Maximum Vertical Stress Research Articles

Large-scale testing facility for heavy haul track

Given the substantially increased demand for increased axle loads of heavy haul trains, there is an imperative need to develop sustainable track infrastructure. When subjected to heavy axle loading, ballast aggregates rapidly break down, compromising the particle friction and associated load bearing capacity. Therefore, understanding the deformation and degradation (breakage) of ballast subjected to various boundary and loading conditions is crucial for improved track design and performance monitoring. Ideally, field testing should be carried out in real-life tracks to avoid laboratory scale and boundary effects, but field tests are often expensive, time-consuming and may disrupt rail traffic, hence not always feasible. A prototype test facility that can simulate appropriate axle loading and boundary conditions for standard gauge heavy haul tracks is presented in this paper. In collaboration with more than a dozen Universities and Industry organisations, Australia's first and only National Facility for Heavy-haul Railroad Testing (NFHRT) has recently been constructed and is now fully operational. This new facility enables a real-size (1:1 scale) instrumented track section to be subjected to continuous cyclic loading simulated via two pairs of dynamic actuators in synchronized operation. The results of a typical test are presented in this paper including the measured track settlement and lateral deformation, transient vertical and lateral stresses, rail and sleeper accelerations, resilient modulus and breakage of ballast. The test results show that an average track settlement of about 14 mm and lateral displacements up to 9 mm are recorded after 500,000 load cycles. Subjected to a 25-tonne axle load, the maximum vertical stress measured at the sleeper-ballast interface is about 225 kPa and this attenuates with depth. The test results of this iconic facility are generally consistent with actual field measurements obtained in heavy-haul tracks located in the towns of Singleton and Bulli in the state of New South Wales, Australia.

Study on the Applicability of an Improved Pile-Beam-Arch Method of Metro Station Construction in the Upper-Soft and Lower-Hard Stratum

The main challenge for metro station construction is to ensure the construction schedule while minimizing the impact on the surrounding environment. Based on the characteristics of the upper-soft and lower-hard stratum in the Dalian city of China, a special structure with six pilot tunnels and three spans of the pile-beam-arch (PBA) method is proposed and applied to the construction of the Labor Park metro station. In this study, laboratory experiments, numerical simulation analysis, and field monitoring data are used to investigate the applicability of six pilot tunnels and three spans of the PBA method in the process of construction. In the process of numerical simulation, the ground surface settlement, arch vertical displacement, the horizontal displacement of rock mass on both sides of the station, and vertical stress of support structure are analyzed. The numerical simulation results are found to be in good agreement with field measurement. Furthermore, the results indicate that, in the construction of six pilot tunnels and three spans of the PBA method, pilot tunnel construction and arch construction have a great influence on the surface settlement. The part of the initial vertical support of the pilot tunnels is removed, which caused the change of the position of maximum vertical stress and the redistribution of vertical stress. Because of reinforcement by side piles on both sides of the station, the horizontal displacement of the rock mass was reduced by 44.76% compared with that without reinforcement. The six pilot tunnels and three spans of the PBA method can effectively control the surface settlement, arch vertical displacement, and horizontal displacement of the rock mass.

Investigation on Surrounding Rock Stability Control Technology of High Stress Roadway in Steeply Dipping Coal Seam

There are a large amount of steeply dipping coal seams deposited in China, the safe and effective extraction of which are the challenge for coal operators due to the complicated geological characteristics, in particular, when the underground roadway is excavated in the steeply dipping coal seams with limited seam distance. The Universal Distinct Element Code (UDEC) was adopted in the present research to explore the stress distribution of surrounding rock of the roadway. Based on the numerical simulation, the damage coefficient was proposed and then used to classify the roof conditions into four groups. After that, the asymmetric support technique was proposed and put into practical applications. It is indicated that the stress concentration on the floor is the main feature of the extraction of steeply dipping coal seams. Moreover, the distributions of the maximum vertical stress and horizontal stress which are much different from each other mainly attributed to the effect of the large dip angle. This research also verified the feasibility of using the asymmetric and partition support technique to maintain the integrity of the surrounding rock, as from the case study conducted at the 12032 longwall coal face of Zhongwei coal mine.

