Experimental Study of the Creep Disturbance Effect and Acoustic Emission Characteristics of Mudstone with Different Moisture Contents

It is important to have a clear understanding of the creep characteristics of water-rich soft rocks under a dynamic load and the evolution of cracks because soft rock roadways in deep mines are very sensitive to disturbances, and instability and damage can easily occur under the impact of disturbances such as mining and blasting. In this study, a self-developed disturbed creep test bench was used to conduct graded loading creep disturbance tests on mudstone specimens with different moisture contents. The results show that an increase in the moisture content leads to a significant increase in the creep failure strain of mudstone, and the accelerated creep rate is greatly accelerated. Moreover, as the moisture content increases, the type of mudstone creep disturbance failure gradually changes from accelerated creep failure to disturbance failure. By analyzing the acoustic emission (AE) characteristics of the mudstone creep disturbance tests, it was found that the increase in the moisture content greatly weakens the AE count and the accumulated energy. In each stage of disturbance, the AE signals jumped, and the stability was restored at the end of the disturbance. As the load increased, the specimen entered the accelerated creep stage, the AE signal increased exponentially, and the internal cracks expanded rapidly until failure occurred. It is of great significance to carry out creep disturbance experiments and to analyze the evolution of the internal cracks in specimens with different moisture contents to maintain the long-term stability of deep soft rocks.

A Test Method for Creep Characteristics of Loess Based on the One-Dimensional Consolidator

When the traditional one-dimensional consolidator is used to study the creep characteristics of loess, due to the hoop effect of the ring cutter, only the attenuation creep stage and stable creep stage of loess can be studied, but the accelerated creep stage cannot be presented. In order to avoid the influence of drilling on the creep characteristics of loess, the paper improves the consolidation instrument by drilling holes along the diameter direction in the center of the sample to provide artificial space for soil failure. At the same time, the sample size is increased to ensure that the diameter of the sample is greater than five times of the diameter of the borehole, so as to avoid the influence of drilling on the creep characteristics of loess. The creep characteristics of loess are studied by step loading (vertical pressure at all levels is 125 kPa, 175 Pa, 225 kPa, and 275 kPa), and the whole creep process characteristic curves of loess under different stress conditions are obtained. An endoscope was placed in the hole to observe the deformation and failure characteristics of loess in different stages of creep. This method makes up for the defect that the traditional one-dimensional consolidator cannot obtain the whole process characteristics of loess creep. At the same time, it has the advantages of simple operation, less external influence factors, stronger data reliability, and can directly observe the changes of loess creep soil. It has a beneficial role in promoting the experimental research of loess creep characteristics.

Effect of straw reinforcement on the shearing and creep behaviours of Quaternary loess

In addition to the shearing behavior of soil, the creep character is also considered crucial in determining the long-term shear strength. This especially holds true for the loess that possesses the metastable microstructure and is prone to landslide hazards. This study explored the potential application of straw reinforcement to enhance the shearing and creep properties of the Quaternary loess. The mechanism responsible for the straw reinforcement to elevate the peak shear strength was revealed. Furthermore, three creep characters, namely attenuating creep, non-attenuating creep, and viscous flow were identified in this study. The unreinforced and reinforced specimen behaved in a different manner under identical shear stress ratio condition. The reinforced specimen was superior in limiting the particle relative movement within the shear plane than the unreinforced specimen. The chain reaction of interparticle contact loss, accompanied with excessive viscous displacement, rapid weakening of creep resistance, and eventually accelerated creep displacement, provided an evidence for the formation mechanism of slow-moving landslide. The long-term shear strength using the isochronal stress–strain relationship may be used for optimising the design of high-fill embankment works.

Viscoelastoplastic Displacement Solution for Deep Buried Circular Tunnel Based on a Fractional Derivative Creep Model

Time-dependent deformation of surrounding rock is a common phenomenon for tunnels situated in soft rock stratum or hard rock stratum with high geo-stress. To describe this phenomenon, a creep model combining the Abel dashpot and a non-Newton viscous element was adopted, and the analytical solution about the viscoelastoplastic deformation for circular tunnel was obtained based on this creep model. Then, the auxiliary tunnel of Jinping II hydropower station was taken as an example to reveal the influence of creep parameters on the creep deformation. The research shows that (1) the creep model can well describe the whole creep stage of rocks, that is, the decay, constant, and accelerated creep stages, (2) the creep deformation has a positive relation with the value of fractional order of Abel dashpot and the order of the non-Newton viscous element, and (3) the creep curves between test results and analytical solutions are well consistent with each other, which demonstrate the validity of the analytical solution.

