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Wang, Chong; Lai, Yuanming; Zhang, Mingyi; Li, Shuangyang
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Knowledge of particle shape and configurationdependent thermal conductivity is necessary to investigate heat or water transfer in geomaterials, especially under freezing states. Thermal conductivity of a porous medium is affected by its matrix components and particle shapes. However, the particle shapes in geomaterials are various, and the effect of the ice phase on the thermal conductivity may increase substantially as ice content increases from unfrozen to freezing. In this study, a generalized thermal conductivity model for geomaterials is proposed based on phase transition theory and geometry approximation with respect to unfrozen and freezing states. Volumetric contents of each component and shape factors are required to predict thermal conductivities by the model. In order to evaluate the model, test results from both ours and previous literatures are employed to evaluate the calculated ones, and they match very well. In addition, compared with the other two models, i.e., a physical model and an empirical model, the proposed model is more reasonable and effective.
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Russo, Gianpiero
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2 Citations
The paper presents experimental results of two Osterberg’s cell load tests (OLTs) and three conventional load tests (COLTs) in the same subsoil conditions on Continuous Flight Auger (CFA) piles carefully monitored during construction stages. The instrumentation along the pile shaft in all the tests allows interesting comparisons of both global behaviour and local load transfer. Significant differences in the stiffness of the soilpile system with the different test procedures is outlined. The main differences between the two test procedures occur at the two opposite ends of the pile, as could have been expected, while the observed behaviour in the middle part of the tested piles is close for the two models. A relatively simple FEM model has been calibrated on the basis of the OLTs results. The same model is capable of accurately matching the experimental results of the COLTs, proving that the observed differences are not due to random factors. Furthermore, the same model has been used to simulate ideal load tests. Such a reliable simulation shows that both the experimental procedures are actually responsible for significant differences in the behaviour of the soilpile system even in the simple case of a concentrated axial load. Large differences arise in terms of the stiffness of the system with the OLTs providing by far the stiffest response. Despite being intermediate between the OLT and the ILT, the COLTs provide a response of the pilesoil system, which is on the average about two times stiffer than the Ideal test, where the force applied on top of the pile does not depend on a tangible reaction system. Care should be thus taken when considering the results of such tests in the prediction of the settlement of a piled foundation. Correction factors should be applied to the experimentally observed behaviour.
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By
Mimouni, Thomas; Laloui, Lyesse
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25 Citations
Using pile foundations as heat exchangers with the ground provides an efficient and reliable energy source for the heating and cooling of buildings. However, thermal expansion or contraction of the concrete brings new challenges to the design of such structures. The present study investigates the impact of temperature variation on the mobilised bearing capacities of geothermal piles. The mechanisms driving the variations and redistribution of mobilised bearing forces along geothermal piles are identified using ThermoPile software. The EPFL and Lambeth College test piles are modelled and analysed as realscale experiments. Three simple representative cases are used to investigate the impact of oversizing geothermal piles on their serviceability. It is found that the mechanisms responsible for the variations and redistribution of mobilised bearing forces along the piles are unlikely to cause geotechnical failure, even if the ultimate bearing force of a pile is reached. Furthermore, oversizing geothermal piles compared to conventional piles can have a negative impact on their serviceability.
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By
Kim, Donghee; Ryu, Dongwoo; Lee, Changho; Lee, Woojin
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3 Citations
The uncertainty in the spatial distributions of consolidation settlement (s_{c}) and time (t_{p}) for Songdo New City is evaluated by using a probabilistic procedure. Ordinary kriging and three theoretical semivariogram models are used to estimate the spatial distributions of geolayers which affect s_{c} and t_{p} in this study. The spatial map of mean (μ) and standard deviation (σ) for s_{c} and t_{p} are determined by using a firstorder second moment method based on the evaluated statistics and probability density functions (PDFs) of soil properties. It is shown that the coefficients of variation (COVs) of the compression ratio [C_{c}/(1 + e_{0})] and the coefficient of consolidation (c_{v}) are the most influential factors on the uncertainties of s_{c} and t_{p}, respectively. The μ and σ of the s_{c} and t_{p}, as well as the probability that s_{c} exceeds 100 cm [P(s_{c} > 100 cm)] and the probability that t_{p} exceeds 36 months [P(t_{p} > 36 months)] in Sect. , are observed to be larger than those of other sections because the thickness of the consolidating layer in Sect. is the largest in the entire study area. The area requiring additional fill after the consolidation appears to increase as the COV of C_{c}/(1 + e_{0}) increases and as the probabilistic design criterion (α) decreases. It is also shown that the area requiring the prefabricated vertical drains installation increases as the COV of c_{v} increases and as the α decreases. The design procedure presented in this paper could be used in the decision making process for the design of geotechnical structures at coastal reclamation area.
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By
Zhang, Yulong; Shao, Jianfu; Liu, Zaobao; Shi, Chong; Saxcé, Géry
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This paper is devoted to numerical analysis of strength and deformation of cohesive granular materials. The emphasis is put on the study of effects of confining pressure and loading path. To this end, the threedimensional discrete element method is used. A nonlinear failure criterion for intergranular interface bonding is proposed, and it is able to account for both tensile and shear failure for a large range of normal stress. This criterion is implemented in the particles flow code. The proposed failure model is calibrated from triaxial compression tests performed on representative sandstone. Numerical results are in good agreement with experimental data. In particular, the effect of confining pressure on compressive strength and failure pattern is well described by the proposed model. Furthermore, numerical predictions are studied, respectively, for compression and extension tests with a constant mean stress. It is shown that the failure strength and deformation process are clearly affected by loading path. Finally, a series of numerical simulations are performed on cubic samples with three independent principal stresses. It is found that the strength and failure mode are strongly influenced by the intermediate principal stress.