Determining the resilient modulus of sandy subgrade using cyclic light weight deflectometer test

Resilient modulus is one of the important parameters used in the analysis and modelling of subgrade behaviour and it is directly related to the safety, economic and lifetime of roads structures. The determination of Resilient Modulus (MR) using repeated load triaxial (RLT) test is expensive, cumbersome and time consuming. Therefore, a new research approach has been adopted in this study based on performing cyclic light weight deflectometer (LWD). The objective of the cyclic loading is to bring the soil to its near elastic behaviour and then measure the dynamic deformations moduli (Evd). The cyclic LWD tests were performed in the field and in the laboratory and the measured Evds values were compared to the resilient moduli measured by RLT tests conducted under similar testing conditions. The number of LWD test cycles required to bring the soil to its elastic state can be decided directly during the test by using a new multifunctional LWD manufactured solely for the purpose of this study. It has been found that the required number of LWD repetitions (cycles) to conduct Evd moduli close to MR moduli depends on many influencing factors, namely, the moisture contents, lateral confinement and the maximum vertical axial stress levels during testing. Based on this study, it can be inferred that the cyclic LWD test can be used to predict simply and quickly the resilient behaviour of the tested soil with good precision and reliability.

Experimental study on vertical stress distribution underneath granular silos

Present study is aimed to experimentally scrutinize the horizontal distribution of vertical stress underneath granular silos using a high-sensitivity stress sensor mat system. Two granular materials of contrasting particle shape but similar size were utilized to construct granular silos by the alleged local source method. The experimental results indicate that the average vertical stress is agreeable with the value predicted by Janssen's law; however, the distribution of measured vertical stress is found to be non-uniform over the horizontal plane, which is contrary to the assumption of Janssen's law. The distribution non-uniformity is aggravated by a more elongated particle shape. The maximum vertical stresses occur at vicinity of enclosing wall and the vertical stresses lower than the average value appear at the center zone with fluctuations. We proposed an empirical equation with three parameters for characterizing the distribution of vertical stresses normalized by the average value given by the Janssen's law.

Numerical simulation research on response characteristics of surrounding rock for deep super-large section chamber under dynamic and static combined loading condition

The stability control of surrounding rock for large or super-large section chamber is a difficult technical problem in deep mining condition. Based on the in-site geological conditions of Longgu coal mine, this paper used the dynamic module of FLAC3D to study the response characteristics of deep super-large section chamber under dynamic and static combined loading condition. Results showed that under the static loading condition, the maximum vertical stress, deformation and failure range are large, where the stress concentration coefficient is 1.64. The maximum roof-to-floor and two-sides deformations are 54.6 mm and 53.1 mm, respectively. Then, under the dynamic and static combined loading condition: (1) The influence of dynamic load frequency on the two-sides is more obvious; (2) The dynamic load amplitude has the greatest influence on the stress concentration degree, and the plastic failure tends to develop to the deeper; (3) With the dynamic load source distance increase, the response of surrounding rock is gradually attenuated. On this basis, empirical equations for each dynamic load conditions were obtained by using regression analysis method, and all correlation coefficients are greater than 0.99. This research provided reference for the supporting design of deep super-large section chamber under same or similar conditions.

The feasibility of PSO–ANFIS in estimating bearing capacity of strip foundations rested on cohesionless slope

