Mechanical Properties Of Plain Soils And Improved Soils Subjected To Heavy-haul Train Load

Heavy-haul railway traffic has been considered to be the main trend of the development of railway freight by the international community due to its characteristics of greater quality of traction,larger axle load and higher annual traffic volume.In order to meet the requirements of increasing freight traffic volume year by year in our country,heavy load has become the inevitable trend of national railway freight transport.However,the design theory,disaster analysis and prevention techniques for heavy-haul railway are significantly lagged behind the requirements of rail freight traffic in China,resulting in frequent occurrence of differential settlement of embankment,and bringing the rail freight and the safety of the people associated in threaten.In the present paper,the high embankment filling soils in the position of K44+522 in the third section of Bazhun heavy-haul railway were taken and reinforced with randomly distributed discrete polypropylene fibers and the stabilizer.The variations of static properties,dynamic properties and stabilities of plain soils and improved soils with various influencing factors were investigated using static and dynamic triaxial compression tests.Then,a design suggestion of subgrade bed structure of heavyhaul railway was proposed,and an appropriate incremental elasto-plastic constitutive model was developed.The influencing factors for plain soils involved water content,compaction degree and loading frequency.For fiber reinforced soils,fiber content and fiber length were taken into account.And,for stabilized soils,stabilizer content and curing time were counted in.Furthermore,confining pressure level was considered for all types of soils.The details of main contents and progress of the present paper are listed as follows:(1)Common static triaxial compression tests(CTC)were carried out to systematically investigate the deviatoric stress-axial strain relations of plain soils and improved soils.The applicability of Duncan-Chang model to the strain hardening curves of plain soils and fiber reinforced soils was verified,where initial tangent modulus and ultimate deviatoric stress were selected as model parameters.And,the applicability of Prevost strain softening model to the strain softening curves of stabilized soils was verified,where initial tangent modulus and residual strength were selected as model parameters.The variations of model parameters with various influencing factors were analyzed.Notably,the existence of optimum fiber content and optimum fiber length were found.Based on the aforementioned investigation,empirical models of model parameters for plain soils and improved soils accounting for various influencing factors were developed,respectively.(2)The variations of backbone curves,dynamic shear moduli and damping ratios of plain soils and improved soils with various influencing factors were investigated using multi-stage loading dynamic triaxial tests.The applicability of Hardin hyperbola model to the backbone curves of soils under various influencing factors was verified,where the maximum dynamic shear moduli and ultimate stress amplitudes were selected as model parameters.The variations of failure stress,maximum dynamic shear modulus,ultimate stress amplitude,and damping ratio with various influencing factors for plain soils and improved soils were formulated as empirical models,respectively.For fiber reinforced soils,the moderating effects of confining pressure on fiber contribution were accounted for in the models.Notably,the relation between optimum fiber content and confining pressure level was formulated with an empirical expression.(3)A series of dynamic triaxial tests with long-term constant amplitude repeated cyclic loading were conducted to investigate the dynamic stabilities of plain soils and improved soils.More concerns were focused on the variations of the relations between cumulative plastic strain and cyclic loading number,dynamic strength and critical dynamic stress of the three types of soils.Critical dynamic stresses were obtained by numerical fitting,and the influences of various influencing factors on the critical dynamic stresses of plain soils,fiber reinforced soils and stabilized soils were formulated as empirical models,respectively.Furthermore,normalization properties of dynamic strength with respect to static strength were analyzed and normalized dynamic strength models corresponding to the three types of soils were developed,respectively.The model parameter,critical value of normalized dynamic strength,can be used to determine the design value of dynamic strength,serving as a design basis,when static strength is known.One kind of multi-layered strengthened subgrade bed structure is developed,where surface layer consists of strengthened layers of stabilized soils and fiber reinforced soils,bottom layer are composed of transition layer of fiber reinforced soils and plain soil layer.The applicability of the structure under heavy-haul train action of 30 t axle load were discussed.(4)The strengths and weaknesses of many constitutive models of soils were discussed so as to find an appropriate one capable of reproducing the inconstant Masing effect and Ratchet effect of plain soils and improved soils under repeated cyclic loadings.Using the merits of subloading surface hardening rule and doubleyield-surface model composed of shear and volume yield surfaces,the subloading surface hardening rule based double-yield-surface model was developed,and the determination methods of model parameters were proposed.Moreover,taking the advantage of the secondary development function of large finite element analysis software ABAQUS,the constitutive model involving the Masing effect and Ratchet effect of plain soils and improved soils was programmed as a VUMAT subprogram called CLDYNA.The subprogram was used to reproduce the testing cases,and the simulation accuracy of CLDYNA to the cumulative plastic strains of plain soils and improved soils were verified.