International Journal of Geomechanics

The joint leakage in a shield tunnel is a common defect and difficult to be accurately quantified due to the complex deformation mode and stress state, which seriously threatens the service performance of the tunnel lining. The waterproofing of the joint is divided into two stages, which are separately provided by the sealing gaskets in the joint and the normal stress on the joint interface. Combined with the two-stage characteristics of the joint waterproofing, an analytical leakage model is proposed to deal with the joint leakage. In this model, the maximum water head loss or critical water head flowing through the sealing gaskets is calculated by a general formula, in which the parameters are determined by indoor tests. The proposed analytical model for the joint leakage in the shield tunnel not only considers the joint deformation modes, including joint opening and joint dislocation, but also considers the stress state of the joint interface. The joint leakage analysis of a typical shield tunnel in Shanghai indicates that both the high external water head and the joint dislocation can reduce the critical joint opening of the initial leakage, and the critical joint opening of the waist joint is much smaller than that of the top joint. The average hydraulic pressure acting on the joint interface can be negligible, because it is less than 5% of the concrete stress at the joint. Based on the proposed joint leakage model, the waterproofing capacity of the tunnel-lining ring can be optimized by moving the sealing gaskets along the joint interface.

The aim of this study is to develop a method to simulate the behaviors of clayey gravel during water erosion by using computational fluid dynamics–discrete element method. The fluid phase was solved using the “fixed coarse-grid” scheme based on Navier–Stokes equations, and the solid phase was described by an assemblage of discontinuous particles in the Particle Flow Code in three dimensions (PFC^3D) modeling framework. A fictitious clay method was introduced, in which the suffosion of clay matrix was translated to the strength degradation of bond according to a degradation law. The degradation law was embedded into PFC^3D by the development of additional code in the Python language. The method was first validated by comparing with experimental results. Then, a parametric study was carried out to investigate the influence of three different factors, particle-size distribution (PSD), soil porosity, and pressure gradient, on the erosion behavior of soil. Results show that the proposed model offers a promising method to predict the erosion behavior of clay soil at the particle level. The numerical results generally match the empirical criteria, for example, it demonstrates that the initiation of the erosion process is significantly influenced by material properties (PSD, porosity) and loading condition (hydraulic gradient), and also reveals clearly defined critical levels for the three parameters, below which through-piping cannot be expected.

Subgrade rutting failures in matting structures present a unique challenge in expedient airfields. Mats are often constructed over existing soft subgrades and can experience aircraft with large gross loads and high tire pressures. It is important to understand and predict the critical factors that contribute to deformation in these thin structures. This paper presents the results of a comprehensive study on the critical factors contributing to rutting in the AM2 matting system under static loading, using a combination of full-scale instrumented testing, layered elastic analysis (LEA), and three-dimensional finite-element (FE) modeling. The FE model of the AM2 mat system is built by implementing a user-defined constitutive model, multimechanical model (MMM), to simulate the soft soil subgrade response. The MMM is a recently developed elastoplastic kinematic hardening model capable of capturing the soil behavior under complex stress histories. The results of the FE model are compared against those attained from a set of linear and nonlinear LEA and full-scale testing. The FE results show a good agreement with the LEA results and the measured data from earth pressure cells and single depth deflectometers. The FE model is then used in a series of sensitivity studies to explicate effects that factors such as loading conditions, subgrade cover material, mat joint load transfer, and mat–soil interface conditions have on deformation response. Results show that incorporating the load transfer mechanism occurring at the mat joints and varying the mat–soil interface condition affect the near surface subgrade deformation and stress responses that contribute to rutting failures. Findings of this study show that although the LEA procedures are the basis of current airfield design methodologies, critical design features and the corresponding deformation responses can be better modeled using the FE approach.

Ultimate soil resistance that acts on a seismically induced caisson foundation and its distribution along the caisson height have been formulated by considering passive wedge failure analysis for both dry and submerged conditions. The method for limiting the equilibrium of forces acting on a three-dimensional idealized passive wedge developed due to seismicity has been adapted in this study. Using this process, seismic inertia forces will be computed by considering the surrounding soil as a viscoelastic material that overlays a rigid stratum subjected to a harmonic shaking. For a very large value of caisson width, the comparison of this analysis with a 2D seismic earth pressure problem illustrates the compatibility of this methodology in the plane strain condition. Variation in earth pressure coefficients and their distribution for different seismic acceleration coefficients, soil properties, excess pore pressure ratios, and caisson aspect ratios will be discussed in this analysis. The reasonable agreement of the distribution of total passive pressure along the wedge height under the submerged condition with the experimental results shows the applicability of the proposed formulations as a design approach for caissons in sandy soil.

