a detailed knowledge of multiple soil-structure interactions. The Geotechnique Symposium in Print took place at the Institution of Civil Engineers on 14th September 2017 to discuss the wider aspects of tunnelling in urban locales. The papers included here bring together important international research presented at the symposium and featured in Geotechnique . Topics of discussion, amongst others, included - case studies from Crossrail, CTRL and Barcelona metro projects - open-face tunnelling effects on non-displacement piles in clay - influence of building characteristics on tunnelling-induced ground movements - impact of new tunnel construction on structural performance of existing tunnels. Tunnelling in the Urban Environment offers practitioners and researchers alike with important coverage of the increasingly complex and varied challenges engineers have to face when constructing tunnels in urban centres
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The influence of a time delay between sequential tunnel constructions
In congested urban areas twin-tunnelling projects have an associated requirement for accurate predictions of any ground movements that may arise in order to minimize possible damage to existing structures. Much previous research has been carried out concerning the ground movements around single tunnels in greenfield conditions and this, along with field observations, forms the basis of most ground movement predictions. For twin-tunnel projects the prediction of overall ground movements is often based upon superposition of two equal settlement troughs. Previous research by the current authors has shown that this assumption may be invalid for most cases. The research presented in this paper considers the influence on observed ground movements of a construction process that involves a delay between the construction of each tunnel, compared with tunnels constructed simultaneously. A series of four centrifuge tests is described and the results are presented in terms of the ground movements measured at the surface
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Novel apparatus for generating ground movements around sequential twin tunnels in over-consolidated clay
The tests consisted of a sequential twin-tunnel construction with varied centre-to-centre spacing in overconsolidated clay. Relatively complex apparatus facilitated a predefined volume loss whilst monitoring surface settlement, tunnel support pressures and pore-water pressures. The surface data were assessed against published estimation methods with the results highlighting some inconsistencies
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An apparatus for centrifuge modelling of a shaft construction in clay
Tunnelling systems have been widely used for housing transport links and service lines. The preferred method for accessing tunnels is via narrow vertical excavations (referred to as ‘shafts’) because of the advantages it provides in the congested urban environment. These shafts can exist as staircases, lifts or for ventilation purposes. The settlements and ground movements associated with shafts are influenced by the method of construction, type of soil and geometry of excavation (i.e. diameter size and depth). Literature describing the ground movements arising from shafts is relatively limited when compared with other geotechnical construction events (for example tunnelling). Therefore, apparatus has been designed, fabricated and tested to explore the behaviour of a clay ground model when constructing shafts. The apparatus outlined in this paper is a novel approach for the centrifuge modelling of short-term shaft-sinking induced settlements in clay
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The effect of a forepole umbrella system on the stability of a tunnel face in clay
A new series of three-dimensional centrifuge model tests using soft clay has been conducted using the geotechnical centrifuge facility at City University London. These tests aim to quantify the reinforcing proficiency of different arrangements of steel pipes in a Forepoling Umbrella System (FUS). The results highlight some interesting effects of the FUS on tunnel stability and the spread of ground movements in the vicinity of the tunnel heading
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Twin-tunnelling-induced changes to clay stiffnesses
Tunnels used for transportation in urban environments are often constructed in pairs. Projects in which tunnels are constructed sequentially and within close proximity are referred to as ‘twin tunnelling’. Case studies and recent research indicate that the prediction of settlements for such schemes cannot be determined using existing simple methods derived from consideration of a single tunnel. To establish the reasons for the observed variation in settlements, a series of centrifuge tests was undertaken on various twin-tunnel arrangements in overconsolidated clay. The tests consisted of preformed cavities from which a specific quantity of supporting fluid could be drained, with precision, creating a predetermined magnitude of tunnelling volume loss. Data were obtained for surface and subsurface displacements, changes in pore-water pressure near the tunnels and the support pressure within the tunnels. The systematic use of cavity contraction models was found to be an informative method of explaining the observations. Use of an elastic–perfectly plastic cavity contraction model coupled with observations from the experiments enabled the shear stiffness of the clay around the tunnel to be described. Further analysis demonstrated a reduction in shear stiffness of the soil prior to and during the second tunnel excavation, explaining the increase in volume loss observed in that event
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Explanation for twin tunnelling-induced surface settlements by changes in soil stiffness on account of stress history
In this article, a group of representative centrifuge tests were selected for numerical modelling to explain the surface settlements induced by sequential twin tunnelling. Both Modified Cam Clay model (MCC) and Three-Surface Kinematic Hardening model (3-SKH) were adopted in the simulation, which indicated the use of 3-SKH model conduced to mimicking more closely centrifuge model response. Via performing more contrastive numerical analyses with 3-SKH model, the influence of the first tunnel event on the stiffness of the soil around the second tunnel was quantitatively investigated, whereby the mechanism behind the observed surface settlements was finally made clear
