Characteristics Of Stress Field Research Articles

An enhanced numerical model for considering coupled strain-softening and seepage effects on rock masses surrounding a submarine tunnel

The seepage of groundwater and the strain-softening of rock mass in a submarine tunnel expand the plastic region of rock, thereby affecting its overall stability. It is therefore essential to study the stress and strain fields in the rocks surrounding the submarine tunnel by considering the coupled effect of strain-softening and seepage. However, the evolution equation for the hydro-mechanical parameters in the existing fully coupled solution is a uniform equation that is unable to reproduce the characteristics of rock mass in practice. In this study, an updated numerical procedure for the submarine tunnel is derived by coupling strain-softening and seepage effect based on the experimental results. According to the hydro-mechanical coupling theory, the hydro-mechanical parameters such as elastic modulus, Poisson's ratio, Biot's coefficient and permeability coefficient of rocks are characterized by the fitting equations derived from the experimental data. Then, the updated numerical procedure is deduced with the governing equations, boundary conditions, seepage equations and fitting equations. The updated numerical procedure is verified accurately compared with the previous analytical solution. By utilizing the updated numerical procedure, the characteristics of stress field and the influences of initial pore water pressure, Biot's coefficient, and permeability coefficient on the stress, displacement and water-inflow of the surrounding rocks are discussed. Regardless of the variations in hydro-mechanical parameters, the stress distribution has a similar trend. The initial permeability coefficient exerts the most significant influence on the stress field. With the increases in initial pore water pressure and Biot's coefficient, the plastic region expands, and the water-inflow and displacement increase accordingly. Given the fact that the stability of the tunnel is more sensitive to the seepage force controlled by the hydraulic parameters, it is suggested to dewater the ground above the submarine tunnel to control the initial pore water pressure.

Rejuvenation of La-based metallic glass by controlling different modes of relaxation

Metallic glasses (MGs) have multiple dynamic relaxation modes, among which α and β relaxations are two typical modes. The rejuvenation of MGs usually refers to the increase in the total enthalpy of structural relaxation. However, the control mechanism of single-mode relaxation and the contributions of different relaxation modes to the total enthalpy are still unclear. We investigated the effects of stress, temperature and their coupling fields on α and β relaxations through experimental and molecular dynamics (MD) simulations. We found that the temperature field activated more sites for β relaxation, while the prolonged stress leads to the aggregations and connections of a quantity of localized atomic clusters and even activates α relaxation. The coupled field integrates the characteristics of stress and temperature fields and results in a clear splitting of α and β relaxations. Our work provides useful insights into the single-mode regulation of α and β relaxations in MGs.

Coupling CPFE-CA simulation for grain refinement in ultrasonic elliptical vibration diamond cutting of polycrystalline Cu

While microstructure evolution is commonly observed in severe plastic deformation of polycrystalline metals, modulating the cutting-induced grain refinement in subsurface of polycrystalline metals is promising for promoting the performance of machined surface. In this study, we demonstrate the effectiveness of applying ultrasonic vibration assistance (UVA) in effectively grain refinement of polycrystalline Cu in ultra-precision diamond cutting by experiments and multiscale coupling simulations. Specifically, ordinary cutting (OC) and ultrasonic elliptical vibration-assisted cutting (UEVC) experiments of polycrystalline Cu are carried out, and subsequent cross-sectional characterizations of microstructure evolution in subsurface by metallurgical microscope and electron backscatter diffraction, as well as instrumented nanoindentation tests, are performed, which jointly demonstrate significantly promoted grain refinement in subsurface and increased machined surface hardness by UVA, due to increased dislocation density that is beneficial for the nucleation and growth of dynamic recrystallization. In particular, the multi-scale coupling of Crystal Plasticity Finite Element (CPFE) simulation and Cellular Automata (CA) method is firstly established for exploring the microstructural evolution during UEVC and OC of polycrystalline Cu, which is capable of elucidating the underlying correlation of grain refinement behavior in subsurface with characteristics of stress and strain fields in cutting area. Furthermore, the influence of amplitude on the propensity of grain refinement is experimentally and theoretically evaluated, which suggests a critical amplitude of 4 μm that leads to a maximum reduction in grain size by 80.9 % and a maximum increase in machined surface hardness by 55.8 % in UEVC from that in OC, because of the mostly pronounced strain accumulation and dislocation activity. The findings reported in this study demonstrate the effectiveness of applying ultrasonic vibration assistance for modulating the grain refinement accompanying strengthening of machined surface in ultra-precision diamond cutting of polycrystalline metals.

