International Journal of Geomechanics

Abstract

Studies about the strain-softening behavior of unsaturated soils published in the literature during the past three decades are summarized under three categories; namely: (i) mechanical characteristics and micromechanisms, (ii) prediction models for shear strength, and (iii) numerical methods for modeling strain-softening behavior of unsaturated soils. In addition, the influence of the soil–water characteristic curve and time effects on the strain-softening behavior of unsaturated soils are discussed. Various experimental studies related to the strain-softening behavior of unsaturated soils are summarized to interpret the mechanical behavior characteristics and micromechanisms of the strain-softening under large shear deformation. The widely used empirical/semi-empirical prediction models from the literature for interpreting the peak, critical, and residual shear strength of unsaturated soils are comprehensively summarized considering the influence of soil fabric and water phase on the shear strength. Several numerical methods (i.e., conventional plasticity, bounding surface plasticity, disturbed state concept, and elasto-viscoplasticity) of modeling the strain-softening behavior of unsaturated soils are discussed, highlighting their strengths and limitations. The comprehensive details summarized in this paper related to the strain-softening behavior is valuable for the rational analysis and design of geostructures in unsaturated soils that undergo large shear deformation.

Practical Applications

In several scenarios, unsaturated soils exhibit strain-softening behavior when they undergo a large deformation during the shearing stage. In other words, there is reduction in the shear strength from a peak to a residual value associated with the evolution of a shear band where the dilation is localized. Shear deformations behavior in geo-infrastructures constructed in unsaturated soils are sensitive to the wetting conditions associated with rainfall infiltration. Therefore, shear strength in the geostructures varies between peak and residual value according to the magnitude of corresponding shear deformation. For this reason, the strain-softening behavior should be considered for reliable analyses and design of geo-infrastructure in unsaturated soils undergoing large shear deformation. This paper provides a comprehensive review of the studies about the physical mechanisms and numerical modeling methods of strain-softening behavior of unsaturated soils. The contents summarized in this article assist geotechnical engineers to understand how the strain-softening occurs in unsaturated soils under shearing, and how the shear strength reduction due to strain-softening behavior can be modeled in engineering practice applications.

Abstract

This study attempted to improve the understanding of the effect of mechanical mismatch on the shear behavior of an interface or discontinuity between different rocks under direct shear. First, general and simplified analytical solutions for the shear stress (τ) distribution on a rock interface or discontinuity were determined. Subsequently, the τ distributions on the rock interface that were acquired from the analytical solutions were demonstrated via numerical simulations. Finally, the effect of mechanical mismatch between the rocks on τ distribution, shear strength, and potential location of crack initiation at the interface was investigated. The results indicated that it was difficult to widely use and promote the developed general solution. This is due to the lengthy expression for the solution and the complexity that was involved in the determination of the integration constants. The simplified solution omits the coupling effect between the interfacial τ and normal stresses (σ). In addition, it could accurately describe τ on a rock interface. The internal moment that was produced by shear forces (P) could significantly affect the τ distribution on a rock interface. Nonuniformly distributed τ is observed on the interface between different rocks and thicknesses. Furthermore, the nonuniformity increased with the increase in the mechanical or thickness mismatch. The τ and its concentration result from the loading of P. The noncoaxial P could produce compression and bending effects. The τ primarily results from the compression effect, and the bending effect affects the τ concentration. Cracks would not initiate simultaneously from both ends of the interface due to the dissimilar τ concentrations. For the sinistral sense of shear, cracks would first initiate from the right interface end if Young’s modulus of the upper rock (E₁) is smaller than that of the lower one (E₂). The results could guide a profound understanding of the shearing behavior of the rock interfaces or discontinuities between different rocks.

Practical Applications

This study presented general and simplified analytical solutions to calculate the τ distribution on the interface or discontinuity between different rocks that were subjected to direct shear. The accuracy and reliability of the analytical solutions were validated against a series of two-dimensional (2D) finite-element models (FEM) of direct shear. The τ was distributed nonuniformly and concentrated asymmetrically on the interface between different rocks. The τ concentrations were different at both ends of the interface. The difference increased with the increase in the mismatch in Young’s modulus (E) between the rocks on either side of the interface. The mechanical mismatch between the rocks could influence the τ distribution, shear strength, and potential crack initiation location at the interface. The results could be important to better understand the shear behavior, strength characteristics, and fracture morphology of the interfaces between different rocks. Furthermore, these could further guide disaster prevention and stability evaluation of rock engineering.

