International Journal for Numerical and Analytical Methods in Geomechanics最新文献

Stimulating long and persistent fractures from multiple perforations in horizontal wells plays a vital role in enhancing the recovery of hydrocarbons from unconventional reservoirs. However, interaction among fractures may lead to dramatic nonuniformity, but the mechanism that drives the competition still eludes explanation. We proposed an improved two-dimensional discrete element model to simulate fluid competition and stress interaction among perforations in the same fracturing stage. The fluid partitioning is implemented by dynamically dividing the injected fluid into different perforations to maintain pressure consistency and fluid conservation. The model is validated by comparing the induced stress, fracture aperture, and the evolution of the fracture height and the injection pressure with theoretical models. The influences of the perforation friction, fluid viscosity and injection rate are examined systematically. Simulation results reveal that fluid competition tends to stimulate one dominant fracture with other perforations suppressed. The effect of increasing the perforation friction for promoting the fluid partitioning is not remarkable while using more viscous fracturing fluid helps to initiate more fractures at the perforations. With a higher injection rate all fractures can propagate to the borders but the asymmetrical fracture pattern cannot be avoided. Four typical fracture patterns are distinguished by changing operational parameters.

Climate warming accelerates permafrost thawing, causing warming-driven disasters like ground collapse and retrogressive thaw slump (RTS). These phenomena, involving intricate multiphysics interactions, phase transitions, nonlinear mechanical responses, and fluid-like deformations, and pose increasing risks to geo-infrastructures in cold regions. This study develops a thermo-hydro-mechanical (THM) coupled single-point three-phase material point method (MPM) to simulate the time-dependent phase transition and large deformation behavior arising from the thawing or freezing of ice/water in porous media. The mathematical framework is established based on the multiphase mixture theory in which the ice phase is treated as a solid constituent playing the role of skeleton together with soil grains. The additional strength due to ice cementation is characterized via an ice saturation-dependent Mohr–Coulomb model. The coupled formulations are solved using a fractional-step-based semi-implicit integration algorithm, which can offer both satisfactory numerical stability and computational efficiency when dealing with nearly incompressible fluids and extremely low permeability conditions in frozen porous media. Two hydro-thermal coupling cases, that is, frozen inclusion thaw and Talik closure/opening, are first benchmarked to show the method can correctly simulate both conduction- and convection-dominated thermal regimes in frozen porous systems. The fully THM responses are further validated by simulating a 1D thaw consolidation and a 2D rock freezing example. Good agreements with experimental results are achieved, and the impact of hydro-thermal variations on the mechanical responses, including thaw settlement and frost heave, are successfully captured. Finally, the predictive capability of the multiphysics MPM framework in simulating thawing-triggered large deformation and failure is demonstrated by modeling an RTS and the settlement of a strip footing on thawing ground.

In the context of rock material and modeling uncertainties, the optimization of rock tunnel support systems is often conducted by selecting the most cost-effective solution among several feasible options that typically rely on the engineer's experience, potentially leading to overlooking the most optimal design. To improve such a limitation, this paper presents a multi-objective reliability-based robust design, considering the cost, safety, and design robustness systematically while maintaining the computational efficiency. In this framework, the uncertainty-based reliability constrains is performed using the first-order reliability method (FORM) and an improved Hasofer–Lind–Rackwits–Fiessler recursive algorithm (iHLRF-x). The design robustness, in terms of sensitivity index (SI), is evaluated using the normalized gradient of the system response to the noise factors, which can be efficiently obtained from the output of FORM analysis. Then, the Pareto front revealing the tradeoff between multiple objectives can be directly generated using the proposed optimization framework. To illustrate the effectiveness of this procedure, a set of the optimal design combinations of the shotcrete thickness and installation position for the exampled rock tunnel are obtained, and new perspectives pertaining the success of the reliability-based robust designs are provided.

Particle shape irregularity is a notable feature of granular materials that exerts a profound influence on their mechanical behavior. This study examines the effects of particle overall regularity and surface roughness on the fabric evolution of granular materials using the Discrete Element Method (DEM). By connecting multiple spheres with varying sizes and positions, a diversity of clump particles characterized by distinct overall regularity ($��O�R�$) and surface roughness ($�R_c$) are generated. A series of DEM simulations on drained triaxial compression tests have then been performed on granular assemblies with varying shapes, whereby their characteristics of contact intensity and the anisotropy of various fabric entities defined by contact normal, branch vector, and particle orientation, have been thoroughly investigated. The results show that increasing particle shape irregularity, indicated by smaller values of $��O�R�$ and $�R_c$, is generally associated with an enhanced internal structure within the granular assembly, exhibiting a higher mechanical coordination number and a greater fabric anisotropy. Conversely, in granular assemblies with relatively high overall regularity, the fabric anisotropy is notably reduced, and this reduction cannot be compensated by enhancements in particle surface roughness. The evolution of two contact-related fabric anisotropies is analyzed in relation to particle orientation-based fabric anisotropy, which is more profoundly influenced by particle overall regularity, underscoring its significant role in fabric evolution of granular materials.

A non-iterative stress-displacement solution is proposed in this paper to investigate surrounding rock mass with multiple non-circular deep tunnels. Compared with the Schwarz alternating method, the proposed method only spends one-time matrix operation, showing considerable efficiency and accuracy of the matrix solution. First, a matrix solution is formulated to obtain the conformal mapping function for the non-circular tunnel. Then, in consideration of multiple boundary conditions, the generalized complex variable theory and fast Fourier transform (FFT) algorithm are used to formulate a matrix solution of potential functions. The stress and displacement fields around multiple non-circular tunnels can be further determined from the matrix solution. A series of numerical examples are conducted to verify the proposed method, perform the convergency study, and discuss the effects of tunnel geometries, distances, and arrangements. The convergency study shows that the more accurate conformal mapping function requires more sampling points in the FFT. In addition, the high stress zone would be amplified if the arrangement of multiple tunnels makes the high stress zone adjacent or overlapped.

