Study On Palaeo-soil Swelling Characteristics And Microstructure Of Yinxi High-speed Railway Zaosheng No.3 Tunnel

With the continuous development of my country's economy,the construction of high-speed railways in the western loess area has become a top priority for driving economic growth in the west The Yinxi High Speed Rail passes through the ancient soil layer in the Loess Plateau in the form of a tunnel.The engineering properties of the ancient soil are complex,and its expansibility greatly increases the difficulty of tunnel construction.In this paper,the expansive palaeosol of Yinxi High-speed Railway Zaosheng No.3 tunnel was taken as the research object.Through the data collection,laboratory test,nuclear magnetic resonance test and electron microscope scanning test,the physical characteristics,expansive characteristics and microstructure of the palaeosol were studied.The results are as follows:(1)Through the measurement of basic indexes and the analysis of mineral composition,the physical property indexes and mineral components of ancient soil were obtained,and the swelling of ancient soil was discriminated.(2)By carrying out unloaded swelling rate test,swelling force test,loaded swelling rate test and diy-wet cycle test on ancient soil,the swelling characteristics of ancient soil under different water content were obtained,and the undisturbed soil and remolded soil were carried out Compared.Tests show that the unloaded swelling rate and swelling force decrease with the increase of initial water content,and the unloaded swelling rate and swelling force of remolded soil are significantly higher than that of the original soil;The swelling rate of undisturbed soil and remolded soil decreases with the increase of the overlying load,and the swelling rate of remolded soil under any load is greater than that of undisturbed soil;Under dry-wet cycle conditions,the unloaded swelling rate of ancient soils has a positive correlation with the number of cycles,but the growth rate shows a downward trend.(3)Through the NMR test,the ancient soil T2 distribution curve and spectral area were analyzed,and the relaxation time was converted into pore distribution law,and the relationship between pore distribution and soil state was studied.The results show that the T2 spectrum is in the state of three peaks,the ancient soil has the largest proportion of the T2 spectrum area,followed by the second peak,and the third peak,and the pores are mainly mesopores.(4)The microscopic changes of undisturbed soil and remolded soil under different initial moisture contents were analyzed by electron microscope scanning experiment,and it was found that the initial moisture content increased,the average particle abundance,particle fractal dimension,average pore abundance increased,and the average particle circle The degree of shape,the average circularity of the pores,and the fractal dimension of the pores decrease.(5)Based on the principle of gray correlation method,the correlation of ancient soil unloaded expansion rate,expansion force and microstructure parameters was evaluated through correlation model analysis,and it was found that the fractal dimension played a major role in the expansion characteristics of ancient soil.