The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale underground heater test designed to study coupled thermal-hydrological-mechanical-chemical behavior in unsaturated fractured and welded tuff. As part of the international cooperative code-comparison project DECOVALEX, four research teams used four different numerical models to simulate and predict coupled thermal-hydrological-mechanical (THM) processes at the DST. The simulated processes included above-boiling temperature changes, liquid and vapor water movements, rock-mass stress and displacement, and THM-induced changes in fracture permeability. Model predictions were evaluated by comparison to measurements of temperature, water saturation, displacement, and air permeability. The generally good agreement between simulated and measured THM data shows that adopted continuum model approaches are adequate for simulating relevant coupled THM processes at the DST. Moreover, TM-induced rock-mass deformations were reasonably well predicted using elastic models, although some individual displacements appeared to be better captured using an elasto-plastic model. It is concluded that fracture closure/opening caused by change in normal stress across fractures is the dominant mechanism for TM-induced changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. This indicates that TM-induced changes in intrinsic permeability at the DST, which are within one order of magnitude, tend to be reversible.

/pdf/coupled-thermal-hydrological-mechanical-analyses-of-the-52w2giynvv.pdf

Coupled thermal-hydrological-mechanical analyses of the Yucca Mountain Drift Scale Test - Comparison of field measurements to predictions of four different numerical models

Mastering the formation mechanism and dynamic evolution pattern of bed separation zone during repeated mining is the key to the success or failure of separation grouting injection technology. Taking a mine in Ningdong as the engineering background, using the research methods of theoretical analysis, numerical simulated, and field measurement, four overburden structure models of single or multiple key strata with repeated mining and single or multiple key strata without repeated mining were constructed. The evolution pattern of bed separation zone was studied from four aspects: bed separation zone development mechanism, formation position, development form, and change of clearance. The results showed that when a single key stratum of repeated mining exists between coal seams, it is difficult to form a large separation area because there is no key stratum during the upper coal mining, and a separation area will be formed below the key stratum when mining the lower coal. When the key stratum exists above all coal seams, a large separation zone will be formed under the key stratum when the upper coal is mined. When mining the lower coal, when the coal seam spacing is less than 4 times of the mining height of the lower coal, the crack will penetrate the upper coal floor. If the key stratum is not completely broken, secondary separation may occur. When multiple key strata are all located above all coal seams, there will be bed separation zone under the high-level key stratum. When multiple key strata and coal seams are distributed at intervals, the separation below the high-level key stratum during upper coal mining and the separation below the low-level key stratum during lower coal mining are formed. Because the high-level key stratum bends and sinks after being affected by the mining of coal 1, the clearance of bed separation during mining coal 2 is slightly less than that during mining coal 1, which is contrary to the pattern that the greater the bed separation clearance increases with the increase of mining height.

/pdf/research-on-formation-mechanism-and-evolution-pattern-of-bed-5tospr8r.pdf

Research on Formation Mechanism and Evolution Pattern of Bed Separation Zone during Repeated Mining in Multiple Coal Seams

Heterogeneity is an important characteristic that affects the mechanical behavior of rock. In the present work, a statistical rock mesoheterogeneity model based on the Weibull distribution function is introduced into the discontinuous deformation analysis (DDA) method to simulate the mechanical failure of heterogeneous rock, in which the general heterogeneity degree is controlled by a heterogeneity index and the mechanical property of each subblock element is randomly assigned. Brazilian disc and uniaxial compressive rectangular specimens are simulated as examples. Results show that it is more reasonable to consider the heterogeneity of elasticity properties (the elastic modulus and Poisson’s ratio) and strength properties (the tensile strength, cohesion, and friction angle) simultaneously in the heterogeneity model. It is also shown that with a larger heterogeneity index, which means a lower degree of heterogeneity, the reproducibility of the macroscopic response curves of a specimen gets better, while the exact cracking always differs but with less scattered cracks, and the global fracturing failure pattern and mode are weakly influenced by the heterogeneity. Moreover, with the increase in the heterogeneity index, the macroscopic equivalent modulus and strength get larger and approach those of a homogeneous specimen. This work indicates the importance of heterogeneity for rock mechanical behaviors including the macroscopic equivalent response and the fracturing failure. By the subblock DDA method to simulate fracturing realistically, the fracturing failure process of heterogeneous rock can be successfully reproduced, which builds good foundation for the simulation study of heterogeneous rock fracturing in practical problems, e.g., coal and rock fracturing in fluidization mining in the future.

