Back Analysis Of High Loess Slope Shear Strength Parameters

Currently, when using the limit equilibrium method or numerical analysis method toevaluate the stability of high loess slope, the core issue is the value of shear strengthparameters. However due to the inner properties of loess and disturbance during sampling,transporting and texting, the shear strength parameters obtained from laboratory test on loesssample changes greatly, resulting in great uncertainty of slope stability analysis. Therefore themethod of acquiring parameters through back analysis is drawing increasing attention, andalso plays a role in some cases. This paper employed back analysis to systematically study theshear strength parameters of high loess slope.Engineering geological conditions of Loess Plateau, like topography, lithology,geological structure and groundwater, distribute regionally. Therefore the Loess Plateau wasdivided into eitht regions on the basis of its engineer geological conditions. The limit stateslope is the one that has distinct deformation, of which the relationship between the heightand the width is statistically studied. At last the relationship is displayed by an exponentialequation.Take the safety factor of limit state slope as1. Using the Morgenstern-Price methodology,multi-stage inversion with each10m high bring comprehensive shear strength parameters ofloess slope with the height between20m and120m. analysis of these parameters show thatwith the increase of slope height, the frictional angle decreases nonlinearly greatly while thecohesion increases linearly greatly.Direct shear tests with multi-stage vertical stress were conducted to study the shearstrength properties of loess slope with different height. Results show that shear strength canbe calculated by the linear Mohr-Coulomb strength criterion when the value of vertical stressis below400kPa. While when the value is above600kPa, shear strength increases nonlinearly,meaning it cannot be calculated by Mohr-Coulomb strength criterion. Besides under highstress condition, friction angle increases but cohesion increases. Particle analysis tests werealso conducted, revealing that during consolidation with high stress and shearing, there wasarticle breakage which is more serious during consolidation. Besides analysis of the particleeffect on shear strength shows that the higher the clay particle content is, the more obvious the trend of friction angle decrease and that of cohesion increase are. Test results prove thatchange of inversion strength parameters in every region with slope height is correct. What’smore, both test results and back analysis demonstrate that it is with great caution to use linearMohr-Coulomb strength criterion and it cannot get reliable parameters until the test tresscorresponds to actual stress.Based on the inversion strength parameters, it again utilized the Morgenstern-Pricemethodology to calculate safety factors of slopes with different aspect ratios in the eightregions and also utilized Monte-Carlo methodology to figure up failure probability of theseslopes under6groups of variation coefficient. On the basis of the results, two figures wereavailable. One displays the relationship between safety factor and slope height, the otherdisplays the connection of failure probability and slope height. The two figures can help tomake preliminary judgment on slope stability and also help to determine height and width ofslopes to be designed and its reliability.