Study On Stability Evaluation And Prevention Measures Of Ancient Landslides In A Bridge Site Of Yanjiang Expressway

Landslide is one of the main types of geological disasters in highway construction in mountainous areas in China.In order to meet the needs of line alignment and environmental protection,it is inevitable to arrange the line in bad sections such as landslide.When the highway passes through the landslide accumulation body,the stability of the landslide affects the design,construction and operation of the highway engineering.Once the landslide occurs,it can directly bury the highway,destroy the roadbed,bridge and culvert and other facilities,cause traffic interruption,even force the road to change the line,and directly threaten the safety of the construction personnel and construction equipment,resulting in significant economic losses.Among them,the landslide has a significant adverse effect on the bridge pile foundation,and the excessive displacement of the bridge pile foundation directly affects the stability of the bridge superstructure.In this paper,the ancient landslide group of a bridge site along the Yanjiang Expressway is taken as the research object.Based on the study of the formation mechanism of the ancient landslide,the landslide stability and corresponding prevention and control measures are studied,which is of great significance to ensure the safety construction of Expressway and the stability of bridge pile foundation.Based on the investigation of the engineering geological conditions and structural characteristics of the landslide area,this paper analyzes the genetic mechanism of the landslide by means of geological mechanics,and evaluates the stability of the landslide by combining qualitative and quantitative methods.The stability of landslide before and after pile foundation construction and the displacement of bridge pile foundation are analyzed by numerical simulation.According to the results of the study,reasonable prevention and control measures are put forward,and the effectiveness of the treatment project is demonstrated by numerical simulation.The research results are as follows.(1)The landslide developed in the slope of limestone intercalated with carbonaceous shale,and its slope structure is obviously controlled by geological structure.The landslide area is located between two NWW trending F11-4 and F12-1 thrust faults,close to the footwall of F12-1 fault.Affected by the fault,the attitude of the rock stratum in the landslide area is SW,and the dip angle is 25° and it forms a gently inclined bedding structural slope.Affected by the traction fold of F12-1 fault in the front edge,the rock stratum at the foot of the slope becomes gentle,even in the inverted slope.(2)There are three ancient landslides in the area,which disintegrated many times in the later period,forming obvious "round chair" landform.The 1# landslide is in "strip shape" with a volume of about 3.2 million m3.The debris with good cementation of breccia is formed in the later stage of the accumulation body,and the landslide occurs earlier.The 2# landslide is irregular "long strip" with width at the top and narrow at the bottom on the plane,with a square volume of about 8.9 million m3,the 3# landslide is in a long "tongue" shape on the plane,narrow at the top and wide at the bottom,with a square volume of about 8.52 million m3.The formation time of the landslide is later than that of the landslide.The landslide accumulation body is mainly composed of silty clay with fragments of limestone,and the sliding zone is a layer of silty clay formed by argillization at the top of carbonaceous shale.(3)As a whole,the slope is of a gently inclined bedding structure.Under the tectonic action,the carbonaceous shale is prone to interlaminar dislocation.Due to long-term groundwater immersion,argillization occurs and weak intercalation is formed.With the rise of the earth’s crust,the valley is cut down,and the weak intercalation is exposed at the foot of the slope.Under the action of rainstorm,the rain water infiltrates into the slope along the joint fissures,which leads to the increase of the self weight of the rock and soil mass,the decrease of the shear strength of the weak interlayer,the increase of the sliding force and the decrease of the anti sliding force,and the sliding tensile deformation failure occurs.(4)By combining qualitative analysis and quantitative evaluation,the stability of the whole and local landslide under various working conditions is analyzed.The calculation results show that 1# landslide is stable under natural conditions,basically stable under rainstorm and earthquake conditions,and the middle and rear local slopes are basically stable under various working conditions.The 2# landslide is in a stable state under natural conditions,basically stable under rainstorm and earthquake conditions,and the rear and front local slopes are basically stable under various working conditions.The 3# landslide is in a stable state under all working conditions.Because the stability coefficient of 1# landslide and 2# landslide is less than the safety factor in rainstorm and earthquake,the proposed bridge passes through the middle and rear part of landslide group,and its safety reserve does not meet the design requirements of highway bridge pile foundation.(5)Flac3D software is used to simulate and analyze the stability of 1# landslide pile foundation under various working conditions and the displacement distribution of bridge pile foundation.The results show that 1# landslide pile foundation is stable under natural conditions and basically stable under rainfall and earthquake conditions.The maximum horizontal displacement of bridge pile foundation pile top is 1.4619 × 10-2m,which exceeds the horizontal displacement limit of 6 mm for horizontal sensitive buildings and 10 mm for non sensitive buildings specified in article 5.7.2.2 of technical code for building pile foundation(JPG94-2008)requirement.(6)On the basis of finding out the characteristics of landslide development,rock and soil mass composition and slope structure characteristics,combined with landslide formation mechanism analysis and stability evaluation,a scheme of anti slide pile retaining engineering + interception and drainage engineering on the upper part of pile foundation is proposed.The effectiveness of the treatment project is demonstrated by numerical simulation.