Vertical Compressive Stress Research Articles

Optimized gradient boosting models and reliability analysis for rock stiffness [formula omitted]

The Extreme gradient boosting algorithm XGBoost has been confirmed to be an accurate method for predicting rock stiffnesses and anisotropic parameters from basic input features such as rock porosity, density, vertical compression stress, pore pressure and burial depth (Nguyen-Sy, T., To, Q.D., Vu, M.N., Nguyen, T.D. and Nguyen, T.T., 2020. Study the elastic properties and the anisotropy of rocks using different machine learning methods. Geophysical Prospecting, 68(8), 2557–2578). This study has the following contributions: reducing the R2-error score (that is, 1-R2) by 35 %, RMSE by 21 % and MAE by 16 % comparing to the previous study by considering an advanced CatXG hybrid boosting model in combination with the optimizer Optuna for predicting C13 (the most difficult stiffness to accurately predict); 2-conduct a reliability analysis for the predicted stiffness C13 with respect to the randomness of the input features. We also discuss the use of C11 or C33 as additional input features for accurately predicting C13 as well as the prediction of the related anisotropic parameter δ. This significant improvement of predicted stiffness C13 is extremely important because it encourages petrophysical engineers to use machine learning for predicting the elastic stiffnesses of rocks.

Experimental analysis of the cyclic behavior of rammed earth walls reinforced with arundo donax natural fiber

This experimental study analyzes the seismic behavior of rammed earth walls with Arundo donax natural fiber inclusions, also called "caña brava” (REWC), and without inclusions (REW). The experimental program consists of two stages: physical and mechanical characterization of materials, where properties such as particle size, density, Atterberg limits, and compressive and flexural strength were determined; and dynamic tests, which included compressive and cyclic load tests on REW and REWC walls. The walls were subjected to cyclic loading with derivatives between 0.2 % and 1.4 % in-plane combined with a constant vertical compressive stress of 13 kPa to simulate seismic forces in the presence of gravity loads. The results regarding loading, hysteretic response, energy dissipation, stiffness degradation, and failure mode are compared. In addition, the envelope curves of the load-displacement hysteretic responses are analyzed using a capacity spectrum approach. It is observed that the inclusion of Arundo donax natural fiber negatively affects the mechanical properties of the walls and their hysteretic response. It is also observed that the energy dissipation is affected by the inclusion of Arundo donax natural fiber. However, the stiffness behavior is similar in all the walls. Rigid body failure modes are identified in all the walls, but the walls, including Arundo donax natural fiber, form failure planes that divide the wall into two bodies.

Fracture stratigraphy in oil-rich shale of the Upper Xiaganchaigou Formation (upper Eocene), western Qaidam Basin

Natural fractures in wells from oil-rich shale of the Upper Xiaganchaigou Formation in Qaidam Basin, the largest Cenozoic sedimentary basin in the Tibetan Plateau, vary systematically by stratigraphic position, allowing reconstruction of fracture generation processes during regional tectonic deformation. Compacted fractures, layer-bounded fractures, and low-angle (or bed-parallel) fractures were widely identified in oil-rich shale cores obtained from vertical wells. The layer-bounded fractures can be subdivided into two sets: one prevalent in light-colored (low gamma-ray) stiff layers and arrested by dark-colored (high gamma-ray) compliant layers, the other set primarily confined to the dark-colored compliant layers. The strikes of layer-bounded fractures, revealed by horizontal well FMI logs, are dominantly N60°-80°E and N10°-30°E within the light and dark intervals respectively. These strikes are consistent with natural hydraulic fracturing of the light layers during the Oligocene and the dark layers during the Middle Miocene-Holocene, based on a regional anticlockwise rotation of the compressional strain orientation. The abutment of layer-bounded fractures against low-angle (or bed-parallel) fractures in dark layers implies that the low-angle fractures were hydraulically fractured under a vertical least compressive stress during the Early Miocene, which produced the most intense tectonic compression. Layer-bounded fractures generally remain unmineralized in the dark layers but were mineralized in the light layers, suggesting that fractures confined in light layers would not decrease the sealing capacity of the shale oil system and fractures confined in dark layers can increase the pore space of shale oil storage. The Oligocene and Early Miocene fractures represented mechanical flaws that affected the initiation of Middle Miocene-Holocene fractures, implying that all natural fractures may alter hydraulic fracture stimulations to some extent.