Surface and Deep Structure of the Hanshan–Wuwei Basins in the Lower Yangtze Region: Implications for Mesozoic Tectonic Evolution of the South China Block

AbstractIndicating the tectonic features of the Hanshan–Wuwei basin can reconstruct the framework of the basins formed in Mesozoic and further understand the Mesozoic tectonic evolution of the South China Block. Studies on surface structure, regional stress field and deep geophysical characteristics of the Mesozoic Hanshan–Wuwei basin in Lower Yangtze region were carried out. NE–NNE trending folds and faults developed in the northern margin of the basins. The reconstruction of tectonic stress fields indicates four stress stages dominating the basins' evolution including NW–SE compression, N–S compression, NW–SE extension and NWW–SEE compression. 2D seismic profiles reveal coexistence of thrust, strike‐slip and normal faults in the basin. Combined with regional geological studies, the geodynamic processes for the formation of the Hanshan–Wuwei basin can be divided into five stages: 1) During the Late Triassic, EW trending foreland basin was formed by N–S compression; 2) From Mid‐Jurassic to Late Jurassic, continuous compression strengthened the foreland deformation and formed thrust nappes. In this stage, the integrated foreland basin was compartmentalized or fragmented, and transferred to the broken foreland basin; 3) NE‐trending sinistral strike‐slip movement at the beginning of the Early Cretaceous; 4) Regional extension resulted in normal faults and rift basins developing in the Late Cretaceous; 5) The NWW–SEE compression at the end of the Late Cretaceous caused NW sinistral strike‐slip faults to form, which partly transformed the rift basin.

Characteristics of waterproof failure and optimal width of narrow coal pillars under the coupled effects of mining, excavation and seepage

The failure of waterproof coal pillars under the coupled effects of mining, excavation and water seepage is a significant factor contributing to sudden water inflow accidents in underground roadways. Investigating the instability characteristics and optimal width of waterproof coal pillars holds vital significance for water control and resource protection in mines. This study focus on the rational width of waterproof coal pillar at Dongzhuang Coal Mine in Shanxi Province. Using FLAC3D, a fluid–structure interaction numerical model of waterproof coal pillar was established, revealing the coupling characteristics of stress fields, plastic zones, and seepage zones within coal pillars under the influence of mining, excavation and water infiltration weakening. Furthermore, the stability characteristics of waterproof coal pillars with different widths were compared. The results are as follows: (1) Under the combined action of overlying strata pressure and water pressure from the gob, the coal mass on the water-inflow side of coal pillar is the first to fail. Additionally, with the infiltration of water, the elastic modulus, cohesion, and friction angle of the coal mass in the seepage zone decrease. (2) The lifecycle of waterproof coal pillar can be divided into three stages: working face mining, water infiltration from the gob, and roadway excavation. Based on this, the connectivity between plastic zones and seepage zones serves as the critical condition for the stability of waterproof coal pillar was proposed. (3) When the width of waterproof coal pillar is 3 m and 5 m, plastic zones become connected, forming a water-conducting channel. When the width of waterproof coal pillar is 7 m, 9 m, and 11 m, seepage zones and plastic zones are not connected, and the coal pillar exhibits load-bearing and water-barrier properties.

