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Publisher: Springer-Verlag   (Total: 2355 journals)

 Acta Geotechnica   [SJR: 1.818]   [H-I: 22]   [7 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 1861-1133 - ISSN (Online) 1861-1125    Published by Springer-Verlag  [2355 journals]
• A geotechnical perspective of raw earth building
• Authors: Domenico Gallipoli; Agostino Walter Bruno; Céline Perlot; Joao Mendes
Pages: 463 - 478
Abstract: Much research has been devoted over the past 30 years to the development of construction materials that can lower the environmental and economic costs of buildings over their entire life by reducing embodied energy, minimizing air conditioning needs and cutting down demolition waste. In this respect, raw earth is an attractive material because it is natural and largely available. In its simplest form, this material consists of a compacted mixture of soil and water which is put in place with the least possible transformation. Raw earth construction has been practised in ancient times but has only recently been rediscovered thanks to modern technology, which has improved fabrication efficiency. If properly manufactured, raw earth exhibits comparable mechanical characteristics and better hygro-thermal properties than concrete or fired bricks. After a brief historical overview, we discuss the advantages of raw earth construction in terms of environmental impact, energy consumption and indoor air quality together with the main obstacles to its wider dissemination. We also review the hydro-thermo-mechanical behaviour of raw earth in the context of the recent geotechnical literature, by examining the dependency of key parameters such as strength, stiffness and moisture retention on: (a) material variables (e.g. particle size and mineralogy), (b) manufacturing variables (e.g. density and stabilization) and (c) environmental variables (e.g. pore suction, ambient humidity and temperature).
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0521-1
Issue No: Vol. 12, No. 3 (2017)

• Granular contact dynamics with elastic bond model
• Authors: J. Meng; J. Huang; D. Sheng; S. W. Sloan
Pages: 479 - 493
Abstract: This paper proposes an elastic bond model in the framework of contact dynamics based on mathematic programming. The bond model developed in this paper can be used to model cemented materials. The formulation can be reduced to model pure static problems without introducing any artificial damping. In addition, omitting the elastic terms in the objective function turns the formulation into rigid bond model, which can be used for the modeling of rigid or stiffly bonded materials. The developed bond model has the advantage over the explicit DEM that large time step or displacement increment can be used. The tensile and shear strength criteria of the bond model are formulated based on the modified Mohr–Coulomb failure criterion. The torque transmission of bonds is introduced based on rolling resistance model. The loss of shear or tensile strength, or torque transmission will lead to the breakage of bonds, and turn the bond into purely frictional contact. Three simple examples are first used to validate the bond model. Numerical examples of uniaxial and biaxial compression tests are used to show its potential in modeling cemented geomaterials. Numerical results show that elastic bonds are indeed necessary for the modeling of cemented granular material under static conditions.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0481-5
Issue No: Vol. 12, No. 3 (2017)

• DEM simulations of sandstone under true triaxial compressive tests
• Authors: K. Duan; C. Y. Kwok; X. Ma
Pages: 495 - 510
Abstract: Numerically simulated true triaxial compression tests (σ 1 ≥ σ 2 ≥ σ 3) are conducted in this study to elucidate the failure mechanism of sandstone using 3D discrete element method (DEM), in particular the effect of the intermediate principal stress (σ 2). Eight series of tests (σ 3 = 0, 10, 20, 30, 40, 50, 70, and 100 MPa) are conducted. Within each series, σ 2 is varied from σ 2 = σ 3 to σ 2 = σ 1 from test to test. For each test, σ 1 is raised monotonically to failure while keeping σ 2 and σ 3 constant. The DEM simulations reveal the effect of σ 2 on the variations of peak stress, Young’s modulus, failure plane angles, the brittle–ductile transition, and the evolution of failure modes, the effect beyond the well-understood effect of σ 3. The simulation is in qualitative agreement with the results obtained experimentally. Detailed analyses performed on the particle-scale responses further the understanding of the microscopic mechanisms. The distribution of contact force becomes more homogeneous with the increase of σ 3, which leads to the resulting damage being more localized rather than diffused. The interaction between contact force distribution and coalescence of cracks determines the processes and patterns of fracturing in the sample scale. σ 2 is found to affect the microscopic stress distribution as well as structure evolution, and this effect weakens with the increase of σ 3.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0480-6
Issue No: Vol. 12, No. 3 (2017)