Theoretical Research and Shaking Table Test on Nominal Aspect Ratio of the Isolated Step-Terrace Structure

With the installation of rubber isolation bearings in the upper and lower ground layers, an isolated step-terrace structure was created. Considering the ultimate bearing capacity of the rubber bearing under tension as the critical condition, a comprehensive framework was established to evaluate the overturning failure mechanisms present in isolated step-terrace structures. The bound of nominal aspect ratio was identified as the principal control index within this framework, which incorporates critical parameters such as height ratio (α), width ratio (β), vertical tensile stiffness to compressive stiffness ratio (γ), seismic coefficient (k), and nominal vertical compressive stress (σ0) to provide a thorough analysis of the structural responses and potential failure scenarios. In order to further investigate this matter, a scaled model of an isolated step-terrace concrete frame structure featuring two dropped layers and a single span within an 8° seismic fortification zone was meticulously crafted at a 1:10 similarity ratio. Subsequently, a series of shaking table tests were conducted to analyze the structural response under seismic excitation. The findings indicate that: utilizing the bound of nominal aspect ratio as a metric to gauge the anti-overturning capacity of isolated step-terrace structures is a justified approach. In instances where the height ratio remains constant, the bound of nominal aspect ratio for both positive and negative overturning trended upward with an increase in the width ratio. Notably, the bound of nominal aspect ratio for positive overturning consistently registered lower values compared to that of the negative overturning, underscoring the heightened susceptibility of step-terrace structures to positive overturning. Moreover, in scenarios characterized by higher height and width ratios, the structural integrity remained unscathed by any overturning effects arising from insufficient tensile strength in rubber bearings. Furthermore, the bound of nominal aspect ratio exhibited an ascending trend as the seismic coefficient, nominal vertical compressive stress, and vertical tensile stiffness to compressive stiffness ratio decreased. The outcomes derived from the shaking table test not only confirm the impressive seismic performance of the structure, but also, by closely examining the instantaneous stress variations within the upper and lower isolation layers of the model, substantiate the existence of a positive overturning hazard in scenarios marked by higher seismic coefficients (k). This observation aligns seamlessly with the theoretical projections, thereby substantiating the efficacy of the structural overturning failure theory through direct empirical verification.

Investigation of the Influence of Boundary Conditions on the Deformability of a Model of Soil Base of limited Width in The Form of a Linearly Deformable Elastic Half-Plane

Abstract The paper examines the available approaches to the distribution of vertical compressive stresses over the depth of the soil base from a uniformly distributed load on the surface. The paper investigates the effect of boundary constraints in plan of the model of a continuous linearly deformable (elastic) layer of finite width on its distribution capability and stress-strain performance from a loaded foundation on the surface. Numerical calculations of the stress-strain state of a uniformly loaded foundation with different relative bending stiffness (flexibility), which interacts with a layer of a certain depth (compressible depth) and strict horizontal constraints in terms of width (the width dimensions of the model), were performed using the SCAD software. Based on the analysis of the results of numerical calculations for the plane problem (plane-strain deformation), the width of the model of a linearly deformable layer of finite width, which considers the distribution of compressive loads over the depth at an angle of α≈20÷25° to the vertical from the edges of the loaded foundation, was justified. In this case, the width of the model according to the angle of distribution of α≈20÷25° has virtually no effect on the average settlements and the maximum moment forces of the foundation in comparison with the increase in the width of the model.

Experimental Study of the Mechanical Properties of Full-Scale Rubber Bearings at 23 °C, 0 °C, and -20 °C.

