Study On Stability Of High Subgrades Due To Rainfall Infiltration

The studying of highway hazards is closely connected with the development of highway constructions. Large-scale Highway subgrades are booming in mountain regions with more and more highways being constructed in our country. The heavy rain with certain duration can cause serious damages to the highway, such as slope sliding. This paper, firstly, analyzed the stability of BaiShi earth dam with complicated seepages, and then calculated extensively the stability of the highway subgrade due to rainfall infiltration with different theories. The slope stability mechanism and failure mode are deeply dealt with in the paper. 1. A FEM program is made to calculate the complicated seepage of the earth dam of BaiShi reservoir and a new finite element algorithm called abandoned element method is developed to solve the free surface of the unconfined seepage. This method is verified to have good precision and convergence by the comparison of calculation results with electrical resistance simulation method, and can be applied to all kinds of unconfined seepage. The unit discharges are calculated by FEM method. In addition, the difference between calculated and measured water levels is dealt with. Accordingly, the measures of seepage stability are proposed in this paper. At last, the stability of the earth dam is analyzed under the rapid drawdown. The calculating method proposed in the paper provides a good reference for the design and running of the BaiShi earth dam. 2. In this paper, a seepage model of infiltration into unsaturated highway subgrades was proposed and the corresponding analyzing program of stability of the bank was made on the basis of the simplified Bishop's method, which can calculate the different depths of infiltration into the subgrade. The stabilities of popular highway subgrades are calculated with the program easily, and the finite methods of seepage and elasto-plasticity verified the results calculated by the limit equilibrium program. Some valuable conclusions have been drawn for the design and construction of the highway subgrade.3. A two-dimensional unsteady seepage FEM program was developed on the basis of a saturated-unsaturated seepage mode. A suitable SWC curve was applied and the unsaturated hydraulic conductivity was predicted with Corey-Green method. Transient seepage fields due to rain infiltration were computed for popular highway subgrades and the water content and matric suction in the slope was analyzed. The computed transient pore water pressures were then used for subsequent elasto-plastic FE program. An extended Mohr-Columbia failure criterion was used to allow for shear strength variations as a result of changes in matric suction and the FE stress fields were gained accordingly. The global safety factors were calculated by the stress integration along a slip line and a concept of "small slip failure" was introduced in stability of the embankment slope. Effects of various rainfall events, angleφb and the anisotropic permeability ratio caused by construction of the embankment on the seepage and stability were analyzed. 4. In this paper, reliability analysis of highway subgrades is presented using two methods of uncertainty adapted "JC" method and Monte Carlo simulation method (MCSM) based on the view of "mean rainfall" and the corresponding program was developed which can account for relative randomly distributed variables. According to published data, the effect of changes of saturation due to rainfall infiltration on soil strength parameters is dealt with. In addition, the sensibility factors affecting the slope stability are analyzed, and also the rules of reliability of soil slope stability are researched a little more deeply. At last, a reference reliability index is proposed for the highway subgrade stability.