Analysis Of Stability And Slope Ratio Optimization Of Excavated Slope Of Gully Land Consolidation Project In The Loess Hilly Area

The poor geological conditions,serious soil erosion,scarcity of farmland and fragile ecology had severely restricted local sustainable development of the Loess Plateau in China.the large-scale gully land consolidation project has been firstly taken to solve the influence and limitation of regional environment on economic development in Yanan city.Lack of scientific guidance in gully land consolidation project had led to the increasingly prominent problem of slope diseases,and which poses challenges to local disaster prevention and reduction,agriculture and sustainable socio-economic development.This paper takes Nangou gully land consolidation project in Yan'an city as the research background,on the basis of field investigation,drilling survey and seepage test,numerical simulation was used as the main research method,combined with indoor geotechnical test,CTC and RTC triaxial shear test,the influence of rainfall and excavation unloading on the stability of loess slope was studied.Through the qualitative analysis of slope displacement,shear strain and plastic zone distribution after different excavated schemes,the potential instability mechanism of loess slope was discussed.Based on the numerical simulation,the stability of loess slopes with different slope shapes were calculated,and the functional relationship between the stability coefficient and slope height,slope rate of loess slopes were put forward.Optimum design schemes for excavated loess slopes with different slope heights were proposed.The research results can provide scientific basis and guidance for loess slope design and disease prevention of gully land consolidation project in the Loess Plateau.The main results and conclusions are as follows:?1?A series of field investigation,drilling survey and single-ring seepage tests were performed,determining the types of disease and stratum structure of loess slope at Nangou,Yan'an city.It was concluded that the permeability coefficient k of the excavated slope increased with the increase of the topography,while the k in the middle of the filling area was obviously larger than that on both sides,and the soil k in the excavated area was 2-3 times as much as that in the ditch filling area.?2?The CTC and RTC triaxial shear tests of indoor undisturbed and saturated loess were carried out,which was shown that the stress-strain changed characteristics of loess with confining pressure and water content,revealing the softening after rainfall and unloading of soil would reduce the strength of shallow loess,which would lead to the erosion and local instability of shallow slope.?3?Based on the Geo-studio finite element numerical simulation and considering the fluid-solid coupling effect,the influence of rainfall on the stability of loess slope was analyzed,which revealed the cause of local instability of shallow loess slope caused by rainfall.The relationship between rainfall infiltration depth and rainfall satisfied the following exponential function:y=a+be^cx.?4?Based on the comparative analysis of sensitivity and orthogonal test method,it was concluded that the sensitivity of slope shape to the stability of loess slope was:single-stage slope ratio>step width>single-stage slope height.F_S decreased linearly with the increase of single-stage slope height,increased linearly with the increase of step width,and decreased exponentially and negatively with the increase of single-stage slope ratio.?5?Based on Midas/GTS finite element numerical simulation analysis,the local slip mechanism of shallow loess slope after excavating was revealed as follows:graded excavation step-stress?stress concentration at the foot of graded step slope?cracks at the foot of graded steps?crack propagation?rainfall??shallow crack penetration of multistage slope?local slip instability.The plastic deformation caused by stress concentration at the foot of slope can be reduced by slowing down single-stage slope rate,increasing step width,single-stage slope height and excavating large platform,and the stability of slope can be improved.?6?Based on the numerical simulation,the functional relationship between slope stability factor F_S with slope height and slope gradient were derived.The fitting formula was as follows:F=a?tan????^b,coefficients a and b were functions of slope height,and the comprehensive slope rate of slope excavation can be determined by this formula.