Determination of mine fault activation degree and the division of tectonic stress hazard zones

This article conducts a comprehensive study on the activation characteristics of faults in the mine and analyzes the distribution patterns of the original rock stress field. Through quantitative research and analysis, we determine the partitioning characteristics of tectonic stress in the mine field under the dual effects of fault activation and original rock stress. The study also reveals the significant impact of different fault activation characteristics and different tectonic stress partitions on the stability of roadway surrounding rock. Using the Mohr–Coulomb strength criterion as a foundation, we investigate the mechanisms of fault activation and establish a mathematical model for fuzzy comprehensive evaluation. This model enables us to determine the strength level of fault activation in coal seam 9 of the Limin coal mine and construct a geological structure model. It has realized the transformation of fault activation degree from qualitative evaluation to quantitative evaluation. The stress state analysis software is used to draw the division of tectonic stress dangerous areas under the synergistic effect of fault activation and original rock stress. We then analyze the impact on the stability of roadway surrounding rock in these different hazardous areas. Utilizing the fuzzy comprehensive evaluation method, we take into account the impact of faults on the distribution characteristics of stress fields and the stability of roadway surrounding rock. This approach enables us to more accurately and comprehensively determine the hazardous areas of tectonic stress in the mine field under the dual effects of faults and original rock stress.

SR-M−GAN: A generative model for high-fidelity stress fields prediction of the composite bolted joints

Predicting the stress fields induced by composite bolted joints constitutes a valuable research endeavor aimed at enhancing the structural integrity of equipment in the aerospace and maritime fields. The stress field characteristics of composite bolted joints may undergo changes due to potential mechanical behaviors, such as damage, constitutive relationships and so on, which exhibit complexity and pose challenges for prediction. Nevertheless, modeling the intricate stress field distribution directly from experimental data poses a formidable challenge, necessitating a substantial investment in materials and resources. Traditional finite element numerical methods have the capability to model the distribution of the stress fields. However, to obtain a high-fidelity solution of this distribution, it is imperative to generate a fine-grid mesh and allocate the significant computational resources. The incorporation of computational techniques in coarse −grid would enhance the efficiency of computations, but these approaches compromise the accuracy of the calculations. In this paper, an innovative deep learning framework is introduced. The network named as the SR-M−GAN is drew inspiration from the conditional generative adversarial network (cGAN) used in the image super-resolution. The primary aim of SR-M−GAN is to mitigate the intrinsic trade-off between result fidelity and computational complexity. The model proposed in this paper effectively reconstructs high-fidelity stress field data from calculations on coarse grids and exhibits excellent reconstruction metrics. Simultaneously, the model only incurs additional computational costs during the deployment of the network model and training. When compared to experimental data, the model continues to demonstrate superior performance in both accuracy and computational efficiency. Therefore, this method offers a high-fidelity data augmentation approach for stress analysis in composite bolted joints, laying the groundwork for large-scale analysis of stress field characteristics in composite bolted joints.

Evolution of stress field and plastic failure characteristics non-isobaric narrow gas storage spaces

The uncertainty of stress distribution and plastic damage threatens the safety of underground gas storage facilities. The bi-directional computational mechanics model is established first, the analytical solution for stress calculation and the implicit equation of plastic zone boundaries are derived. Then, the distribution of stress field in non-isobaric narrow gas storage spaces are studied, and the distribution law of plastic zone is revealed. The results show that: (1) The analytical solution of the surrounding rock of the non-isobaric gas storage roadway is derived, and the implicit equation of the boundary of the plastic zone is obtained; (2) The maximum principal stress exhibits an axisymmetric distribution, gradually decreasing with the increase of the distance; (3) As the stress P0 increases, the pressure equilibrium point increases; as λ increases, the pressure equilibrium point decreases first and then increases; (4) λ is more sensitive to roof failure than the two gangs; When λ is less than 1, the “elliptical” plastic zone appears at the roof, manifested as tensile and shear failure; when λ is 1, the “circular” plastic zone appear, manifested as shear failure; when λ is greater than 1, the elliptical plastic zone appears at the both sides, manifested as shear failure.

