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Journal articleAdam A, Buchan AG, Piggott MD, et al., 2015,
Adaptive Haar wavelets for the angular discretisation of spectral wave models, Journal of Computational Physics, Vol: 305, Pages: 521-538, ISSN: 1090-2716
A new framework for applying anisotropic angular adaptivity in spectral wave modelling is presented. The angular dimension of the action balance equation is discretised with the use of Haar wavelets, hierarchical piecewise-constant basis functions with compact support, and an adaptive methodology for anisotropically adjusting the resolution of the angular mesh is proposed. This work allows a reduction of computational effort in spectral wave modelling, through a reduction in the degrees of freedom required for a given accuracy, with an automated procedure and minimal cost.
Journal articleElliott GM, Jackson CA-L, Gawthorpe RL, et al., 2015,
Late syn-rift tectono-stratigraphic evolution of Vingleia Fault Complex, Frøya High, offshore Mid-Norway, Basin Research, ISSN: 1365-2117
The growth and linkage of normal faults controls the sedimentology and stratigraphic architecture of syn-rift deposits; the facies and stratigraphic architecture of syn-rift deposits thus provides insights into the temporal and spatial development of syn-depositional fault arrays. In exhumed ancient rifts, exposures are typically too small to allow the erosional history of the source area to be linked to the stratigraphic record of the adjacent basin and, in the subsurface, seismic resolution is too poor and borehole data to sparse to allow the rift structural style, and sedimentology and stratigraphy of syn-rift deposits to be constrained. As a result, many rift basin tectono-stratigraphic models are largely conceptual and have yet to be tested by observations from natural examples. In this study we integrate 3D seismic reflection data with biostratigraphically constrained core and well data to elucidate the tectono-stratigraphic evolution of Middle-to-Late Jurassic syn-rift deposits adjacent to the Vingleia Fault Complex in the southern Halten Terrace, offshore Mid-Norway. The syn-rift succession is dominated by shelf-to-basinal mud and siltstones of the Melke and Spekk formations, although isolated coarse-clastics do occur. The spatial and temporal distribution of these coarse clastics is complex due to syn-depositional movement on the Vingleia Fault Complex. For example, coarse clastics are found either fringing basement highs, as footwall-derived fan-delta conglomerates, or adjacent to Jurassic fault scarps, as submarine fans and shoreface sandstones. High rates of accommodation generation led to the vertical stacking of aggradational fan deltas in the immediate hangingwall of the Vingleia Fault Complex, with these coarse clastics rapidly (<5 km) pinching out into fine-grained offshore deposits. On the footwall of the Vingleia Fault Complex, which is characterised by relatively low rates of accommodation generation, hangingwall shorelines developed downdip o
Journal articlePelecanos L, Kontoe S, Zdravkovic L, 2015,
A case study on the seismic performance of earth dams, Geotechnique: international journal of soil mechanics, Vol: 65, Pages: 923-935, ISSN: 0016-8505
The seismic non-linear behaviour of earth dams is investigated by using a well-documented case study and employing advanced static and dynamic coupled-consolidation finite-element analysis. The static part of the analysis considers the layered construction, reservoir impoundment and consolidation, whereas the dynamic part considers the response of the dam to two earthquakes of different magnitude, duration and frequency content. The results of the analysis are compared with the recorded response of the dam and exhibit a generally good agreement. The effects of the narrow canyon geometry, the reservoir impoundment and the elasto-plastic soil behaviour on the seismic dam behaviour are investigated. Finally the implications of the adopted constitutive modelling assumptions on the predicted response are discussed.
Conference paperTsaparli V, Kontoe S, Taborda D, et al., 2015,
Numerical Modelling of Multi-directional Earthquake Loading and Its Effect on Sand Liquefaction, 6th International Conference on Earthquake Geotechnical Engineering
Earthquakes generate multi-directional ground motions, two components in the horizontal direction and one in the vertical. Nevertheless, the effect of vertical motion on site response analysis has not been the object of extensive research. The 2010/2011 Canterbury sequence of seismic events in New Zealand is a prime example among other earlier field observations strongly corroborating that the vertical acceleration may have a detrimental effect on soil liquefaction. Consequently, this study aims to provide insight into the influence of the input vertical motion on sand liquefaction. For this reason, two ground motions, with very different frequency contents, are used as the input excitations. Non-linear elasto-plastic plane strain fully coupled effective stress-based finite element analyses are conducted to investigate the occurrence of liquefaction in a hypothetical fully saturated Fraser River Sand deposit. The results indicate that the frequency content of the input motion is of utmost importance for the response of sands to liquefaction when the vertical loading is considered.
