Experimental Study On Rock Mechanics Based On Deep Fracture In Typical Banks

The southwest is located in the first Terrain gradient band where is rich in hydropower resources. A series of built and ready to build large-scale hydropower project located in the area. Because the region is located in the eastern fringe of Qinghai-Tibet fault block, Qinghai-Tibet Plateau in recent millions years continued to uplift , the Geo-environmental conditions is special complex in the region. Therefore, in these areas for large-scale, very large projects, a series of constraints engineering design, construction and operation of major engineering geological problems need to be studied and resolved, one of which is the deep fracture in the banks. In this paper, the east side of the south-west of the Qinghai-Tibet Plateau is the main research subjects, by choosing the deep fracture revealed in the Yalong Jinping I Hydropower Station, Dadu Pubugou, Shen Xigou, Shuang Jiangkou large-scale hydropower projects as research material, using inductive and deductive philosophy thinking way, stress the geological scene prototype research and geological processes analysis, attach importance to the process of formation and evolution of natural slope and the environmental geomechanics of deep fracture, apply modern numerical simulation technology and rock mechanics theory and method, integrated research the deep fracture'development characteristics and formation mechanism from the four aspects: geological prototype research, numerical simulation of inversion, rock mechanics experiments, Theoretical Analysis of Damage. A set of deep fracture in the basic framework of research and technical methods of system was initially established. Achieved the following main results:(1) By systemic studying the deep fracture in the typical banks, summarized the general development characteristics of deep fracture. That is :①More cracks developed in the distance 70 ~ 120m above the bottom of the slope rock mass;②Most cracks were banded output, there is relatively complete rock between the rupture zones of the rock;③It is very clean in the Cracks, almost no secondary Mud ;④The development degree of cracks increases with the elevation increasing and decreases with the horizontal depth increasing;⑤The formed time of cracks gets old with the elevation increasing and gets fresh with the horizontal depth increasing;⑥These cracks formed along the steep inclination of tectonic structure which generally parallel with the slope.etc.(2) By comprehensive analyzing deep fracture'geological environment in the typical banks , put forword to the geological environment conditions which are deep fracture can be generated that must be met by:①The hard and brittle lithology which can store a higher strain energy in the process of structural transformation;②The geological structure (such as body folds) and structural conditions (lithology and tectonic fractures) which conducive to the release and storage of strain energy ;③As a generalized "load" could lead to slope compressive deformation and rock mass have a strong rebound of the Heights stress conditions;④Rapid crustal uplift (rapid incised valley) can lead to strongly release the rock mass strain energy.etc.(3) Put forward the formation mechanism of deep fracture. In the valley (or superimposed horizontal valley) process of formation and evolution of landforms, going with regional erosion and the process of valley cutting, the banks to adjust to changing stress field, caused rock slope by pre-stored internal strain energy release strongly and produced the difference unloading rebound to the direction of the overhead,then formed the deep fracture,which belong to epigenetic time_dependent structures.(4) According to the process of the formation and evolution of rock slope, determined the transformation mode of rock slope, rock slope inward from the slope table is divided into there zone : surface reformation zone, epigenetic reformation zone , structural reformation zone . Surface reformation zone can be broken down unloading cracking zone and tightly compressing zone ; eqigenetic reformation zone is made up of deep fracture zone and inter-plate girder zone .Had done a Statistical analysis to the rock stress , sound wave, point load, fracture density, fracture aperture and so on .(5) By analyzing the measured stress of typical slope indicated that there is a big geological tectonism in the development region of deep fracture, belong to the high(middle) stress area, rock stress is mainly composed of the horizontal tectonic stress.(6) Based on the changing characteristics of the measured stress with the level of depth, the rock slope stress field can be divided into four zone: stress decreasing zone, stress increasing zone, stress rebounding zone and stress stability zone. Stress decreasing zone can be compare with the scope of unloading zone.Stress rebounding zone can be compare with the scope of deep fracture formation region. Analysis showed that the bottom boundary of stress rebounding zone represent the impact depth of the valley stress field. Statistics reveal that the depth is about 150 ~ 387m in China's south-west valley region.