Study Of Subgrade Structure Design Theory And Key Technology On Heavy Haul Railway Of Large Axle Load

Currently, heavy haul railway transportation has become an important trend of the world railway development due to its large carrying capacity, high efficiency, and low cost. However, its major technical parameters such as high axle load, short distance between axles and long train marshalling lead to substantial increase in the dynamic loading threshold value of the subgrade, in the depth affected by the action of dynamic load, and in the number of times of continuous actions imposed on the subgrade respectively, which in turn intensify the dynamic interaction of the train/line system, accelerate the cumulative deformation and exacerbate the fatigue damage of the subgrade. Therefore, in comparison with the normal speed railway, more stringent requirements should be proposed for the design and construction of the subgrade bed of large axle load and heavy-haul railway. The Chinese Heavy-haul Railway Design Specification (draft for approval) has explicitly stipulated such requirements for the types of subgrade bed structure modes and the soil thickness of each structure in the selection of filling materials and control of compaction quality. Nevertheless, many problems are spotted in the Specification. Firstly, there are no mechanical analysis-based design and calculation methods and such simplification of subgrade bed structure mode constrains its adaptability in different geographical environment. In addition, the technical parameters of the subgrade bed in the Specification like the structure thickness, fillings and compaction control criteria, are proposed with reference to The High-speed Railway Design Specifications rather than in accordance with the actual conditions for heavy haul highway, thus the selection of such indicators shall be improved. Furthermore, obvious differences in the criteria for the thickness, filling materials and compaction of subgrade bed structure are detected between the technical standards of the subgrade bed structure proposed by China and those by other countries with advanced heavy haul railway technologies, and especially the surface thickness of the reinforced subgrade bed turns out to be much higher. Such discrepancies have hampered the sound development of China’s heavy haul railway technologies. Technically, whether changes in the track stiffness of the heavy haul railway pose an impact to the effect of the sleeper sharing wheel load forces or not is to be verified and analyzed. So far no schemas for live load have been prepared for the subgrade according to its own bearing characteristics as a kind of soil structure, thus the subgrade structure analysis and design are performed currently with reference to the ones for bridge structure. However, the subgrade structure obviously differs from the bridge structure in the stress characteristics and structure properties, and the original mode of live load for trains has proven to be out of date, thus researches shall be conducted to specify the mode of load imposed on the subgrade bed so as to provide a basis for the design and analysis of the subgrade structure. At present, China’s railway subgrade bed structure design theory is still in its infancy, in which systematic design methods based on the structure analysis are yet to be established. As for the heavy-haul railway, the cumulative plastic deformation and damages to the structure exert a prominent effect on the lines under the action of high-frequency cyclic dynamic load imposed on the subgrade. Therefore, the effect of the high-frequency cyclic dynamic load shall be taken into consideration in improving design theories and technologies for the heavy-haul railway subgrade so as to meet the stipulated technical requirements of strength, bearing stiffness and long term service performance. In summary, researches on the design methods and technologies for the subgrade bed structure of the large axle load and heavy haul railway are of great significance, and the results are expected to offer practical guide to the design of the heavy haul railway subgrade.After the study and analysis of design technologies of the heavy haul railway and their applications at home and abroad, research methods including theoretical analysis, numerical simulation and the on-site real train test are applied, and an integrated design and calculation method for the subgrade bed structure is proposed according to the three-factor control principle of dynamic strength, bearing stiffness and high-frequency and long-term stability. Relevant researches are performed to analyze the effect of the sleeper sharing wheel load forces under the technical conditions of the heavy haul railway, the performance of the track structure transferring load imposed by the heavy vehicles, characteristics of the dynamic action on the subgrade under the load of large axle load vehicles, and main factors affecting the spatial distribution pattern for the subgrade bed structure bearing the vehicle load. Based on the effect of the vehicle load-track system-subgrade bed structure coupling action, an analysis model and the mode and parameters for the subgrade surface bearing the vehicle load are developed. A design and calculation method is proposed for the spatial distribution pattern of the load on the subgrade bed structure of the heavy haul railway, comprehensively reflecting the loading characteristics of the large axle load vehicle and the performance of heavy track structure transferring the vehicle load. The load technical parameters are specified for the design of the subgrade bed structure of large axle load and heavy haul railway, the vertical multiple-layer construction mode and the soil thickness of each structure are optimized, and the dynamic strength of fillings on each layer of the subgrade bed structure, the technical standards for the dynamic carrying capacity and the expression of K30 are improved. The main work and conclusions are as follows:1. The Approximate Calculation Method