Abstract: The mechanical property of frozen saline sandy soil is complicated due to its complex components and sensitivity to salt content and confining pressure. Thus, a series of triaxial compression tests were carried out on sandy samples with different Na2SO4 contents under different confining pressures to explore the effects of particle breakage, pressure melting, shear dilation and strain softening or hardening. The test results indicate that the stress–strain curves exhibit strain softening/hardening phenomena when the confining pressures are below or above 6 MPa, respectively. A shear dilation phenomenon was observed in the loading process. With increasing confining pressure, the strength firstly increases and then decreases. By taking into consideration the changes between the grain size distributions before and after triaxial compression tests, a failure strength line incorporating the influences of both particle breakage and pressure melting is proposed. In order to describe the deformation characteristics of frozen saline sandy soil, an elastoplastic incremental constitutive model is established based on the test results. The proposed model considers the plastic compressive, plastic shear and breakage mechanisms by adopting the non-associated flow rule. The breakage mechanism can be reflected by an index related to the initial, current and ultimate grain size distributions. The hardening parameters corresponding to compressive and shear mechanisms consider the influence of particle breakage. Then the effect of particle breakage on both the stress–strain and volumetric strain curves is analyzed. The calculated results fit well with the test results, indicating that the developed constitutive model can well describe the mechanical and deformation features of frozen saline sandy soil under various stress levels and stress paths. In addition, the strain softening/hardening, contraction, high dilation and particle breakage can be well captured. PubDate: 2019-03-21

Abstract: Reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) is a newly developed binder for soil stabilization/solidification. It can be used as an alternative of conventional hydraulic binders for the stabilization/solidification of heavy metal-contaminated soils. Acid rain exposure has been found to weaken the heavy metal immobilization in solidified/stabilized soils in the long term. However, very limited studies have been conducted to investigate the effect of acid rain on the physical and strength properties of GGBS–MgO-stabilized heavy metal-contaminated soils. In this study, GGBS–MgO-stabilized lead (Pb)-contaminated soils are subjected to the semi-dynamic leaching test using the simulated acid rain (SAR) as leachant, which has pH values of 2.0, 3.0, 4.0, and 5.0. Deionized water (pH 7.0) is also used for comparison. Dry density, soil pH, needle penetration depth, and unconfined compressive strength (qu) of the solidified/stabilized soils are measured after the leaching test. The results show that exposure to SAR yields reduction in qu and soil pH, whereas the needle penetration depth increases and dry density changes only marginally. The qu of the solidified/stabilized soils increases noticeably with increased GGBS–MgO content. It is also found that the stabilized Pb-contaminated soil exhibits lower qu than its clean soil counterpart. Furthermore, a quantitative relationship is proposed to correlate the normalized needle penetration resistance with the normalized qu. Finally, a multiple linear regression is performed to statistically reveal the dependence of soil strength on three independent variables, which are SAR pH, Pb concentration and binder content. PubDate: 2019-03-20

Abstract: Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field. PubDate: 2019-03-18

Abstract: The small-strain elastic shear wave velocity ( \(V_S\) ) is a basic mechanical property of soils and is an important parameter in geotechnical engineering. Recently, \(V_S\) has been adopted as one of the indices for development of liquefaction charts. This implies that if a parameter affects \(V_S\) , it may also affect liquefaction resistance. Some of the parameters whose effects have been accounted for include relative density, stress state and geologic age. An important parameter that affects both liquefaction resistance and \(V_S\) is fabric. Quantification of in situ fabric is still an open problem and hence, considerable judgement is needed in order to map laboratory test results to field conditions. In this paper, we conduct numerical simulations at the grain-scale to investigate the effect of fabric on \(V_S\) . We start by showing that two granular assemblies, with the same stress state and void ratio but different fabrics, can exhibit different trends in liquefaction behavior. Furthermore, via a numerical implementation of the bender element test, we obtain two distinct trends of \(V_S\) anisotropy for the two granular assemblies. Finally, we consider three different fabric measures based on contact properties and explore correlations between \(V_S\) anisotropy and fabric anisotropy. We also look at fabric tensors of the ‘strong’ and ‘weak’ network, respectively, of the granular assemblies. Our results suggest that for liquefiable soils, i.e., recent Holocene-age deposits with negligible cementation and with a stress history of seismic loading, a knowledge of \(V_S\) anisotropy can give information about fabric anisotropy. A knowledge of in situ fabric could help in more accurately mapping laboratory test results to field conditions. PubDate: 2019-03-18