/pdf/dda-simulation-study-on-mechanical-failure-of-heterogenous-3kmvmnom6b.pdf

DDA Simulation Study on Mechanical Failure of Heterogenous Rock

Combined effects of microwave irradiation and water cooling on the deformation and failure behaviours of CSTBD granite

Effective control of high-position hard roofs (HHRs) is important to meet the requirements of safe and efficient mining. Vertical-well hydraulic fracturing (VWHF) is an innovative control method developed for releasing strong ground pressure behavior induced by HHRs. Understanding the hydraulic fracture extension law and its spatial form is critical for VWHF applications on HHRs. To address this problem, we have developed a large-scale three-dimensional fracturing simulation apparatus. The effective sample size is 2,060 by 1,200 by 1,200 mm. The primary loading subsystem maintains full closed-loop control of stress and displacement and can perform one-way, two-way, three-way, and step-wise independent loading and unloading with a maximum three-dimensional loading capacity of 10 MPa. The apparatus can achieve gas sealing of 3 MPa. The time-space distribution law of multiple parameters in the apparatus can be monitored using 240 simultaneous test channels, and experimental phenomena can be observed in real time using the visual monitoring system. The fracturing subsystem can achieve flow infinite transformation between 0 to 107 mL/min and water pressure up to 51.7 MPa. The apparatus simulates hydraulic (supercritical carbon dioxide or liquid nitrogen) fracturing of coal-rock mass under complex stress and a gas-containing environment. To the best of our knowledge, these are the first large-scale three-dimensional fracturing simulation experiments performed on hard roof sandstone samples. We analyze the distribution and change of acoustic emission signal and stress before and after fracturing and determine the hydraulic fracture propagation law, which reflects characteristics of hydraulic fracturing hard roof processes in a large-scale fracture network. These experiments validate the capability and reliability of the newly developed apparatus. The results provide strong technical support for VWHF to weaken HHRs.

Novel Large-Scale Three-Dimensional Fracturing Experimental Apparatus and Application to Vertical-Well Hydraulic Fracturing of Hard Roof

With the further development of deep rock mechanics engineering, such as the exploitation and utilization of geothermal resources, the exploitation of deep mineral resources, and the safe disposal of nuclear waste, the study of mechanical properties of deep high-temperature rock is gaining the attention of the researchers. However, not only the high temperature but also the cooling condition/method that will be used in the construction such as drilling cooling will also greatly affect the mechanical properties of the rock. In this paper, the mechanical behaviour and the evolution of the mechanical properties of the high-temperature (600°C–1,000°C) granite under different cooling methods are studied. The following conclusions can be obtained: (1) The peak stress of the granite decreases with the heating temperature. Compared with natural cooling, water cooling has a more significant effect on strength degradation. (2) The increase of the heating temperature increases the maximum axial strain of the granite. The water cooling method more greatly induces the maximum axial strain of granite than the natural cooling. The maximum axial strain of the specimen under the water cooling reaches 117.3% of that under natural cooling (800°C). (3) The elastic modulus of the granite decreases with the heating temperature. Water cooling will have a stronger effect on the reduction of the elastic modulus than natural cooling. The maximum difference value (2.02 GPa) of the elastic modulus under the different cooling methods occurs at the temperature of 800°C. (4) Poisson’s ratio of the granite increases with heating temperature, and the cooling method does not have an evident effect on it. The relationship between Poisson’s ratio and the heating temperature under different cooling methods can be described using the linear model. (5) According to the influence of the temperature on the peak stress, the elastic modulus, and Poisson’s ratio, the heating temperature domain can be divided into the unapparent zone, the significant zone, and the mitigation zone. (6) The thermal stress due to the nonuniform temperature field and the different thermal expansion coefficients is incompatible. Such incompatibility stresses the essences of the degradation of the mechanical properties of the granite.

/pdf/an-experimental-study-on-the-mechanical-properties-of-high-2tqufuoxdv.pdf

An Experimental Study on the Mechanical Properties of High-Temperature Granite under Natural Cooling and Water Cooling

The excavation of coal mine often encounters the fault problem. In this paper, the analytical study of the pressure of the fault that the work face may cross is carried out. The evolution of the stress and moment of the fault is investigated. Afterwards, a numerical study is performed. The stress and displacement of the rock mass near the work face and the fault are calculated under each situation (the initial fracture and periodical fracture of the immediate roof, the forward direction fault, and the reverse direction fault), and the changes in the contact pressure and stress near the fault during the excavation are analyzed. The effect of searching tunnel on the mining pressure is also studied. The calculation results indicate that although the searching-tunnel can decrease the stress at the roof of the fault (reverse direction fault), it may increase the stress at other positions. It can therefore be concluded that, for the forward direction fault, the effect of the searching tunnel to decrease the contact pressure and dissipate the energy is limited; on the contrary, it will aggravate the fragmentation degree and make the support more difficult, while, for the reverse direction fault, the excavation of searching tunnel can not only provide a better understanding of the characters of the fault but also have a positive effect on the ground control.

/pdf/analytical-and-numerical-study-of-the-ground-pressure-of-the-55b9blntt8.pdf

Yanan Gao

Papers

An Experimental Study on the Mechanical Properties of High-Temperature Granite under Natural Cooling and Water Cooling

Analytical and Numerical Study of the Ground Pressure of the Work Face Crossing the Fault