The effects of initial stress conditions and rock breaking parameters on rock fragmentation were investigated through indentation tests and numerical analysis with a discrete element method (DEM). Different initial stress conditions and the thrusting velocity of the cutter were the focus in the design of indentation tests. Some indicators were used to reflect the instability of the thrusting force, rock breaking efficiency, failure patterns, and the fluctuation of force-penetration response. The indentation results show that rock failure patterns have a closed correlation with initial stress conditions and loading rate. All tests showed several distinct chipping phases, and obvious chiseled pit and crushed rock could be observed on the top surface of each rock specimen. Moreover, rock brittleness was evaluated by the fluctuation of force-penetration curves; this study also revealed the changing law of peak and mean thrusting force. In addition, the numerical simulation of indentation tests reproduced the development process of rock fragmentation and crack propagation. Increasing confining stress restrained the propagation of vertical and lateral cracks, whereas the axial stress had the effect of restraining the propagation of radial cracks. However, there is a critical confining stress that causes different results of fractured depth. Therefore, this investigation is beneficial for optimizing the cutting parameters to promote rock breaking efficiency.

The aim of this paper is to propose a closed-form solution to the poromechanics problem of stress and pore pressure distribution around noncircular openings at great depth subjected to hydrostatic water pressure and far-field geostresses. The problem is solved by superposing the effects of fluid and solid skeleton obtained over simple circular geometries and generalizing the obtained expression to various geometries of mined ore bodies using complex variable functions and conformal mapping techniques. The principal stresses obtained analytically over the opening boundary and within the domain are compared with the results of finite-element analysis to verify the proposed approach. The comparison conducted for a representative noncircular opening indicates good agreement between the analytical and numerical methods. Hence, a parametric study is used to investigate in detail the stress variation under different opening dimensions, heterogeneous initial total stress conditions, and far-field pore pressure values. The proposed solution could be instrumental in the design of underground openings and deep mass alterations (that include local anthropogenic damage zones). It could also be used to provide reinforcement solutions where stresses can reach the mechanical stability thresholds.

The objective of this paper is to present simplified methods for determining the three-dimensional (3D) system capacities of pile foundations based on the lower and upper bound theorems of plasticity. The failure surface for a typical long offshore pile subjected to 3D head loads is derived based on the lower bound (LB) theorem of plasticity, and the LB capacity of a pile system is then proposed. Using the failure mechanism determined from the elastic solution in the LB analysis of a pile system, the upper bound (UB) capacity of the pile system is obtained from the energy-balancing equation. An actual offshore platform is selected as a case study to demonstrate the proposed methods. The current methods extend the planar limit analysis by incorporating global torsion and out-of-plane failures and can be useful in designing and assessing pile systems.

Expansive black cotton soil (BCS) exhibits volumetric instability when exposed to moisture fluctuation, rendering loss of geomechanical strength. The present paper evaluates the effectiveness of stabilizing BCS with two different types of fibers, polypropylene (PF) and glass fiber (GF), in conjunction with envirosafe alkali-activated binder (AAB) at different proportions of fly ash and slag in alkaline mixture. AAB is produced by blending an alkali-activator solution of sodium silicate and sodium hydroxide with aluminosilicate precursors (Class-F fly ash/slag) at 0.4 water to solid (w/s) ratio. PF and GF are varied from 0% to 0.4% with 5% AAB in BCS. Unconfined compression strength (UCS), indirect tensile strength (ITS), and California bearing ratio (CBR) tests are carried out as performance indicators of geomechanical strength of both fiber-reinforced AAB treated BCS. The research also focuses on proposing an optimum dosage of fly ash–slag ratio and fiber reinforcement in AAB-BCS. Monte Carlo Simulation is used to determine the reliability index for each case. The influences of varying dosages of fibers and fly ash–slag proportions in BCS show a significant improvement in shear strength and tensile properties. The reliability analysis also shows that the amount of fibers and fly ash–slag proportions are essential factors in UCS, ITS, and CBR strength for both PF and GF reinforcement.

