Time-varying Stability Analysis Of Slope Based On Dynamic Deterioration Of Geomaterials

Slope stability analysis involves hydropower, transportation, mining, civil engineering and other infrastructure construction fields. It is one of the most important research subjects in Geotechnical Engineering and Engineering Geology, which has a long history of development. As a well-studied research topic it still remains great potential and is both pure and complicated at the same time.As the most common form of geotechnical structures in nature, slope has obvious evolution characteristics in the process of geological history. The course from formation and development to failure is a complicated dynamic mechanics process, which essentially determined by the contradictory relationship between stresses and strength of geomaterials of the slope. Externally the environmental forces (seismic force, seepage force, human activity, etc.) change the stress state inside the slope, and internally as a result of environmental factors geomaterial deteriorates gradually, thus exert a combined impact on the slope evolution process. The dialectical and unified contradictory relationship between strength and stress leads to slope instability whether to decrease strength or increase stress. The ratio of strength and stress determines the evolution condition of the slope, which is the original definition of factor of safety. Due to mutual influence of external dynamic environmental force and internal time-varying strength parameter, the factor of safety of slope is bound to present a distinct characteristic of dynamic change. Therefore, a time-varying dynamic viewpoint and a method based on the dialectical unity of internal and external cause are required to accurately study the dynamic state of slope stability in the research.This dissertation presents a time-varying stability analysis of slope based on dynamic deterioration of geomaterials, choosing seismic action and reservoir water fluctuation as the representative influencing factor of exogenetic force, and acquiring the mathematical expression of the two exogenetic force’s effect on the geomaterials mechanical parameters via experimental study, thus carry out the time-varying stability analysis of slope under the synergic effect of internal and external cause. The research of this dissertation expands the factor of safety to a time-related function, and promotes the slope stability evaluation from the traditional non time-varying system category to a time-varying one comprehensively, consequently enables the analysis to cover the life evolution cycle of the slope. The research output will provide theoretical guarantee to the successful construction and safe operation of foundation works and have important theoretical significance as well as engineering practical value.The time-varying stability analysis of slope based on dynamic deterioration of geomaterials is an expanding issue of slope project which involving the simulation environment experiment of geomaterial deterioration, the building of dynamic deterioration mathematical models, the algorithm of slope dynamic stability under seismic action and reservoir water fluctuation, the reliability of time-varying stability and many other content. Through field research, laboratory test, theoretical research and numerical analysis, this dissertation carries out a systematic research on the subject, and the main contents and findings are as follows:(1) According to different definitions of slope factor of safety, its calculation method can be concluded to three types, respectively based on traditional limit equilibrium, stress field of sliding surface and strength reduction method. And the research course of slope stability can be divided into three phases:engineering geology qualitative analysis, mechanical mechanism quantitative calculation and evolution mechanism dynamic study. On that basis, a theoretical proof of stability analysis of the time-variation of slope dynamic stability under the synergic effect of internal and external cause is made, based on the basic definition of time-varying system. The study shows that the evolution of the slope is a quite complex dynamic mechanical process, and under the action of external dynamic force, the factor of safety of slope is dynamically change over time, but this dynamic factor of safety still do not have the characteristics of time-variation. The slope system belongs to the category of the time-varying system only when the interaction of internal and external cause is uniformly taken into account. In this case, slope’s output dynamic factor of safety can be defined as the time-varying factor of safety.(2) With the study of dynamic failure mode and instability mechanism of rock slope in Wenchuan earthquake-affected region, the concept of joint’s vibration deterioration is put forward. On that basis the joint’s cyclic shear experiment is conducted, by which the quantitative study of the cyclic amplitude, the cyclic numbers and the relative velocity’s influence on the strength of joint are carried out. Based on which the mathematical model of vibration deterioration effect is established. With the study of bank slope’s stability and deformation evolution law under the action of dynamic seepage in the Three Gorges Reservoir Region, the concept of geomaterial’s water weakening is put forward. On that basis the geomaterial’s dry-wet circulation experiment and saturation experiment are conducted, by which the quantitative study of the dry-wet circulation numbers and the duration of the saturation’s influence on the strength of geomaterial are carried out. Based on which the mathematical model of water weakening effect is established.(3) The analysis method of dynamic stability of rock slope is studied.