(7) The distribution of stress field is closely related to the process of surface- epigenetic reformation in rock slope . Based on the formation mechanism , bank stress field can be divided into there regions from outside to inside : Stress field slopes, Transition zone and Tectonic stress field. They can be compare with the surface reformation zone, epigenetic reformation zone and structural reformation zone. Analyzing the stress distribution characteristics in various zones, put forwoed that the slope stress field based mainly on the self-weight stress field. Transtion zone is that region where the tectonic stress field transit gradually into the self-weight stress field . Tectonic stress filed is made mainly up of tection stress, this viewpoint can be validate according to the Maximum principal stress angle changing with the depth .(8) Using modern numerical simulation techniques, check and analyse the stress field of the slope characteristics of the sub-band, and to explore the impact that the tectonic stress ; At the same time, also analyzing the basic characteristics of evolution of stress field and the deep fracture formation process. In the geological process, according to the changing characteristics of crack unit stress Mohr circle, explore formation times of the deep fracture ,and then, draw a conclusion that come close to the slope table and cracks of high-elevation are earlier ,respectively, than the deep fracture and cracks of low-elevation in the formation period, and verify the results of geological analysis.(9) The basis is the actual state of stress changes in the formation process of deep fractures for experimental design , to carry out comparison of unloading rock mechanics test in a different confining pressure and different unloading rates . Revealed by the test in unloading conditions :①With the destruction of confining pressure increasing, specimen failure damage to the transition from tensile to shear failure, and at the same confining pressure, with the unloading rate increasing, the tensile damage also more serious;②Specimen show the progressive damage of characteristics, usually near the surface of the specimen there are the tension cracks, the general shear failure surface developed from the parts of tensile fracture surface , they are tension-shear nature;③Unloading have a great impact for the specimen horizontal strainε3 and volumetric strainεV, into the unloading stage, horizontal strains'gradient changes significantly increased , volumetric strain changed from compression to expansion ;④The stress - strain curves of the majority of the samples have a large drop after the curves'peak. Show that the destruction of the specimen have a small plastic deformation at the time of unloading , damage to more sudden and brittle features ;⑤There are larger difference between the behind-peak and the front-peak in stress-strain curves, stress - strain curves are more smoothness before the peak and more uneven after the peak . Show that the stress-strain of samples are more complex after the peak;⑥The majority of the sample stress - strain curve after the peak was "Ⅱ-type", showing that the brittle characteristics;⑦confining pressure impact on the specimen strength more larger than loading , and the same initial confining pressure, the specimen strength and deformation modulus decreased with the unloading rate increasing ;⑧with unloading rate increasing, the sample shear strength parameters c values increased andφvalues of decreased, compared with the loading conditions, the shear strength parameters c values decrease andφvalue increased ; etc . Based on these test results , making a reasonable explanation for the deformation and failure characteristics of the deep fractures.(10) Through assumed that the rock micro-element strength and distribution, constructed the damage evolution equation and constitutive model on nominal stress-strain formulation, and using it to compare and analyse the characteristics of damage evolution of rock in different experimental conditions, the development and evolution of samples can be divided into three stages in the loading and unloading process . The first phase compare with the forepart of the curves'Yield limit , the above stress - strain curve is near a line , rock micro–element is elastic deformation mainly, only a very small number of rock micro-element damaged, damage evolution was slowly increasing from 0 momentum, the curves'slope increased slowly. The second phase compare with the sample curves'Yield limit→residual strength, stress - strain curve into a nonlinear, rock is plastic deformation mainly, a large number of rock micro–element began to yield-damage, damage evolution was rapidly increasing in momentum, the curves'slope increased quickly. Finally, it remains stability in a certain level. The third phase compare with the sample curves'residual strength, the curves'macro-rupture occurred, but there is still a certain degree of capacity, a small number of rock micro-element continue to yield-damage, the rate of damage evolution gradually slow down, the curve slope gradually decreased and tends to 1. Revealed initially :①In the same confining pressure, the rate development of sample damage is more faster in unloading condition than the loading conditions ;②the same confining pressure, the rate development of damage increases with the unloading rate increasing ;③the samples'strength is only related with the rate of damage evolution , but they have nothing to do with the total number of rock micro-element which have damaged when the sample damaged; etc. And based on the results of the various types of analysis, a reasonable explanation have made for the characteristics of development and distribution of deep fractures and the characteristics of deformation and damage of deep fractures.