International Journal of Geomechanics

A new instability criterion based on the strength reduction method (SRM) is proposed, where the slope is in the limit equilibrium state when the critical slope contour determined by the slip-line field theory and the slope intersect at the toe of the slope. The proposed method was validated using a published case. Compared with the traditional instability criterion, the SRM with the displacement finite-element method, the SRM based on the Davis algorithm, and finite-element limit analysis, Spencer’s method, the proposed method is more suitable for slope stability analysis. The safety factor was calculated separately from the critical slip surface. As such, the critical slip surface is determined without involving an optimization or iteration. The proposed method recognizes the objective quantification of the instability criterion.

During the excavation of large-scale underground caverns, in which dynamic feedback analysis is required, the efficiency and accuracy in determining mechanical parameters of surrounding rock masses have significant influences on the safety and effectiveness of construction. In this study, a novel intelligent displacement back-analysis method is proposed to determine the geomechanical parameters. In this method, the parameter determination is transformed into a global optimization problem, which treats the error between in situ measured displacements and numerically calculated displacements as an objective function and regards geomechanical parameters as decision variables. To solve this optimization problem featuring high nonlinearity, multiple peak values, and high computation cost, an intelligent optimization algorithm combining the particle swarm optimization (PSO) technique and the Gaussian process machine learning (GP) theory is developed, and then, the algorithm is used to cooperate with the finite difference method (FDM) to form the method called PSO-GP-FDM for displacement back-analysis. Subsequently, the PSO-GP-FDM method is applied to the back-analysis of rock mass parameters for the Tai'an Pumped Storage Power Station. With the obtained mechanical properties of rock masses, the FDM-based numerical modeling can reproduce very well the in situ measured displacements in this hydropower station after excavation, demonstrating that the PSO-GP-FDM method is feasible to obtain reasonable mechanical parameters of surrounding rock masses. With excellent global optimization ability and high computational efficiency, the proposed method is suggested for displacement back-analysis of geomechanical parameters of underground caverns.

Rock abrasivity is the main factor that causes erosion of excavation tools and is usually quantified by the Cerchar Abrasivity Index (CAI). Although Cerchar abrasivity tests are easy to perform, they are time consuming and require a relatively high volume of rock samples. Having good correlations of CAI values and other faster and simpler tests is therefore of great interest, since it results in time and budget savings when controlling excavating tool wear. Based on the results of 73 andesitic rock samples coming from the central area of Ecuador, this paper presents a series of artificial neural networks developed to find a good estimation of CAI values of andesitic rocks from their petrographical properties. The network showing the best performance (R² equal to 97%) is identified and a detailed process to estimate CAI value using the network developed is described.

The first caving of top coal and the first weighting of roof strata in Longwall Top Coal Caving (LTCC) mining were analyzed in a parametric study by using a discontinuum modeling technique with plastic rock material. The analysis demonstrated that an increase in the value of cover depth decreases the stability of main roof strata, while the impact of horizontal stress on the main roof strata stability is not consistent under different stress magnitudes. The study confirmed that an increase in the value of immediate roof rock strength, coal strength, coal elastic modulus, and coal bedding spacing improves the stability of top coal before its first caving, whereas an increase in the value of top coal thickness, cover depth, and horizontal stress magnitude facilitates the commencement of top coal caving. Based on the parametric study's result, a new criterion for the prediction of face distance where top coal starts to cave was statistically developed and named Top coal First Caving Distance (TFCD). TFCD is successful in using immediate rock strength, coal elastic modulus, coal bedding spacing, top coal thickness, and horizontal stress magnitude for a reliable prediction of the face distance. The findings of this paper are helpful in better understanding basic caving/weighting mechanics, predicting face/roof instability risks, and assessing coal loss in LTCC, which contribute to improving safety and productivity in thick seam extraction.

The convergences of surrounding rock masses are important indexes which can be used to evaluate the stability levels of underground rock excavations. The predictions and control of convergences are essential tasks to ensure that safety measures are met in such rock excavation processes as mining, tunneling, and underground constructions. Meanwhile, many uncertainties are associated with the predictions of convergences due to the complexity and nonlinearity of rock mechanical behaviors. However, such uncertainties had not been considered in previous studies. In this paper, a novel method was proposed to predict the convergences and uncertainties of surrounding rock masses in underground caverns. In the present study, a relevance vector machine (RVM) was adopted to build the relationships between the convergence displacement and mechanical parameters of the rock masses. Numerical simulations and experimental designs were used to provide the samples for the RVM model. Then, Monte Carlo simulation (MCS) was utilized to simulate the uncertainties of the convergences based on the RVM model. The proposed method was verified using different examples with the simulated uncertainties. The results showed that the proposed method had the ability to reasonably predict the convergences and their related uncertainties. When compared with the deterministic method, the proposed method was found to be more rational and scientific and had also conformed to rock engineering practices. Therefore, the proposed method provided a scientific way to predict convergences and displacement uncertainties of surrounding rock masses in underground caverns.