• Use of discrete element modeling to study the stress and strain
distribution in cyclic torsional shear tests
• Authors: José Francisco Wilson; Esteban Sáez
Pages: 511 - 526
Abstract: In this research, a torsional cyclic shear test was modeled using the 3D discrete element method (DEM). The results are compared against experimental data and micro-mechanical aspects of the soil during the loading are discussed. The aim of the work is to study the homogeneity of strains during this laboratory test and to compare the micro-mechanical behavior of the soil sample for different strain levels. The experimental investigation was performed using a synthetic soil material made of glass beads, which simplifies the modeling and calibration since normal interaction forces do not induce rotation of the particles. Both the model and experimental tests used the same grading distribution and particle size. We showed that the hysteresis cycles can be properly reproduced in terms of shape and magnitude. Thus, we obtained a robust estimation for the secant shear modulus and damping ratio at different strain levels. With this, it was possible to build stiffness degradation and damping increase curve to compare it with experimental data obtained from torsional shear tests. Based on this validation of the DEM model, we discuss the micro-mechanical behavior of the soil and its relation with the macroscopic parameters obtained. It is shown that shear strain distribution on the sample is not uniform and that large values of strains concentrate close to the top of the sample as top rotation increases, which differs from the standard assumption of a constant value across the height of the sample. Additionally, it is observed that at 0.8 times radius, the cumulative torque reaches approximately 90% of the total torque applied to sample.
PubDate: 2017-06-01
DOI: 10.1007/s11440-017-0526-4
Issue No: Vol. 12, No. 3 (2017)

• Three-dimensional DEM investigation of critical state and dilatancy
behaviors of granular materials
• Authors: Wei Zhou; Jiaying Liu; Gang Ma; Xiaolin Chang
Pages: 527 - 540
Abstract: The critical state is significant to the mechanical behaviors of granular materials and the foundation of the constitutive relations. Using the discrete element method (DEM), the mechanical behaviors of granular materials can be investigated on both the macroscopic and microscopic levels. A series of DEM simulations under true triaxial conditions have been performed to explore the critical state and dilatancy behavior of granular materials, which show the qualitatively similar macroscopic responses as the experimental results. The critical void ratio and stress ratio under different stress paths are presented. A unique critical state line (CSL) is shown to indicate that the intermediate principal stress ratio does not influence the CSL. Within the framework of the unique critical state, the stress–dilatancy relation of DEM simulations is found to fulfill the state-dependent dilatancy equations. As a microscopic parameter to evaluate the static determinacy of the granular system, the redundancy ratio is defined and investigated. The results show that the critical state is very close to the statically determinate state. Other particle-level indexes, including the distribution of the contact forces and the anisotropies, are carefully investigated to analyze the microstructural evolution and the underlying mechanism. The microscopic analysis shows that both the contact orientations and contact forces influence the mechanical behaviors of granular materials.
PubDate: 2017-06-01
DOI: 10.1007/s11440-017-0530-8
Issue No: Vol. 12, No. 3 (2017)

