International Journal of Geomechanics

Finite-difference method (FDM) is applied herein to solve the three-dimensional (3D) contaminant transport model to predict pollutant migration through clay liner or clay deposit in waste landfill. The seven pollutant species typically found in municipal and industrial wastes are considered in this study. Physical processes such as advection, dispersion, diffusion, geochemical reactions, and first-order decay are considered in the governing equation. The computer program CONTAMINATE-3D was developed to solve the 3D analysis of the contaminant migration model. A comparative study between one-dimensional and 3D analysis is presented in this paper based on the proposed methodology considering unidirectional flow and dispersion in three directions. Based on the present 3D analysis, design charts for liners and concentration profiles of contaminant species passing through liner media were developed to facilitate the designers’ work. Contaminant plumes were also developed within the aquifer considering unidirectional and uniform groundwater velocity to illustrate the effect of the vertical-plane patch source of constant concentration on the neighboring areas of the landfill site using the proposed solution technique of 3D contaminant transport model.

This paper pertains to the development of a simplified model for an infinite beam subjected to a concentrated load moving with constant velocity and resting on geosynthetic-reinforced granular bed over soft soil improved with stone columns. The granular fill has been modeled as a Pasternak shear layer while naturally occurring saturated soft soil has been idealized by a Kelvin-Voigt model, stone columns by Winkler springs, and geosynthetic as rough elastic membrane. Nonlinear behavior of granular fill, stone columns, and the soft soil has been represented by means of hyperbolic constitutive relationships. An iterative finite difference scheme has been adopted for obtaining the system response by employing the Gauss-Seidel technique. The system response has not been found to be sensitive at lower velocities of the applied load. Damping has been found to influence the response at higher velocities. Ultimate shear resistance of granular fill has a negligible effect on the response of the system.

The construction of railway embankments on loose beds using reinforcing elements results in a modification of the embankments slope, which significantly reduces the amount of earthworks. In addition, reinforcement of both bed and embankment is essential to increase the load-bearing capacity and control the settlements. A solution for high railway-embankment stabilization and enhancing the operational axle load is the use of micropiles in the embankment toe to transmit the applied loads to the firm underlying layers and avoid the deep sliding of loose subgrade. This paper presents three experimental models of embankments of 10 m in height on a scale of $1/20$ to set up a number of loading tests: one based on a non-reinforced embankment and two others based on reinforced embankments that are stabilized with two different arrangements of micropiles. During laboratory tests, the data, including the load-bearing capacity of embankments, displacements of embankment crest and bed surface, and axial strain of micropiles were measured using the instrumentation tools. In the next step, three numerical models were developed by using the PLAXIS-3D code based on the FEM. Then, a comparison was made between the experimental and numerical data to verify the outputs of the numerical analyses. In the procedure of numerical analyses, elastoplastic behaviors of embankment material and bed were defined based on the Mohr-Coulomb failure criterion, and micropiles were considered as linear elastic elements. Finally, after accomplishing a series of sensitivity analyses on the geometric parameters of micropiles, their efficiency factor was classified in the process of arrangement optimization.

The cone penetration test is widely used to interpret the undrained shear strength of soil. The interpretation methods are commonly based on bearing capacity, cavity expansion, or strain-path methods. In this paper, the cone penetration test is analyzed using the large deformation FEM and contact mechanics. The full penetration process from the ground surface is modeled. The three-parameter Tresca model is used to represent the undrained soil behavior in a total stress analysis, to derive a cone factor equation for interpretation of the undrained shear strength. This equation is then compared with existing solutions in the literature as well as experimental data. The complexities that arise in more sophisticated soil models are elaborated in an effective stress analysis using the Modified Cam Clay model.

Embankment construction on soft ground has increased considerably over recent years as a result of the increase in infrastructure development activities and because of the unavailability of suitable land. Geosynthetic-reinforced pile-supported (GRPS) embankments provide an effective and reliable solution to the problem of constructing embankments over soft ground. The combination of geosynthetic reinforcement and piles can alleviate the uneven surface settlements on the embankment crest while reducing the embankment load transferred to the soft foundation soil. This paper presents a numerical analysis based on the FEM carried out on a GRPS embankment. Analysis was carried out in both a two-dimensional (2D) plane strain condition for different 2D idealizations of piles and in a three-dimensional (3D) condition. The interaction between geosynthetic and soil was taken into consideration during the analysis. The results obtained for the 2D models are compared with the 3D model results. The stress transferred to the piles and foundation soil, lateral displacements, development of settlements at the base of the embankment on both the foundation soil and piles, and the generation and dissipation of excess pore water pressures during and after construction for both 2D and 3D models are discussed.

The bearing capacity of unsaturated soils is commonly estimated using the conventional shear-strength parameters determined from drained loading conditions regardless of soil type and drainage condition. However, the use of drained shear-strength parameters to interpret the bearing capacity of unsaturated fine-grained soils is not reliable because there are uncertainties with respect to the drainage conditions of both the pore-air and pore-water pressures. In the current study, a series of model footing tests is conducted in statically compacted unsaturated fine-grained soils. The model footing test results are interpreted using the modified effective-stress and total stress approaches considering the influence of matric suction. In addition, the validity of the modified total stress approach is tested using in situ plate-load test results in unsaturated fine-grained soils. The advantages, disadvantages, and limitations of using the modified effective-stress and modified total-stress approaches for engineering practice applications are discussed in this paper.

