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Geomechanics and Geotechnical Engineering
QIAO WEIGUO, SONG WEIJIE, LIN DENGGE
QIAO WEIGUO, Professor, Ph.D. (corresponding author), e-mail: qiaowg1@163.com, SONG WEIJIE, Ph.D., LIN DENGGE, Professor, College of Civil Engineering and Architecture, Shandong University of Science and Technology; Shandong Provincial, Key Laboratory of Civil Engineering Disaster Prevention and Mitigation. Qingdao, 266590, China
New advances in research of the micro crack seepage properties of rocks
Abstract: Micro fracture seepage of rock properties latest research results were reviewed and discussed the applicability of the relevant micro-cracks and reliability of conclusions. Analysis showed that: In the study of seepage characteristics of micro-cracks in determining the mechanical opening, joint roughness coefficient and other parameters of the theory, experiment and numerical simulation conducted played a crucial role. Experimental study on micro-cracks is still in the exploratory stage, high-precision measuring instruments applied research in the field among the fissure to be strengthened. The conclusion of this paper also provides some reference and direction for the study of micro fracture seepage characteristics. Key words: rock mechanics, micro-cracks, mechanical aperture, flow characteristics, review.
Introduction
In deep underground engineering, the rock burst, the large deformation of surrounding rock, water inrush and other natural disasters occur frequently, which are caused by the micro-cracks development in surrounding rock. Fractures are mainly formed by the geologic structure changes and weathering, the micro-cracks open width is between 0.1 p,m and 10 p,m, of course, scholars at home and abroad has yet to reach for the seepage characteristics of rock fracture and research so far. As a branch of fractures, microcracks on the related research is far from enough, therefore, it is imperative to carry out the study of the seepage characteristics of rock micro-cracks. This paper aims to the hydraulic properties of micro-cracks and seepage characteristics of theoretical and experimental progress were analyzed, and the seepage characteristics change rule of the micro-cracks in order to further explore.
1. Hydraulic properties of micro-cracks 1.1. Micro-cracks aperture
The opening of the micro-cracks include mechanical aperture and hydraulic equivalent aperture [2]. Mechanical dynamics refers to the width of the cracks on the geometric measurement, the definition is usually equal to the average opening (E), the average opening are defined as follows:
E=^-JL0x№E(x-y)dxdy- (1)
where, Lx, Ly are the size of the joint; E(x, y) is function of local coordinates.
© Qiao Weiguo, Song Weijie, Lin Dengge, 2016 [35] www.dvfu.ru/en/vestnikis
The hydraulic equivalent opening refers to which is obtained by analysis of seepage calculation and conform to the cubic law of the crack opening [3, 4], which is defined as follow:
i
e = (^)3 , (2)
\igwz
where, Q is a given hydraulic gradient through the fissure flow; w is the width of between two parallel plate; v is the movement of the fluid viscosity coefficient; g is the acceleration of gravity.
As the hydraulic equivalent opening need to be obtained by the experimental method, the experimental conditions and methods have a great influence on the mechanical opening degree of micro-cracks. Therefore, the emphasis on the mechanics of micro-cracks is discussed.
The mechanical opening degree of micro fracture is in the micron level, which is measured by the physical method. So it is very important to use the effective method to measure the micro fracture aperture. At present, the main use of nuclear magnetic resonance and CT scan of these two methods to describe the mechanical opening of the fracture.
Nuclear magnetic resonance imaging has been widely used in medical diagnosis, petroleum exploration and other fields. It has the characteristics of nondestructive and rapid detection, which plays an important role in the detection of rock physics. The important parameters such as the opening degree, roughness, permeability, contact area and so on can be used for real time non-destructive testing, which can effectively solve the technical bottleneck of micro fracture mechanics. The pore structure of the rock was experimentally studied by nuclear magnetic resonance technique, and the relationship between the coefficient of the conversion model and the porosity and permeability of the rock was obtained by Zhao Jie [5], and the fracture structure of the coal body was detected by using the nuclear magnetic resonance technique, such as Pan Yishan [6]. It has been preliminarily applied in the detection of micro cracks in rock by the visible nuclear magnetic resonance technique.
