| Slice methods are widely used in landslide stability analysis and control engineering design field, which solving stability coefficient based on static equilibrium conditions and assumptions of interslice force directon. With regard to the influence of assumption on stability coefficient, there are little differences among slice methods as long as the two limited conditions are satisfied first proposed by Morgenstern and Price. However, assumption of interslice force directon is a key factor that affects the accuracy of stability coefficient because most semi-precision slice methods can not meet the two limited conditions. The error of residual sliding force is large when calculated by inappropriate method and safety factor which the same of semi-precision slice methods causing huge waste or hidden danger, even accidents in landslide control engineering design. Therefore, it is necessary to conduct research on optimization of slice methods based on comparative analysis and choosing value standards of safety factor according to different semi-precision slice methods.According to formulas of semi-precision slice methods and precision slice methods, data statistical analysis is used to research the relationships between stability coefficients and recommend a more precise and reasonable slice method. Then, the values of safety factors for different semi-precision slice methods are determined under the same conditions of horizontal residual sliding forces of last silce. The following research contents can be acquired for the study:(1)General recurrence formulas of semi-precision slice methods and precision slice methods were deduced on the basis of setting interslice tensile stress to be zero. And improved particular solutions were obtained by assuming interslice force directons. Besides those, the principle and characteristics of practical recurrence method and precision recurrence method were researched.(2) Stability coefficients of fifty gathered landslides under typical working conditions were calculated by semi-precision slice methods and precision slice methods utilizing LSTT software. Semi-precision slice methods contain transfer coefficient method, improved transfer coefficient method, US army corps of engineer’s method, Lowe method, simplified Janbu method, practical recurrence method and simplified Bishop method. Precision slice methods include Spencer method, Morgenstern-Price method, Morgenstern-Chen method and precision recurrence method.(3) The relationships between stability coefficients calculated by semi-precision slice methods, precision slice methods and Morgenstern-Chen method were concluded by correlation analysis and regression analysis using SPSS software, meanwhile, the deviation values of them were explored by frequency analysis to recommend a more precise and reasonable slice method.(4) Residual sliding forces of aforesaid fifty landslides were calculated by improved transfer coefficient method when safety factors were given from 1.05 to 1.50 with every increased 0.05. In addition, correlation analysis, frequency analysis, T test and regression analysis were done applying SPSS software to find out the function relationships between safety factors which solved by other semi-precision slice methods under the same conditions of horizontal residual sliding forces of last silce.The following results can be reported for the study:(1) As with transfer coefficient method, improved transfer coefficient method which applicable to retrogressive landslide is not suitable for the landslide whose front anti-dip angle is large or the dip angle of trailing edge is lager than 35°, though the deficiency of transfer coefficient method has been overcame to some extent. The stability coefficient calculated by US army corps of engineer’s method tends to be larger for the landslide whose average angle of slip surface is large.(2) In most case, the size order of stability coefficients calculated by semi-precision slice methods is:transfer coefficient method, improved transfer coefficient method, US army corps of engineer’s method, Lowe method, practical recurrence method, simplified Bishop method, simplified Janbu method. The stability coefficients calculated by US army corps of engineer’s method and Lowe method tends to be similar, while the stability coefficients calculated by simplified Bishop method is slightly larger than simplified Janbu method.(3) The size order of stability coefficients calculated by precision slice methods is:precision recurrence method, Spencer method, Morgenstern-Price method 1,Morgenstern-Chen method, Morgenstern-Price method 2, Morgenstern-Price method 3 in most case. The deviation values of stability coefficients calculated by precision slice methods are generally no more than 2%.(4) It is recommended that practical recurrence method is the optimized method used in landslide stability analysis and control engineering design because of high precision and simple calculation process.(5) The deviation values of safety factors for different semi-precision slice methods augment as the values of safety factors increase under the same conditions of horizontal residual sliding forces of last silce. The values of safety factors for Lowe method, US army corps of engineer’s method, practical recurrence method and simplified Janbu method are 0.78%,1.62%,1.62% and 5.55% less, while for transfer coefficient method are 1.78% larger on average than improved transfer coefficient method.(6) If giving the value of safety factor for one semi-precision slice method arbitrarily, safety factors for other semi-precision slice methods can be determined by necessary linearity function. For example, the values of safety factors for improved transfer coefficient method, US, army corps of engineer’s method, Lowe method, simplified Janbu method and practical recurrence method are 1.2254,1.2054,1.2154,1.1579 and 1.2068 when the safety factor for transfer coefficient method is 1.25.The innovation point of this paper is that the optimized slice method has been obtained and the value of safety factors has been constructed based on statistical analysis of stability coefficients calculated by slice methods and safety factors solved by semi-precision slice methods under the same conditions of horizontal residual sliding forces of last silce, which will be of guiding significance to be standard and scientific in landslide stability analysis and control engineering design. |
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