| The study on the stability of block rock slope under seismic loading is a hot and hard issue. It affects much on the construction of large-scale hydraulic and hydropower station, mountain high way, railway, nuclear power station. The deep researches into dynamic response rule, failure mechanism, stability evaluation and so on are of great significance. Not only can it push the development and creativity of slope stability theory under earthquake, but also support the planning, designing and constructing of geotechnical facility.Rock mass is composed of rock blocks that divided by the discontinuity network. Discontinuity is an important factor that determines the process and rule of the propagation of seismic wave. It is the foundation of research into rock slope dynamic response and dynamic stability to study the propagation characteristics of seismic wave in joint rock mass.Earthquake dynamic response of slope includes acceleration, velocity, displacement, dynamic stress, strain response, etc. In a broader sense, except for the standard parameters above, some abstract and human defined variables like dynamic factor of safety can also be viewed as the slope dynamic response to seismic. Of all the physical quantities and semaphores that can stand for the slope dynamic response, acceleration is the basic one. Because for one thing, velocity and displacement can be obtained by acceleration integral and double integral for time, respectively. For another, the relationship between dynamic stress and acceleration, dynamic stress and quality of any infinitesimals in the slope are in accordance to Newton’s second law. Dynamic stress and strain can be connected by modulus. In this sense, dynamic stress, dynamic strain and acceleration are also connected. Actually, the dynamic responses in slope are all induced by acceleration of seismic. Therefore, the study of dynamic response rules in time and space of acceleration is becoming the basic problem in this field, and it is of great value in evaluate the slope dynamic stability.In calculation of dynamic stability, permanent displacement analysis and factor analysis of dynamic stability are two main methods in evaluation of dynamic stability. Permanent displacement can reflect more precisely the dynamic response of slope to earthquake compared to factor of stability. Theoretically, the standard of permanent displacement is more reasonable than that of factor of stability. However, much insufficient exists in it. First of all, naturally, permanent displacement analysis is a relatively rough method. In calculation of slope permanent displacement caused by earthquake, slope material deformation, rock strength under dynamic situation and deformation character are not taken into consideration. Second, average displacement can be obtained through the permanent displacement analysis, which can not reflect the real condition. Third, stability factor of slope will not be received through the analysis and it will cause much inconvenience in stability evaluation. So, permanent displacement analysis and dynamic strength reduction method are combined here to get the factor of stability. Permanent displacement is taken as the index of dynamic failure, and strength reduction method is used to calculate the factor of stability. Currently, two criterions can be used to determine dynamic failure. One is the relation curve of permanent displacement and reduction coefficient. The other is whether the permanent curve is divergence at the key point after earthquake. Two criteria are not so perfect and a new calculation method will be put forward here.Commonly, slopes with rocks of integrity, inclining to the slope and less amount of discontinuities are relatively stable. But under the effect of strong earthquake, the rocks become less composed and more discontinuities appear. Shear strength reduced rapidly, so is the whole slope stability. Now, not so much work has been done on the aspect of earthquake caused dynamic response to that kind of rock slope and failure manners. As long as how strong the destructive can be, it appears to be urgent and important to take research into the study of the problem.Study in this paper mainly aims at block rock mass slope.3DEC simulation is used to take further study into the dynamic response rule, stability analysis and failure mode. It aims to reveal the earthquake affect on block rock slope and its mechanism. In this way, theoretical criterion can be provided to evaluate more precisely the dynamic stability of rock slope. Main contents of this paper are listed as follows:(1) Based on3DEC simulation model, the characters of propagation in single joint rock mass is analyzed. First, the numerical model of harmonic seismic wave propagating in homogeneous rock is built. Model and dynamic analysis parameters are chosen according to real parameters, which include boundary conditions, size of grid cells and time step of dynamic calculation and so on. Next, the propagation model of seismic in single joint rock is built and another factor is taken into consideration. The factor is the influence of discontinuity stiffness on propagation rules when seismic wave vertically oblique and incident on structure plans. The feasibility of simulation is taken into consideration and combined much work done by others. It is quite reliable to utilize3DEC simulation to analyze the dynamic problems of joint rock mass.