International Journal of Geomechanics

The wasted iron tailings have poor engineering properties. Using polypropylene fiber and cement to modify iron tailings and in road engineering is an efficient way to utilize iron tailings as a resource. Dynamic triaxial tests were used to investigate the cumulative deformation and damage evolution of fiber cement–modified iron tailings (FCITs) with different fiber contents (W_f) and dynamic static ratios (R_cr) under cyclic load. In a study of the cumulative deformation of FCIT, it was found that increasing R_cr results in the cumulative deformation of FCIT. However, adding fibers to FCIT could minimize cumulative deformation by up to 70%. Furthermore, with an increase of R_cr, FCIT gradually changed from plastic shakedown to plastic creep and finally realized the deformation behavior of incremental failure. A plastic shakedown creep prediction formula with an error within ± 10% could be established according to the deformation behavior. The plastic shakedown creep prediction model could be further developed, and it was found that FCIT has the best deformation resistance when W_f = 0.75%. In the study of FCIT damage evolution, it was observed that adding fibers can improve the failure form of iron tailings under IF (incremental failure) conditions. The cumulative damage variable formula proved that the fiber significantly improves the cumulative damage of FCIT. However, when W_f > 0.75% and W_f < 0.75%, FCIT cumulative damage increased faster when W_f = 0.75%. W_f = 0.75% had the best effect on reducing FCIT cumulative damage.

The mapping of failure that uses acoustic emission (AE) is an advanced approach when tracking the location of cracks in rock materials under progressive stress. However, attenuation of the acoustic wave significantly affects the identification of the acoustic source. The isotropic attenuation model suggests a constant attenuation in all rock material directions, which provides unrealistic failure mapping for anisotropic rocks. This study investigated the impact of high attenuation on the mapping failure of a composite (sandstone–shale–sandstone), intact sandstone, and shale specimens that used AE under axile stress. The results proved that wave attenuation is the primary obstacle when obtaining a compatible failure map to the real specimen failure. Multiple setups of AE sensors were implemented to quantify the attenuation values for a wave that propagated across composite joints and shale bedding planes. The amplitude and energy attenuation (EA) values increased by 85% and 47%, respectively, when the bedding plane of the shale was from 0° to 90°, which reflected anisotropic behavior. The energy and amplitude reduction reached 99% and 39%, respectively, by propagating across a single joint. In addition, behind a double joint, no signal was received. However, the application of a load perpendicular to the joint interface improved the acoustical–wave characteristics. Therefore, when the rock material anisotropy was increased, there was lower accuracy when mapping specimen failure, because the attenuation varied at each angle between the source and sensor. In addition, the EA method suggested in this study presented hits location more precisely compared with the P-velocity (Vp) method. These findings could contribute to the development of an anisotropic attenuation model for field and laboratory application of AE.

Over the past decades, many advances have occurred for theories on large-strain nonlinear consolidation of soft soils with regard to traditional drainage boundaries, which are completely permeable and impermeable cases and which may not truly reflect the whole process of drainage boundary. Thus, some further considerations related to a time-dependent drainage boundary are adopted in the analysis of soft soil consolidation by incorporating biologarithmic compressibility and permeability models. Also, the analytical solutions are obtained by means of variable substitution and Laplace transform. Additionally, the proposed solution is degenerated and compared with the existing solution and the laboratory tests to validate its correctness and feasibility. Finally, the soil consolidation behavior is analyzed under different values of the interface parameter, nonlinear compressibility and permeability, and external loading. The results indicate that the drainage boundary permeability is largely determined by the interface parameter; the larger the interface parameter, the better the boundary permeability. Besides, the consolidation rate decreases with the increasing value of I_c(α − 2) when the external loading remains constant. While the parameter I_c(α − 2) is a constant value, the consolidation rate varies with the external loading. Therefore, appropriate parameter values are more conducive to the full consolidation of soft soil foundations.

On the premise of following the concept of culture heritage conservation, the technique of wooden bolt anchorage plays an important role in the conservation of rammed earthen sites. The existing researches on single anchoring cannot reflect the mechanical behaviors of an anchor group. To solve this problem, this study proposes a load-transfer method achieved by a universal joint combined with a load-bearing platform. Three rammed earth models were tested by continuous loading, with embedded length, bolt interval, and margin as variables. The failure mode, ultimate pulling resistance, load–displacement relationship, and interfacial strain distribution of the anchorage system were analyzed. Results show that the two bolts present sequential failure. Meanwhile, the failure of wooden bolts is not in a single mode; instead both interfacial debonding and soil-bearing capacity loss were observed in soil at the same time. The bolts are mostly in plastic-slipping state under working conditions. The strain is unevenly distributed at the bolt—grout interface and highest near the loading side. Spacing effect and marginal effect are important factors affecting the pulling resistance of two bolts, and their importance is higher than the quality of a single bolt. In addition, numerical simulations on behaviors of two wooden bolts, considering the interval distance, margin distance, embedment length, and layered property of rammed earth, were established by “interface” element, which supplemented and expanded the model test results. Results of this paper could provide deeper understanding of the macroscopic performance and internal mechanism of a two-bolts group.