Journal articleMonroy R, Zdravkovic L, Ridley AM, 2015,
Mechanical behaviour of unsaturated expansive clay under K-0 conditions, Engineering Geology, Vol: 197, Pages: 112-131, ISSN: 0013-7952
The mechanical response of unsaturated soils with significant amounts of active clay minerals can be highly stress path dependent. Traditionally, the Axis Translation Procedure has been used to study these materials in the laboratory. This technique, however, does not fully replicate conditions in the field, nor is it able to test soils during the important process of desaturation and resaturation. A novel osmotic oedometer has been developed at Imperial College London to test unsaturated soils under atmospheric pressure. With this equipment, it has been possible to continuously record changes in vertical and radial stress, gravimetric water content, degree of saturation, matrix suction, and void ratio, throughout a test. The Paper presents results from tests carried on samples of compacted London clay using the new oedometer and standard oedometers. The full data set gives an insight into the mechanical response of unsaturated expansive clay along complex stress paths. Results are interpreted using an existing framework for unsaturated expansive clays. Because of inherent limitations in the method of testing, some of the data needs to be interpreted with care. Nevertheless, the response recorded along different stress paths was found to be consistent and in agreement with framework predictions.
Journal articleCui W, Gawecka KA, Taborda DMG, et al., 2015,
Time-step constraints in transient coupled finite element analysis, International Journal for Numerical Methods in Engineering, Vol: 106, Pages: 953-971, ISSN: 1097-0207
In transient finite element analysis, reducing the time-step size improves the accuracy of the solution. However, a lower bound to the time-step size exists, below which the solution may exhibit spatial oscillations at the initial stages of the analysis. This numerical ‘shock’ problem may lead to accumulated errors in coupled analyses. To satisfy the non-oscillatory criterion, a novel analytical approach is presented in this paper to obtain the time-step constraints using the θ-method for the transient coupled analysis, including both heat conduction–convection and coupled consolidation analyses. The expressions of the minimum time-step size for heat conduction–convection problems with both linear and quadratic elements reduce to those applicable to heat conduction problems if the effect of heat convection is not taken into account. For coupled consolidation analysis, time-step constraints are obtained for three different types of elements, and the one for composite elements matches that in the literature. Finally, recommendations on how to handle the numerical ‘shock’ issues are suggested.
Journal articleCraske J, Debugne ALR, van Reeuwijk M, 2015,
Shear-flow dispersion in turbulent jets, Journal of Fluid Mechanics, Vol: 781, Pages: 28-51, ISSN: 0022-1120
We investigate the transport of a passive scalar in a fully developed turbulent axisymmetric jet at a Reynolds number of Re = 4815 using data from direct numerical simulation. In particular, we simulate the response of the concentration field to an instantaneous variation of the scalar flux at the source. To analyse the time evolution of this statisticallyunsteady process we take an ensemble average over 16 independent simulations. We find that the evolution of Cm(z, t), the radial integral of the ensemble-averaged concentration, is a self-similar process, with front position and spread both scaling as √t. The longitudinal mixing of Cm is shown to be primarily caused by shear-flow dispersion.Using the approach developed by Craske & van Reeuwijk (J. Fluid Mech., vol. 763, 2014,pp. 538–566), the classical theory for shear-flow dispersion is applied to turbulent jets to obtain a closure that couples the integral scalar flux to the integral concentration Cm. Model predictions using the dispersion closure are in good agreement with the simulation data. Application of the dispersion closure to a two-dimensional jet results in an integraltransport equation that is fully consistent with that of Landel et al. (J. Fluid Mech., vol.711, 2012, pp. 212–258)