of Sleeper share Train Load On Gauss FunctionAccording to Winkler continuous and uniform elastic foundation beam, rail vertical deflection curve shape remain unchanged, along with the train load move and moves, proposed the approximate calculation method of sleeper share train load on Gauss Function. Studies have shown that:load sharing ratio is equal to the multiplied of the sleeper location corresponding to the Gauss function distribution coefficient of subgrade reaction force and the sleeper spacing; Single train load is applied just above the sleepers, the share rate of adjacent 5 sleeper is 9.24%、24.14%、33.24%、24.14%、9.24%, the results have been verified by the field test and model test, compared with the empirical method, the precision of engineering applications have been greatly improved; With heavy railway ballasted track roadbed structural finite element calculation results comparison shows, the method is also applies to acquire the ratio of sleeper share train load under the condition of heavy railway.2. Analysis on the characteristics for Subgrade Bed of Heavy Haul Railway bearing train loadAccording to the type of rolling stock and structural features, this paper sums up the main type of locomotive vehicle load model of China’s railway. Based on the principle axial spacing of train vehicle type of live load and the main type of track structure pillow spacing in our country to match, generalized train subgrade axle type of live load about axial spacing is 2400 mm and 1800 mm under the condition of passenger train and freight train running, compares and analyzes characteristics of stress distribution based on subgrade structure under the single, double and four axle load mode. After comparing with subgrade stressunder thestandard live load for railway train(2005), especially one of it’sschemata which is called ZH(Chinese freight railway live load), as a conclusion, the key factors’major and minor relationship which affects the subgrade stress distribution characteristics is axle load>axle distance>axle number; In addition, this paper presents four axis load model for overloaded railway bed’s structure analysis and calculation model for integration oftrackway, compared with uniaxial load,the max difference is about 14.8kPa, the depth of main influented area is 1.5～2m from subgrade surface, and the relative error is 67%～87%, which increases with the increase of depth; After comparing with measured value, the theoretical calculating valuefor both the max peak value of dynamic stress in subgrade surface(the limit dynamic stress of subgrade surface) and dynamic stress in subgrade(the frequently encountereddynamic stress of subgrade) is a little bigger,which means it should be safefrom the perspective of structure design.3. Numerical Calculation and Analysis for Subgrade Bed of Heavy Haul RailwayA finite element model of roadbed structure with ballast track was constructed with ABAQUS program. The model was simulated by linear elastic finite element in three-dimensional space. The paper studied the distribution rule of train load on the sleepers under conditions of various top-filling of subgrade bed (class A group of packing or graded broken stone), various axle numbers (single, double and four axle wheels), various loacations of wheel load (right above the sleeper for single wheel load, at 1/2,1/3,1/4,1/5, or 1/6 for double sleepers), various wheel load (25t,30t,35t), and various stiffness of the rubber spring under trail (70kN/mm,80kN/mm,90kN/mm). What’s more, the dynamic characteristics of the structural stress, deformation of heavy haul railway subgrade were studied with various top-filling of subgrade bed (class A group of packing or graded broken stone), wheel load (25t,30t,35t), and axle wheel.4. Study of Subgrade Structure Design Principle and Technology Standard on Heavy Haul Railway of Large Axle LoadCombining with the experimental analysis of the cumulative plastic deformation state of existing typical subgrade filler under the action of cyclic loading, obtained the dynamic strength parameters of the filler corresponding to the different cumulative deformation state of each layer of the subgrade bed structure, or the mathematical expression of dynamic bearing capacity and the long-term stability loading threshold value based on the K30, acquired the cyclic deformation modulus of the subgrade filler in the actual work conditions after the strain modification and the cyclic loading correction to the deformation modulus derived from the K30 test which figured out the design parameters of filler states. Pursuant to the principles of engineering structure design and analysis, the control principle and calculation method of the subgrade structure of heavy-haul railway ballasted track were proposed to meet the technical requirements like the dynamic strength, bearing stiffness and high-cyclic and long-term stability, and put forward a proposal for the thickness of surface layer of subgrade with graded broken stone over bottom layer of subgrade with foundation coefficient K30 carrier changes, optimized the bed structure in the way of multi-layer constructing type in vertical direction and arrived at the structural system formed by the upper surface layer、the lower surface layer、the upper bottom layer、the middle bottom layer and the lower bottom layer. In response to the problem of capacity concerns of the graded gravel under the super axle load condition (40t), put forward strengthening measures that adopted the high-strength and water-proof asphalt concrete instead of graded broken stone as the strengthened filler of surface layer. The double-layer structure mode of "closure on the upper layer and drainage on the lower one" is proposed to meet the waterproofing and drainage requirements of the upper layer of the subgrade bed, in which the dense-framework graded gravel with weak water permeability at the coefficient of 10-4cm/s is used on the upper layer and the void-framework graded gravel at the water permeability coefficient of 10-2cm/s is adopted on the lower layer.