Xianbiao Mao

The Mechanical Properties of Mudstone at High Temperatures: an Experimental Study

Temperature is one of the main factors affecting the mechanical properties of rocks (Wang 1995). Rock mass involved in projects such as nuclear waste disposal, geothermal energy generation, and underground development in large cities is generally under high temperature conditions. Engineers need to know the rock’s mechanical parameters for excavation of underground rocks, design of supports, and for stability analysis of the surrounding rock. Therefore, the strength of the rock and its deformation behavior at high temperatures need to be understood (Wai et al. 1982; Al-Shayea et al. 2000). Since the 1970s, many researchers have investigated the effects of temperature on the mechanical properties of rocks (Yin et al. 2012a, b; Heuze 1983; Lau et al. 1995). Chen et al. (2012) measured the peak stress, peak strain, and elastic modulus of granite subjected to thermal treatment from 20 to 1,000 C. They found that the peak stress and elastic modulus of heated granite decreased as the heating temperature increased, while the peak strain increased. Xu et al. (2008) studied the mechanical characteristics of granite under the action of temperatures ranging from room temperature to 1,200 C and found that mechanical characteristics did not show obvious variations below 800 C; strength decreased suddenly above 800 C and bearing capacity was almost lost at 1,200 C. Dwivedi et al. (2008) reviewed the thermo-mechanical properties of granites from India and other locations. Through a series of physical and mechanical tests on salt rock at different temperatures (20–240 C), Liang et al. (2006) found that the ultrasonic velocity of the samples declined with rising temperature, while the uniaxial compressive strength and axial strain increased, whereas the tangent modulus had an opposite trend. Ferrero and Marini (2001) applied microscopic analysis to study crack densities in limestone and marble samples at temperatures up to 600 C. They found a correlation between the increase in open porosity due to new fractures and the crack density for both rocks. Zhao et al. (2012) studied the thermal deformation and failure mode of large-size granite specimens at high temperatures and pressures, and obtained the behavior of the thermodynamic parameters of the specimens, such as Young’s modulus, for various temperatures. Other studies (Zhang and Mao 2009; Mao et al. 2009; Luo and Wang 2011; Wu et al. 2005) concluded that the strength of most rocks decreased with increasing temperatures and that the drop in strength depended on the rock type. Although much knowledge has been gained through theoretical and experimental studies of granite, salt rock, and sandstone, few experimental studies on the mechanical properties of coal measures mudstone at real-time have been carried out because of limitations due to experimental L. Zhang Civil Engineering Department, Xuzhou Institute of Technology, Xuzhou 221008, Jiangsu, China

Thermal Properties of Mudstone at High Temperature

AbstractAn experimental study of the thermal damage properties of mudstone at high temperature can serve as a reference for problems in rock mechanics under high temperature, such as in oil exploration, geothermal resource development, and nuclear waste treatment. In this study, a uniaxial compression test of mudstone was conducted using the MTS 810 electrohydraulic servo system and MTS 652.02 high-temperature furnace under different temperatures ranging from 25 to 800°C to analyze the thermal damage properties of mudstone at high temperature. The main results of the study are as follows: the ductility of mudstone is improved under the effect of high temperature, and the failure mechanism exhibits a transition trend from brittleness to ductility; the elastic modulus increases from normal temperature to 400°C and then decreases rapidly when the temperature is over 400°C; and by defining the damage by tangent deformation modulus and combining the test data from uniaxial compression tests of mudstone at high...

Nonlinear Dynamics Mechanism of Rock Burst Induced by the Instability of the Layer-Crack Plate Structure in the Coal Wall in Deep Coal Mining

The instability of layer-crack plate structure in coal wall is one of the causes of rock burst. In the present paper, we investigate the formation and instability processes of layer-crack plate structure in coal wall by experiments and theoretical analysis. The results reveal that layer-crack plate structure formed near the free surface of the coal wall during the loading. During the formation of the layer-crack plate structure, the lateral displacement curve of the coal wall experiences a jagged variation, which suggests the nonlinear instability failure of the coal wall with a sudden release of the elastic energy. Then, a dynamic model for the stability analysis of the layer-crack plate structure was proposed, which takes consideration of the dynamic disturbance factor. Based on the dynamic model, the criterion for dynamic instability of the layer-crack plate structure was determined and demonstrated by an example. According to the analytical results, some control methods of dynamic stability of the layer-crack plate structure was put forward.

