Mechanical Behaviors Of Coarse Grain-fines Mixture Of High-speed Railway Embankment Under Cyclic Loadings

Recently,a rapid development of high-speed railway has occurred in China.During the construction of high-speed railway,the embankment was directly placed on the subgrade without any separation layer.Under long-term traffic loadings,a coarse grain/fines layer is formed by the interpenetration of the coarse grains from the embankment and the fines soils from the subgrade.Field investigation of this layer shows that the coarse grain content decreases over depth.Being the intermediate component between the embankment and subgrade,this layer plays an important role in the load bearing and transmission.Thus,investigation of its mechanical behavior is of vital importance.Since directly testing such mixture with large grains is limited with common laboratory equipment,the consideration of a scaled material at smaller size is advisable.In addition,at different sites,the coarse grains contained in the interlayer can present a large variability in terms of grain size distribution.In this study,the scaling and gradation effects of coarse grains on the mechanical behavior of coarse grain/fines mixture were systematically investigated through static and cyclic triaxial tests and μCT scanning tests.In addition,based on the relevant cyclic testing results,permanent strain and resilient modulus model which can consider the effects of stress level,number of cycles,coarse grain content and uniform coefficient were proposed.With these models,the cumulative settlement of the high-speed railway embankment composed of these soils were calculated.Based on the calculated results,suggestions of selecting the embankment materials were given.The main contents and conclusions are listed as follows:(1)To study the effect of grain size,two kinds of grains,namely,in-situ grain and scaled grain S1 were prepared.The maximum grain sizes of the in-situ grain and scaled grain S1 were 63 mm and 20 mm,respectively.To study the effect of grain size distribution,by reducing the proportion of the smaller grains of scaled grain S1,two other scaled grain S2 and S3 were designed and prepared.The Cu values of S1,S2 and S3 were 2.72,2.01 and 1.12,respectively.(2)Large-scale cyclic triaxial tests were carried out on the in-situ grain/fines mixture(in-situ grain sample)at six volumetric coarse grain contents fv(0%、5%、10%、20%、35%and 45%).Two dynamic parameters(namely permanent strain and resilient modulus)were obtained.By comparing these two parameters with those obtained previously by Wang et al.(2017,2018a)from small-scale cyclic triaxial tests on scaled grain/fines mixture(scaled grain sample)with scaled grain prepared by applying the parallel gradation method,the effect of grain size were evaluated.Results showed that,for the in-situ grain samples and the scaled grain samples,when fv=0%-20%,the soil skeleton was dominated by fines soils,while when fv>35%,the soil skeleton was dominated by coarse grains.Correspondingly,the changing rates of permanent strain and resilient modulus with at these two fv ranges were distinctly different.In the case of fine-fine contact structure,on the whole,the permanent strains and resilient modulus values of the in-situ grain samples are consistent with those of the scaled grain samples,suggesting that the grain size effect was negligible.In the case of grain-grain contact structure,the permanent strains and resilient modulus values are found to coincide globally at the two scales,also suggesting that the grain size effect was small,the slight differences between the two scales being attributed to the irregular grain sliding and the distribution of the fines soils(3)Small-scale static triaxial tests were carried out on scaled grain/fines mixture(scaled grain sample)with five fv values(5%、10%、20%、35%and 45%)and three Cu values(2.72、2.01 and 1.12).The effect of grain size distribution on the static behavior was evaluated.Results showed that at three Cu values(2.72、2.01 and 1.12),the samples with small fv values(5%,10%and 20%)corresponded to fine-fine contact structure,and the samples with large fv values(35%and 45%)corresponded to grain-grain contact structure.With the decrease of Cu value,the characteristic volumetric coarse grain content fv-cha increased.In the case of grain-grain contact structure,the decrease of Cu value caused the increase of large grains,leading to increases in peak deviator stress,friction angle,poisson’s ratio and dilatancy angle.In the case of fine-fine contact structure,the decrease of Cu value caused the decrease of total coarse grain quantity,leading to decreases in peak deviator stress,friction angle,poisson’s ratio and dilatancy angle.The change patterns of Young’s modulus and cohesion with increasing Cu were the same for the two soil fabrics-Young’s modulus and cohesion decreased with the increase of Cu.(4)Small-scale cyclic triaxial tests were carried out on scaled grain/fines mixture(scaled grain sample)with two fv values(35%and 45%)and three Cu values(2.72、2.01 and 1.12).The effect of grain size distribution on the cyclic behavior was evaluated.Note that compared with the static triaxial tests,the fv values corresponding to fine-fine contact structure were not considered.This is because according to the results of the static triaxial tests,when the sample was under the confining pressure equal to the in-situ one(σ3=30 kPa),the effect of Cu on the mechanical behavior was negligible in the case of fine-fine contact structure.The results of dynamic tests shows that the quantity of large grains increased with the Cu decreasing,adding difficulties to the rearrangement of the coarse grains and thus leading to a decrease of permanent strain.When Cu decreased,the total quantity of coarse grains decreased,reducing the interactions between coarse grains,leading to a decrease in resilient strain and an increase in energy loss.Correspondingly,the resilient modulus decreased and the damping ratio increased.(5)Based on the testing results of dynamic behaviours(permanent strain and resilient modulus),the permanent strain model proposed by Gidel et al.(2001)and the modified resilient modulus model proposed by Uzan(1985)were separately extended at fine-fine contact structure(fv=0-20%)and grain-grain contact structure(fv=35%and 45%).The extended models can consider the effects of stress level(△p and Δq),coarse grain content fv,coefficient of uniformity Cu,and so on.With these models,the cumulative settlement of the high-speed railway embankment with different fv and Cu values were calculated.Based on the calculated results,it is suggested that coarse grain-fines mixture with fv values of 35%and 45%can be used as the materails for embankment construction.