• Validation of discrete element method by simulating a 2D assembly of
randomly packed elliptical rods
• Authors: Zhaofeng Li; Yu-Hsing Wang; Xia Li; Quan Yuan
Pages: 541 - 557
Abstract: This paper aims at establishing the predictive capability of the discrete element method (DEM) by validating the simulated responses of granular systems against experimental observations at both the macroscale and the microscale. A previously published biaxial shearing test on a 2D assembly of randomly packed elliptical rods was chosen as the benchmark test. In carrying out the corresponding DEM simulations herein, the contact model was derived and then validated using finite element analysis; the associated parameters were calibrated experimentally. The flexible (membrane) boundary was modeled by a bonded-particle string with experimentally calibrated parameters. An iteration procedure was implemented to replicate the initial packing and also to satisfy the boundary conditions in the experiment. Overall, the DEM simulation is found effective in reproducing the stress–strain–volumetric response, the statistical observation on the fabric anisotropy and the strain localization. Furthermore, the closer the numerical packing is to the experimental one, the closer the response is reproduced, demonstrating the significance of the initial packing reconstruction. Still, there are some minor differences between the experiment and simulation, reflecting the limitations associated with the particle number and the measurement resolution used in the experiment when reproducing the initial packing.
PubDate: 2017-06-01
DOI: 10.1007/s11440-017-0542-4
Issue No: Vol. 12, No. 3 (2017)

• Quantitative analysis of piping erosion micro-mechanisms with coupled CFD
and DEM method
• Authors: Hui Tao; Junliang Tao
Pages: 573 - 592
Abstract: Piping, as one of the critical patterns of internal erosion, has been reported as a major cause for failures of embankment dams and levees. The fundamental mechanism of piping was traditionally investigated through experimental trials and simplified theoretical methods in macroscale. Nevertheless, the initiation and progressive evolution of piping is a microscale phenomenon in its essence. The current understanding of the micro-mechanism of piping erosion is limited due to a lack of quantitative analysis and visualized evidence. And in fact, seepage flows can affect the soil fabrics and the development of contact forces between particles. But how these fabrics and contact forces evolve under a critical hydraulic gradient is still not fully understood. In this paper, the detailed process of piping erosion is investigated by using a coupled computational fluid dynamics and discrete element method (CFD–DEM) approach. The treatment of soil–flow interactions in CFD–DEM is explained by exchanging the momentum between the two phases. During the simulation, the piping erosion process is initiated by incrementally ascending differential water head across the soil samples. The three main stages of piping erosion (initial movement, continuation of erosion and total heave) can be identified from monitoring the particle velocity and positions. In addition, the evolution of contact force, hydraulic force, coordination number and void fraction is inspected to provide insight into the micro-mechanism of piping erosion. Two cases are simulated, one with a uniform particle size and a relatively uniform porosity distribution and the other with specific particle size and porosity distributions. An interesting finding from this study is that piping does not always initiate from the free surface and the evolution of piping depends heavily on the particle size and porosity distribution.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0516-y
Issue No: Vol. 12, No. 3 (2017)

• Combined spheropolyhedral discrete element (DE)–finite element (FE)
• Authors: Christopher T. Senseney; Zheng Duan; Boning Zhang; Richard A. Regueiro
Pages: 593 - 603
Abstract: This paper presents a combined spheropolyhedral discrete element (DE)–finite element (FE) computational approach to simulating vertical plate loading on cohesionless soils such as gravels. The gravel particles are modeled as discrete elements, and the plate is modeled as a deformable FE continuum. The simulations provide a meaningful step toward better understanding how deformable bodies transmit loads to granular materials. The DE–FE contact algorithm is verified through comparison with an analytical solution for impact between two symmetric bars. A parametric study is conducted to ensure boundary effects are not significantly influencing the simulations. Numerical simulations are compared to experimental test results of lightweight deflectometer loading on a gravel base course with satisfactory agreement. Future developments of the approach intend to simulate wheel loading of military aircraft on unsurfaced airfields.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0519-8
Issue No: Vol. 12, No. 3 (2017)