①A new analysis method of dynamic stability of rock slope is proposed based on permanent displacement ratio theory, for the purpose of improving the defects of existing methods, such as the permanent displacement method is not being able to provide a unified failure criteria and the dynamic stability factor method possessing no capability to reflect the dynamic deformation of slope under seismic loading. The proposed method obtains the corresponding seismic permanent displacement by continually adjusting the strength reduction factor. And slope failure is considered takes place when the seismic permanent displacement reaches a certain proportion of the slide plane length, and the corresponding strength reduction factor is defined as the dynamic stability factor. The method is fully combined the advantages of seismic permanent displacement method and dynamic stability factor method, and provides an index which has a clear physical meaning to evaluate the dynamic stability of rock slope based on dynamic strength reduction theory, therefore the method has a great application foreground.②Another new dynamic stability analysis method of rock slope is presented based on the Kinetic Vector Method (KVM), for the purpose of improving the defects of limit equilibrium method which cannot take the slope material’s dynamic features and seismic loading’s fluctuation characteristics into account. The dynamic analysis is carried out based on three dimensional distinct element code (3DEC), and the kinetic inertial force of the slope under seismic loading can be obtained via the calculation of the net vectorial nodal force of the finite difference grid. By that the new method not only inherits the traditional limit equilibrium analysis but also has fully nonlinear dynamic analysis capabilities. Then time series of the factor of safety of rock slope for whole earthquake process can be obtained by dynamically calculation based on limit equilibrium theory. Finally, for the evaluation of the entire dynamic stability of the slope, dynamic factor of safety (DFOS) is proposed and defined as a numerical value corresponding with a given rate of probability guarantee based on reliability theory.(4) A series of quantitative researches of rock slope’s dynamic stability are carried out by the proposed Kinetic Vector Method (KVM), which include the influence factors of the wave characteristics of seismic loading (such as frequency, amplitude, evolution rule and waveform), the incidence direction of horizontal seismic wave and the composite mode of spatial seismic wave.①The quantitative study on the wave characteristics of seismic loading’s influence on rock slope’s stability shows that the dynamic stability of rock slope increases with the increasing of the seismic loading’s frequency, and decreases with the increasing of seismic loading’s amplitude; the descending order of dynamic stability of rock slope under actions of different evolution modes of seismic loading are:stable equivalent wave> increasing-then-decreasing wave> ever-increasing wave=ever-decreasing wave; and the descending order of dynamic stability of rock slope under actions of different waveforms of seismic loading are as:ideal triangular wave> ideal simple harmonic wave> half-impulsive wave> full-impulsive wave.②The quantitative study on the incidence direction of horizontal seismic wave’s influence on rock slope’s stability shows that the condition where the incidence direction is uniform to the main sliding direction is most harmful to the dynamic stability of rock slope, however the condition where the incidence direction is orthogonal to the main sliding direction is most beneficial.③The quantitative study on the composite mode of spatial seismic wave’s influence on rock slope’s stability shows that the vertical seismic wave has significant effects on the dynamic stability of rock slope. Based on that, the phase differences and the frequency combination modes of spatial seismic wave’s influence on rock slope’s dynamic stability are well studied.(5) Based on the dividing stages of rock slope’s failure process under earthquake, the vibration liquefaction and vaporization effect is presented as an explanation to the mechanism of high speed earthquake landslides. The limit liquid pore pressure theory and the Fourier law of thermodynamics are adopted for the theoretical research of the vibration liquefaction and vaporization effect, while the Sunjiayuan landslide in Wenchuan earthquake-affected region as analysis examples. The research indicates that, because of the action of the vibration liquefaction and vaporization effect, the limit liquid pore pressure bears70%of the total normal stress in the sliding zone and the vaporization pressure bears another19.2%during the sliding stage, therefore the effective stress on the sliding zone of Sunjiayuan landslide is decreased to10.8%of its original effective stress. The decreasing of normal stress leads to the reduction of frictional resistance of sliding zone, and by which the sliding block obtains a high sliding speed at the early stage of slope’s failure process. Consequently, the successful case study shows that the vibration liquefaction and vaporization effect is a reasonable explanation of high speed earthquake landslides.(6) The time-varying stability analysis method of bank slope is studied.①A time-varying stability analytical algorithm based on reliability theory is proposed, in which the randomness of geomaterial’s degradation is fully taken into account, and a dual index system is established based on reliability theory to evaluate the time-varying stability of bank slope, The evaluation index:Reliability factor of safety (RFOS) is defined as the product of the mean factor of safety and the corresponding time-varying reliability.②A time-varying stability numerical algorithm based on dynamic seepage field is also proposed, in which the dynamic seepage field of bank slope during the reservoir operation is obtained by finite element numerical algorithm, and the dynamic seepage is regarded as the driving factor of bank slope’s evolution. Therefore, by the dynamic numerical algorithm, the time-varying stability and deformation evolution law of bank slope under the combined action of hydraulics effect and water weakening effect are well studied.(7) A series of quantitative researches of bank slope’s time-varying stability are carried out with the proposed numerical algorithm.