International Journal of Geomechanics

Unbound granular materials (UGMs) are widely used as road-base and subbase materials for bearing traffic loading. Significant road settlements can be caused by cyclic traffic loading after several years of operation. To investigate the settlements of base and subbase induced by traffic loading, a series of large-scale cyclic triaxial tests were conducted under low-amplitude and high-cycle loading. Special attention was paid to the effects of the grain gradation on the strain accumulation of UGMs under cyclic loading. Test results showed that the gradation had a considerable effect on the accumulated deformation of UGMs under various cyclic loadings. Based on the experimental results, an improved high-cycle accumulation (HCA) model, incorporating a new function to consider the influence of grain gradation, was proposed. Good agreement was observed between the results predicted by the improved HCA model and the validation-test results.

Landslides triggered by rainfall are a major multiple geological hazard on Qin-ba Mountain. Debris landslides in the shallow layer make up a large proportion of all landslides. A flume experiment was conducted to successfully initiate shallow debris landslides using artificial rainfall in Zhouzhi County, Qin-ba Mountain. The results confirmed a strong correlation between the deformation of the slope and pore-water pressure and moisture content. Slope deformation and failure were observed, and a possible mechanism was interpreted based on the experimental results. The primary effects of rainfall on shallow debris landslides on Qin-ba Mountain are the generation of a seepage force and a decrease in the effective stress of the sliding surface as a result of increasing pore-water pressure. The effect of rainfall on the shear strength parameters is inconspicuous, which might be attributed to the relatively high saturation of debris soils under natural conditions. The failure type in Test 1 is considered to be a progressive failure, which is a typical failure mode for homogeneous shallow debris slopes on Qin-ba Mountain, particularly for slopes exposed to artificial excavation. The deformation in Test 2 with higher density, merely taking the form of surface erosion, usually occurs in the regions with the least artificial disturbance. The two deformation types can be primarily attributed to differences in the soil density; the higher density reflected the smaller number and size of inside pores and led to lower soil permeability, which further influenced the distribution of the pore-water pressure, direction of seepage force, and slope deformation. Therefore, higher soil density changed by compaction is beneficial for the stability of shallow debris slopes on Qin-ba Mountain, even with high pore-water pressure. In subsequent work, a series of experiments with various densities should be conducted to explore the dividing point between the two deformation modes, and in addition, tests with larger slope gradients should also be considered.

Ballast containing large aggregate particles with uniform gradation is an essential layer in a railway substructure to facilitate load distribution and drainage. Although it is constructed or maintained in accordance with specifications, ballast is characterized by inherent randomness, not only in particle properties, such as mineralogy, size, and shape, but also in particle arrangement. Because of this randomness, the initial configurations of ballast particle assemblies vary from place to place in the field as well as from specimen to specimen in the laboratory. This paper presents a study on the influence of the aggregate scale randomness, which is the initial particle arrangement, on the geomechanical behavior of ballast. A series of large-scale, laboratory, triaxial shear-strength and repeated-load permanent-deformation tests was performed at the University of Illinois at Urbana-Champaign for the study. Numerical simulations using the discrete-element method (DEM) were also performed to understand better the effect of different initial particle arrangement on the basis of the identical particle scale properties of shape and size distribution. The ballast aggregate particles were modeled as three-dimensional (3D) polyhedral elements according to the properties of particles used in the laboratory tests, and then, the same set of particles was used to simulate laboratory tests with different initial particle arrangements for each specimen. The results from both the laboratory experiments and numerical simulations confirm that the initial particle arrangement may significantly affect the strength and deformation behavior of ballast. Hence, multiple tests or simulations with different particle-packing arrangements are needed to study ballast mechanical behavior. However, a stable average response can be obtained with a minimum of three laboratory experiments or numerical simulations.

