Study On Column-Net Structure Embankments Of Ballastless Track In High Speed Railway

The high-speed railway is the main embodiment of science and technologyin the modern age, and the important sign of the railway modernization. Thehigh-speed railway has won fine economic and social benefits due to thecharacteristics such as high speed, good. security, fine comfortableness, andeconomies. With the raising speed on existing lines and construction of theexpress passenger line in our country, stability and comfort is highly required inthe railway line. Simultaneously, because of the augmentation of the rate oftraffic flow, the maintenance and repair of traditional ballasted track becomemore difficult. While currently, the unballasted track structure has been widelyapplied in high-speed railway abroad, because of the advantages such as the goodbulk property, the fine stability, the strong firmness and durability, smalldeformation and slow accumulated deformation for orbit. It has never beenpracticed to construct the segment of unballasted track in our country, so theapplication of ballastless track to soil subgrade is inmature. To meet the demandof PDL construction, the study of Ballastless Track subgrade Engineering must becarried out, especially the key technology of ballastless track soil subgradeEngineering:The adjustment of Unballasted Track Railway Line can only be donethrough the rail fastenings system. Unballasted Track Railway must meet thestrict requirement of ground settlement, differential settlement and flexdeformation, if the Unballasted Track will be constructed on soil subgrade.Subgrade residual settlement is difficult to avoid owing to soil characteristics.Therefore, for reaching the strict standard in differential settlement andaccumulation settlement, subgrade fillings should reach high standards, and at thesame time more strict requirements of foundation treatment and constructiontechnology are needed, which increase the construction cost. The large scale, longline, complex geological conditions, shortness of fine fillings make theconstruction of high-speed railway more difficult either in terms of cost or interms of sources of fine fillings. Moreover, the abandoned soil produced byrelacement is greatly influential to ecological environment. According to the above various factors, selecting and using right foundation treatment become oneof the key factors of the construction of the high speed railway.Column-Net Structure Embankment is a kind of enforcing treatment in thefield of soft foundation, which came into being in Japan in the 1970s'. It hasattracted much attention due to the outstanding merit in small vertical settlement,short stabilization time and easy control of the construction quantity. It is onekind of comparatively new foundation treatment methods, which is applied latelyin China. A few researchers have carried out limited research work, and havegained benefieial results which are still far form being systematical, and theory isfar behind construction practice.Based on comprehensive experimental section of unballasted track onSui-Yu Railway Line, the research of Column-Net Structure Embankmentreinforcing soft soil foundation has been carried out. Research methods mainlyinclude the design calculation theory analysis, centrifuge model test, large scalemodel test, field real train test, numerical analysis, and field Long-termobservation test. Main contents of this research are soil arching mechanism inreinforced bedding, strain and stress of pile and net, accumulated settlement andresidual settlement of subgrade, and dynamic response law of low embankmentunder the action of vehicle. Main research result as follows:(1) Definition, composition, strengthening mechanism, and applicationscope of Column-Net Structure Embankments has been elaborated deeply, andinterrelation of flexible arch of Column-Net Structure Embankments and cushionof composite foundation has been analyzed. Based on the equilibrium equation invarious systems of forces, calculation methods for geogrid have been deducedunder certain assumptions, and influencing factors have been discussed. JapanCalculation Method and deduced Parabola Calculation Method are applied tocalculate tension stress of geogrid, but results show an obvious differencebetween the two calculation means. The research indicates the ParabolaCalculation Method is more reasonable.(2) Column-Net Structure Embankments under layered buried embankmentand train running has been simulated firstly in home and abroad by numericalanalysis software FLAC3D V3.0 with two heights of embank and eight differentkinds of pile spacing. Research show that the pile spacing has large effect to settlement, stress of pile and net, forming of soil arch. When the pile spacing isbig, accumulated settlement and the force in pile is big, and it is difficult to formsoil arch. In terms of the settlement control of Column-Net StructureEmbankments, 2.0 meters of pile spacing should be the optimal pile spacing incomprehensive experimental section of unballasted track on Sui-Yu RailwayLine.(3) Through centrifugal model tests of Column-Net StructureEmbankments for ballastless truck with a geometrical scale of 1:80 in theDK134+820 section of comprehensive experimental section of unballasted trackon Sui-Yu Railway Line, the pile-soil interaction and settlement of two kinds ofColumn-Net Structure Embankments, whose pile spacing is 2 m and 3 mrespectively, are studied. It is shown that the requirement of residual settlementof unballasted trick can be met. The settlement increases when pile spacingchanges from 2m to 3 m. Two tests show that the beating capacity of the twostructures with different pile spacings has a great potential, if Column-NetStructure Embankments is designed based on settlement control.(4) Model test of Column-Net Structure Embankments was firstly designedwith a geometrical scale of 1:13 to investigate the effectiveness in the controllingof soft ground settlement of Express Passenger Line in china based onexperimental section of unballasted track on soil subgrade in the Dk134+820seetion of high embankment in comprehensive experimental section ofunballasted track on Sui-Yu Railway Line. The dimensional analysis method isused. Mechanism of flexible arch, strain and stress of pile and net, and settlementof subgrade are analyzed and researched in high embankment. The researchresults indicate that the accumulated settlement and residual settlement of soilsubgrade reinforced with Column-Net Structure Embankments is small enough tomeet the design standard of settlement. The net of the structure plays a large roleof load distribution, and the pile is able to reinforce the structure in verticaldimension. The pile beating capacity is composed of lateral friction resistanceand end bearing resistance; the existence of soft soil interlayer in the foundationmay result in negative friction, and the neutral point lies in the interface under thesoft soil layer.(5) Long-term observation tests in site are conducted in two kinds of different high embankment, and dynamic tests in construction site is conducted inlow embankment in comprehensive experimental section of unballasted track onSui-Yu Railway Line. The research results show that the post-constructionsettlement of soil subgrade reinforced with Column-Net Structure Embankmentsis small enough to meet the design standard of the construction of the unballastedtrack on the soil subgrade. The research of field real train test shows that axletoad has remarkable influence to stress of subgrade for ballastless track, andsome effect on acceleration rate. Whether in the condition of train or wagon,speed has little effect to the embankment response. After the attenuation within 3meters height of embankment, neither dynamic stress or acceleration isinfluential to reinforced bedding of Column-Net Structure Embankments.(6) On the basis of comparison and analysis of the research result obtainedabove, design calculation method of Column-Net Structure Embankments hasbeen summarized. In addition, the investigation when the embankment level islower than soil arch height is also carried out. At last, rational limitation ofminimum altitude of embankment is also gained, which means the height ofembankment should be higher than soil arch height in order to ensure safety andstabilization of Column-Net Structure Embankments.