The safety and durability of rock mass engineering face rigorous testing in cold regions, prompting experimental study of the effect of chemical corrosion and freeze–thaw (F–T) cycles. By taking the actual environment of the joint rock in the hydro-fluctuation belt of a typical bank slope in the Three Gorges Reservoir region of China as an example, the interactive relationship between chemical corrosion and F–T cycles is studied. An F–T cycle test scheme is adopted to explore the F–T degradation of mechanical properties and the damage degradation mechanism of mode-I fracture sandstone immersed in different chemical solutions. Experimental results show that there is significant F–T damage that deteriorates mechanical properties under different chemical corrosion attacks but that peak strain and yield properties of the specimen increase. In the first 0–20 F–T cycles, the deterioration of the mechanical properties of sandstone immersed in the alkaline Na₂SO₄ (pH = 12.0) solutions is the smallest; that in the acid Na₂SO₄ (pH = 3.0) solutions is the greatest. With the increasing number of F–T cycles, the F–T degradation degree of the mechanical properties in Na₂SO₄ (pH = 12.0) solutions gradually increase, but that is still smaller than that in the acid Na₂SO₄ (pH = 3.0) solutions, and is relatively great compared with that in neutral solutions. The damage to the microstructure inside sandstone gradually increases with repeated F–T cycles; the edges and corners of the mineral grains gradually disappear or become smoother. In addition, the roughness of the mineral grains gradually decreases, eliciting a looser structure. Statistical analysis reveals an obvious relationship between the damage degree of mechanical parameters and ion concentration in chemical solutions, with higher ion concentrations leading to greater deterioration degrees of physical and mechanical parameters. Correlations among physical and mechanical properties of specimens, F–T damage variable degree, and ion concentration are obtained.

This paper gives a set of time-dependent displacement solutions for the case of a vertical concentrated force acting in the interior of fractional derivative viscoelastic soils. First, a three-dimensional fractional derivative viscoelastic model is introduced to describe the creep behavior of soils. Then based on the well-known Mindlin's elastic solutions and the correspondence principle from static elasticity to quasi-static viscoelasticity, the proposed precise solutions are systematically obtained in the Laplace transform domain. Subsequently, the proposed displacement solutions are validated against three previously published solutions, and all of the comparisons show a good agreement. Finally, parametric study and discussion are given, to analyze in detail the influence of parameters on the solutions. It is indicated that the displacements for fractional derivative viscoelastic models are smaller in the early stage and larger in the later stage than those for conventional viscoelastic models, and the proposed displacement solutions are more general.

Ground source energy systems (GSES) utilize low enthalpy geothermal energy and have been recognized as an efficient means of providing low carbon space heating and cooling. This study focuses on GSES where the exchange of heat between the ground and the building is achieved by circulating a fluid through heat exchanger pipes. Although numerical analysis is a powerful tool for exploring the performance of such systems, simulating the highly advective flows inside the heat exchanger pipes can be problematic. This paper presents an efficient approach for modeling these systems using the finite-element method (FEM). The pipes are discretized with line elements and the conductive–advective heat flux along them is solved using the Petrov–Galerkin FEM instead of the conventional Galerkin FEM. Following extensive numerical studies, a modeling approach for simulating heat exchanger pipes, which employs line elements and a special material with enhanced thermal properties, is developed. The modeling approach is then adopted in three-dimensional simulations of two thermal response tests, with an excellent match between the computed and the measured temperatures being obtained.

Cumulative plastic deformations under cyclic loading can be a key consideration in the design of caissons and short piles. In the case of caissons and piles serving as anchors for floating offshore structures, the possibility of loss of embedment due to upward ratcheting of the anchor under cyclic loading is a particular concern. This paper presents a finite-element investigation of a caisson subjected to cyclic lateral and inclined loading. A cyclic hardening model is employed to permit simulation of cumulative plastic deformations during loading below the ultimate load state. An asymmetric Fourier analysis was employed as a highly efficient alternative to a full three-dimensional analysis, and results using the two approaches were found to be in good agreement. A realistic range of material parameters was established through back-analysis of laboratory model tests published previously. Parametric studies were performed to assess the effects of small-strain soil stiffness, subyield stress–strain behavior, load inclination, soil–pile interface adhesion, and load reversals. Pile performance for cyclic loading with nonuniform load amplitudes was also investigated. The aforementioned factors were found to have a moderate to large influence on the magnitude of cumulative displacements under cyclic loading.

Shield tunnels are widely used in urban underground construction. The stability of the tunnel face has become a crucial issue in shield tunneling. In this paper, the anisotropy of friction angles is taken into account in the upper theorem of limit analysis of the tunnel face. A formula of the anisotropic friction angles is established and applied in the limit analysis of the tunnel face, and the anisotropic parameters can be determined by the simple triaxial compression tests. The results are compared with existing works to validate the reasonability of the proposed method. Two types of constant friction angles are adopted to compare the anisotropic effects on the support pressures distributed uniformly on the tunnel face and the failure mechanism. The results show that the anisotropy of friction angles will lead to an increase in the support pressures and obviously influence the shape and range of the failure pattern. The anisotropy of friction angles should be considered in the stability analysis of the tunnel face.

Uniaxial compressive strength (UCS) and tensile strength are essential parameters of rocks, but the direct measurement of these two parameters is time-consuming and requires high-quality samples. As such, academics brought attention to the indirect tests (such as point load test, ultrasonic wave velocity test, and Schmidt hammer test) that could be used to estimate UCS and tensile strength quickly. The empirical equations for estimating UCS and tensile strength of rocks in study cases remain poorly understood as previous equations based on indirect testing methods possess a lithological heterogeneity–dependent nature. Laboratory experiments (e.g., uniaxial compressive test, Brazilian test, point load test) and regression analysis were performed to acquire the correlation equations for calculating UCS or tensile strength of rocks based on indirect tests. Significant linear equations were derived and UCS or tensile strength can be estimated by point load test with acceptable discrepancy. The comparison study showed that using improper empirical equations to predict UCS and tensile strength can yield significant errors. A modified basic quality (BQ) rock mass classification system was also presented to conveniently classify the rock mass in study cases.