The concrete cutoff wall is the main seepage control measure for concrete-face rockfill dams (CFRDs) constructed on alluvium foundations. The cutoff walls are concealed structures, their working behavior influenced by alluvium foundation deformation and the seepage effect as critical factors. Cutoff wall behavior directly affects the foundation seepage control effect and the dam’s structural safety. The behavior of concrete cutoff walls in CFRDs built on alluvium foundations was investigated in this study through statistical analysis and numerical simulation. The cutoff wall behavior was statistically reviewed based on deformation data from several case histories. A numerical calculation model for the mechanical behavior of the cutoff wall, considering the foundation seepage–creep coupling effect, was established. A nine-parameter creep model was adopted to simulate the foundation creep behavior. The foundation seepage–creep coupling effect was simulated using a hydromechanical coupling analysis method. The cutoff wall behavior was analyzed using a combination of measured data and numerical results. The effects of the foundation seepage–creep coupling effect, dam construction speed, cutoff wall construction sequence, and depth of the cutoff wall behavior are also discussed.

High pressure and high temperature (HPHT) oil and gas pipelines, the burial depths of which are determined by the mitigation requirement of upheaval buckling deformation or legislation requirement, are commonly buried below the ground surface. Uplift resistance of soils surrounding the buried pipeline should be considered into the design of upheaval buckling behavior. Conventional design guidelines and current analytical models based on either dry soil or fully saturated soil are generally applied to predict the soil uplift resistance behavior. Therefore, the HPHT pipelines are mostly buried in unsaturated onshore soils. This paper presents a series of small pipe–soil interaction experiments for measuring the reaction that is acted on the buried pipelines in unsaturated backfill soils, and its purpose is studied to evaluate the impacts of soil moisture content, pipeline burial depths, and tension loading levels on the soil uplift resistance behavior. A new simplified formula based on the uplift experiment results is specially established to predict the uplift resistance of soils. Further, the uplift failure mechanism and pipe–soil interaction behavior are effectively applied to predict the upheaval buckling behavior of the pipelines buried in unsaturated soils.

A series of triaxial tests were carried out on two types of coral sand to study the inherent anisotropy behavior under triaxial stress conditions. Notably, monotonic drained and undrained tests and cyclic drained tests were conducted. At the same time, the effect of inherent anisotropy on particle breakage and the relationship between the relative breakage and the input energy were investigated. The test results show that the coral sand demonstrates very evident anisotropic behavior under triaxial stress conditions. In the monotonic tests, the effective peak friction angle and compressibility of coral sand decrease with the depositional angle, and the anisotropic behavior is more significant at low confining pressures. The axial and volumetric strains of the low-depositional-angle specimens increase faster with the number of cycles in the cyclic drained tests. In addition, the relative breakage increases with the confining pressure but decreases with the depositional angle. A unique relationship exists between the relative breakage and the input energy for the specimens with the same depositional angle, but the relative breakage decreases with the depositional angle at the same input energy.

How to generate slip surfaces efficiently is crucial for the stability analysis of slopes. Traditional approaches for generating slip surfaces rely directly on the geometric features of slopes, which makes it complicated to generate arbitrary slip surfaces. Moreover, the resulting surfaces are usually not smooth in many cases. This study proposes a method for generating an arbitrary slip surface utilizing the superposition of the integral of the logistic function. This technique can transform the traditional geometry problem into a parameter selection one, in which three groups of parameters in the function determine the shape of the slip surface. The higher-order continuity and differentiable property of this function ensure the smoothness of the slip surfaces. The paper investigates the impact of relevant parameters on the slip surface’s pattern and provides the upper and lower bounds of the control variables. This method further develops a procedure for producing arbitrary slip surfaces. The factor of safety and the critical slip surface are calculated using the Morgenstern–Price method and the biogeography-based optimization. After constructing the slip surfaces by this method, two cases in Association for Computer Aided Design show that this method is applicable and verify its calculation accuracy by comparing the factor of safety and the critical slip surface. The results indicate that this approach presents a new direction for the slip surface structure, which can be a general method extending from two to three dimensions.

