| Red soft rock that mainly belongs to continental red rock series from Jurassic to Neogene is widely distributed in China, simply intitule Red-sandstone granular soil. Its lithology is commonly classified as mudstone, sandstone, shaly sand, sandy mudstone, siltstone, etc. Regionally distributed Red-sandstone granular soil is largely used as embankment fillings. However, the engineering practice shows that Red-sandstone granular soil embankment generally exsists some prominent disadvantages such as poor stability, easily cracking, uneven settlement, etc. It causes negative influences to the driving comfort and safety. Strengthening technology for Red-sandstone granular soil plays a important role in the application of red soft rock fillings to embankment engineering in red beds area. In this paper, therefore, the mechanical behaviors and seismic stability of reinforced Red-sandstone granular soil retaining wall were researched deeply by in-situ tests, laboratory experiments, theoretical analysis and numerical simulations, combining with the construction of reinforced Red-sandstone granular soil embankment in highway from Xiangtan to Hengyang in Hunan province and the following projects:Study on new reinforced structure in Highway and its demonstration engineering supported by Hunan Province Transportation Department Research Project (200612) and Study on mechanical mechanism and design method of reinforced highway embankment supported by Hunan Provincial Department of Education Project (08c199). The main researches had been carried out as follows:(1) Large-scale horizontal push-shear field tests were carried out aiming at red soft rock fillings of southern Hunan province, test sites on Western Tanheng highway of Hunan province. Mechanical characteristics, deformation properties and failure mechanism of Red-sandstone granular soil embankment when acted thrust were analyzed. Simplified method for three-dimensional sliding surface of its field push-shear sample was proposed. Based on the theoretical analysis of three-dimensional thrust-sliding limit equilibrium method, the computation formulas for the large-scale field horizontal push-shear test considering three-dimensional sliding surface was inferred, and the shear strength parameters of red-sandstone granular soil were calculated. (2) Gabion mesh and high strength geogrid were adopted to strengthen red soft rock fillings. A series of large-scale triaxial tests were carried out on gabion mesh and high strength geogrid reinforced Red-sandstone granular soil with different reinforcement layers, different compaction and different water content. Microscopic reinforcement mechanism, strength and stress-strain characteristics of reinforced Red-sandstone granular soil were studied. The effects of reinforcement layers, water content and compaction on stress-strain relationship and strength properties of red-sandstone granular soil were analyzed. The reinforcing effects were evaluated by introducing the strength ratio parameter. Optimization of reinforcement scheme was put forward based on the comparisons between geogrid and gabion mesh reinforced red-sandstone granular soil.(3) Utilizing PFC3D program and discrete particle flow theory, particle numerical model of Red-sandstone granular soil reinforced with gabion meshes was builded by programming secondary development using FISH. The micro-mechanical parameters of the particle model were calibrated according to large-scale triaxial experiments, tensile tests of gabion mesh and pull-out tests. On this basis, reinforcement micro-mechanism and micro-mechanical properties of reinforced Red-sandstone granular soil were studied, the impacts of confining pressure and reinforcement layers on its micro-mechanical properties were also analyzed. Its displacement distribution and shear zone development law were found at different strain levels. All of these provide nature method for understanding reinforcement mechanism of reinforced Red-sandstone granular soil.(4) Comprehensive observations of the test section of reinforced Red-sandstone granular soil retaining wall in western Tanheng highway were accomplished by various testing elements. The distribution pattern such as deformation of the wall face, soil pressure of the wall back, tensile strain of the gabion mesh layers was obtained by the observations. Using FLAC3D program, the three-dimensional coupling numerical model of stone cage wall face, filling, gabion meshes and interfaces was builded to calculate and optimize the stress and strain of the retaining wall in different influencing parameters, and some controlled design parameters of Red-sandstone granular soil embankment reinforced with gabion meshes which should be taken into account were presented. (5) Numerical simulation using dynamic analysis module of finite difference procedure FLAC3D was conducted on a full-scale gabion-reinforced Red-sandstone granular soil retaining wall subjected to horizontal seismic, and the numerical model wall was validated by comparison of the numerical and the measured seismic experimental results. The dynamical responses of the retaining wall which subjected to seismic waves with different amplitude and reinforced with different vertical spacing reinforcements, such as horizontal and vertical displacement, acceleration and failure mode were analyzed. On the basis of the above analysis, some measures and suggestions such as the seismic-induced displacement control standard, the reasonable vertical spacing of the gabion mesh reinforcement in different seismic region, and the calculation formula for earthquake acceleration amplification factor were proposed for the seismic design of gabion-reinforced soil retaining wall.(6) Based on the assumption of deform rupture shape of the wall which is directed toward the extensibility and inextensibility of reinforcements, a limit equilibrium method identified as horizontal slice method was presented to analyze the internal stability of reinforced retaining wall subjected to horizontal and vertical seismic loads. Formulas about the required tensile force and length of reinforcements to maintain the internal stability of the wall were deduced. In this horizontal slice method, the sliding wedge of reinforced retaining wall was divided into a number of horizontal slices. The effects of variation of parameters such as backfill soil friction angle, horizontal and vertical seismic acceleration coefficients on the stability of the reinforced soil wall were studied. It offered practical method for seismic design of reinforced retaining wall.(7) Considering the effects of reinforcements on soil lateral stiffness and reinforcement-soil friction, the reinforced embankment was simplified as a multi-mass system which was connected by springs, dampers, and reinforcement-soil interfaces. Based on the d'alembert theorem, dynamic equation of the multi-mass system under horizontal seismic was deduced. The dynamic system represented by the dynamic equation was expressed by the state space method. According to the state equation, a numerical simulation model was established using SIMULINK so that the dynamic response of reinforced embankment can be obtained. It offered simplified method for earthquake response prediction of reinforced retaining wall.
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