Showing papers in "Computers and Geotechnics in 2016"

Application of distinct element methods to simulation of hydraulic fracturing in naturally fractured reservoirs

Abstract: The Distinct Element Method (DEM) represents a rock mass as an assembly of blocks (polygonal or polyhedral). Contacts between blocks correspond to discontinuities (i.e., fractures or joints) that can exhibit non-linear mechanical behavior, including slip and opening. If flow in rock fracture is approximated using the lubrication equation, coupled hydro-mechanical DEM models can be used for simulation of rock mass treatment by fluid injection. However, this approach has a limited capability for simulating fracture propagation. The synthetic rock mass (SRM) concept overcomes this limitation. In SRM, the bonded particle model (BPM), which is an assembly of circular or spherical particles bonded to each other, represents deformation and damage of intact rock. If pre-existing discontinuities are represented in the BPM, the resulting model, referred to as SRM, has the capability of simulating hydraulic fracturing in naturally fractured reservoirs. The model delivers a pattern of hydraulic fractures that evolves in response to both intact rock fracturing and sliding and opening of pre-existing joints.

Efficient slope reliability analysis at low-probability levels in spatially variable soils

Abstract: Direct Monte Carlo simulation for the reliability analysis of slope stability with spatially variable soil properties suffers from a serious lack of efficiency when the probability of failure, p f , is low (e.g., p f 10 - 6 ). Based on the multiple response surfaces and Subset simulation, this paper proposes an efficient approach for the estimating of small probabilities of slope failure in spatially variable soils. An improved Cholesky decomposition technique is presented for the simulation of the globally non-stationary random fields of spatially variable soil properties in the multiple soil layers. Two slope examples are investigated to demonstrate the effectiveness of the proposed method. The efficiency of the proposed approach for parametric sensitivity analysis is also highlighted.

Effect of particle shape and validity of Gmax models for sand: A critical review and a new expression

Abstract: Shear modulus of soils at small strains, G max , is an important parameter in the design of geo-structures subjected to static and dynamic loading. Numerous models have been proposed in the literature for the prediction of G max for saturated and dry sands. In this work, a novel approach is proposed, based on the concept of state parameter, for the examination of the validity of G max equations in capturing the effects of void ratio and confining pressure on the small-strain behavior of granular soils. Four expressions of G max from the literature are examined. It is shown that while the expressions examined may predict the measured values of G max with some level of accuracy, dependencies to the state parameter are observed when the results are plotted against the state parameter. This is attributed to the exclusion of the effect of particle shape in the determination of the model parameters. To alleviate this deficiency, a new expression of G max for dry and saturated sands is proposed and validated using a comprehensive set of resonant column test data performed at a range of initial void ratios, confining pressures, particle shapes and grain size distributions.

Numerical study of the response of a group of energy piles under different combinations of thermo-mechanical loads

Abstract: Energy piles are rapidly gaining acceptance around the world because they represent a renewable and clean source of energy that can be used for the heating and cooling of buildings, and the de-icing of infrastructures. This technology couples the structural role of pile foundations with an energy supply using the principle of shallow geothermal energy. The exploitation of geothermal energy represents an additional thermal load that is imposed to the foundation and the surrounding soil. Because the primary role of energy piles is the stability of the overlying structure, this aspect must be ensured even in the presence of the additional thermal load. This study summarises the results of 3-D thermo-hydro-mechanical finite element analyses that investigated the behaviour of a group of energy piles for which field data were available. This allowed the nearly unique validation of the numerical approach with experimental data and a confirmation of the reliability of the results. The work provides a summary of the foundation behaviour under both conventional and extreme thermal loading conditions with reference to a geothermal operation of the piles for cooling and/or thermal energy storage applications within one season. The interaction between the piles is studied and the thermally induced group effects analysed. Attention is dedicated to the vertical stress and displacement developments in the piles. The results presented in this study outline crucial aspects that may be considered by engineers for the geotechnical and structural designs of such geostructures.

Reverse modelling of natural rock joints using 3D scanning and 3D printing

Abstract: In order to overcome the deficiency of natural joint specimens with the same surface morphology for experimental studies, we present a technical method for replicating natural joint specimens that incorporates two advanced techniques – three-dimensional (3D) scanning and 3D printing – using computer-aided design (CAD) as the ‘bridge’. This method uses an optical scanning apparatus and CAD techniques to reconstruct a virtual joint specimen with the natural rock’s joint morphology, and a 3D printer then manufactures a physical mould based on the virtual joint specimen for casting concrete or plastic specimens quickly and accurately. Quality verification clearly indicated that this method reduces the experimental errors originating from the discrepancies between replicating specimens containing natural joint’s morphology.

