Study On The Mechanism Of Mud Pumping In The Roadbed And Its Remediation Of Ballastless High-speed Railway

Ballastless slab track characterized by higher track geometry,better ride comfort and less maintenance has been widely used in high-speed railway(HSR)in the world.The growing requirements on timely delivery of passengers have given an increasing rise to the train axle load,operating train speed and frequency of train departure.Simultaneously,the extreme precipitations occur frequently owing to the global warming.Under the coupling action of the dynamic high-speed train loads and intensive precipitations,the mud pumping takes place in the ballastless HSR,which affects the ride comfort and even endangers the safety of train operations in a worse case.Therefore,it is of great importance to conduct corresponding investigations on mechanism of mud pumping formation in ballastless track and to propose a targeted mud pumping remediation method for ensuring the ride comfort and safety of high-speed train operations.In this study,based on the intercity HSR line of Shanghai to Nanjing in ballastless track,the filed investigations,in-situ soil core tests,numerical simulations using the finite element method and the full-scale physical model test were carried out to reveal the formation mechanism of mud pumping in ballastless track,and accordingly an optimized polyurethane chemical injection(PCI)method was put forward for mud pumping remediation.The results obtained allow the following conclusions to be drawn:(1)Both the field observations and soil core tests were carried out in the field,and the characteristics of mud pumping in ballastless track were also summarized.The results showed that mud pumping was mostly initiated at both ends of the concrete base(the expansion gap),then extended into the area within 2 m along the track direction,whilst no mud pumping occurred at the middle of the concrete base.Both the ground water and the corresponding capillary water height in the testing filed were analyzed,and the maximum capillary water height in the testing filed was far away from the upper surface of the subgrade,so the precipitation was identified as the main source of water supply for triggering mud pumping.The results of XRD tests on soil samples of roadbed,subgrade and pumped mud showed that finer particles in the pumped mud originally came from the roadbed instead of the underlying subgrade.The infiltrated rainwater was all retained in the roadbed and finally formed the standing water according to the mass water content analysis on soil cores.Based on the grain size distributions(GSD)analyses on roadbed materials,essentially the formation of mud pumping was a progressive process of soil particles migration including the particles accumulation and particles loss in the roadbed,i.e.,the finer particles firstly accumulated at the lower roadbed,then gradually migrated to the upper roadbed,and finally were pumped out from the expansion gap and side cracks along the concrete base.(2)A three-dimensional dynamic coupling finite element model(FEM)of vehicle-track-subgrade was established using the ABAQUS software,and the dynamic responses of the track-subgrade system were analyzed under normal subgrade conditions.The obtained results indicated that a whipping effect similar to the cantilever beam was formed at the end of concrete base in ballastless track,which made initial detachments from the concrete base and the roadbed.The detachments provided free infiltration routes for rainwater into the roadbed,and the infiltrated rainwater gradually saturated the roadbed.The flapping action caused by the whipping effect drove the accumulated finer particles to pump out from the detachments and finally formed mud pumping in ballastless track.It is a mutual promotion of deterioration between the track vibrations(referring to the whipping effect)and the mud pumping,forming a vicious cycle.(3)A full-scale physical model of ballastless slab track was established by considering the expansion gap and precipitation device,and simultaneously a comprehensive testing system was formed with multiple sensors arrangement in the physical model.Both precipitation simulation and moving train tests were carried out on this physical model and the mud pumping was reproduced in the laboratory.The mechanism of rainwater infiltration was discovered and the moving train loads facilitated the infiltration of rainwater,promoting the formation of mud pumping and its subsequent development.(4)The full-scale physical model test of mud pumping in ballastless track was carried out,and both the static and dynamic performances of the track-subgrade structure before and after mud pumping together with the forming mechanism of mud pumping were analyzed.It can be found that both static and dynamic performances of the track-subgrade structure were decreased greatly after mud pumping.Under the moving train loads,the pore water pressure was gradually accumulated and formed a pore water pressure gradient from the bottom to upper in the saturated roadbed along the vertical direction,which increased with the increasing of train speed.The pore water pressure gradient drove the finer particles to migrate upward and the up-migrated particles accumulated at the roadbed surface.During the dissipation of pore water pressure at the roadbed surface,the finer particles were pumped out and finally formed the mud pumping.(5)Based on the Terzaghi seepage theory,an evaluation method was proposed to analyze the particles stability of roadbed materials taken from the investigating site in different mud pumping conditions.Accordingly,a calculation method for evaluating the critical hydraulic gradients on the movement of roadbed particles in different diameters were derived,which were verified by the testing results obtained from the physical modeling test.Additionally,based on the statistical analyses on soils suffered mud pumping from the documented researches,a evaluation metod by considering plasticity index and liquid limit of soil prone to mud pumping was put forward.(6)A cyclic uniaxial test on the polyurethane stabilized aggregates(roadbed materials)was carried out,and both the mechanical properties and the long-term performances of the stabilized sample were analyzed.An optimized polyurethane chemical injection method on mud pumping remediation was put forward based on the characteristics of particle stability of roadbed and the formation process of mud pumping.Simultaneously,based on the laboratory full-scale physical model test and the in-situ remediation test of the PCI method,the applicability and effectiveness of the PCI method was successfully verified on mud pumping remediation in ballastless track.