| With the national development focus shifted to mountainous regions of the Southwest, the infrastructure construction is in great demand. High-fill and deep-cut are commonly used in highway construction due to the limitations of topographical feature and geological condition of the mountainous area. Also, concerning to the local conditions, investment-saving and environment protection, a great amount of soil-rock mixture is used in foundation filling. However, it is difficult for design and construction in light of its complicated mechanical characteristics caused by the variance and the uncontrollable characteristic of particle and the effect of rainfall to its engineering properties. Thus, in order to provide some guidance and help for the relevant theoretical research and engineering construction, the research about the mechanical properties of rock-soil mixture and the according foundation stability analysis were carried out by this paper based on the support of the National Natural Science Foundation (51078137).Firstly, a series of lab large-scale direct shear test was conducted through Geotest direct shear apparatus based on orthogonal experimental design. These tests took into account variable factors such as water content, soaking time, wetting/drying cycles, stone content, rock property and soil property, etc. Tests results indicated that the shear strength of soil-rock aggregate mixture was caused by mutual embedding, mesh and friction of aggregated rock. Also, it was shown that rock content is the single primary factor for influencing the strength property of rock-soil aggregate mixture; with the rise of which of the test specimen, the internal friction angle increases linearly. In3varied water effect situations, water content has the most significant impact for the strength of soil-rock aggregate mixture; it is also highly influential for the shape of shear stress-shear displacement curve, and the shear strength will not continue to weaken when the specimen was soaked for more than48hours or under4wetting/drying cycles. The shear strength of specimen affected by rock when its rock content is not less than55%.Secondly, large-scale triaxial tests were conducted under different stress path: conventional triaxial tests, constant p tests and constant stress ratio tests to study the mechanical deformation properties of soil-rock mixture. Rock content is primary influencing factor for the shear strength of rock-soil mixture while the change of stress path has little influence on its shear strength. Confining pressure and stress path are two external factors for the volumetric strain characteristics of soil-rock mixture. The specimen presented dilatation under low confining pressure and contraction under high confining pressure. Besides, the εv～ε1curve differs under different stress path. Rock content and degree of compaction are two internal factors for the volumetric strain characteristics of soil-rock mixture. Dilatancy becomes prominent with the increase of rock content or degree of compaction and vice versa.Thirdly, a number of soil-rock mixture foundation were filled in a special model tank, and the horizontal push-shear test and vertical shear test were carried out under normal condition, rain-fall soaked and dry-wet cycling. The soil-rock mixture foundation would show structural both in push-shear test and vertical shear test under normal condition; and the dry-wet cycling had little influence on the fracture surface and shear strength of the foundation; by contrast, the rain-fall soaked had great influence on the fracture surface, shear strength, p-s curve, failure mode and ultimate load of the subgrade, especially, the samples’c, φ value would greatly reduced.What’s more, based on the discussion on the shear deformation characteristics of the shear process and shear deformation mechanism of soil-rock aggregate mixture, a shear damage model reflecting the characteristics of residual strength after failure was established by analyzing the micro-mechanical upon shear element of the structural plane and interface. In this model, the shear element was divided into two parts: damaged material and undamaged material, the shear load was supported by the two parts together, and the damage material provides bearing capability for residual strength; and according to the comprehensive analysis of method for shear element strength, a new statistical damage evolvement model was developed by introducing the statistical damage theory and supposing the strength of shear element obeyed Weibull distribution. Moreover, a new statistical damage constitutive model which can simulate the whole shear deformation process of soil-rock aggregate mixture was established, which not only could well reflect the characteristic of strain-softening of soil-rock aggregate mixture, but also could describe the impact on shear deformation process by normal stress and reflect the deformation characteristics of residual strength.Then, based on the intensive study on soil-rock mixture foundation deformation and mechanics mechanism, the stress exposed by the foundation was divided into two parts:isopiestic stress and deviatoric stress, the nonlinear characteristics of the subgrade deformation was considered, the idea of step-loading and the layer-wise summation analysis theory of foundation settlement were introduced, and the incremental generalized Hooke’s law and the incremental Duncan-Chang constitutive model were adopted, thereby, the subgrade displacement analysis model with the initial porosity ratio, poisson’s ratio, cohesive strength and internal friction angle as the model parameters was established. Then, the method for determining the parameters of the model is discussed via the back analysis based on the adaptive genetic simulated-annealing algorithm. Compared with the other existing methods and the computation of an engineering example, it shows that the proposed method is reasonable and preferable, which could meet the needs of the construction.Finally, the bearing mechanism and failure mode of soil-rock mixture filled foundation was deeply analyzed on the base of existing research. Meanwhile, the new unilateral sliding failure mode considering the asymmetry characteristics of foundation was established by slip line field theory. Then, based on above research with the upper limit analysis theory, a mobile permissible velocity field and the ultimate bearing capacity calculation formula of soil-rock mixture subgrade were developed, a new method to determine ultimate bearing capacity of foundation is put forward by use of the sequential quadratic programming optimization algorithm. Through the calculation of bearing capacity of foundation in different gradient and compared with the result of the upper bound finite element method based on nonlinear programming, it is shown that the proposed method is reasonable and feasible. |
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