Unloading Tests Research Articles

Discussions on the Complete Strain Energy Characteristics of Deep Granite and Assessment of Rockburst Tendency

In deep mining, the rockburst hazard has become a prominent problem. Rockburst is difficult to be predicted, and it gives a severe threat to mining safety. In this paper, the triaxial compressive tests with an acoustic emission (AE) device and full cyclic loading and unloading tests were carried out, respectively, to present characteristics of linear elastic strain energy and peak strain energy. Also, to consider the time-delay strain characteristics of granite found in the abovementioned tests, a new stage loading and unloading test with dual monitoring systems was designed and performed. Through 20 days’ time-delay strain monitoring, the peak-strength strain energy was further modified. The results showed that the peak strain energy is approximate 1.2-1.3 times than linear elastic strain energy under the same confining pressure, and after considering the time-delay strain effect, the modified maximal strain energy value of the deep granite significantly increased. The peak strain energy values are further enhanced from 1.0 × 104 J/m3 to 1.8 × 104 J/m3, respectively. At last, by taking advantage of the strain energy index model of rockburst, the tendency and intensity of rockburst were assessed contrastively.

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.

Mechanical properties of soft soils experiencing lateral unloading under initial excess pore water pressure

The mechanical properties of soft soils were investigated through triaxial lateral unloading tests under different initial excess pore water pressures and confining pressures. The lateral unloading stress–strain, strength parameters, and initial unloading moduli of soft soils are presented and discussed in this study. The results show that soft soils abruptly undergo unloading failure. The lower confining pressure and the higher initial excess pore water pressure can result in intensive unloading failure behavior. The unloading strength of soft soils linearly decreases as the initial excess pore water pressure increases. The stress–strain curve is characterized as strain hardening. The cohesion and initial unloading moduli of soft soils drastically reduce when the initial excess pore water pressure exceeds 20 kPa. The conclusions obtained in this study can provide a theoretical reference for the design and numerical analysis of underground spaces in soft soils.

Deformation characteristics and failure modes of sandstones under discontinuous multi-level cyclic loads

Rock often suffers from discontinuous fatigue damages in geotechnical engineering, so it is of great significance to study deformation characteristics and failure modes of sandstone under discontinuous multi-level cyclic loading and unloading. By conducting continuous (only dynamic loading) and discontinuous (combined static and dynamic loading) multi-level cyclic loading and unloading tests, this study explored deformation characteristics and failure modes of sandstone. The test results demonstrated that the sandstone mainly suffered from large deformation dominated by dilation in circumferential direction when it was damaged after compression. After multi-level loading and unloading, evolution characteristics of deformation parameters showed characteristics of stepwise growth. However, the introduction of static loading changed such characteristics. In the process of combined static and dynamic loading, due to short action time, dynamic loading mainly promoted nucleation and sudden changes of cracks, while static loading affected crack development process mainly in the early compaction stage and late crack propagation and coalescence stage. Moreover, static loading showed small effects in elastic stage. In the compaction stage, the compacted micro-cracks reopened and cracks were affected by inertial damages in crack propagation stage and interconnected more thoroughly, so that the sandstone was finally more fragmented.

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.

Mechanical behaviour and damage evolution of Beishan granite considering the transient and time-dependent effects of excavation unloading

A series of triaxial unloading tests on integrated rock, triaxial compression tests and creep tests on unloading-cracked rock were carried out on dry and saturated specimens. The spatial and temporal distribution of microcracking process were obtained with the help of 3D acoustic emission (AE) monitoring system. The experiment data revealed that the saturation had a degeneracy effect on the transient failure strength of integrated granite, and the failure strain at high confinement. The creep characteristics of unloading-cracked granite in lateral direction were found to be greater than in axial direction. At high confinement, the critical strain of creep failure, the creep deformation and the lateral creep strain rate were shown to be degraded by the saturation. When the applied stress surpassed the level of 90% of the compressive strength of unloading-cracked specimen, the variation of AE event showed a consistent tendency with the evolution of lateral strain, and the AE events were found to be mainly accumulated along the macroscopic failure surface. The AE signal was noticed to be more sensitive than the macroscopic deformation to the occurrence of creep failure. Before the appearance of large increase in lateral strain, the rapidly increase of AE activity had occurred.