Damage accumulation of coal caused by cyclic loading and creep impacts may ultimately lead to rock failure and dynamic disasters. Quantifying this behavior is crucial for evaluating the mechanical response under creep and cyclic loading processes. Here, creep tests were conducted after cyclic loading and unloading at 10 MPa confining pressure. The elastic modulus and Poisson’s ratio indicated fatigue damage during the entire loading process. The elastic moduli in the unloading stage were slightly higher than those in the loading stage. The gap in Poisson’s ratio decreased in the cyclic loading procedure. The axial and volumetric strains exhibited a slightly negative exponent trend. The stress–strain response indicated hardening and softening effects under constant axial stress. The hardening effect dominated the peak strength at lower axial stresses. The creep-hardening deformation was 0.17 and 0.21 under 88 and 100 MPa, respectively. However, the softening effect played a more significant role at higher applied stresses. Additionally, the peak strength decreased by approximately 2.7 MPa when the axial stress increased from 100 to 102 MPa. The coal near the mining-induced stress boundary experienced cyclic damage, and the creep stress gradually decreased. Thus, the mining-induced stress boundary evolution rate initially increased and was mitigated as the mining-induced stress reduced. Next, the boundary expanded outward and gradually stabilized until the applied loading did not lead to obvious rock failure. Finally, the mining-induced stress boundary in the entire gob may be flat during the long-term evolution. Considering the creep failure time and choosing a suitable time to determine the boundary are essential for preventing dynamic disasters.

Abstract

There has been a rapid development of the nuclear industry on account of climatic change caused by global warming. The most favorable rocky sites for nuclear power plants are exhausted worldwide, and now nuclear facilities are proposed on soil sites. In a soil site, a combined piled raft foundation (CPRF) system can provide the desired level of performance for a nuclear structure under seismic loading conditions. The combined system of the nuclear structure with the CPRF involves complex dynamic interactions between the pile, soil, raft, and the superstructure. In the existing literature, the dynamic behavior of the CPRF system supporting massive and stiff structures is not well studied and investigated. Due to the highly nonlinear dynamic characteristic of the CPRF system supporting massive and stiff reactor buildings, this paper adopts interface nonlinearities (separation and sliding) through the master and slave concept, concrete and reinforcement material nonlinearities through the concrete damage plasticity model for piles and superstructures, and an equivalent linear approach considering soil degradation and damping with Mohr–Coulomb postyielding hardening behavior for the soil. After the successful validation of the modeling approach with the results of the experimental testing, the different aspects of the CPRF of reactor building are investigated, such as amplification, pile and raft behavior, pile–raft coefficient, separation and sliding, contact pressure, nonlinearity in the piles, and the response of the superstructure. This study indicates that the peripheral piles of the CPRF system could prevent the separation of the raft from the soil. The numerical result shows that damage in the CPRF system occurs near the top portion of the pile. The presented procedure to develop the combined model for the seismic analysis in the time domain could help to formulate guidelines for safety-related nuclear structures resting on the CPRF system.

Practical Applications

Several countries are planning and building new nuclear reactors to meet their increasing demand for clean electricity. The most favorable rocky sites to construct nuclear power plants are exhausted worldwide; therefore, some upcoming nuclear reactors are supported on a soil site. The seismic safety of nuclear power plants is paramount because their failure may cause disasters. Hence, the problem of soil–structure interaction becomes vital for nuclear power plants (NPPs) due to the building of nuclear structures under unfavorable ground conditions. The present study describes the methodology to assess piled raft–supported NPP behavior under extreme earthquake events. The presented approach will be helpful to consultants and designers working in the nuclear field. The present study will help to formulate guidelines for the seismic safety of NPPs that rest on soil sites.

In land reclamation projects, the scheme of dredged clays with a multiplicity of horizontal drains is often adopted to shorten the vertical drainage path for seepage flow of pore water, but studies of consolidation theory on layered clay–sand reclamation are relatively inadequate. In this investigation, a novel semianalytical solution is obtained, applying the boundary transform technique based on the plane-strain consolidation theory. The correctness of the presented solution is investigated by comparing against results obtained from finite-element analyses and field tests. Subsequently, a detailed parametric analysis is conducted to explain how the consolidation efficiency is affected by geometric and physical parameters of soft soils and horizontal drains. It is revealed that the rate of consolidation increases with the horizontal permeability of dredged clay but decreases with the width of reclaimed land. For the effect of horizontal drains, the consolidation efficiency increases with the thickness, horizontal permeability coefficient, and layout number. Moreover, the optimal design method of layered horizontal drains is described in detail, and a figure considering the optimal thickness, layout number, and position of horizontal drains is provided as theoretical guidance for use in design.

