International Journal of Geomechanics

This paper aims to present and validate a numerical technique for the simulation of the overtopping and onset of failure in rockfill dams caused by mass sliding. This goal is achieved by coupling a fluid dynamic model for the simulation of the free surface and through-flow problems together with a numerical technique for the calculation of the rockfill response and deformation. Both the flow within the dam body and in its surroundings are taken into account. An extensive validation of the resulting computational method is performed by solving several failure problems on physical models of rockfill dams for which experimental results have been obtained by the authors.

This paper explores the mechanism of deformation noncoaxiality from the particle scale. A multiscale investigation has been carried out with the particle-scale information obtained from discrete element simulation on granular materials. The specimens were prepared anisotropically and sheared in various loading directions. The deformation noncoaxiality, i.e., noncoincidence between the principal stress direction and the principal strain increment direction, was observed. The directional statistical theory has been used to study the anisotropies in material fabric and particle interactions, and to characterize them in terms of direction tensors. Based on the stress-force-fabric relationship, the stress direction was determined from these direction tensors. In monotonic loading, it was observed that the force anisotropy is always coaxial with the loading direction, i.e., the strain increment direction, while the fabric anisotropy only gradually approaches the loading direction. This noncoincidence between the fabric anisotropy and the strain increment direction is found to be the main cause of deformation noncoaxiality. An expression to calculate the degree of noncoaxiality from the ratio and the direction deviation between the fabric and force anisotropies has been proposed.

A numerical simulation of centrifuge model tests was carried out to develop an understanding of the behavior of geogrid-reinforced soil barriers (GRSBs) of landfill covers subjected to differential settlement. The influence of the axial stiffness of the geogrid, soil-geogrid interface friction, overburden pressure, and thickness of the soil barrier on the overall performance of GRSBs was investigated. Results from the study indicate that unreinforced soil barriers (URSBs) experience tensile stresses and strains throughout their thickness at the zone of maximum curvature; however, with the inclusion of geogrid within the soil barrier, the depth of the tension zone was found to be reduced significantly. A significant reduction in the magnitude of tensile stresses and strains in particular below the location of the geogrid was noticed with an increase in the axial stiffness of the geogrid. The results demonstrate that the geogrid layer mobilizes higher tension and thereby transfers lesser bending stress to the portion of soil barrier placed below the geogrid layer. The magnitude of the mobilized tensile load of the geogrid was found to be directly proportional to the magnitude of the axial stiffness of the geogrid, overburden pressure, and thickness of the GRSB. The normalized depth corresponding to zero horizontal tensile stress was found to decrease with an increase in the axial stiffness of the geogrid, which indirectly suggests that the depth of tension cracks can be considerably reduced with an increase in axial stiffness of the geogrid. This study also suggests that the integrity of GRSBs subjected to differential settlements can be retained only when a geogrid having adequate tensile load-strain characteristics along with an adequate overburden pressure is provided. The results of numerical analyses of GRSBs subjected to differential settlements were observed to corroborate well with the physically observed centrifuge test results.

A simplified solution procedure is proposed for estimating the dynamic response of a pile group partially embedded in a layered saturated soil and subjected to horizontal harmonic loading. In the proposed procedure, the transfer matrix method is first applied to solve the vibration equation of a single pile, from which the stiffness of the single pile is obtained. The existing lateral displacement of the saturated half-space is introduced to build the attenuation function, and the balance equation of passive pile is built accordingly. The simple Winkler model is used to derive the dynamic pile-soil-pile interaction factor. Based on the dynamic interaction factor, the horizontal impedance of the pile group is obtained by using the superposition principle. The calculated dynamic interaction factor and impedance of the pile group are in agreement with earlier results for an idealized case, verifying the correctness of the proposed method. The main parameters, such as the embedment ratio, pile separation distance, modulus of the first soil layer, and permeability, are considered in the numerical study, which may provide geotechnical and earthquake engineers with valuable reference for pile foundation design.

Consolidation is one of the most important concepts in geotechnics. The pioneer ideas of both Terzaghi and Biot, and its attendant phenomena have been shown to be widely applicable to a wide variety of soils. However, when structured and collapsible soils are taken into account, the standard consolidation theory ultimately requires complementary considerations to correctly predict a soil’s behavior. In particular, indirect determination of permeability by means of the consolidation coefficient, $c_v$, tends to highly underestimate the real hydraulic behavior for these types of soil. In the present paper, a modification in the understanding of the consolidation process is proposed to adjust the prevailing theory to the reality of both structured and collapsible soils. In short, it is considered that every soil may suffer microcollapses that increase pore-water pressure momentarily during the consolidation process. The mathematical basis for the new theory is developed and a new parameter, the collapsibility index $\eta$, is introduced. The new understanding provides an alternative explanation to the apparent low permeability observed in consolidation tests. Also, an experimental data set is analyzed in light of the concepts hereby developed. The latter analysis reveals that the alternative approach proposed predicts well the behavior of the experimental data considered.