The diaphragm wall is a common structure in geotechnical projects and is placed in the soil to control different flows. Considering that the soil environment is a sulfate environment, the materials of this wall (plastic concrete) are exposed to sulfate attack. The proximity of the sulfate environment and plastic concrete leads to a loss of efficiency of the diaphragm wall. As a result, waste products, such as leachate, pass through the diaphragm wall and contaminate other parts of the soil, or subsurface water resources are scattered in different layers, and it becomes impossible to exploit them. In this research, nanosilica has been added to plastic concrete mortar to improve the mechanical properties and permeability of plastic concrete against the sulfate environment. Therefore, the mechanical properties and permeability of plastic concrete without and with nanosilica (1.5, 3, 4.5, and 6 cement weight replacement) at different ages (7, 14, 28, 56, and 90 days) have been investigated and finally compared with similar results of plastic concrete in the typical environment. In this research, the results of uniaxial and triaxial compressive strength tests and the elastic modulus were studied, to determine the mechanical properties, and the permeability coefficient was obtained by performing a triaxial permeability test. Scanning electron microscopy and X-ray diffraction analysis were also used for microscopic inspection of plastic concrete specimens under sulfate attack. The results show that mechanical properties, such as compressive strength and elastic modulus, increase at an early age and deteriorate over the long term. The permeability coefficient was also significantly reduced, while the changes of elastic modulus were controlled. Finally, microstructural experiments showed a significant effect of nanosilica on specimen cavity filling, justifying the results obtained in mechanical and permeability experiments.

Today, extreme weather conditions occur globally due to climate change, and roads with traditional impermeable pavement could cause environmental problems, such as urban flooding and heat islands. Although porous pavement is an important environmentally friendly alternative pavement structure for roads, it has many disadvantages and is not yet widely used. Therefore, this paper reported the use of stone columns in the soil subgrade of porous pavement to improve the performance of roads. Soil property tests were conducted to evaluate the permeability of subgrade soil in a field area at different depths. Stone columns with different heights and various aggregate sizes were installed in the soil subgrade in the laboratory and the field, with the concrete footing on top of the stone columns as the bedding course of the pavement. Quantitative comparisons of the permeability of stone columns in the laboratory and the field were undertaken. In addition, the temperature changes in the stone columns and concrete footings before and after watering were tested to investigate the influence of the stone columns on the thermal behavior of roads. The test results showed that stone columns improved the water drainage in porous pavement and contributed to temperature reductions in urban roads. Therefore, the use of stone columns is recommended in porous pavement to improve water drainage and mitigate urban heat islands.

Wide-grading particles on loose soil slopes will experience scouring erosion under the coupling of runoff and seepage. In this paper, the particle critical incipient velocity was calculated based on an analysis of the rolling dynamics, and a runoff-seepage coupled nonlinear mathematical model was established. In the model, the Navier–Stokes equation and the Brinkman–Darcy equation were used to describe runoff and seepage motion, respectively. The expressions for the interface velocity and safety factor of the incipient particle motion were obtained by introducing a stress-jumping boundary condition where the velocity is continuous and the stress is jumping, with an incipient motion probability distribution. An indoor flume-scouring experiment on the wide-grading gravel particles was conducted, and the absolute errors of the particle incipient probabilities between the theoretical and experimental results were found to be less than 0.14. Furthermore, the existing theoretical model is a special case of the present model when the jumping coefficient is equal to 0. The parameter analysis results show that increasing the runoff depth, jumping coefficient, slope angle, porosity, and permeability aggravates the erosion of wide-grading particles, while increasing the equivalent diameter, unit weight, and exposure angle has the opposite effect. This study can provide guidance on wide-grading soil erosion due to runoff and seepage.

A series of model tests were carried out to determine the effects of reinforcement type and footing location on the performance of two-tiered mechanically stabilized earth walls (T-TMSEWs) under strip loading. By preventing and allowing the footings to tilt, the influence of the degree of footing freedom was also examined as the third variable. Particle image velocimetry showed that the use of a two-tiered configuration in mechanically stabilized earth (MSE) walls and a decrease in the soil–reinforcement interaction and stiffness changed the slip surface geometry and prevented the development of deep slip surfaces in the lower tier. It was found that although the decrease in reinforcement stiffness and its interaction with soil decreased the bearing capacity of the strip footings on two-tiered MSE walls, it also reduced the lateral displacement of the T-TMSEWs under footing loading. Also, allowing the footing to tilt was found to be an effective solution for minimizing the deformation of the backfill surface and the induced lateral pressure. Moreover, comparison of the results with analytical methods showed that the construction of MSE walls in two-tiered configurations reduced the lateral pressure in the upper tier. This became more noticeable with a decrease in the soil–reinforcement interaction and reinforcement tensile stiffness and an increase in the distance from the footing to the wall crest.