We propose a new approach for performing drained and undrained loading of elastoplastic geomaterials over large deformations using smoothed particle hydrodynamics (SPH), a meshfree continuum particle method, combined with the modified Cam Clay (MCC) model of critical state soil mechanics. The numerical approach draws upon a novel one-particle two-phase penalty-method based formulation for handling undrained loading in saturated soils, which allows tracking of the buildup of pore-water pressures under combined shearing and compression. Large-scale parallelized simulations are employed to accommodate a significant number of degrees of freedom in a three-dimensional setting. After verification and benchmark testing, the SPH based formulation is used to analyze the propagation of reverse faults through fluid-saturated clay deposits and the rupture of strike-slip faults across earthen embankments. The computational methodology tests the robustness of the meshfree approach in situations where the soil tends to dilate on the ‘dry’ side of the critical state line and to compact on the ‘wet’ side, but cannot, because of the incompressibility constraint imposed by undrained loading. Our results extend the current understanding of fault rupture modeling and further demonstrate the potential of our framework together with the SPH method for large deformation analyses of complex problems in geotechnics.

Most rock slope stability analysis methods are based on the Hoke–Brown criterion under a two-dimensional state of stress, which somewhat ignores the effect of intermediate principal stress. In this paper, by introducing the improved three-dimensional H-B criterion into the Meridian plane, the tangent line of a point on the H-B strength envelope is regarded as its instantaneous equivalent M-C parameter. Based on this, a formula for solving the equivalent M-C strength parameters under a three-dimensional stress state is established to describe the shear strength parameters of rock mass, considering different stress states. Taking three kinds of rocks as examples, the influence of the intermediate principal stress on their strength parameters is analyzed. On this basis, the realization scheme of the nonlinear strength reduction method is established, the stability of the slope is studied, and an example verifies the feasibility of this method. This method fully considers the nonlinear characteristics of rock slope strength parameters under a three-dimensional stress state and provides a solid basis for slope stability analysis.

This paper presents a size-dependent energy-based strain burst criterion linking strength, elasticity, fracture energies and specimen size effect with stress state due to changes in boundary conditions. It proposes the concept of a ‘Burst Envelope’, a surface in three-dimensional principal stress space derived based on energy storing and dissipation characteristics of a rock sample, taking into account the size of the specimen and potential localised failure pattern. A scalar burst index is also proposed to quantify the bursting scale. To illustrate and verify its functioning, a numerical modelling framework based on the distinct element method and employing a new cohesive-frictional contact model is used to perform virtual strain burst experiments under different polyaxial loading-unloading scenarios, mimicking various underground excavation scenarios. The obtained results are in good agreement with the theoretical prediction of burst occurrence. On that basis, the variation of burst possibility and magnitude are investigated with key factors, including confinement level and the material's elastic, strength and fracture properties. The effect of the specimen's aspect ratio and size on the rock burst potential is elaborated and verified using virtual strain burst experiments, facilitating the linking of the proposed theoretical framework with the evaluation of in-situ strain bursts in rock masses around underground openings.

The structured sand presents significant interparticle bonding and anisotropy, resulting in significant differences in the physical and mechanical properties from the pure sand. This study proposes a new surrogate model based on the concept of multi-fidelity residual neural network (MR-NN) as an alternative to DEM simulation for predicting the mechanical behaviours of structured sand with different initial anisotropy and saving largely computational costs. The model is initially trained using low-fidelity (LF) data to focus on capturing the main underpinning correlations between macroscopic mechanical parameters with inter-particle properties and anisotropic state variables (i.e., microscopic fabric and tilting angle of sand), where the LF data are generated from previously proposed anisotropic macro-micro quantitative correlation. Subsequent training on sparser high-fidelity (HF) data is used to calibrate and refine the model with HF data generated from DEM simulations for anisotropic structured sand. Feedforward neural network (FNN) is adopted as the baseline algorithm for training models. The macroscopic anisotropic parameters of structured sand predicted by the surrogate model are compared with DEM simulations to examine its feasibility and generalization ability. Furthermore, the robustness of the surrogate model is examined by discussing the effect of LF data on the performance of MR-NN. The superiority of the MR-NN is further verified by comparing the performance of the trained MR-NN with the one-shot trained FNN based on the same HF data. All results demonstrate that the proposed surrogate model can provide a fast and accurate simulation of the anisotropic parameters of structured sand.

Based on Biot's dynamic wave theory and Novak's plane strain model (PSM), an analytical model for the horizontal vibration of a partially embedded pipe pile group (PEPPG) in layered saturated soil (LSS) that considers the soil-plugging effect was established: First, the frequency-domain analytical expressions of the saturated surrounding soil (SSS) and soil plug were obtained by introducing operator decomposition theory and the variable separation method. Second, the analytical solution of the horizontal impedance of a partially embedded pipe pile in the LSS is derived using the coupled condition of the pile–soil interface and transfer matrix method. Finally, an analytical solution for the horizontal impedance of PEPPG in LSS was derived by introducing the horizontal dynamic interaction factor of piles and the superposition method. The derived solution is then reduced and compared with existing theoretical solutions to verify its rationality. In addition, further parametric analysis was conducted to investigate the influences of pile spacing, embedment ratio, properties of layered SSS, and soil-plugging on the horizontal vibration characteristics (HVCs) of PEPPG.