International Journal of Geomechanics

The ballastless track system (BTS) is widely adopted in the construction of high-speed railways (HSRs), in which the vehicle-induced cyclic stresses are evenly transferred to subgrade at a smaller amplitude compared with the conventional ballast track system. In this study, both static and cyclic triaxial tests were performed on a subgrade fill procured from a HSR construction site in Mainland China. The subgrade fill contained high fine content (d < 0.075 mm). The stress-strain-strength relationship from static tests provided a reference for the cyclic tests. The permanent axial strain and resilient modulus under cyclic HSR loading were studied based on the stress levels in the field. Based on the test data, a new empirical equation is proposed to predict the accumulated permanent axial strain with cycle number and low cyclic deviator stress. A new method for determining all parameters in this equation is proposed and verified by fitting test data. The results show that the predictions from the newly proposed relationship are in good agreement with the experimental results. The resilient behavior of the fill under cyclic loading is also investigated. It is found that under low-stress levels the resilient modulus slightly decreases with the increase of stresses, which is not well described by existing models.

It is acknowledged that porosity and voids encapsulated in rocks significantly affect the mechanical behavior of specimens from laboratory observations. In this study, a proposed conceptual porosity model was idealized to evaluate the change of voids fraction caused by straining, and the equivalent porosity was introduced to indirectly characterize material properties on a macroscopic scale. A micromechanics-based analytical method was developed to track the progressive failure of specimens induced by the localization strains; in particular, the shear-failure factor was assumed to be a cumulative distribution function of the shear straining. On this basis, a phenomenological constitutive model for low-porosity rocks, generally within 5%, was further developed with only a few model parameters. In addition, predictions of multiaxial stress-strain relation and volumetric strains were examined by comparing the observed results from laboratory data. A parametric study was carried out to address how the variation of porosity impacts mechanical behaviors of rocks in the proposed model.

The present study predicts the long-term settlement behavior of pile groups using two different viscoelastic soil models, a linear five-parameter model (a Hookean spring element connected in parallel with two Maxwell elements in series) and a nonlinear five-parameter model (a Hookean spring element and two Maxwell elements all connected in parallel). The reliability of the models was assessed by comparing the results with available experimental and numerical data, and it was found to be within an error of 18% for the linear model and within an error of 3% for the nonlinear model. Parametric studies for 3 × 2 and 3 × 3 pile groups were carried out on the basis of parameters such as the distribution of shear stresses around the shaft over time, load resisted by the base over time, displacement of each pile element, and the effects of an enlarged base, the pile slenderness ratio, and pile group spacing. Interaction factors for the 3 × 2 and the 3 × 3 pile groups obtained by the linear and nonlinear five-element models were compared and shown to have a maximum difference of 7%.

An improved response surface method (RSM) is presented for reliability analysis of a tunnel face in broken soft rocks. To fully consider the variation of the slip surface caused by the random variables in the process of reliability analysis, the method uses an implicit performance function, and it is adjustable according to each possible combination of the random variables. The improved RSM and the traditional method, which is based on a deterministic performance function (a fixed slip surface), are compared. In practical projects, random variables are often correlated, and normal distributions are not advisable under some circumstances. To satisfy the practical requirements, the proposed method was extended into the space of nonnormal correlated random variables. Sensitivity analysis was performed by examining different combinations of the coefficient of variation (COV) of random variables. Results show the following: (1) A significant improvement was observed in the reliability analysis using the proposed method, especially when a large uniform force was applied to the tunnel face. (2) Beta distributions are suggested in lieu of normal distributions to exclude the negative values that probably occur as random data. (3) The stability of the tunnel face tended to be underestimated without consideration of negative correlation of random variables. (4) The stability of the tunnel face was more sensitive to the internal friction angle than to the cohesion, and the sensitivity was intensified when the negative correlation was considered. In summary, the improved RSM adequately considers the uncertainties in engineering and leads to a more reliable stability assessment of the tunnel face.