(2) Through much numerical simulation calculation, the influence rules of discontinuities on rock mass slope dynamic response are studied. The analysis of earthquake dynamic response to rock mass slope is a rather complicated problem. Except for the complication of seismic loading, rock materials, physical and mechanical properties and the distribution of discontinuities, scale and other factors play a role in the response. It is not reasonable to take all the factors above into consideration. In this paper, the physical and mechanical properties and the distribution of discontinuities are taken into account. On the aspect of physical mechanism, structure stiffness is the main factor for its influence on transmission and reflection coefficient of seismic wave. In addition, on the aspect of discontinuity contribution, occurrence, starting position, and density of structures are the studying points. The results indicate:1) Change of structure occurrence will lead to the different directions of reflection and refraction of seismic wave. And then the distribution of field energy of seismic wave will be changed. It finally reflects on the spatial variation of slope amplification factor PFA contours. As for dip layered slope, when dip angle of structure is relatively small, structures have a reduction effect on seismic propagation. When the angle is relatively big, it has the opposite effect. The dynamic stability of slope decreases with the increase of dip angle. The existing of anti-dip layered slope will make the propagation of seismic in rock slope decay. As structure dip angle increases, the strength of discontinuity decreases. The dynamic stability of anti-dip layered structure is better than that of dip layered structure.2) The starting position of structure is a big factor on seismic dynamic response of rock slope. The higher the position is, the stronger the response is.3) With the increasing of structure stiffness, the strength of seismic dynamic response has the tendency increase. The effect of structure stiffness on reflected and transmitted waves is mainly on the relationship of energy distribution, but not impact can be seen at the propagation path of them in rock slope. When the incident wave is shear wave, the seismic dynamic response to structure shear stiffness is more sensitive than that to normal stiffness.4) More intense the discontinuities are, worse the integrity the rock slope is, and stronger the seismic dynamic response is, which means the poorer the dynamic stability of slope is.(3) Orthogonal experiment design method is applied, and evaluation on the sensibility of the maximum value of PGA amplification factor in rock mass slope through numerical calculation and statistics. Though the maximum value of PGA amplification factor can not reflect the whole condition of dynamic response to rock slope comprehensively, it can to some degree determine the strength of it. The changing rule of maximum PGA amplification factor is analyzed under structure effect. Based on this, taken into consideration the correlations and random linkage in structure factors, range analysis and variance analysis are used to put the sensibility of maximum PGA amplification factors in order. The sensibilities of dip angle starting position, shear stiffness, normal stiffness and distance of discontinuities are in descending orders. Therefore, when there is structure with big dip angle and develops shallowly, the dynamic response will be stronger. Under the effect of seismic loading, this type of dynamic stability problem in rock slope will be more prominent.(4) On the limitations of the first failure criterion in dynamic strength reduction method in practical, a calculation method of dynamic stability in rock mass slope based on permanent displacement is put forward. Under seismic loading, the permanent displacement of earthquake increase with strength reduction factor. When the factor is relative small, its increase amplitude will not be obvious and when it exceeds a certain value, the increase amplitude of earthquake permanent displacement will increase rapidly and show a change in the form of an exponential function. On how to set the dynamic stability factor on the relation curve of seismic permanent displacement and reduction factor, the current solution is take the corresponding reduction factor of turning point. But many subjective factors exist in determining the turning point, making the method more insufficient in utilizing. A new concept of permanent displacement is proposed. That is when the permanent displacement along rock slope caused by earthquake reaches a certain ratio of the length of slip surface, the slope will be in unstable state. Applied this approach to the study of block rock slope in Wenchuan earthquake area, the dynamic stability factor is1.04under the Wenchuan earthquake load. The result is in consistent with geological survey condition and verifies the practicability of this method.(5) On the limitations of the second failure criterion in dynamic reduction method, a calculation method of dynamic stability factor in rock slope considering structure degradation. When the second criterion is taken to analyze the dynamic stability in rock mass slope, the dynamic stability factor that obtained through static strength parameters is the same with the static stability factor, which means the effect of amplitude of seismic load on dynamic stability factor can not been seen here. Based on relative research achievement about the friction coefficient decays with the cumulative displacement and relative rates that done by others, a calculation method of dynamic stability factor in rock slope considering structure degradation is proposed and corresponding implementation process is given. Projects in Wenchuan earthquake area is taken for illustration. The results indicate it is feasible to put the method into calculation of dynamic stability in rock mass slope.(6) The Tiger Mouth rock mass slope is a typical slope that is controlled by dominant joint plan in Wenchuan earthquake area. The5.12Wenchuan earthquake makes the discontinuities in the slope connect with each other, which lead to the formation of wedge and it will slide towards the free face under the effect of seismic. A heavy landslide happened, making the Min River block and traffic close. Based on the block rock mass slope of Tiger Mouth, a numerical model is built in3DEC software. The evolution process of the slope getting deformed under Wenchuan earthquake has been restored, and the dynamic mechanism has been analyzed in detaisl. It brings reference to the study on the failure mode of the same type rock mass slope under heavy earthquake. |
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