Unloading Rate Research Articles

Experimental Investigation on Deformation and Strength Behavior of Marble with the Complex Loading-Unloading Stress Path

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading-unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high-stress underground engineering.

3D Finite Element Pseudodynamic Analysis of Deficient RC Rectangular Columns Confined with Fiber Reinforced Polymers under Axial Compression.

This paper utilizes the advanced potential of pseudodynamic three-dimensional finite-element modeling to study the axial mechanical behavior of square and rectangular reinforced concrete columns, confined with fiber reinforced polymer (FRP) jackets and continuous composite ropes in seismic applications. The rigorous and versatile Riedel-Hiermaier-Thoma (RHT) material model for concrete is suitably calibrated/modified to reproduce the variable behavior of characteristic retrofitted columns with deficient internal steel reinforcement detailing, suffering nonuniform local concrete cracking and crushing or bulging and bar buckling. Similarly, the 3D FRP jacket or rope confinement models may account for damage distribution, local fracture initiation and different interfacial bonding conditions. The satisfactory accuracy of the reproduced experimental stress-strain envelope behavior enables the analytical investigation of several critical design parameters that are difficult to measure reliably during experiments. Additional parametric analyses are conducted to assess the effects of steel quality. The significant variation of the field of developed strains on the FRP jacket at the ultimate and of the developed strains and deformations on steel cages among different columns are thoroughly investigated. This advanced analytical insight may be directly utilized to address missing critical parameters and allow for more reliable FRP retrofit design of seismic resistant reinforced concrete (RC) columns. Further, it allows for arbitrary 3D seismic analysis of columns (loading, unloading, cyclic or loading rate effects or preloading) or addresses predamages.

Effect of initial minimum principal stress and unloading rate on the spalling and rockburst of marble: a true triaxial experiment investigation

Hard rock often performs as brittle failures, such as cracking, spalling, and rockburst, induced by excavation in deep underground engineering. To understand the effect of the minimum principal stress and unloading rate on the spalling and rockburst of marble, unloading experiments were carried out by using true triaxial equipment combined with a high-speed camera, acoustic emission instrumentation, and scanning electron microscope. The experimental results showed that the failure process of marble specimens was more stable and less inclined to exhibit dynamic ejection with a low strain energy release per unit time under a low unloading rate. At the same time, the initial minimum principal stress controlled the marble’s failure mode by changing its mode from predominantly shear failure to shear-tension failure as the minimum principal stress decreased, similar to the spalling behaviors of surrounding rock observed in the underground tunnel. What’s more, the dynamic rockburst of the marble specimen was simulated under the condition with a high unloading rate, high initial minimum principal, and free face on the specimen. Based on these experimental investigations, a flowchart was also presented to estimate the possible failure mode of marble specimens under different initial minimum principal stress and unloading rate conditions, which would be conducive to the disaster prevention of hard rock in deep underground engineering.

Shear behaviour of sand-concrete interface with side post-grouting considering the unloading effect

Side post-grouting bored-pile foundations are widely used in geotechnical engineering. However, few studies have focused on analyzing the shear behaviour of soil and concrete while considering the grouting process. A total of 48 groups of self-developed, large-scale direct shear experiments were conducted to investigate the shear behaviour of sand-concrete interface with side-grouting, focusing on the effect of soil stress unloading and the on-site roughness measurements of bored pile. The influence of the grouting volume V, unloading rate α and interface roughness R on the shear behaviour are discussed, and the grouting modes and shear band deformation are investigated. The experimental results indicated that the grouting volume has a limited effect on improving the shear strength, and an optimal grout quantity will exist in post-grouting engineering. In addition, the unloading has a non-negligible influence on the grouting modes.

Consideration of anisotropies related to bedding planes in the modelling of thick-walled hollow cylinder tests on Boom Clay

Tunnel excavation in Boom Clay formation, Belgium, highlighted an anisotropic behaviour related to its bedding planes. Hollow cylinder tests mimicking tunnel excavation under laboratory conditions confirmed the isotropic convergence of samples cored perpendicular to the bedding and the anisotropic behaviour of samples cored parallel to the bedding. This article investigates the possibility to reproduce these observations by numerical modelling considering anisotropies related to weakness planes. The behaviour of samples cored perpendicular to the bedding is simulated using an isotropic elastoplastic model. Despites of its simplicity, it shows good agreements with the experimental results. The results highlighted two competing phenomena: the hydro-mechanical coupling occurring during the unloading on the one hand, and the drainage towards the central hole on the other hand. Accordingly, the unloading rate is found to influence the displacement and the plastic zone extent. Strength and hydraulic anisotropies are considered in the simulations of samples cored parallel to the bedding. However, numerical results are opposite to the experimental observations, both in terms of the maximum displacement orientation and the plastic zone extent. Stiffness anisotropy should be considered as it might affect the hydro-mechanical coupling and therefore influence where the plastification begins.