• Particle shape effect on thermal conductivity and shear wave velocity in
sands
• Authors: Changho Lee; Hyoung Suk Suh; Boyeong Yoon; Tae Sup Yun
Pages: 615 - 625
Abstract: This study presents the correlations between quantified shape parameters and geotechnical properties for nine sand specimens. Four shape parameters, sphericity, convexity, elongation and slenderness, were quantified with two-dimensional microscopic images with the aid of image processing techniques. An instrumented oedometer cell is used to measure compressibility, thermal conductivity and shear wave velocity during loading, unloading and reloading stages. As the particle shape inherently determines the initial loose packing condition, initial void ratio and shape parameters are well correlated with compressibility. The applied stress in soils increases the interparticle contact area and contact quality; round particles tend to achieve higher thermal conductivity and shear wave velocity during stress-induced volume change. Multiple linear regression is implemented to capture the relative contributions of involved variables, revealing that the thermal evolution is governed by the initial packing density and particle shape. The experimental observations underscore the predominant effect that particle shape has on the geomechanical and physical properties upon stress-induced soil behavior.
PubDate: 2017-06-01
DOI: 10.1007/s11440-017-0524-6
Issue No: Vol. 12, No. 3 (2017)

• Centroid sliding pyramid method for removability and stability analysis of
fractured hard rock
• Authors: Wei Wu; Xiaoying Zhuang; Hehua Zhu; Xingen Liu; Guowei Ma
Pages: 627 - 644
Abstract: This paper proposes a new method using centroid sliding pyramid (CSP) to identify the removability and stability of fractured hard rock in tunnel and slope engineering. The new method features two geometrical and topological improvements over the original key block method (KBM). Firstly, all the concave corners are considered as starting points of cutting process when a concave block is divided into a set of convex blocks in the original KBM. Only the concave corners formed by two joint planes are used for partitioning a concave block in the presented method and concave corners with free planes are excluded. Secondly, joint pyramid for removability computation in the original KBM is generated using all of the joint planes, while CSP is calculated only from the joint planes adjoining the free planes. The cone angle θ of CSP is the vectorial angle formed by the two candidate sliding surfaces of this CSP. Removability analysis of a block is transformed into calculating the cone angle of CSP. The geometrical relationship is simplified, and data size for removability computation is reduced compared with the original KBM. The provided method is implemented in a computer program and validated by examples of fractured rock slopes and tunnels.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0510-4
Issue No: Vol. 12, No. 3 (2017)

• Fast shear behavior of granular materials in ring-shear tests and
implications for rapid landslides
• Authors: Yao Jiang; Gonghui Wang; Toshitaka Kamai
Pages: 645 - 655
Abstract: The high mobility of rapid landslides is one of the most important subjects of both theoretical and practical interest to engineers and scientists. The idea that ultralow resistance could explain the high mobility inspires researchers to examine the shear behavior of granular materials under a wide range of conditions, but the response of granular materials to fast loading rates is largely unknown. The motivation for this study was to examine several fundamental issues of particle properties and mechanical conditions on the fast shear behavior of granular materials. Two granular materials were studied in the oven-dried state and were sheared by employing a ring-shear apparatus. Results indicated that angular particles (silica sand) had higher shear strength parameters than spherical particles (glass beads). In addition, the dilative process was observed during shearing, which depended on normal stress and particle shape. A slightly negative shear-rate effect on shear strength was observed for both granular materials under a certain range of shear rates. Furthermore, cumulative shear displacement had a significant effect on the degree of particle crushing. Fast ring-shear tests also revealed that shear rate had a slightly negative effect on particle crushing. Based on these experimental results, the possible applications of dynamic grain fragmentation theory to assess the high mobility of rapid landsliding phenomena were discussed. It was indicated that the magnitude and release rate of elastic strain energy generated by grain fragmentation played important roles on the dynamic process of landslide mobility.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0508-y
Issue No: Vol. 12, No. 3 (2017)

