Study On Reasons For Lower Strength Of Granite In High Earth Stress Area

Beginning with the abnormal phenomena of a large number of low strength as to the granite in Shuang jiangkou dam site, on the basis of a great deal of site investigation and analysis, environmental occurrence of the granite such as geological background, engineering geological conditions, characteristics of stress field, geological structure, etc. are studied. Combined with the petrology, construction characteristic, later transformation and the rock structure characteristic of dam site granite, a lot of macro-mechanical test and Micro and Fine Structure characteristic analysis are carried out aim at the granite of lower strength and higher strength respectively. The main several research results can be obtained as follows.(1) By a great deal of site investigation and analysis, the systematic test for the rock mass, the weathering burden features, stress field, groundwater condition, rebound and wave velocity of the granite at different height and depth are carried out, and based on this, the characteristic of geological environment is been quantized, and the correlation analysis between the characteristic of geological environment and the intensity of saturated single axis is proceeded.(2) By measuring the basic physical index of the porphyritic-like granite in Shuang jiangkou dam, in consideration of the stress field condition, setting different ambient pressure and carrying out the penetrability test aim at the porphyritic-like granite which has different intensity respectively, disclosing the relation among the cranny growth situation, the confining pressure situation and the granite penetrability characteristic, analyzing the relation between the intensity variation and micro-crack of granite in Shuang jiangkou dam.(3) By carrying out the study on the Acoustic Emission (AE) test in the monotonic loading process and the cyclic loading process for the granite, disclosing the change characteristic of the acoustic emission rate during the crack initiation-extension-perforation process of the granite crack, the difference of the strength in the failure process of granite causes the difference of the failure rate release. It indicates that the failure process for the lower strength granite is a gradual steady process, and the failure process for the higher strength granite is centralized fierce one.(4) Based on the conventional triaxial test, unloading confining pressure triaxial tests in different confining pressure, different unloading rate and unloading start point, it discloses that the unloading rock failure characteristic of the granite and the parameter change characteristic in the failure process. Especially, for the elastic modulus, the test indicates that during the test process elastic modulus is not constant but variety has obviously turning point. The test process could be separated into loading section, unloading section and rebound section, and the disparity between each section is relative big, elastic modulus in the loading section is usually three or four times to elastic modulus in the unloading section, and elastic modulus in the rebound section is smaller than elastic modulus in the unloading section.(5)According to the memory test of loading and unloading confining pressure circularly, it indicates that there exist the stress hysteresis phenomenon and hysteresis trapezoidal during the unloading process, just the formation mechanism is different from the hysteresis loop during the uniaxial loading and unloading process, the change characteristic and the formation mechanism during the unloading confining pressure process are complex more. The formation mechanism of a closed trapezoidal is disclosed in the view of the energy variety characteristic during the deformation and failure process.(6)According to the analysis of Scanning Electron Microscope (SEM) and mineral contents, it indicates that in micro structure aspect there are mainly three reasons for lower strength of granite. Firstly, the test result indicates that the saturated uniaxial strength decreases with the feldspar content increases, and the numerical test result indicates that in three minerals the mica content is most sensitive to the compressive strength, the contents of feldspar and mica of the granite in Shuang jiangkou dam site are higher that that in other area. So, the higher content of feldspar and mica is one of the microscopic reasons for the strength of the granite in Shuang jiangkou dam site is lower than that in other area.Secondly, according to the analysis of the uniaxial test, conventional triaxial test, unloading triaxial test of the granite in Shuang jiangkou dam site and SEM of the destroyed section of the field rock burst rubbings, the inner cleavage plane of feldspar in the granite in Shuang jiangkou dam site is extraordinarily growing and very dense, and lots of feldspar even showes scaly and acicular cleavage. It discloses that the main destroyed mode of the granite in Shuang jiangkou dam site is not the along grain destroy between each mineral particles but the transgranular destroy occurs along with the feldspar cleavage plane. It is another reason for lower strength of granite in Shuang jiangkou dam site, and also is the main reason for adit residue in Shuang jiangkou dam site is silty sand.Thirdly, because the essential differences of the micro-rupture mechanism between the granite triaxial compression test and unloading confining pressure triaxial test, when conventional loading although there is a relative great difference in elastic modulus between minerals, the mica acts as coordination function. So, the mica content is beneficial to the granite conventional triaxial test but opposite to the unloading confining pressure triaxial test.With the unloading of the confining pressure, due to the elastic modulus of mica is lower than other minerals, it offers the volume rebound space for other minerals. Because the difference rebound between minerals, it is favorite to the production and development of crack among minerals and in the inner mineral. The conventional triaxial test indicates that the granite in Shuang jiangkou dam site is easy to be influenced by the confining pressure. When the confining pressure is higher than 25MPa, the strength of the granite in Shuang jiangkou dam site is even higher than that in other area, and with the confining pressure rising the elastic modulus increasing correspondingly. It is the better interpretation to the function of mica.(7) The numerical microscopic experimental method system of graphics and image is established with the technique of image processing and FLAC which is a large-scale simulation software. According to the real structural model of the granitee, the law of stress distribution of various minerals caused by external forces and the failure process of crack appearing—expanding—run-through are simulated. On the basis, the sensitivity analysis of the mineral's composition and their relevant parameters are carried out..Finally, based on the systematic study, it can be considered that the reasons for low strength and great discreteness are as follows. In the macroscopic aspect, it is influenced by geological environment including ground stress field, geological structure, epigenetic Reformation, rock mass structure, etc. In the microcosmic aspect, it depends on contents of feldspar and mica, cleavage distribution in feldspar, fracture distribution in each mineral. In general, the basic condition for the fracture of granite unloading rock mass is the growing feldspar cleavage plane and the relative high mica content. The internal motive force which caused differences of stress field and internal crack of granite is high earth stress and good energy storage capability of granite, with rapid rappincise of valley and strongly release of tectonic and slope stress. The external motive force for the development of the inner crack of granite in the dam site area is late reformation(epigenetic reformation) which caused further release of slope and surrounding rock stress in high ground stress environment.