Study On The Repair Mechanism Of Ballastless Track Subgrade Settlement By Foaming Polyurethane Grouting Materials And Lifting Technology

The settlement of high-speed railways has always been a major problem affecting the smoothness,ride comfort and safety.The uneven settlement of the roadbed has an adverse effect on the stress and stability of the ballastless track structure,causing geometric irregularities in the line structure,and trains can only pass through at a speed limit,which seriously affects the normal operation.When the settlement of the ballastless track subgrade is small,the rail pads are generally used as fastener adjustments to meet the track regularities.If the irregularity caused by the settlement of the roadbed exceeds the adjustment capacity of the fasteners,it will affect the comfort,and in severe cases,the safety of the train.Therefore,to restore smoothness,reliable ballastless track lifting measures must be adopted before the fastening capacity reaches the limit.The traditional lifting technology is difficult to meet the needs of non-middle driving during the window maintenance time,due to its large excavation damage,heavy equipment,poor timeliness and low efficiency.Polymer grouting technology is characterized by its light weight,adjustable expansion ratio,good durability,flexible organization,fast construction speed,small damage to the structure(small grouting aperture),and can be uplifted for many times.It has been widely used in lifting projects at home and abroad,such as municipalities,airports,highways and water conservancy reinforcement.It has shown significant advantages and irreplaceable effects,which has achieved good technical and economic effects,especially in engineering fields such as factory ground subsidence repair,building rectification and highway subsidence and lifting.However,polymer grouting and lifting technology suitable for highway cannot be used for high-speed railways.Except the difference of subgrade structures and materials,high-speed railway maintenance operations have higher requirements for time and accuracy than highways and airport roads.At present,in the field of high-speed railway ballastless track subgrade settlement treatment,researches on the polymer grouting technology application is not much,and there is a lack of in-depth study on the settlement mechanism and construction technology of foamed polyurethane.Aiming at the filling of foamed polyurethane grouting material,the lifting mechanism and construction technology of the ballastless track subgrade,the following research work is carried out:(1)The reaction principle of foaming polyurethane and the action mechanism of additives were systematically analyzed,the composition types of slurry were selected and determined,the slurry mix ratio test was carried out.And the influence of the types and amounts of synthetic main compound and additive on the foaming performance,apparent physical and mechanical properties of the material is comprehensively analyzed.The expansion force test was carried out on the slurry with the optimal formula,and the change law of the expansion force of the foamed polyurethane with the density and time of the curing body was studied.The results show that the rigid foam polyurethane prepared by the reaction of POP2140 and4118P has no shrinkage,higher initial strength,and more uniform foam structure.When the amount of isocyanate-PM200 is twice that of polyol,the early strength and apparent physical form of the foam are the best.When triethylenediamine is used as the catalyst,the recommended dosage is 0.83%?1.67%of the amount of isocyanate and polyol in the formulation,which can control the initial reaction time within 15s.If 1,4-butanediol is used as crosslinking agent,too much or too little of it will lead to cracking of the polyurethane consolidation.When the amount of polyol and isocyanate is 20 phr and 40 phr respectively,the optimum dosage is about 4.5 phr.The trend of the expansion force of the polyurethane slurry foaming with time can be divided into three stages:slow growth stage,sharp increase stage,and stable stage.At the sharp increase stage,the expansion force can be increased to about 80%of the maximum expansion force,and the duration is about 15-20s.The maximum expansion force of the foamed polyurethane reaction is linearly related to the density of the consolidated body.When the sample density is 0.532 g/cm³,the corresponding maximum expansion force is 1.528 MPa.There are three stages in the curing of foamed polyurethane:rapid heating stage,slow heating stage and constant temperature stage.The reaction stops in10 minutes,which means the curing is basically completed,so it can be considered that the main strength of the material can be exerted after 10 minutes of reaction.(2)The compression and bending specimen mold and impermeability test device of foamed polyurethane were designed and developed,the compressive,flexural and impermeability of foamed polyurethane samples with different densities were tested.The influence of sample density on the mechanical property indexes such as compressive strength,compressive elastic modulus,bending strength,fracture deflection,initial seepage pressure and permeability coefficient were analyzed,and the evolution mechanism and failure mode under compression and bending were studied.The results show that the compressive strength and elastic modulus increase with the increase of density,strength and density accord with power function,elastic modulus and density conforms to quadratic function.The range of compressive strength is 0.82 MPa?8.95 MPa,while the density range of material sample is0.084 g/cm³?0.36 g/cm³.When the sample density range is 0.323g/cm³?0.36g/cm³,the range of the compressive modulus of elasticity varies from 177.5MPa to 219.7MPa,which is similar to that of graded crushed stone.The compression and bending failure modes are related to the density.When compressed,and the density is less than 0.19g/cm³,the deformation will be large but without macro cracks;once the density is greater than 0.19g/cm³,oblique shear-slip fracture cracks will occur.Under bending,the deflection deformation of the low-density specimens gradually increase with the increase of the load,showing a certain degree of plasticity.When the density is greater than 0.263g/cm³,brittle failure and cracks occur,which locates in middle of the specimen,approximately 45 degrees.The permeability coefficient decreases with the increase of the density,and the overall value range is in the order of 10^-6?10^-8 m/s,which is far less than that of graded gravel on the surface of high-speed railway subgrade.