• Dynamic instabilities under isotropic drained compression of idealized
granular materials
• Authors: T. Doanh; N. Abdelmoula; L. Gribaa; T. T. T. Nguyên; S. Hans; C. Boutin; A. Le Bot
Pages: 657 - 676
Abstract: The compressibility behaviour of loose and contracting granular assemblies, normally consolidated and overconsolidated, under isotropic drained compression is investigated in this paper. Short cylindrical samples of water-saturated monodisperse glass beads, initially assembled in loose state by moist-tamping technique, are isotropically compressed in a classical axisymmetric triaxial machine. Very loose glass bead samples experience numerous unexpected events, sometimes cascading, under undetermined triggered effective isotropic stress in loading and in unloading, while the classical compressibility behaviour of granular material is recovered once these events ignored. Each event, resembling the stick–slip instability during shear in triaxial compression, is characterized by a transient dynamic phase I with very fast drop of effective isotropic stress $$\sigma ^{'}$$ due to an excess pore pressure development at nearly constant volume and constant axial strain, followed by a quasi-static phase II with gradual increase in axial $$\varepsilon _\mathrm{a}$$ (contraction) and volumetric $$\varepsilon _\mathrm{v}$$ (compaction) strain, and a full progressive recovery of $$\sigma ^{'}$$ to the previous level before event. A short-lived liquefaction with null $$\sigma ^{'}$$ measured in the first phase I results in a local collapse state. Collapse events also happen on unsaturated moist and dry states. Rare events even occur during the unloading of subsequent isotropic compression cycles. The effects of triggered isotropic stress are discussed, the instability characteristics measured, the comparison with stick–slip instability made and the hypothesis of micro-structural instability with local collapse of contact networks and rapid micro-structural rearrangement argued.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0514-0
Issue No: Vol. 12, No. 3 (2017)

• Modeling of unsaturated granular flows by a two-layer approach
• Authors: Xiannan Meng; Yongqi Wang; Chun Wang; Jan-Thomas Fischer
Pages: 677 - 701
Abstract: Flows of partially saturated grain-fluid mixtures over complex curved topography are commonly observed in nature. However, comprehensive understanding of the physics behind them is to date out of reach. To investigate their dynamic process, a two-layer approach is proposed, in which the fluid-saturated granular layer is overlaid by the pure granular material. More specifically, the lower layer is described by a two-phase mixture theory of density preserving solid and fluid constituents. For the upper layer, the single-phase granular mass is treated as a frictional Coulomb-like continuum, and the dilation effect and the influence of the interstitial air are ignored. The capillarity effects and grains-size segregation are not considered in both the layers. The lower and upper layers interact at an interface which is a material surface for the fluid phase, but across which the mass exchange for the granular phase may take place. The granular mass exchange across the layer interface is parameterized by an entrainment type postulate. In addition, the classical jump conditions are employed to connect both layers at the interface dynamically. Furthermore, we perform the depth-averaged technique for the saturated grain-fluid mixture lower layer and the pure granular upper layer, respectively, to simplify the governing equations established. It is demonstrated that the resulting model equations can be reduced to most of the existing single-layer pure granular flow models and saturated two-phase single-layer debris flow models. Numerical solutions demonstrate that the present two-layer model can describe flows of partially saturated grain-fluid mixtures and the transition process of a saturated grain-fluid mixture into an under-saturated state.
PubDate: 2017-06-01
DOI: 10.1007/s11440-016-0509-x
Issue No: Vol. 12, No. 3 (2017)

• Advance in the penetrometer test formulation to estimate allowable
pressure in granular soils
• Authors: Jesús Díaz-Curiel; Sandra Rueda-Quintero; Bárbara Biosca; Georgina Doñate-Matilla
Abstract: In this paper, we present a modification of the existing mathematical formulation used to obtain the allowable bearing pressure from dynamic penetration tests in order to extend its applicability to the design of shallow foundations. The conventional relationships adopted to obtain the allowable bearing pressure from penetrometer tests have a discontinuous gradient, and they are limited to a depth less than the footing width. The aim of this work was to find a relationship that permits the estimation of this pressure in cohesionless soils, from the results of dynamic probing super heavy tests, through a single non-piecewise and continuous relationship that remains valid up to depths several times the footing width. This equation was applied as part of the geomechanical characterization survey undertaken for the construction of an elevated helipad in the centre of the Iberian Peninsula. The survey results were considered satisfactory, and the construction was completed without structural problems.
PubDate: 2017-07-08
DOI: 10.1007/s11440-017-0565-x