Abstract: The paper describes an explicit coupling procedure for efficient consolidation analysis. Each time step is divided into a flow step followed by a drained mechanical step. The flow step keeps the total mean stress increment fixed and solves a diffusion problem based on piecewise constant pressure data. The procedure can be added to purely mechanical finite element codes and does not require a fully coupled element type. Details on discretization and implementation are provided as well as results of numerical tests. PubDate: 2019-03-18

Abstract: The increasing understanding of the connection between particle morphology and mechanical behaviour of granular materials has generated significant research on the quantitative characterisation of particle shape. This work proposes a simple and effective method, based on the fractal analysis of their contour, to characterise the morphology of soil particles over the range of experimentally accessible scales. In this paper, three new non-dimensional quantitative morphological descriptors are introduced to describe (1) overall particle shape at the macro-scale, (2) particle regularity at the meso-scale, and (3) particle texture at the micro-scale. The characteristic size separating structural features and textural features emerges directly from the results of the fractal analysis of the contour of the particle, and is a decreasing fraction of particle dimension. To explore the meaning of the descriptors, the method is applied first to a variety of Euclidean smooth and artificially roughened regular shapes and then to four natural and artificial sands with different levels of irregularity. Relationships are established between the new morphological descriptors and other quantities commonly adopted in the technical literature. PubDate: 2019-03-16

Abstract: The transversely isotropic (TISO) constitutive and frost heave models for the freezing of fine-grained soils are more accurate than the isotropic model and simpler than the orthotropic models. First, in combination with the mesoscopic composition of freezing soils, a mechanical model for the interaction between the equivalent ice lens and the soil in frozen soils is established based on the series and parallel models in the theory of composite mechanics. Second, the TISO constitutive model together with the analytic expression of five elastic constants is provided for analysis of the freezing soils. Third, a preliminary elastoplastic model for TISO freezing soil is established based on the Hill plastic model. Fourth, the heat–moisture–deformation coupling TISO model and the hydrodynamic frost heave model are derived according to a thermodynamics equation, a soil water motion equation, and generalized Hooke’s law. Synchronization and uniformity of the TISO constitutive model and the TISO frost heave model are realized for analyzing the interaction between permafrost soils and buildings. Finally, an indoor standard frost heave test and the frost heave of a prototype canal are simulated based on the above models. The numerical results indicated that the models presented in this paper accurately described the frost heave and revealed the interaction between permafrost and buildings. PubDate: 2019-03-08

Abstract: Many shallow foundations are constructed within the soil layer above the groundwater table, where the soil remains unsaturated, and the failure of shallow foundation is mostly related to shear failure. The shear strength of the unsaturated soil is one of the main engineering properties required in geotechnical designs. Previous researchers suggested that the shear strength of the unsaturated soil depends on matric suction in the soil. The shape of the soil–water characteristic curve (SWCC) has a significant effect on the characteristics of unsaturated shear strength with respect to matric suction. In this paper, a new model was proposed for the estimation of the unsaturated shear strength from SWCC. In this new model, meniscus was considered to transfer soil suction into both additional net normal stress and additional cohesion. Based on the categorization from soil science, water in soil can be categorized into three groups: (1) gravity water, (2) capillary water and (3) hygroscopic water. The elemental analysis on the contractile skin indicated that only the capillary water in the soil can transfer stress into soil skeleton. Consequently, the SWCC is modified by considering capillary water only for the estimation of unsaturated shear strength. In the derivation, unsaturated soil is considered as four-phase material. Finally, a new mathematical equation for the estimation of the unsaturated shear strength was proposed and verified with the experimental data from the published literature. In addition, the proposed equation does not consist of any empirical parameter and can be used to predict the shear strength of unsaturated soil. PubDate: 2019-03-07

