Study On The Stability And Its Influence Factors For Highway Bedding Rock Slopes

Based on generalized model of highway bedding rock slope and engineering examples, the slope stability, factors sensitivity and reliability and so on were studied by using strength reduction of FEM, limit equilibrium method, single factor analysis and orthogonal design method of factors sensitivity, first order-two moment method and response surface method of reliability analysis. In the slope stability calculation, based upon the difference of safety coefficients obtained between general FEM program and traditional limit equilibrium method, and the influences of parameters selection on the results of FEM were quantitatively analyzed, a new transformation method of safety coefficients by different Drucker-Prager yield criterions, and matching method between Drucker-Prager yield criterions and Mohr Coulomb criteria based on value of the internal frictional angle were proposed. In the factors sensitivity analysis of layered slope, the idea of using parameter increment was put forward when considering the factors level, and the applicability of variance analysis method in slope sensitivity was verified. In the analysis of slope reliability, the idea of combining factors sensitivity was put forward, the application of factors sensitivity in slope reliability was studied, and reference for engineering design was proposed. According to the model calculation and analysis of engineering examples, the design principles of slope protection was suggested. The main contents and research results as follow:Using nonlinear FEM strength reduction method, stability analysis was made on the model of bedding rock slope with a group of runned-through plane of weakness. Results show it is disadvantage for the stability when the slope steeper than the dip angle of rock stratum and the presence of free surface, and the safety coefficient decreases with the increase of slope height. The influence of load on the top of slope to safety coefficient depends on the magnitude of load and relative position between load and the runned-through plastic zone. Retaining wall or anchor rods can increase the safety coefficient of slope by block and intercept the runned-through plastic zone, then the plastic zone moves to other more weak structure plane, only when the plastic zone is runned through can anchor rod play a role.The calculation results of highway bedding rock slope safety coefficient obtained by general FEM program are unsafe, for the results of FEM based on Drucker-Prager yield criterion are about22%higher than the results of traditional limit equilibrium method based on Mohr Coulomb criteria. In the finite element analysis, the calculation results are more conservative when the dilatancy angle is ignored. When using the associated flow rule, the distribution range of plastic zone reduced obviously than that of the non associated flow rule. The value of Young’s modulus and Poisson’s ratio almost have no effect on slope stability, but the cut of Young’s modulus will make the displacement of slope increase obviously, the scope of plastic zone will increase with the reduce of Poisson’s ratio.By study on Drucker-Prager yield criterion and traditional Mohr Coulomb criteria shows, in slope stability by FEM based on different Drucker-Prager yield criterion, ratios of D-P radii can be used as conversion coefficient of safety factors under different Drucker-Prager yield criterion. When D-P yield criterion are to be matched to Mohr Coulomb criteria, traditional method is always from the perspective of "average", the matching conclusion from "average" may have a larger deviation for a specific slope. A new method is proposed in this paper:the ratios of safety factors under Mohr Coulomb hexagon circumcircle Drucker-Prager yield criterion(DP1) and M-C criteria vary with the value of the internal frictional angle, putting the numerical value of internal frictional angle and the ratios on two-dimensional space of internal frictional angle～ratios of D-P radii, it can get the best matching yield criterion by comparing the y-intercept under corresponding value of internal frictional angle among these curves, this method is more specific and the results will be more accurate.For the sensitivity analysis of homogeneous slope, it may use the different level of parameters directly; but in the case of multi-layered rock mass, it is difficult to use the method mentioned above, instead a method of the available parameter increment is suggested. The comparison of single factor analysis, range analysis and variance analysis shows, the conclusion obtained from these methods are basically the same, but the effect of variance analysis is best.Study of slope reliability analysis based on factors sensitivity shows, whether the strength parameters or geometric parameters, if there sensitivity are larger and there exists some variability, its influence to slope stability analysis of reliability cannot be ignored, or there will have a significant impact on the analysis results. The higher sensitivity level of factors selected as random variable, the larger failure probabilities, firstly determining the sorting of parameters sensitivity, and choosing random variable according to the variation coefficient of parameters, the results of slope reliability analysis maybe more reasonable. The slope reliability analysis combined with factors sensitivity can not only influence the final results greatly; to some extent, it is also related to the computational workload and complexity. For reliability analysis of bedding rock slope and other complex slope, reasonable application of the sensitivity analysis results can eliminates the un-sensitive factors and simplify the original complex analysis of slope reliability.Analysis of typical bedding landslide in Jiande section of Hang Qian Expressway shows, the formation elements of highway bedding rock slope can be attributed to the layered rock, adverse trend and occurrence of rock, plane of weakness and free surface. The general design principle for bedding rock slope protection is "retaining at the bottom and slow slope above" that is strengthen the retaining of lower part of the slope, choosing reasonable slope ratio according to the obliquity of rock, when conditions permit, slowdown and reduce the height of slope as far as possible, the surface of slope should be timely protected, when there is load on the top of slope, it is necessary to identify the relative position between load and potential sliding surface, when the slowing of slope is restricted, engineering measures should be used to enhance the protection according to the actual situation.