Polyurethane grouting materials with different compositions for the treatment of mud pumping in ballastless track subgrade beds:properties and application effect

Mud pumping in subgrade beds under ballastless tracks will deteriorate the dynamic performance of infrastructure under railway lines,reduce the smoothness of the railway lines,and seriously affect the comfort and safety of the trains.Due to their good mechanical properties,twocomponent polyurethane materials can be used for grouting to treat the fouling problems caused by ballastless track mud pumping.To develop a polyurethane formula suitable for the treatment of ballastless track mud pumping,we first performed indoor experiments to investigate the mechanical properties and gelation time of polyurethane elastomers synthesized with different raw material composition ratios,to determine an optimal composition ratio of the raw materials.Then,we conducted a dynamic field test to verify the remediation effect of the polyurethane material fabricated according to the design ratio.The results showed that polyurethane grouting material with the selected design ratios improved the contact characteristics between the surface layer of the subgrade bed and the base plate in the area,coordinating the dynamic response between the track structure and the subgrade bed.Thus,the obtained polyurethane grouting material could be used to renovate mud pumping areas of ballastless tracks with a good treatment effect.

Mud pumping in high‑speed railway:in‑situ soil core test and full‑scale model testing

Mud pumping induced by moving train loads on rainwater-intruded roadbed causes intensive track vibrations and threatens safety of high-speed trains.In this paper,a vehicle–track–subgrade finite element model was established to analyze the dynamic responses of a ballastless track,and results showed that the concrete base and roadbed were detached because of the whipping effect arising from the rainwater intrusion channel.An in-situ soil core test showed that the intruded rainwater accumulated in roadbed to form standing water and saturated the roadbed.The flapping action of the concrete base caused by the whipping effect led to mud formation mixed with fine particles and rainwater,which migrated upward under the pore-water pressure(PWP)gradient.Mud pumping resulted from continuous particle migration in the saturated roadbed under moving train loads:under normal roadbed condition,coarse and fine particles were uniformly distributed in the roadbed;in early period of mud pumping,fine particles migrated downward to bottom of the roadbed because of the rainwater infiltration flow;in middle stage of mud pumping,fine particles migrated upward and gathered at the roadbed surface under PWP gradient;in later period of mud pumping,fine particles were entrained and removed with the dissipation of excess PWP.Moreover,a full-scale physical model was established to reproduce mud pumping,and polyurethane injection remediation against mud pumping was validated on this physical model.The remediation method was applied to an in-situ mud pumping.The deviation of the vertical track profile reduced remarkably and remained at a low level within half a year,showing a good long-term service performance of the polyurethane remediated roadbed.

The mechanism and effects of subgrade fluidisation under ballasted railway tracks

The rapid growth in railway infrastructure and the construction of high-speed heavy-haul rail network,especially on ground that is basically unsuitable,poses challenges for geotechnical engineers because a large part of the money invested in the development of railway lines is often spent on track maintenance.In fact around the world,the mud pumping of subgrade fines is one of the common reasons why track performance deteriorates and track stability is hindered.This article presents a series of laboratory tests to examine following aspects of mud pumping:(1)the mechanisms of subgrade fluidisation under undrained condition,(2)the effects of mud pumping on the engineering characteristics of ballast,and(3)the use of vertical drains to stabilize subgrade under cyclic loads.The undrained cyclic triaxial testing on vulnerable soft subgrade was performed by varying the cyclic stress ratio(CSR)from 0.2 to 1.0 and the loading frequency f from 1.0 to 5.0 Hz.It is seen from the test results that for a specimen compacted at an initial dry density of 1790 kg/m3,the top portion of the specimen fluidises at CSR=0.5,irrespective of the applied loading frequency.Under cyclic railway loading,the internal redistribution of water at the top of the subgrade layer softens the soil and also reduces its stiffness.In response to these problems,this paper explains how the inclusion of vertical drains in soft subgrade will help to prevent mud pumping by alleviating the build-up of excess pore pressures under moving train loads.

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.