Mud pumping in the roadbed of ballastless high-speed railway

International audienceMud pumping that is deliberately avoided in the design of a ballastless high-speed railway still occurs and induces 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; this infiltrated 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 favourable conditions to form mud pumping under dense high-speed train loads. Soil particles of less than 7·1 mm in diameter migrated during mud pumping; these first accumulated at the lower roadbed, then gradually migrated to the upper roadbed, actuated by the hydraulic gradient generated, and were 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 treated in a timely fashion. 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

Remediation of mud pumping in ballastless high-speed railway using polyurethane chemical injection

International audienceMud pumping induced by repeated train passages is one of the most serious distresses not only for ballasted tracks but also for ballastless tracks. In this study both field and laboratory tests were performed to find out the main influencing factors and to further develop a new remediation method. Field observations showed that mud pumping in ballastless track mostly occurred at both ends of concrete base (the expansion joints), triggered mainly by the whipping effect of track vibration due to repeated train passages. Based on the results from cyclic uniaxial test on polyurethane-aggregate specimen, showing an excellent resilience behavior and a high and stable quasi-static elastic modulus under the stress amplitude of 80 kPa for 3 million cycles, a remediation method with Polyurethane Chemical Injection (PCI) was developed for mud pumping in ballastless track, which consisted in injecting polyurethane chemical materials into roadbed at an angle of 45° to get a large stabilized area. Field testing and monitoring showed that after PCI remediation, the vibration acceleration decreased by 16.7% at the end of track slab and by 9.1% in the middle of track slab in 12 h; the deviator of the vertical track alignment was reduced from −9.58 mm to −0.08 mm in 3 days. A longer 6-month filed monitoring showed that the vertical track alignment deviator remained in a relatively low level (less than −0.62 mm) after PCI remediation, and there was no mud pumping reoccurrence in the PCI remediated area, indicating the performance of the proposed remediation method