This study aimed to assess particle swarm optimization (PSO) optimized with an adaptive network-based fuzzy inference system (ANFIS) for the prediction of the ultimate bearing capacity of strip footing resting on a sloping crest. To make datasets, a 2524 finite element model (FEM) simulation analysis was performed. In this sense, the k-fold validation technique selected for the selection of the testing and validation purposes. Also, the variables of the ANFIS algorithm (i.e., the number of clusters), and the PSO algorithm (the population size were optimized using a series of trial and error process. The second main objective of the current study was to assess the applicability of PSO–ANFIS in estimating the ultimate bearing capacity of the strip footing resting on a single cohesionless slope when it was subjected to an external vertical applied stress. The input parameters were the soil properties (Stype) (classified into five different classes from the weakest, shown as S1, to the strongest type, displayed in higher order), slope angle (β), setback distance ratio (b/B) (the distance of the footing from the slope crest to the footing width), and vertical settlement (Uy) of the footing, while the main output was the applied vertical stresses (Fy) that can be applied on the footing. Note that the ultimate bearing capacity (Fult) is the maximum vertical stress when Uy = 0.1 B. A thorough comparison between the measured FEM simulations, the proposed ANFIS and PSO–ANFIS models were carried out to demonstrate their effectiveness in predicting a reliable Fult resting on cohesionless slopes.

EFFECT OF ORIGINAL STRESS STATE OF THE ROCK MASS ON DISTRIBUTION OF DISPLACEMENTS AND STRESSES IN THE VICINITY OF SUPPORTED MINE WORKING IN UNSTABLE ROCKS

The statement of the problem of finding the stress-strain state of support and rocks surrounding the mine working in case of cushioning with phenol resins is used to determine the peculiarities of its formation under the influence of gravitational and hydrostatic original stress fields. It is found that maximum vertical compressive stresses arise in vertical support units at gravitational field, which exceed by half their values for a hydrostatic field. However, as the height in the mine roof filled with phenol resins increases, maximum vertical stresses for both options become comparable. In arch support units at hydrostatic original state of the rock mass, the values of horizontal tensile stresses are by half less than at gravitational original field.

Influence of wheel polygonal wear on wheel-rail dynamic contact in a heavy-haul locomotive under traction conditions

For a heavy-haul locomotive within a wheel repairing period, wheel polygonal wear with different operating mileages is obtained by field testing. The test results show that the maximum radial runout of the wheel can increase to 0.87 mm and accompany with the typical damage of wheel tread shelling. Taking the wheel polygons as input excitation, the locomotive-track coupled dynamic model is established, which is verified by the comparisons of test and calculated wheelset vertical acceleration in time and frequency domains. The variable wheel-rail friction coefficient is introduced so as to consider the dry and wet rail conditions. The wheel-rail dynamic contact characteristics under the traction and dry-wet rail surface conditions are analysed simultaneously. It is found that the wheel polygon deteriorates the locomotive traction performance and induces the obvious wheel-rail slipping with large tangential stress, especially in wet rail condition. In dry condition, the wheel-rail could contact generally in the adhesion state. But the longitudinal creep forces fluctuate locally with some larger amplitudes closed to the adhesion force, which is mainly attributed to the excitation of serious wheel polygon. Comparing with the results of the newly repaired wheel, the maximum wheel-rail vertical force, longitudinal force, normal stress and tangential stress at the end of wheel repairing period can increase by 55 kN, 28 kN, 240 MPa and 470 MPa in sequence. The wheel-rail slipping and high-stress state in traction condition should be the dominant factors contributing to the wheel damage of tread shelling.

Mechanical Response of Buried Polyethylene Pipeline Beneath the Roadway and Its Influential Factors

In order to investigate the 2D mechanical response of flexible buried pipeline beneath roadway under static loads, taken high density polyethylene as object, ANSYS finite element commercial software is employed to construct pipe-soil interacting coupled model. By means of newly developed model, the influence of burial depth, diameter of pipe and modulus of backfilled earth on deformation and mechanical characteristics of pipe is studied. Furthermore the viscoelastic properties of tubular product are considered to simulate creep deformation of pipe under constant loading. These results indicate that: (1) the maximum vertical displacement and Von Mises stress appear at top of pipe and side of internal surface of pipe, respectively; (2) the viscoelastic properties significantly improve the safety of pipe; (3) even if burial depth does not have significant effect on mechanical characteristics of pipe, there is optimal burial depth; (4) the greater the diameter of conduits, the larger modulus backfilled earth, the more difficult it is to destroy pipe breaks.

