Research For The Anisotropy Of Quartz Mica Schist And Its Effect On The Stability Of The Surrounding Rock

For the rock, the formation of primary or secondary structural surface, such asbedding, foliation, joints and faults, etc, are naturally emerged in course of theirdiagenetic and post-secondary evolution. Precisely because of the presence of thesediagenesis defects and structural damage, rock presents obvious anisotropy. Quartzmica schist whose anisotropy in mechanics is mainly controlled by mineralcomposition and arrangement of the combination of rock, inherently contains a largeamount of schistosity, which has a significant impact on the underground cavernstability. In this paper, quartz mica schist in Danba hydropower station is researched.By microscopic and macroscopic mechanical properties test, combined with a largenumber of on-site investigation and analysis, along the main line of engineeringgeological analysis and evaluation quartz mica schist rock anisotropy and its influenceon the stability of surrounding rock is accurately grasped by using a variety oftheories, such as structural geology, mineral petrology, rock mechanics, fracturemechanics, anisotropic theory, etc. Through the above research, the paper mainlyachieves the following:(1) A large amount of geological information is collected from the geologicalsurvey on site. Since the transformation superimposed has experienced multipleepisodes metamorphism activities during the Indochina period, the Yanshanian periodand the Himalayan period, tectonism is extremely complex in the area. The area ismainly distributed by medium-high mountain landform eroded, the maximumgeostress whose value should be in the power of medium-high class are close tohorizontal direction. Groundwater that is mainly constituted by a certain depth offissure water buried in bedrock is not developed. The CPD1adit surrounding rock,namely quartz mica schist with foliation intensively developed, is a lower strengthsoft rock. Since the excavation of CPD1adit, without support measure, the overallstability of Surrounding rock has been favorable.(2) By casting grinding test and scanning electron microscopy quartz micaschist mineral composition and microstructure were studied, revealing themicroscopic mechanism of its anisotropy performed. Firstly, through the statistical analysis of42groups of grinding test form CPD1adit, it is noted that the minerals ofsurrounding rock in adit mainly include mica flaked and quartz granular and theircontent change regularly with adit depth. Secondly, the use of SEM performed onquartz mica schist indicates that the flake and granular minerals keep alignment andthere are multiple directional micro-weak surface, such as grain boundaries, latticefaults, fissures, etc, in the contact surface of grains or internal. Finally, thecombination of geological origin analysis reveals fabric unit of quartz mica schist hasgone through a long geological time, making mineral particles and micro-weaksurface with the lattice preferred orieniation(LPO) and shape preferredorientation(SPO), which results in the emergence of anisotropic characteristics ofquartz mica schist from the macro and micro perspective.(3) Combined with rock anisotropy theory, quartz mica schist macroscopicphysical and mechanical anisotropy were studied through the use of uniaxialcompressive strength test, triaxial unloading tests, in-situ direct shear tests and thescene acoustic test. Uniaxial compression test shows that the uniaxial compressivestrength and elastic modulus of quartz mica schist has a tendency to increase after thefirst decrease with the increase of β values, the curve is “U” shape, when β vlues isabout30°, both of them reach a minimum; Specimen failure mode is mainly tensilefailure and its failure characteristics are controlled by foliation and loading direction.Triaxial unloading result shows that both of strength and deformation of quartz micaschist exist anisotropic properties, and its anisotropy becomes weakening trendgradually with the confining pressure increases; specimen failure mode is all shearfailure with wearing layer. The scene acoustic test shows that the P-wave velocity ofquartz mica schist is anisotropic, that is to say, When traveling along the foliation,P-wave velocity reach the maximum, When direction of traveling is perpendicular tothe foliation, P-wave velocity reach the Minimum, P-wave velocity in the otherdirection are between the both; experimental data also indicate P-wave velocity ofsamples increase significantly and their anisotropy index show a decreasing trendfrom the upper to the lower. The scene rock direct shear test shows that shearparameters and deformation characteristics of quartz mica schist samples aresignificantly anisotropic; When shear direction is parallel to foliation, peak shearstrength parameters of sample f=0.61, C=0.84MPa, shear strength parameters f’=0.45,C’=0.30MPa, almost every sample cut layer damage phenomenon; When sheardirection is perpendicular to foliation, peak shear strength parameters of sample f=0.76, C=2.75MPa, shear strength parameters f’=0.69, C’=0.23MPa, the shear surfaceof most of samples have1~3cm of ups and downs.(4) Through field geological survey, related geological datas of CPD1aditwere collected, and based on fracture mechanics theory, the analysis of factorsaffecting the stability of surrounding rock was carried out from both of various microand macro perspective. From the microscopic point of view, mineral composition ofquartz mica schist has a more pronounced effect on the stability of surrounding rock,the higher the mineral content of quartz, the better rock mechanical properties, inother words, the better the stability of surrounding rock. In addition, stressconcentration is prone to emerge along the foliation direction after adit excavation, somicro-cracks in the direction areprone to extend or slip along the particles. From amacro perspective, the factors that influence the stability of surrounding rock ismultifaceted. Among them, the rock strength, geostress and excavate typessignificantly impact on the stability of surrounding rock and are controlling factor forrock stability adit.(5) Through on-site geological survey, combined with a large statisticalanalysis for indoor and field test datas. Firstly, the author believe that quartz micaschist foliation is a special surface structure, the nature of various aspects has beenmore fully reflected in the P-wave velocity of surrounding rocks and rock saturateduniaxial compressive strength and other parameters. therefore, foliation should not beinvolved with the division and assessment of rock parameters form other structuralplane, such as joints, faults, etc. Meanwhile, the structure type of adit surroundingrock is named as“massive-like”structure. Secondly, based on hydropower engineeringsurrounding rock engineering geology and surrounding rock classification preliminarycriteria, create Danba hydropower underground caverns initial impression standard bycorrecting for high stress, rock structure and foliation occurrence. Finally, accordingto the classification of engineering geology and rock the actual rock type, rockdeformation and rock classification criteria, based on the quality of Danba soft rocktype, rock structure type and completeness of rock, combined with on-site geologicalsurvey and monitoring data, establish soft rock classification system(SRCS) which issuitable for Danba underground cavern.