Study On Characteristics Of Seepage Voids In Corase-grained Soil Filler And Mechanism Of Mud Pumping In Subgrade Bed Of Ballastless Track Filling Graded Crushed Stone For High-speed Railway

The coarse-grained soil,with graded crushed stone or A/B fillers as the representative,is the primary material for filling high-speed railway subgrade.Its excellent physical and mechanical properties would guarantee the bearing and anti-deformation capacities of subgrade bed during the operation of railway line.However,due to the problems that coarse-grained soil has wide particle composition,relatively large void size,high permeability and complicated restraint state between coarse particles and fine ones in it,it is necessary to carry out the research on characteristics of seepage void in coarse-grained soil filler as well as mud pumping disease of graded crushed stone which strengthens the surface layer of subgrade bed of ballastless track.The results can optimize subgrade structure design,improve engineering construction quality and reduce operation and maintain costs of railway line.After summarizing the existing theoretical studies and engineering practice achievements,a several important properties about coarse-grained soil,like the characteristics of seepage voids,seepage stability and sidewall leakage in seepage test,were studied by laboratory tests and theoretical analysis.Moreover,the formation condition and discriminant method of mud pumping in subgrade bed of ballastless track for high-speed railway under rainfall infiltration were researched.For the transitional coarse-grained soil which has dense skeleton structure but ambiguous seepage failure type,the property of seepage path was analyzed by seepage deformation test in order to propose a method for accurately distinguishing the seepage failure type.Base on the composition of the void between soil particles of coarse-grained soil and rigid wall of permeameter,a method of correcting the test value of hydraulic conductivity and a way of boundary treatment in seepage test were presented so as to avoid the effect of permeameter wall.According to the simulation tests of mud pumping and rainfall infiltration-drainage analysis about subgrade bed of ballastless track,the formation condition and mechanism of mud pumping were elaborated,and a judgement method of mud pumping and a optimiztion subgrade bed design for infiltration and drainage of subgrade bed were also studied.The specific contents are as follows:1.Research on seepage deformation test of the transitional coarse-grained soil filler and criterion of failure type judgementFor coarse-grained soil filler with dense skeleton structure,its fines content generally falls in between 25%and 35%.Although this kind of soil possesses good physical and mechanical properties,the seepage failure type expresses transitional type like piping or boiling,which are hard to discriminate.By conducting a series of seepage deformation tests,the behavior of variation in seepage failure type with changing of compaction degree for the transitional coarse-grained soil filler were studied.For the defect of the current"pore diameter method"and"variable cross-section method"that ignores or only considers constant tortuosity,a parameter named"equivalent specific surface area"(S'₀)which reflects specific surface area(S₀)and tortuosity(T)was introduced based on Kozeny-Carman equation and the tested hydraulic conductivity,so as to establish an equivalent permeability capillary model.Furthermore,an equal diameter capillary model whose diameter D_e with same porosity(n)and S'₀ of the soil was proposed.According to the relationship between D_eand characteristic particle sizes of gradation curve(d₃),a new criterion called"equivalent permeability-average diameter method"for assessing the seepage failure type of transitional coarse-grained soil filler has been advised.The researches indicate that the seepage failure type of transitional coarse-grained soil is depend on the compaction degree,viz.it would be piping for relatively loose state and the increasing of compaction would affect the continuity of fine particles losing,finally it become boiling.The increasing of density would not only reduce the volume of the void in soil,but also improve the value of tortuosity.The parameter D_e controlled by S'₀ is not merely able to reflect the relation of reality tortuosity and permeability in soil,but also express the influenced of tortuosity on seepage stability.The proposed method provides the same prediction results as the experiment phenomena for the transitional coarse-grained soil.2.Study on mechanism of boundary effect in seepage test of coarse-grained soil filler and the treatment means of permeameter wallThe constant-head seepage test is the routine way to acquire the hydraulic conductivity of corase-grained soil filler,but its test precision is strongly influenced by the boundary effect caused by rigid permeameter wall.On the basis of the composition characteristics of pore V_b between soil particle and permeameter wall,a parameter named boundary void ratio of the soil specimen(e_b)was defined through