Influence of Vertical Prestressed Tensioning Process on Box Girder Web

In order to study the effect of vertical prestressing tension timing and sequence on the vertical stress change of concrete box girder webs, an ANSYS finite element model was established with a large-span continuous rigid frame bridge to simulate the different tensioning sequence of the vertical prestressing steel strands. This paper analyzed the reasonable time of vertical prestressed tension, the stress variation of web plate under different ways of vertical prestressed tension, and the influence of secondary vertical prestressed tension on web crack resistance. The vertical compressive stress at the bottom of the adjacent beam section is obviously smaller than that at the middle of the beam section during segmental tensioning, which leads to the problem that the difference between the free end section and the vertical compressive stress at the end of the beam section is large. During the vertical prestressing of the two lagging sections, the vertical normal stress of the web is more uniform. The maximum vertical compressive stress is 2.7% larger than the overall tension, which is 32.3% less than the maximum vertical compressive stress of the segmented tension. In the case of vertical prestress loss in the initial tension, the reasonable use of the secondary tension construction process can significantly increase the cracking load of the beam.

Numerical modeling of longwall top coal caving method at thar coalfield

Thar Coalfield in Pakistan is the largest reserve of lignite coal in the country, which is outlined by thick coal seams. The preferred mining method for these thick coal seams is the Longwall Top Coal Caving (LTCC). The connection of both the top coal caving and its compactness are still rare; therefore, the capability of top coal caving mechanism is the most significant factor in LTCC that must be adequately explained and examined. Moreover, in order to achieve the ideal coal production, a comprehensive modeling of deformation and induced stress is mandatory. In this study, a 12 m thick coal seam with cutting to caving height ratios like 1:2 and 1:3 has been modelled, and the mechanism of longwall top coal caving demonstrated and front abutment vertical stress distribution in front of face line values were computed with the help of UDEC at Block-IX, Thar Coalfield. The results reveal that a thick layer of top coal can be progressively caved behind the face at the ratio 1:3 instead of 1:2 (which explained the incompetent caving progress of top coal). Similarly, the maximum vertical abutment stress of 20 MPa was observed at 6m in front of the face when cutting to caving height ratio was 1:2 and at 3m in front of face with 1:3 (which is comparatively capable for the face advancement), respectively. Therefore, this numerical modeling study proposes the reasonable height of top coal caving at cutting to caving height ratio 1:3 for the efficient production of thick coal seams at Thar Coalfield.

Numerical Simulation of Hydraulic Fracturing Effect Based on Abaqus

This study simulates the hydraulic fracturing process based on ABAQUS, and obtains the results of hydraulic fracturing fracture propagation under the influence of multi-parameters. Our research shows that:The effect of reservoir modification increases with the increase of injection rate of fracturing fluid. Under the condition of 8m3/min or above, the fracture can communicate with the fault zone 100m away from the bottom of the well. Under the condition of 10cP, the effect of fracture propagation in formation is good. Under the condition of 50cP, the effect of fracture propagation in fracture zone is good. Under the condition of 200cP, the effect of crack propagation is poor. When the injection volume of fracturing fluid is between 0 and 1440m3, the effect of fracture propagation increases gradually. When the injection volume of fracturing fluid is larger than 1440m3, the contribution of continuous injection become weak. The maximum vertical crustal stress results in the fracture extending to the depth. The maximum horizontal crustal stress results in the horizontal propagation of fractured fractures. This study analyzed the influence of formation parameters and hydraulic fracturing parameters on fracture propagation. It also plays an active role in predicting the effect of hydraulic fracturing.

Inversion Method of Initial In Situ Stress Field Based on BP Neural Network and Applying Loads to Unit Body

The initial in situ stress field is the fundamental factor causing the deformation and failure of underground engineering and is an important basis for the feasibility analysis, design, and construction of underground engineering. However, it is difficult to obtain the whole in situ stress field of large‐scale underground engineering in difficult and dangerous areas by field measurement. In view of the fact that the measured in situ stress components (σxx, σyy, σzz, τxy, τxz, τyz) of Sichuan‐Tibet Railway in China are linear with the buried depth, a method is proposed to solve the in situ stress by applying corresponding loads to all unit bodies in the calculation area based on BP neural network and FLAC3D. Through this method, the in situ stress of the tunnel is inverted. The results show that both the maximum principal stress and minimum principal stress increase with the increase of buried depth, and when the tunnel passes through faults or anticlines, the main stress will suddenly drop. Furthermore, compared with the results of the multiple linear regression method, it is found that the proposed method has higher accuracy; especially for the simulation of the maximum horizontal principal stress and vertical stress, the average relative error is reduced by 26.44% and 77.27%, respectively. The research in this paper can provide a new idea for the initial in situ stress inversion of engineering.

