Study On Engineering Geological Characteristics And Pregnant Sliding Mechanism Of Badong Formation Slope With Mudstone Interlayer

Western Hubei-eastern Chongqing area is a region with high incidence of landslide disasters.The widely exposed Badong Formation strata in the area have an obvious control effect on the stability of regional slope and its disaster pregnant evolution.Therefore,the pregnant sliding instability of Badong Formation slope has a significant impact on the engineering construction and personal safety in the area.However,engineering practice and application research show that even if the regional environment is generally prone to disasters(exposure of slippery strata,development of slippery landform and distribution of slippery structures,etc.),in fact,there are still differences in whether the slope of Badong Formation is pregnant and slippery to form landslide,its disaster model and disaster scale.The above differences are closely related to the type of slope structure and the properties of rock and soil mass.Therefore,in order to improve and deepen the understanding of the slope characteristics of Badong Formation with mudstone interlayer,and solve the practical problem of whether this kind of slope can form landslide or not,which is closely concerned in theory and production.In this thesis,the engineering geological characteristics and sliding mechanism of mudstone-bearing interlayer slope in Badong Formation are studied.Based on the landslide information of 107 cases of Badong Formation in Western Hubei-eastern Chongqing area,the basic characteristics of landslide and slope structure characteristics are illustrated,and the engineering geological model of Badong Formation slope is generalized.Then,mechanical tests and numerical tests are carried out on the purple red mudstone and mudstone intercalation in section T2b2,to reveal the variation law of engineering geological characteristics of rock and soil mass,and explore the micro mechanism of the change of macro characteristics of them.Then,on the basis of clarifying the sensitivity of influencing factors and the response law of pregnant sliding deformation under the action of geometric characteristic factors of the slope of Badong Formation with interlayer,the geometric model of easy pregnant sliding is established.Finally,based on the above results,the pregnant sliding model test and numerical simulation of Badong Formation slope with mudstone interlayer are carried out.By analyzing the internal multi-field information response and external macro deformation signs,the slope pregnancy and sliding stages are divided and the evolution mechanism was elucidated.The main research results obtained in this thesis are listed as follows:(1)On the basis of revealing the basic characteristics of Badong Formation landslide and slope structure characteristics,the forward/reverse slope,slope,slope height,geological strata,lithologic structure and rock stratum inclination are selected as the model construction factors,and the factor composition system for the classification of slope geological model is established,and 11 types of Badong Formation slope engineering geological models are generalized.Based on the comparison of the development characteristics of the landslide induced by the slopes belonging to different model categories,the forward slope model with mudstone interlayer composed of T2b2 stratum is selected as the research basis of the slope forming down-shear→disaster pregnant evolution→disaster-induced instability.(2)Based on the results of mechanical and numerical tests of Brazilian splitting and compression failure of purplish red mudstone in T2b2 section,the strain curve method and the improved axial strain difference response method were used to calculate the characteristic strength of mudstone.After the compression deformation stage is divided accordingly,the characteristic strength change and compression failure stage characteristics of mudstone under the action of water were analyzed.By comparing the energy value at the characteristic strength point,energy distribution in each stage and energy evolution in the whole process of load failure of natural and saturated mudstone,the influence law and degree of saturated water on the energy evolution of mudstone are revealed,and the compression damage mechanism of mudstone is clarified from the perspective of energy.The types of water rock interaction during water saturation are analyzed,the characteristics of its energy state are discussed,and the energy interpretation of the water saturation softening mechanism of mudstones in Badong Formation is given.From the perspective of microcrack development and energy analysis of rock sample model,the micro mechanism of energy evolution in the compression failure process of mudstone in Badong Formation is revealed.(3)The effects of confining pressure on σci,σcd,σf were positively correlated,while the effects on σcc were small.The dimensionless parameters of σcc/σf,σci/σf andσcd/σf had no obvious fluctuation with the increase of confining pressure.Water saturation softening and confining pressure effect have significant effects on the energy absorption,energy release and energy dissipation properties of mudstone.The energy evolution mechanism of Ue storage and Ud dissipation coexists in the whole process of rock sample failure,and the strain hardening and strain softening mechanism produced by both of them jointly drove the deformation and failure of mudstone.(4)According to the CU shear test results of mudstone interlayer with different particle gradation and different water content,the influence laws of gradation characteristics and water content on the stress-strain relationship,pore water pressure,shear strength and failure mode are expounded.Based on the PFC2D numerical test results,the meso reasons for the differences in bearing capacity and deformation characteristics of interlayer models with different gradation and moisture content are revealed,which are mainly related to the different model fabric.