This paper numerically examines the effect of foundation shape on the subgrade reaction coefficient. The distribution of the subgrade reaction coefficient on the foundation area and the values of this coefficient in different regions are studied. A method is also introduced to determine the subgrade reaction coefficient of unusually shaped foundations sometimes encountered in practice. It has been shown that it is possible to divide the foundation area into several regions, and the subgrade reaction coefficient of each part is a multiplier of the central point reaction coefficient. The reaction coefficient of a trapezoidal foundation is equal to the reaction coefficient of a rectangular foundation with a similar area, and the length of the rectangle is the same as the height of the trapezoid. Also, the settlements of two foundations are equal. An L-shaped foundation’s reaction coefficient is a product of multiplying the reaction coefficient of a rectangular foundation to a ratio of the rectangular area by an L-shaped area if the settlements of the two foundations are equal. Besides, the reaction coefficient of a ring-shaped foundation is a product of multiplying the reaction coefficient of a circular foundation with the same outside diameter by the ratio of the circular area to the ring area if the settlements of both foundations are the same.

Crack coalescence is an important topic in the research of brittle fracture mechanics. Most of the previous studies have focused on surface or penetrating cracks, while few experimental studies have been performed on internal cracks coalescence. This study fills in the missing data. First, the 3D internal laser-engraved crack (3D-ILC) method was introduced to make internal cracks in glass without causing any damage to the surfaces of the glass. Next, uniaxial tensile tests were performed on the samples with two coplanar internal penny-shaped cracks. A new grid reconstruction algorithm is also proposed to simulate the coalescence and the growth path of two coplanar internal cracks. The crack coalescence process and fracture characteristics were analyzed in detail. The results indicate that the fracture in the test is static fracture, while the fracture is Mode I, based on the characteristics of the fracture morphology. In addition, the Wallner lines were observed after the coalescence of cracks, which were overall elliptical in shape. Before coalescence, the stress intensity factor I (K_I) at the center was higher than that at the outside, indicating the attraction of the twin cracks. Then, after coalescence, the K_I increased significantly at that coalescence site, which indicated that the new crack had rapidly become “elliptical” in shape following the coalescence. The normalized K_II/|K_Imax| and K_III/|K_Imax| of the coplanar cracks were approximately equal to 0 in the simulation, and it was confirmed that the fracture in the test was Mode I.

Natural flaws containing fillings strongly affect the mechanical properties and failure process of rocks in practical engineering. In this work, uniaxial compression tests are performed on filled and unfilled red sandstone specimens containing two parallel dentate flaws. Two monitoring technologies––acoustic emission (AE) and high-speed camera––are employed to record the cracking process of the flawed rocks. The effects of filling and ligament angles on the mechanical properties of the flawed rocks are analyzed. The relationship between AE parameters and stress–strain curves is established in order to evaluate the cracking process. The fracture behaviors between the filled and unfilled specimens are qualitatively compared. An intensity analysis method, based on the historic index and severity, is employed to assess the damage level of the flawed rocks. Simultaneously, a judgmental design criterion, including safety, early warning, and instability, is developed, which can be employed to evaluate the stability of rock structures.

During the excavation of a foundation pit, differences in the stiffness of an enclosure structure with different supports may lead to different deformation modes of the enclosure structure. Without considering the effects of both pore–fluid and chemical reactions in the surrounding soils, the existing simplified analytical method is used to approximate pit sidewall deformation curves under four typical pit enclosure deformation modes, so that the horizontal displacement curves of a neighboring single pile are obtained by the image source method. Based on the results obtained from the simplified analytical method, it is concluded that both the maximum cumulative deformation value and the deformation pattern of a foundation pit enclosure structure can have nonnegligible effects on the horizontal displacement of the adjacent single pile. The maximum horizontal displacement of the single pile in the adjacent pit increases linearly with the control parameters of the pit enclosure structure, while it decreases nonlinearly with either an increase in the horizontal distance from the enclosure structure or a decrease in the excavation depth.