A novel fracture criterion for elastic‐brittle cracked body under compression and shear conditions

AbstractWhen the elastic‐brittle cracked body is subjected to compressive‐shear loading, not only does the phenomenon of wing crack initiation and propagation occur, but there is also a notable incidence of secondary crack initiation and propagation due to shear stress. Moreover, the shear fracture toughness (KIIc) predicted by the fracture criterion in the framework of classical fracture mechanics is usually smaller than the tensile fracture toughness (KIc), which is inconsistent with the actual observations. Therefore, this research firstly establishes a mechanical model of an elastic‐brittle cracked body under biaxial compression considering three kinds of crack parameters and materials' mechanical properties. Then, the stress field characteristics at the crack tip are analyzed. Next, a compression‐shear mixed fracture criterion is proposed. Finally, the effects of the lateral pressure coefficient (λ), critical plastic zone size (rc), friction coefficient (f), and deformation parameters of the crack surface on the failure modes at the crack tip are investigated.

FRACTAL CHARACTERISTICS OF CORE DISKING FRACTURE SURFACES

The morphological characteristics of core disking can reflect the in-situ stress field characteristics to a certain extent, but a quantitative description method for disking-induced fracture surfaces is needed. The fractal geometry was introduced to refine the three-dimensional characteristics of the core disking fracture surfaces, and the disking mechanism was explored through morphological characteristics. The innovative concentric-different scale method was proposed to divide the circular surfaces into square surfaces of different sizes. Furthermore, the applicability of four cubic covering methods for single fractal calculation was studied. Finally, the relative differential cubic covering method and multifractal method were used to calculate the three-dimensional data of core disking fracture surfaces in the Jinping area. The results showed that the concentric-different scale method fits the fractal dimension changing law in the exploration of different surface sizes. Using the differential cubic covering method and the relative differential cubic covering method can obtain good results for smoother surfaces. The disking was caused by tensile stress due to high crustal stress unloading, and the cracks originated from the inside of the core. Fractal theory is conducive to analyzing the morphology of rock fracture surfaces and provides guidance for exploring the failure mechanism and influencing factors.

ПОЛЯ НАПРУЖЕНЬ ТА ГЕОЛОГІЧНА СТРУКТУРА ЗАХІДНОГО ЗАМИКАННЯ ГОРЛІВСЬКОЇ АНТИКЛІНАЛІ ДОНБАСУ ЧАСТИНА 2. ПОЛЯ НАПРУЖЕНЬ І ДЕФОРМАЦІЙ

Background. In tectonophysical studies, particular importance is placed on the mechanism of formation of large complex deformation structural elements, and the Horlivka anticline of the Donets Basin serves as such an example. Its importance lies not only in its complex structural composition and development mechanism but also in its potential impact on safe and efficient coal mining. This study aims to analyse the geological structure, stress and strain fields of the western closure of the Horlivka anticline that could be useful in forecasting geological factors for coal mining operations. Methods. The paleostress analysis was performed on the collected fault orientation and kinematic data using Gushchenko's kinematic method based on the analysis of tectonic displacement vectors along slickensides. Mesoregional stress field characteristics were reconstructed by statistical processing of local stereographic solutions. The relative age chronology and staging of tectonic stresses were studied using the stress monitoring method. The characteristics of the principal axes of the total strain field were processed using the GEOS software. Results. Mesoregional stress field is characterised by a subhorizontal NW–SE-oriented maximum principal stress axis and a subhorizontal NE–SW-oriented minimum principal stress axis. This stress regime is classified as a strike-slip regime and is the youngest one (Alpine orogeny) for the Donets Basin. The evolution of tectonic loading conditions of the studied structure is characterised by a deformation series of six stress regimes from the oldest (normal) to the youngest (strike-slip). Stretching axis of the strain ellipsoid is oriented NW and N–S, and shortening axis is oriented NE, nearly orthogonal to the anticline axis. Strike- and oblique-slip faulting regimes of the total strain field were determined for most of the study area, and the deformation has occurred under shear conditions, as indicated by the Lode–Nadai coefficient. Conclusions. A structural pattern of deformation elements, including a conjugate strike-slip fault system of the shear zone, a dome-shaped fold, and longitudinal thrusts in its limbs, may be interpreted as a single pattern of structural paragenesis developed by right-lateral displacements along the longitudinal strike-slip fault system within the Horlivka anticline paraxial part. Fragments of mutual symmetry between the stress and strain fields can be taken as evidence of their genetic relationship.

