Optimization of critical slip surface in unsaturated slopes subjected to rain infiltration by CBO algorithm

Abstract

Slope stability analysis is one of the most important issues in the safe design of infrastructures. This is mainly due to the historical hazards arising from instability of soil mass, leading to serious consequences including both fatality and financial loss. According to the field observations, rainfall is one of the most important factors stimulating the instability of unsaturated slopes. Nonetheless, most classical solution methods ignore the unsaturated conditions by simply assuming the soil conditions to be dry or completely saturated. In order to cope with this limitation, commercial software has been improved for simulation of two-phase flow under unsaturated conditions such as the GeoStudio software package. However, the scientific challenge as one of the limitations of commercial software is the optimization of critical slip surface under transient precipitation. Therefore, the main objective of this research is to develop a homemade computer code in MATLAB based on the Colliding Bodies Optimization (CBO) algorithm so that it can be used for studying complex two-phase flow problems more efficiently in terms of time. The code can effectively calculate and optimize the factor of safety against stability considering the effects of precipitation applied to the slope boundaries by employing Richards’ equation. Calculations of factor of safety are performed by assuming the circular wedge geometry and Bishop's method. According to the algorithm, the centroid of the wedge of slip circles in space is randomly guided and as a result, the coordinates of the circle collision points are obtained within the slope. If the consistency conditions are satisfied, the wedge is divided into parts and the factor of safety is calculated by finite difference method. This process will continue until the optimal factor of safety is obtained. Finally, the results of the code are validated against the output of GeoStudio 2018. The results confirm that the new method is robust in predicting the critical conditions much more rapidly than the software. Although changes in the factor of safety are minor for the simple validation example, the most important feature of the new code is that it reduces the CPU occupancy by 71% on average, independent of the type of CPU.

Keywords

• unsaturated slope stability
• rainfall
• optimization
• colliding bodies optimization algorithm
• finite difference method