Geological Core Ground Reorientation Technology Application on In Situ Stress Measurement of an Over‐Kilometer‐Deep Shaft

As mining progresses to depth, engineering activities face the extreme challenge of high in situ stress. To efficiently measure the deep in situ stress before engineering excavation, an innovative deep in situ stress measurement method capable of the geological core ground reorientation technology and acoustic emission (AE) technology was proposed. With this method, nonorientation geological cores collected from the thousand‐meter‐deep borehole were reoriented based on the spatial spherical geometry model and borehole bending measurement principle. The distribution of deep in situ stress of an over‐kilometer‐deep shaft in the Xiangxi gold mine was investigated with real‐time synchronized MTS 815 material testing machine and PCI‐II AE instrument. The results show that the in situ stress changes from being dominated by horizontal stress to being dominated by vertical stress with depth. The horizontal maximum principal stress and vertical stress gradually increase with depth and reach a high‐stress level (greater than 25 MPa) at a depth of 1000 m. The direction of the maximum principal stress is near the north. Meanwhile, to analyze the accuracy of the measured in situ stress comparatively, the stress relief measurements were performed at a depth of 655–958 m in the mine, using the Swedish LUT rock triaxial in situ stress measurement system. The distribution of deep in situ stress obtained by the stress relief method agrees well with that by the AE method, which proves the reliability of the AE in situ stress testing method based on the geological core ground reorientation technology.

Construction Mechanics of Ultra Deep Ventilation Shaft in Subway Station under Complex Engineering Environment

Take one subway station of Chongqing rail transit as background, 3D FEM models of sequential excavating to large-section and ultra-deep shafts under complicated environment were set up, and influence of shaft construction to the stability of shaft wall rock, slab-pile walls, high-voltage tower foundation and intersection of shafts and air flues was analyzed. The analysis result showed: wall rock displacement near the contour line of shaft and air flue changed significantly and the maximum displacement was 10.84 mm; vertical displacement caused by shaft and air flue excavating affected ground surface and the maximum ground surface settlement was 20 mm; shaft excavating brought large stress to shaft walls and the maximum vertical stress was 2.94 MPa; plastic zone of wall rock mainly distributed in the junction of shaft and air flue, outward excavation contour line for 6-10 m, and the distribution range was large. The maximum principal stress of shaft and air flue during initial support at the junction of air flue vault and curve walls was large, and the maximum stress was 6.09 MPa; the maximum and minimum principal stress of slab-pile retaining walls before shaft and air flue excavating was -1.60 MPa and 0.32 MPa, then increased to -1.78 MPa and 0.63 MPa respectively after excavating, additionally, shear failure would not happen; the maximum displacements from X-scale, Y-scale and Z-scale of tower’s foundation were 0.32 mm, 0.86 mm and 1.20 mm respectively. Furthermore, the difference of inhomogeneous deformation was small, and shaft construction had no effect to high-voltage towers.