(3)Based on the research on the rheological properties of the foamed polyurethane slurry,a foamed polyurethane grouting model experiment was carried out to analyze the diffusion behavior of the foamed polyurethane slurry in the rigid board-graded broken stone contact layer structure.The study proposed the diffusion mode of foamed polyurethane in a graded broken rock under pressure and combined the elastic-plastic mechanics and small hole expansion theory to deduce the initial pressure for the occurrence of splitting cracks.The results show that the A and B components of the foamed polyurethane slurry react within a short time after contacting,and the volume expands rapidly.As the viscosity increases,the state of the mixed liquid successively goes through the following four stages:liquid,flow-shaped expansion,shape-stable plastic,and consolidated body with certain strength.The flow characteristics of the foamed polyurethane slurry are mainly manifested in the curing time characteristic,which is the length of time for the four stages of reaction.The foaming characteristics of foamed polyurethane slurry are greatly affected by temperature.With the temperature increasing,both the initial reaction time and foaming curing time decrease.By adjusting the temperature of the slurry,the reaction time of the foamed polyurethane slurry can be adjusted between 0-15s.The foamed polyurethane slurry mainly presents a special diffusion form of flake splitting in the graded broken stone.At the same time,the solidified slurry has a strong cementing effect on the graded broken stone so.In the diffusion range,the upper and lower surfaces of the sheet-shaped solidified body are bonded with a certain thickness of graded broken stone particles to form a sandwich structure.(4)The study analyzed the mechanical behavior of the foamed polyurethane grouting in the lifting stage and established the calculation model of the vertical lifting effect formed by the foamed polyurethane grouting in the graded broken stone.The multiple lifting mechanisms of foamed polyurethane grouting for the contact layer of the ballastless trackbed is revealed.Through the establishment of an isometric model test of the ballastless track subgrade,the influence of different grouting modes and grouting parameters such as grouting pressure and grouting amount on the elevation is studied,and the grouting elevation process is explored.The results show that the grouting lifting process can be statically divided into three stages,which is the slight uplift when the grout starts to split,the local uplift during the grout splitting process,and the overall uplift of the flaky split layer.The foamed polyurethane slurry forms split flakes on the contact layer of the ballastless track bed or near the surface of the graded broken stone.With the subsequent grouting,the depth of the split layer will gradually increase.Eventually,a root-like split distribution form is formed.(5)Using foamed polyurethane grouting uplift technology,the track structure uplift rate is relatively stable,and the height stability time is closer to the grouting end time,which can better achieve the goal of average and controllable uplift during grouting.The grouting mode determines its lifting rate,and the duration of grouting(the amount of grouting)affects the final lifting height.With the increase of the grouting time in the grouting mode,the uplift rate of the ballastless track structure increases.The grouting mode is an injection for the 20s,with a pause of 5s,and the maximum uplift rate of the upper ballastless track structure is 1mm per8s.The grouting pause time in a cycle is within the initial reaction time of the grouting,and the grouting pause time has a certain impact on the lifting rate.Extending the grouting pause time can appropriately reduce the lifting rate of the track structure.By increasing the grouting pressure,the grouting uplift rate can be increased to a certain extent.After the lifting height reaches the maximum value,the deformation rebound is very small and doesn't exceed 2mm.Therefore,it shows that the use of foamed polyurethane grouting technology to raise the ballastless track structure is relatively stable.In the graded broken stone,the foamed polyurethane slurry spreads in an elliptical shape,and the diffusion radius is 65?80cm.It is recommended that the spacing range of the grouting holes doesn't exceed 1.3m in the actual grouting and lifting engineering application.When the grouting lifting height is relatively large,continuous lifting along the two sides of the track structure will cause voids under the middle of the base plate because the diffusion range of the foamed polyurethane is limited.Therefore,when the designed lifting height is greater than 10mm,a hole should be drilled in the middle of the track slab,and high-strength polymer mortar with good fluidity should be injected for the purpose of filling and compacting.(6)The analysis and research put forward the specific performance parameter requirements of the two-component foamed polyurethane grouting integrated equipment,and proposed the lifting principle and linear design plan of the ballastless track,and obtained a complete set of construction technology for the grouting and lifting of the ballastless track subgrade which included the monitoring plan,grouting process technology and controlling skylight point in operation time.In the section of small lift and stable settlement,the line elevation should be lifted to the original design position or the position where both ends are connected.For sections with unstable settlements,the settlement can be reserved for the section in combination with the settlement rate in the linear design of the lift.Fasteners are used for transition at both ends of the line,but the lift should not exceed the design elevation by 10mm.When the settlement is large and the construction period is relatively tight,the linear design should be combined with the settlement linear in the form of fitting the longitudinal slope.The construction of the ballastless track foamed polyurethane grouting lifting technology is roughly divided into three stages,that is,the preparation phase,the lifting phase,and the repair phase.The grouting process technology includes arranging and marking grouting holes,drilling,installing grouting pipes,grouting equipment debugging and material performance confirmation,grouting uplift,and filling,real-time dynamic monitoring of line elevation position,removal of grouting pipes,line precision Tuning and repair of track closure layer.The grouting material is light in weight and can be moved with the grouting equipment without setting up a stockyard.The complete construction process doesn't involve large-scale equipment,at the same time,the construction equipment is simple and the construction site is small,so the purpose of rapid grouting operations is achieved.