Abstract: Biogrouting is a ground improvement technique, which utilizes microorganisms. The numerical simulation of biogrouting is important to ensure efficient operation and to assess the applicability to the target ground. In this study, we compared syringe-scale biogrouting with biogeochemical simulation. Parameters suitable for practical applications were included. The rate constant and half-saturation constant of the reaction rate law in ureolytic bacteria Pararhodobacter sp. strain SO1, obtained from the simulation based on the urease activity test, were 1 × 10−8 mol/mg/s and 0.635 M, respectively. To achieve the same mineral precipitation in measurement and simulation, a setting in which only the calcite precipitated was used. In the sequential simulation of the solidification test, a variation in discharged Ca2+ concentration was reproduced by introducing an “adjustment index”, which considers the microbial biomass contributing to the reaction. Moreover, for the re-injection test, in which microbes were injected again to further improve the biogrout strength, the settings were validated by the sequential simulation followed by predictive simulation on different injection dates. The results indicate that by conducting a biogeochemical simulation of calcite precipitation for biogrouting using ureolytic bacteria, the strength of biogrout can be predicted and managed. PubDate: 2019-03-06

Abstract: An experimental study on internal erosion of coarse-grained soils collected from the Rhine River is presented. The tests performed on laboratory column aim to assess the potential of such soils to suffusion and to characterize their stability and the variation of the soil physical parameters during the suffusion process. An experimental device (large vertical column with 60 cm of height and 26 cm of diameter) has been developed, which allows the application of upward flow to non-cohesive soils under controlled hydraulic loading. The investigation of the parameters affecting the suitability of the soils to suffusion leads to the identification of the hydraulic gradient that initiates the migration of particles to the outlet. The results show an increase in permeability, which is related to the migration and the washing out of fine particles in the upper layer. The particle size distribution of the downward soil layer after test is performed, and the analysis corroborates the localization of particles suffusion. The grain size analysis of the outlet shows that eroded particles are smaller than 500 µm and their size rather increases with increasing hydraulic load. Usual methods based on geometrical criteria proved to overestimate the susceptibility to suffusion of soils from the Rhine, and, therefore, one consider that, for such soils, it is preferable to carry out laboratory tests to evaluate the suffusion process. PubDate: 2019-03-06

Abstract: Internal erosion is a complex phenomenon which represents one of the main risks to the safety of earthen hydraulic structures such as embankment dams, dikes or levees. Its occurrence may cause instability and failure of these structures with consequences that can be dramatic. The specific mode of erosion by suffusion is the one characterized by seepage flow-induced erosion, and the subsequent migration of the finest soil particles through the surrounding soil matrix mostly constituted of large grains. Such a phenomenon can lead to a modification of the initial microstructure and, hence, to a change in the physical, hydraulic and mechanical properties of the soil. A direct comparison of the mechanical behaviour of soil before and after erosion is often used to investigate the impact of internal erosion on soil strength (shear strength at peak and critical state) using triaxial tests. However, the obtained results are somehow contradictory, as for instance in Chang’s study (Chang and Zhang in Geotech Test J 34(6):579–589, 2011), where it is concluded that the drained strength of eroded soil decreases compared to non-eroded soil, while both Xiao and Shwiyhat (Geotech Test J 35(6):890–900, 2012) and Ke and Takahashi (Geotech Test J 37(2):347–364, 2014) have come to the opposite conclusion. A plausible explanation of these contradictions might be attributed to the rather heterogeneous nature of the suffusion process and to the way the coarse and fine grains are rearranged afterwards leading to a heterogeneous soil structure, a point that, for now, is not taken into account, nor even mentioned, in the existing analyses. In the present study, X-ray computed tomography (X-ray CT) is used to follow the microstructure evolution of a granular soil during a suffusion test, and, therefore, to capture the induced microstructural changes. The images obtained from X-ray CT reveal indeed that fine particles erosion is obviously not homogeneous, highlighting the existence of preferential flow paths that lead to a heterogeneous sample in terms of fine particles, void ratio and inter-granular void ratio distribution. PubDate: 2019-03-06