①The amplitude of reservoir fluctuation is selected as the external influence factor to carry out the quantitative study on its influence. The results shows that the fluctuation of slope’s stability is mainly caused by change of reservoir water level, and a positive correlation can be used to describe the relationship between the amplitude of stability’s fluctuation and the amplitude of water level’s variation, meanwhile the influence of water weakening effect on slope’s stability increases with increasing of amplitude of water level’s variation. Hence the fluctuation of water level plays a negative effect on the stability of bank slope.②The slope Angle is selected as the geometric influence factor to carry out the quantitative study on its influence. The results shows that the overall stability of bank slope decreases with increasing of the slope angle, the influence of water weakening effect on slope’s stability also decreases with increasing of the slope angle③The salvage value of degradation coefficient is selected as the mechanical influence factor to carry out the quantitative study on its influence. The results shows that the salvage value of saturation deterioration coefficient has a greater influence on bank slope time-varying stability while the salvage value of dry-wet cycle degradation coefficient has a weaker one. And there is a positive correlation between the salvage value of saturation deterioration coefficient and dry-wet cycle degradation coefficient, one’s influence on slope’s stability increases with increasing of the other one’s value.(8) The engineering practicability of this dissertation is testified by two case studies. Xingwenping rock slope in Wenchuan earthquake-affected region is presented to demonstrate the feasibility and correctness of the KVM which takes the vibration deterioration of joint into account, and a three-dimensional model is established according to the actual terrain by3DEC, then the waveform data of the5-12Wenchuan earthquake are selected as the seismic loading in this case study, which is recorded on the BHN channel, the BHE channels and the BHZ channel of CD2station in Chengdu by China Earthquake Administration, and a result of FD=1.04is obtained and matchs the actual situation well. On the other hand, the Huangtupo riverside slump-mass No.I is presented to verify the practicability of the bank slope’s time-varying stability numerical algorithm. A double sliding zone geological model is established based on the information of geological structure and spatial form by exploration tunnel group, then the dynamic seepage field is obtained by saturated-unsaturated seepage finite element method. The calculation shows that:during the reservoir operation process, the change of seepage line lags behind reservoir water level. The seepage line presents a concave shape during the increasing of reservoir water level, and a convex shape during the declining. The calculation of time-varying stability shows that during the reservoir operation process both the stabilities of shallow and deep landslides are influenced closely by the water level’s fluctuation; however, the stability of the shallow one is fluctuated wildly and influenced easily. With the increase of time, the mean and amplitude of fluctuation of both landslide’s stabilities gradually decreases and tends to be stable. The quantitative study on water level drop velocity’s influence on bank slope’s stability shows that, the stability of shallow landslide is more sensitive to the water level drop velocity, and its critical instability velocity of water level drop is about2.5m/d. The calculation of deformation shows that the deformation evolution process of slumping-mass is fairly regular, and can be summarized as follows:fast in the beginning and then slow down gradually, from the outside to the inside and from leading edge to trailing edge. The deformation calculation results are verified by the measured data of3monitoring points of GPS. The main innovations of this dissertation are as follows:(1) The mathematical model of joint’s vibration deterioration effect and geomaterial’s water weakening effect are established via experiments. The joint’s vibration deterioration mathematical model can be dynamically determined by three dynamic response values:the relative speed between rocks V(t), the numbers of cyclic shear K(t) and the amplitude of cyclic shear J(t). Meanwhile, the geomaterial’s water weakening mathematical model can be dynamically described by three dynamic deterioration coefficient:cohesion coefficient of Wet-dry cycling u(n), friction angle coefficient of Wet-dry cycling co(n) and saturation deterioration coefficient ω(t).(2) A new method for analyzing dynamic stability of rock slope is presented, which taking vibration degradation of joints into account. In this method, the mathematical model of vibration degradation effect is applied to rock slope’s dynamic stability analysis. And at each dynamic calculation step, mechanical parameters of joints are refreshed according to the real-time values of dynamic response. The earthquake inertial force of any moment can be obtained via net vectorial nodal force’s vector calculating, therefore the time series of factor of safety can be got for whole earthquake process by dynamic calculating. For the purpose of evaluating the entire dynamic stability of slope, dynamic factor of safety (DFOS) is proposed and defined as a numerical value corresponding with a given rate of probability guarantee based on reliability theory.(3) Two time-varying stability analysis methods for bank slope are presented:time-varying analytical algorithm based on reliability theory and time-varying numerical algorithm based on dynamic seepage field. The analytical algorithm takes randomness of geomaterial’s degradation into account, and the reliability factor of safety (RFOS) based on reliability theory is proposed as a time-varying stability evaluation index. The numerical algorithm is based on the dynamic seepage field of bank slope during reservoir operation, and the mechanical parameters of geomaterial are dynamically refreshed according to the water weakening mathematical model. Consequently the time-varying stability and deformation evolution law of bank slope can be analyzed by dynamic calculation.