Energy Evolution Law of Ore-Bearing Rock during Unloading under High Static Stress and Frequent Disturbance

Based on the complex engineering environment in deep rock engineering, preloaded high axial pressure, unloading of axial pressure, and impact loading were used to simulate high in situ stress, unloading of excavation, and blasting disturbance, respectively; the experimental study on frequent impact disturbances under unloading of high static load was carried out, which aimed at revealing the energy evolution law of rock. First, the equations of elastic property, plastic energy, and incident energy in the impact process are discussed by theoretical analysis. Then, it is found by further investigation that dynamic stress-strain curve and envelope curve of rock are with the same change tendency. The initial stage was short straight stage, and then linear stage appeared and was less influenced by unloading rate. The plastic energy, the ratio of reflected energy to incident energy, and the energy consumption per unit volume of rock increase during impact. The higher the preloaded axial pressure is, the greater the ratio of reflected energy to incident energy is, the smaller the ratio of transmitted energy to incident energy and the average energy dissipation per unit volume of rock are. When the unloading rate increased, the elastic energy generated by impact gradually increased, the plasticity gradually weakened, the ratio of transmitted energy to incident energy increased, and the ratio of reflection energy to incident energy decreased.

Installation Time of Ground Support during Tunnel Excavation: A Novel Graph Methodology

Installation time of ground support in many caverns excavation is mainly based on analytical and empirical method, instead of the robust numerical simulations. A novel graph method is proposed to determine the installation time of the support upon finite difference method (FDM) simulation. A three dimensional numerical model for the diversion tunnel of Guandi hydropower station is developed. The curve d-r between the convergence d and the excavation unloading rate r is built up upon one-step excavation simulation. Meanwhile, the curve d-l between d and the distance from the tunnel face l, is established via the step-by-step simulation. Using the dimensionless convergence λ instead of d in both curve d-r and curve d-l, curve λ-r and curve λ-l can be yielded to achieve the composite graph r-λ-l. When r = r0, l can be chose as l0 along the identical λ in the comprehensive graph r-λ-l, where r is the excavation unloading rate at the critical plastic state and l0 is the distance from the tunnel face to the support installation location. The proposed graph method is demonstrated to meet well with common sense upon the diversion tunnel of Guandi hydropower station and can be conveniently realized in other engineering practice.

Influence of asymmetric out-of-roundness coupled with wear of subway wheels on vehicle dynamics

Out-of-roundness and tread wear are common types of damage to subway wheels, which greatly affect the dynamic performance of subway vehicles. In light of the coupling and asymmetry of out-of-roundness and tread wear found in real-world subway wheels, an analysis method that considers asymmetric out-of-roundness coupled with tread wear was proposed for vehicle dynamics. A vehicle dynamics model featuring asymmetric out-of-roundness coupled with tread wear was used to investigate the influences of asymmetric wheel damage and coupled damage on the dynamic performance of the vehicle system. The vehicle’s dynamic performance was simulated under different conditions, including asymmetric out-of-roundness and symmetric out-of-roundness, uncoupled damage and coupled damage, asymmetric coupled damage and symmetric coupled damage. Then the estimated data was compared against the measured data. The study finds that the vertical wheel/rail contact force, lateral wheel/rail contact force, derailment coefficient and wheel unloading rate increased in the case of asymmetric out-of-roundness. In the presence of coupled damage, the degree of tread wear had a relatively great influence on the lateral wheel/rail contact force, axle lateral force, and derailment coefficient, but had little influence on the vertical contact force. Compared to symmetric coupled damage, asymmetric coupled damage had a greater influence on peak vertical vibration acceleration and stability index, and their values are closer to the measured values in the case of asymmetric coupled damage. This suggests that the dynamics model that considers asymmetric out-of-roundness coupled with tread wear can provide more accurate results as guidance on the maintenance and overhaul of subway wheels.