Abstract: This paper presents analytical solutions for predicting one-dimensional diffusion of an organic contaminant through a triple-layer composite liner system comprising a geomembrane (GM), a geosynthetic clay liner (GCL), and a compacted clay liner (CCL). We consider two different bottom boundary conditions, i.e., fixed-concentration bottom boundary and semi-infinite bottom boundary, for which the methods of separation of variables and Laplace transform, respectively, are used to obtain the analytical solutions. The proposed analytical solutions are then verified against the CST3 numerical model and an analytical solution available in the literature. Using the verified analytical solutions, a series of parametric studies is conducted to investigate the effect of several relevant parameters on contaminant transport through the GM/GCL/CCL liner system. The results indicate that the CCL thickness, the CCL distribution coefficient, and the effective diffusion coefficient of CCL have significant impact on contaminant diffusion in the GM/GCL/CCL liner system, whereas the effective diffusion coefficient of the GCL, the diffusion coefficient of the GM, and the partition coefficient of the GM have negligible effect on contaminant diffusion in the GM/GCL/CCL liner system. The analytical solutions presented herein can be used to aid the design of a triple-layer composite liner system and the verification of other numerical models. PubDate: 2019-03-06

Abstract: The paper deals with numerical computations of hydraulic variations and groundwater flow changes in continuous permafrost due to climate change and open-pit mining in the cold regions of Northern Canada. The work is a case study in connection with the proposed Kiggavik project in Nunavut, Canada. A major challenge in simulating fluid flow through partially frozen porous media is how to define the transient hydraulic conductivity as a function of temperature. An indirect approach based on the similarity between soil freezing and soil–water characteristic curves for unsaturated soils is implemented to define the relation between unfrozen water content and cryogenic suction which is linked to temperature by the Clausius–Clapeyron equation. Richard’s equation is then used to model fluid flow in partially frozen ground conditions. Finite element numerical modelling results of a worst-case climate change scenario indicate that although the permafrost could disappear completely in about 750 years with an increase in \(7\,^{\circ }\) C in the mean annual ground surface temperature during the next 100 years, the groundwater level around tailings facility would be approximately 35 m below ground surface. Hence, tailings pore water would not likely to migrate upwards to reach the ground surface even after the permafrost is completely disappeared. These conclusions are certainly within uncertainties and limitations of the analysis. PubDate: 2019-03-05

Abstract: The influence of non-plastic fines on undrained monotonic behaviour of sand was investigated over a wide range of fines content (FC = 0–40%), global void ratio and initial mean effective stress ( \( p_{0}^{\prime } \) = 100 kPa to 500 kPa), by performing triaxial compression tests with isotropic and anisotropic consolidation. For the sand–silt mixtures, steady-state line in the e − log(p′) space was found to be dependent on fines content with the existence of a limiting fines content, which defines the transition from a “fines in sand” to a “sand in fines” soil fabric. It is demonstrated that by means of the concept of equivalent granular void ratio, e*, all steady-state/critical state data points can be well described by a unique relationship in the e* − log(p′) space called the equivalent steady-state line (EG-SSL), regardless of fines content. The equivalent granular state parameter (ψ*), defined in terms of e*, and the EG-SSL can be effectively adopted for predicting the undrained monotonic behaviour of sand–silt mixtures and the onset of static instability, irrespective of fines content and initial state of the sand–silt mixtures. PubDate: 2019-03-05

Abstract: Behaviour of granular soils subjected to internal erosion involves complex coupling between solid–fluid interaction, skeleton deformation and microstructural evolutions. This paper presents a micro–macro investigation on suffusion in idealized gap-graded and well-graded soils using the coupled computational fluid dynamics and discrete element method. The interaction between soil particles and seepage flow is modelled via momentum exchange between two phases. The progressive loss of fine particles subjected to upward seepage flow at various hydraulic gradients is investigated. The fines content, volumetric contraction and void ratio are monitored to identify the changes of macroscopic states of the soil skeleton. In addition, the microstructural evolution is tracked via particle-scale descriptors such as coordination numbers and force chain statistics. Several clogging–unclogging events which are responsible for the sudden changes of fines content and skeleton response are observed during suffusion. A parametric study indicates that the initial fines content and the hydraulic gradient significantly affect the kinetics of suffusion. Microstructural analyses reveal that the removal of fines is accompanied by the reduction in weak contact pairs and particles with low connectivity. PubDate: 2019-03-01