The CT scanning technique is similar to that of nuclear magnetic resonance, and can detect the opening degree of the rock samples without damage. However, the accuracy of CT scanning technology is relatively low, generally can reach micron level, nuclear magnetic resonance can be reached nanometer scale, CT has some limitations, but also has a targeted, so that the experiment can be finished more quickly and efficiently. Among them, The CT scanning technology of rock micro-cracks structure were analyzed by Verhelst F.; GE Xiurun, in the uniaxial compression experimental conditions, by CT in the crevices of the rocks fine view propagation rule of real-time scanning. CT scan technology has played an important role in the detection of micro cracks in rock.
1.2. Roughness of micro cracks
At present, the evaluation system of micro fracture roughness is not perfect, the main evaluation index is: the joint roughness coefficient (JRC), fractional dimension (D) and fracture surface relief, et al. [7], because of the mechanical characteristics of JRC and the fracture is closely related, so it is widely adopted, now the following main introduce the JRC.
JRC is rock joint surface geometry of the quantitative description of the parameters, which have an important impact on the flow characteristics of micro-cracks, and the formula of JRC is as follows:
tanA^ = JRClg— , (3)
where, JCS is the joint surface compressive strength; an is the normal stress; A^ is the increment for friction angle.
There are two ways to determine the JRC: one is the shear test method; the other is the comparative method. Of course, these two methods can not accurately determine the crack of JRC. In 1977, N. Barton and V. Choubey proposed a typical roughness profile with 10 grades [7], so the existing test methods are based on the cracks, and the JRC value is not valid for the natural or unknown fracture morphology.
2. Micro fracture seepage characteristic theory
Because of the difference and complexity of rock fractures, the earliest indoor experiments are carried out by using two smooth parallel plates on behalf of cracks. The most representative is M. HoMU3e successfully put forward the cubic law in 1951 [8], that is the flow and the gap width of the slit is proportional to the three time. The formula:
Q = £—/ . (4)
^ v 12 J v '
However, in practice, the fracture surface is rarely smooth, so many scholars pay more attention to the experimental study of the fracture, and the correction is based on the cubic theorem, which is mainly concentrated on the research of the crack caused by the roughness. In order to solve the deviation caused by the roughness, Y. W. Tsang put forward the use of a tuning parameter to solve and get a good verification. With the further study of the seepage experiment, the relationship between the Barton and other scholars have found that the flow and the gap width is not the three time. The gap width is far greater than 3, while the Lomize and other scholars have found that the gap width index n is less than 3. At present, the basic law of natural rough fracture seepage has not been fully unified understand, for the obvious difference between the 3 kinds of relations, Xu Guangxiang will be summed up as follows: the cubic law, the super cubic law and the lower cubic law [9].
Many scholars have carried out extensive research on the relationship between hydraulic opening degree and mechanical opening degree. The current recognition of Y.W. Tsang and P. A. Witherspoon put forward a model of the combination of the cave - raised [10], which is based on the nail bed model and the model of the cave. Of course, all the models are based on the assumption that the joint roughness coefficient (JRC) is determined, so it is very important for the determination of JRC.
3. New progress of the experiment of micro fracture flow in rock
Because of the high accuracy requirement of the structure, material composition and defect of the rock, it can be measured in real time. And the magnetic resonance and CT scanning technology in the field of geotechnical engineering are late. At present, the related experiments are not perfect, the relevant nuclear magnetic resonance theory and the accuracy of CT scanning are still to be further explored.
Pan Yishan et al., Using magnetic resonance imaging to obtain magnetic resonance images of water saturated coals, observed the flow of main fissure research coal and water in coal sample in the flow channel; Xie Heping et al., with the help of CT scan obtained density distribution information and statistical characteristics of the internal material in the process of rock damage under uniaxial compression, through the statistical features of density change and fractal index to describe evolution of behavior of micro cracks. Can be seen that the micro fracture seepage characteristics experiment can be realized with the aid of the high precision instrument.