A fractal model based on a new governing equation of fluid flow in fractures for characterizing hydraulic properties of rock fracture networks

Abstract: This study presents a fractal length distribution model of fractures in discrete fracture networks (DFNs), adopting a fractal dimension Df that represents the geometric distribution characteristics of fractures and another fractal dimension DT that represents the tortuosity of fluid flow induced by surface roughness of single fractures in DFNs. A new governing equation for fluid flow in single fractures based on the cubic law was incorporated into this fractal model. Fluid flow in 1290 DFNs with different geometric characteristics of fractures and side lengths was simulated and their equivalent permeability was calculated. The results show that the values of a, which is the power law exponent of the fracture size distribution, calculated by the proposed fractal model are consistent with those reported in similar previous studies. The flow rate of a DFN changes proportionally with e 6 - D T where e is the aperture, which agrees better with the in-situ measurements reported in literature than the prediction of classical cubic law (e3). The equivalent permeability of DFNs is more sensitive to the random number utilized to generate the fracture length than the ones used to generate the orientation and center point of fractures. With the increment of Df, the size of the representative elementary volume (REV) decreases. When the size of a DFN is larger than the REV, the variation of equivalent permeability induced by the random number holds constant. When Df

Evolution of soil arching; 2D DEM simulations

Abstract: A series of model tests were performed using a 2D multiple trapdoor system, similar to a 2D unreinforced piled embankment. Three soil arching evolution patterns were observed in the tests: the triangular expanding pattern, the tower-shaped evolution pattern, and the equal settlement pattern. The model tests were simulated using the discrete element method (DEM), and the micromechanical parameters were calibrated using the stress–strain curves from plane strain compression tests. Noncircular particle clumps were adopted to achieve a high frictional behavior of the sand. The calculated deformation and stress distribution well-matched the results of the model tests. The three evolution patterns were observed in the DEM simulations, and additional calculations were performed to specify the application conditions. The DEM simulations showed that the contact forces between the particles formed a strong force chain network enveloping the triangular and tower-shaped deformation regions. Another, much weaker, force chain within the deformation region supported the strong force chain outside.

An evolutive elasto-plastic model for cemented paste backfill

Abstract: An evolutive elasto-plastic model is developed in this research work to address the vital role of binder hydration in the evolution of the mechanical behavior and properties of cemented paste backfill (CPB). Double hardening/softening parameters, which include effective incremental plastic strain and degree of binder hydration, are adopted. A non-associated plastic potential function based on the dilation angle is employed to formulate the plastic deformation and dilation of CPB. Mechanical parameters, such as cohesion, internal friction and dilation angles, stiffness, and Poisson’s ratio are expressed as functions of the degree of binder hydration. The developed model is implemented in a finite element code, COMSOL Multiphysics, and then validated against experimental data obtained from laboratory tests performed in this study and by other researchers. The validation results show good agreement between the predicted and experimental results, thus confirming the capabilities of the new constitutive model to well capture binder hydration induced evolution of the mechanical behavior and properties of CPB.

Numerical simulation of microstructure of brittle rock using a grain-breakable distinct element grain-based model

Abstract: A distinct element grain-based method (GBM) was developed to simulate the microstructure of rock-like materials. Using this method, a UDEC-GBM model can be readily constructed with a given mineral composition, allowing independent assignment of specific properties to both the grains and grain boundaries. Both intra-granular cracks cutting through the grains and inter-granular cracks developed along grain boundaries can be captured. These features allow a full incorporation of both geometric and mechanical heterogeneity at grain scale for simulating brittle rocks. The validity of the proposed UDEC-GBM approach was verified by simulating a low-porosity sandstone under compression and direct-shear tests. The UDEC-GBM was proved to be capable of reproducing many of the characteristics associated with brittle fracture in low-porosity sandstone. It was found that the model with unbreakable grains trends to under-estimate the crick initiation threshold, highlighting the importance of the incorporation of breakable grains when modeling micro-structure of brittle rocks. The numerical experiments suggested that examining the extent of the tensile stress zones alone may lead to a biased evaluation of tensile cracking at crack initiation. The tensile stress magnitude must also be taken into consideration. It was also found that a synthetic sandstone sample with relatively low ground boundary strength produces a more ductile post-peak behavior. Microscopic tensile strength of the grains has limited influence on the failure mechanism of the synthetic specimen under unconfined compression loading. The proposed GBM approach provides a very useful tool for studying grain-scale micro-mechanics of brittle rocks.