Antidip bedding rock slopes always contain a developed set of joints that dip into the free surface; however, sometimes they have some gently inclined cross joints, and the persistence ratio (k) of these cross joints exerts important influences on the stability and failure modes. In this work, a limit-equilibrium model combined with a fracture mechanics (FM) method was established to predict the stability of antidip bedding rock slopes with very low-persistent cross joints. Rock layers that might undergo toppling failure were analyzed with the theories of Mode I fracture in FM; layers with the potential for shear–sliding failure were studied by solid mechanics and theories for Mode II fracture from FM. Then, a practical slope and a model test (reported in previous studies) were used to verify the feasibility of the proposed method. The calculated results were consistent with the actual state of the slope and the experimental result, which indicated that a limit-equilibrium model combined with an FM method could be used to predict the stability of such slopes. To further approve the rationality of the method, a contrastive analysis was conducted that used two other methods. The results showed that the predicted factors of safety (FS) of the proposed method were conservative and on the safe side. The proposed method could be used by engineers to evaluate and design such rock slopes.

A novel micromechanics-based model in brittle rocks is presented during the confined tensile fracture. This model is formulated by using the stress intensity factor of crack under distinct loading modes in the fracture mechanics theory. The confined tensile fracture mechanisms with and without initial crack friction are established. The development process from the confined tensile fracture without initial crack friction to that with initial crack friction is studied. The stress–strain constitutive relationship describing the transformation mechanism from the unloading of axial stress at a compressive state to the loading of axial stress at a tensile state is also studied during the confined tensile fracture. The tension–compression transformation mechanism codetermined by the friction of the initial crack, the angle of initial crack inclination, and the confining pressure is proven. With the decrease of the angle of initial crack inclination or the increase of the confining pressure, the confined tensile fracture constitutive model tends to be dominated by the tensile fracture mechanism with initial crack friction. The confined tensile fracture of brittle rocks tends to happen at the larger angle of initial crack inclination. The reasonableness of the presented model is checked by contrasting the experimental data. Effects of the friction, density, inclination angle and size of the initial crack, and the confining pressure on the tensile relation curve of strain and stress, the tensile relation curve of crack length and stress, the tensile elastic modulus, the tensile stress of crack initiation, and the tensile strength are discussed.

Abstract

A variational method, using a lateral force as a functional and incorporating equilibrium in-the-small, is proposed for limit equilibrium analysis, in which a conventional safety functional in the form of a quotient and the need to use the method of slices or Lagrange multipliers can be avoided. The lateral force is a real force acting on a retaining wall, but is a fictitious force in slope stability analysis. Furthermore, the fictitious force is related to the factor of safety. The force is a push, pull, or null when the slope is unstable, stable, or critical. By setting this lateral force to zero, a critical stability state can be obtained. The proposed variational method is capable of reproducing the classical solutions and yielding new useful analytical results. The proposed method can be a viable alternative technique, because of its effectiveness and ease of use is comparable to the conventional variational approaches.

Practical Applications

The limit-equilibrium analysis technique is widely adopted in the practice of slope stability and earth pressure on retaining walls. Among the existing techniques in limit-equilibrium analysis, the variational calculus method from a mathematical perspective is more accurate than traditional techniques. However, this method was greatly limited due to the cumbrous solution on the quotient form functional. The proposed method uses a lateral force as functional, incorporating equilibrium in-the-small. It can simplify the solution significantly and the assumption of slip surface or relation of forces can be avoided. Results obtained from the proposed method can be used in the design of clay slopes, seismic stability of cohesionless slopes, lateral earth pressure on retaining walls under seismic conditions, and lateral earth pressure on rough retaining walls. The charts can be used to conveniently assess the stability of slopes or earth pressure on retaining walls under certain conditions in the preliminary design. The suggested method can also be further developed into a broader range of practical applications.