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.

Constitutive Model of Lateral Unloading Creep of Soft Soil under Excess Pore Water Pressure

Presented in this paper is a study on the lateral unloading creep tests under different excess pore water pressures. The marine sedimentary soft soil in Shenzhen, China, was selected in this study. The results show that the excess pore water pressure plays a significant role in enhancing the unloading creep of soft soil. Higher excess pore water pressure brings more obvious creep deformation of soft soil and lower ultimate failure load. Meanwhile, the viscoelastic and the viscoplastic modulus of soft soil were found to exponentially decline with creep time. A modified merchant model and a combined model of the modified merchant model and plastic elements are used to simulate the viscoelastic and the viscoplastic deformation, respectively. Therefore, a lateral unloading creep model of soft soil is developed based on the modified merchant model. The accuracy and applicability of this model were verified through identifying the parameters in the model. Research results are of particular significance to the numerical simulation of underground space excavation in soft soil areas which considers the effects of excess pore water pressure.

Effects of prior cyclic loading damage on failure characteristics of sandstone under true-triaxial unloading conditions

To investigate the effects of prior cyclic loading damage in rocks on subsequent unloading failure characteristics under true-triaxial conditions, a series of complicated unloading tests incorporating the damage-controlled cyclic loading path and stress σ3 unloading path was conducted using a true-triaxial test system. The experimental results reveal that the prior cyclic loading damage has an impact on the strength and deformation characteristics, energy conversion and failure mode. As the number of prior cyclic loads increases, the unloading strength and Young's modulus increase firstly and then decrease, while the peak unloading strain, as well as the ratio (η) of crack damage stress to peak unloading stress, exhibits a descending trend. The energy storage capacity of rock samples is dramatically reduced as cycle number increases to 10 and enlarged slightly with a further increase in cycle number. Both shear fracture and tensile fracture appear in each rock sample under this unloading condition, as the prior cyclic loading number increases, the dominant failure mode of rock samples changes from tensile failure to mixed tensile-shear failure, then to shear failure, while the failure angle ranging from 65° to 80° deceases firstly and then turns to rise.

Unloading-Induced Crack Propagation of Two Collinear Unequal Length Flaws in Brittle Rocks

The propagation and coalescence of numerous discontinuous joints significantly contribute to landslide instability during excavation unloading. The tip expression of stress intensity factors of two collinear unequal length cracks in a typical rock mass under unloading conditions was calculated based on the superposition principle and fracture mechanics to determine the meso-influence law of intermittent joint interaction in the slope under the action of excavation. The effects of many factors on this interaction were also analyzed theoretically. Unloading tests were conducted on rock-like specimens with two collinear unequal length cracks in addition to numerical simulation and theoretical analysis. The results show decreased interaction between the two cracks with increased crack distance, increased influence of the main crack on a secondary crack with increased length of the main crack, and decreased influence of the secondary crack on the main crack with decreased length of the secondary crack. Wing tensile cracks first appear at the tip of flaws, and the propagation of these cracks occurs with the generation of secondary tensile cracks and shear cracks during unloading. Propagation and coalescence between cracks lead to tension and shear mixed failure of a rock bridge, and tensile cracks appear near the unloading surface. The axial initiation and peak stress of a crack increase with increased flaw distance, and the theoretical calculations were confirmed by lateral unloading test results.

