Nonlinear Cyclic Stress-Strain Relations of Soils

A cyclic torsional shear testing system was developed to measure the dynamic properties of soils at a wide range of strain levels (10-4 ~ 1%). Use of proximity transducer and pneumatic actuator in a closed loop system enabled us to measure the deformation at very small strains. A new simple nonlinear model of G/Gmax = 1/(α+γβ) agreed well with the test results of various geologic materials. In this model, parameter a represents the strain at which the stiffness starts to decrease, and parameter α controls the rate of the stiffness degradation. Loose sands had larger α and β, whereas clays and mudstones had smaller α and β. A unique relationship of β = 0.2logα + 0.3 was also found from the compiled data

The role of constitutive models in MPM simulations of granular column collapses

The granular column collapse is a well-established experiment which consists of having a vertical column of granular material on a flat surface and letting it collapse by gravity. Despite its simplicity in execution, the numerical modelling of a column collapse remains challenging. So far, much attention has been dedicated in assessing the ability of various numerical methods in modelling the large deformation and little to the role of the constitutive model on both the triggering mechanism and the flow behaviour. Furthermore, the influence of the initial density, and its associated dilatancy and strength characteristics, have never been included in the analyses. Most past numerical investigations had relied on simple constitutive relations which do not consider the softening behaviours. The aim of this study is to illustrate the influence of the constitutive model on the on-set of failure, the flow behaviour and the deposition profile using the material point method (MPM). Three constitutive models were used to simulate the collapse of two granular columns with different geometries and for two densities. The results of the simulations showed that the constitutive model had a twofold influence on the collapse behaviour. It defined the volume of the mobilised mass which spread along the flat surface and controlled the dissipation of its energy. The initial density was found to enhance the failure angle and flow behaviours and was more significant for small columns than for larger ones. The analysis of the potential energy of the mobilised mass explained the existence of two collapse regimes.This project has received funding from the European Unions Seventh Framework Programme for research, technological development and demonstration under grant agreement no PIAP-GA-2012-324522 and the Swiss National Science Foundation under grant agreement P1SKP2 158621.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11440-016-0436-

The applicability of microbially induced calcite precipitation (MICP) for internal erosion control in gravel-sand mixtures

Seepage-induced internal erosion in earth-filled embankment dams has been attracting the attention of civil engineering researchers and practitioners for decades. Microbially induced carbonate precipitation (MICP), owing to its proved performance in soil enhancement and permeability control, can potentially be used for internal erosion control. This paper examines the applicability of MICP for internal erosion control in gravel–sand mixtures using a large one-dimensional column test apparatus which incorporates the implementation of MICP. Visual observations, erosion characteristics and hydro-mechanical behaviours of non-MICP and MICP treated gravel–sand mixtures were investigated through a series of constant-pressure erosion tests. Test results confirm that MICP treatment can reduce the cumulative erosion weight, erosion rate and axial strain relative to non-MICP soil. The magnitudes of hydraulic conductivity for all tested samples before the erosion process fall into a range from 5·5 × 10−5 to 8·0 × 10−3 m/s. After the erosion process, non-MICP soils and MICP treated soils with low cementation concentrations experience a significant increase in hydraulic conductivity. Furthermore, a hydro-mechanical coupling analysis was conducted and different erosion modes were identified for low and high concentrations of cementation solution. Fundamentally, the efficiency of internal erosion reduction is controlled by the calcium carbonate precipitation content within the tested soils. Higher precipitation content can facilitate the formation of larger clusters of cemented sand particles, thus reducing the likelihood of erosion. The first author also extends thanks to the Cambridge Commonwealth, European & International Trust for the financial support in the PhD studentship.This is the author accepted manuscript. The final version is available from ICE Publishing via http://dx.doi.org/10.1680/jgeot.15.P.18

Effects of Soil Fabric on Undrained Behavior of Sands

The undrained behavior of sands in monotonic triaxial compression and extension tests was simulated using the Distinct Element Method (DEM). Soil specimens were prepared at different initial soil fabrics but at similar void ratios and the effects of soil fabric on the undrained behavior were investigated. The DEM results show that soil fabric and its change have profound effects on the undrained response of sands. They also provide some insights for the interpretation of the published experimental data that show the effects of specimen reconstitution methods and of preshearing on the undrained behavior of sands

Experimentally observed evolution between dynamic patterns and intrinsic localized modes in a driven nonlinear electrical cyclic lattice

Locked intrinsic localized modes (ILMs) and large amplitude lattice spatial modes (LSMs) have been experimentally measured for a driven 1-D nonlinear cyclic electric transmission line, where the nonlinear element is a saturable capacitor. Depending on the number of cells and electrical lattice damping a LSM of fixed shape can be tuned across the modal spectrum. Interestingly, by tuning the driver frequency away from this spectrum an LSM can be continuously converted into ILMs and visa versa. The differences in pattern formation between simulations and experimental findings are due to a low concentration of impurities. Through this novel nonlinear excitation and switching channel in cyclic lattices either energy balanced or unbalanced LSMs and ILMs may occur. Because of the general nature of these dynamical results for nonintegrable lattices applications are to be expected. The ultimate stability of driven aero machinery containing nonlinear periodic structures may be one example.Comment: 7 pages 7 figure

Influence of GSHP system design parameters on the geothermal application capacity and electricity consumption at city-scale for Westminster, London