Numerical investigation of failure evolution for the surrounding rock of a super‐large section chamber group in a deep coal mine

AbstractThe stability control of surrounding rock for a large or super‐large section chamber group is a difficult technical problem in deep mining conditions, and this stability control has become one of the most important factors restricting the safety of a large‐scale coal mine. Based on the in‐site geological conditions of a super‐large section chamber group that functions for a coal gangues separation system in the Longgu Coal Mine, we first researched the stress, deformation, and failure characteristics of the in‐site chamber group surrounding rocks by using the FLAC3D software. Simulation results showed that the maximum vertical stress, deformation, and failure range of the surrounding rock for a super‐large section chamber group are larger than those of an ordinary section chamber. In addition, the roof subsidence and the plastic zone radius at the intersection are obviously larger than that of other parts, which increase by approximately 50.8% and 44.4%, respectively. Therefore, the chamber group intersection should be taken as the key area for surrounding rock control. Then, the influences of two key parameters of the chamber group, spacing and angle, were researched in detail to help in the design of the chamber group. Results show that when the chamber spacing is 80 m, the interaction begins to occur. When the angle is 70°, the stress of the surrounding rock and roof subsidence reach the minimum. Thus, the optimum chamber group parameters are determined to be a spacing of 80 m and an angle of 70°. Finally, from the perspective of chamber group stability, the optimal chamber group parameters can better meet the normal use for the super‐large section chamber group. This research provides a reference for the design of a super‐large section chamber group under the same or similar conditions and provides a strategy for controlling the surrounding rock.

Tire-Pavement System Modelling and Triaxial Contact Stress Analysis

In the pavement design, the tire load is usually assumed to be a circular or rectangular evenly distributed load, which is actually not like this. In order to provide data reference for the development of tire-pavement contact stress intelligent test instrument, it is necessary to model the tire-pavement system, and construct tire-pavement 3D finite element model using large finite element software. Firstly, establish three-dimensional model of tire and road surface, and impose certain loads and constraints. Secondly, the triaxial contact stress of the tire-pavement under static conditions is analyzed. Then the triaxial stress under traction conditions is analyzed, and the triaxial stress under different conditions of use of the tire is compared and analyzed. The results show that as the load of the tire gradually increases, the maximum vertical contact stress and the maximum longitudinal stress gradually decrease, but the maximum lateral pressure gradually increases. When the inflation pressure of the tire is gradually increased, the contact stresses in the three directions are correspondingly increased. However, the maximum vertical contact stress and maximum longitudinal stress increase more obviously than the maximum transverse contact stress. In addition, the transverse contact stress could be ignored under traction conditions.

Optimization of parameters affecting horizontal cracking in continuously reinforced concrete pavement (CRCP)

Field evaluation of distresses in continuously reinforced concrete pavement (CRCP) indicated punch-out distress associated with horizontal cracking at the depth of the longitudinal steel is the most severe performance problem in CRCP. The developed 3-D model was used to perform a parametric analysis to determine the effects of critical loading location, concrete properties, and longitudinal steel design on horizontal cracking potential. The maximum vertical tensile stresses in the concrete were slightly affected by the coefficient of thermal expansion of the concrete. The critical tensile stresses in the concrete were observed to decrease as the base modulus, slab–base friction, slab thickness, and transverse crack spacing increase. The vertical tensile stresses significantly decreased when the longitudinal steel spacing decreased. The use of varying longitudinal steel spacing and reducing the depth of steel may be one of the ways to reduce the horizontal cracking potential without changing the steel ratio of the slab.

Analysis of mechanical properties of transition segment of road and bridge based on high-strength foam concrete

The settlement difference in the transition section of road and bridge may lead to traffic accidents, which has many negative effects. Back filling of foam concrete behind abutment is a good method of treatment. In this study, a kind of high-strength foam concrete was designed for back filling of transition section of road and bridge, and the mechanical properties of the transition section of road and bridge were analyzed. It was found that only a large number of micro cracks grew in high-strength concrete under a large number of load cycles, and structural damage did not happen, indicating the high-strength foam concrete had high strength and long service life; the static base pressure was about 40 KPa, indicating a small burden on the structure. Under the vehicle load, the vertical displacement of the transition section of road and bridge with back filling of high-strength foam concrete behind abutment was about 0.7 m and the maximum vertical stress was about -40 KPa, which was significantly lower than that of the ordinary concrete, and the stability was favourable, indicating that the high-strength foam concrete had good vibration absorption effect. The analysis results verifies the value of the high-strength foam concrete as the backfill material of the transition section of road and bridge, providing some theoretical basis for its application and popularization.