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance. For this, their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions. The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies. However, little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading (FL-CSU) conditions that influence rock failure. In this paper, we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography (CT) scanning technique and the GCTS RTR 2000 rock mechanics system. Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble. The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests, and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests. In FL-CSU tests, as the unloading rate increases, the dissipated energy grows and elastic energy reduces. CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension. It shows that the crack pattern after FL-CSU tests depends on the unloading rate, and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released. The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.

Mechanical Characteristics of Coal Samples under Triaxial Unloading Pressure with Different Test Paths

In order to understand the influence of unloading path on the mechanical properties of coal, triaxial unloading confining pressure tests with different initial confining pressure and different unloading rate were carried out. The test results show that the triaxial unloading strength of coal samples under different test conditions is lower than conventional triaxial tests, but the brittleness characteristics are more obvious. This result indicates that the coal samples are easily damaged under unloading conditions. In the axial loading stage of the confinement unloading tests, the axial strain plays a leading role. However, during the confining pressure unloading stage, the circumferential deformation is large, which is the main deformation in this stage. Higher unloading rates of confining pressure are associated with shorter times between the peak stress position and sample complete failure. This shows that samples are more easily destroyed under higher unloading rates and the samples are more difficultly destroyed under lower unloading rates. In addition, with increasing unloading rate, the peak principal stress difference and confining pressure at failure decrease gradually, whereas the confining pressure difference at failure increases gradually. Compared with conventional triaxial compression tests, the cohesion of coal is reduced and the internal friction angle is increased under the condition of triaxial unloading test.

EXPERIMENTAL INVESTIGATION ON MULTI-FRACTAL CHARACTERISTICS OF ACOUSTIC EMISSION OF COAL SAMPLES SUBJECTED TO TRUE TRIAXIAL LOADING–UNLOADING

Coal–rock dynamic disasters seriously threaten safe production in coal mines, and an effective early warning is especially important to reduce the losses caused by these disasters. The occurrence of coal–rock dynamic disasters is determined by mining-induced stress loading and unloading. Therefore, it is of great significance to analyze the precursory information of coal deformation and failure during true triaxial stress loading and unloading. In this study, the deformation and failure of coal samples subjected to true triaxial loading and unloading, including fixed axial stress and unloading confining stress (FASUCS), are experimentally investigated. Meanwhile, acoustic emission (AE) during the deformation of coal samples is monitored, and the multi-fractal characteristics of AE are analyzed. Furthermore, combined with the deformation and failure of coal samples, the precursory information of coal deformation and rupture during true triaxial stress loading and unloading is obtained. Finally, the relationship between multi-fractal characteristics and damage evolution of coal samples under FASUCS is discussed. The results show that the multi-fractal spectral widths of AE time series under the conditions of FASUCS with different initial confining stresses or unloading rates are quite different, but the dynamic changes of multi-fractal parameters [Formula: see text] and [Formula: see text] are similar. This indicates that the microscopic complexity of AE events of coal samples under different conditions of FASUCS differs, but the macroscopic generation mechanism of AE events has inherent uniformity. The dynamic changes of [Formula: see text] and [Formula: see text] can reflect the stress and damage degree of coal samples. The dynamic change process of [Formula: see text] well accords with the damage evolution process of coal samples. A gradual decrease of [Formula: see text] corresponds to a slow increase of damage, while a sharp increase of it corresponds to a rapid growth of damage. At the same time, the mutation point of damage curve at distinct stress difference levels shares the same variation trend with the [Formula: see text] mutation point. The change of [Formula: see text] can reflect the damage process of coal samples, which can be used as precursor information for predicting coal–rock rupture. The finding is of great significance for the early warning of coal–rock dynamic disasters.

