| China's southwest, northwest, southeast and central-south regions have a wide range of distribution of red beds. As a special material, red-mudstone shows different engineering mechanics nature with the general subgrade fillings because it's intensity is low, and it's particles are easily soften and collapsed to meet water. So it faces with a serious lack of quality fillings for railway construction in these areas. At present, China's railway construction is in the development of high-speed, and "state railway network and long-term planning" carrys out some adjustment which plans that in 2020, operating mileage of national railway is to be extended to 130,000 km, with an increase of 29,000 kilometers. Up to now, construction of 75,000 kilometers has been completed, with 54,000 kilometers left to be built, and a considerable number of them will be across the red beds areas. If red-mudstone can be used to fill subgrade in high-speed railway, we can save a lot of projects investment at the same time to reduce a large number of abandoned side, and protect the environment.However, there is a big controversy for the applicability of using red mudstone to fill railway subgrade in engineering field. The relative shortage of engineering properties research for red-mudstone material, and unreasonable use of the subgrade structure and technology indicators make the embankments arise some diseases, such as subsidence, cracking, collapse of the slope, and so on. Because of the high strength and deformation requiments of subgrade for high-speed railway, the fillings should have some characteristics, such as good compaction, big density, high strength, small deformation, stability, durability, economic and so on. So it's less for red-mudstone to be used as fillings of subgrade in high-standard railway. So some people believe that red-mudstone can't be used to fill subgrade in high-speed railway because it's water stability is difficult to be solved. Some people suggest that only improved red-mudstone can be used. At the same time, some people want to attempt to solve the applicability and technology for using red-mudstone to fill subgrade in high-speed railway through detailed study on the characteristics of red-mudstone, reasonable design and strict construction under the guidance of the theory of subgrade design. Under this situation, the scientific research projects, "experiment study on key technology for ballastless track under line project in Suining-Chongqing line" and "experiment study on dynamic performance of red-mudstone subgrade bed in Dazhou-Chengdu line" are carried out.In the past, the research of red-mudstone was mainly focused on engineering geology, rock structure characteristics, rock mechanics properties and weathering, but the road performance of this material was less studied. In recent years, the road performance studies on red-mudstone have emerged one after another, which include the crack properties, mineral composition, cement materials, mechanical properties, permeability and compaction process. The studies on dynamic properties of red-mudstone and being used to fill the subgrade bed are blank. Combined the above two scientific research projects, and based on detailed investigation of present research situation for red-mudstone, the research work is carried out. First, compaction, mechanical and water stability characteristics of red-mudsone are researched. Second, preliminary design of red-mudstone subgrade is made under the guidance of the theory of subgrade design and domestic or overseas experience. Through centrifugal model test, cyclic loading test at the scene, subgrade settlement observation test on-site and numerical analysis, the the dynamic responses, settlement law, and the stability of red-mudstone subgrade are detailedly researched. The results of them are used to prove and improve the design technology for red-mudstone subgrade. Based on the experimental results and experience, the key design technology and construction technology for red-mudstone subgrade are put forward. I hope all of these could provide a strong reference for the popularization and application of red-mudstone in high-speed railway.All of them have made a number of innovative achievements, including the following major areas:1. Through making research on historical application and research situation of red-mudstone in railways and roads areas at home and abroad, a detailed summary of the causes of red beds, distribution, mineral composition, chemical composition, structure properties, crack, expansion, contraction, mechanical properties of red mudstone, subgrade diseases and mechanism of them is made. On the basis of investigation results and the properities of subgrade engineering, targeted, a series of research new works have be carried out.2. The research.on physical and mechanical properties, such as compaction, strength and deformation characteristics, and water stability of representative red-mudstone samples from Suinig-Chongqing line and Dazhou-Chengdu line is made. The results indicate that compared to general railway fillings, the compacted red-mudstone has following features: big density, high strength, small deformation, soften and weak expansion to meet water. So we should take some measures to make use full of it's advantages and avoid it's shortcomings.3. According to the properties of subgrade bed in high-speed railway, dynamic strength, critical dynamic stress, cumulative deformation characterastics of red-mudstone of compacted red-mudstone are studied using 3-axis vibration test method for the first time, and in accordance with the test data, the dynamic accumulative deformation prediction model of red-mudstone is created. They lay the foundation for the dynamic design for red-mudstone subgrade.(1) In the condition of 25 kPa and 50kPa confining pressure, the critical dynamic stress of red-mudstone is 50kPa and 175kPa respectively; the dynamic strength increased with confining pressure and decreases with the time increases; the dynamic modulus decreases with strain increasing and confining pressure decreases; the damping ratio increases with the strain increases, and decreases with the confining pressure increasing at same strain. The actual dynamic stress of subgrade is less than red-mudstone's critical dynamic stress, which indicates that the compacted red-mudstone will not destroy and it's deformation will be stable.