Intelligent prediction of rock bursts has great significance in rock mechanics research and a high value in engineering applications. An intelligent rockburst prediction method based on a Bayes-optimized convolutional neural network (BOCNN) was proposed. First, an exploratory analysis of data was conducted using joint distribution diagrams and the heat map of the correlation matrix to establish a high-quality data set of rockburst engineering cases and a parameter system for rockburst prediction. Second, six rockburst prediction models were built by combining machine learning algorithms, such as random forest, k-nearest neighbor (KNN), and Bayes, deep learning (CNN1d and CNN2d), and BOCNN. In addition, we used accuracy, precision, recall, F1 score, receiver operating characteristic curve, Taylor diagram, and the probability indicator of prediction results as indicators to evaluate the accuracy of the models. A comparative analysis of the six rockburst prediction models was conducted to explore rockburst prediction models with good robustness, generalization performance, and high accuracy. Moreover, 11 comparative models were established for comparative analysis. Then, we used the MATLAB tool to build an intelligent rockburst prediction system and applied the findings to the Jiangbian Hydropower Station in Sichuan Province, China. The results of the study show that the intelligent rockburst prediction system can provide technical support for predicting rockburst hazards in mining, transportation, and water conservancy and hydropower projects and a scientific basis for later construction and the design of support structures.

The performance of geogrid reinforcement is largely influenced by the interlocking effect between the geogrid and soil. To enhance the performance of geogrid reinforcement in sand, a novel biocementation-enhanced geogrid reinforcement method was proposed. Biocementation could be carried out in situ via microbially-induced carbonate precipitation (MICP). To understand the mechanisms of the biocementation enhancement effect, the stress–dilatancy behavior of coarse sand (CS) that was treated with the biocementation-enhanced geogrid reinforcement method was studied through a series of triaxial tests. The test results showed that the interlocking effect between the geogrid and CS could be effectively improved when the CS around the geogrid was biocemented by the calcium carbonate (CaCO₃) that was generated in situ through MICP. The stress–dilatancy behavior of geogrid-reinforced CS was significantly enhanced after biocementation. In addition, the interlocking effect that was provided by the geogrid could mitigate or even avoid brittle failure, restrain the further development of dilation, and enhance the mobilization of the friction strength (q_f) of the biocemented coarse sand (Bio-CS) during shearing.

In this research, we used a slip-line method for a typical failure mechanism simulated with adaptive finite-element software to investigate the active earth pressure of narrow and layered backfills against inverted T-type retaining walls rotating around the base mode. The slip-line field calculation models for the inverted T-type retaining wall with narrow and layered backfills were established considering the effect of the heel bottom plate and the inverted T-type retaining wall characteristics. Subsequently, the limit equilibrium and finite difference methods were used to solve the stress state of each point. The earth pressure acting on the stem and the second failure surface can be obtained by converting the boundary conditions. The proposed method was verified by comparing its results with those from finite-element limit analysis and existing theoretical solutions. Furthermore, several extensive parametric studies have investigated the effects of backfill width, heel length, and interface strength. The results show that increasing interface strength and decreasing backfill width help reduce the earth pressure against the stem and second failure surface. The change in the length of the heel bottom plate exhibited a dual effect.

Reasonable estimation of the bearing capacity of bulbs is essential for the design of squeezed branch pile (SBP) foundations, and existing analytical methods cannot be applied to SBPs. This study investigated the bearing capacity of bulbs by model tests, field tests, and theoretical and numerical analysis. From the laboratory model loading test and transparent soil model test assisted by particle image velocimetry technology, the bearing features of SBPs and the displacement field around a bulb under failure state were investigated, respectively. It was found from the tests that the bulb’s bearing ratio is up to approximately 50% out of the total applied load to the pile at failure. Furthermore, by referring to Meyerhof’s theory of bearing capacity for deep foundations, an analytical method was proposed to calculate the bearing capacity of a bulb for an SBP considering its unique geometry. Finally, field tests with a real-size SBP and numerical study were conducted to verify the proposed method. The ultimate bearing capacity (UBC) of the bulb estimated by the proposed method has an acceptable difference of 4.21% from the model test and 17.6% from the numerical field cases. The proposed solution can be used as an effective explicit solution to evaluate the UBC of the single bulb of SBPs considering the pile’s dimensions and soil parameters.

For reproducing the heterogeneous structure of granites, a novel three-dimensional grain-based model based on particle flow code (GBM3D-PFC) was proposed in this paper. Then, dynamic semicircular bending tests under different impact velocities were conducted on the numerical samples with different grain sizes by the coupled split Hopkinson pressure bar system. On the one hand, with increasing grain size, the dynamic flexural tensile strength and proportion of transgranular cracks increase, while the total crack number decreases. On the other hand, as the loading rate increases, the dynamic flexural tensile strength and transgranular crack proportion increase. Based on the GBM3D-PFC, the evolution of the load applied to the sample can be reasonably described. The relationship between the contact force network and the sample’s strength can be established. The microcracking behavior obtained by numerical simulations can be used to explain the variation of the macroscopic mechanical parameters.