Journal articleOrmo J, Melero-Asensio I, Housen K, et al., 2015,
Scaling and reproducibility of craters produced at the Experimental Projectile Impact Chamber (EPIC), Centro de Astrobiología, Spain, Meteoritics & Planetary Science, ISSN: 1086-9379
Journal articleXu S, Freeman SPHT, Rood DH, et al., 2015,
Decadal <sup>10</sup>Be, <sup>26</sup>Al and <sup>36</sup>Cl QA measurements on the SUERC 5 MV accelerator mass spectrometer, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, Vol: 361, Pages: 39-42, ISSN: 0168-583X
We quantify the routine performance and uncertainties of cosmogenic 10Be, 26Al and 36Cl QA measurements made on the SUERC 5 MV accelerator mass spectrometer since 2004. Our analysis compiles data from primary (NIST SRM4325 for 10Be, Purdue Z92-0222 for 26Al and Purdue Z93-0005 for 36Cl) and secondary (Nishiizumi's series for 10Be, 26Al and 36Cl) reference samples with 10Be/9Be, 26Al/27Al and 36Cl/Cl ratios ranging between 10-11 and 10-13. Our decadal datasets indicate that the 10Be, 26Al and 36Cl secondary standard samples have average standard deviations 1.1%-2.4%, 1.1%-2.8% and 3.0%-3.1%, respectively. The average statistical uncertainties based on counting statistics are 1.0%-1.8%, 0.9%-2.8% and 2.5%-2.7% for 10Be, 26Al and 36Cl, respectively. These indicate additional uncertainties (0.6%-1.6% for 10Be, 0.5%-2.4% for 26Al and 1.4%-1.7% for 36Cl) above those calculated from counting statistics alone. The average differences between the measured and the nominal values are within ±1% in 13 of 14 secondary samples.
Journal articleMonteux J, Collins GS, Tobie G, et al., 2015,
Consequences of large impacts on Enceladus' core shape, Icarus, Vol: 264, Pages: 300-310, ISSN: 1090-2643
The intense activity on Enceladus suggests a differentiated interior consisting of a rocky core, an internal ocean and an icy mantle. However, topography and gravity data suggests large heterogeneity in the interior, possibly including significant core topography. In the present study, we investigated the consequences of collisions with large impactors on the core shape. We performed impact simulations using the code iSALE2D considering large differentiated impactors with radius ranging between 25 and 100 km and impact velocities ranging between 0.24 and 2.4 km/s. Our simulations showed that the main controlling parameters for the post-impact shape of Enceladus’ rock core are the impactor radius and velocity and to a lesser extent the presence of an internal water ocean and the porosity and strength of the rock core. For low energy impacts, the impactors do not pass completely through the icy mantle. Subsequent sinking and spreading of the impactor rock core lead to a positive core topographic anomaly. For moderately energetic impacts, the impactors completely penetrate through the icy mantle, inducing a negative core topography surrounded by a positive anomaly of smaller amplitude. The depth and lateral extent of the excavated area is mostly determined by the impactor radius and velocity. For highly energetic impacts, the rocky core is strongly deformed, and the full body is likely to be disrupted. Explaining the long-wavelength irregular shape of Enceladus’ core by impacts would imply multiple low velocity (<2.4 km/s) collisions with deca-kilometric differentiated impactors, which is possible only after the LHB period.