The philosophy of designing piles as settlement-reducing elements causes the number of piles required beneath a wide foundation to be significantly reduced compared with conventional design methods. In this study, the behavior of deep soil mixing (DSM) columns used as settlement-reducing elements under the foundation of circular liquid reservoir tanks with large diameters (more than 50 m) in sandy soils of the Persian Gulf coast under axial loading has been investigated using a parametric-based finite element method (FEM) analysis in a Midas GTS NX environment. The parametric study includes 1 case with no improvement and 48 cases of improved soil with various configurations. The loading conditions are similar to those of a standard liquid tank with a large diameter. Variables such as area improvement ratios, DSM column floating ratios, and cushion layer thickness have been investigated. It has been found that increasing the thickness of the cushion layer, the floating ratio, and the area improvement ratio each contributes a certain amount to the overall reduction of the total and differential settlement values. However, under certain conditions, some factors, such as the area improvement ratio, have been found to have a reduced improvement effect. These factors, and their influence on the whole system, have been thoroughly investigated numerically in the parametric study, and in the two validation cases, the results of the FEM analysis have been compared with measured data from the field and small-scale tests. The results of gathered data from field tests and parametric studies have also been compared with empirical formulas provided by other researchers.

This paper attempts to investigate the bearing capacity of a strip footing that rested on unsaturated homogenous sands and used upper bound finite-element limit analysis in association with linear optimization. The analysis is based on suction stress-based (σ^s) effective stress (σ′) analysis. Unlike the saturated soil, the size of the yield surface varies with the vertical position, and therefore, proper modifications are carried out in the upper bound formulations to incorporate the unsaturated soil property functions. A detailed investigation is carried out to estimate the load-carrying capacity of a strip footing that vary the mechanical strength, unsaturated soil properties, water table depth (h_w), and climatic conditions. The numerical solutions are presented in terms of bearing capacity factors [BCF (N_γs, N_qs)] for the unit weight (γ_w) and surcharge pressure (q_s). The nodal velocity patterns are drawn for several cases, and the presented solutions are compared with the available results. The design charts provided in this paper might be useful to design engineers.

The effect of soil density on the soil–water characteristic curve (SWCC) is becoming a topic of universal interest due to the heterogeneity of soils and environmental variables. In this study, a simple and effective model based on the idea of translating the particle-size distribution curve to the SWCC is proposed for predicting SWCC change with initial density. There is only one new parameter introduced, and it is easily calibrated using two SWCCs obtained from test data. The SWCCs for the same soil at different initial void ratios can be estimated using the developed model. Several existing models are also thoroughly examined, with an emphasis on the advantages and disadvantages of each model. The validity of the proposed model was then verified by comparing it to three other models and experimental data for eight different types of soils. The proposed model also outperforms other existing models in this extensive study, providing good and consistent prediction performance across various soils. The proposed model is then applied to different engineering challenges involving the estimation of bearing resistance of unsaturated soils. Three typical examples chosen for illustration in this paper are the effect of soil density variation on unsaturated shear strength, bearing capacity of a shallow foundation, and ultimate bearing resistance of an energy pile. The findings of the investigation reveal that the proposed model can be utilized to solve a variety of problems involving soil–structure interaction in unsaturated soils.

Abstract

The small pipe–roof–beam method has better adaptability to ultra-shallow buried subway stations. Five groups of similar model tests were carried out to explore the pipe–roof deformation, surface settlement, and beam strain caused by excavation using this method. Based on the test results, a 3D numerical model was established to study the pipe–roof deformation under different cases. The following conclusions were obtained: as the height-to-span ratio increases, the pipe–roof deformation, and surface settlement increase and, afterward, decrease. Increasing the number of beams can significantly reduce the pipe–roof deformation and surface settlement, and has a greater effect on the surface settlement. The soil-arching effect occurs above the pipe–roof during excavation, and is strengthened with an increase of height-to-span ratios. The difference between the pipe–roof deformation caused by excavation is within 2% in two cases: free at both ends and fixed at only one end. It is not recommended to add support to only one end in actual construction. Adding support at both ends can reduce the pipe–roof deformation by about 40%. The research conclusions have guiding significance for the development of the small pipe–roof–beam method.

Practical Applications

The development of subway construction has gradually become one of the themes of urban development. Finding ways to build subway stations quickly, economically, and safely has gradually become the focus of attention. The pipe–roof method is a method of constructing urban underground space, which is widely used throughout the world. However, there are many shortcomings in the existing pipe–roof method. This paper proposes the small pipe–roof–beam method that can save the construction period on the premise of ensuring safety, which is currently being applied to Shifu Road Station of Shenyang Metro Line 4 in China. In this paper, the law of surface settlement and pipe–roof deformation caused by excavation is obtained by combining the method of laboratory test and numerical simulation. The research can be the theoretical basis for the subsequent application of this method. The matching relationship between the pipe–roof and the beam will be investigated in the next study, which will provide guidance for the optimal design of subway stations.