In this study, the effects of ambient temperature on the horizontal mechanical performance of isolated rubber bearings were investigated using high-speed reciprocating loading methods. A comprehensive series of 54 experimental trials are performed on the full-scale (900 mm-diameter) isolation rubber bearings, encompassing a range of temperatures (-20 °C, 0 °C, and 23 °C), shear pressures (50%, 100%, and 250%), and frequencies (0.20 Hz, 0.25 Hz, and 0.30 Hz). Because the compression-shear tests were conducted at high velocities and pressures (specifically, vertical compressive stress of 15 MPa), the equipment used in these tests was capable of generating substantial inertial and frictional forces. Appropriate correction methodologies for the precise determination of mechanical performance metrics for bearings are presented. Then, a comprehensive investigation of the effects of various loading conditions on the characteristic strength, post-yield stiffness, horizontal equivalent stiffness, and equivalent damping ratio of LRB900 (lead-core rubber bearings 900 mm-diameter) and LNR900 (linear natural rubber bearings 900 mm-diameter) is conducted. The empirical results show a discernible relationship between these characteristics and ambient temperature as the number of loading cycles increases, except for the equivalent damping ratio. Finally, empirical fitting formulations incorporating the influence of ambient temperature are presented for each performance indicator. These formulas are intended to assist designers in performing seismic design analyses by allowing them to take into consideration the effects of ambient temperature comprehensively.

Damage evolution and failure mechanism of masonry walls under in-plane cyclic loading

Masonry structures often exhibit poor seismic performance during earthquakes owing to their low material strength, structural integrity, and ductility. To address the mechanism of in-plane damage and collapse of masonry walls subjected to earthquake ground motion, three full-size brick walls, one with structural columns and two without structural columns, were designed and constructed in this experimental study to quantitatively investigate the effect of structural columns on the seismic performance of masonry walls. Quasi-static tests of masonry walls subjected to cyclic lateral loading were conducted using electrohydraulic servo actuators. The in-plane damage evolution and failure mode of the masonry walls under lateral loading were investigated. A numerical model with a novel plastic damage constitutive model of masonry was established. The accuracy of this model and the calculation results were verified by comparing them with the crack development and mechanical property curves obtained from the experiments. Based on the numerical model, a parametric analysis was conducted to investigate the influence of critical factors, such as mortar strength, vertical load, and height-to-width ratio, on the seismic performance of masonry structures. The test results showed that structural columns can effectively improve the performance of masonry structures. Properly installed structural columns can improve the peak load-bearing capacity, ductility, and energy dissipation of masonry structures. The masonry damage constitutive model used in this study could simulate the damage to masonry wall specimens well and reflect the hysteretic curves of the specimens within an acceptable range of accuracy. Thus, it provides a valuable reference for physical experiments. An appropriate mortar strength and vertical compressive stress can effectively enhance the load-bearing capacity of masonry structures within a certain range. For structural column walls. Walls with mortar strengths (average compressive strength) of 12.5 MPa, 10 MPa, 7.5 MPa, and 5 MPa showed an increase in peak bearing capacity of 26.2 %, 23.9 %, 18.5 %, and 9.4 %, respectively, compared to walls with mortar strengths (average compressive strength) of 2.5 Mpa. The aspect ratio had a significant impact on the shear load-bearing capacity of the masonry walls. An increase in the aspect ratio led to a transition from shear failure to flexural failure and a subsequent decrease in shear load-bearing capacity. For masonry walls with structural columns, increasing the diameter of the longitudinal steel in the structural columns can enhance the load-bearing capacity of the masonry wall, resulting in a significant improvement in its overall performance.

Cyclic shear test and finite element analysis of TRC-reinforced damaged confined masonry walls

Due to the low level of seismic fortification during construction, masonry walls are highly vulnerable to earthquakes, but some walls remain in service after repair. To investigate the strengthening effect of textile-reinforced concrete (TRC) on the seismic performance of seismically damaged confined masonry (CM) walls, tests were conducted on four masonry walls, and a finite element model was established for parametric analysis. A TRC facing improves the overall performance of the wall and increases the ductility of the wall. TRC reinforcement technology increases the peak load of CM walls by 19.9%-21.7%, the ductility factor by 42.1%-52.8%, and the cumulative energy dissipation by 95%-118%. The more severe the damage to the wall is, the less effective the strengthening effect of the TRC reinforcement. As the number of layers of textile increases, the load-bearing capacity, deformation capacity and energy dissipation capacity of the walls increase. When the vertical compressive stress increases, the increase in the shear bearing capacity and energy dissipation of walls due to the TRC facings first decreases and then increases. Moreover, TRC facings have limited effectiveness in reinforcing walls with high aspect ratios and high mortar strengths.

