Stability Analysis And Prevention Measures For Slopes With Residual Soil-sandstone Binary Structure Under Rainfall Conditions

Binary slope is consist of overlying soil layer and bottom rock mass,it is commonly seen in the cut slope building which built in mountains or hilly areas with loose accumulation existed in shallow surface,the slope with residual soil-sandstone binary structure is one kind of binary slope.Binary slope is easy to evolve into landslide under the influence of rainfall as the main factor,thus causing a certain degree of economic losses,casualties and ecological damage.Therefore,on the basis of analyzing the stability of the binary slope under rainfall,it is of great significance to carry out the binary slope control project.In this paper,we choose the binary slope of cutting slope construction that in a resettlement area of Huichang county as the research object,we studied the instability by using field investigation,laboratory soil test,theoretical analysis,slip surface stress method and numerical modeling and put forward prevention measures,we obtained the following main conclusions:（1）The average liquid limit,the average plastic limit and the plastic index of residual slope soil are 36.88%,23.63%and 13.25,respectively.It is a silty clay with low liquid limit and the average permeability is 3.47×10^-6m/s.The optimal initial water content is15.8%,when the initial water content is less than or more than 15.8%,the cohesion decreases and the angle of internal friction presents the characteristics of"first sharply decreases,then slowly decreases".（2）In the process of rainfall,the volume water content,pore water pressure,stress,displacement and safety factor of shallow soil in slope are easily affected by different rainfall conditions（rainfall intensity,rainfall duration and rainfall type）.The volume water content and pore water pressure of shallow soil gradually increase with the increase of rainfall intensity or rainfall duration,which leads to the increase of the maximum shear stress at the surface of the slope,the expansion of the shear stress concentration range at the foot of the slope（the shear stress value also increases）,the overall decrease of effective stress in the slope and the great change of the displacement in the first-grade slope soil.The decline of slope safety factor is positively correlated with rainfall intensity and duration.Under the condition of rainfall intensity at 160mm/d,80mm/d,40mm/d and20mm/d（the duration of rainfall is 6 days）,the decline amplitude is 8.80%,6.21%,3.71%and 1.95%,respectively.When the rainfall lasted for 6 days（rainfall intensity is 160mm/d）,the decline amplitude was 8.80%and showed the characteristics of"fast in the early period and slow in the late period".Different rainfall types have different influences on the soil volume water content and pore water pressure.The decreasing type is the largest,followed by isointensity type and unimodal type,and the increasing type is the least.The decreasing type of rainfall causes the most obvious changes in shear stress,effective stress and soil displacement at the foot of slope.At the same time,it is found that the decreasing rainfall has the greatest influence on the slope safety factor.Under this condition,the decline amplitude of the safety factor is 7.32%,which is larger than that of the unimodal（6.30%）,isointense（6.21%）and incremental（5.93%）rainfall conditions.（3）In the absence of rainfall,compared with the slope without support,the safety factor of the slope after strengthening 10m anti-slide pile at different pile sites A,B and C（corresponding to the upper,middle and lower part of the slope respectively）increased19.98%,21.09%and 0,respectively.For the same pile site B,the safety factor of the slope after strengthening 8m,9m and 11m anti-slide pile increased 0.37%,21.09%and 21.09%,respectively.However,under the condition of the same rainfall（holding time for 6 days）,compared with the slope without support,the decline of the slope safety factor after strengthening anti-slide pile is different（the decline of the slope after strengthening 10m anti-slide pile at different pile sites A,B and C is 6.01%,6.04%and 5.83%,respectively）.The safety factor of the slope at the same pile site B after strengthening 8m,9m and 11m anti-slide pile is 5.35%,6.04%and 6.04%,respectively.Therefore,fully considering the safety and economy,the optimal reinforcement scheme is to arrange 9m anti-slide pile in the middle of the slope.