In nature, soil layers possess large variability during their geological process. This variability may also lead to differences in the location and thickness of nonliquefiable and liquefiable soil layers. In practice, the impact of liquefaction on the pile can be ignored at depths greater than 20 m due to high confining stress levels and a lack of liquefaction triggering data. On the other hand, this approach may underestimate design loads in many cases, especially in deep-seated and embedded engineering structures. This paper presents the bending response of piles installed through liquefiable layers located at depths beyond 20 m, and parametric analysis was conducted for a wide range of liquefiable layer thicknesses and depths by using OpenSeesPL (version 3.0.2) software. The numerical results were evaluated considering the inelastic concrete pile behavior under different earthquake records and different peak ground accelerations. The findings show that liquefaction can lead to a failure of piles even at depths greater than 20 m, and thus, a design consideration of piles may require a more comprehensive view considering the liquefiable layer depth and thickness effect.

Steel piles are widely used in deep foundation systems to transfer significant structural loads to strong soil layers. A steel pile may buckle or fail by forming multiple plastic hinges at different locations. The pile’s plastic hinge distributions and failure mode are considerably affected by soil–pile interactions (SPIs). A proper consideration of SPIs is essential for the successful design of a steel pile foundation. In practice, empirical calculation or numerical modeling analysis methods are commonly utilized. Nevertheless, the conventional design methods are cumbersome in the modeling process and may be inaccurate in considering SPIs. This paper develops a new integrated soil–pile element to explicitly reflect SPIs and capture steel piles’ inelastic behaviors based on the plastic hinge method and the Euler–Bernoulli beam theory. Two virtual nonlinear springs, the concentrated rotational and distributed lateral springs, are used to simulate the plastic hinge deformations and the soil responses, respectively. The rotational and lateral springs are assigned at both ends and the Gauss integration points of an element, respectively. The Gaussian quadrature method is utilized to consider the soil reactions and generate the soil stiffness matrix. Based on the numerical integration and condensation methods, the virtual springs are directly integrated into the element formulation, simplifying the programming and practical modeling process. The updated Lagrangian approach is adopted for describing the element kinematic motions. The element tangent and secant relations between nodal forces and displacements are derived based on the potential energy principle. Several verification examples are presented to demonstrate the capability of the proposed element.

Rock bolting is one of the most effective and economical methods for jointed rock mass reinforcement. Therefore, the elastic constitutive relationship in an anchored rock mass, which could well reflect the rules of strength and deformation, should be studied. An orthotropic elastic model, which considered varying anchorage angles (α) and the corresponding mechanical parameters, was investigated based on the theory of mechanics of composite materials. To verify the suitability of the model and the mechanical parameters, a WAW-3,000 kN servo universal hydraulic testing machine was used to undertake a series of uniaxial compression tests on the anchored layered rock specimens. The mechanical parameters of composite specimens, such as elastic modulus (E), Poisson’s ratio (ν), peak strength, and its variation with α were obtained. The results showed that the anchored effect of a bolt on the layered rock mass was very significant and anisotropic, for instance, increments in the elastic parameters in three orthogonal directions were quite different with the increase in α from 0° to 90°, which mainly depended on the relative volume content of the component materials in a certain direction. Comparing the calculated E with the experimental values, the variation rules for the mechanical parameters maintained good consistency, and this proved that the calculated equations in this work were reasonable and had acceptable precision. The spillway slope of a reservoir was simplified and simulated and the optimal α was obtained, which further demonstrated the reasonability and applicability of the proposed model from the viewpoint of engineering practice.

The wear of the disc cutter results in an increase in contact area with the rock, so when the wearing of the cutter exceeds its maximum limits, it should be replaced. However, this is not done on time because of cost concerns. Therefore, this study will investigate the effects of disc cutter wear on tunnel boring machine (TBM) operational parameters such as thrust force, torque, penetration rate, and specific energy as well as how it impacts advance rate and project time in hard rock TBMs. Finite-element modeling has been performed on 17″ disc cutters, calibrated with the results of linear cutting machine test (LCM). The wear values varied within the range of 0–20 mm and different penetration depths. Numerical results of normal cutting forces are compared with the previously suggested theoretical model, which shows good agreement. According to numerical models, normal and rolling forces are significantly increased as disc cutter wear increases. Increases in vertical and rolling forces are 130% and 75%, respectively. Changes in the side force on the disc cutters indicate that with the wear of the cutters, the result of the side forces is asymmetric and from the inside of the cutter. It causes the bearing system of the disc cutter to fail. The intensity of these asymmetric forces decreases as the installation radius of the disc cutter increases. Further, the wear of disc cutters significantly increases the specific excavation energy, resulting in a significant reduction in excavation efficiency. Finally, disc cutter consumption information for the Kani Sib water conveyance tunnel is also provided, which confirms the findings.

