503 research outputs found
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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 âshaftsâ) 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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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 âtwin tunnellingâ. 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 elasticâperfectly 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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Test development for the investigation of soil disaggregation during slurry tunnelling
Slurry tunnelling uses a water based slurry to aid in tunnel face support and transportation of the excavated material. Unlike other tunnelling methods this requires expensive surface separation plant to remove the excavated ground from the slurry. Incorrect specification of this plant can lead to significant delays and added cost to a tunnel drive. Due to the tight budgets and space constraints this can cause contracts to become unprofitable, in particular where small diameter slurry tunnels are excavated by pipe jacking. For this reason accurate prediction of the size distribution of the soil particles and lumps in the disaggregating slurry is required. This research concerns the development of test procedures and methods of soil classification that will enable improved predictions of the degree to which soils/weak rocks will disaggregate during the slurry tunnelling process
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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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System to measure three-dimensional movements in physical models
A newly developed imaging system is presented, which measures three-dimensional (3D) deformations of a soil surface in geotechnical experiments involving physical modelling. The method adopts the computer vision technique âstructure from motion and multi-view stereoâ delivered by an open-source software MicMac. Three, 2 megapixel industrial cameras were synchronised and used to capture images of a deforming soil surface. The images were used to reconstruct the observed scene to a high-density, accurate 3D point cloud. A new method has been developed to process the obtained 3D point clouds and images to determine the 3D displacement vectors. The procedure is highly automatic which allows large data sets to be processed with minimal manual intervention. Two series of quantification experiments were carried out to assess the performance of the system which has shown the overall accuracy to be within 0·05âmm over a field of view of 500âĂâ250âmm. An example application is presented to demonstrate the capabilities of the 3D imaging system
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A study on performance of three-dimensional imaging system for physical models
A study by Le et al. (2017) reported the application of computer vision techniques structure from motion (SfM) and multi-view stereo (MVS) to measure three-dimensional soil displacements at the surface of physical models. However, little information exists on the significance of the camera resolution and the number of images to the measurement performance. This study assesses the measurement performance of the SfMMVS, provided by an open source software Micmac, with input images taken by two different types of camera including DSLR (18Mega-pixel) and mobile phone cameras (12Mega-pixel). Rigorous quantifications were carried out to examine the precision of the image analysis, in measuring vertical and horizontal displacements, over a region of interest of 420x200mm. The measurement precision, achieved by different numbers of images, ranged from 0.06mm to 0.03mm. The results from this paper can be useful for researchers to select appropriate camera that satisfies their measurement requirements
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Mechanisms for the disaggregation of soil cuttings in slurries
A series of tests has been undertaken on a variety of different soils to understand how soil cuttings disaggregate when pumped from the tunnel face to the separation plant during slurry tunnelling for pipe jacked tunnels. It is important to understand this process to ensure that the separation plant can be optimised to remove the maximum amount of soil from the slurry prior to the reuse of the liquid. In pipe-jacking operations this liquid is normally water, which is recirculated after the soil has been re-moved to minimise and if possible eliminate the production of liquid waste. The paper will present results from a series of âmixing testsâ, devised by the authors to investigate disaggregation, and also from standard laboratory tests undertaken to establish basic soil properties such as soil strength. The âmixing testsâ simulate the softening of the cuttings due to the presence of water and the shear forces applied to the slurry by the pumps and have al-ready allowed the effect of these factors to be quantified. This series of tests will demonstrate how the mechanism of disaggregation varies with strength resulting in different proportions of disaggregated soil particles in slurries subjected to the same mixing time and shear forces
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