The coefficient of earth pressure at rest (K₀) for normally consolidated clays increases nonlinearly with increasing consolidation pressure toward a steady value under high pressure rather than remaining constant. Analytical expressions for evaluating pressure-dependent K₀ were derived from three representative critical state soil models: modified Cam-clay model (MCC), original Cam-clay model (OCC), and clay and sand model (CASM). In formulations, the authors relaxed a well-adopted assumption that stress ratio is kept constant during one-dimensional (1D) compression. It is found that the constant stress ratio, corresponding to the well-adopted assumption, is essentially a limit value of the stress ratio as predicted by MCC and CASM under high pressure during 1D compression. The predicted relationship between K₀ and consolidation pressure is significantly affected by the critical state stress ratio. Without considering the effect of high pressure, the value of K₀ may be considerably underestimated. The results predicted by the proposed formula based on CASM agree well with experimental data, showing the capability of this formula for predicting pressure-dependent K₀.

This paper investigates the posterosion geomechanical behavior of internally unstable granular material due to removal of fines caused by erosive forces of water flow. Posterosion undrained behavior of a gap-graded internally unstable soil was investigated for a range of erosion durations and inflow velocities using a triaxial-erosion apparatus. Test results indicated that the undrained behavior of the original specimen changed from a strain hardening behavior to a flow-type behavior with limited deformation after internal erosion. The initial peak strength improved and the flow potential decreased during the initial stage of erosion. This observed increase in initial peak strength is believed to be the result of a better interlocking between the coarse particles posterosion. In contrast, the slip-down movement of the particles due to an increase in the posterosion void ratio postponed the dilation tendency. Test results also suggested that even erosion of a small percentage of fine particles improved the mechanical frictional behavior of the soil. However, there was a threshold value for the loss of fine particles at which this positive effect deteriorated. This might have been due to formation of local metastable structures and/or overcoming contractive behavior after loss of the semiactive fines and a considerable increase in the global void ratio. Shear strength results, rate of erosion, and local vertical strains together suggest that the intergranular void ratio is a powerful index in evaluating the posterosion mechanical behavior of internally unstable soils.

In geotechnical engineering, determination of the bearing capacity of a shallow foundation in nonhomogeneous soil is an intriguing task. In the present study, an effort was made to evaluate the ultimate bearing capacity of shallow strip footing using the limit-equilibrium method in a pseudostatic environment. The foundation rests on two layered soils with varying cohesion, friction, and unit weight. The particle swarm optimization (PSO) algorithm was used to optimize the solution. The failure mechanism was assumed as linearly varying with depth with a different wedge angle at each layer. The bearing capacity of a shallow strip footing in two-layered soil was conveyed as a single coefficient for a coincident resistance of unit weight, surcharge, and cohesion. The effect of various parameters on the variation of bearing-capacity ratio was studied. A comparative study of the present study was applied with respect to different analytical solutions and numerical studies.

Nontraditional low-carbon additives are widely used in the sustainable treatment of problematic soils for construction and pavement materials. This study investigated the mechanical and microstructural properties of white kaolin (low strength clay) and green bentonite (high swelling clay) treated with a low-carbon sodium silicate-based liquid additive. The mechanical tests included unconfined compressive strength (UCS), direct shear and one-dimensional compression tests. Microscale assessments, including a field emission scanning electron microscopic (FESEM) test, nitrogen-based Brunauer, Emmett, and Teller (N₂-BET) surface area analysis and particle size analysis (PSA), were performed on the treated specimens to investigate the modification of soil structure, including soil fabric and interparticle forces. The performance of the proposed additive is demonstrated by the improvement of shear strength and compressibility of both tested soils. The optimum additive content was found to be 6%, and a significant improvement occurred in the first 7 days of curing. The mechanical property improvement is attributed to the formation of cementitious products and, subsequently, the modification of the soil structure. These cementitious products filled the pores and bonded the soil particles, resulting in an increase in interparticle forces. The sodium silicate-based additive can offer a low-carbon alternative to traditional additives such as cement and lime, which is significant from the engineering and environmental perspectives.

