Study On Compressibility And Consolidation Of Unsaturated Soil And Reinforced Technique On High-speed Railway

Soil mechanics based on saturated theory, which has already been gradually maturing and widely applied to engineering practice. Compared with saturated soil, distributing scope of unsaturated soil is wider in nature. Unsaturated soils have been also applied on engineering practice more widely, which engineering menchanic characteristics are more complicated. However, the theory system of unsaturated soil is still not quite perfect. Engineering applications are also limited. Specailly, because of experimental condition's restriction, researches on unsaturated soils are in the beginning stages in our country. The researches on unsaturated soil mainly included consolidation theory and strength theory. In geotechnical engineering, these two parts controlled distortion and stability for rock and soil mass. Design and construction on high speed railway declined to control settlement and distortion, and satisfied strength and stability requests of system. In order to ensure the safety and smooth degree, the current norms have made explicit and strict rules for post construction settlement of high speed railway. Therefore, according to self-characters, it's very important to establish rational consolidation theories and applicable computation methods.Considering the fact that four phases composite and individual characteristics of different component as well as their differences of saturated soils, the difference between pore-air pressure and pore-water pressure on water-air interface constituted matric suction in addition to the net stress which is imposed on soil skeleton. The saturation degree decided the distribution form and movement rule of pore-air and pore-water in soil, and then influenced the matric suction. On the macro view, the saturation degree directly or indirectly decided consolidation and compress characteristics and permeability. According to existing saturated theory, this text illustrated the consolidation and compression of unsaturated soil in relation to water-soil characteristic through many experiments and computation for deep unsaturated soil (silty clay and silt), which widely distributed along JiaoZhou-JiNan passenger dedicated railway.We researched the permeability coefficient and consolidation parameters of unsaturated soil. Comparing the centrifugal model experiment and long-term settlement observation results, author analyzed the foundation settlement characteristic and stress transfer mechanism, and put forward settlement computation theory applicable to unsaturated soils (silty clay and silt).The textual research work mainly included several contents as follow:(1)Generalizing previous research results of unsaturated soil, author put forward new thoughts and new methods adapting to settlememnt computation and analysis of unsaturated soil foundation for high-speed railway.(2)Reclassified the unsaturated silt and the silty clay through consolidation test and consolidation test under controlling matric suction.We specially researched water-soil characteristics and deformation characters toward unsaturated soils. Comparing the characters of saturated soil, predicted many important parameters, which permeability coefficient and consolidation coefficient influenced the relation between foundation settlement and time. Considering the air diffusion and the weakness of not easy measure, this text brought forward simplified consolidation equation for unsaturated soils, then revised the curves of foundation settlement to come close to engineering practice. On this condition, took advantage of finite element program to analyze the settlement deformation and stress distribution of unsaturated soil under different technique. At the same time, we optimized the subgrade filling technologies and construction time planning.(3)Centrifugal model experiment adopt filling in layers and one-off filling to research the settlement deformation regularity and loading transfer mechanism toward untreated foundation, dynamic compacting foundation and reinforced by cement mixing piles.Results showed, above four models'settlement have achieved more than 70% of total settlement in filling stage and placement stage, embankment settlement have been stable after placing 1.5 years.The embankment settlement by filling in layers is less than one-off filling, which settlement of embankment and foundation for high-speed railway could reduce settlement by controlling the speed of filling and optimizing the filling technology. Centrifugal model experiment respectively tested base center stress and stress under subgrade shoulder, the base stree distribution and additional stress attenuation for double-layer foundation are same with untreated foundation. The measured base center stress is intermediate between proportion load and uniform load, which are relevant to aspect ratio. Combining elastic embankment method, centrifugal model experiment into computation results by Plaxis Element Program, modified calculation methods for the base center stress and the addition stress. Using modified formula calculated foundation settlement solved the solution of Boussinesq was greater than the measured value. At the same time, we could shorten the construction time and reduced the cost of disposing foundation.(4)According to the properties and engineering practical requests of JiaoZhou-JiNan passenger dedicated railway, geotechnical parameters are measured by plate loading test, pressure-meter test, static cone penetration test, standard penetration test. Combining the long-time settlement observation results, author contrasted the geotechnical paramaters between the in-situ tests and laboratory tests, analyzd kinds of in-situ tests'adaptability. For reducing the structural movement and suction fluctuation on foundation in test processes, author presented appropriate in-situ tests and geotechnical parameters toward unasaturated silt or silty clay. Experiment results showed the results of static cone penetration test accorded with value of practice。Beacause standard penetration test has low cost and operational, experiment data have perfect correlation, comprehensively and truly reflected the geotechnical characters of unsaturated soil. In this text, analyzed the datum of standard penetration in typical construction poins, and bought forward the modified fomula for unsaturated silt and silty clay in JiaoDong peninsula and north china section. Above conclusions not only provided wide foreground for the application of standard penetration test on the in-situ test technology for unsaturated soil, but also perfected in-situ tests for the unsaturated soil foundation of high-speed railway.(5)The field long-term observation analyzed the characteristics of settlement and distortion though monitoring vertical settlement, horizontal displacement, deep settlement and pore water pressure.Then author ensured the thickness of compressive layer and inspected the reinforcement effect. At last, validate the settlement computation theories and post construction settlement prediction methods. The observation results showed, the seetlement account for 80%～95.8% of total settlement. The settlement and lateral displacement of unsaturated soil evidently reduced after reinforced by replacement, dynamic compacting and cement mixing piles. Integrity restriction and anti-deformation capability has been enhanced. The subgrade settlement is uniform. And the embankment has favorable stability. Comparing to traditional Boussinesq theory, the settlement of natural foundation and double-layered foundation are close to practice value with modified base center stress and additional stress.(6)Synthesising above test fruit and calculation analysis, inspected the foundation reinforcement effect for replacement, dynamic compacting and cement mixing piles. Field observation and element program computation showed, if the unsaturated foundation is reinforced by replacement and dynamic compacting, the bearing capacity and compression modulus has been enhanced. Therefore, the settlement decreased, but post settlement increased.On the opposite, if the unsaturated foundation reinforced by cement mixing piles, the speed of settlement is slow, and the post settlement is small. So it's bennificial to controll the unsaturated foundation settlement...