Mechanical Characterization And Moisture Migration Of The High-speed Railway Track-bed

The service life of the high-speed railway track-bed is 100 years.During the service life,the track-bed is imposed to the coupling effects of the cyclic loading and extreme climate conditions.Improving the serviceability of the track-bed in consideration of strength,stiffness,permanent deformation and moisture migration,is of paramount importance in ensuring the safety and normal performance of the high-speed railways.In recent years,the high-speed railways developed rapidly in China,but there still exist two problems in the engineering application of the track-bed:the controlling method of physical-treated fine soils in the construction period and the protective method of water related problems in the operating period.In this study,based on these two engineering problems,the mechanical behavior and moisture migration of the high-speed railway track-bed is investigated,by unit cell tests(including micro-structure tests),full-scale model test and numerical modelling.The main research studies include:(1)Unit cell tests.Based on the physical treating method on the fines,6 kinds of samples of the fines matrix with different volumetric inclusion(coarse grains)contents were prepared in the lab.By the ?CT scans,the effect of inclusion content on the soil fabric was investigated.On the other hand,by the monotonic and cyclic triaxial tests,the effect of inclusion content on the monotonic shear strength,permanent deformation under cyclic loading,resilient modulus and damping ratio were also studied.The results show that a characteristic volumetric inclusion content fv_cha exists,separating two zones of the variations of mechanical behaviors with fv.This observation corresponds well with the soil fabrics observed by the ?CT scans:when fv?fv-cha,a the fines dominate the mechanical behaviors of the sample;however,when fv>fv-cha,the inclusions dominate the mechanical behaviors of the sample.(2)Full-scale model test.A full-scale physical model of geo-reinforced pile-supported ballastless track-bed was constructed in a chamber.To simulate the extreme climate condition,the effects of various water levels and cyclic loading on the water content,soil pressure and permanent deformation of the track-bed were investigated.Four testing procedures of water level rising,loading at high water level,water level lowering and loading at low water level were applied.The results indicate that the "soil arching effect" exists in the pile-supported track-bed.This effect is amplified with the rising of water level and loading cycles,while it tends to be stable after enough cycles.At high water level,the whole accumulative settlement of the track-bed is mainly controlled by the saturated zone below the water level,with much less contribution of the unsaturated zone above the water level.Due to stiffness difference between the pile cap and the simulated soil,differential settlement exists inside the subgrade,with larger values above the simulated soil and smaller values above the pile caps.(3)Numerical modelling.A numerical model of high-speed railway track-bed was established.The fissures at the edges of the concrete bases were considered as the infiltration area.The effects of compaction degree and fines content of the subgrade bottom layer on the moisture migration of the track-bed were instigated under natural climate conditions.The results show that after 3 year calculation period,"saturation zone" exists in the subgrade,with the size increasing with the increase of compaction degree(less effect)or fines content(larger effect).Through the analysis of volumetric water content variations with time at different depths of the left-fissure profile,it is indicated that as the compaction degree increases,the arriving time of the wetting front delays and the variation amplitude of volumetric water content decreases.For each case,it all takes a long time(more than 30 days)for the wetting front to pass through the interface of subgrade surface layer and subgrade bottom layer.(4)In combination of the present studies with analytical analysis and existing research references,some technical advice about the physical treating method of fines in the construction period and the protective method of water related problems in the service period are proposed.