Abstract

The stress-induced fracture of brittle rocks, as a result of macrocrack evolution, is closely related to the evolution of microcracks. The study of such damage processes provides information about the mechanical behavior of rock cracks. In this study, we conducted research with respect to macrocracks using hypothetical damage regions constituted by correlated microcracks. A Gaussian mixture model was applied to describe the spatial distribution of microcracks. The Kullback–Leibler divergence was used to characterize the geometric variation of damage regions. The results showed that the robustness of the damage region’s geometry became increasingly higher during the damage evolution and the damage region became unchanged after some time. The robustness of the damage regions could be an indicator of the nucleation of macrocracks. Moreover, a fracture nucleation indicator methodology was developed to calculate the point at which nucleation was formed. This study is considered to enhance the understanding of macrocrack nucleation and it is useful to the application of macrocrack recognition and prediction.

Practical Applications

This study presented an investigation on the robustness of the damage regions associated with macrocracks. The evolution of calculated damage regions can be considered as a process of macrocrack nucleation. The geometric variation of damage regions was studied. Results showed that the geometry of damage regions became increasingly stable during microcracking. A methodology named fracture nucleation indicator was proposed to define the fracture nucleation point, whereby the damage regions showed great robustness. This study was considered to enhance the understanding of a fracture’s nucleation process and the proposed fracture nucleation indicator was a successful approach to quantitatively define macrocrack nucleation time.

In this study, scanning electron microscopy, mercury intrusion porosimetry, and flexible-wall permeability tests were performed on the remolded loess (RL) and the 15% bentonite-modified loess (BML) after wetting–drying cycles. Crack propagation, microstructure damage, pore-size distribution, and saturated permeability were evaluated as a response to wetting–drying cycles. The cracks grow in three stages: crack initiation, rapid crack development, and attenuation. The loess is dominated by linked aerial pores and intergranular pores. With wetting–drying cycles, the number and diameter of intergranular pores grow, and additional aerial pores are generated from intergranular pores. When bentonite is added to the loess, the pore-size distribution curves shift from a trimodal to a bimodal shape. The porosity of samples grows in a declining increment that corresponds well to the increase in the permeability coefficient. Because of the swelling and filling effect of bentonite particles, as well as pore compression at higher confining pressures, the seepage channels are blocked. The permeability of BML after multiple wetting–drying cycles meets the antiseepage requirement of compacted clay liner of landfill that the permeability coefficient should be less than 10⁻⁹ m/s.

Grouting reinforcement methods are usually employed in shield launching sections due to shallow burial depths and poor stratigraphic conditions. Hence, grouting reinforcement provides an increase in buried depth and better formation conditions. The length of prereinforcement is often determined by engineering experience. Excessive ground settlement, collapse, and even buried machine accidents may occur when shield tunneling occurs at the boundary between prereinforced and unreinforced areas. The case of the collapse of the composite stratum at the boundary between prereinforced and unreinforced areas near the shield launching area of the Shumuling shield tunnel in Changsha was analyzed in combination with monitoring data around the collapse area. Based on the slip-line theory and limit analysis method, the evolution pattern instability of the tunnel face soils around the collapse area was analyzed by means of FLAC3D5.0. This model provides a failure mechanism used to analyze the stability of the tunnel face of a shallow circular shield tunneling through the composite stratum at the boundary between prereinforced and unreinforced areas. The limit-supporting force that maintains the stability of the tunnel face was calculated. The rationale behind the proposed failure mechanism was validated by comparing the calculated value of the limit-supporting force with the monitoring value in the chamber of the earth press balance shield machine (EPB). Based on the proposed failure mechanism and considering the influence of the surrounding environment (nearby railroad), a reasonable reinforcement method was selected, and the design reinforcement scheme for the soil in the collapse area was optimized. The research results are expected to provide useful guidance for the stability analysis and safe construction of shield excavation at the boundary between prereinforced and unreinforced areas near shield launching areas.