An elastoplastic model that exhibits hybrid initial and stress-induced anisotropy is developed for layered rock masses. The formulation considers the thermodynamic aspect, and is consistent and rigorous. Both initial anisotropy and stress-induced deformation anisotropy are reflected by a hybrid anisotropic stiffness matrix influenced by the deterioration development degree (DDD) in different directions. By employing a combination of the strength criterion of rock material and rock bedding plane, an anisotropic failure formulation for layered rock mass has been established. The hybrid anisotropic model has been implemented in cellular automata software for the engineering rock mass fracturing process (CASRock). The performance of the anisotropic part is demonstrated by reproducing the deformation and failure characteristics of initial or stress-induced anisotropic behaviors for layered rocks under uniaxial, conventional triaxial, and true triaxial compression and Brazilian splitting conditions. Important features, such as the strength, mechanism, deformation, DDD, and fracturing process variation, can be captured by the proposed model. In addition, a numerical simulation of tunnel excavation in a layered rock mass is performed to study the anisotropic excavation-induced damage zone (EDZ) distribution in the field. The results indicate that the model is able to reproduce the observed failure mode satisfactorily.

Abstract

Thermal operations threaten the safety of geological engineering, especially the rock–concrete interface, as the link of in situ stress transmission is crucial to structural stability. In this study, three-point bending experiments were conducted on sandstone–concrete binary specimens to investigate the thermal effect on fracture behaviors of the rock–concrete bonding interface. The deformation evolution characteristics were analyzed by digital image correlation (DIC). The results demonstrated that the temperature has a remarkable degradation for the bonding capacity of sandstone–concrete interfaces, which can be classified into the rapid-damage stage and steady-damage stage with a critical temperature of 300°C. The phased degradation is closely related to the decline in interfacial adhesion and the thermal damage to concrete, instead of sandstone. By determining the strain inflection point of different positions on the crack path, a new idea is proposed to reveal the crack propagation process. During the loading process, the crack propagates slowly to a certain length and then coalescences violently when subjected to a small load increment. In addition, the crack initiation load is weakened by the elevated temperature or lower interface roughness.

Practical Applications

Thermal hazards (e.g., tunnel fire, coal spontaneous combustion, and radioactive heat in nuclear waste repositories) would weaken the bearing capacity of surrounding rock-lining structures. To ensure the durability of geoengineering and evaluate remediation value, it is essential to determine the mechanical and deformation properties of rock–concrete binary specimens subjected to a high temperature. The study suggests the temperature has a remarkable degradation for the bonding capacity of sandstone–concrete interfaces, which can be classified into the rapid-damage stage and steady-damage stage with a critical temperature of 300°C. The phased degradation is closely related to the decline in interfacial adhesion and the thermal damage to concrete, instead of sandstone. Therefore, the thermal damage not exceeding 300°C would retain the bearing capacity of sandstone–concrete interfaces; otherwise, the concrete lining must be removed and poured again. Meanwhile, roughening the rock surface could improve the interfacial adhesion but the effect is limited.

Hot dry rock (HDR) geothermal energy is clean energy that can meet people’s demand for low carbon. The traditional hydraulic fracturing method is challenging to break HDR dominated by granite, mainly because of HDR’s high temperature and hard characteristics. Microwave irradiation is considered a proven approach to breaking granite. This study investigates the failure mechanism of HDR at 500°C–800°C under microwave irradiation. The experiments cover preheating, uniaxial compression, granulometric analysis, binocular vision monitoring (BVM) technique, and X-ray powder diffraction. The result shows that the uniaxial compression strength of granite decreases the most at 500°C–600°C, which is 62.77% on average. The failure form is from the brittle failure (untreated) to ductile failure (800 °C) with the rise of the microwave irradiation temperature. The microwave makes a particle size in the range of 0.6–2.36 mm the most apparent particle size of granite after uniaxial compression. The BVM technique reconstructs the surface deformation with a 0.7% error. Microwaves cause heat accumulation near the rock near the magnetron, resulting in the formation of crack networks and a molten cavity. The possibility of microwave-assisted fracturing of HDR is discussed at the end of the article.

