Study On The Stability Of High-filled Loess Slope Under The Rise Of Groundwater Level

Yan’an is located in the loess hilly-gully region of Northern Shaanxi Province, which lies in the “Y” type river valley. The city is a typical linear city, where the land for development is in short extremely. In recent years, in order to provide the necessary development space for the economic and social construction, Yan’an has been carrying out land creation projects by cutting hills and filling valleys to create 78.5 square kilometers of flat ground for urbanization since 2012. They are the largest geotechnical engineering projects in collapsible loess areas throughout the word. Most of the projects have to carry out different levels of high fill and dig deep, the ensuing high fill slope deformation and stability problems is particularly prominent. This kind of project will change the original hilly terrain and conditions of discharge of groundwater, which will cause the groundwater to rise in the high fill slope. However, the rise of groundwater level changes the distribution of effective stress and affects the shear strength of the soil in the slope, so that the stability of the slope is affected.The effects of the water content and dry density on mechanical properties and microstructure characters of remodeling loess were discussed in this paper by the indoor experiment, and the necessary parameters in slope analysis were obtained as well. Moreover, this dissertation, taking the Qiaoergou slope as a case study, investigated the impacts of the rise of groundwater level in slope on the seepage field, stress field and stability of the high fill slope by hydrogeological investigation, laboratory experiments, numerical simulation and theoretical analysis. The following main conclusions are obtained:(1) By the compaction test, the optimal water content of the remolded loess was 16%, and the maximum dry density was 1.735g/cm3.(2) This paper uses SEM technology for microstructure scanning under different water content and dry density. The results shows that : the number of large pores decreases and the number of small and medium pores increases with the increase of water content and dry density; the roundness of the pores and particles will decrease with the increase of the dry density, but with the increase of water content, the roundness of the pores decreases and the roundness of the particles increases; the orientation of pores decreases and the ordering property gets better, however, the orientation of particles increases and the ordering property gets worse with the increase of the water content and dry density. At the same time, the changes of the water content and dry density caused the change of the connection and arrangement form of the particles, which had a certain influence on the strength of the remolded loess.(3) The basic rules and fitting formula of shear strength index and water content, dry density are obtained by the indoor direct shear tests. Study shows that the strength of remolded loess has close connection with water content and dry density, a remarkable first order exponential decay relation was found between cohesion and water content, moreover the relation between cohesion and dry density is power function. The internal friction angle is less affected by water content and dry density, and has no obvious function relation among them, but the internal friction angle is shown to decrease with the increase of water content while increase with the increase of dry density. The soil-water characteristic curve of remolded loess under the dry density of 1.6g/cm3 was measured by filter paper method, and it was proved that the soil-water characteristic curve of Yan’an remolded loess can be fitted by van Genuchten model.(4) The influence of groundwater rise on the slope deformation and stability were analyzed by the combination of Qiaoergou high fill slope. The results indicated that the seepage flow in the slope is obvious and the horizontal and vertical displacement of the slope is further increased with the rising of the groundwater level in the slope. In addition, the plastic zone from the lower-middle part of the slope which near the foot of slope upward and inward developed gradually and ultimately from the foot to the middle of the slope holing through. Moreover, the maximum shear strain appears in the foot of slope, and the slope may be cut out from the slope toe which leads to slope failure. Finally, the slope safety factor is decreased from 2.076 to 1.425, and the slope is still in stable condition, but the groundwater rise has negative effect on the stability of slope.The work carried out in this paper can provide a certain theoretical basis for the selection of the filling parameters and technical support for the design, construction and operation of slope engineering. It also has great reference meaning to the similar high fill slope for disaster prevention and mitigation.