the ratio of V_b to the particle volume V_s which concerns the constitution of V_b,and the mechanism of boundary effect was studied.Thereafter,with the help of uniform disks packing principle in plane,a plane geometric calculation model for boundary void ratio is introduced.By comparing e_b of the boundary zone and void ratio(e)of the core zone in soil specimen,a new method to correct the test value of hydraulic conductivity of coarse-grained soil which would not increase the experiment workload,i.e."void size effect"method,was provided.At the same time,for the principle of the equality of e_b and e,a criterion,i.e."equivalent void ratio method",used for estimating the optimal thickness of treatment layer for permeameter wall in seepage test was presented.At last,the relationship between permeameter diameter and boundary effect was discussed.The studies indicated that the"extra void"caused by permeater wall which make e_b greater than e is the source of sidewall leakage through the duration of seepage test.With the different number and locations of particles replaced by the wall in packing unit,there are three typical plane packing patterns of the boundary particles,viz.two particles pattern,three particles with acute packing angle pattern and three particles with obtuse packing angle pattern,of which the corresponding computed values of e_b increases accordingly.The corrected value of hydraulic conductivity and the optimal thickness of treatment layer for permeameter wall are comprehensively determined by gradation,compression degree,particle density of soil specimen,boundary particle accumulation pattern and permeameter diameter.Moreover,the influence of boundary effect on the seepage test result would become less significant as the permeameter diameter exceeds eight times of the maximum particlesize of soil specimen.3.Model test and formation mechanism analysis on mud pumping in the surface layer of subgrade bed strengthened by graded crushed stone for ballastless trackThe mud pumping in the surface layer of subgrade bed strengthened by graded crushed stone for ballastless track is a new kind of high-speed railway subgrade disease in pluvial region for the past few years.Through carrying out a"vertical full scale&plane unit"dynamic model test in the laboratory,the whole process of mud pumping in subgrade bed of ballastless track was simulated quantificationally.Accordingly,the essential formation conditions and factors which influence the damage degree of mud pumping were elaborated.The tests found that the gap formation and ponding between foundation plate of ballastless track and formation surface along with train dynamic-load action are the three essential factors for the formation of mud pumping in ballastless track subgrade.The dynamic-load of train would make the volume of gap compress and cause the ponding water in it to form instant hydrodynamic pressure.The flow washing and carrying the particles in the surface layer of subgrade bed during the dissipation of hydrodynamic pressure is the primary cause of mud pumping.What's more,the increasing of gap height and fines content of graded crushed stone would aggravate the damage degree of mud pumping so that it brings the number of train load action corresponding to mud pumping formation forward and makes more particles move.4.Analysis on discriminant principle of mud pumping formation and measures of infiltration-drainage control in the subgrade bed of slab ballastless trackFor the mud pumping disease in the subgrade bed of slab ballastless track caused by the infiltration of rainwater,the expression of critical rainfall intensity for ponding on the formation surface(I_c)was deduced based on Mein-Larsson ponding theory under rainfall infiltration.According to the ratio of seepage discharge with regard to the foundation plate fractures in mud pumping zone to mud pumping area,a parameter called"equivalent rainfall intensity"(I_e)was introduced.By taking the successive ponding time(t_d)which gotten by I_e>I_c as the condition,a way called"intensity-ponding method"used for discriminating the mud pumping disease in the subgrade bed of slab ballastless track was established.Additionly,aiming at the current drainage measure of ballastless track subgrade bed,an optimization design for infiltration-drainage of subgrade bed in order to prevent mud pumping was proposed,which is based on the princile of satisfying the local rainfall environment along railway line.The researches indicate that the critical rainfall intensity for ponding on the formation surface I_c is controlled by the hydraulic conductivity of graded crushed stone.The value of t_d is mainly affected by rainfall intensity,the leakage fracture size in foundation plate and the permeability of graded crushed stone,during which the number of train load action is relate to the fomation of mud pumping.Besides,there are two phases,i.e.infiltration and drainage,for rainwater flow through the surface layer of subgarde bed,which must meet the requirement of equivalent rainfall on the formation surface<infiltration flow<drainage flow in order to reduce the value of t_d.