Experimental Study on Seismic Performance of Prefabricated Monolithic Concrete–Polystyrene Panel Composite Wall Panels

A normal composite wall panel is a structural component composed of polystyrene insulation boards and concrete surface layers reinforced with steel wire mesh. It can be entirely prefabricated in a factory or constructed with the concrete surface layers cast on-site. A novel prefabricated monolithic concrete–polystyrene panel composite wall panel (CPC wall panel) is proposed in this study. The CPC panel features a middle part that is prefabricated in the factory while the reinforced concrete regions at its two side ends are cast on-site. To evaluate the seismic performance of the wall panel, 18 CPC specimens were designed, manufactured, and quasi-statically tested, through which the structural behaviors, failure mode, and load-bearing capacity were studied. In addition, the influences of the height-to-width ratio and the vertical compressive stress level on the seismic performance of the CPC panels were also investigated. The test results showed that the connectors spaced at 400 mm × 500 mm could ensure the concrete layers on both sides of the polystyrene board worked collectively under seismic conditions. When subjected to lateral loads, the interface between the newly poured concrete and the existing concrete exhibited good bonding. Moreover, the failure mode of the CPC wall panel was largely correlated to the height-to-width ratio that, for specimens having four steel bars of 12 mm diameter and a height-to-width ratio greater than 1, the flexural failure was initially developed, followed by diagonal shear failure. In specimens with a height-to-width ratio of 1, flexural and diagonal shear failures occurred almost simultaneously. For specimens with a height-to-width ratio of less than 1, the final diagonal shear failure was predominant. The longitudinal reinforcing bars at the two ends of the CPC panels could effectively improve their lateral load-bearing capacity, with the enhancement influenced by the height-to-width ratio, the vertical load applied to the wall panel, and the cross-sectional area of the steel bars. In practice, the lateral load-bearing capacity of the CPC panel can be conservatively evaluated using the calculation method of the reinforced concrete shear walls. Finally, the ductility of the CPC specimens was affected by the height-to-width ratio and the axial compressive stress level, such that the specimens with a larger height-to-width ratio and lower axial compressive stress exhibited better ductility.

Study of elastoplastic deformation and crack evolution mechanism of single-crystal ice during uniaxial compression using 3D digital image correlation

Single-crystal freshwater ice was subjected to uniaxial compression tests with different strain rates and crystal orientations, and the real-time full-field deformation and strain were measured by the three-dimensional digital image correlation (3D DIC) method. The mechanism of elastoplastic deformation and crack evolution of single-crystal ice is discussed based on the basal slip. The maximum compressive strength appeared when the crystal orientation (angle between the C-axis and loading direction) θ=0°/90°. When 45°<θ<90°, an inclined bearing layer appeared and caused the specimen to rotate simultaneously. When 0°<θ⩽45°, the vertical compressive stress was sequentially transmitted through the basal plane, two diagonal expansions to the sidewall appeared. The failure mode and compressive strength of ice exhibited a ductile–brittle transition. The strength was positively correlated with the strain rate below the ductile–brittle transition range. The stress concentration at the crack tip was released through basal slip, inhibiting crack propagation, which showed multi-crack ductile failure. When in the ductile–brittle transition range, the cracks could grow stably, leading to axial splitting failure. When the strain rate was higher, the crack tip could not grow stably, producing inclined cracks and resulting in spalling or axial splitting failure, and the ice exhibited a brittle failure mode.

Numerical Modelling and Predicting Performance of Geogrid-Reinforced Low-Volume Unpaved Roads over Soft Subgrades

AbstractPavement performance is usually predicted by large-scale laboratory experiments, expensive field tests and/or comparatively cheaper numerical modelling alternative. In this study, a finite element limiting strain model has been developed to investigate the strain response of both unreinforced and geogrid-reinforced pavements on unpaved roads where the geogrid is placed at the bottom of the unbound granular base layer. A two-dimensional (2D) axisymmetric finite element model (FEM) is used to analyse the behaviour of both unreinforced and geogrid-reinforced granular base. The critical pavement responses (vertical surface deformation, compressive strain and compressive stress at the top of subgrade, etc.) are simulated numerically using ABAQUS. These critical responses are used to develop the pavement performance models (strain limiting models) and to predict long-term service life of pavement or the reduction of thickness of granular base for equivalent service life or a combination of both. The parametric results are then used to develop design charts to aid practitioners to use directly in designs. The numerical model is verified using published literature-based information and found to be reasonable. The results show that the traffic benefit ratio (TBR) of 3.1 at a 20 mm rut depth can be achieved for a thin granular base built over soft subgrade using a medium stiff geogrid.