A city-scale renewable energy network for heating and cooling can significantly contribute to reduction of fossil fuel utilization and meeting the renewable energy targets. Ground source heat pump (GSHP) system is a technology that transfers heat stored over long periods to/from the ground to heat/cool the buildings. In particular, a vertical closed loop GSHP is a viable choice in densely populated urban areas. In this study, an ArcGIS-based simulation model has been developed to examine how many vertical closed loop GSHPs can be feasibly installed at city scale without overusing the geothermal energy underground. City of Westminster, in London, is used as a case study to identify and map areas where GSHPs can serve as a viable option for heating and/or cooling. A parametric study has been conducted to investigate the influence of how space heating and cooling demand is quantified on the potential utility of GSHP systems. The influence of COP variation during operation is also examined. The operational variation of COP influences the electricity consumption of the GSHP systems. Therefore, a comprehensive analysis including the capital cost, C/D ratio distribution, energy demand, and financial risk is highly recommended for district-level planning of GSHP systems.The authors would like to acknowledge the support provided by BP under the project: ‘Potential of low grade geothermal energy at city scale’ and by the Low Carbon Energy University Alliance (LCEUA) of Cambridge University-Tsinghua University-MIT.This is the author accepted manuscript. It is under embargo until 31/07/2016. The final version is available from Elsevier at http://dx.doi.org/10.1016/j.enbuild.2015.07.06

Three-dimensional finite element analysis of the behaviour of cross passage between cast-iron tunnels

The behaviour of cast-iron cross passages in the London Underground was investigated using three-dimensional finite element models. Unlike the behaviour of a full tunnel ring, the structural integrity of a tunnel cross-passage opening relies on support from adjacent linings. In clayey soils, the opening may deform further as the soil stiffness changes from undrained to drained conditions. Degradation of the circumferential bolts and trackbed may also lead to further tunnel movement. A parametric study was conducted to examine the influence of soil stiffness and structural components (e.g., bolts and lintel) on the structural integrity of a tunnel opening. Results show that a lintel effectively transfers the load above the opening to the adjacent linings, and its distortion affects tunnel deformation significantly. If a lintel is not present, both bolts and friction between tunnel segments provide shear resistance to the lining deformation at the tunnel opening against soil loading. Results are compared with field observations made at a critical cross passage in one of the London Underground tunnels. The findings contributed to identifying the critical deformation mechanisms of cast-iron tunnel cross passages, which can be useful during inspection of such structures. This work was supported by National Basic Research Program of China (973 Program: 2011CB013800), National Natural Science Foundation of China (No. 51508403) and the Cambridge Trust scholarship. The authors would like to thank London Underground Limited for providing invaluable the photos and support.This is the author accepted manuscript. The final version is available from NRC Research Press via http://dx.doi.org/10.1139/cgj-2015-027

Simulation of wellbore construction in offshore unconsolidated methane hydrate-bearing formation

The unconsolidated nature of offshore methane hydrate-bearing formation poses challenges to sustainable methane gas production as the weak formation is susceptible to disturbance during wellbore construction. This could contribute to loss of well integrity which could manifest as sand production and error in the interpretation of downhole tests such as mini-frac tests. In this study, a simulation methodology of wellbore construction process is proposed. A finite element model adopting this methodology is developed in order to assess the effect of wellbore construction process on the integrity of the unconsolidated methane hydrate-bearing formation in the Nankai Trough, Japan. The main objectives are (i) to develop a modelling methodology of well construction process for numerical simulations, (ii) to assess the zone and magnitude of well construction-induced stress/strain disturbance in the formation and (iii) to evaluate relative impact of each well construction stage on the integrity of the formation. The results from this study show that the zone of horizontal stress disturbance from the geostatic state due to wellbore construction could extend to more than three times the radius of the wellbore. Following the wellbore construction, the deviator stress is concentrated in the hydrate reservoir sublayers with high hydrate saturation while plastic deviatoric strain has accumulated in the sublayers with low hydrate saturation. The results also show that modelling of cement shrinkage process is crucial in predicting the concentration of deviator stress in the high hydrate saturation layers

Water absorption and shrinkage behaviour of early-age cement in wellbore annulus

Controlling cement shrinkage in a wellbore is important in maintaining its integrity. Although numerous laboratory experiments on the water absorption and shrinkage behaviour of oil well cement have been reported in the past, such behaviour in the wellbore annulus with consideration of pore water migration from the surrounding formation has seldom been examined. In this study, using a cement shrinkage model calibrated against available experimental data, a coupled hydromechanical finite element analysis of a cement-formation model is conducted to simulate the water migration, absorption and shrinkage behaviour of early-age cement placed in the annulus of a wellbore. The objectives of this study are (i) to identify the threshold permeability value of the formation above which there is no longer a bottleneck for pore water to flow into the cement and (ii) to estimate a reasonable range of cement bulk shrinkage volume in wellbore annulus geometry. Results show that the threshold permeability of the formation would be around 0.1 mD for three different types of cement examined in this study: Class G cement, rapid setting (RS) cement and Schlumberger optimized particle size distribution (OPSD) technology cement. The bulk shrinkage volume varies from 0.01% to 2.4% depending on cement type and formation permeability (1 mD to 0.1 μD). The proposed methodology facilitates the simulation of water migration/absorption and shrinkage behaviour of well cement in different formations