Experimental Study on Mesodamage of Migmatite Under Cyclic Loading and Unloading

To study the mesodamage of migmatite under cyclic loading, single cycle loading and unloading tests of migmatite were carried out under different confining pressures (σ3) and stress ratios (p), and acoustic emission (AE) technology was used to monitor the evolution of microcracks. Additionally, the mesodamage of migmatite was analyzed from the porosity and T2 spectrum distribution measured by nuclear magnetic resonance (NMR) spectroscopy. The results show that the peak stress ratios (pmax) of migmatite under different σ3 are different, and as the value of σ3 increases, the migmatite gradually undergoes a change from tension to shear failure. Moreover, with the increase in σ3, high-frequency signals gradually appeared, indicating that the increase of σ3 to a certain extent limited the generation of large size microcracks. The variations in damage variables defined by the number of AE hits and porosity are the same, “steep increment type,” and increase suddenly when approaching pmax. Furthermore, σ3 slightly affects the pore throat distribution of migmatite. In addition, σ3 has a certain inhibitory effect on the porosity of migmatite and accumulation of mesodamage. However, when σ3 exceeds a critical value, the inhibitory effect becomes a promoting effect.

Mechanical Behavior and Permeability Evolution of Coal under Different Mining-Induced Stress Conditions and Gas Pressures

The gas permeability and mechanical properties of coal, which are seriously influenced by mining-induced stress evolution and gas pressure conditions, are key issues in coal mining and enhanced coalbed methane recovery. To obtain a comprehensive understanding of the effects of mining-induced stress conditions and gas pressures on the mechanical behavior and permeability evolution of coal, a series of mining-induced stress unloading experiments at different gas pressures were conducted. The test results are compared with the results of conventional triaxial compression tests also conducted at different gas pressures, and the different mechanisms between these two methods were theoretically analyzed. The test results show that under the same mining-induced stress conditions, the strength of the coal mass decreases with increasing gas pressure, while the absolute deformation of the coal mass increases. Under real mining-induced stress conditions, the volumetric strain of the coal mass remains negative, which means that the volume of the coal mass continues to increase. The volumetric strain corresponding to the peak stress of the coal mass increases with gas pressure in the same mining layout simulation. However, in conventional triaxial compression tests, the coal mass volume continues to decrease and in a compressional state, and there is no obvious deformation stage that occurs during the mining-induced stress unloading tests. The theoretical and experimental analyses show that mining-induced stress unloading and gas pressure changes greatly impact the deformation, failure mechanism and permeability enhancement of coal.

Mathematical methods to unloading creep constitutive model of rock mass under high stress and hydraulic pressure

The main purpose of this paper is to study the unloading mechanical characteristics of rock mass unloading under high stress and high water pressure. An unloading creep test of deep sandstone was conducted under different initial stresses and hydraulic pressures by using the TFD-2000 rock triaxial rheological test system. Based on the experimental results, we can explore unloading creep properties and time-dependent deformation characteristics of rock mass. Deformation characteristics of rock masses during the attenuation and stable creep stages under different stresses and hydraulic pressure conditions were analyzed, and the variation laws of creep parameters in different creep stages were studied and fitted by a mathematical model. The applicability of the fitting results was discussed. A nonlinear unloading creep constitutive model and mathematicalmethods of rock mass under high stress and high hydraulic pressure was proposed based on element model theory. Parameters of this nonlinear creep constitutive model were recognized. Results demonstrated that the nonlinear deformation characteristics run through the entire creep stage. During the attenuation and stable creep stages, elastic deformation parameters generally present exponential functional variations with the increase in time and hydraulic pressure. In the accelerating creep stage, viscous deformation parameters also present an exponential functional variation. The test curve fits with the model curve obtained by the optimal numerical analysis software 1stOpt, which verifies the accuracy and applicability of the model. This study can provide theoretical references for accurate prediction of time-dependent deformation of deep rocks under high stresses and high hydraulic pressures.

