Experimental Study On Typical Filler For Passenger Dedicated Railway Subgrade Roadbed By Static And Dynamic Triaxial Test

As an important part of the basis of passenger dedicated railway unballasted track structure, the subgrade roadbed should be designed and constructed in accordance with a high standard of earth structure, and it requires more request than in the past, such as the roadbed fill material, compaction standard, deformation control, thus it should have sufficient strength, stiffness, uniformity and long-term stability. So research about physical-mechanical property of the filler of subgrade roadbed is urged. The paper shows the physical and mechanical parameters of the filler of subgrade roadbed based on a series of comprehensive laboratory tests,study on effects of the dynamic-static strength and cumulative plastic deformation on physical properties and load status, base on test and data analyse,the content study and conclusion are following:1) The test was made on the roadbed fill material, including the particle size analysis test, relative density test, compaction test, compression test and permeability test. Obtained results reveals following facts: the main fill materials that used for the subgrade roadbed of unballasted track structured have a good grading as per the specification, including the well graded gravel and A&B group packing. The permeability coefficient of filler K>i×10^-4cm/s, being a free drainage and non-sticky coarse grained soil. It is can be compacted, property large-scale compaction test results more reasonable than standard compaction. Compression coefficient increases with the increasing of compactness, the compressibility of the sample will increase after immersion in water. Critical hydraulic gradient increased whereas permeability coefficient decreased with the decrease of compactness.2) According to the direct shear test and triaxial compression test, the paper studies the properties of the two kinds of samples, discusses compactness, water-bearing state and drainage condition which affect its shear strength. The initial researchment is: its shear strength increased with the increased of compactness and confining pressure, the cohesion is increases significantly and the variation of inner friction angle is not apparent. Shear strength index of direct shear test is larger than triaxial compression test. The variation of shear strength index is not apparent when the low degree of density and repeated shears. The cohesion as gradually decreased as cutting frequencies increased even die out completely, the inner friction angle decreases slightly, when the high degree of density and repeated shears. Undrained shear strength larger than drained shear strength, when pressure is not evident(σ₁< 350kPa),because of the unsaturated soils and artificial compacted soils has the suction and negative pore water. The shear strength and initial secant modulus (ism) of samples with the optimum water content are larger than the one after immersion in water, under drained conditions, as well as the cohesion decreased and the inner friction angle is not apparent. The shear strength of different kinds of well graded gravel with different compactness is more than A&B group packing when pressure is not evident(σ₁<120kPa).3) By means of cyclic triaxial experiments, cumulative plastic deformation curves are divided into three regions in the semi-logarithmetic coordinates: elastic region, yield region and damage region. By assumption and reasonable predigestion, there is a threshold cumulative plastic deformation curve between yield region and damage region, the slope for the curve increases linearly with stress cycle number, its orresponding dynamic stress amplitudes is called ultimate dynamic strength. According to the above method, there is a threshold cumulative plastic deformation curve between elastic region and yield region, the slope for the curve is an constant at the beginning, the cumulative plastic deformation decrease gradually after a certain number of times of loading, then deformation gradually turned to be stable later on, it called equi-strain rate in the semi-logarithmetic coordinates failure criterion, its orresponding dynamic stress amplitudes is called critical edge dynamic strength. Under dynamic loading which is less than critical edge dynamic strength, it is considered that specimen deformation consists mainly of elastic deformation and only small part was the cumulative plastic strain, moreover deformation reaches stable quickly that satisfy the post-construction settlement exacting requirements of high-speed railway unballasted track structure. With critical edge dynamic strength, a design method of unballasted track structure subgrade on control strain increment is advanced. The idea is to control accumulative effect such as accumulative deformation subgrade. When dynamic stress amplitudes is close to critical edge dynamic strength, sample structure destruction was not happen, this definition as distinguished from the damage in general.4) This paper studies the properties of the two kinds of samples, discusses compactness, water-bearing state, frequency of loading and initial axial pressure action which affect its critical edge dynamic strength by using the dynamic triaxial test. The experimental results indicate that: its critical edge dynamic strength increases with the increased of compactness and increases linearly with confining pressure, the samples with the optimum water content are larger than the one after immersion in water, but at a modest rate(about 13%- 18%). Critical edge dynamic strength of well graded gravel and A B group packing respectively increases 51% and 25% , when frequency of loading grown from 1Hz to 5Hz. To exert a initial axial pressure upon the sample is equivalent to anisotropic consolidation condition cause the dynamic strength is higher. The dynamic-static strength ratio of samples with the optimum water content is within 0.272-0.415. the tests indicate that to compare with the critical edge dynamic strength of well graded gravel which immersed in water, the ultimate dynamic strength is about 1.38 to 1.60 times, the dynamic-static strength ratio is within 0.379-0.478.