International Journal of Geomechanics

Matting systems are used for temporary applications on soft soils to reduce ground pressure exerted by aircraft, heavy equipment, vehicles, and construction material. They have been used for military airfields, construction platforms, and similar applications. Previous evaluation studies of matting systems have typically consisted of full-scale testing, with only a limited amount of numerical modeling found in the literature. This paper presents results of full-scale accelerated testing of 21 test sections encompassing five matting systems, five soil-support conditions, and two aircraft loadings. One of the soil-support conditions was instrumented and tested in conjunction with three matting systems and one aircraft loading. Three-dimensional finite-element modeling was performed on the instrumented sections using the measured test data for calibration. Good matches of measured soil stresses were obtained with the model for two of the mats, whereas the model underpredicted stresses in the third mat. Modeling of the type performed in this paper was capable of correctly ranking the performance of the matting systems modeled relative to the full-scale test results.

The elastic solution of a loaded cross-anisotropic half-space is dependent on the type of anisotropy that is governed by whether the characteristic equation has real and distinct or equal or complex roots. Most previous solutions have been for the case of real and distinct roots and most have also been incomplete. This work presents complete expressions for all stresses and displacements in a homogeneous, linearly elastic cross-anisotropic half-space caused by a surface vertical point load for all three types of anisotropy, which are believed to be more compact and simpler than those given by the only currently existing complete solution. In the present work, only 38 intermediate parameters need to be calculated in order to define all stresses and displacements as compared with the 62 parameters required in the other existing complete solution. Also, an important discovery is made: the surface settlement of a half-space is given by the same formula irrespective of the type of anisotropy, and this parallels the previous discovery that the contact stress of a rigid punch on a half-space is independent of anisotropy. This overrides the current notion that different formulas for the settlement apply when the characteristic equation has real roots and when it has complex roots. Hence, it is concluded that all existing formulas for the surface settlement of a cross-anisotropic half-space caused by distributed surface loads—for the case of real and distinct roots—are valid for all types of anisotropy. It is discovered that a much-publicized solution for the problem of a surface vertically loaded cross-anisotropic half-space is in error. Parametric studies carried out show that all elastic constants strongly influence the horizontal normal stresses and radial displacement. It is believed that the compact formulas presented herein will be appealing to engineers in all parts of the world.

This paper discusses reflection and transmission phenomena of a three-dimensional (3D) plane quasi-P (qP) wave incident at a triclinic half-space superimposed by a fluid layer underneath another distinct triclinic half-space. The closed-form expressions are obtained for reflection and transmission coefficients of corresponding reflected and transmitted waves. These reflection and transmission coefficients are computed numerically and shown graphically for fixed values of relevant parameters. Certain problems have been discussed as special cases to the present problem. The findings of the current study can be used to analyze problems with similar geometry in different elastic mediums, such as monoclinic, transversely isotropic, and isotropic.

Typical triaxial compression experiments were revisited to investigate the essential mechanical behavior of rockfill materials to be reflected in constitutive modeling, such as the nonlinear dependence of the strength and the dilation on the confining pressure and the accumulation of permanent strains during cyclic loading. The mathematical descriptions of the axial stress-strain behavior during initial loading, unloading, and reloading were formulated, respectively, which enables us to represent the hysteresis loops directly without recourse to complex concepts and parameters. The axial stress-strain model was then incorporated into the constitutive framework of generalized plasticity for the modeling of cyclic behavior of rockfill materials. This task was fulfilled by defining the elastic modulus, the plastic flow direction tensor, the loading direction tensor, and the plastic modulus for different loading conditions. In particular, the plastic flow direction tensor was derived based on a stress-dilatancy equation considering the influence of loading direction, and the representation of the plastic modulus was established in terms of the tangential modulus and the elastic modulus by using the special constitutive equations under axisymmetric stress states. The cyclic model proposed in this study has three distinct features. First, the hysteresis behavior and the accumulation of permanent strains were unified and described under the framework of generalized plasticity. Second, all the loading phases were treated as elastoplastic processes so that no purely elastic regions exist in the principal stress space. Third, the introduction of two aging functions for the consideration of the hardening effect facilitates the controlling of the magnitudes of permanent strains. There are in total 13 parameters in the model, all of which can be determined easily from (cyclic) triaxial compression experiments. To check the capabilities of the model in reproducing the monotonic and cyclic behavior, typical triaxial compression experiments were simulated with the constitutive equation. Satisfactory agreement between the experimental results and the corresponding model predictions lent sufficient creditability to the effectiveness of the proposed model, which further motivates us to extend the model for more complex stress paths and apply the model in practical engineering in the future.