• Detailed comparison of nine intact rock failure criteria using polyaxial
intact coal strength data obtained through PFC 3D simulations
• Authors: Peng-fei He; Pinnaduwa H. S. W. Kulatilake; Xu-xu Yang; Dong-qiao Liu; Man-chao He
Abstract: Study of intact rock failure criteria is an important topic in rock mechanics. In this study, applicability of nine different intact rock failure criteria is investigated for intact coal strength data. PFC3D modeling was used to simulate the laboratory polyaxial tests for cubic intact coal blocks of side dimension 110 mm under different confining stress combinations. A modified grid search procedure is proposed and used to find the best-fitting parameter values and to calculate the coefficient of determination (R 2) values for each criterion. Detailed comparisons of the nine criteria are made using the following aspects: R 2 values, σ 1 − σ 2 plots for different σ 3, shapes on the deviatoric plane, linearity or nonlinearity on the meridian planes. Through the comparisons of R 2 values, σ 1 − σ 2 plots and meridian lines, the modified Wiebols–Cook and modified Lade criteria were found to fit the intact coal strength data best. The nine failure criteria are categorized into three types based on the appearances on the deviatoric plane.
PubDate: 2017-06-17
DOI: 10.1007/s11440-017-0566-9

• Cyclic degradation and non-coaxiality of soft clay subjected to pure
rotation of principal stress directions
• Authors: Jian-Gu Qian; Zi-Bo Du; Zhen-Yu Yin
Abstract: Foundation soils are often under non-proportional cyclic loadings. The deformation behaviour and the mechanism of non-coaxiality under continuous pure principal stress rotation for clays are not clearly investigated up to now. In order to study the effect of pure principal stress rotation, a series of cyclic undrained tests on Shanghai soft clay subjected to cyclic rotation of principal stress directions keeping the deviatoric stress constant under the pure rotation condition were conducted using hollow cylinder apparatus. Based on this, the evolutions of excess pore pressure and strains during cyclic loading were investigated, together with the effects of the intermediate principal stress parameter and the deviatoric stress level on stress–strain stiffness and non-coaxiality. The result can provide an experimental basis for constitutive modelling of clays describing the behaviour under non-proportional loadings.
PubDate: 2017-06-15
DOI: 10.1007/s11440-017-0567-8

• A modified normalized model for predicting effective soil thermal
conductivity
• Authors: Hailong He; Ying Zhao; Miles F. Dyck; Bingcheng Si; Huijun Jin; Jialong Lv; Jinxin Wang
Abstract: Effective soil thermal conductivity (λ eff) describes the ability of a multiphase soil to transmit heat by conduction under unit temperature gradient. It is a critical parameter for environmental science, earth and planetary science, and engineering applications. Numerous models are available in the literature, but their applicability is generally restricted to certain soil types or water contents (θ). The objective of this study was to develop a new model in the similar form of the Johansen 1975 model to simulate the λ eff(θ) relationship of soils of various soil textures and water contents. An exponential type model with two parameters is developed and a new function for calculating dry soil thermal conductivity is presented. Performance of the new model and six other normalized models were evaluated with published datasets. The results show that the new model is able to well mimic λ eff(θ) relationship of soils from sand to silt loam and from oven dry to full saturation. In addition, it has the best performance among the seven models under test (with root-mean-square error of 0.059 W m−1 °C−1, average deviations of 0.0009 W m−1 °C−1, and Nash–Sutcliffe efficiency of 0.994). The new model has potential to improve the reliability of soil thermal conductivity estimation and be incorporated into numerical modeling for environmental, earth and engineering studies.
PubDate: 2017-06-10
DOI: 10.1007/s11440-017-0563-z

