Mud pumping in the roadbed of ballastless high-speed railway

Mud pumping intendedly avoided in the design of a ballastless high-speed railway still occurred and induced extraordinary track vibrations. In this study, in situ investigations and laboratory tests were performed to disclose the initiation and development of mud pumping detected in the field. The in situ investigations indicated that mud pumping principally appeared at both ends of the concrete base up to a maximum distance of 2 m. Precipitation, instead of groundwater, was found to be the water source triggering mud pumping, which infiltrated into the graded gravel roadbed through the detachments of the ends of the overlying concrete bases due to the whipping effect. Once mud pumping occurred, the vibrations of concrete bases were aggravated and caused severe track settlements under train loads. The results of laboratory tests indicated that the infiltrated rainwater was retained in the roadbed above the less permeable subgrade, and the roadbed contained an unstable particle skeleton with excessive plastic fine particles, both of which provided favorable conditions to form mud pumping under dense high-speed train loads. Soil particles less than 7.1 mm in diameter migrated during mud pumping, which first accumulated at the lower roadbed, then gradually migrated to the upper roadbed actuated by generated hydraulic gradient, and finally pumped out through the detachments around the expansion gaps, thereby resulting in large amounts of voids in the roadbed and a vicious cycle if not timely treated. These features of mud pumping in ballastless tracks differ from those of ballasted tracks and will benefit the development of remediation measures and improvement of slab track designs.

Recent studies on railway-track substructure at TTCI

Railway-track substructure is the foundation of the railway-track infrastructure and consists of four major components: ballast, sub-ballast, subgrade and drainage. Safety and performance of the track infrastructure, to a large degree, depends on the performance of the track substructure. Adequate support from the track substructure is the most critical element needed for good track performance. When properly constructed and maintained, the ballasted track is the most cost-effective track structure for railway operations, especially for heavy-haul freight operations. Good track-substructure support is characterized by good drainage and strong resistance of the ballast, sub-ballast and subgrade layers to excessive deformation and failures under repeated dynamic wheel loads. As the track substructure plays such important roles, research concerning track substructure has been broad and extensive around the world, with many universities and research institutes conducting various studies and investigations. This paper provides an overview of the recent research conducted at Transportation Technology Center, Inc. (TTCI), a subsidiary of the Association of American Railroads (AAR), in the following areas: ballast mud pumping and its effects on track performance, remediation of subgrade mud pumping, remediation of ballast pockets, ground-penetrating radar (GPR) for inspecting track substructure, and development of software tools with focus on track-substructure functions and management.