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Ground movements generated by sequential Twin-tunnelling in over-consolidated clay
The expansion of urban populations comes with an associated demand for increased public transport. An often utilised solution is to construct a rapid transit system within tunnels. Generally, a pair of tunnels are constructed within relative close proximity. The construction of these tunnels will generate ground movements which have the potential to cause damage to existing surface and subsurface structures. Modern tunnelling practice aims to reduce these movements to a minimum; however there is still a requirement for accurate assessment of 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. This presumes that the movements generated from the construction of the second tunnel are not influenced in any way by the presence of the first tunnel. A series of plane strain centrifuge model tests have been conducted to explore the validity of superposition as a prediction method. The tests consisted of a sequential twin-tunnel construction with varied centre-to-centre spacing in over-consolidated clay. Relatively complex apparatus facilitated a predefined volume loss whilst monitoring surface settlement, tunnel support pressures and pore-water pressures. The measured data were assessed against superposition for surface vertical settlements in the plane perpendicular to an advancing tunnel face. The results highlight some inconsistencies with the superposition method
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Ground movements associated with twin-tunnel construction in clay
The rising population in urban environments comes with an associated demand for increased public transport. Due to the level of surface congestion an often utilised solution is to construct rapid transit systems within tunnels. Any sub-surface construction will generate ground movements which have the potential to cause damage to existing surface and sub-surface structures. Urbanisation and congested cities have driven the need for accurate predictions of tunnelling-induced settlements and has produced many publications (e.g. Peck, 1969; Cording & Hansmire, 1975; Clough & Schmidt, 1981; O’Reilly & New, 1982; Attewell & Yates, 1984; Cording, 1991; Mair et al., 1993 and Mair & Taylor, 1997). Largely, however, these empirically based prediction methods are concerned with single tunnel greenfield, arrangements. Generally, mass rapid transport systems comprise of a pair of tunnels constructed within relative close proximity. This is known as twin-tunnel construction. A number of case studies have shown a relative difference in the settlements due to each tunnel construction (e.g. Cooper et al., 2002; Cording & Hansmire, 1975 and Nyren, 1998). These were further investigated by numerical studies which support these observations (e.g. Addenbrooke & Potts, 2001 and Hunt, 2005). Analyses that use isotropic linear elasticperfectly plastic soil models have tended to produce wider surface settlement troughs then observed by the Gaussian distribution (Mair et al., 1981). It is that clear valuable insight could be gained from a physical model based study. Therefore, a series of plane strain centrifuge tests was carried out investigating twin tunnelling-induced settlements in overconsolidated clay. Apparatus necessary to perform these tasks required a significant amount of time to develop and was relatively complex. The main variables were the spacing between the tunnels, both horizontally and vertically, and the magnitude of volume loss. The tests were conducted at 100g where the cavities represented two 4m diameter tunnels at (usually) a depth of 10m at prototype scale. The tests utilised novel apparatus designed during the research to enable the simulation of the construction processes related to volume loss in separate sequential tunnels.
The results presented are in regards to the prediction of ground movements in the plane perpendicular to advancing tunnels and the significant findings of the research are as follows: -
1. Single tunnel surface and sub-surface settlement troughs are well represented by Gaussian distributions, however, the twin-tunnelling predictions can be improved by modifying the settlements solely due to the second tunnel construction.
2. The magnitude of volume loss from the new tunnel construction had increased due to the presence of the first tunnel. This effect was lessened by larger spacings between the tunnels.
3. Second tunnel settlements can be predicted using equations by Peck (1969), O’Reilly & New (1982) and Mair et al. (1993) but with the modifications. The surface and sub-surface settlement distributions towards the existing tunnel were observed to be wider than a single tunnel
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An addendum for particle image velocimetry in centrifuge modelling
Image-based analysis is a growing field in geotechnical engineering and has a wide range of applications from measuring particle size distribution to observing the three dimensional internal deformation of granular materials. In centrifuge modelling, pre-failure deformation can be captured from the visible vertical plane of the models. This paper describes a new set-up for measuring a two dimensional displacement field using particle image velocimetry (PIV). The system makes use of the texture (intensity of pixel) of images to determine the accurate pattern of pre-failure ground movements. A complementary metal oxide semiconductor (CMOS) sensor is employed rather than a charged couple device (CCD) sensor used in previous studies. Three examples are presented: (1) synthetic experiment using a sliding bed equipped with LVDT behind the window of a strong box to make and measure controlled displacements; (2) strip footing test on glass ballotini under vertical loading; (3) ground movement generated during a shaft construction in clay. These examples provide a range of tests with artificial texture and demonstrate the most important parameters governing the experimental set-up and image analysis. The possibility of using PIV for grain scale investigations is dis-cussed. The study highlights the benefits of new technology and provides guidelines to minimise artefacts in image processing
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