Centrifuge modelling on long-term behaviour of tunnels in transitional ground

Long-term settlement of tunnels has caused concerns about its influence on tunnel safety and serviceability. Aiming to investigate the long-term behaviour of tunnels against the background of Shanghai metro line, two cases of centrifuge modelling were conducted, with efforts to expose the mechanism affecting the consolidation of the ground. Evenly layered ground and transitional ground strata were set for each case separately and the settlement, lining load and pore water pressure were checked against elapsed time up to 20 years. The results verified some previous findings concerning the settlement and lining load development trend, however, it was also shown that the transitional ground made the tunnel response more complicated. The research is expected to provide some basis for further research on other affecting factors, such as lining permeability. © 2010 Taylor & Francis Group, London.

Influence of fluid viscosity on the response of buried structures in earthquakes

The use of high viscous pore fluid has been widely established to match the rate of excess pore pressure generation and subsequent dissipation in dynamic centrifuge tests. The appropriate viscosity is linked to the geometric and gravity scaling factors which corresponds to the use of pore fluid of 'N' cSt in a 'N'g centrifuge test. The use of either water (1 cSt) or pore fluid lower than 'N' cSt can influence the behaviour of soil liquefaction in a centrifuge test. In this paper, the floatation of a tunnel following soil liquefaction is investigated using pore fluids with two different viscosities. The results show that the uplift displacement of the tunnel is significantly affected by the pore fluid viscosity. © 2010 Taylor & Francis Group, London.

Particle Image Velocimetry analysis in dynamic centrifuge tests

The Particle Image Velocimetry (PIV) technique is an image processing tool to obtain instantaneous velocity measurements during an experiment. The basic principle of PIV analysis is to divide the image into small patches and calculate the locations of the individual patches in consecutive images with the help of cross correlation functions. This paper focuses on the application of the PIV analysis in dynamic centrifuge tests on small scale tunnels in loose, dry sand. Digital images were captured during the application of the earthquake loading on tunnel models using a fast digital camera capable of taking digital images at 1000 frames per second at 1 Megapixel resolution. This paper discusses the effectiveness of the existing methods used to conduct PIV analyses on dynamic centrifuge tests. Results indicate that PIV analysis in dynamic testing requires special measures in order to obtain reasonable deformation data. Nevertheless, it was possible to obtain interesting mechanisms regarding the behaviour of the tunnels from PIV analyses. © 2010 Taylor & Francis Group, London.

Centrifuge and discrete element modelling of tunnelling effects on pipelines

The paper presents centrifuge test data of the problem of tunnelling effects on buried pipelines and compares them to predictions made using DEM simulations. The paper focuses on the examination of pipeline bending moments, their distribution along the pipe, and their development with tunnel volume loss. Centrifuge results are obtained by PIV analysis and compared to results obtained using the DEM model. The DEM model was built to replicate the centrifuge model as closely as possible and included numerical features formulated specially for this task, such as structural elements to replicate the tunnel and pipeline. Results are extremely encouraging, with deviations between DEM and centrifuge test bending moment results being very small. © 2010 Taylor & Francis Group, London.

Highly efficient spin transport in epitaxial graphene on SiC

Spin information processing is a possible new paradigm for post-CMOS (complementary metal-oxide semiconductor) electronics and efficient spin propagation over long distances is fundamental to this vision. However, despite several decades of intense research, a suitable platform is still wanting. We report here on highly efficient spin transport in two-terminal polarizer/analyser devices based on high-mobility epitaxial graphene grown on silicon carbide. Taking advantage of high-impedance injecting/detecting tunnel junctions, we show spin transport efficiencies up to 75%, spin signals in the mega-ohm range and spin diffusion lengths exceeding 100μm. This enables spintronics in complex structures: devices and network architectures relying on spin information processing, well beyond present spintronics applications, can now be foreseen. © 2012 Macmillan Publishers Limited. All rights reserved.

Seismic analyses of shallow tunnels by dynamic centrifuge tests and finite elements

The increments of internal forces induced in a tunnel lining during earthquakes can be assessed with several procedures at different levels of complexity. However, the substantial lack of well-documented case histories still represents a difficulty in order to validate any of the methods proposed in literature. To bridge this gap, centrifuge model tests were carried out on a circular aluminium tunnel located at two different depths in dense and loose dry sand. Each model has been instrumented for measuring soil motion and internal loads in the lining and tested under several dynamic input signals. The tests performed represented an experimental benchmark to calibrate dynamic analyses with different approaches to account for soil-tunnel kinematic interaction. © 2009 IOS Press.