Experimental study on deformation failure and crack propagation evolvement of jointed rock bridge under different unloading rates

Experimental study on deformation failure and crack propagation evolvement of jointed rock bridge under different unloading rates

The mechanical behavior of rock salt under different confining pressure unloading rates during compressed air energy storage (CAES)

Compressed air energy storage (CAES) is a promising energy conversion storage technology and underground salt caverns are recognized as the appropriate storage places for it. The reduction of air during the withdrawal periods leads to the surrounding rock being unloaded, which threatens the stability of the salt cavern. To investigate the mechanical behavior of surrounding rock under different unloading rates, the confining pressure unloading tests in six unloading rates have been conducted on rock salt. Results show the rock salt undergoes volume expansion at the beginning of unloading due to the reduction of elastic compression strain. The volume strain expands rapidly beyond a threshold, i.e. accelerated strain point. The stresses at failure point and accelerated strain point show that a low unloading rate is beneficial to enhance rock strength. Nevertheless, larger inelastic strain appears in a lower unloading rate. The effect of the unloading rate on rock salt failure mode is that rapid unloading will promote crack growth. Moreover, the relationship between the unloading rate of surrounding rock and the air withdrawal rate is established. The outcome derived from the research can provide some instruction and inspiration for air withdrawal rate design in CAES.

Acoustic Emission Experimental Research of the Damage Characteristics of Raw Coal under Different Loading and Unloading Rates

In this study, triaxial load failure experiments of coal samples under different strain rates and different confining pressure unloading rates were carried out using an RTX-1000 rock triaxial apparatus, and the acoustic emission characteristic parameters of a Micro-II acoustic emission imaging acquisition instrument were used to study the acoustic emission characteristics and damage deformation law of coals under different conditions. Damage models were constructed on the basis of the characteristic parameters to analyze the damage law of coal samples. Experimental results show that the acoustic emission (AE) counts and AE energy of the coal samples decrease, but the peak AE counts and peak AE energy increase with the increase in strain rates. The cumulative AE counts decrease from 9902 times to 6899 times, the peak counts increase from 209 times to 431 times, the cumulative AE energy decreases from 6986 aJ to 3786 aJ, and the peak AE energy increases from 129 aJ to 312 aJ. The overall level of the AE count rates and the AE energy of the coal samples decrease, but the peak AE counts and peak AE energy increase with the increase in unloading rates. The cumulative AE counts decrease from 18,689 times to 16,842 times, the peak AE count rates increase from 245 times/s to 535 times/s, the cumulative AE energy decreases from 9846 aJ to 7430 aJ, and the peak energy increases from 257 aJ to 587 aJ. The damage models are constructed on the basis of AE counts, and the comparative experimental and theoretical curves are analyzed to obtain a higher fitness close to 1. The damage threshold increases from 0.30 to 0.50 and from 0.34 to 0.55, and the damage amount increases from 0.50 to 0.60 and from 0.34 to 0.62 with the increase in strain rates and unloading rates. The research results have practical significance for revealing the mechanism of disaster occurrence in actual engineering excavation and proposing engineering measures to prevent coal rock damage and disaster occurrence.

An Uplifted over Logistics Costs Efficiency by the Hub and Spoke System at Cikarang Dry Port

This research aimed to discovered and analyzed those efficiency of logistics costs by hub and spoke system at Cikarang dry port that provides ports which included logistics services with logistics companies and supply chains so it would contributes to reducing logistics costs and dwelling time at Tanjung Priok Port. This research uses Factor Analysis and AHP (Analitycal Hierarchy Process) method as a method to find out all factors which influence most towards logistics performance and how to reduce those logistics costs at Cikarang Dry Port. Factor Analysis result starting from 14 elements into 8 elements which divided into 3 factors such as transportation, administration and inventory costs. The weight of loading and unloading costs is 0.24709, the weight of container costs is 0.20384 and the weighting of stacking fees is 0.14429. So AHP results was obtained from factors and elements of logistics costs Cikarang Dry Port which has most influence are F1 (loading and unloading costs), F6 (custom service fees), F8 (forwarding service costs), F2 (goods inspection service costs), F4 (stacking fees) , F12 (service quality that needs to be improved), F3 (container tariffs), F5 (loading and unloading labor rates).

Deformation characteristics of granites at different unloading rates after high-temperature treatment

Naturally cooled granite specimens pretreated at 800 °C were separately subjected to conventional triaxial compression tests and triaxial confining pressure unloading tests under a constant deviatoric stress. Via these tests, the deformation characteristics of granite at different confining pressure unloading rates were explored. The research results showed that the granite specimens were more easily damaged at an intermediate unloading rate than at a high or low unloading rate. The strain–confining pressure compliance results quantitatively revealed that rock failure during unloading was caused by intense radial deformation and volumetric dilatancy. With the increase in the unloading rate, the strain–confining pressure compliance first increased and then decreased. In the confining pressure unloading process, initially, the elastic modulus of the rocks linearly decreased with the decreasing confining pressure. In the case that the confining pressure was unloaded to the critical point where rock failure occurred, the elastic modulus of the rocks dramatically decreased. Additionally, Poisson’s ratio gradually decreased with the reduction in confining pressure. However, approaching the critical point where rock failure occurred during unloading, Poisson’s ratio remarkably decreased. Afterwards, Poisson’s ratio increased to 0.5 and then remained unchanged until the specimens became damaged.