Seismic behavior of autoclaved aerated concrete masonry walls reinforced with glass-fiber geogrid

Reinforcing horizontal mortar joints with steel bars can improve the seismic performance of autoclaved aerated concrete (AAC) masonry walls. Nevertheless, the diameter of steel bars can require a large thickness of the mortar joints, which in turn results in the thermal bridging effect. Glass fiber geogrids (GFGs) can be employed to replace steel bars to reduce the thickness of mortar joints, and thus can mitigate the thermal bridging effect. Despite considerable advancements in understanding the mechanical performance of GFGs in masonry walls, their effect on the seismic performance of AAC walls remains unknown. Therefore, this study investigates the effect of different GFG configuration rates on the seismic performance of AAC masonry walls using reciprocating load tests. Furthermore, the effect of different vertical compressive stresses on the seismic performance of GFG-reinforced AAC masonry walls was studied via comprehensive finite element simulations. The test results show that the ultimate load-bearing capacity and ultimate displacements of the wall specimens were enhanced as the GFG configuration rate increased, approaching that of benchmark specimens reinforced with steel bars. On the other hand, the cracking and ultimate loads, along with ultimate displacements of GFG-reinforced AAC masonry walls were significantly improved compared to those of the unreinforced AAC masonry walls. It was also evidenced that the crack formation, failure mode, ductility and energy dissipation capacity of the specimens simulated through the finite element method were highly comparable to that of experimentally tested specimens. Accordingly, the vertical compressive stresses were in the range of 0.1 MPa ∼ 0.6 MPa, while the seismic performance of GFG masonry walls was improved with the increase in vertical pressure. The results of the current study confirm that embedding GFG in horizontal mortar joints can delay the cracking of AAC masonry walls and improve their seismic performance. Ultimately, an empirical equation was developed to calculate the seismic shear strength of GFG-reinforced AAC masonry walls.

Using fracture-based continuum modeling of coupled geomechanical-hydrological processes for numerical simulation of hydraulic fracturing

This paper describes numerical simulation of hydraulic fracturing using fracture-based continuum modeling (FBCM) of coupled geomechanical-hydrological processes to evaluate a technique for high-density fracturing and fracture caging. The simulations are innovative because of modeling discrete fractures explicitly in continuum analysis. A key advantage of FBCM is that fracture initiation and propagation are modeled explicitly without changing the domain grid (i.e. no re-meshing). Further, multiple realizations of a preexisting fracture distribution can be analyzed using the same domain grid. The simulated hydraulic fracturing technique consists of pressurizing multiple wells simultaneously: initially without permeating fluids into the rock, to seed fractures uniformly and at high density in the wall rock of the wells; followed by fluid injection to propagate the seeded fracture density hydraulically. FBCM combines the ease of continuum modeling with the potential accuracy of modeling discrete fractures and fracturing explicitly. Fractures are modeled as piecewise planar based on intersections with domain elements; fracture geometry stored as continuum properties is used to calculate parameters needed to model individual fractures; and rock behavior is modeled through tensorial aggregation of the behavior of discrete fractures and unfractured rock. Simulations are presented for previously unfractured rock and for rock with preexisting fractures of horizontal, shallow-dipping, steeply dipping, or vertical orientation. Simulations of a single-well model are used to determine the pattern and spacing for a multiple-well design. The results illustrate high-density fracturing and fracture caging through simultaneous fluid injection in multiple wells: for previously unfractured rock or rock with preexisting shallow-dipping or horizontal fractures, and in situ vertical compressive stress greater than horizontal. If preexisting fractures are steeply dipping or vertical, and considering the same in situ stress condition, well pressurization without fluid permeation appears to be the only practical way to induce new fractures and contain fracturing within the target domain.