(2) Considering the influences of density, moisture content, lateral stress, dynamic stress amplitude and frequency for accumulative deformation of compacted red-mudstone, the dynamic accumulative deformation model is put forward. Based on the predict model and dynamic FEM, the deformation of the red-mudstone layer (1.7m thick) of subgrade is 3.94mm. 4. Under the guidance of the design theory of subgrade in high-speed railway, and combined with the characteristics of red-mudstone, new structures and design parameters of red-mudstone subgrade for ballastless track and ballast track, which are proved reasonable by experiment results. The red-mudstone subgrade bed for ballast track composes of 0.6m thick graded gravel, 0.2m thick sand with a composite geomembrane layer, 1.7m thick red-mudstone, and red-mudstone embankment. For ballastless track, the structures could be:①the subgrade composes of 0.7m thick graded gravel, 2.3m thick AB group fillings, and red-mudstone embankment. This type of structure can be used for plate ballastless track or sleeper buried ballastless track.②the subgrade composes of 0.4m thick graded gravel, 2.3m thick AB group fillings, and red-mudstone embankment, or composes of 0.7m thick graded gravel, 0.2m thick waterproof layer, and red-mudstone embankment. This type of structure can be used for double block ballastless track.5. The paper puts forward to a new type of structure and design parameters of red-mudstone subgrade that composed of 0.6m thick graded broken stone, 0.2m thick sand included a composite geomembrane layer and 1.7m thick red-mudstone. By using cyclic loading test on-site for the first time, the size and distribution of dynamic response such as dynamic stress, displacement, acceleration of red-mudstone subgrade in a variety of working condition are studied. The experimental result is similar to which calculated in dynamic finite element method.(1) As a major excitation equipment at the scene, ZSS50 cyclic loading equipment can meet the requirements of loads with different frequencies and amplitude which can simulate the dynamic actions of the train in different axle load of trains and at different speeds on track and subgrade .It is an important equipment to study the dynamic character of track and subgrade.(2) The maximum dynamic stress, dynamic displacement, and acceleration of red-mudstone subgrade are 43.89kPa, 0.755mm and 6.201 m/s~2; that of the red-mudstone layer are 20.58kPa, 0.529 mm and 4.166 m/s~2; At the same frequency, the axle load of trains is the main factor influencing the dynamic performance of subgrade and the impact of rainfall is limited.6. Using methods of centrifugal model test, subgrade settlement observation on-site, cyclic loading tests at the scene and numerical analysis, the different types of deformation of subgrade such as compaction settlement, cumulative settlement, are analyzed. The compaction settlement of red-mudstone subgrade in gravity is caused by multiple factors such as red-mudstone coarse granule disintegration, rheological change, the consolidation of unsaturated soil, lateral deformation and so on. The method of percentage (the ratio of compaction settlement quantity and the thickness of the subgrade) is suggested to analyse the compaction settlement of red-mudstone subgrade. According to the results of red-mudstone subgrade and field subgrade settlement observation of improved soil and centrifugate model test, the compression ratio of red-mudstone subgrade is 0.2-0.46%o, the compression ratio of improved red-mudstone subgrade is 0.12-1‰, the compression ratio of red-mudstone layer is 0.67‰, the compression ratio of improved red-mudstone layer is 0.65‰in the condition of design and construction technology of subgrade for high-speed railway at present. In the design, 1‰can be as the compression ratio of red-mudstone subgrade for high-speed railway, and the time for subgrade settlement tend to stablity may be 5 months to 7 months.7. The depth of the soften soil of subgrade caused by atmosphere rainfall or flooding is directly affected by the compaction quality of red-mudstone soil. If less coarse, big density, small porosity, the soil permeability will be small, and the atmosphere rain can only impact limited depth of subgrade. The expansion rate of red-mudstone soil is smaller, and it will decreases when the load increases. As taking some waterproof and drainage measures to red-mudstone subgrade, soil and rain are separated frome each other, so we can make the conclusion that the soften and expansion characteristics of compacted red-mudstone has a very limited impact for subgrade in high-speed railway.8. Considerating the collapse and soften properties, compaction quality, the further broken of coarse particles caused by construction machines, period and other factors, the standard of red-mudstone particle size in construction is identified, that is, the largest particle size after collapse or broken should be less than 15cm. Considerating the compaction, strength, slope stability, settlement performances, we conclude that the strength and deformation characteristics of red-mudstone can meet the requirements of embankment in high-speed railway. Considerating the dynamic strength, dynamic accumulated deformation, dynamic responses of red-mudstone subgrade bed, as well as the subgrade bed structure, we conclude that the dynamic characteristics of red-mudstone soil can meet the requirements of subgrade bed in high-speed railway. Considerating soften and expansion characteristics of red-mudstone, the waterproof and drainage measures of subgrade, we conclude that the water stablity characteristics of red-mudstone soil will impact red-mudstone subgrade a little. Finally, we can believe that using red-mudstone to fill subgrade in high-speed railway is appropriate.9. The compacted, strength, water stability characteristics of the improved red-mudstone are studied and the work performance of the improved red-mudstone subgrade is researched by the methods of centrifugate model test, subgrade settlement observatio, cyclic loading test on-site. The results indicate that the improved red-mudstone has the characteristics of high strength, lower deformation, good water stability, uneasy softening in the water, and can satisfy the requirements of strength, stability and durability. It indicates that using red-mudstone as the fillings of subgrade in high-speed railway is feasible.10. According to the characteristics of red-mudstone as well as the improved soil and the engineering experience, the paper sums up the key technologies of design and constructon of red-mudstone subgrade in high-speed railway which includes some contents related with design and construction, such as selection principle of fillings, quality standard for compaction, structure and shape of red-mudstone subgrade, drainage, reinforcement, slope protection, construction techniques and so on. All of them are proved reasonable by various tests. The design and construction technology of red-mudstone and it's improved soil subgrade provide a strong reference for the popularization and application of red-mudstone in high-speed railway, and have significant social and economic benefits. The results reach internationally advanced level.
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