Conclusions and Outlook
The study of the seepage characteristics of rock micro fracture is still in the exploration stage, and the relevant results are based on a large number of experimental studies. The results are verified by theo-
retical derivation and numerical simulation. By analyzing the existing research results, the conclusions can be drawn as follows:
1) The mechanical opening degree of the micro fracture is between 0.1 ^m and 10^m, which determines the mechanical opening degree of the micro fracture is depended on the high precision test equipment;
2) The theory of micro fracture seepage flow has been improved and developed based on the theory of cubic law. The relationship between the opening degree and the mechanical opening degree has been obtained; but the evaluation system of the roughness of the micro fracture is still not perfect, and the JRC value of micro cracks cannot be obtained by the existing methods;
3) A large number of experiments show that the micro fracture seepage characteristics experiment in the nuclear magnetic resonance imaging and CT scanning technology and other high precision instrument is a good way to achieve.
REFERENCES
1. Xie Heping. Damage mechanics of rock and concrete. Beijing, Science Press. 1994:106-108.
2. Jiang Yujing, Li Bo, Wang Gang, et al. New advances in experimental study on seepage characteristics of rock fractures. Chinese Journal of Rock Mechanics and Engineering. 2008;27(12):2377-2386.
3. Witherspoon P.A., Wang J.S.Y., Iwai K., et al. Validity of cubic law for a deferrable rock fracture. Water Resources Research. 1980;16(6):1016-1024.
4. Ping R.J. Rock joint and rock mass behaviour during pressurized hydraulic injection. Cornwall, UK, Camborne School of Mines, 1986.
5. Zhao Jie, Jiang Yizhong, Wang Weinan, et al. Investigation of rock pore structure using NMR technology. Well Logging Technology. 2003;27(3):185-188.
6. Shi Qiang, Pan Yishan. A method of nuclear magnetic resonance imaging analyzed in the crack and fluid pass way of coal body. Coal Mining Technology. 2005;10(6):6-9.
7. Barton N., Choubey V. The shear strength of rock joints in theory and practice. Rock Mechanics and Rock Engineering. 1977; 10(1-2): 1-54.
8. Lomize M. Filtering in fractured rocks. M., Gosenergoizdat, 1951. (in Russ.).
9. Xu Guangxiang, Zhang Yongxing, HA Qiuling. Super-cubic and sub-cubic law of rough fracture seepage and its experimental study. Journal of Hydraulic Engineering. 2003;(3):74-79.
10. Tsang Y.W., Witherspoon P.A. The dependence of fracture mechanical and fluid flow properties on fracture roughness and sample size. Journal of Geophysical Research. 1983;88(B3): 2359-2366.
The report was delivered at the 5th Russian-Chinese Scientific Technical Forum Deep Level Rock Mechanics and Engineering, August, 5-7th, 2015, Weihai, CPR. On its basis, the author has written an article especially for FEFU: School of Engineering Bulletin.
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НАУКИ О ЗЕМЛЕ. Геомеханика и строительная геотехнология
УДК 622.011:622.83
ЦЯО ВЭЙГО, СУН ВЭЙЦЗЕ, ЛИНЬ ДЕНГЭ
ЦЯО ВЭЙГО - профессор, кандидат технических наук, e-mail: qiaowg1@163.com; СУН ВЭЙЦЗЕ - кандидат технических наук; ЛИНЬ ДЕНГЭ - профессор, Колледж гражданского строительства и архитектуры, Шаньдунский университет науки и технологии. Циндао, 266590, Китай
Новые результаты исследования фильтрационных свойств микротрещин горных пород
Аннотация: Приведен обзор результатов исследований фильтрационных свойств микротрещиноватых горных пород численными и экспериментальными методами. Показана большая роль шероховатости берегов трещин и других факторов при построении адекватных моделей. Экспериментальные исследования микротрещин все еще находятся на начальном этапе, поэтому необходимо развивать подобные исследования, в том числе прикладные, с применением высокоточных измерительных приборов. Выводы данной статьи показывают направления и пути изучения характеристик микрофильтрационных разрушений.
Ключевые слова: механика горных пород, микротрещины, механическая диафрагма, характеристики потока, обзор.