Seismic Performance of Desert-sand Autoclaved Aerated Concrete Block Wall Constrained by Precast Integral Construction Column

Considering the structural types and construction techniques used in common residential buildings in Xinjiang, the paper proposes a novel masonry structure, i.e., desert-sand AAC blocks constrained by a precast integral construction column. In order to study the seismic performance of the structure, five wall specimens with different constraints are designed and fabricated and further compared and analyzed in terms of failure mode, hysteretic performance, energy dissipation capacity, ductility, stiffness degradation, and other performance indexes. It has been determined from the results that under horizontal low-cycle repeated loading, the main failure cracks of the desert-sand AAC block wall constrained by the precast integral construction column are intersecting oblique cracks, and the cracks in wall failure penetrate along concrete blocks. The stiffness degradation rate of the wall constrained by the water-resistant gypsum construction column gradually decreases with increasing vertical compressive stress. The wall constrained by the water-resistant gypsum construction column with arranged steel bars has a desirable ductility. Under the same vertical compressive stress, the wall constrained by the water-resistant gypsum precast construction column with arranged steel bars has a significant energy dissipation capacity from cracking to the ultimate state. (4) A restoring force model is built for desert-sand AAC block walls constrained by the precast integral construction column using a three-fold line model, offering technical support for the elastoplastic seismic analysis of structures.

Similarity Model Test on Stress and Deformation of Freezing Pipe in Composite Strata during Active Freezing

In order to prevent the problem of freezing pipe fracture in the process of artificial freezing construction, taking the main shaft freezing project of Shandong Yuncheng Mine as the background and based on the similarity theory, the similarity model test of freezing pipe in the composite stratum of the active freezing section is carried out. The test results show that the distribution of freezing temperature field in sand layer and clay layer is “W”-shape, and the temperature at the interface of outer ring pipe is slightly higher than that of the inner ring pipe. The change rate of freezing temperature can be divided into three stages: rapid decreasing section, slow decreasing section, and stable section. Compared with the clay layer, the sand layer has the shorter freezing closure time and the lower freezing average temperature. The frozen pipe is always in the state of vertical compression in the clay layer, while in the state of vertical compression first and then vertical tension in the sand layer. The maximum compressive strain in the clay layer is −305 με, which is equal to the vertical compressive stress of 64.1 MPa. The maximum tensile strain in the sand layer is 406 με, which is equivalent to the tensile stress of 85.2 MPa. The bending direction of different freezing pipes is different in the different soil layers. The maximum bending strain of the frozen pipe in the clay layer is 779 με, which is 2.6 times the vertical strain at the same location, corresponding to the bending stress of 163.4 MPa and reaching 0.69 times of the yield limit of frozen pipe, while the bending strain in the sand layer is very small. The vertical strain and bending strain of freezing pipes at the interface of soil layers are very small, but the bending strain is still dominant.

Theoretical analysis of two cracks in sandstone under the simultaneous action of pressure and shear stress

AbstractThe cracked rock mass could slide along the weak side under the action of compressive loads and shear loads in landslides; thus, the stability evaluation of the crack in rock mass media is of particular importance. The formulas of stress intensity factors (SIFs) for each crack tip are theoretically derived by implementing complex function theory. The results of the analytical solutions are then compared with the predicted results of the finite element analysis via ABAQUS software, revealing a slight relative error. Photo elastic experiment and static compression tests are conducted to verify the correctness of theoretical results. The obtained results reveal that the internal pressure leads to the increasing SIFs of mode II at crack tips. Further, the larger the difference between the horizontal and vertical compressive stresses, the more likely the occurrence of shear failure in the cracked rock mass. For the two cracks, the most dangerous point is the inner crack tip of the longer crack.

Experimental validation of a non-linear train-track-bridge dynamic model of a stone arch railway bridge under freight traffic

ABSTRACT This article presents the validation of a non-linear FE numerical model of a multi-span stone arch railway bridge based on experimental tests and under in-service freight trains. Static loading tests allow evaluating the bridge response in terms of vertical displacements in the arches, opening/closure deformations on specific block joints of the arches, and vertical compressive stress variations in the piers. The bridge FE model is developed by combining the potentialities of a global continuous homogeneous model, based on FEM and Drucker-Prager model, and a local modelling approach based on a dedicated non-linear contact model. The freight vehicle modelling is based on a flexible FE approach, and the validation of the dynamic behaviour of the train-bridge system involved the comparison between numerical and experimental responses. All the numerical responses are in very good agreement with the experimental responses. Finally, a simulation of the dynamic behaviour of the train-bridge system is performed for realistic scenarios of freight traffic considering speeds between 40 and 140 km/h.