Abstract: This paper studies the effect of interfacial areas (air–water interfaces and solid–water interfaces) on material strength of unsaturated granular materials. High-resolution X-ray computed tomography technique is employed to measure the interfacial areas in wet glass bead samples. The scanned 3D images are trinarized into three phases and meshed into representative volume elements (RVEs). An appropriate RVE size is selected to represent adequate local information. Due to the local heterogeneity of the material, the discretized RVEs of the scanned samples actually cover a very large range of degree of saturation and porosity. The data of RVEs present the relationship between the specific interfacial areas and degree of saturation and gives boundaries where the interfacial area of a whole sample should fall in. In parallel, suction-controlled direct shear tests have been carried out on glass beads and the material strength has been corroborated with two effective stress definitions related to the specific air–water interfacial areas and fraction of wetted solid surface, respectively. The comparisons show that the specific air–water interfacial area reaches the peak at about 25% of saturation and contributes significantly to the material strength (up to 60% of the total capillary strength). The wetted solid surface obtained from X-ray CT is also used to estimate Bishop’s coefficient χ based on the second type of effective stress definition, which shows a good agreement with the measured value. This work emphasizes the importance to include interface terms in effective stress formulations of unsaturated soils. It also suggests that the X-ray CT technique and RVE-based multiscale analysis are very valuable in the studies of multiphase geomaterials. PubDate: 2019-02-15

Abstract: A multi-disciplinary approach is adopted in the present work towards investigating bio-cemented geo-materials which extends from sample preparation, to microstructural inspection and mechanical behaviour characterization. We suggest a new way to induce “cell-free” soil bio-cementation along with a comprehensive description of bio-improved mechanical and microstructural properties. We utilize the soil bacterium Sporosarcina Pasteurii in freeze-dried, powder—instead of vegetative—, state and determine overall reaction rates of “cell-free” microbial-induced calcite (CaCO3) precipitation (MICP). We further investigate strength and stiffness parameters of three base geo-materials which are subjected to MICP under identical external bio-treatment conditions. Different trends in the mechanical response under unconfined and drained triaxial compression are obtained for fine-, medium- and coarse-grained sands for similar range of final CaCO3 contents. Pre- and post-yield dilatancy–stress relationships are obtained revealing the contribution of dilatancy in the achievement of peak strength. Medium-grained sand yields higher dilatancy rates and increased peak strength with respect to fine-grained sand. Further, insight into the bio-cemented material’s fabric is provided through scanning electron microscopy, time-lapse video microscopy and X-ray micro-computed tomography with subsequent 3D reconstruction of the solid matrix. A qualitative description of the observed precipitation behaviours is coupled with quantified microscopic data referring to the number, sizes, orientations and purity of CaCO3 crystals. Results reveal that MICP adapts differently to the adopted base materials. Crystalline particles are found to grow bigger in the medium-grained base material and yield more homogenous spatial distributions. Finally, a new workflow is suggested to ultimately determine the crucial contact surface between calcite bonds and soil grains through image processing and 3D volume reconstruction. PubDate: 2019-02-15

Abstract: A screw pile has higher end bearing capacity than any other straight pipe piles due to its larger helix with respect to central shaft. However, larger helices are not frequently used as it will bend and may reduce the actual bearing capacity of the ground. In the present study, the effect of helix bending deflection on the load settlement behaviour and ultimate bearing capacity is investigated. To achieve the objectives, model scale pile load tests were conducted. The effect of helix bending on the load settlement behaviour at higher stress level was also investigated in this research. The helices with different helix-to-shaft-diameter ratios and thicknesses were used, so that clear difference of deformed and non-deformed screw piles in the load settlement behaviour can be observed. Dry Toyoura sand in dense state was used as a model ground. It is observed from test results that the helix bending deflection starts affecting the load settlement behaviour of the ground if it is more than the critical helix bending deflection. The ratio of critical helix bending deflection to outstand length decreases with increase in helix-to-shaft-diameter ratio, and its relationship is presented in this study. It is also observed that the Roark’s formula for flat circular plate having uniform load over a very small circular area with fixed outer edges showed good agreement with the measured helix bending deflection. In order to estimate the optimum helix thickness, the well-agreed equation is also modified with respect to critical helix bending deflection. PubDate: 2019-02-11