To investigate the acoustic emission (AE) characteristics and damage mechanical behaviors of Huashan granite, two groups of uniaxial compression tests were carried out: the first group of tests was performed on samples after thermal treatment without a cooling-off period (i.e., tested at real-time temperature; RT samples); and a second group of tests was performed on samples after the samples had cooled to room temperature (i.e., tested at post-temperature, PT samples). The experimental results showed that AE energy increased with the increase of treatment temperature, and it became quite large for temperatures over 500°C. The AE accumulative energy counts of the RT sample were higher than the PT sample. It was found in uniaxial compression tests of heat-treated granite that the mutation point in the ∑N/∑E curve could be taken as the indicator of the crack damage threshold (${\sigma }_{cd}$). The peak strain (the strain at the peak of the stress–strain curve) of the RT sample was larger than the PT sample, implying that the ductility of the former was more obvious, while peak stress and elastic modulus were smaller. For treatment temperatures of 500°C and below, the damage of the sample increased exponentially with external load, whereas as the temperature increased to 700°C, the damage-time curve was hyperbolic without an obvious inflection point. The sample under uniaxial compression exhibited axial splitting failure at low temperatures, and the failure mode became more complicated as the temperature rose.

Prediction of rock mass strength is important for estimating stability in rock engineering projects. In this paper, a novel strength criterion was proposed to determine the strength of a jointed rock mass subjected to a complex stress state. First, a series of tests were carried out on specimens with different joint geometries and under different confining pressures. From these tests, the factors affecting the strength of the jointed rock mass were obtained. The experimental results showed that normalized strength decreases initially and then increases as joint inclination angle increases, and it increases consistently and nonlinearly with increasing normalized confining pressure. Based on the experimental results, an improved nonlinear strength criterion was presented to describe the combined influence of the two aforementioned factors on jointed rock mass strength, and methods for determining the involved parameters were also introduced. Furthermore, multiaxial test data from the literature and biaxial test data from this paper were used to validate the feasibility and reliability of the new criterion. The results showed that the new criterion can predict the strength of jointed rock with high accuracy.

Seismic ground motion is a dynamic random process, and it has obvious random uncertainty. The traditional reliability analysis of bedding rock slopes under earthquake excitation is usually based on pseudostatic method and dynamic time-history analysis method, both of which ignore the random nature of earthquake excitation. Furthermore, the existing power spectral density (PSD) method for seismic response can consider the random nature of seismic activity using finite element software, but the process of calculation is time-consuming and laborious, and the calculated response results are difficult to use directly in seismic reliability analyses of slopes. This paper presents a fast algorithm for the random seismic response of bedding rock slopes using the pseudoexcitation method. Based on the calculated results, a calculation method for a dynamic fuzzy degree of reliability of bedding rock slopes under random earthquake excitation is presented using the first-excursion failure criterion, taking into account uncertainties in the strength parameters and structural limit states. A comparison with the existing method is performed to verify the proposed fast algorithm. Then, the effectiveness of this proposed calculation method is studied by comparison with a general method. A further sensitivity analysis is presented to reveal the influences of seismic ground motion parameters and fuzzy uncertainties (in strength parameters and structural limit state) on the reliability of bedding rock slopes.

Rockburst is a serious instability problem in Canadian deep underground excavations and is related to the postpeak behavior of hard and brittle rocks. Rockburst potential can be estimated by comparing the postpeak stiffness of the failed rock and the prepeak stiffness of the surrounding rock. Estimation of the postpeak behavior of hard and brittle rocks can help engineers evaluate and predict potential severe instabilities. The complete stress-displacement surface (CSDS) model is an analytical model (a constitutive model for rock joints) that estimates rockburst proneness. This paper aims to verify the application of the CSDS solution for estimating the postpeak curve (postpeak stiffness) in intact hard rocks by experimental results. To verify the model properties used in the model, different types of rock mechanics tests have been performed. The results show that the CSDS model can be used to estimate the postpeak behavior of medium to hard rocks.