Achieving slope stabilization by placing anti-slide piles has become an effective slope-reinforcement technique in geotechnical engineering. Based on the limit analysis method, this study investigated the 3D stability of unsaturated inhomogeneous piled slopes, incorporating three failure patterns: toe-failure, face-failure, and base-failure. The suction and effective unit weight profile under different effective saturation were considered and the soil cohesion was assumed as varied linearly with depth. A theoretical analysis of the lateral force acting on a row of rigid anti-slide piles with the same spacing in a row through plastically deforming soils is described while considering the soil suction and inhomogeneity. In addition, a new formula was developed to calculate the lateral forces provided by a row of piles. The genetic algorithm (GA) was used to obtain the minimum value of the factor of safety F_S and the corresponding critical slip surface. The results of the study were compared with those of the existing literature to verify the feasibility of the proposed approach. In addition, the influences of key parameters on F_S of 3D piled slopes were investigated by parametric analysis. The numerical results indicate that piled slope stability will be underestimated when the 3D effects and suction are not considered, and soil inhomogeneity has a negative impact on piled slope stability. It was also found that the probability for the occurrence of face-failure increases with increasing inhomogeneity and decreasing suction-induced effect, and the occurrence of face-failure significantly reduces the pile reinforcement effect. In addition, when the anti-piles are located at the slope toe, the base-failure mechanism yields the critical values of F_S in most cases. This study can provide several reasonable suggestions for engineering applications and a theoretical basis for further studies on the stability of slopes reinforced with piles.

Understanding gas migration behavior in host rocks is of importance to the safety evaluation of deep geological repositories for nuclear waste. Experimental results reported in the literature show that gas flow in saturated claystone is through a highly localized network of dilatant pathways and that water is barely displaced. To explain the specific gas migration behavior, a two-scale approach is developed. A subcritical criterion for microcrack propagation is proposed to represent the time-dependent damage at the macroscale. The passage from microscale to macroscale is implemented through an asymptotic homogenization method. The solid mechanics is coupled with the fluid flow through pore pressure variation and an intrinsic permeability model, which implicitly accounts for the fracture opening induced permeability change. The developed model is tested against both laboratory and in situ gas injection experiments in the literature. Some key experimental findings, such as the development of preferential gas pathways and the fully saturated state are explicitly captured by the poroelastic damage model. Model results explain that the highly localized fracture pathways are the major places where gas and water interact with each other, and as a result the whole rock is almost kept fully saturated, which helps us get in-depth understanding of this gas transport mechanism.

A generalized failure criterion is proposed to describe the rock failure under polyaxial compression. The physical meaning of the new criterion is that rock failure will occur when the distortional strain energy of the material reaches some critical value that linearly increases with the effective mean stress σ_m,3 on the fracture plane striking in the σ₂ direction. When w = 0.5, the new criterion reduced to the linear Mogi criterion. When w = 0 and 1, it reduced to two failure criteria corresponding to the upper bound and lower bound of the failure loci on the deviatoric plane, respectively. The linear Mogi criterion is an average of the upper and lower bounds. One new failure with w = 0 was convex on the deviatoric plane in contrast to the Mogi criterion with concave failure envelopes on the deviatoric plane. The new criterion was validated against polyaxial compression test data of eight different rock types at various stress states. The new criterion achieved a very good fit to the test data of each rock type and was better than the linear Mogi criterion.

Abstract

This paper presents the results of a laboratory study aimed at optimizing the attachment angle of a microanchor to a sand–geogrid interface under direct shear mode. Well- and poorly graded sands, together with a single configuration comprising 24 microanchors attached at angles of 0°, 30°, 45°, 60°, and 90° to the interface are considered. All samples were prepared under dry conditions at 80% relative density and tested under normal pressures of 12.5, 25, and 50 kPa using a 300 × 300 × 170-mm direct shear box. The results show that the enhancements achieved for fine uniform sand samples were greater than for equivalent well-graded nonuniform sand samples. The optimum microanchor attachment angle for maximum interaction enhancement at the interface with well-graded sand varied between 30° and 60°, whereas this angle was 45° with the fine uniform sand. Microanchors attached at 0° and 90° angles adversely affected interaction. The greatest enhancements—of 13% and 21%—were, respectively, achieved for the well- and poorly graded sand–microanchored-geogrid samples compared with normal sand–geogrid samples. These improvements are attributed to the mobilization of passive sand resistance in front of the microanchors during shearing, with sand particle size and grading, together with normal pressure, also proving to be influential factors on the interactions. Improving sand-reinforcement interactions can reduce the amount of backfill and reinforcement required, thus reducing costs.

Practical Applications

Soils are abundantly used in the construction of earth structures. To improve the bearing capacity, reduce deformation, increase stability, and the tensile characteristics of soils, various methods, including soil reinforcement using geosynthetics, are used. Geosynthetics, such as geogrids, are widely used for reinforcing embankments and slopes, for example. The behavior of reinforced soil structures is largely governed by complex interaction mechanisms that develop between the reinforcement and the fill material. Soil–geosynthetic interactions comprise: (1) soil–geogrid skin friction; (2) soil–soil shear strength in apertures; and (3) bearing resistance against transverse ribs. To develop an understanding of the interaction mechanisms, direct shear or pullout tests are usually performed. In direct shear tests, Factors (1) and (2) are dominant, with Factor (3) being dominant in pullout tests. Passive soil resistance against transverse ribs under pullout accounts for 75% to 90% of the pullout resistance, whereas in direct shear, it contributes 10% to 12% to the overall shear resistance. To increase the contribution of passive resistance in direct shear mode, researchers have investigated different methods, including the attachment of elements as anchors to the geogrid surface. Improved interaction leads to increased stability in earth structures, reduces backfill and the amount of reinforcement required, and saves costs.

