Study On The Mud Pumping Mechanism And Reinforcement Effect For Graded Gravel In High-Speed Railway Ballastless Track Surface Layer Of Subgrade Bed

High-speed railway ballastless track structure has been in serive for about over 10 years. Based on the field investigation, it has been found that some deteriorations of the structure occurred, such as ageing crack of CA mortar layer, ballastless track plate cracking, longitudinal expansion joint of concrete base plate cracking and subgrade bed at neighbouring area of expansion joint experiencing mud pumping, etc. It is the most typical among the deteriorations that mud pumping appeared at the area of before and after longitudinal expansion joint in subgrade bed and at the contacting surface between the concrete base plate and the subgrade bed. In the ballastless structure, after the appearance of cracks at longitudinal expansion joints of the concrete base plate and at the contacting surface between the concrete base plate and the subgrade bed, rain water can penetrate into the subgrade and result in the development of excess pore water pressuredue to the motivation of vehicles. The cyclic process of development-dissipation of excess pore water pressure causes the subgrade material erosion and results in the movement of fine aggregate particles in the subgrade. Furthermore, pore water penetrating through the cracks can force fine aggregates out of the subgrade and therefore lead to themud pumping. This phenomenon can cause reduction of effectiveness of the subgrade supporting the concrete base plate, resulting in differential settlement of the surface layer, a decrease of vehicle-track-subgrade interaction, and deteriorations of operational and safety levels.At present, the research on mud pumping mechanisms and treatment methods for high-speed railway ballastless track is becoming popular and also an urgently challenging issue in the field of operation and maintenance of high-speed railway. Therefore, to treat the disease of mud pumping of ballestless track subgrade for high-speed railway, it is both theortically and practically significant to conduct an in-depth study on the deformation characteristics of graded gravel of ballestless track subgrade, mud pumping mechanisms and associated treatment techniques.Aiming at advancements in the knowledge of widespread mud pumping disease of ballastless track surface layer of subgrade bed for high-speed railway, the research was carried out by means of theory analysis, large-scale static, dynamic triaxial tests and indoor large-scale model tests with following achievements::1) The experiments including particle grading size analysis, compaction and permeability tests of the graded gravel in the surface layer of the subgradefor the high-speed railway ballastless track show that the two types of tested subgrade materials with the percentage ratio of coarse aggregate (40%-75%) meetthe criteria of aggregation parameter and their gradation curves both satisfy the requirement in China code of design for high-speed railway. Moreover, the two materials with a permeability coefficient of KT> i 10-4 cm/s, have relatively high permeability conforming Darcy's law. The maximum density decreases with an increase of the fine aggregate content, with which whilethe optimum water content increases.2) Large-scale static triaxial tests were conducted to investigate the influence of dry density of ratio, water content and gradation of aggregates on the static strength of graded gravel. The results illustrate that the strength increases with the increase of the dry density of ratio. However, when the dry density of ratio increases to a critical value, the aggregates can be crushed with the compaction, leading to a reduction of the shear strength associated with an increase of fine aggregate content. The shear strength of graded gravel drops sharply with the increase of water content and the resistance of graded gravel against deformation is significantly weakened especiallywhen it is fully saturated. Also, compared with impacts of dry density ratio, gradation of aggregates on the shear strength, the water content has the most significant influence.3) The results of the dynamic triaxial experiments of graded gravel indicate that:At the small dynamic stress, graded gravel deforms most elastically with minority of the cumulative deformation; but its resilient deformation and the cumulative deformation increase obviously with the increase of dynamic stress. The development trend of the cumulative deformation curves versus the number of load cycles can be divided into 2 states including:attenuation state and breakdown state. The curve of the attenuation and breakdown state can be expressed, respectively through the model Stewart (1986), and the model Monismith (1975) with a good fitting effect.4) In the same condition of confining pressure and dynamic stress, the cumulative deformation of graded gravel gradually increases with the increase of water content, loading frequency and fine aggregate content; whereas the critical dynamic stress and the shear strength decrease with the increase of water content, loading frequency and fine aggregate content. Particularly, the increase of water content affects most on the cumulative deformation, the critical stress and the shear strength.5) The results of the large scale model test of ballastless railway structure subjected to dynamic load indicate that:At optimum water content ?opt= 5.22%, dynamic stress, vibration displacement, motion velocity, dynamic acceleration and dynamic pore water pressure of graded gravel gradually decreased according to increase of the number of loading and then becomes stable; when the graded gravel subgrade is at the saturated state with the water content of ?s= 7.86%, the subgrade gradually experiences mud pumping. The development process of mud pumping could be divided into three stages as following:slow pumping stage, intense pumping stage and disengaging stage. With the gradual deteriating of mud pumping, the contact surface of concrete base plate bottom and subgrade bed increasingly becomes empty sling or disengaging phenomena, leading to the differential settlement, even seriously affecting the flat degree and causing track slab cracking etc. Based on dynamic response charteristics of the subgrade before and after mud pumping, the research proposes applying dynamic response index as evaluation criteria to analyze the effect of mud pumping on the dynamic characteristics of ballastless railway.6) Graded gravel subgrade subjected to dynamic load caused excess pore water pressure happened, expressing development-dissipation process of excess pore water pressure. Furthermore, cracking phenomena gradually occurred at contact surface between ballastless track concrete base plate and waterproof layer, resulting in permeability line between the waterproof layer and the subgrade of graded gravel. The permeability line under dynamic load transported fine aggregate particles to the waterproof layer, causing mud pumping. It is found that the reason causing mud pumping is comprised simultaneously by aggregate gradation, water and dynamic load.7) This study devlopes a set of grouting equipment simulating reinforcement process and uses this equipment to pour high polymer chemical pulp into the damaged subgrade caused by mud pumping. The dynamic characteristics of the treated subgrade under the cyclic dynamic loading are examined and the effectiveness of grouting reinforcement is analyzed. The results indicate that grouting reinforcement not only improves contacting condition between ballastless track concrete base plate and subgrade bed but also recoveres supporting interaction between the layers. Also, a waterproof layer for the subgrade bed iss created as the result of the grouting reinforcement. Moreover, high polymer chemical pulp reactes with free water to fill up the voids of the graded gravelinduced by the loss of the fine aggregate particles and helpes recovering the strength and the stiffness of the subgrade.