At the 2nd JCST author sharing meeting, Mr. Xue Dongjie from China University of mining and Technology (Beijing) was invited to explain the cover article on the excavation induced stress drop in damaged coal consulting a combined yield and failure criterion published in the 1st JCST in 2020, and had a Q & A exchange.
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Q-questions
Why is the horizontal mining stress unloading, which is not consistent with the numerical simulation results?
A
Thank you for the teacher's question. It's a very good question. It must be traced back to the source. Is it from engineering or mechanics. In the early stage, such as Airey (1974; 1977), Barton (2019) mentioned an important experiment in the relevant literature as the similar simulation experiment. Of course, this experiment must come from the long-term observation of the field practice, so the similar simulation experiment is still very important, but it can not always stay in the early 20th century, and should have the voice of Chinese scholars. There is a common sense problem now, that is, the engineering pressure relief caused by excavation. How to reflect it is the later mechanical modeling problem. So from the evolutionary point of view, observation is in front, modeling is in the back, right? At present, our research group has not found much solid on-site support basis for the direct experimental evidence in this respect, but the most important thing is that we have not found the opposite argument, so we can not 100% think it is no problem. We think that the key to solve this problem is to develop the distributed in-situ stress measurement technology, and break through from the principle of in-situ stress measurement.
The second is mechanical modeling. The position perpendicular to the free surface is the free surface, obviously 0. How to transition to the original in-situ stress state, a very easy assumption is monotony, monotony is simple, directly connected by line segments. If the vertical stress is also operated in this way, it will be troublesome. Of course, there is also the main stress. Why do we say so? An important observation of the early collapse thought is that the self stability occurs across the backward rock strata. Just like the animal drilling, why the pangolin and earthworm drilling are not killed? It is obvious that the load lost in the excavation must have been transferred. Where is the transfer? Horizontal load The distribution is also continuous without obvious stress concentration, which is only vertical stress, so there must be stress concentration in another direction, we think this observation is extremely important.
The contradiction lies in how the numerical simulation is difficult to simulate the horizontal stress distribution consistent with the engineering observation. In fact, there is another problem that the pressure relief is also difficult to simulate. In the early days, I believe that scientists in mechanics, especially in numerical simulation, did not pay attention to the essential difference between geotechnical materials and geological materials. As I said in my report, simply transplanting the knowledge of material mechanics into geological materials did not conform to the nature of mining dynamics. At present, a common problem in numerical simulation is that the corresponding problem of stress-deformation, such as FLAC and other subsequent evolution problems such as true deformation, must be zeroed, which should be considered in numerical simulation, but it is still not well solved. Another problem is that mechanical modeling is generally continuous, and even discrete theory has the substantive problem of crack expansion. One of the ideas I repeatedly stressed in my report is that the nature of damage is the topological and geometric connectivity of discontinuous cracks. If the macro parameters are not paid attention to, it is difficult to simulate good results. For example, the general range of Poisson's ratio is 0 to 0 0.5, but it is a continuous material. Of course, the rock is generally below 0.3, but for the damaged rock mass, how to answer the applicability of Poisson's ratio in the process of rock to rock transformation involves two questions: first, the determination and evolution of the rve size defined by Poisson's ratio; second, the problem of breaking through the Poisson's ratio range, whether the negative Poisson's ratio is greater than 0.5, here We provide some literature, please refer to:
[1] Xue DJ, Zhou J, Liu YT, Gao L. on the excavation induced stress drop in damaged coal considering a combined yield and failure criterion. International Journal of Coal Science & technology. 2020, 7 (1): 58-67
This paper focuses on the problem of plastic and failure stress drop caused by Mohr circle movement
[2] Xue Dongjie, Zhou Hongwei, Ren Weiguang, Li Dongping. Study on the mechanism of crack growth and energy propagation based on Steiner minimum tree similarity simulation [J]. Journal of coal industry, 2015, 40 (3): 541-547. (EI)
This paper focuses on the essential difference between the topological connection of similar simulation and the self stability of rock and soil
[3] Xue DJ, Zhou HW, Liu JF, Zhou J, Liu YT, Zhao, YW, Zhang L. Experimental research on the structural behavior of fractured coal under uniaxial compression. Energies. 2018, 11(10): 2538. (Q1-SCI/EI-IF2.707)
This paper focuses on the problem of mechanical parameter mutation caused by geometric structure
Q
Is the direction of bearing pressure vertical or tangential? A bearing pressure is also a special term for mining, especially for coal mining industry. The use of the word "bearing" reflects the wisdom of the miner: rock self stability, or common bearing, which is also an observation from the idea of early caving method. So historically speaking, the depth of scientific research of miners, especially coal miners, is ahead of other industries. Many methods of construction are from mining. The research of mining is in-depth, and other industries are standardized. Obviously, gravity is easy for engineers and scientists to feel, so the supporting pressure can be equivalent to the vertical pressure in engineering. But in mechanics, we need to consider comprehensively. In our teaching, whether it's tunnel or roadway, circular excavation is the easiest solution. So according to the two sides, we need to consider tangential stress. According to the working face, whether it's horseshoe or rectangle, we need to consider vertical stress distribution in front. So this angle is a distribution of the horizontal plane based on the three-dimensional space, because it is perpendicular to the gravity direction, if it is any other angle, such as the top and bottom plate, it needs to carry out angle conversion. We have solutions in relevant literature, please refer to:
[1] Xue DJ, Wang JQ, Zhao YW, Zhou HW. Quantitative determination of mining-induced discontinuous stress drop in coal. International Journal of Rock Mechanics and Mining Sciences. 2018;111:1-11.(Q1-SCI/EI-IF3.78)
This paper focuses on the solution of mining stress drop
Q
How to understand yield and failure from mechanical point of view? A
Thank you for your question. In fact, this question is very difficult to answer. The research on Yield in the field of artificial materials is much deeper than that of geotechnical materials, and the mechanism of yield must be answered. This involves many questions, such as micro nano dislocations (such as crystals), cross-scale connectivity (such as salt rock). The core question is how to understand the plasticity of rock or rock mass, which is a certain difficulty. The conventional observation is based on the stress-strain relationship curve, but this is a precondition. The macro scale of cylinder and cube is much larger than that of the micro nano structure in the material class, so the observation here is more based on the average observation of macro statistical mechanics. As far as matters are concerned, I also mentioned in the report that for the geometric connectivity of discontinuous fractures, plasticity is the connectivity at the micro scale, and destruction is the connectivity at the macro level, so this involves a cross scale connectivity problem. The work of connecting networks of different scales must be answered with the help of percolation, renormalization and other advanced theories. Therefore, plasticity can be regarded as a prelude to failure, a process quantity rather than a state quantity. Please refer to:
[1] Xue DJ, Zhou HW, Zhao Y, Zhang L, Deng L, Wang X. Real-time SEM observation of mesoscale failures under thermal-mechanical coupling sequences in granite. International Journal of Rock Mechanics and Mining Sciences. 2018;112:35-46.(Q1-SCI/EI-IF3.78)
[2] Xue Dongjie, Zhou Hongwei, Wang Chaosheng, Gao Hailian. Study on percolation model of fracture evolution of overlying strata [J]. Journal of China University of mining and technology, 2013, 42 (6): 917-922. (EI)
[3] Xue Dongjie, Zhou Hongwei, Ren Weiguang, Liu Yaqun, Chen Chaofan. Pillar collapse model and instability of super high mining in shallow seam [J]. Journal of coal science, 2015, 40 (04): 760-765. (EI)
[4] Xue Dongjie, Zhou Hongwei, Ren Weiguang, Zhang Bofu, Liu Yaqun, Zhao Yufeng. Evolution of mining fissures and formation mechanism of bench cut in shallow deep thin bedrock coal seam group [J]. Journal of coal science, 2015, 40 (08): 1746-1752. (EI)
[5] Xue Dongjie, Zhou Hongwei, Ren Weiguang, Zhang Miao, Zhao Biao, Zhong Jiangcheng. Study on multifractal distribution of deep joint spacing of Beishan granite [J]. Geotechnical mechanics, 2016, 37 (10): 2937-2944. (EI)
Q
You have also published a lot of literature on seepage mechanics. Please ask how to understand the influence of water on rock strength
Thank you for your question. This is an important question. It is not easy to answer this question. Some rocks or soils are rich in minerals that are easy to react or combine with water, which is easy to understand. However, some hard rocks, such as granite, have obvious strength drop according to our fluid solid coupling experiment, which is not easy to understand. Although the principle of effective stress and physicochemical reaction can reveal some phenomena, our understanding of water is not comprehensive in general, and we look forward to cooperation and joint research.
Q
Professor Xue, please find out the difference between pre peak and post peak uninstallation a
Thank you for your question. There are many references in this field for the conventional triaxial experiment. Here we focus on the recovery of unloading before and after the peak of mining dynamic behavior. The most important purpose of unloading is to see the boundary and contribution degree of elastic deformation and plastic deformation. An important indicator is the storage and release amount of elastic energy. One of the differences between the mining dynamic behavior and the material mechanical behavior is the in-situ stress, the in-situ stress without deformation or the in-situ stress with memory deformation in the history. Obviously, it will not release the energy completely after unloading. If so, there will be no stress concentration. However, the key problem is how much is transferred, which can not be explained well by the material mechanical behavior, Now we have several important phenomena, such as rock burst, rock burst, coal and gas outburst and so on. Where are the boundaries of dynamic disaster sources? It's impossible to name one kind of rock. It can be seen that both soft rock and hard rock have dynamic disaster phenomena. The current definitions are not quantitative definitions, but only an observation of phenomena, lacking scientific basis support. For example, at the same depth, when rock burst occurs in this place, why does the adjacent area not occur? Their in-situ stress and other properties are similar; another similar phenomenon is the problem of landslides on the ground, the same area, the collapse of this mountain peak, and why the adjacent similar mountain peak does not collapse. Are they very different? Obviously not, but internal The critical scale of fracture connection is different. Some of them reach their own critical state. Once the critical size of interference or interference changes after a period, the macro whole will change. Moreover, according to the current classical stress-strain relationship curve, the calculated energy value is always small, which is not enough to trigger such a violent dynamic disaster of rock mass. Then where does this part of energy come from? Obviously, it is the transferred energy, without considering the stress drop, without considering the discontinuous stress distribution, it is impossible to calculate the accurate energy transfer at all, and there is a miscalculation on the bearing pressure concentration, more