A dual-porosity finite-element model is presented for stability analysis of boreholes drilled in fractured porous media with the consideration of elastoplastic damage response. Governing differential equations adopted in this study can account for the elastoplastic deformation and continuum damage. The approximate solution for the governing differential equations was obtained numerically by applying FEM. Performance of the numerical model was evaluated through several published numerical/analytical examples. Simulation results demonstrate that the FEM model is robust and is capable of capturing essential issues for borehole stability analysis, such as responses of effective stress, damage evolution, fracture propagation, and stability analysis for boreholes under different conditions. Numerical results indicate that borehole failure starts with the initiation of microcracks (damage occurs), progresses with further extension of fractures (damage zone), and is accompanied by redistribution of fluid pressures in dual porosity and variation of stress around boreholes. Also, a more deformable geoformation may be characterized by more damage developing in the vicinity of the wellbore wall, indicating that elastic analysis gives an underestimation in terms of effective stress and an overestimation in terms of fluid pressure responses. In the case of borehole stability under hydraulic injection, this study produced a smeared damage zone (equivalent to fracture zone) instead of a single fracture along a specific direction, which manifests as a shorter damage zone.

Layered soils are often encountered in nature; hence, it is important to determine the coefficient of permeability in them, due to its effect on ground contamination and the recharging of water basins. While porosity–permeability relationships in pure clays and sands are well established in the literature, the permeability of natural sedimentary basins remains highly uncertain. Theoretical equations based on Darcy’s law are often used to determine the coefficient of permeability in layered soil. However, the results obtained from these equations are often found to be different from the measured values of permeability using field testing. Moreover, there is a very limited body of literature available on the validity of Darcy’s law in layered soils. With this in view, experiments were carried out in the laboratory to determine the coefficient of permeability in stratified soils (single, double, and triple-layered sands). The measured values of permeability were then correlated to the characteristic frequencies of elastic shear and/or compression waves obtained from bender/extender tests. Using Biot’s theory of wave propagation and Darcy’s law, mathematical equations were further developed to predict the permeability of layered soils. Based on the results obtained from the present study and the data available in the literature, it was demonstrated that the permeability of layered soils can be predicted reasonably well from these equations.

Electrokinetic geosynthetics (EKG) has been used as an electrode to perform electroosmotic consolidation for the improvement of soft soil. To use previous analytical theories to analyze the drainage and consolidation behavior of soil mass, the band-shaped EKG electrodes need to be converted to cylindrical electrodes; however, no definitive principle for this conversion is available. In the present study, six commonly used equivalent equations for a prefabricated vertical drain (PVD) were proposed for the conversion of the EKG electrode, and a numerical model was developed to evaluate their validity in terms of electric current, average degree of consolidation, and surface settlement. The effects of electrode configuration and electrode dimensions are also discussed based on the numerical results. Considering the overall performance on the analysis of the electroosmotic consolidation process, the equivalent equations were recommended to convert the band-shaped electrode to a cylindrical electrode. The electrode configuration and electrode dimensions showed a significant effect on the electroosmosis process. The hexagonal electrode configuration presented larger electric current, average degree of consolidation, and settlement than the parallel and rhombic electrode configurations. With the increase in electrode width and thickness, the electric current, average degree of consolidation, and settlement increased.

Rock mechanics parameters for underground engineering cannot be rapidly or comprehensively tested on site; thus, this article presents a method for determining rock mechanics parameters using drilling parameters obtained during a field drilling experiment. Based on the rock fracture characteristics, a rock-cutting mechanical model was developed that does not assume a damaged rock surface. In this model, the ultimate load of the rock under an oblique cutting force is derived based on slip-line theory. The relationship between the cutting torque and the rock mechanics parameters (hereafter referred to as the CT-R relationship) is then obtained. The results of laboratory tests were compared with theoretical analysis, and the mean deviation ξ was 16.8%, verifying the rationality of the rock-cutting mechanical model and the CT-RP relationship. The CT-RP relationship established in this article can provide a theoretical basis for the inversion of the rock mechanics parameters by the drilling parameters.

Although soil–rock mixtures (S-RMs) exist extensively on Earth’s surface, their mechanical behaviors remain elusive. In this study, a series of large-scale direct-shear tests was conducted to investigate the mechanical behaviors of S-RMs composed of weathered basalt with different rock block contents, including stress–strain relationship, dilatancy, particle breakage, and shear strength. The results indicate that the S-RM shows more and more pronounced strain-hardening behavior as its rock block content increases. For S-RMs with different rock block contents, there is a power law relationship between the stress ratio and the displacement-increment ratio before the peak shear strength is reached. The horizontal shear stress depends strongly on the volumetric strain of the S-RM. Namely, volume compression results in closer contacts among particles, enhancing the strength and strain-hardening behavior of the S-RM, whereas volume expansion disperses particles, reducing the strength and enhancing the strain-softening behavior. It is also shown that particle breakage can exert significant control over the mechanical properties of the S-RM. Particle breakage becomes more and more severe as the rock block content and the normal stress increase, and then it reaches a critical state. In addition, it is remarkable that the relationship between the shear strength and the normal stress follows a power law instead of a linear one.