The falling weight deflectometer (FWD) testing develops a deflection basin on the pavement surface. Depths of this deflection basin from the center of the falling weight are measured at different radial offsets. These deflections are used for the backcalculation of the pavement layer moduli. Most of the available backcalculation software uses the layered elastic theory and static load to calculate moduli from known pavement surface deflections. However, the FWD test load is dynamic, and layer materials may show nonelastic behavior. Layered elastic theory in these types of software cannot characterize dynamic response of the pavement. Also, elastic theory is unable to accurately predict the surface deflection whenever stress developed in any pavement layer exceeds the yield point. For this reason, this study has performed a finite-element analysis of the airport pavement under the FWD test considering the dynamic load and materials plasticity. The analysis presented here includes elastoplastic behavior of pavement layer materials. Both axisymmetric and quarter cube models have been developed in ABAQUS. Time-deflection histories are simulated to match the FWD test data. A comparison is made between the dynamic, static, and field deflection basins. Contours of vertical deflection and strain are also plotted to observe their distribution on both the axisymmetric and quarter cube models. Analysis results show that the time-deflection histories are in close agreement with the field data. The axisymmetric model yields better results than the quarter cube model. Deflections from the static analysis are greater than the dynamic analysis for an identical set of the layer modulus of elasticity. A uniform distribution of strain is observed from the static analysis in both of the geometries. However, the dynamic analysis does not show similar distribution because of the time-dependent response.

Drilled-displacement (DD) piles are installed using a drilling tool consisting of a partial-flight auger and a displacement body. This tool is inserted and advanced in the ground by both a vertical force and a torque. Despite the widespread use of DD piles throughout the world, most of the design methods available for calculation of shaft capacity were developed solely on the basis of results of pile load tests for which only the pile head capacity was known (no instrumentation that allowed separation of shaft and base loads was used in those tests). The shaft capacity of a pile depends on the stress state of the soil surrounding the pile that results after its installation. Proper analysis of the impact of installation of DD piles on the stress state in the in situ soil is fundamental to the development of reliable design methods. This paper presents the results of one-dimensional, quasi-axisymmetric finite-element analysis (FEA) performed with an advanced sand constitutive model that capture the essential stages of the installation and loading of DD piles in sand. In addition, a set of equations is proposed for the estimation of the unit limit shaft resistance of DD piles installed in sand that takes into account the initial soil state and the rate of penetration of the drilling tool into the ground during pile installation.

The existence and characteristics of Roscoe and Hvorslev state boundary surfaces for unsaturated silty soil have been investigated by conducting controlled-suction triaxial tests on samples of an unsaturated compacted silty soil. The experimental tests were performed in a specially designed double-walled Bishop and Wesley triaxial apparatus. In the experiments, the soil samples were subjected to isotropic consolidation to a virgin state (under constant suction) followed by unloading to a predefined overconsolidation ratio (OCR). The Roscoe and Hvorslev state boundary surfaces for the unsaturated soil were studied by performing drained triaxial shearing tests on the samples with specified OCRs. The results show that the Roscoe and Hvorslev surfaces also exist for unsaturated silty soil. The location of these surfaces in the normalized space $\left(q/p_e^{\prime }\right):\left(p^{\text{net}}/p_e^{\prime }\right)$ is dependent on the suction. The Roscoe surface shrinks and expands with decreasing and increasing suction, and the Hvorslev surfaces are nearly parallel for different suctions.

This paper presents a bounding surface plasticity model that can be used to simulate complex monotonic and cyclic loading paths. A new mapping rule that only uses the last stress reversal point is introduced to describe the stress-strain behavior of granular soils during loading and unloading. This mapping rule is easy to implement and is suitable for highly erratic cyclic loading conditions, e.g., those induced by earthquake or traffic loading. The application and performance of the model are demonstrated using the results of experimental tests with various stress paths conducted under both monotonic and cyclic loading conditions. The study shows the efficiency of the new mapping rule in capturing the characteristic features of the behavior of granular soils under various loading paths.

The undrained behavior of sand with fines under both monotonic and cyclic loading was investigated experimentally by triaxial testing. The testing program covers fines content ranging from 0 to 30%, and a wide range of initial void ratios and confining stresses. The spectrum of cyclic loading includes one- and two-way cyclic loading, and in the latter case, the cyclic loading may be symmetrical or nonsymmetrical. The influence of fines content can be captured by using the equivalent granular void ratio, $e^{\ast }$, in lieu of the void ratio, $e$. All steady-state data points can be described by a single relationship referred to as the equivalent granular steady-state line. This study proposes the concept of the equivalent granular state parameter, ${\psi }^{\ast }$, for capturing the combined effects of the stress state, density state, and fines content. The observed behavior can then be related to $e^{\ast }$ and/or ${\psi }^{\ast }$. Irrespective of fines contents, the behavior under monotonic loading can be classified into three types, and ${\psi }^{\ast }$ is a predictor of the behavior type. Cyclic liquefaction, when triggered by adequate cyclic stress and load cycles, can occur in three different forms. Irrespective of the fines content, ${\psi }^{\ast }$ determines the form of cyclic liquefaction. Furthermore, under certain conditions, the monotonic and cyclic behavior can be correlated.