Material point method to simulate the evolution characteristics of loading damage in fractured sandstone

Cracks in rocks can lead to different load responses and mechanical properties. Primary and secondary cracks are altered during loading, with varying degrees of impact on the failure of underground engineering rock masses. However, the precise modeling of fracture initiation and propagation resulting in failure remains challenging, and the macroscopic evolution characteristics of cracks and stress fields have not been accurately described. In this study, a numerical simulation tool based on the material point method (MPM) and a strain softening model were used to establish a plane loading model with different cutting angles. A simulation of the damage evolution of fractured sandstone under constant-rate loading was conducted for different confining pressures and cutting angles. The results show the following. 1) A comparison of the simulation and experimental results indicates that the proposed numerical method accurately simulates the macroscopic crack generation and the complex physical processes of fractured sandstone during loading. 2) The stress concentration area originates from both ends of the crack and extends toward the edge in a diagonal direction until the crack has penetrated the rock. 3) The ranking of the cutting angle based on the sandstone strength is 0° > 90° > 60° > 30°. The cumulative dissipated energy exhibits an inflection point near the peak stress, and the increase after this point is primarily attributed to friction at the crack surface. The damage degree and dissipated energy of the rock in different stages correspond to the rock’s strength.

Complete constitutive model of CFRP including continuous damage in low strain rate compression and temperature generation in high strain rate impact

AbstractThis paper reports a method to construct the complete constitutive model of carbon fiber‐reinforced plastic (CFRP) by distinguishing the strain rate state and modifying the material stiffness. In quasistatic (low strain rate) compression, the initial nonlinear effect caused by the material defects was described by introducing an nonlinear increasing factor. In dynamic (high strain rate) impact, the nonlinear strengthening effect was described by introducing the dynamic amplification factors of stress and strain based on the reference state. Considering the instantaneous temperature rise obtained from the impact work–temperature generation equation and the influence of temperature on modulus and strength, the coupled thermomechanical constitutive models combining the strain rate effect and the temperature effect were established. The user‐defined material subroutines (VUMAT) were developed, and then these constitutive models were verified by finite element analysis (FEA). Finally, the developed material subroutine was applied to predict the impact penetration of CFRP laminates, and the results show that the opening holes, damage and energy dissipation are in good agreement with the reference experiment. This work comprehensively analyzed the construction method of CFRP constitutive models, which would provide a guidance for the coupled thermomechanical behavior under dynamic impact.Highlights An anisotropic continuum mechanical constitutive behavior considering strain rate effect and damage evolution behavior was established. A macroscopic anisotropic constitutive model including the coupled impact temperature generation‐mechanical behavior was established. Combined with the incremental finite element method, the three‐dimensional user‐defined material subroutines (VUMAT) were developed and verified. The thermomechanical impact penetration behaviors of CFRP laminates was predicted, and the stress field, temperature field and failure characteristics were visualized.

Characteristics, causes and significance of subsurface stress field of Beijing earth fissures group

The development of earth fissures in a certain area often has certain genetic correlations, which is called ‘group-developing phenomenon’ of earth fissures. So far, 41 earth fissures have developed in the Beijing area in the North China Plain. Surveys and exploration studies have shown that these earth fissures have a good correspondence relationship with the activities of the adjacent control faults. Accordingly, they can be divided into 9 zones. The mechanical analysis of these fissures groups reveals that they are roughly formed through 3 models, namely, ‘contemporaneous model’ (dip-slipping movement), ‘secondary model’ (strike-slipping movement) and ‘companion model’ (basin extension movement). The characteristics of this type (fault-controlled) earth fissures in combination with their distribution regularities in the area can be used to indicate that the distribution of subsurface tectonic stress fields is as a whole an extension of the NW-SE-trending, which is not completely consistent with the near E-W-trending extension of deep material migration. These analyses show that under the background of extensional deformation of the basin, the subsurface stress field characteristics of superficial formations are more complex, which not only embodies the movement form (unity) of the deep structure in whole, but also partially reflects the adjustability of the regional tectonic stress field.