A rock burst can cause a serious disaster. For the mitigation of rock burst-induced disaster, the possibility assessment of a rock burst for an underground excavation is a key undertaking and also a great challenge because rock burst failure involves a dynamic and large deformation process. In this paper, a numerical simulation method of rock burst using discontinuous deformation analysis (DDA) is proposed by showing the advantage of DDA in handling the large deformation of this dynamic process. First, the problem of using the number of the adjacent boundary in the original algorithm of the DDA program was determined and solved, which is critical and important in analyzing artificial mesh models. The DDA model was used for simulating a rock burst process and was verified by a true triaxial test in which the critical stress for rock burst had been clarified. Then, the effects of rock hardness and brittleness on critical stresses were clarified quantitatively. The critical stresses of a rock burst were estimated for rocks with various values of the elastic modulus, Poisson’s ratio, cohesion, friction angle, and tension strength. Finally, a strategy is proposed for examining the critical depth of a rock burst for various types of rocks under different tectonic levels. Rocks are classified by the mechanical properties of them, such as elastic modulus, Poisson’s ratio, cohesion, friction angle, and tensile strength. The depth condition was converted to initial stress. The tectonic condition was represented by the ratio of horizontal-to-vertical components of the initial stress. Through a large set of simulations, the critical depths of rock bursts in seven types of rocks were estimated for different tectonic levels. The results show that the proposed rock burst analysis using DDA simulation is effective and useful.

Seismic fragility analysis is a way to study the vulnerability of structures considering various uncertainties. In the paper, a seismic vulnerability analysis of rock mountain tunnels is proposed. The uniform design method (UDM), a computationally economic method for experiment design, is adopted here. Through the UDM, experiment samples are generated considering variability of the variables. Different tunnel depths, tunnel scales, rock masses, and lining thicknesses are considered. Verified dynamical finite element analysis is carried out to develop probabilistic seismic demand models. The fragility curves are established based on probabilistic seismic demand models. It is concluded that (1) UDM can generate good samples considering different sources of uncertainties; (2) a comparison between the established vulnerability curves and the empirical ones validate the feasibility of the developed methodology.

Among the most used ground improvement techniques is the mitigation of soil liquefaction by installation of stone columns. To minimize soil liquefaction, the area ratio of the stone columns should be evaluated properly. The available solutions for determining the area ratio of stone columns were developed without consideration of the effects of the stiffness of the columns. However, stone columns have a larger drained elastic modulus than the surrounding soil. The aim of the present paper is to provide a simplified solution for determining the drainage capacity of stone columns during soil liquefaction considering stiffness and limited permeability of the stone columns. Equal strain condition is considered at all depths, i.e., the same vertical deformation of the stone column and surrounding soil is considered at all depths. The present solution shows that the susceptibility of soil liquefaction increases due to the reduction of the stiffness and permeability of the stone column. It is also observed that the maximum pore water pressure ratio decreases by 20–60% due to the stiffness effect of the stone column. Design charts are presented based on the results of the developed solution. Charts can be used to determine the area ratio of the stone columns. Through the presentation of a design example, the present solution is compared with available design methods and the variances between these methods are discussed.

To investigate the mechanical characteristics of gravel under general stress conditions, a large-scale static and dynamic true triaxial apparatus was developed at Tsinghua University. The largest specimen was 200 × 200 × 400 mm, and the applied pressure can meet the research requirements of large-scale geotechnical structures. A set of verification experiments with cyclic loadings of different directions and frequencies was performed to verify the accuracy of the loading system. Then, a static true triaxial experiment with constant mean principal stress, p, and generalized shear stress, q, was carried out to study the deformation characteristics of gravel under changing Lode’s angle, θ. The results show that changing Lode’s angle, θ, causes plastic deformations in all principal stress directions. In addition, a series of experiments with both axial and lateral cyclic loadings was carried out to study the deformation characteristics of a gravel specimen under different lateral dynamic conditions. Results from the experiments indicate that lateral dynamic loads significantly change the residual and reversible strains in all three principal directions.