DEM modelling of cone penetration tests in a double-porosity crushable granular material

Abstract: A three-dimensional discrete element model is used to investigate the effect of grain crushing on the tip resistance measured by cone penetration tests (CPT) in calibration chambers. To do that a discrete analogue of pumice sand, a very crushable microporous granular material, is created. The particles of the discrete model are endowed with size-dependent internal porosity and crushing resistance. A simplified Hertz–Mindlin elasto-frictional model is used for contact interaction. The model has 6 material parameters that are calibrated using one oedometer test and analogies with similar geomaterials. The calibration is validated reproducing other element tests. To fill a calibration chamber capable of containing a realistic sized CPT the discrete analogue is up-scaled by a factor of 25. CPT is then performed at two different densities and three different confinement pressures. Cone tip resistance in the crushable material is practically insensitive to initial density, as had been observed in previous physical experiments. The same CPT series is repeated but now particle crushing is disabled. The ratios of cone tip resistance between the two types of simulation are in good agreement with previous experimental comparisons of hard and crushable soils. Microscale exploration of the models indicates that crushing disrupts the buttressing effect of chamber walls on the cone.

3D multiscale modeling of strain localization in granular media

Abstract: A hierarchical multiscale modeling approach is used to investigate three-dimensional (3D) strain localization in granular media. Central to the multiscale approach is a hierarchical coupling of finite element method (FEM) and discrete element method (DEM), wherein the FEM is employed to treat a boundary value problem of a granular material and the required constitutive relation for FEM is derived directly from the DEM solution of a granular assembly embedded at each of the FEM Gauss integration points as the representative volume element (RVE). While being effective in reproducing the complex mechanical responses of granular media, the hierarchical approach helps to bypass the necessity of phenomenological constitutive models commonly needed by conventional FEM studies and meanwhile offers a viable way to link the macroscopic observations with their underlying microscopic mechanisms. To model the phenomenon of strain localization, key issues pertaining to the selection of proper RVE packings are first discussed. The multiscale approach is then applied to simulate the strain localization problem in a cubical specimen and a cylindrical specimen subjected to either conventional triaxial compression (CTC) or conventional triaxial extension (CTE) loading, which is further compared to a case under plane-strain biaxial compression (PBC) loading condition. Different failure patterns, including localized, bulging and diffuse failure modes, are observed and analyzed. Amongst all testing conditions, the PBC condition is found most favorable for the formation of localized failure. The CTC test on the cubical specimen leads to a 3D octopus-shaped localization zone, whereas the cylindrical specimen under CTC shows seemingly bulging failure from the outlook but rather more complex failure patterns within the specimen. The CTE test on a uniform specimen normally ends in a diffuse failure. Different micro mechanisms and controlling factors underlying the various interesting observations are examined and discussed.

Three-dimensional numerical analysis of a DCM column-supported highway embankment

Abstract: This paper presents a three-dimensional numerical analysis of a well-monitored DCM (deep cement mixing) column-supported embankment for a highway in Thailand. A coupled three-dimensional mechanical and hydraulic numerical model was used for this analysis, and the results are compared with the field measurements, including settlement, load distribution between soil and DCM columns, excess pore water pressure and lateral movement. The numerical model reasonably simulated the performance of the DCM column-supported embankment compared to the observed data. From the field measurements, the surrounding soils carried approximately half an embankment load, while the rest was transferred to the stronger soil layers below. The computed maximum bending moment in the column under the toe of the embankment was located at the interlayer between the fill material and soft clay due to the existence of thick fill material layer. A parametric study was conducted to investigate the influence of five key factors on the performance of the DCM column-supported embankment. The parametric study indicated that the factor of safety against bending moment failure ( FS bending ) seems to be the most important factor of safety to design in this study. The embedded column was more suitable than a fixed column to support high embankment load due to the higher moment capacity of the column with the same improvement area ratio a r , while the reduction ratios of settlement ( R s ) and lateral movement ( R l ) were not significantly different. If the floating column is chosen, sufficient improvement depth to produce the column tip laterally restrained is necessary to not only reduce the lateral movement but also increase the FS bending . The d r of greater than 0.7 is suggested in this study.