Creep Properties of Expansive Soils under Triaxial Drained Conditions and its Nonlinear Constitutive Model

The creep behaviors of expansive soils play an important role in landslide prediction and long-term stability analysis. In this paper, triaxial drained compression creep tests of expansive soils were conducted on the improved stress-controlled triaxial apparatus. The test results show that only transient deformation and attenuation creep occur with low deviator stress, and the increment of axial strain increases exponentially with deviator stress increasing; while deviator stress reaches a certain value, attenuation creep, steady creep and accelerated creep all occur in a creep curve. Meanwhile, the volumetric strain presents the shear shrinkage characteristic at the initial stage of loading, and the shear shrinkage is small. With the extension of loading time, the volumetric strain gradually varies from shear contraction to dilatancy. When entering the accelerated creep stage, the development rate of volumetric strain increases sharply. Besides, isochronous stress-strain curves of expansive soils indicate that their creep process possesses nonlinear characteristics, and the nonlinear degree is related to creep time and stress level. Imitating the empirical formula of cyclic cumulative deformation of clay, a new nonlinear creep model is presented, which may well describe the creep property of expansive soils. Furthermore, critical failure stress could be obtained based on the proposed creep model. The ratio of the critical failure stress to conventional shear failure stress ranges from 70% to 80%, with average of 75.56%, therefore, critical failure stress may be estimated by conventional triaxial tests with the margin of error 5.5% within.

Analysis and numerical calculation of a coupled creep and strain-softening model for soft rock tunnels

Proper mechanical model selection is critical in tunnel support design and stability analysis, especially to reflect the creep and strain-softening behavior of soft rock. We present a coupled nonlinear Burgers strain-softening (NBSS) model and numerical calculation method to investigate the coupled effects of creep and strain-softening of soft rock tunnels. The nonlinear elastic-viscous model is used to simulate the steady creep behavior of mudstone, and the nonlinear viscoplastic strain-softening model is used to simulate the accelerated creep behavior and post-peak strength attenuation behavior. The experimental results show that the viscoplastic parameters and post-peak softening parameters of mudstone are highly sensitive to confining pressure and exhibit nonlinear characteristics. The accelerated creep curve obtained by the numerical calculation is consistent with the experiments, which verifies the model reliability. We use the NBSS and nonlinear Burgers Mohr-Coulomb (NBMC) models to calculate the plastic zone distribution characteristics and deformation law. The distribution of the plastic zone calculated by the NBSS model is larger with more localized fractures. The NBSS model is useful for studying the evolution of stress and displacement fields of complex surrounding rock mass, which provides important theoretical guidelines for the support design and stability analysis of soft rock tunnel engineering.

Accelerated Creep Test (ACT) Qualification of Creep-Resistance Using the WCS Constitutive Model and Stepped Isostress Method (SSM)

In this study, a qualification of accelerated creep-resistance of Inconel 718 is assessed using the novel Wilshire-Cano-Stewart (WCS) model and the stepped isostress method (SSM) and predictions are made to conventional creep data. Conventional creep testing (CCT) is a long-term continuous process, in fact, the ASME B&PV III requires that 10,000+ hours of experiments must be conducted to each heat for materials employed in boilers and/or pressure vessel components. This process is costly and not feasible for rapid development of new materials. As an alternative, accelerated creep testing techniques have been developed to reduce the time needed to characterize the creep resistance of materials. Most techniques are based upon the time-temperature-stress superposition principle (TTSSP) that predicts minimum-creep-strain-rate (MCSR) and stress-rupture behaviors but lack the ability to predict creep deformation and consider deformation mechanisms that occur for experiments of longer duration. The stepped isostress method (SSM) has been developed which enables the prediction of creep deformation response as well as reduce the time needed for qualification of materials. The SSM approach has been successful for polymer, polymeric composites, and recently has been introduced for metals. In this study, the WCS constitutive model, calibrated to SSM test data, qualifies the creep resistance of Inconel 718 at 750°C and predictions are compared to CCT data. The SSM data is calibrated into the model and the WCS model generates realistic predictions of stress-rupture, MSCR, damage, and creep deformation.

Study on long-term uniaxial compression creep mechanical behavior of rocksalt-mudstone combined body

Rocksalt and mudstone are usually under common stress in salt storage caverns, resulting in different mechanical properties from pure rocksalt and mudstone. To accurately obtain the creep mechanical characteristics of rocksalt-mudstone combined body, we have made three different combinations. The long-term creep experiment of bedded rocks can more closely reflect the long-term mechanical behavior of surrounding rock of salt storage caverns. The experimental results indicated that the long-term creep curve of the combined body includes initial and steady creep stages, and even includes accelerated creep stage. The strain of mudstone layer in the combined body was lower than that of rocksalt because of the higher strength. With the increase of the height ratio of mudstone, the creep strain of the combined body and each rock layer decreased, but the creep rate increased. A new nonlinear creep-damage constitutive model was proposed, which can well describe the creep evolution characteristics of the experiment. Compared with the fitting curves of classical Burgers and Nishihara creep constitutive models, it is revealed that the proposed model is most consistent with the experimental data. The duration of the long-term creep experiment under lower stress has a highly significant effect on the accuracy of predicting rock creep results. This research will contribute to a deeper understanding of the long-term creep characteristics of bedded rocks in salt storage caverns.