Study On Dynamic And Static Properties Of Railway Ballast Bed With TDA

Railway ballast bed is usually composed of broken rock blocks with edges and corners,which can provide bearing capacity,vertical and horizontal resistance for sleepers and ensure sufficient drainage of ballast bed.However,under the long-term cyclic load of train,the mechanical and physical state of railway ballast bed is an irreversible process of continuous degradation from the beginning of service.The degradation of ballast bed can easily lead to a series of diseases,such as ballast bed hardening,mud boiling,uneven settlement and so on,which seriously affect the traffic safety and transportation efficiency,and greatly increase the cost of line maintenance.In recent years,a few scholars at home and abroad have filled the ballast layer with appropriate content of tire-derived aggregate(TDA),and this method can reduce the degradation of ballast,track settlement and stiffness of ballast bed,improve energy dissipation,and reduce the effect of train on ground vibration.It also has a good application potential in improving the performance of ballast bed,and has significant environmental and economic benefits.However,under the repeated train load,the macro mechanical properties of the ballast bed with TDA are affected by the relative size of ballast particles and TDA,the content of TDA and other key factors.At the same time,due to the short time of this method,the relevant experiment conditions are relatively single and lack of systematicness,especially the mechanism of adding TDA in ballast in order to delay the degradation of ballast bed is not clear.Based on this,the indoor static and dynamic triaxial unit tests are carried out by using GCTS large-scale vibration triaxial apparatus.Under the condition of reflecting the actual stress situation of the ballast,the dynamic and static characteristics of ballast bed are revealed by adding TDA with different contents and particle sizes into the ballast sample.In addition,a three-dimensional laser scanner combined with image analysis method are used to analyze the geometric changes of crushed stone ballast particles before and after the test.The influence of TDA on delaying the degradation of ballast particles is further analyzed from the micro level,and the mechanism of TDA on delaying the degradation of railway ballast bed performance is preliminarily clarified.The main contents and research results are as follows:(1)The peak strength and initial elastic modulus of the ballast sample decrease with the increase of TDA contents,and the residual strength goes through three stages:decrease-increase-decrease with the increase of TDA contents,and the maximum residual strength occurs when the volume ratio of TDA is 10%.The ductility of the sample increases with the increase of TDA contents,that is,the weakening degree of the strength of the sample decreases after the sample is destroyed.(2)The cumulative axial plastic strain increases with the increase of dynamic stress amplitude and loading frequency.In addition,the cumulative plastic strain increases rapidly in the initial loading stage,and the strain is stable with the increase of the number of cycles.The dynamic elastic modulus increases with the increase of dynamic stress amplitude and load frequency,and the damping ratio increases with the increase of dynamic stress amplitude,and grows gradually first and then tends to be stable with the increase of cycle times.(3)When TDA are added with different particle sizes and contents,the curve of cumulative axial plastic strain of ballast samples with the number of cycles will show different development patterns.Combined with the plastic shakedown theory,the development forms are divided into plastic shakedown state,plastic shakedown and ratcheting behavior state,and plastic collapse state.(4)The dynamic elastic modulus of ballast sample decreases with the increase of TDA contents when range particle size of TDA is 8～16 mm are added.The dynamic elastic modulus increases with the increase of TDA contents when range particle size of TDA is 16～25 mm are added and the volume ratio does not exceed 10%.When the volume ratio exceeds 10%,the dynamic elastic modulus decreases with the increase of TDA contents.It is not obvious to reduce the dynamic elastic modulus by adding large particle size TDA.The evolution law of dumping degree of hysteresis curve of ballast sample with TDA contents is consistent with the analysis of dynamic elastic modulus.Generally speaking,the damping ratio of the sample increases with the increase of TDA contents,and the damping ratio decreases sharply at first and then remains almost constant gradually with the increase of cycle times.(5)The elastic strain energy and dissipated energy of the ballast sample increase with the increase of TDA contents,and the dissipated energy is gradually close to the elastic strain energy,which indicates that the energy dissipation efficiency is gradually improved.In addition,the dissipation capacity of small particle size TDA is more significant at the same content.With the increase of TDA contents,the BBI value of ballast break index decreases gradually,while the cumulative plastic strain increases continuously.The addition of TDA can redistribute the energy of ballast sample,the accumulated plastic strain consumes more energy,and the energy consumed by the breaking of ballast particles is reduced,which is helpful to reduce the degradation of ballast.(6)The effects of different TDA particle sizes and contents on the dynamic characteristics of the ballast samples were compared under cyclic loading,according to the cumulative axial plastic strain,dynamic elastic modulus,damping ratio,dissipated energy and the BBI value of ballast break index,the optimum content are obtained as follows:the optimum volume ratio of range particle size of TDA is 8～16mm is 10%,and that of range particle size of TDA is 16～25 mm is 15%.It is concluded that the effect of large particle size TDA is better when it is applied to the railway ballast bed by comparing the macro mechanical indexes of two kinds of particle sizes TDA under their respective optimum content.(7)When the TDA volume ratio is 0% and 10%,the maximum abrasion depth of the obviously degraded ballasts is 2.5～8.4 mm and 2.9～4.1 mm respectively.The average maximum abrasion depth is 4.650 5 mm and 3.512 0 mm respectively,and the overall average abrasion depth is 0.244 5 mm and 0.176 9 mm respectively.It can be seen that adding TDA can reduce the average maximum abrasion depth of ballast particles and the overall average abrasion depth.The 3D sphericity variation rate of the obviously degraded ballast particles in the ballast samples with TDA also decreases,which indicates that ballast abrasion can be alleviated by adding the TDA from the micro-scale.