DEM simulation of bonded granular material. Part I: Contact model and application to cemented sand

Abstract: A three-dimensional bonded contact model is developed in this study to simulate the mechanical behavior of bonded granular material using the Discrete Element Method. Two types of bonded contact are identified, i.e. parallel bond contact where the bond material deposits at a physical contact (two particles being in contact) and serial bond contact where the bond material deposits at a virtual contact (two particles being close but not in contact). The strength and stiffness of the bond material are incorporated into a previous unbonded contact model where the complete interactions in the normal, tangential, rolling and torsional directions are included. A grain is simplified as a sphere and the bond material is idealized in shape as a cylinder with concave ends complementary with the surfaces of the two bonded spheres. The effect of such bond geometry is considered in terms of bond strength and stiffness. The bond failure criterion can capture the coupled effect of normal force, shear force, rolling moment and torque on bond breakage. Parametric studies in DEM simulations show that the bond material properties, the bond content and the bond distribution all influence the behavior of cemented sand. With an increase in bond content or decrease in confining pressure, the mechanical behavior of cemented sand evolves from dilation-dominant where the peak state coincides with the maximum dilation rate, to bond-dominant where the peak state coincides with the maximum bond breakage rate. Microscopic statistics shows that the contributions of shear force and rolling moment to bond failure are statistically equal, both significantly greater than the contribution of torque.

Three-dimensional slope reliability and risk assessment using auxiliary random finite element method

Abstract: This paper aims to propose an auxiliary random finite element method (ARFEM) for efficient three-dimensional (3-D) slope reliability analysis and risk assessment considering spatial variability of soil properties. The ARFEM mainly consists of two steps: (1) preliminary analysis using a relatively coarse finite-element model and Subset Simulation, and (2) target analysis using a detailed finite-element model and response conditioning method. The 3-D spatial variability of soil properties is explicitly modeled using the expansion optimal linear estimation approach. A 3-D soil slope example is presented to demonstrate the validity of ARFEM. Finally, a sensitivity study is carried out to explore the effect of horizontal spatial variability. The results indicate that the proposed ARFEM not only provides reasonably accurate estimates of slope failure probability and risk, but also significantly reduces the computational effort at small probability levels. 3-D slope probabilistic analysis (including both 3-D slope stability analysis and 3-D spatial variability modeling) can reflect slope failure mechanism more realistically in terms of the shape, location and length of slip surface. Horizontal spatial variability can significantly influence the failure mode, reliability and risk of 3-D slopes, especially for long slopes with relatively strong horizontal spatial variability. These effects can be properly incorporated into 3-D slope reliability analysis and risk assessment using ARFEM.

Numerical investigation of pile installation effects in sand using material point method

Abstract: The installation of displacement piles in sand leads to severe changes in the stress state, density and soil properties around the pile tip and shaft, and therefore has a significant influence on the pile bearing capacity. Most current numerical methods predicting pile capacity do not take installation effects into account, as large deformations can lead to mesh distortion and non-converging solutions. In this study, the material point method (MPM) is applied to simulate the pile installation process and subsequent static pile loading tests. MPM is an extension of the finite element method (FEM), which is capable of modelling large deformations and soil-structure interactions. This study utilizes the moving mesh algorithm where a redefined computational mesh is applied in the convective phase. This allows a fine mesh to be maintained around the pile tip during the installation process and improves the accuracy of the numerical scheme, especially for contact formulation. For the analyses a hypoplastic constitutive model for sand is used, which takes into account density and stress dependent behaviour. The model performs well in situations with significant stress level changes because it accounts for very high stresses at the pile tip. Numerical results agree with centrifuge experiments at a gravitational level of 40 g. This analysis confirms the importance of pile installation effects in numerical simulations, as well as the proposed numerical approach’s ability to simulate installation and static load tests of jacked displacement piles.

Uncertainty reduction and sampling efficiency in slope designs using 3D conditional random fields

Abstract: A method of combining 3D Kriging for geotechnical sampling schemes with an existing random field generator is presented and validated. Conditional random fields of soil heterogeneity are then linked with finite elements, within a Monte Carlo framework, to investigate optimum sampling locations and the cost-effective design of a slope. The results clearly demonstrate the potential of 3D conditional simulation in directing exploration programmes and designing cost-saving structures; that is, by reducing uncertainty and improving the confidence in a project's success. Moreover, for the problems analysed, an optimal sampling distance of half the horizontal scale of fluctuation was identified.