Study on Evolution Characteristics of Stress Field of Stepped Fault

With the development of mining to the deep, the ground stress increases correspondingly, and the geological conditions become more and more complicated. Mine dynamic disasters mainly occur near faults. Numerous studies have been conducted on single faults, but few studies have been done on stepped faults formed by double faults under complex geological conditions. Based on this, with the 3110-working face of Zhong Tai Mining in He bi as the engineering background, Single fault model a and different step fault models b, c, d, e and f were established by Flac3D numerical simulation software. The characteristics of stress field evolution of single fault and step fault under the influence of deep mining are studied. The results show that the stress distribution in the upper wall of the fault forms a stress concentration zone, and in the lower wall forms a stress weakening zone. Under the influence of excavation disturbance, the vertical fault distance and the horizontal distance between faults will affect the stress evolution characteristics of the stepped fault.

Multiscale fractures, stress and reservoir quality characterization: The Mississippian Meramec and Osage intervals, STACK play, Central Oklahoma

This study characterized in-situ stress-fields and multi-scale fractures in the low porosity and low permeability Mississippian-aged “Meramec and Osage” intervals in the Anadarko Basin of central Oklahoma. As the two reservoirs' productivity is influenced by fractures, understanding fracture nature, frequency and distribution is important for the development of these reservoirs. The study implemented an integrated approach to characterize in-situ stress-fields and related fractures utilizing 3D seismic data, cores, petrophysics, and formation micro-imaging logs. Density of fractures was interpreted from integrated analyses of seismic, petrophysical, core, and FMI log data. Fracture orientations were determined from the seismic and FMI log data. Detected fractures in the “Meramec” interval preferentially occur in pervasively calcite-cemented rock with low porosity. Because of the adverse effect of calcite cementation, fractures did not significantly improve reservoir quality within the “Meramec” interval. In contrast, detected fractures in the “Osage” interval added sufficient porosity and permeability to overcome negative cementation effects and improved the overall reservoir quality. This study revealed a rotation of the horizontal stress orientations from ancient stress regimes to the present-day in-situ stress regimes. The study demonstrated the application of integrated seismic and non-seismic data in fracture and stress-field characterization, improved our understanding of the effects of fractures on reservoir quality to reduce risks associated with exploring low-porosity/low-permeability mixed carbonate and siliciclastic reservoirs.

Thermo–Solid Coupling Analysis of Bionic Piston for a Mud Pump in Tunnel Engineering

With the development of mud shield tunnel construction technology, the demands on the working performance of a mud pump are becoming higher and higher. As one of the critical components of a mud pump that is easy to wear, the performance of the piston directly affects the operational efficiency and lifespan of the mud pump. The bionic shape of the piston was designed under the guidance of non-smooth surface characteristics of natural organisms to enhance friction and wear performance as well as longevity. The stress field and temperature field characteristics of the pistons with three bionic structures (pit, stripe, and prismatic) were analyzed based on finite element simulation. The stress field analysis results indicated that, for the prismatic shape and pit shape pistons, the maximum stress was concentrated in the lip regions, and both of them bore large stress at the root. For the stripe-shaped piston, the stress was dispersed on both sides of the stripe structure, the stress at the root was small, and the stress gradient along the axial direction was relatively gentle. The stripe-shaped bionic structure can significantly improve the stress distribution state on the piston surface, and the optimal stripe width was recommended to be between 1 and 1.5 mm. The temperature field analysis results indicated that, for the stripe-shaped piston, the surface temperature and heat flux were the smallest, and the temperature gradient was relatively smaller than that of pit-shaped and prismatic-shaped pistons, so it was easier to dissipate heat. When the stripe width was 1.5 mm, the temperature distribution was the most uniform, and the heat flux in localized areas was the smallest, so the heat generated by friction was relatively easy to discharge in the unit area.

