Mud pumping under railtracks: mechanisms, assessments and solutions
Mud pumping under railway tracks has received increasing attention from academic and practical perspectives in recent decades, however, the actual mechanisms and possible solutions are still not understood or well established. Frequent investigations in countries such as Japan, Canada, the USA, China, Australia, the UK, and other European regions where railway systems are the largest and most advanced, indicate that mud pumping still leads to high annual maintenance costs. On this basis, a thorough review is therefore essential, so this paper presents a systematic and comprehensive review of mud pumping in railways. In particular three primary aspects of mud pumping are addressed: (i) the phenomena and mechanisms; (ii) assessments; and (iii) solutions. The review shows the three essential factors that trigger mud pumping, i.e., excess fines, excess water, and cyclic loads. While excess fines can be induced by subgrade fluidisation, ballast breakdown and external sources, the excess water is mainly due to insufficient drainage in the foundations. Given these 3 factors, different contexts where mud pumping can be instigated are summarised such as subgrade fluidisation and infiltration, peat boils from soft roadbeds and upward migration of non-subgrade fines. Unfavourable weather condition, poor sleeper-ballast contact and stress/strain concentration at particular sections such as rail joints, switches, crossings and transition zones can accelerate the inception of mud pumping. In all cases, the generation of excess pore pressure is the driving mechanism. The study also summarises the laboratory and in-situ techniques currently used to assess mud pumping. 4 major groups of mud pumping solutions are highlighted with their advantages and disadvantages: (1) clean, modify and renew problematic layers; (2) enhance drainage condition; (3) geosynthetics; and (4) chemical stabilisations.