Characteristics of unloading damage and permeability evolution of sandstone under hydro-mechanical coupling

A triaxial unloading mechanical test is conducted on sandstone considering different initial confining pressures and unloading rates under the effect of hydraulic coupling. The energy evolution of sandstone unloading failure is studied, and the influences of the initial confining pressure and unloading rate on the deformation characteristics and permeability evolution of sandstone are discussed. It is found that the initial confining pressure and unloading rate have a significant impact on the axial and radial strains of the sandstone, and the radial deformation is more sensitive to the unloading process. The energy dissipation of the sandstone sample increases with the increase of the confining pressure and is nonlinearly correlated to the unloading time. The permeability during the unloading deformation of the sandstone decreases with the increase in the initial confining pressure, indicating that the initial confining pressure of unloading has an inhibitory effect on the permeability of the sandstone. The permeability decreases with the increase in unloading rate, indicating that the faster the increase in the unloading rate, the greater the dissipated instantaneous elastic energy, which is accumulated and released during failure. The higher the degree of fracture in the sample, the larger the peak value of permeability.

A modified Mori–Tanaka approach incorporating filler-matrix interface failure to model graphene/polymer nanocomposites

A novel modified Mori-Tanaka (M-T) approach to modeling the stress-strain response of graphene nanocomposites subjected to large deformation is described. It is hypothesized that slip at the filler-matrix interface occurs, and this limits the stress which can be transferred to the filler through the interface. Although interfacial slip has a significant influence on the mechanical response of polymer nanocomposites, the M-T method, commonly employed to model nanocomposites, does not accommodate this effect. A pseudo-filler phase is defined to represent the filler when interfacial slip occurs, and this is modeled using finite elements to simulate a single filler flake encapsulated by the interface, interacting with the matrix. The stiffness of the pseudo-filler is determined by examination of the variation of stress in the filler when interfacial slip occurs; this interfacial slip model is incorporated into the M-T method. Quasi-static tensile loading and unloading tests were conducted on polyvinyl alcohol (PVA)-graphene oxide (GO) nanocomposite samples, and significant enhancement of the stress-strain response of the nanocomposite was observed by the addition of GO; e.g. a 5 wt% GO addition increases the initial elastic modulus of PVA from 330 MPa to 1.29 GPa. The modified M-T method was used to simulate the stress-strain response of the nanocomposites, and the theoretical results compare well with experimental stress-strain data for both loading and unloading.

Evolution of the mechanical and strength parameters of hard rocks in the true triaxial cyclic loading and unloading tests

Rock fracture process is associated with the degradation and mobilization of the mechanical and strength properties of rock. A number of true triaxial cyclic loading and unloading tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) to investigate the evolution of the rock properties as a function of accumulated damage. In addition to stress and strain measurements, acoustic emission (AE) signals were monitored and used to associate different stages of crack development with different stages of the mechanical and strength parameter evolution. Young's modulus and five strain increment ratios were calculated from the measured increments of stresses and strains in each cycle. It was found that hard rocks exhibited profound deformation anisotropy and directional dilation. A general trend of decreasing volumetric strain reversal stress and peak stress with increasing plastic strains was observed for all the tested specimens. The evolution of the strength properties of rock under true triaxial compression was investigated using a linear 3D hard rock failure criterion and was found to be generally consistent with the cohesion weakening and frictional strengthening concept. The influence of the intermediate principal stress on the evolution of the rock properties was also discussed.

Strength-weakening effect and shear-tension failure mode transformation mechanism of rockburst for fine-grained granite under triaxial unloading compression

The occurrence of rockburst is closely related to initial high stress state and excavation unloading effects. To investigate the effects of three-dimensional (3D) stress state and unloading rate on the rockburst mechanism, triaxial unloading compression tests were performed on cubic fine-grained granite specimens (50 mm × 50 mm × 50 mm). Two unloading rates (0.08 and 40 MPa/s) were employed in the tests with seven initial 3D confining pressures (5, 10, 20, 30, 40, 50, and 60 MPa). For comparison, triaxial and biaxial compression tests were conducted at the same initial confining pressures. The results show that, under the same confining pressures, the variations in peak strength of granite specimens are in the following order: triaxial compression test > biaxial compression test > triaxial unloading compression test under low unloading rate > triaxial unloading compression test under high unloading rate, which indicating that unloading induces an obvious strength-weakening effect on fine-grained granite. In the triaxial unloading compression test with the same unloading rate, a higher confining pressure results in a greater rock bearing strength and stress drop. More energy will be stored in the rock, and the rockburst intensity will be more serious when the rock is destroyed. When the confining pressure is constant, the lower unloading rate is beneficial for improving the rock bearing strength, and more elastic energy will be accumulated and stored in the rock. Once rockburst occurs, the energy released by rock is larger and the degree of rockburst is stronger. The failure mode of fine-grained granite specimens in triaxial unloading compression tests transforms from shear-tension failure to tension failure with increasing confining pressure, which is the main reason for the weakening of rock strength. Establishing a strength criterion for 3D rock considering the unloading effect is a key for understanding the mechanism of rockburst.