An experimental program was designed to study the effects of cross-anisotropy under three-dimensional conditions on fine Nevada sand using tall prismatic specimens. Eighteen drained tests were performed using a true triaxial apparatus. Specimens were created with both horizontal and vertical bedding planes by dry pluviation, then a freezing technique was used to solidify the specimens while they were mounted in the true triaxial apparatus. The specimens were then sheared at various $b$-values, where $b=({\sigma }_2-{\sigma }_3)/({\sigma }_1-{\sigma }_3)$, in the three sectors of the octahedral plane. The stress-strain behavior and strength under various loading conditions on the octahedral plane are presented. Results show similar strength results in Sectors I and II. Effects of cross-anisotropy are shown when comparing the friction angles in Sectors I and II with those in Sector III. The strength is highest in Sector I, where the friction angle in triaxial compression ($b=0.0$, vertical specimen with horizontal bedding planes) is 5.3° higher than the triaxial-compression friction angle in Sector II/III ($b=0$, horizontal specimen tested with the horizontal bedding planes in the vertical direction). In triaxial extension ($b=1.0$), the friction angle is 2.5° higher in Sector I/II than that in Sector III. Shear band inclinations in these three sectors were essentially unaffected by the cross-anisotropy.

This paper describes the theory behind a recent extension of a two-dimensional (2D) boundary-element code, FRACOD, to enable simulations of either coupled fracture (F)-hydraulic (H) processes or coupled F-thermal stress (T) in rocks. This extension is the next step in the ongoing development of a three-dimensional (3D) fracture mechanics code that couples F-H-T processes and predicts fracture initiation and propagation under thermal and hydraulic loadings. The original FRACOD simulated both mode I (tensile) and mode II (shear) fracture propagation that only involved mechanical processes in rock masses. In this study, the F-T coupling in FRACOD was developed using an indirect boundary-element method based on fictitious heat sources. The F-H coupling in FRACOD focused on fluid flow in explicit rock fractures using a cubic law. An explicit iteration method is used to simulate the fluid flow process in fractures and its interaction with mechanical deformation. Several verification and application cases have been included in the paper that demonstrate the effectiveness of the coupled functions. The extended code has been applied to the liquefied natural gas (LNG) underground storage experiment in South Korea and the Äspö pillar stability experiment (APSE) pillar spalling experiment in Sweden, and these applications are reported elsewhere.

In this paper, the propagation behavior of an induced hydraulic fracture in the presence of a natural fracture is described. A complex meshing and finite-element technique are employed to couple a poroelastic formation, a hydraulic fracture, and an arbitrarily oriented natural fracture. Possibilities of fracture deviation, arrest, and crossing for various angles of approach are investigated under different scenarios of in situ stress contrast, rock strength, and natural fracture geometry (length). Results of this study show that orientation of natural fracture and its length have a profound effect on induced hydraulic fracture propagation. It has been observed that, in most cases, the induced hydraulic fracture crosses short natural fractures ($\text{<}10\hspace{0.17em}\text{m}$). As the induced hydraulic fracture approaches the natural fracture, fluid leak off increases, and consequently, the width of the induced fracture at the wellbore (fracture mouth) decreases. Once the induced hydraulic fracture breaks out of the natural fracture, the fluid leak off decreases, thus increasing the width of the induced fracture. It has been also observed that propagation of induced fracture is blocked or diverted by the presence of a long natural fracture ($\text{>}10\hspace{0.17em}\text{m}$). With an increase in the injection rate, however, the induced fracture is likely to cross a long natural fracture ($\text{>}10\hspace{0.17em}\text{m}$). The new understandings derived from the fully coupled poroelastic model have many beneficial applications, including design and optimization of hydraulic fracture treatments in naturally fractured reservoirs (tight gas and shale gas reservoirs) and permeability enhancement by fluid-induced shear displacement of fracture surfaces [enhanced geothermal systems (EGS)].

The authors present a numerical limit analysis on the slope stability in this paper using the rigid finite-element method (RFEM). The novelty of this study is the consideration of the rotational component of the centroid velocity for each element, as well as a generalized overturning failure criterion governing the element rotation. By combining the generalized rotation failure criterion with the Mohr-Coulomb failure criterion, the RFEM-based upper and lower bound limit analysis is formulated as a typical primal and dual linear programming problem and is solved effectively by a primal-dual interior-point method. The proposed formulation and methodology are validated by three classical soil or rock slope stability problems. Numerical results confirm the necessity of considering the rotation mechanism in RFEM-based limit analysis for slope stability in order to achieve realistic predictions.