• Numerical modelling of a field soil desiccation test using a cohesive
fracture model with Voronoi tessellations
• Authors: Y. L. Gui; W. Hu; Z. Y. Zhao; X. Zhu
Abstract: Numerical modelling of a field soil desiccation test is performed using a hybrid continuum-discrete element method with a mix-mode cohesive fracture model and Voronoi tessellation grain assemblages. The fracture model considers material strength and contact stiffness degradation in both normal and tangential directions of an interface. It is found that the model can reasonably reproduce the special features of the field soil desiccation, such as curling and sub-horizontal crack. In addition, three significant factors controlling field desiccation cracking, fracture energy, grain heterogeneity and grain size are identified.
PubDate: 2017-05-30
DOI: 10.1007/s11440-017-0558-9

• Influence of gravel segregation on gluing solution solidification in a
railway ballast
• Authors: Chung Fang; Yusin Lee; Yung-Jung Lin; Li-Sin Lu; Po-Chen Chen
Abstract: Influence of gravel segregation on gluing solution distribution in a realistic railway ballasts is investigated numerically. The rheological characteristics of the gluing solution are accounted for by using a coupled elasto-visco-plastic stress model with an asymptotic time increasing viscosity. The realistic railway ballast is accomplished by using the vertex identification technique, in which the regulations of the gravel composition in ballast practice are followed. Specifically, a uniformly distributed one and a segregated one induced by the difference in gravel size and gravel shape are generated. The rheological model of the gluing solution and twofold ballasts are incorporated into the finite element code of ANSYS $$^{\copyright }$$ to study the flow characteristics and time-dependent solidification of the gluing solution. Relatively concrete vertical and lateral gravel–gravel adhesion in twofold ballasts can be generated by using denser network arrangement of the gluing solution; the vertical penetration in the uniform ballast is slightly deeper than that in the segregated ballast, while a slightly reverse tendency in the lateral extension near the upper and central parts appears. However, the difference in the solidified gluing solution distributions in both ballasts is essentially insignificant, indicating that the influence of the gravel segregation needs not necessary to be taken into account in gluing practice when percolation is not taken into account.
PubDate: 2017-04-05
DOI: 10.1007/s11440-017-0544-2

• Three-dimensional DEM analysis of single geogrid-encased stone columns
under unconfined compression: a parametric study
• Authors: Meixiang Gu; Jie Han; Minghua Zhao
Abstract: Three-dimensional discrete element method (DEM) was employed in this study to analyze the behavior of single geogrid-encased stone columns under unconfined compression. Four important parameters were investigated to understand and evaluate their effects on the behavior of the encased columns by seven DEM models. The biaxial geogrid used as an encasement material for stone columns was simulated using parallel-bonded particles, and the aggregate in the stone column was simulated using graded particles. Both the macroscopic responses (e.g., vertical pressure–strain curves) and the microscopic interactions (e.g., contact force, coordination number, and sliding fraction) of the columns under unconfined compression were analyzed and are presented in this paper. The numerical results show that the geogrid encasement with high tensile stiffness could provide high confining stresses and then effectively increased the bearing capacity of the column. The short column yielded quickly even though its column modulus at a small deformation was relatively high. The modulus of the column slightly decreased with an increase in the column diameter due to high circumferential strains mobilized in the geogrid encasement. The column with large aggregate was stiffer and deformed less than the column with small aggregate. Selecting aggregate with a size larger than the geogrid aperture size was an effective way to achieve better interlocking between the aggregate and the geogrid and to minimize mass loss for the geogrid-encased stone column under loading. Due to limited deformation allowed by the geogrid encasement, a coefficient of radial stress equal to half of the coefficient of passive earth pressure was suggested to estimate the ultimate bearing capacity of the geosynthetic-encased stone column.
PubDate: 2017-04-04
DOI: 10.1007/s11440-017-0547-z

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