Overconsolidated clays are often encountered in geotechnical engineering; therefore, developing a sophisticated model that can accurately describe the mechanical behavior of overconsolidated clays has high practical value. Based on the transformed stress and the subloading surface concept, this paper presents a generalized critical-state model, with special focus on the volume change under three-dimensional loading conditions. Because existing experimental results show that not only the shear strength but also the volume change of clays depend on the loading conditions, a new stress–dilatancy equation (relationship between stress ratio and plastic strain increment ratio) defined in the transformed stress space is established. Then, by introducing the subloading surface concept, a new critical-state model is proposed. The new model not only describes the strain-softening behavior of overconsolidated clays, but also takes into consideration the influence of intermediate principal stress on strength and deformation of clays. The performance of the model is validated by triaxial compression and extension tests and true triaxial tests on normally consolidated and overconsolidated clays.

The problem of scale-size dependency of the mechanical properties of intact rock has been extensively investigated with a particular focus on uniaxial compressive strength, whereas a limited number of studies have included point-load and tensile testing. Such mechanical properties, in particular tensile strength, are of fundamental importance in the hydraulic fracturing design process. Despite advancement in such experimental researches, analytical investigation has not attracted the same amount of attention. In this paper, the scale-size dependency of intact rock under point-load and indirect tensile testing conditions was studied experimentally and analytically using different rock types with various geological origins, including sedimentary, igneous, and metamorphic. From the comparison of the experimental results, it was concluded that all rock types follow the generalized size-effect concept where strength decreases with an increase in size. Also, it was confirmed that under indirect tensile testing with an increase in size, the failure of rock transfers from pure tension to a combination of shearing and tension. In addition, the applicability of different size-effect models (statistics, fracture energy, and multifractals) to experimental data was assessed. It was thus concluded that the models with two constants fit the point-load strength data more accurately. Also, it was found that the size-effect model based on fracture-energy theory provided the best fit to the tensile strength data, whereas the statistical size-effect model provided the least best fit.

The heat-transfer phenomenon in dry sands, which is generally looked at from the perspective of conduction, was studied with the thermal flux method (TFM) proposed in the literature. However, when results from the TFM were compared with those obtained from the numerical simulation, a substantial disparity was noticed. Through careful analysis of the results, it was realized that there is a significant contribution from the convective heat and possible ingress of lateral heat into the column, which would violate the assumption that the heat migration is one-dimensional (conductive). To quantify the ingress of lateral heat in the column, the TFM was suitably modified, designated as the modified thermal flux method (MTFM). It was found that the results from the MTFM match quite well with those obtained from the numerical simulation, with adequate boundary conditions. In short, MTFM facilitates understanding the of the mechanisms of heat transfer in dry sands by conduction and convection, as demonstrated in this technical note.

The Qinghai–Tibet Power Transmission Line (QTPTL), crossing about 550 km of permafrost region, was constructed between July 2010 and October 2011. The long-term thermal-mechanical stabilities attract extensive attention because of global warming and engineering activities. Conventional calculation and prediction of the thermal-mechanical characteristics of a frozen soil foundation for the QTPTL always involve deterministic analysis. This study aimed to investigate the uncertain thermal-mechanical characteristics of a transmission tower foundation by a stochastic analysis method. Considering the effect of stochastic temperature on the stochastic deformation, a stochastic coupling computer program was compiled. The random temperature fields and random deformation fields of foundation soils for a transmission line tower in a permafrost region were calculated and predicted for the next 30 years. This work provides a novel method to analyze the stochastic thermal-mechanical characteristics of a frozen soil foundation for a transmission line tower in permafrost regions. The average standard deviations of temperature grew from 1.1 to 1.6°C on January 15, and the maximum standard deviations of temperature were grew from 3.0 to 3.5°C. The average standard deviations of vertical displacement grew from 0.46 to 0.79 cm on January 15, and the maximum standard deviations of vertical displacement grew from 1.81 to 21.2 cm. The increase of standard deviations implies that the results of conventional prediction methods for temperature and deformation may be deviate from the exact value. These results can clarify the stochastic thermal-mechanical characteristics of frozen soil foundations for the QTPTL in permafrost regions.

