Analysis On Loess Collapsibility Of Zheng-Xi High-Speed Railway

Loess collapsibility often does great harm to project construction activities, may causing soil slope collapsing and sliding, the railway roadbed subsidence and other undesirable phenomena of engineering geology, which may produce highly destruction to project. Zheng-Xi high speed railway is the first (350 km/h at best ) high speed railway construction in loess area in China, with high subgrade settlement requirement that the final deformation is controlled within±1.5 cm In order to improve the quality of road construction, collapsibility of loess along the high-speed railway needs further study and analysis, so that we master its engineering properties.First of all, due to the wide distribution of loess and its serious collapsibility along high-speed railway , field large-scale water immersion tests have been carried out in typical site in addition to laboratory collapsibility test. Collapsibility deformation changes with time and the laws are manifested in the following phases: the initial steady section, the steep section, the mid-flat section before stopping water as well as steep section and the last gentle section after stopping water; the collapsibility rate changes from small to large and then to small, becoming more and more stable during immersing water ,in stopping water stage, the rate changes from large to small and then becomes stable; the settlements of the measuring points are different in test pit, the general trend is the farther from the central pit, the smaller the settlement with the largest settlementof the pit's center; With the increase of immersion time, cracks develop from the nearby to the farther, and the local to the whole, and from slow to rapid development process. Secondly, the loess microstructure changes before and after collapse were qualitatively analysed though SEM images, and based on fractal theory a quantitative analysis of the loess microstructure changes was carried out to the master pore characteristics of the sample ,showing that total number of pores increase, the increase is mainly accounted for small pores, and the number of large and medium-sized pores decrease, a decrease in the total pore areas; also discussed fractal dimension changes of the sample before and after collapse. several issues about image processing technology and fractal theory in loess microstructure decription , including the choice of better amplification factor of SEM image, image processing technology, the extraction and calculation of pores, the scope of the image, the number of samples and so on. Based on the above analysis of the impact factors of loess collapsibility and the mechanism, we conclude that the loess collapsibility must be with the porous and loose structure, and none water resistant inter-linking of loess grain. Finally, in order to prerdict and evaluate loess collapsibility, Elman model was established based on field water immersion test, the model results showed higher prediction accuracy; based on laboratory test and comprehensive analysis main effecting factors of the collapsibility, four indexes closely related with collapsibility were selected, namely, the natural water content (w), dry density (ρ_d), void ratio (e₀), plasticity index (I_P), ANFIS model and multiple linear regression model were established for forecasting collapsibility coefficient; By comparison of the results of the two models, it shows that ANFIS model is superior to the other one.