Deeply excavated expansive soil canal slopes can have instability risks during operations. The deformation mechanism is analyzed using internal and external factors, such as engineering geology, hydrogeology, rainfall, and groundwater levels in the middle expansive soil canal slope in the central line of the South-to-North Water Diversion Project, Taocha District, Nanyang City, Henan Province, China. The stability of the canal slope and the effect of the reinforcement and disposal measures are evaluated with a geological survey, manual inspection of the defects, safety monitoring data analysis, hidden danger geophysical detection, and stability numerical simulation. The following conclusions are drawn: (1) the surface-refilled cement-treated soil failed to isolate the water vapor exchange between the expansive soil and the atmosphere; and (2) the stagnant water in the upper layer is replenished by precipitation, leading to the increase in the groundwater level during the rainy season and the decrease in the groundwater level during the dry season. Shear creep deformation occurs along the fissure of the second to fourth grade canal slopes. The potential sliding surface consists of a gentle dip angle leading edge and a steep dip angle trailing edge. The arrangement of the antisliding piles on the water-passing section and the crack surface distribution had an effect, and the potential shear outlet is in the first grade berm, the deformed body is still developing, and the trailing edge is not apparent. The potential failure mode of the slope is deep sliding, and the current safety factor is 1.002. A combination of groundwater discharge and micropile reinforcement is required to improve the stability of the canal slope. After the drainage and reinforcement are implemented, the slope’s deep antislide stability safety factor increases to 1.462, which met the safety requirements. The results could provide a reference for the operation management and reinforcement governance of similar projects.

The properties of rock vary with spatial location, and spatial variability is a critical source of uncertainty that affects the safety of rock engineering. However, the key parameter characterizing spatial variability—the scale of fluctuation—is rarely studied in rock engineering. One of the reasons for this phenomenon is the lack of a simple and practical test method to determine the scale of fluctuation of rock. This deficiency hinders the development of reliability-based design in rock engineering. An evaluation method of the scale of fluctuation in rocks based on the compressive wave velocity of drilling cores is proposed to address this problem. First, drilling cores at the Yingliangbao hydropower station were collected, and the wave velocities were measured based on ultrasonic tests. Then, three different statistical methods, namely, the method of moments, maximum likelihood estimation, and Bayesian analysis, were used to evaluate the scale of fluctuation in rocks based on measured wave velocities. Results show that the optimal autocorrelation models that describe the autocorrelation structures of Yingliangbao granite are a single exponential function or cosine exponential function, and that the corresponding scales of fluctuation of Yingliangbao granite range from 6.33 to 12.44 m. In addition, the stationarity of the wave velocity is verified by the modified Bartlett test, and the scale of fluctuation based on in situ ultrasonic tests and the treatment of wave velocities at joint locations is discussed. This study presents a practical and reliable method to evaluate the scale of fluctuation in rocks and provide a reference for reliability evaluations in rock engineering projects.

The variation of clay undrained shear strength with consolidation stresses is of paramount importance to geotechnical engineers. For several decades, the normalized shear strength of clays has been the cornerstone of geotechnical design in many applications, such as staged construction of embankments. Recently, different trends of shear strength dependency on consolidation stresses have emerged indicating nonlinear variation. Resedimentation has been a useful and reliable method to investigate stress dependency by generating lab test specimens of fine-grained soils with a minimum sample to sample variation for benchmark studies. Previous work on resedimented Nile silty clay (RNSC) has shown unique features, such as high sand fraction and contradicting index properties used for soil classification. The shear strength of Nile silty clay (NSC) and the effects of the consolidation stresses are explored through a laboratory program on resedimented samples from material excavated within the Nile River flood plains. A portion of the material was processed using mechanical energy, which exhibited a significant increase in the liquid limit and a larger clay-sized fraction of particles from grain-size distribution tests. The measured engineering parameters are compared with comparable studied soils and established empirical correlations to provide insights on mechanical behaviors of the NSC. NSC is found to have a rapid decrease in the undrained shear strength ratio and a rapid increase in the lateral stress ratio with increasing consolidation stress.

This work reassesses the state parameters (ψ) inferred from in situ cone penetration testing (CPT) using normalized tip resistance (Q_p) from the Rose Creek tailing impoundment in Canada that was previously introduced in the literature. For this reassessment, a recently proposed rescaling equation is used to modify the Q_p−ψ trendline calculated using CPTwidget software (v2.5). These reassessed ψ values are compared with independent values calculated from in situ gravimetric water content (ψ_GWC) and the critical state line (CSL) measurements. A good agreement between the inferred ψ and the in situ sample ψ_GWC is obtained using the rescaling equation.