Corner separation effects for normal shock wave/turbulent boundary layer interactions in rectangular channels

Experiments are conducted to examine the mechanisms behind the coupling between corner separation and separation away from the corner when holding a high-Machnumber M∞ = 1.5 normal shock in a rectangular channel. The ensuing shock wave interaction with the boundary layer on the wind tunnel floor and in the corners was studied using laser Doppler anemometry, Pitot probe traverses, pressure sensitive paint and flow visualization. The primary mechanism explaining the link between the corner separation size and the other areas of separation appears to be the generation of compression waves at the corner, which act to smear the adverse pressure gradient imposed upon other parts of the flow. Experimental results indicate that the alteration of the -region, which occurs in the supersonic portion of the shock wave/boundary layer interaction (SBLI), is more important than the generation of any blockage in the subsonic region downstream of the shock wave. © Copyright 2012 Cambridge University Press.

Micro-measurement and monitoring system for ageing underground infrastructure (Underground M3)

Advances in the development of computer vision, miniature Micro-Electro-Mechanical Systems (MEMS) and Wireless Sensor Network (WSN) offer intriguing possibilities that can radically alter the paradigms underlying existing methods of condition assessment and monitoring of ageing civil engineering infrastructure. This paper describes some of the outcomes of the European Science Foundation project "Micro-Measurement and Monitoring System for Ageing Underground Infrastructures (Underground M3)". The main aim of the project was to develop a system that uses a tiered approach to monitor the degree and rate of tunnel deterioration. The system comprises of (1) Tier 1: Micro-detection using advances in computer vision and (2) Tier 2: Micro-monitoring and communication using advances in MEMS and WSN. These potentially low-cost technologies will be able to reduce costs associated with end-of-life structures, which is essential to the viability of rehabilitation, repair and reuse. The paper describes the actual deployment and testing of these innovative monitoring tools in tunnels of London Underground, Prague Metro and Barcelona Metro. © 2012 Taylor & Francis Group.

The response of buildings to tunnelling: A case study

The response of buildings to tunnelling induced ground movements is an area of great importance for many urban tunnelling projects. This paper presents the response of two buildings to the construction of a 12 m diameter sprayed concrete lining (SCL) tunnel with face reinforcement, in Italy. Soil and structure displacements were monitored through extensive instrumentation. The settlement response of the two buildings was found to differ significantly, demonstrating both flexible and rigid response mechanisms. Comparison of the building settlement profiles with greenfield settlements enables the soil structure interaction to be quantified. Encouraging agreement between the modification to the greenfield settlement profile displayed by buildings and estimates made from existing predictive tools is observed. Potential issues for infrastructure connected to buildings, arising from the embedment of rigid buildings into the soil, are also highlighted. © 2012 Taylor & Francis Group.

Experimental study into the flow physics of three-dimensional shock control bumps

This paper describes a fundamental experimental study of the flow structure around a single three-dimensional (3D) transonic shock control bump (SCB) mounted on a flat surface in a wind tunnel. Tests have been carried out with a Mach 1.3 normal shock wave located at a number of streamwise positions relative to the SCB. Details of the flow have been studied using the experimental techniques of schlieren photography, surface oil flow visualization, pressure sensitive paint, and laser Doppler anemometry. The results of the work build on the findings of previous researchers and shed new light on the flow physics of 3D SCBs. It is found that spanwise pressure gradients across the SCB ramp and the shape of the SCB sides affect the magnitude and uniformity of flow turning generated by the bump, which can impact on the spanwise propagation of the quasi-two-dimensional (2D) shock structure produced by a 3DSCB. At the bump crest, vortices can form if the pressure on the crest is significantly lower than at either side of the bump. The trajectories of these vortices, which are relatively weak, are strongly influenced by any spanwise pressure gradients across the bump tail. Asignificant difference between 2D and 3D SCBs highlighted by the study is the impact of spanwise pressure gradients on 3D SCB performance. The magnitude of these spanwise pressure gradients is determined largely by SCB geometry and shock position. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

The influence of tunnelling on piled foundations

A study of the influence of tunnelling on piled foundations was recently completed at the University of Cambridge. The study focussed on tunnelling near driven piles in dense sand and was carried out by means of centrifuge modelling. This paper presents a summary of the main findings, describing the mechanism controlling tunnelling-induced pile behaviour, a zone of influence around the tunnel where piles might be affected and recommendations for tunnelling near piles in practice. Both single piles and pile groups are considered.