Study on transient unloading loosening simulation test of excavation of jointed rock mass

Based on the abrupt displacement of a jointed surrounding rock mass during blasting excavation of deep underground caverns, a set of transient unloading loosening simulation tests was designed for multi-joint rock masses. Through the linear regression analysis of the unloading displacement of jointed rock masses, the opening displacement of a joint is determined to be proportional to the square of the initial unloading stress. The numerical simulation method is used to apply horizontal and vertical in situ stresses to the jointed rock mass model to study the opening displacement in the jointed rock mass at different lateral pressure coefficients of in situ stress. The results indicate that, with the unloading of the horizontal stress, a larger lateral pressure coefficient of the in situ stress leads to a larger opening displacement in the jointed rock mass. When the vertical stress is unloaded after the horizontal stress is totally unloaded, a lower lateral pressure coefficient of the in situ stress leads to a larger opening displacement. The influence of the stress lateral pressure coefficient on the open deformation of the jointed rock mass is obviously higher than that of the unloading rate on the rock mass.

Effects of foundation settlement on comfort of riding on high-speed train-track-bridge coupled systems

In this study, the principle of minimum potential energy has been used to deduce the mapping relationship between beam deformation and rail deformation with consideration of the constraint effects of a subgrade system. Based on the above mapping relationship, a dynamic simulation analysis on the train-track-bridge coupled system has been carried out. Taking the index of riding comfort as the investigation object, in combination with the specified limits in the relevant specification, an analysis has been carried out to determine the control threshold of beam deformation of high-speed railways. According to the simulation results, the dynamic response of different types of high-speed trains shows significantly different sensitivities to the pier settlement and the beam faulting. The threshold value in settlement control in the case of simultaneous settlement of two adjacent piers is smaller than that in the case of the single-pier settlement. Under the operating condition of beam faulting, the wheel unloading rate is more sensitive to the faulting of the beams than the vertical acceleration and Sperling’s ride index. To avoid the vertical acceleration specified limit of car body, it is recommended to maintain the pier settlement to less than 9.7 mm, and the beam faulting to less than 5.3 mm.

3-D Numerical Study on Progressive Failure Characteristics of Marbles under Unloading Conditions

3-D particle-based discrete element method (PB-DEM) was employed to numerically study the mechanical and progressive failure characteristics of pre-fissured marble specimens under conventional triaxial unloading conditions. The microscopic parameters of PB-DEM for marble materials were calibrated using comparison with the previous experimental data. To systematically investigate the mechanical properties and the progressive failure characteristics of pre-fissured marble specimens under the unloading conditions, a series of numerical specimens were simulated. The effects of fissure geometric conditions, initial confining pressures, and unloading rates on the mechanical and failure behaviors were explored via simulations. The present numerical results indicate that peak strength increased as the initial confining pressures increased or the unloading rate decreased. Crack coalescence types and the ultimate failure modes in the pre-fissured marble specimens were significantly affected by the unloading stress paths. The present numerical results provide a better understanding of unloading mechanical and failure characteristics to scientists and engineers in rock mechanics and rock engineering.

Influence of excavation schemes on slope stability: A DEM study

Slope failure due to improper excavation is one of common engineering disasters in China. To explore the failure mechanism of soil slope induced by toe excavation, especially to investigate the influence of excavation unloading path and rate on slope stability, a numerical slope model was built via particle flow code PFC2D. The development of crack and strain during excavation were obtained and used to evaluate the deformation characteristics. Furthermore, excavation types representing different unloading paths and rates were compared in terms of crack number and strain level. Results indicate that crack number and strain level induced by horizontal column excavation are much greater than those of vertical column excavation and oblique excavation. The crack number and strain level increase with excavation unloading rate. Besides, the feasibility of taking the average strain of slope surface and the average value of maximum strain along monitoring lines to represent the global deformation characteristics were discussed. This study can provide a theoretical guidance for slope monitoring and preliminary optimal selection of excavation scheme in the design and construction of slope engineering.