Study On The Strength Reduction Method For Slope Described By Linear And Non-Linear Failure Criteria

Slope stability analysis is the most basic and important stage in slope engineering, which is also the most difficult and emergent problem need to be solved. In the recent year, with the development of compute technology, the strength reduction method (SRM) is becoming a new trend in the area of slope stability analysis. When compared to the traditional limit equilibrium method (LEM), SRM not only meets the mechanical equilibrium condition, but also takes into consideration of the stress-strain relationship; During calculation, SRM does not have to make any assumption, it can search for the critical slip plane and find out the minimum safety factor for the slope in the automatic way, while the failure of slope and development of plastic zone can be reflected in the result of SRM, as well, which indicates that SRM has more advantages than LEM for it is based on the more precisely theories. Although many people have done much work on SRM, and get many achievements. The SRM is still far away from the perfect state. So the present paper has done the studies on some details for SRM which needs to be renovated. The main contents and conclusions are:(1) The different definitions of safety factor for strength reduction method are discussed, and the relationship of safe factor for SRM and LEM is obtained, which shows the consistency of definitions for SRM and LEM;(2) The impact of cohesion c and internal friction angleφto the stability of slope are analyzed, which studied the influential factors for stability of slope in the microscopic mechanism of shear strength; the equivalent influential angleθe is proposed; whenβ=θe, the c andφhave the same impact on the stability of slope; whenβ<θe,φhas the larger impact on the stability of slope than that of c; whenβ>θe, c has larger impact on the stability of slope than that ofφ; the equivalent influential angleθe is suitable in different homogenous slopes, whose value isθe=34.1°;(3) Three critical failure criterions, the connectivity of plastic zone criterion, the convergence of numerical computations criterion and the mutation of displacement criterion, are founded in the same homogenous slope; The elasto-plastic analysis is done to the stability of slope by the method of fast lagrangian analysis of continua three dimensions (FLAC3D). Safety factors obtained from the three criterions are compared with that from Janbu's procedure of limit equilibrium. The analysis results show that, the automatic characteristic for the connectivity of plastic zone criterion is not obvious, it need observation and judgment from people; The calculation theory of the numerical computations convergence criterion is easy to understand, and convenient to be programmed, with great precision for its result is in small differences with that from LEM; the physical meaning of displacement mutation criterion is clear;(4) In order to analyze the rationality of the chosen point's location and the displacement mode, numerical calculation models are founded for isotropic soil slope and joint rock slope, separately. Some monitoring points are located in slope while some in the internal of slope. The relationships between displacement and reduction factor are analyzed. According to the displacement mutation characteristic of displacement-reduction factor curves, hyperbola fitting equations are founded. The fitting results show that (a) fitting data are in good correlation with the monitoring data; (b) the points on and below the critical slip line can be chosen as the monitoring points of displacement mutation criterion; the top of slope is recommended to be chosen as the monitoring point in the universal slope with the consideration that the top of slope is sure to be below the critical slip line and its location is easy to obtained; If the location of slip plane is not certain, it is not proper to choose the toe of slope as the monitoring point; All the three displacement modes can lead to the same safety factor for the same monitoring point;(5) A numerical model was founded in plane strain mode by FLAC3D for homogeneous soil slope, whose parameters were reduced until slope reached the critical state. Then FISH program was used to get the location data of slip plane from displacement contour lines, which are used to determine the critical slip plane. Furthermore, method to determine multiple slip planes is also proposed by setting different height of elastic areas;(6) The influences of cohesion, internal friction angle, tensile strength, dilation angle and elastic modulus to the safety factor and slip plane of slope are obtained, which show that, cohesion and friction angle have the largest impact on the safety and slip plane; the parameter ofλcφ= ccr/(γhtanφcr) is defined, and the variation trends of slip plane with cohesion and internal friction angle are unified; The safety factor increases with the increase of tensile strength; The influences of dilation angle and elastic modulus to the safety factor are obtained for different practical situation, which prefect the past dealing method for influence of these parameters to the stability of slope; (7) The dual-spring element is adopted to simulate the characteristic of cable. A numerical calculation model for slope reinforced by cable is founded by FLAC3D. The excavation procedure of slope is simulated during the calculation. The procedure of SRM and parameters of cable to the stability of slope are discussed, the analysis results can give some guidance for both science research and engineering project;(8) In order to apply the SRM in Hoek-Brown criterion, and make the result the same with that from Mohr-Coulomb criterion, the relationship between parameters of Hoek-Brown criterion and Mohr-Coulomb criterion is analyzed; the equivalent cohesion and friction angle are adopted to calculated the safety factor of slope, which explores a new way to indirectly calculate the safety factor of slope based on the Hoek-Criterion; The new is convenient and practical to be processed;(9) The SRM is combined with the ubiquitous-joint criterion; the influential factors of the stratified rock slope are analyzed according to the numerical calculation, which expands the application of SRM;(10) The SRM in Hoek-Brown criterion is further expanded in the three dimensional stability analysis; Rock slope in one surface mine is chosen as the analysis object. Three dimensional numerical analysis model is founded by FLAC3D, the safety factor and failure mode of slope are calculated, the deformation responses of different areas in slope are recorded in the macroscopic way, which can give guidance for engineering practices;(11) The failure modes of stratified rock slope are simulated by FLAC3D, during simulation, the elastic-plastic elements are adopted to simulate the deformation of weakness plane; the results from the numerical simulation are in accordance with that from laboratory tests, which extends the application area of SRM with FLAC3D.