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By
Tatone, Bryan S. A.; Grasselli, Giovanni
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8 Citations
Discontinuities in brittle geomaterials, including concrete and rock, represent localized zones of weakness and enhanced hydraulic transmissivity that often control the hydromechanical behavior of the medium. The shearing of discontinuities and the resulting morphological changes can significantly alter this behavior. In this work, a procedure is developed to characterize sheared discontinuity replicas as a function of the applied normal load using Xray microcomputed tomography (microCT) imagery. A specimen design and testing procedure that facilitates CT scanning is presented along with an image processing procedure to quantify the morphological changes in the specimens. Subsequently, the results of direct shear testing and imagebased measurements of mean fracture aperture, surface area, median effective aperture, and the preferential orientation of fracture void space are presented and discussed. Application of the procedure developed herein yields characteristics of the morphology of sheared discontinuities that were previously not possible to obtain or that were time consuming to collect with destructive sectioning methods.
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By
Liu, Wei; Zhao, Yu; Shi, Peixin; Li, Jiaoyang; Gan, Penglu
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This paper investigates the face stability of shielddriven tunnels shallowly buried in dry sand using 1g largescale model tests. A halfcircular tunnel model with a rigid front face was designed and tested. The ground movement was mobilized by pulling the tunnel face backwards at different speeds. The support pressure at tunnel face, settlement at ground surface, and internal movement of soil body were measured by load cells, linear variable differential transducers, and a camera, respectively, and the progress of face failure was observed through a transparent lateral wall of model tank. The tests show that, as the tunnel face moves backwards, the support pressure at the tunnel face drops sharply initially, then rebounds slightly, and tends to be stabilized at the end. Similarly, the ground surface settlement shows a threestage variation pattern. Using the particle image velocimetry technique, the particle movement, shear strain, and vortex location of soil are analyzed. The variation of support pressure and ground surface settlement related to the internal movement of soil particles is discussed. The impact of the tunnel face moving speed on the face stability is discussed. As the tunnel face moves relatively fast, soil failure originates from a height above tunnel invert and an analytical model is developed to analyze such failure.
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By
Wichtmann, T.; Niemunis, A.; Triantafyllidis, Th.
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14 Citations
The presented results of cyclic triaxial tests on sand demonstrate that the cumulative effects due to small cycles obey a kind of flow rule. It mainly depends on the average stress ratio about which the cycles are performed. This socalled “cyclic flow rule” is unique and can be well approximated by flow rules for monotonic loading. Amongst others it is shown that the cyclic flow rule is only moderately influenced by the average mean pressure, by the strain loop (span, shape, polarization), the void ratio, the loading frequency, the static preloading and the grain size distribution curve. A slight increase of the compactive portion of the flow rule with increasing residual strain (due to the previous cycles) was observed. These experimental findings prove that the cyclic flow rule is an essential and indispensable concept in explicit (Ntype) accumulation models.
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By
Agus, Setianto Samingan; Arifin, Yulian Firmana; Tripathy, Snehasis; Schanz, Tom
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15 Citations
This paper presents results of an experimental work to determine a relationship between swelling pressure and suction of heavily compacted bentonite–sand mixtures. For comparison, tests were also carried out on heavily compacted bentonite specimens. A series of swelling pressure tests were performed using multistep constantvolume method where suction of the specimens tested was reduced in a stepwise manner toward a zero value. The suction reduction was induced using vapor equilibrium and axistranslation techniques. It is shown that compacted specimens did not exhibit any collapse upon suction decrease and exhibited maximum swelling pressures at zeroequilibrium suction. The development of swelling pressure with decreasing suction of the specimens showed threshold suctions below which a further reduction in suction yields an increase in the swelling pressure of the same magnitude. The magnitude of threshold suction was found to be a function of bentonite content in compacted specimens.
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By
Wang, Pei; Arson, Chloé
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A method is proposed to calculate the distribution of energy during the quasistatic confined comminution of particulate assemblies. The work input, calculated by integrating the loaddisplacement curve, is written as the sum of the elastic deformation energy, the breakage energy and the redistribution energy. Experimental results obtained on samples subjected to compression stresses ranging between 0.4 and 92 MPa are used to calibrate the model. The elastic energy stored in the samples is obtained by simulating the compression test on the final particle size distributions (PSDs) with the discrete element method and by extracting the contact forces. A PSD evolution law is proposed to account for particle breakage. The PSD is related to the total particle surface in the sample, which allows calculating the breakage energy. The redistribution energy, which comprises the kinetic energy of particles being rearranged and the friction energy dissipated at contacts, is obtained by subtracting the elastic energy and breakage energy from the work input. Results show that: (1) at least 60% of the work input is dissipated by particle redistribution; (2) the fraction of elastic deformation energy increases, and the fraction of redistribution energy decreases as the compression stress increases; (3) the breakage energy accounts for less than 5% of the total input energy, and this value is independent of the compressive stress; (4) the energy dissipated by redistribution is between 14 and 30 times larger than the breakage energy.
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