A New Strain-Softening Constitutive Model for Circular Opening considering Plastic Bearing Behavior and Its Engineering Application

Geomaterials generally show strain-softening characteristics after peak-load. Based on the triaxial test for sandy mudstone, a simple elastopeak plastic-strain-softening-damage model (EPSDM) was proposed. Compared with the traditional strain-softening model, EPSDM shows obvious plastic bearing characteristics before strain softening. Then, the closed-formed solution of circular opening was deduced based on the newly proposed model. A plastic shear strain increment was introduced as the extension constraint condition of peak plastic zone. The solution correctness of EPSDM was also verified by comparing with other research results. In addition, the solution based on EPSDM could degenerate for a series of results obtained by elastobrittle plastic model (EBM), elasto-strain-softening model (ESM), and elasto-perfectly plastic model (EPM) under certain conditions. Hence, it could be regarded as a unified solution. Finally, the research results denoted that when the inner pressure was fully released, the maximum postpeak failure radii and surface displacement of surrounding rock indicated the characteristics of EBM>ESM>EPSDM>EPM. Therefore, the plastic bearing behavior could effectively decrease the postpeak failure zone radii and surface displacement. The dilation coefficient noticeably influenced postpeak failure range and surface displacement, particularly the damage zone radii and tunnel wall convergence. The research results can provide very important theoretical bases for evaluating the tunnel stability and support design reliability for underground engineering.

Research on water retention and microstructure characteristics of compacted GMZ bentonite under free swelling conditions

In this study, water retention tests under free swelling conditions were performed to investigate the water intake (or loss) behaviour of compacted GMZ bentonite. First, the water retention characteristics were investigated, and then the microscopic pore structure was observed by environmental scanning electron microscope (ESEM). The results indicate that GMZ bentonite has a strong swelling (or a limited shrinkage ability) due to water intake (loss). The suction behaviour of GMZ bentonite is similar to MX80 bentonite and FEBEX bentonite. We also find that the confinement conditions can affect the suction behaviour of the material, especially at high relative humidity (RH). Additionally, a mathematic model can fit the mass change data very well. Microscopic tests show that the granular sensation of GMZ bentonite is obvious for a sample at low RH. With the increase in RH, the surface of GMZ bentonite becomes more smooth. The differences in the porosities calculated by the macroscopic and microscopic tests can be attributed to image resolution. The inter-laminar pores and intra-aggregate pores cannot be observed by the ESEM method. In addition, ESEM observation can provide an intuitive basis for the further research of the seepage property of GMZ bentonite.

A New Unified Solution for Circular Tunnel Based on a Four-Stage Constitutive Model considering the Intermediate Principal Stress

Based on the triaxial test, the elasto-perfectly plastic strain-softening damage model (EPSDM) is proposed as a new four-stage constitutive model. Compared with traditional models, such as the elasto-brittle-plastic model (EBM), elasto-strain-softening model (ESM), elasto-perfectly plastic model (EPM), and elasto-peak plastic-brittle plastic model (EPBM), this model incorporates both the plastic bearing capacity and strain-softening characteristics of rock mass. Moreover, a new closed-form solution of the circular tunnel is presented for the stress and displacement distribution, and a plastic shear strain increment is introduced to define the critical condition where the strain-softening zone begins to occur. The new analysis solution obtained in this paper is a series of results rather than one specific solution; hence, it is suitable for a wide range of rock masses and engineering structures. The numerical simulation has been used to verify the correctness of the EPSDM. The parametric studies are also conducted to investigate the effects of supporting resistance, residual cohesion, dilation angle, strain-softening coefficient, plastic shear strain increment, and yield parameter on the result. It is shown that when the supporting resistance is fully released, both the post-peak failure radii and surface displacement could be summarized as EBM > EPBM > ESM > EPSDM > EPM; the dilation angle in the damage zone had the highest influence on the surface displacement, whereas the dilation angle in the perfectly plastic zone had the lowest influence; the strain-softening coefficient had the most significant effect on the damage zone radii; the EPSDM is recommended as the optimum model for support design and stability evaluation of the circular tunnel excavated in the perfectly plastic strain-softening rock mass.

Effects of Heating Rate on the Dynamic Tensile Mechanical Properties of Coal Sandstone during Thermal Treatment

The effects of coal layered combustion and the heat injection rate on adjacent rock were examined in the process of underground coal gasification and coal-bed methane mining. Dynamic Brazilian disk tests were conducted on coal sandstone at 800°C and slow cooling from different heating rates by means of a Split Hopkinson Pressure Bar (SHPB) test system. It was discovered that thermal conditions had significant effects on the physical and mechanical properties of the sandstone including longitudinal wave velocity, density, and dynamic linear tensile strength; as the heating rates increased, the thermal expansion of the sandstone was enhanced and the damage degree increased. Compared with sandstone at ambient temperature, the fracture process of heat-treated sandstone was more complicated. After thermal treatment, the specimen had a large crack in the center and cracks on both sides caused by loading; the original cracks grew and mineral particle cracks, internal pore geometry, and other defects gradually appeared. With increasing heating rates, the microscopic fracture mode transformed from ductile fracture to subbrittle fracture. It was concluded that changes in the macroscopic mechanical properties of the sandstone were result from changes in the composition and microstructure.