The vertical uplift capacity of strip plate anchors buried in cohesive-frictional soil adjacent to a crest of slopes has been computed by using finite-element lower-bound limit analysis with a second-order conic optimization technique. The uplift capacity is expressed in terms of dimensionless uplift factors F_γ and F_c from the contribution of soil self-weight and soil cohesion, respectively. The influence of the anchor edge distance from the crest of slope (e), internal friction angle of the soil (ϕ), slope angle (β), slope height (h), and embedment ratio of the anchors (d/b; d = embedded depth of anchor, and b = width of anchor) on the uplift capacity have been examined. The magnitudes of uplift factors were found to decrease continuously with an increase in the angle and height of the slope, whereas they increased with increasing soil friction angle, anchor edge distance from the crest, and the embedment ratio of the anchors. The critical edge distance of the anchor, beyond which the uplift factors remained almost constant with further increase in the edge distance, was been found to increase with the increase in ϕ, β, and d/b, and a negligible variation was observed for a change in the height of the slope.

The stability of tunnel roof is investigated in nonhomogeneous soils using the theory of plasticity. Based on the nonlinear failure criterion, the expressions of tunnel roof in nonhomogeneous soils are deduced, and the influence of pore pressure is taken into account. The soils are nonhomogeneous in the vertical direction only. Due to the complexity of the differential expressions obtained, a multilayer method is proposed that divides the soil mass into finite layers. Numerical results for tunnel roof stability are obtained through iterated calculation. The results show that the height of the collapsing block increases with increases in the variation of cohesion with depth, while its length increases with increases in the cohesion of the soil mass at the tunnel roof. The rate of increase of cohesion with depth plays an important role in the stability of tunnel roof.

The discrete element method (DEM) was used to investigate the coefficient of earth pressure at rest, K₀, of granular soils. The results indicate that K₀ decreases as the void ratio decreases and the vertical stress increases. The K₀ of specimens prepared by different methods may be quite different despite the void ratio and vertical stress being the same. The analysis reveals that at a particulate level, the coordination number of the soil determines the K₀. Both DEM simulation and laboratory data indicate that there is a good relationship between the K₀ and the small strain shear stiffness.

In the study described in this paper, a general particle dynamics (GPD) code was developed to simulate rock fragmentation by tunnel-boring machine (TBM) cutters in mixed-face ground. In the GPD code, a novel algorithm of the contacts at the boundary between two different media was proposed. The mechanism of rock fragmentation by two disc cutters and the effects of cutter spacing on rock fragmentation in mixed conditions were investigated. With increasing cutter spacing, the propagation pattern of Hertzian cracks varied and an optimal cutter spacing was found. The mechanism of rock fragmentation by a disc cutter and a drag bit and the effects of the acting position of different TBM cutters on rock fragmentation in mixed conditions were studied. The optimal acting position of the disc cutter and the drag bit was found to benefit the effectiveness of TBM rock fragmentation.

This study investigated the additional vertical stress (Δσ) change occurring under the centerline of a continuous footing on geotextile-reinforced soil. For different reinforcement parameters and conditions, load tests and finite-element model (FEM) analyses were performed. It was determined that the presence of reinforcements reduces the additional vertical stress values. There is a good correlation in some cases between experimental measurements and FEM results, but the values obtained are different from the values proposed based on theoretical considerations developed for uniform materials. Each additional reinforcement layer affects Δσ, and different parameters can affect the values of Δσ. It was determined that the reinforcements create a discontinuity in the triangular failure wedge under the footing, thereby affecting the additional stress developed.