This paper presents a study using the FEM on the effect of interference on the settlement of two closely spaced rough rigid strip footings resting on the surface of linearly elastic finite and infinite nonhomogeneous soil beds with the modulus of elasticity linearly varying with the depth. Both symmetric and asymmetric cases with respect to its geometry and applied loading have been considered in the analysis. The finite-element (FE) mesh was created using four-noded isoparametric elements. The results are presented in terms of the interaction factors, which are defined as the ratio of the settlement of the interfering footings with the corresponding value of the settlement of the isolated footing identifying the influence of the size of the footings, loading on the footings, and clear spacing between the footings. The settlement of the interfering footings is observed to be greater than the settlement of the isolated footing of the same width and loading. With an increase in the stiffness of the soil, the effect of interference on the settlement is reduced.

Calculation of seismic earth pressure is important for design of retaining walls in earthquake-prone areas. Existing analytical methods for computation of seismic passive earth pressure do not consider the effect of Rayleigh waves, although they constitute about 67% of the total seismic energy. In this paper, a new dynamic approach is proposed by considering primary, shear, and Rayleigh waves for estimation of seismic passive earth pressure by satisfying all the boundary conditions including a zero-stress condition at the ground surface. The limit equilibrium method is used to calculate seismic passive earth pressure for a rigid retaining wall supporting cohesionless backfill with critical combinations of seismic accelerations. The seismic influence zone obtained in this study is about 23 and 14% larger compared with available pseudostatic and pseudodynamic methods, respectively, which indicates the effect of Rayleigh waves. Also, there is a decrease of 22 and 3% in seismic passive earth pressure coefficient when the present results are typically compared with pseudostatic and pseudodynamic methods, respectively. The present results are more critical for calculation of seismic passive earth pressure by considering all major seismic waves in the proposed new dynamic approach.

Constitutive models based on the multilaminate framework naturally account for strain-induced anisotropy, but inherent anisotropy needs additional considerations. A recently introduced feature is the capability of the model to take into account anisotropic elasticity in the small strain range. The implications of taking into account anisotropy of small strain stiffness for the analysis of geotechnical problems is addressed in this paper by solving two relatively simple benchmark problems: a deep excavation and a strip footing. It follows from these studies that, if displacements and thus strains remain moderate (which is often the case for these types of problems), the influence of small strain stiffness anisotropy is more pronounced compared with problems where relatively large strains occur. However, the influence of stiffness anisotropy can be approximately accounted for by using the average of the axial stiffnesses.

This paper considers the prediction of spudcan punchthrough failure in sand over clay. Centrifuge model tests published in the literature reveal that the failure mechanism is complex and changes as the spudcan pushes through the sand and the sand plug below it is pushed into the clay. This paper assesses the relevant provisions of the new ISO standard and proposes a simple adjustment to the present predictive equations. Results are compared with data published in the literature.

Soil consolidation by prefabricated vertical drains (PVDs) relies on some soil properties that are spatially variable, such as the coefficient of permeability. However, the design of soil consolidation via PVDs has been traditionally carried out deterministically and thus can be misleading because of the ignorance of the uncertainty associated with the inherent spatial variation of soil properties. In this paper, the effects of spatial variability of soil permeability on soil consolidation by PVDs are investigated stochastically, and the corresponding uncertainty associated with the degree of consolidation is statistically quantified and analyzed.

The present paper deals with an experimental study and an analytical model set up to predict the shear resistance of clay reinforced by short fibers in plane strain and axisymmetric cases. The primary theoretical framework of this study is an extension of the Michalowski model, which is applied to reinforced sandy soils. According to the experimental findings, the failure of soil composite material is caused by slippage of the fibers. Thus, interface behavior between single fibers and the clay has been investigated by means of pullout tests. Taking into account the fiber content, interface behavior, and soil failure criterion, a model that enables the prediction of the failure of clay/fiber composite has been produced. The comparison with the results of undrained triaxial compression tests performed on reinforced clay has shown that the axisymmetric model is best suited for reproducing the failure criterion of the reinforced material.

Earlier researchers have demonstrated that the compaction effort and water content, type of soil and mineralogy, void ratio, fabric, and stress history influence the soil-water characteristics curve (SWCC) of fine-grained soils to a great extent. However, it should be noted that compacting the soil and retrieving specimens from the compacted mass for suction measurements is a tedious task. Also, many times, identical soil specimens with similar compaction states may result in varied pore-size distribution characteristics. Hence, for establishing the SWCC, soil specimens in the form of slurry of adequate consistency would be quite handy, particularly for maintaining uniformity in specimen preparation. With this in view, the current study was carried out to understand the influence of initial water content on the SWCC and to recommend its best suited value for conducting experiments. It has also been noted that suction measurement devices, such as pressure membrane extractor and the dew point potentiameter WP4, have limitations associated with the thickness of the specimen, which may become an important and deciding parameter. Hence, in this study, SWCCs were established for commercially available kaolin and bentonite clays by varying the initial water contents and thickness of their specimens. Details of the methodologies adopted for these investigations are presented in this paper. These investigations reveal that the initial water content influences only the initial portion of the SWCC for soil suction less than 500 kPa, while the specimen thickness does not significantly influence the shape of the SWCC. It has also been demonstrated that specimens prepared from the initial slurried state facilitate uniformity in suction measurements.