Stability Analysis Of Soil Slopes Under Actual Stress State

Two limitations are associated with the methods generally used in stability analysis of slopes in practice. Firstly, too much focus is put on calculating the factor of safety (Fs), while the mechanism of slope failure is always ignored. Secondly, slope stability analysis in the design stage is always divorced from slope stability control in the construction stage. In this dissertation, slope stability analysis method under actual force condition is investigated systematically.The distribution of stress and strain (displacement) components in slopes, which are constructed by filling and cutting respectively, from finite element analysis shows that slope stress history has considerable influence on its stability. Limit equilibrium (slice methods) methods (LEM), however, cannot take this influence into account. Though the finite element shear strength reduction technique (FE-SSRT) overcomes some shortcomings associated with LEM (such as force/moment balance only at slices is considered and the stress analysis is lack in precision), it can only obtain Fs due to that the stress analysis is just corresponding to the slopes under limit equilibrium yet.On account of the shortcomings associated with the methods generally used in stability analysis of slopes mentioned above, the finite element stability analysis method for slope's actual stress state, namely FEM-ASS, is extended into effective stress analysis. Consequently, it can take the influence of the stress and porous stress changes due to the external force (including filling and cutting) on slope stability into consideration. A soil parameter analysis shows that the stress distribution is independent on Young's modulus for the slopes under actual force condition, and that Poisson's ratio has slight effects on the results of FEM-ASS. Furthermore, some modifications are made for FEM-ASS to improve the algorithm efficiency.Some search directions with low effects are eliminated for the Monte Carlo approach, which can be used to search for the critical slip surface, to improve the efficiency. Furthermore, a new optimization approach, named Particle Swarm Optimization (PSO), is implied into slope stability analysis, and a complete algorithm is purposed to search for the critical slip surface. Numerical studies and an application to LaiLongShan landslide show that PSO has the ability to search for the critical slip surface with considerable effectiveness and efficiency, and can be easily implemented and incorporated with LEM as well as FEM-ASS.According to the discussion on the definitions of Fs, a new definition, based on "internal virtual work" corresponding to the movement trend of slicing mass, is purposed. It can avoid theambiguousness of the physical meaning of FEM-ASS's Fs when the slip surface is neither linear nor circular. With the "internal virtual work" Fs definition, finite element kinematic method (FEKM) is purposed for slope stability analysis. It can analyze slope stability with the movement trend of slicing mass, and can overcome the defect that stability analysis can only be considered at limit equilibrium for LEM and FE-SSRT. The stability analysis of strength-strain relatively soil slopes can be performed by FEKM.Combined with a strain-softening model, the stability of strain-softening soil slopes is analyzed by FEKM. A curve of Fs as to the movement trend of slicing mass is obtained, which overcomes the shortcoming that only the factor of safety associated with peak or residual strength parameters can obtained for LEM and FE-SSRT. The Fs curve has a peak, whose value is less than that obtained if peak strength parameters used. At that state, not all the points along the slip surface are at the peat strength. It is recommended that the stability of strain-softening soil slopes should be evaluated by the peak value of the Fs curve. The process of stress state at the points along the slip surface as to the movement trend of slicing mass is simulated by FEKM, and the mechanism of progressive failure of strain-softening soil slopes is interpreted to some extend.The stability of Duhu embankment under actual force condition during its construction process (filling and consolidation) is analyzed by FEM-ASS with PSO, and the influence of stress history on slope stability is investigated. Some variables, such as displacement, under actual stress state can be predicted by FEM-ASS, which helps the engineers to control slope stability in practice. The mechanism of soil slope failure is interpreted from a microcosmic view, such as the stress path and the inclination of the primary stress at some key points on the slope.