Differential Settlement Analysis And Compaction Standard Research Of Cut-fill Roadbed

In the embankment filled treatment, there are there difficult problems. One is Vehicle Bump Approaching Bridge (VBAB), the other is the stability of height embankment, and another is differential settlement of the cut-fill roadbed.The difference of strength and stability because of the difference of physical and mechanical properties between the cutting and filling can induce the differential settlement after the road construction. And if it can not be treated properly, cracks will produce on the pavement, and then the pavement will be premature damaged. So there is very impotent significance about the research to the differential settlement of the cut-fill roadbed.The research depends on the Nadanbo-Dongfeng highway in Jilin province. The way passes through mountain terrain, with 300 to 600 metres above sea level. So there're many cut-fill problems. By the investigation, the cut-fill roadbed in the location of K92+400 is selected as basic analysis. And according to the investigation information, on the surface this is low liquid limit clay, and beneath it is weathered rock.At present,the layer-wise summation method is adopted to calculate embankment settlement which is stipulated in the standard. In the layer-wise summation method, subsidiary stress under the center of foundation is obtained based on the nestification of Boussinesq's solution, it will greater than the other subsidiary stress which is not under the center of foundation. So the settlement value is greater using this subsidiary stress to calculate. And in the layer-wise summation method, based on the assumption that the soil mass under the foundation is in completely lateral confinement condition, the soil only can be compressed vertically. So this method is not preciseness. So in the research, the two-dimensional elasto finite element is used to analyze the settlement of cut-fill roadbed.The cut-fill roadbed in the location of K92+400 is analyzed firstly. The analysis indicated that the greatest settlement value position is located in filling side, but is not in the middle of roadbed.Under the request compactness when constructing, the differential settlement is 15.2mm after excavating steps. There is thin layer low liquid limit clay on the original surface, its compression compared with the fill and sand is slightly higher. When it is not excavating steps the differential settlement is slightly greater than excavating steps, the value is 16.6mm.Subsequently in order to analyzing the influence of step to the differential settlement purely, the supposition that there's only had two kind of lithologic- fill and sand is adopted. And now, according to the finite element results, after excavating steps, the differential settlement is slightly increased.The fill is mixed by the sand and clay on the surface which is excavated down from the hill. After roller compaction, it has bigger modulus of deformation, cohesive force, and internal friction angle. In This article, the four kind of situations has been considered that the fill compactness respectively is 87%, 90%, 93%, and 96%, the physics mechanical properties of the fill has carried on the analysis. In this research, the relations between modulus of deformation and compaction, between cohesive force and compaction, and between internal friction angle and compaction have been analyzed. The analysis indicated that along with compactness advancing, the fill's modulus of deformation, the cohesive force and internal friction angle presents the tendency which gradually increases first and when compactness reached to 93%, the curve becomes smooth.Considering the influence of foundation soil, the foundation soil is divided into compressible stratum and incompressible stratum. And then the relation of differential settlement of cut-fill roadbed and compactness, and the depth of fill have been analyzed separately. When the depth of fill respectively is 3.5m, 5.5m, 7.5m, and 11m, the finite element method is separately used to analyze the differential settlement. And the analysis indicated that the differential settlement presents the tendency along with the depth of fill increase linearity.The roadbed of 11m depth of fill has been taken as the major analysis object. The construction request compactness standard is that the compactness of embankment below roadbed bottom 0~80cm is 96%, the compactness of embankment below roadbed bottom 80~150cm is 93%, and the compactness of embankment below 150cm is 90%. In construction request compactness condition, the differential settlement has been analyzed. According to the results, the differential settlement of the cut-fill roadbed is6.56mm when foundation soil is compressible, and it is18.42mm when foundation soil is incompressible. So the differential settlement question in the compressible stratum situation is more obviously. In order to analyze the relations of compactness and the differential settlement, the compactness of embankment is supposed to be uniform. And the settlement of the roadbed is separately calculated as the compactness is 87%, 90%, 93%, and 96%.The research indicated that when the foundation soil is incompressible stratum, the roadbed subsidence is nearly produced by the fill own subsidence, and the roadbed subsidence largely in the filling side, and the differential settlement is produced by the compression deformation of fill own.To the compressible stratum, the roadbed subsidence is produced by the fill and foundation settlement, and the settlement of foundation is much greater. When the compactness of is lower, the fill own subsidence is greater, and the settlement of roadbed is largely in the filling side. Along with the increase of compactness, the fill own subsidence is gradually reduce, and the largest settlement of roadbed is more and more approaching the location of cut and fill. And the research indicated that no matter in the incompressible stratum or in compressible stratum, when compactness increasing from 87% to 93%, the differential settlement value gradually reduced. But, when compactness enhances to 90% after, the differential settlement changed rarely. Therefore compactness of 93% may regard as a marginal value. And we can get the conclusion that enhancing compactness blindly can increase the construction difficulty and the project investment, moreover can not reduce the differential settlement obviously. So based on the research, compactness of 93% is suggested to the embankment below 150cm constructions.Finally, there are several proposals to the compaction standard on the cut-fill roadbed. Embankment below roadbed bottom 0~80cm needs higher compactness in order to increase the bearing capacity of roadbed to the pavement, so compactness of 96% is suggested. And embankment below 150cm needs higher compactness in order to reducing the embankment consolidation under the load function, thus reducing pavement subsidence in using process. And the differential settlement is the main reason which courses the pavement damage. So compactness of 93% is suggested to the embankment below 150cm constructions to the cut-fill roadbed, but much higher compactness is not suggested.The conclusions from this research have certain instruction significance to the cut-fill roadbed design and construction of highway in the future.