Journal articleAllen PA, Armitage JJ, Whittaker AC, et al., 2015,
Fragmentation model of the grain size mix of sediment supplied to basins, Journal of Geology, Vol: 123, Pages: 405-427, ISSN: 1537-5269
Journal articleCastelltort S, Whittaker AC, Verges J, 2015,
Tectonics, sedimentation and surface processes: from the erosional engine to basin deposition, Earth Surface Processes and Landforms, Vol: 40, Pages: 1839-1846, ISSN: 1096-9837
Journal articleWan X, Liu JG, Yan H, 2015,
The Illumination Robustness of Phase Correlation for Image Alignment, IEEE Transactions on Geoscience and Remote Sensing, Vol: 53, Pages: 5746-5759, ISSN: 0196-2892
Phase correlation (PC) is a well-established imagematching algorithm. It is robust to the variation of image contrastand brightness, but whether it is invariant to local illuminationchange, particularly sun angle change, is yet to be investigated.This paper presents our study to prove and demonstrate therobustness of illumination invariance of the PC algorithm viamathematical analysis and image matching experiments. First, a3-D space named slope-aspect-intensity (SAI) is proposed to characterizethe 3-D relationship between image intensity and terrainslope/aspect angles for a given illumination geometry. Based onthe SAI space, the mathematical relationship between PC crosspowerspectra and illumination direction (e.g., solar azimuth andzenith angles) is analyzed. The impact of illumination angle variationbetween two images for matching is then rigorously investigatedvia experiments using simulated terrain shading imagesfrom a digital elevation model and real optical images taken underdifferent sun illumination conditions. Our study confirms that PCis robustly invariant to illumination and therefore can achievereliable matching between images taken under different solarillumination conditions for various remote sensing applications.
Journal articleAbushaikha AS, Blunt MJ, Gosselin OR, et al., 2015,
Interface control volume finite element method for modelling multi-phase fluid flow in highly heterogeneous and fractured reservoirs, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 298, Pages: 41-61, ISSN: 0021-9991
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Journal articleAghakouchakn A, Sim WW, Jardine RJ, 2015,
Stress-path laboratory tests to characterise the cyclic behaviour of piles driven in sands, Soils and Foundations, Vol: 55, Pages: 917-928, ISSN: 0038-0806
Recent publications have emphasised the importance of addressing cyclic behaviour when designing piled foundations. Laboratory tests may be conducted to provide site-specific cyclic soil characteristics, but questions arise concerning: (i) how to take into account the pile installation process and (ii) how to apply the results to assess pile capacity and deformation responses under cyclic loads. This paper describes an investigation into the cyclic behaviour of Dunkerque and NE34 Fontainebleau sands, performed to support and to help analyse field-scale and model pile cyclic loading tests on the same soils. Forty computer-controlled, locally instrumented, cyclic and static triaxial tests were performed, following testing schemes that were developed to reflect the conditions adjacent to the pile shafts. Assessments were made of how the cyclic variations in stress imposed during installation and the period allowing for the two types of sand to creep following such ‘installation’ effects, affect the response to subsequent cycling. Constant-volume cyclic tests, involving up to 4500 cycles, were imposed from alternative sets of initial conditions that revealed the relationships among the cyclic deviatoric amplitude, the changes in mean effective stress and the number of cycles as well as the permanent strain accumulation and the cyclic stiffness characteristics. Monotonic compression and extension tests are also presented for both sands to help frame their strength, stiffness and critical state properties. A synthesis with previously obtained cyclic pile test trends confirms the practical applicability of the results obtained.
Conference paperMartinez Calonge D, Gawecka KA, Zdravkovic L, et al., 2015,
Development of a new temperature-controlled triaxial apparatus for saturated soils, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
In recent years, the study of the Thermo-Hydro -Mechanical (THM) behaviour of geomaterials has become a growing area in geotechnical engineering due to the increasing interest in energy geostructures and underground nuclear waste disposal. Advanced laboratory testing is essential in gaining an understanding of the THM behaviour of soils and solving these complex geomechanical problems. This paper describes the development of a new triaxial apparatus at the Imperial College Geotechnics Laboratory, capable of testing saturated soils at temperatures up to 85°C and pressures up to 800kPa. In order to aid its design, numerical analysis of the thermal response of the cell was conducted using the Imperial College Finite Element Program (ICFEP) with its newly developed THM capabilities.
