Analysis of macroscopic strength and meso-scale particle rotation characteristics of railroad ballasts fouled by different levels of coal dust#br#
WANG Meng1，XIAO Yuanjie1，2，ZHANG Chongchong1，YANG Tao1，TAN Pan1，LU Mingjiao3
(1. School of Civil Engineering，Central South University，Changsha，Hunan 410075，China；2. MOE Key Laboratory of Engineering Structures of Heavy Haul Railway，Central South University，Changsha，Hunan 410075，China；
3. School of Civil Engineering，Tongji University，Shanghai 200092，China)
Abstract：Fouled ballast can commonly cause severe defects of ballasted trackbed including mud pumping and uneven settlement，thus further endangering stable and safe operations of trains. To address this engineering challenge，ballast specimens with different fouling levels were prepared in the laboratory by adopting the classic fouling index(FI)，and subsequently subjected to monotonic triaxial compression tests. The newly-invented wireless，self-powered，and smart sensors(SmartRock) were placed at different positions inside ballast specimens to measure real-time particle rotation data. The macroscopic shear strength behavior of ballast specimens with different fouling levels were analyzed comparatively and linked to meso-scale ballast particle movement. The results show that increasing coal dust fouling level could cause gradual transition of macroscopic behavior from strain hardening to strain softening. When the fouling index(FI) value ranged from 10% to 15%，both peak deviator stress at failure and apparent cohesion reached their minimums，respectively. The rotation of ballast particles inside the triaxial specimens mainly occurred in the vertical planes，whereas the vertical-plane rotations of ballast particles located closer to the bottom of clean ballast triaxial specimens increasingly attenuated due to the increasing restraint of lateral boundaries on particle movement. When the FI value reached 15%，no discernible rotation patterns were observed for ballast particles inside triaxial specimens，which may be attributable to the loss of inter-particle force-transferring skeleton. When the FI value further exceeded beyond 15%，ballast particle rotations exhibited significantly increasing differences，probably resulting in macroscopic mechanical instability. The uniformity of ballast particle rotations determines macroscopic shear strength behavior to a certain extent. Therefore，the coefficient of variation of the vertical-plane peak Euler angle of ballast particles could be potentially used as a meso-scale indicator of the actual fouling level of ballast beds. The findings are expected to provide theoretical basis and technical reference for Non-destructive evaluation of fouling degree，optimizing ballast-cleaning schedules and implementing intelligent maintenance of ballasted trackbeds.