(5)The numerical test results show that the content of fine particles determines the number and strength of contact force chains,the proportion of tensile/compressive stress chains and the bearing capacity chain structure of the model,and affects the chain structure,distribution and the proportion of strong and weak force chains,so as to make the transmission,distribution and distribution of contact force between particles different,resulting in the difference of shear failure characteristics of the model.The gradation difference caused by the change of particle size of dominant particle group affects the bearing skeleton and force chain structure of the model and its transmission and distribution efficiency,and then determines its bearing characteristics.The influence of moisture content on meso parameters is reflected in the differences in the number,thickness,type proportion and strength distribution of contact force chains in the model,and the mechanical response of the model is different in the macro.(6)The evaluation system of factors affecting pregnancy and sliding of slopes with interlayer is established.After comparing the difference of factor sensitivity results under four kinds of evaluation indexes,it is determined that the factor sensitivity is as follows.Among the first-order factors,the geometric characteristic parameters of slopes have the most significant influence on pregnancy and sliding deformation.Among the second-order factors,slope and interlayer inclination are highly sensitive factors,interlayer thickness and rock mass Poisson’s ratio are relatively sensitive factors,interlayer buried depth,rock mass density and elastic modulus,rock mass cohesion and internal friction angle,interlayer cohesion and internal friction angle are low sensitive factors.According to the response law of slope pregnancy and sliding deformation under the action of geometric characteristic parameters,the easy pregnancy and sliding geometric model of Badong Formation slope with mudstone interlayer established by support vector machine.Which is a quarter elliptical cylinder with a slope of 85°,an interlayer thickness of 0.20m and an interlayer inclination of 40 ° as the center of the elliptical sphere on the bottom surface,with a short axis of 9.8 °(gradient axis),a height of 0.8m(interlayer thickness axis)and a long axis of 12.6 °(interlayer inclination axis).(7)Select appropriate raw materials,influencing factors and their level numbers.Based on this,carry out the proportion test according to the orthogonal design theory.Based on the analysis of sensitivity,significance and quantitative relationship between factors and similar material parameters by range,variance and multiple linear regression,the similar material of purple red mudstone and mudstone interlayer in T2b2 section is developed.The model test reproduces the whole process of slope pregnancy and sliding under the action of Valley undercutting,excavation unloading and interlayer argillization.The analysis results of macroscopic deformation signs and multi field information evolution show that,when the slope is cutting down,the stress adjustment caused by the excavation and unloading of the slope body is the main part in the slope,and there is no obvious deformation.Due to the adverse effects of excavation unloading and interlayer argillization,the pregnant sliding deformation and failure of the slope are severe.The failure types include pull-out groove and wide crack at the rear edge,middle crack network and shear collapse at the front edge.The magnitude and curve shape of displacement,earth pressure and pore water pressure inside the slope vary with the degree of pregnant sliding deformation and failure at the location.(8)The PFC2D software is used to simulate the geological process of slope formation→disaster evolution→disaster instability of Badong Formation slope with single/multi interlayer.By analyzing the evolution characteristics of stress field,displacement field,energy field and microcrack in the whole process of slope pregnancy and sliding,it can be seen that the pregnancy and sliding process of slope with single interlayer has certain stages,but the stage characteristics of each field information are different,and there are differences in the evolution law of multi field information with depth in different pregnancy and sliding deformation periods.The pregnant sliding deformation process of multi interlayer slope shows obvious characteristics of gradual failure from shallow to deep.The evolution law of multi field information from front edge to rear edge is significantly affected by depth,and shows a phased evolution trend.(9)The energy field can fully characterize the change law of the overall state information of the system in the process of slope pregnancy and sliding,and the analysis of the evolution characteristics of multi field information shows that the phased change trend of the energy field is the most obvious.Therefore,based on the energy field evolution curve and combined with the development and distribution of cracks in the slope,the pregnancy and sliding process of the slope with single interlayer is divided into four stages:creep deformation,overall sliding,tensile crack disintegration and accumulation stop,and the pregnancy and sliding process of the slope with multiple interlayer is divided into five stages:creep deformation,shallow sliding,overall sliding,tensile crack disintegration and accumulation stop.The displacement nephogram and model failure characteristics of each stage are analyzed,and the failure phenomenon of Badong Formation landslide of contract type are concluded.Based on this,the pregnant sliding evolution mechanism of Badong Formation slope with interlayer of disaster evolution→disaster instability is clarified.The research results of this thesis identify the types of engineering geological models of Badong Formation slope,deepen the understanding of the characteristics of purplish red mudstone and mudstone interlayer in T2b2 section,solve the problems of whether the slope of Badong Formation with mudstone interlayer is pregnant or not and the difference of disaster mode to a certain extent,and provide guidance for the deformation prediction and prevention design of Badong Formation landslide.