Conference paperCui W, Gawecka KA, Potts DM, et al., 2015,
Numerical modelling of open-loop ground source energy systems, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
The environmental and economic benefits of utilising the ground for extracting and storing heat have been known for a long time. However, only recently have government sustainability policies and rising energy prices encouraged the use of this renewable energy resource. In open-loop systems water is abstracted from one well and re-injected into another after exchanging energy with a building’s heating/cooling system using a heat pump. In order to guarantee a good performance of the system, it is fundamental that the possibility of thermal breakthrough occurring is minimised, i.e. that the temperature of the water being abstracted remains unaffected by the injection of warmer/cooler water at the other well. In this paper, the Imperial College FiniteElement Program (ICFEP), which is capable of simulating fully coupled thermo-hydro-mechanical behaviour of porous materials, was used to perform two-dimensional analyses of open-loop ground source heat systems. The parametric studies carried out highlight the relative impact on the occurrence of thermal break-through of the hydraulic ground conditions andthe geometric characteristics of the system, providing an invaluable insight into possible improvements to the current design procedure.
Conference paperDubasaru V, Zdravkovic L, Taborda DMG, et al., 2015,
Influence of pile raft stiffness on building behaviour in a tunnel-pile clash scenario, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
In a modern urban environment, the underground space becomes increasingly congested due to the high value of the land that forces the new infrastructure projects to be constructed deeper into the ground. For each new project, the potential of both expected and unexpected clashes between new tunnel alignments and the foundations of the existing structures becomes more probable. However, to date, the research on tunnel-pile clashes has been scarce. In the current study, the effects of such a situation are studied by carrying out finite element analyses for a scenario that is typical in the London ground profile. A parametric study was conducted to investigate the influence of the pile raft bending stiffness on the building settlement and the change in piles’ axial forces. It is shown that an increased raft bending stiffness helps to transfer the load from the trimmed pile to the adjacent piles, thus reducing the settlement of the trimmed pile. In the process of tunnel excavation, the pile settles due to the soil-induced downdrag and the loss of both its base and part of its shaft capacity. It is concluded that the tunnel-pile clash has a large impact on the surface structure, piles and tunnel itself.
Conference paperByrne BW, McAdam RA, Burd HJ, et al., 2015,
Field testing of large diameter piles under lateralloading for offshore wind applications, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
Offshore wind power in the UK, and around Europe, has the potential to deliver significant quantities of renewable energy. The foundation is a critical element in the design. The most common foundation design is a single large diameter pile, termed a monopile. Pile diameters of between 5m and 6m are routinely used, with diameters up to 10m or more, being considered for future designs. Questions have been raised as to whether current design methods for lateral loading are relevant to these very large diameter piles. To explore this problem a joint industry project, PISA, co-ordinated by DONG Energy and the Carbon Trust, has been established. The aim of the project is to develop a new design framework for laterally loaded piles based on new theoretical developments, numerical modelling and bench-marked against a suite of large scale field pile tests. The project began in August 2013 and is scheduled to complete during 2015. This paper briefly outlines the project, focusing on the design of the field testing. The testing involves three sizes of pile, from 0.27m in diameter through to 2.0m in diameter. Two sites will be used; a stiff clay site and a dense sand site. Tests will include monotonic loading and cyclic loading. A suite of site investigation will be carried out to aid interpretation of the field tests, and will involve in-situ testing, standard laboratory testing and more advanced laboratory testing.
Journal articlePrelat A, Pankhania S, Jackson CA-L, et al., 2015,
Slope gradient and lithology as controls on the initiation of submarine slope gullies; insights from the North Carnarvon Basin, Offshore NW Australia, Sedimentary Geology, Vol: 329, Pages: 12-17, ISSN: 0037-0738
Slope-confined submarine gullies are present on many continental margins, yet the controls on their initiation and demise are poorly understood because modern or recently active systems are rarely if ever monitored, and exhumed systems, typically formed in very fine-grained successions, are poorly preserved at outcrop. We use 3D seismic reflection and borehole data from offshore NW Australia to investigate long-term (~ 40 Myr) variations in the geomorphology of Eocene-to-Miocene gullies that developed in mixed carbonate–clastic clinothems. Through time, clinoform slope gradient increases from 1.6° to 3.2°, with gullies forming when the clinoform slope exceeds 2.5°. After their inception, gullies increase in width (from 350 m to 770 m) and depth (from 37 m to 60 m). Slope steepening appears to coincide with a change from poorly cemented, fine-grained carbonate to better-cemented, coarse-grained carbonate, implying a secondary, lithological control on slope dip and, ultimately, gully formation.
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