Influence of time-lagged unloading paths on fracture behaviors of marble using energy analysis and post-test CT visualization

Underground excavation usually results in the decrease of horizontal stress in the front of the working face, which may trigger the occurrence of rock bust hazards. Examining rock failure under confining pressure unloading conditions is necessary to reveal the fracture mechanism of rock mass subjected to stress disturbance. Although many investigations have studied the rock mechanical behaviors during unloading, the time-lagged triaxial unloading paths on the rock fracturing mechanisms are still poorly understood. Triaxial unloading experiments and the post-test CT scanning technique were conducted on fine-grained marble samples under different unloading conditions. The stress–strain behaviors, energy evolution characteristics, and the fracture pattern of samples were analyzed. The results indicate that obvious features at various stages can be identified from the deformation history curve of the elastic strain energy (Ue), total strain energy (U), and dissipated energy (Ud). U, Ue, and Ud almost all increase with sample deformation at the time duration stage, and the elastic energy and dissipation energy curves slightly decrease and increase again. After the unloading point, the dissipation energy sharply increases and the incremental rate of elastic energy becomes lower. The elastic strain energy increases with the increasing unloading rate for samples at the confining pressure unloading point and failure point, and it is influenced by the time-lag effect of confining pressure. The crack pattern in the CT images depends on the triaxial unloading rate and time effect, which is in good agreement with the energy dissipation and release analysis. The unloading strategy has an effect on the energy conversion during triaxial unloading tests; in other words, energy dissipation and release are dependent on the unloading rate and time.

K0determination using improved experimental technique

Lateral soil pressures under at-rest conditions are commonly estimated as K0times vertical normal pressure, where K0is obtained from generally accepted empirical formulae, related to the soil's friction angle and the stress history. Results of a programme of laterally, rigidly confined, vertical compression and unloading tests on sand and clay are presented, in which two significant experimental techniques were employed: (a) lateral pressures were measured using null pressure gauges, preventing gauge diaphragm deflection; (b) side-wall friction was measured, and minimised using special friction reduction measures. Comparative tests in which one or both of these measures were not employed indicate the significant errors induced in both the vertical and the horizontal pressures used for K0determination. The bank of experimental data, which formed the basis for the commonly accepted empirical formulae, did not take account of these issues, and they are shown to be inconsistent with values measured in the present study.

Influence of Loading and Unloading Stress Paths on the Deformation and Failure Features of Jinping Marble Under True Triaxial Compression

To study the difference in the failures of laboratories #7 and #8 of the China Jinping Underground Laboratory Phase II (CJPL-II), different loading and unloading stress path tests were carried out under a true triaxial compression condition based on the measured in situ stress of CJPL-II. The strength, deformation and failure mechanisms of Jinping marble under different stress paths were thoroughly investigated. The results showed that under the same stress state as that at sample failure, the bearing capacity and deformation of the unloading stress path were greater than those of the loading stress path. The bearing capacity and deformation in the σ2 and σ3 directions of stress path II and stress path III were similar, and the others decreased from stress path IV to stress path V under unloading conditions. The failure mode results reflect the violence of the unloading failure. The failure features and mechanisms of different stress paths were analysed though the acoustic emission (AE) ring, cumulative energy and strain energy trend. This research may provide a reference for the excavation and support of deep underground works.