The response surface method (RSM) with a quadratic polynomial was used to formulate three equations for the calculation of the maximum reinforcement loads in steel-reinforced mechanically stabilized earth (MSE) walls under operational (working stress) conditions. The RSM models were formulated using control variables found in the simplified stiffness method. The model coefficients were back-calculated from a large database of measured steel-reinforcement loads from full-scale instrumented walls using a least-squares solution. Model uncertainty was quantified using bias statistics for which model bias is defined as the ratio of measured to calculated reinforcement load. The simplest of the three RSM models has only three empirical constants and has the advantage that reinforcement stiffness and soil friction angle are not required as input parameters. The same model was shown to give a predicted load accuracy that exceeds that of the simplified method that is used in current U.S. design specifications and has the same practical accuracy as the simplified stiffness method used for steel MSE walls constructed with frictional soils. The paper shows how the three models can be used in Monte Carlo simulations to compute probabilities of load exceedance at the time of design.

A method for assessing the stability of slurry-supported trenches considering anisotropy and nonhomogeneity using the upper-bound theorem of limit analysis is presented. Only anisotropy and the nonhomogeneity of the cohesion were taken into account in this study. The influence of anisotropy and nonhomogeneity on trench stability was investigated via parametric studies. A series of stability charts was obtained for slurry-supported trenches. It is shown that the results obtained from the upper-bound solutions agree well with those obtained through the variational limit-equilibrium closed-form solutions in isotropic and homogeneous trenches. This article first describes the influences of anisotropy and nonhomogeneity on the required unit weight of slurry, and then the procedures for getting the factor of safety are presented. In addition, the influence of anisotropy and nonhomogeneity on the shape of the trench critical failure surface and on the location of the slip surface of the sliding mass is decribed. The article also gives examples to illustrate the variation in the safety factor with trench depth under different anisotropy and nonhomogeneity conditions.

Several experimental studies have demonstrated that temperature changes may significantly influence the deformation of unsaturated soils. Thus, there is an essential need to develop a predictive framework for unsaturated consolidation capturing the nonisothermal effect. This paper presents an analytical solution to the one-dimensional (1D) consolidation of unsaturated soil deposit in response to temperature variation. A set of governing equations of flow incorporating the nonisothermal condition were first obtained. Then, Fourier sine series and the Laplace transformation were used to derive solutions based on these governing equations. This study highlighted the effect of diurnal temperature variation on pore pressures and soil deformation at different depths while considering two conditions of interest: (1) no external applied load, and (2) application of step loading to the ground surface. In addition, the thermal diffusivity characterizing the consolidation behavior of unsaturated soils was also investigated and is discussed in this paper. It is predicted that a decrease in thermal diffusivity would attenuate the effects of diurnal temperature on the unsaturated consolidation.

Low-permeability slurry trench cutoff walls are commonly constructed as barriers for containment of subsurface point-source pollution or as part of seepage-control systems on contaminated sites. A method to estimate wall thickness in slurry wall design is proposed based on decoupling the advective and dispersive components of contaminant fluxes through the wall. The relative error of the result obtained by the proposed method compared with that by an analytical solution was found to increase as the ratio of the specified breakthrough exit concentration (c*) to the source concentration (c₀) increased. For c*/c₀ of less than 0.1, which covers common practical situations, the relative error was not greater than 4% and was always conservative, indicating that the proposed method provides sufficient accuracy for design. For a given breakthrough criterion (i.e., c*/c₀), the relative error was low for the scenarios having either a low or high column Peclet number, where either dispersion or advection dominates the contaminant migration, respectively, and the relative error was high for the scenario having an intermediate column Peclet number, in which case the coupling effect of advective and dispersive migrations is relatively high.

In this study, the role of municipal solid waste (MSW) behavior in the stress-deformation response of interfaces in a lined landfill system was assessed. MSW was modeled using a constitutive model that considers its time-dependent and degradable characteristics. Effects of compressibility, degradability of MSW, slope geometry, membrane properties, and lift thickness on the interface responses were evaluated. The study implied that in addition to stress levels, MSW characteristics control the mechanical response of lining components.