Liquefaction of sandy soils has led to catastrophic damage to structures during past earthquakes. Implementing simplified procedures such as the shear-stress method is the most prevalent current practice in assessing liquefaction. Because there may be no perfect single simplified procedure, use of multiple methods is reasonable to minimize the risk of uncertainty. This paper presents an alternative simplified method for assessing soil liquefaction that is based on the cumulative kinetic energy density (CKED). Strong ground-motion data were employed, and with the aid of a genetic programming tool, an attenuation relationship was developed for estimating the CKED in liquefiable sites. CKED at a given layer depth was estimated by applying a depth-reduction factor. A simple correlation was developed for estimating the depth-reduction factor by way of equivalent linear site-response analysis on actual liquefiable soil profiles. The performance of the model presented herein was evaluated by implementing two databases of field liquefaction case histories. Probabilistic and deterministic curves are suggested for practice application.

Based on the limit-equilibrium concept, a new method for calculating the active earth pressure acting on a rigid retaining wall that moves horizontally away from soil mass is proposed. Using this method, the trajectory of the minor principal stress resulting from the soil arching effect in retained soil mass is considered, and the arc-shaped axis of the minor principal stress is approximated to a linear axis using the improved horizontal flat-element method. The effects of soil’s internal frictional angle, its unit weight, the wall–soil friction angle relative to the active earth pressure, and the point of application of the resultant active earth pressure are investigated. Finally, the proposed method is applied to two existing tests for rigid retaining walls with different heights. A comparison of the predicted and measured values shows that the proposed method yields satisfactory results.

This study assesses the one-dimensional (1D) consolidation of unsaturated soil subjected to time-dependent loading based on the 1D consolidation theories of unsaturated soil. The differential quadrature method (DQM) is used to produce a general solution that considers various boundary conditions, various initial pore-water and pore-air distributions, and complex time-dependent loading. A special case in which the analytical solution is available in the literature is used for verification and accuracy analysis. It is found that the DQM solution can deliver more accurate results compared with the finite-difference method with a small number of sampling points. The general solution can avoid cumbersome computations in solving eigenequations encountered with the analytical solution. In addition, the proposed solution is more suitable for practical engineering because of its generality in complex initial, boundary, and loading conditions. Finally, the characteristics of the 1D consolidation of unsaturated soils under various initial pore pressure distributions, boundary conditions, and complex time-dependent loading conditions are investigated, and it is found that the initial and boundary conditions have a significant influence on the consolidation of unsaturated soils.

Work in this paper investigates the effects of woody vegetation on soil hydraulic conductivity and the related probability of exceeding deformation-based performance limit states. A summary of results obtained from laboratory and field hydraulic conductivity tests for four levee sites is presented. Limit states, as defined based on the framework of critical state soil mechanics, are developed and simple probability analyses are used to quantify the probability of exceedance under hydraulic loading. A case study of Elkhorn Levee near Sacramento, California, is presented to demonstrate the applicability of the limit states concept. The field and laboratory hydraulic conductivity data obtained from the four levee test sites show no clear trend to support the notion that woody vegetation leads to either high or low values of hydraulic conductivity; site-specific testing is needed to discern such an effect. Even though the levee case used in this study (Elkhorn) was in a marginal condition of stability, based on limit equilibrium analysis, the probability of exceeding LS III after 10 days of a sustained high water level was 2%. This probability, however, increased over time to 37 and 72% after 20 and 30 days of sustained water loading, respectively, which signifies the importance of considering the transient nature of hydraulic loading in making condition assessments after a storm event. The results of modeling a 0.75-m top root layer illustrate the impact of woody vegetation on time duration to exceed a given performance LS level and the importance of accounting for seepage-deformation responses in assessing the vulnerability of levees.

The vertical stresses within granular fills contained in trenches can be determined using Marston’s theory or numerical modeling. Marston’s theory gives only the average value of the vertical stress at a depth. Numerical modeling can be used for determining the vertical stress at any point within the fill. This paper presents vertical stress isobars for trenches filled with granular materials, developed from numerical modeling, and proposed in terms of dimensionless variables. These isobars are similar to the pressure isobars for uniformly loaded circular and strip footings found in literature. These isobars can be used for determining vertical stresses at any point within the granular material contained in trenches and can become a simple and valuable tool. The validity of the isobars was verified through several trenches with randomly selected dimensions and fill properties. It was found that the estimates using the isobars were within 9% of the estimates from the numerical model.

This paper presents a method to locate the global critical slip surface for slope profiles with complex geology. In this method, a readily available concise algorithm based on the Morgenstern-Price method is used to efficiently calculate the factor of safety for a slip surface. This algorithm is combined with an optimization technique to search for global critical slip surfaces based on a set of random initial slip surfaces. The proposed method is applied to analyze three example problems involving slope profiles with various degrees of complexity. This study indicates that the proposed global search strategy can locate the global critical slip surface regardless of the initial slip surface for slope profiles with low to moderate complexity. For more complex geologic slope profiles, however, the proposed search strategy may only be able to identify locally critical slip surfaces depending on the initial slip surface. To ensure that the global critical slip surfaces are identified, more random initial slip surfaces need to be considered. A sound engineering judgment may be combined with the global search algorithm presented in this study to locate global critical slip surfaces more efficiently.