Research On The Deformation Failure Mechanism Of Unloading Slope And Its Prediction With Early Warning In High Geo-stress Region

There are many large infrastructures in the southwest, especially the Large hydropower station and tunnel, and these infrastructures always involved in the mountain ravines. The large-scale unloading deformation of the high and steep rock has became a engineering geological problem in many projects. High and steep slope in the process of geological tectonic history’ often occurrence of high ground stress, in the neotectonic movement and human engineering activities, it brush incised in the river, this called unloading effects. It would cause High geo-stress release inside the slope, result in the stress redistribution, then generate many unloading crack and fault inside the slope. Blind engineering activities usually induce bucking failure in the slope, it has a harmful effect on engineering construction and the human life and property. So it’s very important to have a in-depth research on the slope of high ground stress deformation characteristics and failure mechanism, quantitatively analyse the key factors affecting the slope instability, and get a criterion to warn and forecast the instability. It has a guiding significance for engineering construction, practical construction.The JINPING First Stage Hydropower Station is one of the important cascade hydropower stations in YALONGJINAG Rive, located in the Slope zone of QINGHAI-TIBET plateau to the SICHUAN basin. Because of the Tibetan plateau eastward plate motion and high and steep rock in barrage area, the high geo-stress arise. This paper chose the left bank counter-inclined slope of the JINPING-I Hydropower Station as the research object, combine to the indoor experiment and use the numerical simulation technology to conduct a study on the slope of high ground stress deformation characteristics and failure mechanism and give a evaluation index about warning and prediction of the safety of high and steep slope in this area. The results are as follows:(1) Through the field geological investigation and laboratory test, researched the mechanic parameters of rock mass and distribution feature of lithology, structural surface, joint, stratum, unloading fissure in high and steep slope around the JINGPING-1Stage Hydropower Station, a generalized geological model is established. Based on the objective analysis to bad geological structure of JINPING, the conclusion that the mangy weak geological structure such as X spot bright dike and F5/F8faults had a greater influence on the high slope of JINPING area, and lots of deep cracks developed in the side of slope under the condition of unloading such as tectonic stress field, valley incision, engineering excavation, etc. According to the statistical analysis on270joint in slope project area. DABENLIUGOU material yard, JINPING-I hydropower Station, there are2groups of advantage joints, one orientations is NE20°, another SE85°. They are great harm to the safety of the engineering slope.(2) Using multivariate linear regression analysis method and3d geological model established by numerical simulation technology to a wide range geo-stress inversion analysis of JINPING dam site.According to the simulated result,the geo-stress in JINPING decreases with the increase of elevation as a whole, in shallow surface area,it was mainly influenced by gravity stress, but in deep is tectonic stress; geo-stress in the horizontal ground increases with levels deep. In the middle and lower parts of river bank geo-stress is very large, appearing stress concentration phenomenon, that has a great influence on slope excavation of JINPING-I hydropower station.(3) By means of Triaxial unloading test, The unloading deformation and failure mechanism of sandstone is obtained. According to research result. Established the rock mass deformation mechanism under the condition of unloading that is elastic-ductile-capacity-damage. And detailed analysis of the deformation mechanism at each deformation stage. At the same time, the experimental data display a change rule of unloading rock mass mechanics parameters that is the deformation modulus is increased but Poisson’s ratio decreases as confining pressure increases.(4) Take project profile V-V (0+137) which in the left bank of the dam site area as an example, a geological is set up by UDEC software. And in order to study the engineering excavation unloading effects on high geo-stress of the slope.8Mpa of stress that based on geo-stress parameters inversion was applied on the model boundary.Results showed that the deformation failure mechanism mainly has two aspects:for on thing.the excavation will cause geo-stress released differentiation and redistribution in side of slope, due to geo-stress release. at near the excavation section appeared stress concentration. unloading rebound, bulge deformation, unloading fracture phenomenon as well as loose rock mass deformation.shear rupture stress were appeared in deep slope. Geo-stress redistribution could cause uneven distribution of stress in body of slope, and in high stress zone there will emerged wall caving and the pie of cores, etc., when the deep was excavated, if a large number of geo-stress suddenly released, the phenomenon of rock burst maybe occurred.For another. The upper rock mass of slope Under the action of unloading rebound and was pulled of the gravitational of the lower block have a slight fault or turn, and then squeeze weak structure in the slope, when the pressure is enough, could cut the surface of slope and slide failure.(5) All kinds of fault, unloading fissures, X spot bright dike and bedding of rock slope in JINPIONG have a great harm to the engineering properties of slope.Based on typical slope profile Ⅱ1-Ⅱ1in the left bank of the dam site area, and considering mang bad geological structure such as F5.F8, F42-9faults, Spot bright dike as well as geo-stress using UDEC to study the influence mechanism of Slope stability under the condition of changing the distance of unloading fissures from aspects, strata dip Angle, strata thickness, reservoir water level. And determined the predicted threshold of instability of each single factor. Finally, using the method of orthogonal experiment to study the Multiple factors threshold of in stability under the action of four factors mentioned above. According to analysis, without taking reservoir level into consideration, the most dangerous combination are unloading fissure, the distance is30m,rock thickness is40m, strata dip angle is65°; nevertheless, when considering reservoir level, the most dangerous combination is that unloading fissure distance of30m, when the rock thickness is40m, strata dip angle is65°and water level is1880m., getting the predict the warning indexes of deformation and fracture of high and steep slope in JINPING area.