Investigation of retrogressive and progressive slope failure mechanisms using the material point method

Abstract: Retrogressive and progressive slope failures are a dynamic process, in the sense that they involve a progressively changing scenario. This paper uses the contemporary material point method (MPM), to provide a view of how such failures develop. Two main scenarios are presented: (a) a relatively small slope, which, when subjected to an initial failure, is steepened, leading to the initiation of further failures retrogressing backwards; and (b) a long slope, where an initial perturbation (e.g. an excavation) triggers a series of failures that can retrogressively move up-slope.

Evaluating slope stability uncertainty using coupled Markov chain

Abstract: Geological uncertainty appears in the form of one soil layer embedded in another or the inclusion of pockets of different soil type within a more uniform soil mass. Uncertainty in factor of safety (FS) and probability of failure (Pf) of slope induced by the geological uncertainty is not well studied in the past. This paper presents one approach to evaluate the uncertainty in FS and Pf of slope in the presence of geological uncertainty using borehole data. The geological uncertainty is simulated by an efficient coupled Markov chain (CMC) model. Slope stability analysis is then conducted based on the simulated heterogeneous soils. Effect of borehole layout schemes on uncertainty evaluation of FS and Pf is investigated. The results show that borehole within influence zone of the slope is essential for a precise evaluation of FS statistics and Pf. The mean of FS will converge to the correct answer as the borehole number increases.

The interaction factor method for energy pile groups

Abstract: Prior to this study, no simplified yet rational methods were available for estimating the vertical displacements of energy pile groups subjected to thermal loads. Observing such a challenge, the goal of this study has been threefold: (i) to extend the interaction factor concept from the framework of conventional pile groups to that of energy pile groups, (ii) to present charts for the analysis of the displacement interaction between two identical energy piles over a broad range of design conditions, and (iii) to propose, apply and validate the interaction factor method for the displacement analysis of energy pile groups.

Two-phase Material Point Method applied to the study of cone penetration

Abstract: This paper presents numerical simulations of Cone Penetration Test (CPT) in water-saturated soft soils taking into account pore pressure dissipation during installation. Besides modelling interaction between soil skeleton and pore fluid, the problem involves large soil deformations in the vicinity of the penetrometer, soil–structure interaction, and complex non-linear response of soil. This makes such simulations challenging. Depending on the soil’s permeability and compressibility, undrained, partially drained or drained conditions might occur. Partially drained conditions are commonly encountered in soils such as silts and sand–clay mixtures. However, this is often neglected in CPT interpretation, which may lead to inaccurate estimates of soil properties. This paper aims at improving the understanding of the penetration process in different drainage conditions through advanced numerical analyses. A two-phase Material Point Method is applied to simulate large soil deformations and generation and dissipation of excess pore pressures during penetration. The constitutive behaviour of soil is modelled with the Modified Cam Clay model. Numerical results are compared with experimental data showing good agreement.

Overview of numerical models for interactions between hydraulic fractures and natural fractures: Challenges and limitations

Abstract: The intersection of natural fractures with hydraulic fractures results in formation of complex fracture networks, including non-planar fractures or multi-stranded fractures. On one hand, opening of these natural fractures improves productivity of the formation; on the other hand, coalescence of these fractures into a hydraulic fracture makes pressure analysis and prediction of fracture growth very complicated. Overall, interactions between natural fractures and hydraulic fractures pose more challenges in the fracturing design and its execution. Investigation and understanding of their interaction are crucial in achieving successful fracture treatments in formations with pre-existing natural fracture network. In this paper, we will review the numerical works that have been done in the last decade to model opening of natural fractures during hydraulic fracturing, focusing especially on mechanical models that address propagation of hydraulic fractures in naturally fractures reservoirs. Linear elastic fracture mechanics, cohesive element methods and continuum damage mechanics techniques utilized to understand interaction of hydraulic fractures with natural fractures are discussed here based on their capability to reproduce experimental results and field observations.

