Research On The Stability Of Complex Slopes Based On The Upper Bound Solution

Slope stability is one of the classic problems in hydraulic and geotechnical engineering,and it has always been a hot topic for engineers to analyse it using a simple,fast and applicable theoretical calculation method.However,some of the complex slope stability assessment problems encountered in dam filling and hill reclamation projects lack an appropriate theoretical analysis method.Field research has revealed that the filling earth from upstream deposited silt technique is a common method of reservoir de-risking and strengthening in the loess hills of southern Ningxia,and the resulting composite earth dam contains several weak joints and is susceptible to slope instability due to sudden drops in reservoir water.What is not consistent with this experience,which has been accumulated in practical engineering,is the relative lag in theoretical research.In the national key project for the west-east transmission of electricity across the mountainous region of southern Ningxia,a large number of high non-homogeneous loess slopes are formed in the site of the transmission and transformation station.In order to improve slope stability and ensure project safety,graded slope excavation is usually used in design and construction;the stability analysis of high loess slopes with complex geometric shapes composed of non-homogeneous strata is also a practical engineering problem that needs to be solved.The stability analysis of slopes with cut-fill interface is a problem that needs to be solved in both dam filling and hill formation projects.The cut-fill interface of such slopes is exposed on the slope surface,which is difficult to discuss in a plane stress scenario and can only be analysed by establishing a complex three-dimensional calculation model.Therefore,in this thesis,a series of studies are carried out for the above slope stability problems based on the upper limit method of plastic mechanics limit analysis.Considering the geometrical characteristics of the slope and the characteristics of the internal structure,a damage mechanism satisfying the motion permit is constructed;the internal energy dissipation rate and external power are calculated according to the geometrical relations,and the strength reduction technique is used to reduce the soil shear strength parameters and bring them into the functional equilibrium equation;finally,the upper limit solution of the slope safety coefficient is derived and the sequential quadratic programming method is carried out to find the optimum.On this basis,the actual project in the research project is selected as an arithmetic example,and the accuracy and validity of the theoretical method and numerical simulation method in the paper are verified by comparison.The main research contents and conclusions are as follows:(1)For the structural type of composite earth dam containing multiple weak joints and the operational characteristics of the reservoir,based on the upper limit principle of plastic mechanics limit analysis,two types of motion-permitting damage mechanisms were constructed,and the dynamic changes in reservoir water level and pore water pressure in the slope were involved in the functional equilibrium calculation of the upper limit solution as external loads;then the upper bound solution obtained from the filling earth from upstream deposited silt case was then compared with the numerical simulation results of the finite element limit analysis software OptumG2 and the calculated values of the limit equilibrium M-P method.The results show that the upper bound solution of the slope safety coefficient and the damage mode calculated by the method of this paper are in good agreement with the numerical simulation results of OptumG2 software and M-P method,and the maximum relative error is small,and the sudden drop of the reservoir water level leads to a sharp reduction of the slope safety coefficient,with a maximum reduction of 39%,and the hydraulic effect is significant.(2)Taking the high slope project of Miaoling 750kV substation in the loess hilly region of Ningxia as a case study,two damage mechanisms were constructed based on the upper limit principle of plastic mechanics limit analysis for the complexity of horizontal stratification and multi-stage slope surface of high slope,and the upper limit equations of slope safety coefficients under different damage modes were derived by combining the strength reduction techniques,and finally the optimized solutions for the upper limit solutions were given.After comparing with the numerical solutions of the finite element limit analysis software OptumG2,the results show that the theoretical calculated values of the upper bound solutions of the two damage mechanisms are between the upper and lower limits of the numerical simulation,and the most dangerous potential slip surface drawn after parameter optimisation is quite close to the simulation results.The analysis of the influencing factors shows that the slope safety factor increases approximately linearly with the increase of the internal friction angle and increases significantly;it increases approximately linearly with the increase of the cohesion ratio and then tends to level off;it increases approximately linearly with the increase of the slope step width,but the increase is smaller.(3)The extended horn-shaped logarithmic spiral cone failure mechanism is applied to the three-dimensional stability analysis of slopes in the case of high slope exposure in which the direction of the cut-fill contact zone intersects the slope surface obliquely in the projects of hill formation and embankment filling.The upper limit expression of the slope safety coefficient of the slope containing the cut-fill interface is derived according to the principle of virtual work,and the corresponding calculation procedure is solved by Matlab software.On the basis of this,a simplified calculation method for the three-dimensional stability of high slopes with cut-fill interfaces on the shoulder of dams is proposed.The method of this paper and the numerical simulation method of OptumG3,a finite element limit analysis software,were used to calculate the high slope of the Miaoling 750kV substation site in the loess hilly region of Ningxia,and the existing studies in the literature were generalized into calculation examples for comparison and analysis.The results show that the upper limit analysis theoretical method proposed in this paper and the numerical simulation results of OptumG3 agree well with each other,both in terms of the upper limit solution of the slope safety coefficient and the slope instability slip mode.With the increase of the dip angle a of the cut-fill interface,the safety factor Fs of the slope at the cut-fill interface decreases.In the analysis of the case considering the hydraulic effect,due to the different simplification methods,the calculation results obtained by using the upper limit solution method in this paper are larger than the results of the existing studies,but the two agree to a higher extent in the smaller ratio of width to height B/H.The research results suggest that the cut-fill interface is a potentially weak surface,we should pay attention to the slip-resistance treatment of steeply inclined cut-fill contact surfaces in engineering practice.