This paper focuses on the construction of urban rail transit projects to prevent the risk of water inrush disasters faced during subway construction in cities with abundant groundwater and well-developed karst landforms. In view of the reserved safety thickness for the tunnel face with an anterosuperior karst cave filled with water in urban subway construction, the authors study the influence law and response sensitivity of different types and diameters of the shield machine cutter head and different diameters and water pressure of the anterosuperior karst cave filled with water on the safety thickness. The authors relied on the actual working conditions of the Wangfuzhuang station to the Dayangzhuang station section of the Jinan Metro Line R1 phase I project for this study used the fluid–structure coupling method of COMSOL Multiphysics 5.3 software to simulate the process of shield tunneling and the time of water inrush, and combined the three-dimensional finite-element numerical simulation results and multiple linear regression method to deduce the formula of the reserved safety thickness for the tunnel face with an anterosuperior karst cave filled with water. The conclusions can be used to guide shield excavation in karst areas of urban subways.

The road pavement is a typical three-dimensional (3D) multilayered half-space medium, which consists of the surface courses, base courses, and soil subgrade/foundation. It is critical to understand the dynamic response of the road pavement under vehicular loads for improving structural design efficiency. The actual vehicular loads can be simulated as a harmonic moving load. In general, there are several methods to analyze the dynamic response of the multilayered half-space medium under harmonic moving loads, such as the boundary element method (BEM) and the finite-element method (FEM). However, the previously mentioned methods might face some unavoidable problems, for example, the numerical overflow, when the thickness of the medium is thicker, which causes the wrong result or long computing time. Therefore, the objective of this study is to develop a precise solution to solve the dynamic response of the 3D multilayered half-space medium under a harmonic moving load. In this study, a new type of 3D spectral element method (SEM) was developed and applied to the dynamic response analysis. Details of the mathematical derivation, implementation, and verification were demonstrated in the current paper. Subsequent numerical results, based on a layered road pavement structure constructed on a soil foundation, indicated that both the load speed and the frequency, the damping ratio of the structural materials, and the interlayer condition significantly impacted on the dynamic response of the structure under vehicular loads.

A full-scale shake table testing program was performed with two helical pile groups supporting model structures to better understand the dynamic properties of a soil–pile-group–structure system. Each group had four piles embedded in dry, dense sand confined in a 6.7 m L × 3.0 m W × 4.6 m H laminar box. One group was comprised of 8.8-cm diameter and 3.66-m long helical piles, while the second group had 14-cm diameter and 4.27-m long helical piles. To investigate the influence of the pile-head fixity condition on group behavior, both pinned head and fixed head connections were implemented and tested. White noise excitation and two replicated strong earthquake motions were used to determine the natural frequency and observe the seismic behavior of the soil–pile-group–structure system, respectively. The experimental observations were analyzed to evaluate structural natural frequency and pile-group stiffness and damping. The experimental results were used to calibrate a numerical model that was then used to conduct a parametric study to gain a broader understanding of the seismic behavior of helical pile groups under varying conditions. The experimental and numerical results were compared and the effects of varying properties in a soil–pile–structure system, including their system seismic response, are discussed.

The fluctuation of reservoir water levels has significant impact on bank slope stability near a reservoir. Therefore, it is necessary to analyze the stability of bank slopes with reservoir drawdown. In this paper, a novel method combining the seepage force with rigorous limit equilibrium method is proposed to analyze the stability of bank slopes near the reservoir. First, the calculation method of the phreatic surface and the seepage force is developed. Then, six equilibrium equations are established based on the equilibrium conditions of three forces along the coordinate axis and the equilibrium conditions of three moments around the coordinate axes. Finally, a trust–region–reflective iterative algorithm is used to determine the safety factor of bank slopes from these six equations. Moreover, the effects of drawdown speed, the height of the drawdown, permeability coefficient, and the aquifer thickness on bank slope stability are investigated using the proposed method. Two engineering cases of Ataden I landslide and Guanyinping landslide are used to verify the correctness and practicability of the proposed method.