Development of an implicit material point method for geotechnical applications

Abstract: An implicit material point method (MPM), a variant of the finite element method (FEM), is presented in this paper. The key feature of MPM is that the spatial discretisation uses a set of material points, which are allowed to move freely through the background mesh. All history-dependent variables are tracked on the material points and these material points are used as integration points similar to the Gaussian points. A mapping and re-mapping algorithm is employed, to allow the state variables and other information to be mapped back and forth between the material points and background mesh nodes during an analysis. In contrast to an explicit time integration scheme utilised by most researchers, an implicit time integration scheme has been utilised here. The advantages of such an approach are twofold: firstly, it addresses the limitation of the time step size inherent in explicit integration schemes, thereby potentially saving significant computational costs for certain types of problems; secondly, it enables an improved algorithm accuracy, which is important for some constitutive behaviours, such as elasto-plasticity. The main purpose of this paper is to provide a unified MPM framework, in which both quasi-static and dynamic analyses can be solved, and to demonstrate the model behaviour. The implementation closely follows standard FEM approaches, where possible, to allow easy conversion of other FEM codes. Newton’s method is used to solve the equation of motion for both cases, while the formation of the mass matrix and the required updating of the kinematic variables are unique to the dynamic analysis. Comparisons with an Updated Lagrangian FEM and an explicit MPM code are made with respect to the algorithmic accuracy and time step size in a couple of representative examples, which helps to illustrate the relative performance and advantages of the implicit MPM. A geotechnical application is then considered, illustrating the capabilities of the proposed method when applied in the geotechnical field.

Anisotropically elasto-plastic solution to undrained cylindrical cavity expansion in K0-consolidated clay

Abstract: This paper presents an anisotropically elasto-plastic solution to the undrained expansion of a cylindrical cavity in K 0 -consolidated clay. The elasto-plastic constitutive relationship following the soil yielding process is described by the K 0 -based modified Cam-clay ( K 0 -MCC) model, which can properly reflect the anisotropic effects on the soil behaviour. Following the large strain deformation theory, the problem is reduced to solving a system of first-order ordinary differential equations in the plastic region. The semi-analytical solutions to the radial, tangential and vertical effective stresses are obtained using the Lagrangian method and the elastic–plastic (EP) boundary conditions. In addition, based on the semi-analytical results, an approximate closed-form solution is presented for practical purposes. Extensive comparisons with the isotropic constitutive model-based solutions have been performed to illustrate the effects of the initial stress anisotropy and initial stress-induced anisotropy on the cavity expansion and the stress distributions. The present solution incorporates the anisotropic properties of the natural K 0 -consolidated clay, thereby providing a more realistic theoretical basis for the practical engineering problems such as the pile installation and the pressuremeter tests.

Effect of rubber size on the behaviour of sand-rubber mixtures: A numerical investigation

Abstract: Mixtures of sand and rubber particles were simulated using DEM. Rubber content varied from 0% to 50%. The numerical samples were sheared in the range of 10 −5 –10 −2 % of e 1 . The macro-mechanical response changed depending on the size of rubber particles. As the size of the rubber particles increased, the effect of rubber in the internal structure was attenuated, facilitating the force transmission through sand-sand contacts. Largest rubber particles showed the most advantageous mechanical behaviour. Nevertheless, the selection of both rubber size and content will depend on the intended purpose of use for the mixtures.

Optimization of artificial ground freezing in tunneling in the presence of seepage flow

Abstract: Artificial ground freezing is an environmentally friendly technique to provide temporary excavation support and groundwater control during tunnel construction under difficult geological and hydrological ground conditions. Evidently, groundwater flow has a considerable influence on the freezing process. Large seepage flow may lead to large freezing times or even may prevent the formation of a closed frozen soil body. For safe and economic design of freezing operations, this paper presents a coupled thermo-hydraulic finite element model for freezing soils integrated within an optimization algorithm using the Ant Colony Optimization (ACO) technique to optimize ground freezing in tunneling by finding the optimal positions of the freeze pipe, considering seepage flow. The simulation model considers solid particles, liquid water and crystal ice as separate phases, and the mixture temperature and liquid pressure as primary field variables. Through two fundamental physical laws and corresponding state equations, the model captures the most relevant couplings between the phase transition associated with latent heat effect, and the liquid transport within the pores. The numerical model is validated by means of laboratory results considering different scenarios for seepage flow. As demonstrated in numerical simulations of ground freezing in tunneling in the presence of seepage flow connected with the ACO optimization algorithm, the optimized arrangement of the freeze pipes may lead to a substantial reduction of the freezing time and of energy costs.