In this study, a concrete–cored gravel (CCG) column was adopted as a newly developed composite column technique to support embankments over soft clay. Through a three-dimensional (3D) mechanical–hydraulic coupled numerical model, the performance of a CCG column-supported embankment was studied. The simulation results included settlements, excess pore water pressures, and the load distribution between soils and CCG columns and subsurface settlements, which were compared with those in situ. In addition, the column efficacy (E_p) was evaluated using four existing design methods, and except for an error of 12.9%, the results of the German Design Guideline (EBGEO) agreed well with the measurement, which suggested that it was more feasible than other methods. In addition, two additional soft foundations that were reinforced by gravel columns and concrete piles, respectively were simulated to demonstrate the benefits of the CCG columns when accelerating consolidation. In addition, a parametric study was performed to investigate the consolidation behavior of the CCG columns composite foundation, and compared with other columns, the CCG columns had a higher degree of consolidation and smaller postconstruction settlement, which indicated that the CCG columns could save construction time and their applicability in soft clay. The numerical results revealed that stress transfer and column drainage contributed significantly to the consolidation of the CCG column composite foundation.

The issue of vertical capacity expansion of municipal solid waste (MSW) landfills with reinforced soil walls (RSWs) is addressed in the present investigation. The influence of different conditions of leachate levels in MSW landfills is a major cause of translational failures. Poor hydraulic conductivity, clogging of drainage because of fines, and freezing of drainage in landfills are the crucial factors of the buildup of leachate pressure. Heterogeneity and different fill ages in MSW landfills gradually change the inherent properties of landfills. The assumption-independent component failures of sliding, eccentricity, bearing capacity, tension, and pullout modes in predicting the series system reliability index of RSWs against translational failure under six leachate level conditions may produce large errors because component failures are usually dependent on one another. Therefore, the present paper demonstrates the feasibility of considering dependent failure modes to estimate the dimensions of RSWs to maintain the external and internal stability under six different leachate buildup conditions. The variability associated with the cohesion of solid waste, the apparent cohesion between liner components beneath the wedges, the friction angle of MSW, and the interface friction angle beneath the wedges is considered for the estimation of the lower bound of the series system reliability index. The limit equilibrium method is employed to assess the stability of an RSW for an expanded MSW landfill. Further, the design charts for the optimum values of width and height of the RSW are provided for different leachate levels (h_w) under six leachate buildup conditions by targeting various lower bounds of a system reliability index ≥3.0. The design values of the number of reinforcement layers (n) are also provided corresponding to the optimum dimensions of the RSW subjected to different leachate levels.

Precise, rapid, and reliable prediction of rock strength parameters is of great significance for underground engineering. This paper presents a method for predicting rock strength parameters including the Poisson’s ratio (P), elastic modulus (E), and uniaxial compressive strength (UCS) based on computer drilling jumbo measurement while drilling (MWD) data. First, the distribution characteristics and correlation of MWD data are studied; second, a filtering method of MWD data is proposed, which reduces the influence of operational and mechanical factors; finally, an intelligent prediction model of rock mechanics parameters was established, 30 groups of test data were used for application, and the mean absolute percentage error (MAPE) of prediction results for P, E and UCS are 2.11%, 3.11%, and 2.9%, the determination coefficients (R²) are 0.4346, 0.8241, and 0.6616. Compared with the data before optimization, the accuracy of prediction results is improved significantly, it shows that the deep neural network model can accurately predict rock mass parameters.

Abstract

The effect of water jet kerf depth (H) and cut spacing (S) on the rock-breaking performance of a disc cutter under water jet precutting kerf was investigated through a full-scale linear cutting test and numerical analysis with a discrete element method (DEM). Rock-breaking force, rock chip, and rock-breaking efficiency are used to evaluate the rock-breaking performance. The rock-breaking experimental results show that the rock failure patterns are closely related to H and S, and distinct rock failure patterns are shown for the different rock-breaking parameters. In addition, this study reveals the variation law of rock-breaking force and specific energy (SE) consumption with H and S. The difference in crack expansion between rock-breaking under conventional and precutting kerf conditions was compared with numerical simulations. The lateral cracks tend to expand toward the bottom of the precutting kerf and are more prone to produce large-sized rock chips in the precutting kerf condition. In addition, the increased H promotes the expansion of the vertical and lateral cracks, which resulted in different rock failure patterns. Therefore, the numerical, and experimental results show that the rock-breaking of the disc cutter under the precutting kerf conditions is promising, which significantly improves the rock-breaking performance of the disc cutter in hard rock conditions.