Shock control bump robustness enhancement

Robustness enhancement for Shock Control Bumps (SCBs) on transonic wings is an ongoing topic because most designs provide drag savings only in a relatively small band of the airfoil polar. In this paper, different bump shapes are examined with CFD methods which are validated first by comparison with wind tunnel results. An evaluation method is introduced allowing the robustness assessment of a certain design with little computational effort. Shape optimizations are performed to trim SCB designs to maximum performance on the one hand and maximum robustness on the other hand. The results are analysed and different and parameters influencing the robustness are suggested. Copyright © 2012 by Klemens Nuebler.

Inhomogeneous soils and their effect on ground vibration due to underground railways

Ground vibration due to underground railways is a significant source of disturbance for people living or working near subways. Numerical models are commonly used to predict vibration levels; however, uncertainty inherent to these simulations must be understood to give confidence in the predictions. A semi-analytical approach is developed herein to investigate the effect of uncertainty in soil material properties on the surface vibration of layered halfspaces excited by an underground railway. The half-space is simulated using the thin-layer method coupled with the pipe-in-pipe (PiP) method for determining the load on the buried tunnel. The K-L expansion method is employed to smoothly vary the material properties throughout the soil by up to 10%. The simulation predicts a surface rms velocity variation of 5-10dB compared to a homogeneous, layered halfspace. These results suggest it may be prudent to include a 5dB error band on predicted vibration levels when simulating areas of varied material properties.

Control of a shock-wave/boundary-layer interaction and subsequent subsonic diffuser using a combination of vortex generators and bleed

An experimental investigation has been undertaken in which vortex generators (VGs) have been employed to inhibit boundary-layer separation produced by the combined adversepressure- gradient of a terminal shock-wave and subsonic diffuser. This setup has been developed as part of a program to produce a more inlet relevant flow-field using a small-scale wind tunnel than previous studies. The resulting flow is dominated by large-scale separation, and as such, is thought to be a good test-bed for flow control. In this investigation, VGs have been added to determine their potential for shock-induced separation mitigation. In line with previous studies, it was observed that the application of VGs alone was not able to significantly alleviate separation overall, because enlarged corner separations was observed. Only when control of the corner separations using corner bleed was employed alongside centre-span control using VGs was a significant improvement in both wall pressure recovery (6% increase) and stagnation pressure recovery (2.4% increase) observed. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

The design of multi-element airfoils through multi-objective optimization techniques

This paper presents the development and the application of a multi-objective optimization framework for the design of two-dimensional multi-element high-lift airfoils. An innovative and efficient optimization algorithm, namely Multi-Objective Tabu Search (MOTS), has been selected as core of the framework. The flow-field around the multi-element configuration is simulated using the commercial computational fluid dynamics (cfd) suite Ansys cfx. Elements shape and deployment settings have been considered as design variables in the optimization of the Garteur A310 airfoil, as presented here. A validation and verification process of the cfd simulation for the Garteur airfoil is performed using available wind tunnel data. Two design examples are presented in this study: a single-point optimization aiming at concurrently increasing the lift and drag performance of the test case at a fixed angle of attack and a multi-point optimization. The latter aims at introducing operational robustness and off-design performance into the design process. Finally, the performance of the MOTS algorithm is assessed by comparison with the leading NSGA-II (Non-dominated Sorting Genetic Algorithm) optimization strategy. An equivalent framework developed by the authors within the industrial sponsor environment is used for the comparison. To eliminate cfd solver dependencies three optimum solutions from the Pareto optimal set have been cross-validated. As a result of this study MOTS has been demonstrated to be an efficient and effective algorithm for aerodynamic optimizations. Copyright © 2012 Tech Science Press.