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.
Summary
Shield tunneling is an efficient method of the tunnel construction for the system of underground transportation, but it will disturb the surrounding soil and affect the security of nearby structures. The monitoring for deformation of the structures influenced by the tunneling construction is crucial for structural safety evaluation, especially for the ancient structures. The computer vision‐based structural deformation monitoring approach is a newly developed method that has advantages such as, long‐distance, noncontact, and so on, which provides a promising tool for deformation monitoring of ancient structures. This study applied a computer vision‐based method for field monitoring of three‐dimensional (3‐D) deformation of an ancient tower under the influence of shield tunneling. The target‐free strategy was adopted to eliminate the need for installation of artificial makers on the surface of the tower. The environmental factors during the long‐term monitoring were discussed, especially the influence of temperature variation. Background modification was employed to reduce the error induced by the microchange of camera position. The long‐term 3‐D deformations of the ancient tower including vertical settlement and horizontal deformation in two directions during the shield tunneling construction were obtained. The results indicate that the proposed computer vision‐based method offers an efficient and cost‐effective approach to monitor the 3‐D deformation of the ancient structure and further for structural safety evaluation.
Principal stress rotation induced by moving loads from trains significantly influences railway track settlement accumulation. The stationary cyclic loading commonly adopted to study railway ballast behaviour under repeated train loading cannot fully represent the effects of principal stress rotation, which needs to be properly considered in both laboratory tests and numerical simulations for a better understanding of ballast deformation behaviour. This paper focuses on studying railway ballast deformation behaviour with an emphasis on particle scale interactions under two different loading scenarios – namely, stationary cyclic and moving wheel loading. A ballasted track model consisting of five sleepers was established based on the discrete-element method (DEM) with realistic polyhedron-shaped elements. The numerical model was validated first based on the testing results from a full-scale high-speed railway testing facility at Zhejiang University. Numerical results clearly indicated that moving wheel loading induced larger principal stress rotation than stationed cyclic loading did. Larger principal stress rotation mobilised higher particle rotation and displacement, which further increased particle rearrangements through individual particle rolling and sliding, and potentially could cause accelerated ballast degradation. It is recommended to consider principal stress rotation in ballast settlement predictions to prevent possible underestimation by stationary cyclic loading and its limitations.
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