Practical Applications

Tunnel boring machines (TBMs) are large and complex engineering equipment that are used to complete tunneling operations under removable steel shields. They are widely used in underground space projects, such as subways, water conservancies, and mines. However, with the development of underground space engineering, an increasing number of projects are being carried out under high-strength hard rock geology. When TBMs excavate hard rock, problems, such as low rock-breaking efficiency, slow propulsion speed, and severe tool wear can occur. Therefore, innovative, and efficient rock-breaking methods are necessary for extreme construction conditions. Rock-breaking by water jet-assisted disc cutters could reduce the disc cutter's rock-breaking load and improve the disc cutter's rock-breaking efficiency, which could have broad development prospects. Therefore, in this study rock-breaking experiments with a disc cutter under water jet precutting kerf conditions were carried out on the multifunctional rock-breaking experimental bench, which investigated the effect of H and S on the rock-breaking performance. This could help to further understand the rock-breaking mechanism of a disc cutter under a water jet precutting kerf and provide a reference for the cutter head design of high-pressure water jet-assisted rock-breaking TBMs.

The changes in mechanical behaviors of soils after cyclic loading will lead to the generation of additional settlements. The postcyclic stiffness characteristics of soils are usually analyzed by using conventional cyclic triaxial tests, in which only cyclic deviator stress is applied under undrained conditions; however, the cyclic variations of both axial stress and horizontal stress under traffic loading were observed. Moreover, the pore water was permitted to drain under cyclic loading. Therefore, cyclic triaxial tests under partially drained conditions were performed to investigate the postcyclic stiffness characteristics of remolded laterite clay. The influences of variables, such as the cyclic deviator stress, number of cycles, cyclic confining pressure, and degree of reconsolidation, were evaluated. The postcyclic elastic modulus increases as the cyclic deviator stress, number of cycles, and degree of reconsolidation increase, while it decreases as the cyclic confining pressure increases. Nevertheless, the ratio of the postcyclic elastic modulus with and without cyclic confining pressure decreases as the cyclic confining pressure increases; the postcyclic elastic modulus ratio of the degree of reconsolidation to without reconsolidation increases linearly with the increasing degree of reconsolidation. Based on the result, an empirical formula for the postcyclic elastic modulus, considering the effects of both cyclic confining pressure and the degree of reconsolidation, was proposed. The predicted results match the measured data well, indicating that the formula is valid for the prediction of the postcyclic elastic modulus of laterite clays after cyclic loading.

A nonlinear consolidation model of soils with vertical drains was developed by combining the fact that well resistance varies linearly with depth and exponentially with time. In addition, three variation modes related to horizontal permeability in the smear zone were used to improve this model. The degree of consolidation, excess pore pressure, and settlement of soil layers under multistage loading were simulated by the finite-difference method. The proposed solution was verified through a comparison with the literature to demonstrate its validity. Ultimately, the effects of various parameters associated with well resistance, the ratio of compression index to permeability index, external loading, smear effects, and the distribution of initial average effective stress along the depth on consolidation behaviors were investigated in this study.

An uneconomical design of embedded cantilever retaining (ECR) walls unnecessarily restricts the lateral wall movement, whereas unwarranted wall displacement induced ground settlement can jeopardize the functionality of neighboring structures. Thus, correct estimation of the wall displacement under working conditions is imperative for a safe and economical design. This paper presents an analytical method for the displacement-controlled analysis of ECR walls in cohesionless soils, which can be used to calculate the required embedment depth for a prescribed wall displacement and retaining heights. Alternatively, when the retaining height and the embedment depth of an ECR wall are given, the lateral wall displacement can also be calculated. A displacement-dependent earth pressure mobilization model is proposed to derive the mobilized soil stresses along the wall height. The required embedment depth of the wall is determined by assuming rigid rotation of the wall about a point near the toe and satisfying the horizontal force and moment equilibriums. Analytical formulations are provided to determine the bending moment distribution and the ground settlement. The effect of construction by excavation is also taken into the analysis. The results show that the required embedment depth and the maximum bending moment increase exponentially with decreasing wall displacement. The depth of the pivot point is located at around 0.9 times the embedment depth. The validity of the proposed method is demonstrated by comparing the calculated results with those of the available numerical and experimental studies. The proposed method can provide first-hand design solutions of ECR walls without performing rigorous numerical and experimental studies.