Study On The Slope Stability Of Phyllite Bimrocks Based On The Orientation Of Blocks

In strong tectonic areas such as the eastern margin of the northwest Sichuan province(also known as Maoxian Group area),the phyllite is widely distributed.In this area,phyllite is prone to fragment during the uplifting of the earth crust,rainfall,and gravity,resulting in widely distributed phyllite bimrocks.With the construction of the expressway network planned by Sichuan province,especially the construction of important transportation networks from Sichuan province to Tibet province,such as Wenchuan-Maerkang,Yaan-Kangding,Wenchuan-Jiuzhaigou and Mianyang-Jiuzhaigou expressway will inevitably pass through the phyllite bimrocks area of Maoxian group.However,under the condition of gravity or rainfall,a large number of phyllite bimrock slopes have failed along the Wenchuan-Maerkang and Yaan-Kangding expressways,which not only caused obstacles to the construction and operation of engineering but also threatened the life and property safety of residents.Therefore,it has great significance to study the stability of the phyllite bimrocks slope for the construction and operation of the expressway passing through this area.In addition,it can also provide important theoretical and engineering suggestions for the prevention of geological disasters in such bimrocks.Almost all the literature studied the bimrocks by assuming that the bimrocks are homogeneous geomaterials.However,for phyllite bimrock slopes,due to the oriented mineral texture of phyllite,the phyllite blocks display obvious flat characteristics.The flat phyllite blocks present a similar imbricate texture in the phyllite bimrock slopes,which is different from other bimrock slopes.The similar imbricate texture in phyllite bimrock slopes is defined as the orientation of blocks in this paper.The orientation of blocks makes the existing stability evaluation methods unsuitable for phyllite bimrock slopes.Therefore,it is necessary to further study the deformation and failure mechanism of phyllite bimrock slopes with the orientation of blocks.In this paper,the phyllite bimrock slopes with the orientation of blocks are taken as the research object.The relationship between the orientation of blocks and the flow behaviour of phyllite bimrock slopes,the slope stability of phyllite bimrocks under rainfall conditions,and the influence of the orientation of blocks on the slope stability of phyllite bimrocks are systematically investigated by using the field investigation,laboratory test and numerical simulation.The following innovative results were achieved:(1)Based on the field investigation,the orientation of blocks is defined,and the quantitative index of the orientation of blocks is proposed,namely the orientation angle and orientation ratio.Besides,based on the field investigation data,the thresholds of orientation angle and orientation ratio are determined.Combined with the field investigation data and the phyllite block accumulation test,the influencing factors of the orientation of blocks were studied.The results show that the more vertices of the blocks or the higher the flat rate,the better the orientation of the blocks.Under the experimental conditions of this paper,the smaller the slope angle or the higher the content of fine particles,the better the orientation of the blocks.In addition,the influence of the orientation of blocks on the stability of the phyllite bimrock slopes was studied by the large-scale direct shear tests.The direct shear test showed that the orientation of blocks would greatly reduce the shear strength of the phyllite bimrock slopes,resulting in a significant decrease in slope stability.(2)Through the self-developed large-scale permeability device,a series of researches were carried out on the flow behaviour of phyllite bimrocks with the orientation of blocks.Permeability characteristics of phyllite bimrocks considering the orientation angle and orientation ratio were obtained.Based on the orientation of blocks,the flow behaviour of phyllite bimrocks with different rock contents and block sizes is studied.The results show that the flow behaviour of phyllite bimrocks with the orientation of blocks does not obey the Darcy’s law.For the four experimental conditions in this paper(ie,different orientation angles,orientation ratios,block contents and block sizes),the flow behaviour of phyllite bimrocks can be described by the Forchheimer equation.Besides,the macroscopic permeability coefficient increases with the increase of orientation angle,orientation ratio,and block size.There is a minimum value for the macroscopic permeability coefficient when considering the block content(the block content is 25 %).Combined with experimental phenomena and data,it is believed that when studying the flow behaviour of phyllite bimrocks with the orientation of blocks,the non-Darcy coefficient β can be used as an indicator to judge the seepage failure of phyllite bimrock.The nonlinear least squares method is used to establish the seepage formula considering the block content and the block size,and the accuracy is verified by the NOF method.(3)Based on the flow behaviour of phyllite bimrocks with the orientation of blocks,the effects of slope angle,orientation angle,rainfall duration and rainfall intensity on the stability of phyllite bimrock slopes were studied.With the increase of slope angle or orientation angle,the stability of phyllite bimrock slopes decreases gradually.The longer the rainfall,the worse the slope stability is.In addition,when the slope angle exceeds a certain critical value,the risk of slope instability and failure increases sharply.(4)with regard to further studying the stability of phyllite bimrock slopes,a selfdeveloped physical model device of the two-way inclined was developed.The stability of the slope was studied by using the critical failure angle of the model slope which obtained by this device.According to the different orientations of blocks,model slopes with different orientation angles and orientation ratios were made,and the influence of the orientation of blocks on the stability of the bimrock slope was studied by measuring the critical failure angle of each model slope.The model test results show that the orientation of blocks has a significant effect on the bimrock slope stability,and the orientation of blocks could reduce the stability of the bimrock slope and change the failure characteristics of the bimrock slope.When the orientation angle is parallel to the surface of the bimrock slope,the stability of the bimrock slope is the worst.However,when it is perpendicular to the surface of the bimrock slope,stability is the best.In addition,with the increase of the orientation ratio,the stability of the bimrock slope gradually decreases.(5)Based on the physical model,the numerical simulation model of the phyllite bimrock slope was established.First,the parameter sensitivity in the numerical simulation test was analyzed,and it conclude that the model parameters do not affect the bimrock slope stability when the ratio of the cohesion or friction angle between the block and soil is greater than two.Second,through the numerical simulation test,it is also concluded that under the experimental conditions,the stability of the bimrock slope decreases with the increase of the block content.Third,the flat rate of blocks and the spatial distribution of large blocks also have a significant impact on the stability of the bimrock slope.