| Unique structural characteristic of loess determines its sensitive and complex response to dynamic load such as earthquake, and study on seismic subsidence of loess is of great significance for major infrastructure and urban development in loess areas. In this paper, the properties of loess seismic subsidence have been studied by GDS dynamic triaxial test system with stress control method in State Key Laboratory of Continental Dynamics.Based on the review of studies on basic dynamic characteristics, seismic subsidence experiments and its mechanism in loess dynamics, select typical loess tableland site with support of Baoji seismic zonation project, combining on-site investigation and laboratory tests, as well as qualitative and quantitative analysis, mechanism analysis and modeling have been carried out, and some valuable achievement has been made.Firstly, by means of on-site inspections, static triaxial tests, loess grain size analysis tests, X-RAY spectroscopy anlysis, the stratum structure of typical loess tableland and the physical properties, mineral composition, chemical components and static mechanical properties were studied,which provides a basis for further study.Secondly, The nature of seismic subsidence of undisturbed loess in different depths (1m, 3m,5m) were studied, imposed on equivalent cyclic loading by GDS. The result reveals that seismic subsidence curves of different depths show typical three stages:initial deformation, curved deformation and shear deformation; as the physical and structural differences between samples, seismic subsidence curves tend to have some stage of characteristics; based on the establishment of mathmatical model of subsidence curve, basic parameters were studied. The initial dynamic stress is affected significantly by water content and decreases as it increases, the critical and ultimate dynamic stress is not sensitive to water content but decreases when it is greater than the plastic limit. The basic parameters show a nonlinear relationship with void ratio, and there exists a critical one; as void ratio is larger than which, the basic parameters reduce as it increases. Based on the model analysis a seismic subsidence indicator-sensitivity coefficient was proposed to reflect the occurrence intensity, and the indicator grows with the increase of both water content and void ratio. Loess in 3.0m depth is prone to seismic subsidence with the highest sensitivity coefficient.Further more, based on the analysis of seismic subsidence influence factors and establishment of loess microstructure model, taking advantage of the basic mechanical components for model assumption and abstract, interpret the particle shear micro-slip mechanism and macro-subsidence formation, in terms of the role by both dynamic stress and water. For the first discovered seismic subsidence of loess with micro-cracks in laboratory, a micro-crack propagation mechanism are firstly put forward. A cusp catastrophe model is established based on catastrophe theory and seismic subsidence decription, which promotes our konwlege of seismic subsidence mechanism.Finally, principal component index were constructed by PCA, and based on which a PCA-BP prediction model of subsidence coefficient were established with merits of rapid convergence and high precision prediction, compared with direct BP model. Then select JiaCun tableland as typical loess site, and take layer-wise summation method to evaluate its seismic subsidence. Whenαmax≥300gal, it belongs to moderate seismic subsidence site, asαmax< 300gal minor seismic subsidence site or basically good foundation. Taking seismic microzonation results of Baoji into consideration, the site may produce minor seismic subsidence when suffering earthquakes of both 10% and 2% probability of exceedance in 50 years.
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