Rock joints exert an enormous influence on the permeability of a rock mass because they act as interconnecting networks that provide pathways for fluids to permeate and flow within the rock structure. The apertures in rock joints are irregular in nature and induce flows that cannot be described by the parallel-plate theory based on planar joints or the classical cubic flow relationships. In this study, a two-dimensional (2D) hydraulic aperture distribution was considered to develop a mathematical model for fracture flow. In this approach, the three-dimensional Navier-Stokes equation was integrated over the joint aperture and converted to an equivalent 2D flow model. The proposed model was then solved numerically by adopting a well-known algorithm for coupling the pressure and velocity and implementing it in a computer program. The selected program is capable of predicting the deformation of the joint apertures on normal loading, the resulting flow patterns, and the volumetric flow rates associated with permeability tests conducted using a high-pressure triaxial apparatus that was designed and built at the University of Wollongong. The model output for different conditions of confining stresses and hydraulic gradients was computed, and a good agreement with the experimental results was observed.

Three-dimensional analysis of soil-structure interaction problems considering the response at the particle scale level is a challenging numerical modeling problem. An efficient framework that takes advantage of both the finite- and discrete-element approaches to investigate soil-geogrid interactions is described in this paper. The method uses finite elements to model the structural components and discrete particles to model the surrounding soil to reflect the discontinuous nature of the granular material. The coupled framework is used in this study to investigate two geotechnical engineering problems, namely, strip footing over geogrid-reinforced sand and geogrid-reinforced fill over a strong formation containing void. The numerical model is first validated using experimental data and then used to provide new insights into the nature of the three-dimensional interaction between the soil and the geogrid layer.

This paper presents a new constitutive model for cyclic loading of soil to predict the behavior of soft clays under undrained cyclic triaxial loading. It is inspired by the modified Cam-clay theory, and a new yield surface for elastic unloading is proposed to capture the soil behavior under cyclic loading. Only two additional parameters that characterize the cyclic behavior are used together with the traditional parameters associated with the modified Cam-clay constitutive model. The details of the relevant soil properties, initial states, and cyclic loading conditions are presented, and a computational procedure for determining the effective stresses and strains is demonstrated. The new model is used to simulate cyclic triaxial tests on kaolin, and the model predictions are generally found to be in agreement with the measured excess pore pressures and axial strains. Furthermore, numerous factors that influence the cyclic performance of soft soils can be considered in the new model, such as cyclic stress ratios, preshearing, and cyclic loading frequency. The critical cyclic stress ratio is also predictable using the proposed model in terms of excess pore pressures and axial strains.

Rock mass often exhibits anisotropic properties. Even isotropic rocks will behave anisotropically if cut by regular joint sets. Many rock masses can be regarded as orthogonal anisotropic linear elastic bodies. To date, there is no analytic method available to determine the stresses in an arbitrary-shaped tunnel (except for elliptic holes and circles) in an orthogonal anisotropic rock mass under the action of in situ stresses. To obtain analytical solutions, the conformal transformation method is used, in which with three polar coordinates are introduced. By introducing the power series method, a new analytic method used to determine stresses in an orthogonal anisotropic rock mass with an arbitrary-shaped tunnel is proposed. The analytic solutions are verified by numerical analyses using computer simulation software.

Compaction grouting is widely used for ground improvement, including foundation retrofitting, earthquake disaster mitigation, and settlement compensation. However, until now, it has been developed and used based on an empirical approach and on judgment, rather than on theoretical and analytical methodologies. Estimating the expansion of soils caused by compaction grouting under different depths, soil conditions, and injection pressures is critical to determining the design parameters and construction procedure, as well as to assessing the effectiveness of the ground-improvement measure. Different mechanisms and models are reviewed and evaluated. Graf’s model is among the most appropriate and readily understandable in the view of practitioners; the elastoplastic Mohr-Coulomb constitutive model was deemed appropriate for the finite-element analysis that was conducted in this study. Charts based on the finite-element simulation results of radial expansion and heave in noncohesive soils under different soil and construction conditions are presented to facilitate such an engineering application process. Moreover, the results are compared with case history data and Wong’s analytical failure model. A simplified procedure and examples of how to use the charts are also presented.

The current study was undertaken to evaluate the effect of aggregate shape parameters (i.e., angularity, sphericity, form, and texture) and gradation on the permeability of commonly used aggregate bases in Oklahoma. Aggregates used in this study were collected from three different quarries. For each aggregate type, upper and lower limits of three different gradations, modified AASHTO #57, Oklahoma Aggregate Association (OKAA) Type M, and Oklahoma DOT (ODOT) Type A, were selected. Permeability of 18 different combinations of aggregate types and gradations ($\text{three aggregate types}\times \text{three gradations}\times \text{two gradation limits}$) were tested using a falling-head permeability approach. For a selected gradation, the lower limit exhibited higher permeability values than the upper limit. Also, permeability was found to increase with an increase in effective diameter and void ratio. An increased coefficient of uniformity and fine content resulted in lower permeability, as expected. Furthermore, the shape parameters of different aggregate types were measured using an aggregate imaging system (AIMS). The coefficient of permeability was found to increase with reduced gradient angularity and increased sphericity. The texture index was found to have minimal impact on the coefficient of permeability. A regression model was developed using aggregate shape and gradation parameters to estimate permeability of aggregate bases.

Approximately uniformly spaced cracking occurs in many different materials and processes during thermal or drying shrinkage. Such cracks may occur when a slab of material restrained at the bottom is subjected to a sudden reduction in temperature at the top. Previous research indicates that the crack patterns obtained depend on the Griffith crack length for a shrinking solid. This paper shows that for materials exhibiting elastic-plastic fracture behavior, the fracture energy, Young’s modulus, and tensile strength in combination form a second characteristic length of fracture of the material that has a significant impact on the cracks formed.