Numerical simulation and experimental study on hot rolling forming of spur face gears

Face gear transmission is the transmission mechanism composed of cylindrical gear and face gear. At present, the tooth making method of face gear is still tooth cutting, with low material utilization and low production efficiency. The metal streamlines are cut off, resulting in low fatigue strength of gear teeth. In this paper, the forming of hot rolling was numerically simulated, with which the characteristics of equivalent stress field, equivalent strain field, rolling force variation, and metal flow in the process of rolling were analyzed. Finally, a hot rolling experimental device was set up, by which the face gear test piece was rolled. Then its tooth surface precision was investigated. The research reveals that hot rolling spur face gears is technological feasible and has promising prospects in industrial applications.

Step-over of strike-slip faults and overpressure fluid favor occurrence of foreshocks: Insights from the 1975 Haicheng fore-main-aftershock sequence, China

This study analyzed and summarized in detail the spatial and temporal distributions of earthquakes, tidal responses, focal mechanisms, and stress field characteristics for the M 7.3 Haicheng earthquake sequence in February 1975. The foreshocks are related to the main fault and the conjugate faults surrounding the extension step-over in the middle. The initiation timing of the foreshock clusters and the original time of the mainshock were clearly modulated by the Earth's tidal force and coincided with the peak of dilational volumetric tidal strain. As a plausible and testable hypothesis, we proposed a fluid-driven foreshock model, by which all observed seismicity features can be more reasonably interpreted with respect to the results of existing models. Together with some other known examples, the widely existing step-over along strike-slip faults and associated conjugate faults, especially for extensional ones in the presence of deep fluids, favor the occurrence of short-term foreshocks. Although clustered seismicity with characteristics similar to those of the studied case is not a sufficient and necessary condition for large earthquakes to occur under similar tectonic conditions, it undoubtedly has a warning significance for the criticality of the main fault. Subsequent testing would require quantification of true/false positives/negatives.

Experimental simulation of seepage field distribution for small interval tunnel under varying-head infiltration

In complex hydrogeological environments, it is difficult to achieve a dynamic equilibrium state for the recharge, seepage, and discharge of tunnel groundwater. Especially in case of reservoir flood discharge, local heavy rainfall, extreme dry climate or other emergency situations, the groundwater level will rapidly change, and the tunnel will be under the action of hydrodynamic head height within a certain period of time. Therefore, as urban tunnels in water-rich areas develop towards large cross-section, small spacing and complex sensitive stratum, studying the spatial distribution characteristics of seepage and stress fields in small interval tunnels under variable water head infiltration conditions is an urgent problem to be solved. In this paper, using the self-developed seepage model testing equipment, tunnel seepage tests under different hydrodynamic head heights and heavy rainfall conditions are carried out on the Kexuecheng Tunnel, which is a typical small interval tunnel in complex hydrogeological environment of Chongqing, China. According to the test results, as the burial depth and water level increase, the water pressure at each monitoring point gradually increases. Benefiting from the superimposed drainage effect of adjacent hole, the average water pressure at the inner side is 8 ∼ 12% lower than that on the outer side. The grouting circle and high groundwater level will further aggravate the asymmetric distribution of hydrodynamic pressure on the lining structure. However, as the hydrodynamic head height increases, the effective stress of the stratum decreases, and the earth pressure on the tunnel decreases by 24 ∼ 40%. Under heavy rainfall conditions, the evolution of lining structure pressure can be divided into initial infiltration stage, rapid growth stage, gradually stable stage and slowly decreasing stage. Suffered by the influence of rainstorm on seepage field and stress field, the overall pressure acting on the vault, left haunch, right haunch and bottom of lining structure increase by 45.4%, 132.3%, 117.4% and 147.4%, respectively.