Study On Deformation Instability Evolution Mechanism And Prediction During Excavating Process Of Bedded Rock Slope

Rock bedded slope is quite a common slope type and it is prone to deformation or destabilization under the disturbances of human construction activities,therefore the study on the engineering properties of rock bedded slopes is of great significance to human construction activities.The cases of rock bedded slope failure induced by human activities?which alter its structure or geological conditions?are numerous,some of them even cause catastrophic consequences:Tangyanguang a reservoir slope of Zhexi reservoir located in Hunan province,China,which collapsed on March6th,1961;0ct 9^th 1963 the Vajont landslide in Italy and Jun 13^th,2003,Qianjiangpin landslide on the bank of Three Gorges Dam reservoir,China.In engineering constructions,excavations of bedding rock slopes are prone to slip along the layers,especially as they are with weak intercalated layers,and rainfalls will further increase odds of destabilization.Presently,researchers have obtained abundant achievements related to predictions of bedded rock slope,however most are proposed based on an ideal geological model,conversely,actual deformations and evolutions of slopes are extremely complicated,in addition,physical and chemical changes of rocks and soils exhibit obvious complex nonlinear characteristics.Therefore,more research methods are demanded in predicting and evaluating slope issues.With the aim of predicting the stability of rock bedded slope after excavation,in this article,a side slope of a construction site in Zhijin county Guizhou province was investigated as a typical case of rock bedded slope.Firstly,a field survey was carried out to obtain the structural features of the slope,such as thickness of the rock layers,joint characteristic and mudded intercalation composition etc;Secondly,multiple field tests and laboratory tests were conducted to acquire the mechanical properties of the rockmass and sliding zone materials;Then a geo-mechanical model of the slope was established to analyse its deformation and failure mechanism.Based on the achievement of the field survey and mechanism analysis,a new predicting method of rock bedded slope was proposed.In the end,in order to understand the reaction of a rock bedded slope under different method of excavation,a physical modeling experiment was conducted to stimulate the failure process of rock bedded slope.Deformation zones and failure features of the modeling were studied.All the conclusions and results are listed as follows:?1?Study on engineering geological classification and mechanical properties of weak intercalation in the study area.By geological genesis the weak layers can be classified into three types as geological structure,weathering t,corrosion filled.The first type that can be divided into 4 based on material composition as the whole layered structure,the layered structure,the layered mosaic structure,the layered cataclastic structure and the granular structure.Direct shear tests are conducted using weak layer material samples with different moisture content?w=10%³0%?,medium-sized shear experiment and situ shear test,it found that the shearing strength parameters?c,??decrease with the water content increase,the fitting formulas according to de test data were:?=20.8×ln?w?-6.8,?R=0.986?;c=24.8×ln?w?-12.2,?R=0.990?.The strength of clay film?weak layer thickness less than 0.01mm?is very sensitive of precipitation?before:f_p=3.5⁴.5;f_r=2.5³.5,after:f_p=2.0².6;f_r=1.2¹.6?,the decrease ratios are 22.2%to 65.6%.?2?Three evolutionary mechanisms of slope were established that was based on the initial site of slope instability and the geological characteristics of slope evolution:forward progressive evolution mechanism;backward progressive evolution mechanism and the overall gradual evolution mechanism;The instability mode of the excavated slope includes five types:sliding and fracture,plane sliding,sliding-bending,erosion and washing,rock spalling.?3?Based on the research results of rock bedded slope excavation in a large amount.Firstly,the comprehensive evaluation is carried out for the slope in the study area,the deformation instability mode and characteristics of the slope in the excavation process are summarized;Secondly,10 specific indexes of slope stability are selected for the elements of the geometric characteristics,the engineering geological conditions,and the external environment.The combination weighting is calculated with the mode of on improvement of Analytic Hierarchy Process based on Rough Set theory and the entropy method,meanwhile a method for predicting the stability of slope is established by unascertained measure and cloud model;at last,The engineering practice of stability of excavation slope in the field to prove the rationality and reliability of the established method.?4?Considering different rock structures and excavation conditions of slopes,an evaluation method was established to predict the slope stability.This method is based on deformation data of large frame physical model tests of rock-bedded slopes.The instability threshold value was set as displacement equals 5.0mm;then the instability region can be defined as the area where the displacement surpassed 5.0mm.The tests results show that slope excavation height?H?,rock layer dips???,excavation slope ratio???,spacing of weak interlayers?h?and internal friction angle???are the most influential factors of instability region.Based on test data an experimental formula of instability range was established:?5?For verifying the correction of this new method,it was used to evaluate the situ testing slopes,the prediction results are consistent with the real situation.After confirming the practicality of the method for the situ slopes,the stability of excavation slopes?P1^P7?were evaluated,it shows that the stability grade of C3 and C4 are accounted for 44.8%and 27.6%respectively,namely the slopes stabilities are slightly low,the maximum instability length of excavation is ranged from 0.6H to0.8H,the largest instability region is in the height of 0.4H⁰.6H.