Energy geotechnics: Advances in subsurface energy recovery, storage, exchange, and waste management

Abstract: Energy geotechnics involves the use of geotechnical principles to understand and engineer the coupled thermo-hydro-chemo-mechanical processes encountered in collecting, exchanging, storing, and protecting energy resources in the subsurface. In addition to research on these fundamental coupled processes and characterization of relevant material properties, applied research is being performed to develop analytical tools for the design and analysis of different geo-energy applications. The aims of this paper are to discuss the fundamental physics and constitutive models that are common to these different applications, and to summarize recent advances in the development of relevant analytical tools.

Run-out of landslides in brittle soils

Abstract: One of the factors causing the acceleration of landslides is the loss of strength of the soil involved in the potential unstable mechanism. The travelled distance and the landslide velocity, a key factor in risk analysis, will be determined by the loss of resistant forces. Brittle behaviour, commonly associated with cemented soils, overconsolidated plastic clay formations and sensitive clays, lead to the progressive failure phenomenon explained by the reduction of the strength with increasing strain. In the present study, this phenomenon has been analysed in the case of a saturated slope which becomes unstable by increasing the boundary pore water pressure. A Mohr-Coulomb model with strain softening behaviour induced by increasing deviatoric plastic strain is used. The paper focusses not only on the stability of the slope but also on the post failure behaviour (run-out and sliding velocity). A coupled hydro-mechanical formulation of the material point method has been used to simulate the whole instability process. The influence of the brittleness of the material on the triggering of instability and run-out is evaluated by means of a parametric study varying peak and residual strength. The onset of the failure and the failure geometry are controlled by both peak and residual values. Good correlations between run-outs and brittleness are found. The decay of the strength determines the acceleration of the landslides and the travelled distance.

Upper-bound solutions for the face stability of a shield tunnel in multilayered cohesive–frictional soils

Abstract: Tunneling in urban shallow soft ground is more frequently being carried out using the shield method. An important problem in the stability analysis of a tunnel face is to provide enough support pressure to the excavation face. Among the analytical approaches used to investigate the stability of a tunnel face, limit equilibrium methods or limit analysis methods are widely used. For cohesive–frictional soils, the results obtained from experimental tests and numerical simulations indicate that the failure soil in front of the tunnel face demonstrates two main features, i.e., a shear failure band in the lower part and a pressure arch effect in the upper part. Compared to the limit equilibrium methods, the limit analysis methods have a strictly theoretical basis and provide better results. To better interpret failure features, a new three-dimensional failure mechanism is proposed to analyze the limit support pressure of the tunnel face in multilayered cohesive–frictional soils based on limit analysis methods. The new failure mechanism is composed of five truncated cones that represent the shear failure band and a distributed force acting on the truncated cones that represents the pressure arch effect. The distributed force is calculated using Terzaghi earth pressure theory. The limit support pressures obtained from the new failure mechanism and the existing approaches are compared, and the new mechanism of this paper provides relatively satisfactory results for limit support pressures.

Numerical investigation of the influence of specimen size on the unconfined strength of defected rocks

Abstract: A grain-based distinct element model (GBM) is used to investigate the influence of specimen size on the strength of intact (not defected) and defected rocks. The defected specimens are simulated by integrating a previously calibrated GBM with Discrete Fracture Network models representing defect geometries. The results of scale effect analysis conducted on synthetic specimens show that the strength of intact specimens is independent of specimen size. However, depending on the orientation of defects relative to the loading direction, the strength of defected specimens may decrease, increase or fluctuate with increasing specimen size.

3D characterization of sand particle-to-particle contact and morphology

Abstract: Particle morphology, orientation, and contact configuration play a significant role in the engineering properties of granular materials. Accurate three-dimensional (3D) characterization of these parameters for experiments has historically proven difficult, especially in the context of particle contact with small particle size. This paper describes a computer code that was developed to analyze 3D images of granular materials to measure particle lengths (size), volume, surface area, global centroid location and orientation; it also provides a method to calculate particle contact location and orientation. Measurements from the proposed code can define a state of the granular material’s fabric that can be used as input for micro-mechanics based constitutive models and to validate numerical discrete element simulations. A fabric tensor and its evolution is calculated based on experimental contact normal vectors that were extracted from SMT imaging of an axisymmetric triaxial compression experiment on a natural silica sand known as F-35 sand.