This paper presents the findings from an experimental investigation that was conducted to assess the shear behavior and the changes in the shear behavior and strength parameters for cemented paste backfill (CPB)–CPB interface as well as the effects of key influential factors (normal stress, curing time, interface roughness, and tailings) on the shear characteristics at the CPB–CPB interface. An extensive program of direct shear tests and microstructural analysis will be conducted to gain insights into the shear behavior at the CPB–CPB interface and the fundamental processes that are responsible for the observed behavior. The results show that the shear characteristics at the CPB–CPB interface are a function of the curing time (degree of cementation), the surface roughness of the CPB, normal stress, and chemical composition of the tailings. The shear behavior of a rough CPB–CPB interface is different from that of a smooth interface. A stress hysteresis loop was observed, and the prepeak linear phase of the shear stress–shear displacement response was an elastoplastic process. The shear failure at the CPB–CPB interface follows Mohr–Coulomb failure criterion. In addition, it was found that the CPB–CPB interface was not necessarily a weak plane with respect to the shear failure of the CPB structure. The shear strength at the CPB–CPB interface could be higher than that of the control CPB material (backfill without an interface) and depends on the surface roughness of the CPB and the curing time. An index of the transition curing time for the CPB–CPB interface was proposed to evaluate when the backfill structure would fail first along the CPB–CPB interface. The results presented in this paper will contribute to a better understanding of the shear behavior and the changes in the shear strength parameters of the CPB–CPB interface and guide the development of a constitutive equation for backfill structures that have an interface as well as providing a more cost-effective design for CPB structures.

An unstable rock mass is a common geological hazard in water conservancy and hydropower engineering in southwest China. The mechanical properties of a rock mass joint and the internal and external physical environment are closely related. In this study, the factors to evaluate the stability of a dangerous rock mass and the geological conditions of a highway slope in Yunnan Province will be evaluated. The block discrete-element method (DEM) will be used to calculate the highway slope of a dangerous rock mass. Moreover, three-dimensional numerical analysis will be performed for the strength reduction of the safety factor. The stabilities of the dangerous rock mass under natural, rainfall conditions, roadbed excavation, and exerted active defending net on dangerous rock mass Nos. 1 and 2 will be determined. The safety coefficients under natural, rainfall, and roadbed excavation conditions are 1.36, 1.27, and 1.36, respectively. When an active protective net was applied onto the surface of dangerous rock mass No. 1, the safety coefficients were 1.39 and 1.31 under natural and rainfall conditions, respectively. When the active protective net was applied onto the surface of dangerous rock mass No. 2, the safety coefficients were 1.43 and 1.35 under natural and rainfall conditions, respectively. The potential instability area of dangerous rock mass No. 1 is the rock mass on the bedding joint face. The potential instability area of dangerous rock mass No. 2 is the local block A. The two dangerous rock masses must be monitored during project operation.

Seepage control is a key technical problem in large-scale underground cavern engineering, and seepage analysis plays an important role in realizing engineering optimization design and safety. The long-term seepage control effect of a complex seepage control system on the fractured surrounding rock in the Wunonglong underground powerhouse was investigated by using a finite-element numerical model. The permeability of the fractured rock mass was characterized by using a combination of statistical methods and a field water-pressure test. Seepage behavior was modeled in consideration of the complex seepage control system by using the variational inequality formulation of Signorini’s type and the substructure method for dense drainage holes. A global model of equivalent modeling and a submodel of accurate modeling were combined to reduce the difficulty in mesh generation and to balance calculation time with modeling accuracy. The seepage control effect and its sensitivity to the layout of the seepage control system were comprehensively illustrated based on the numerical results. The validity and reliability of the numerical model were verified via in situ measurements. The rationality of the seepage control system for the fractured surrounding rock in the Wunonglong underground powerhouse and its potential for further optimization were discussed.

This paper presents a study of the mechanical properties of a synthetic transparent clay to be used in physical geotechnical models. The manufacturing processes of a synthetic transparent clay consisting of Carbopol polymers, NaOH powder, and distilled water were briefly introduced. The transparency of the synthetic transparent clay influenced by the sample thickness and the mixture of Carbopol materials was assessed by the modulation transfer function (MTF) method. For the synthetic transparent clay with the 0.75% Carbopol material mixture, the clear visible depth was approximately 30–40 cm. The geotechnical properties of the clay with this mixture were measured in detail. The permeability (permeability test) and thermal conductivity (thermal conductivity test) of the synthetic transparent clay with a specific U10 mixture were 5.3 × 10⁻⁷ cm/s and 0.62–0.72 W · M⁻¹ · K⁻¹, respectively. These analyses demonstrated that synthetic transparent clay exhibits macroscopic geotechnical properties comparable to those of very soft clay and could be used to simulate submarine landslides.