Vertical Stress Induced Anomalous Spectral Shift of 13.17° Moiré Superlattice in Twist Bilayer Graphene

The electronic states of the twist bilayer graphene (TBG) moiré superlattice are usually regulated by the rotation angle, applied electric field, applied magnetic field, carrier concentration and applied stress, and thus exhibit novel physical properties. Squeezing, that is, applying vertical compressive stress to the graphene layers, has profound significance in regulating the photoelectric properties of the moiré superlattice and constructing optical nanodevices. This paper presents the photoelectric properties of a TBG moiré superlattice with a twist angle of 13.17° and tunability under vertical stress. Interlayer distance decreases nonlinearly with compressive stress from 0 to 10 GPa, giving rise to weakened interlayer coupling compared to a Bernal-stacked graphene bilayer and an enhanced repulsive effect between the layers. The calculated Bloch wave functions show a strong dependence on stress. With the increase in stress, the band gaps of the system present a nonlinear increase, which induces and enhances the interlayer charge transfer and leads to the redshift of the absorption spectrum of the moiré superlattice system. By analyzing the differences in the Bloch wave function and charge density differences, we explain the nature of the physical mechanism of photoelectric property change in a stress-regulated twist superlattice system. This study provides a theoretical basis for the identification of piezoelectric properties and the stress regulation of photoelectric devices based on TBG, and also provides a feasible method for regulating the performance of TBG.

Focal Mechanisms and Stress Field Characteristics of Microearthquakes in Baihetan Reservoir in the Downstream Area of Jinsha River

The Baihetan Reservoir was impounded on 6 April 2021, after which the water level rose significantly. Notably, after one week of impoundment, microseismic activities were prominent around the reservoir area, which was highly associated with the water level change. From 6 April 2021 to 31 December 2021, over 7000 microearthquakes were recorded by the seismic stations in the vicinity of the reservoir, including 12 ML &gt; 3 events. The maximum was the 21 December 2021 ML3.9 earthquake in Qiluogou town, Sichuan. The post-impoundment seismic events were clustered in Hulukou town in the Qiaojia Basin, with an overall “Y-shaped” pattern. In this study, taking advantage of the high-frequency waveform matching approach, the pre- and post-impoundment focal mechanism solutions totaling 207 ML &gt; 2 earthquakes are successfully obtained. The impoundment-induced stress change is analyzed, and the iterative joint inversion method is used to invert the stress field. Major results and conclusions include the following: (1) After impoundment, the number of normal fault earthquakes remarkably increased in the reservoir area; (2) Impoundment has led to a vertical compressive stress field and horizontal tensile stress field in the area where microearthquakes occurred. It is necessary to pay close attention to possible moderate-to-strong earthquakes in the future.

Performance analysis of subgrade in asphaltic rail track design and Indonesia’s existing ballasted track

AbstractDue to subgrade-related concerns, the performance of Indonesia’s ballasted track continues to be significant impediments for the Indonesian railway stakeholders’ intention to increase the speed of passenger train operations. This study aims to examine the vertical compressive stress in the subgrade of Indonesia’s ballasted track and two asphaltic rail track designs, asphaltic overlayment and asphaltic underlayment, under various cyclic loading conditions based on three different train speeds, 120 (low speed), 240 (medium speed), and 360 kph (high speed). The AC layer thicknesses for each asphaltic rail track design are as follows: 0.1, 0.2, 0.3, and 0.4 m for asphaltic underlayment, and 0.075, 0.15, 0.225, and 0.3 m for asphaltic overlayment. 2D finite element models and simulations were used to capture and predict the subgrade’s vertical compressive stress performance. The most obvious finding to emerge from this study is that the asphaltic overlayment track has a greater capacity for transmitting and decreasing stresses from the top structure to subgrade layer than the asphaltic underlayment track and the Indonesia’ ballasted track, respectively. This research can shed light on the prospective application of asphaltic rail track to the Indonesian rail network for the faster passenger trains operation.