Experimental Study On Dynamic Characteristics And Microstructure Of Soft Clay Under Cross-river Subway Loading

During the construction of urban subways,it is inevitable to cross special areas such as rivers,lakes and even the ocean.Especially in the southeastern coastal areas of China,the watershed network is dense and the soft clay is widely distributed.Many cross-river subways are often constructed in deep soft soil layers.The dynamic load generated by the subway train could lead to uneven settlement of the foundation.The frequent changes of the warp soil above and the water level aggravate the post-construction settlement of the subway tunnel.Since the cross-river subway tunnel is under water,excessive deformation of the tunnel will lead to leakage,misplacement and other metro hazards,which will affect the safty of the subway.Therefore,the in-depth study of the dynamic characteristics of soft clay under the cross-river subway load can provide a basis for the long-term settlement control of the cross-river subway.In this paper,bidirectional dynamic triaxial test is designed,and the characteristics of pore pressure,cumulative strain and permeability of soft clay under cyclic loading of cross-river subway are studied.The effects of effective confining pressure and radial cyclic stress ratio were analyzed,and the pore pressure and axial cumulative strain models of soft clay under cyclic loading were established.At the same time,the NMR test system is used to quantitatively study the size and distribution of pores in soft clay samples before and after cyclic loading.To understand the variations of mechanical properties of soil from the perspective of microstructure,the correlation analysis between microscopic parameters and macroscopic dynamic characteristics is made.The conclusions are as follows:(1)The confining pressure of the soil around the cross-river subway changes frequently.The effect of confining pressure on the simulation of the cross-river subway load by dynamic triaxial test cannot be ignored.In existing researches,the two-way dynamic triaxial test is rarely used to simulate the subway loading,and the waveform used is usually not the real case.(2)Under the cyclic loading of cross-river subway,the growing of pore pressure is firstly rapid,then it slows down and becomes stable.Increasing the effective confining pressure during cyclic loading,the pore water pressure will rise;the stability value of pore water pressure will increase with the increase of radial cyclic stress ratio;in real case,if the subway cyclic load is regarded as one-way cyclic loading,it will underestimate the amount of excess pore water pressure accumulated in the soil.The quadratic logarithmic model is used to simulate the pore pressure development of soft clay under cyclic loading.The model could simulate the effects of radial cyclic stress ratio and effective confining pressure,which can provide as a reference for post-construction settlement prediction of cross-river subway.(3)The axial strain of soft clay under the loading of cross-river subway increases rapidly at first,and then the rate of strain accumulation gradually slows down and stabilizes at the end of loading.The larger the effective confining pressure,the smaller the cumulative strain generated by the soil,indicating that the confining pressure has a reinforcing effect on the soil;the larger the radial cyclic stress ratio,the smaller the axial cumulative strain of the soil.The explicit relationship between strain accumulation of soft clay under cross-river subway loading is proposed,which could provide some guidance for long-term post-construction settlement control of cross-river subway.(4)After the cyclic loading,the permeability coefficient of the soil sample decreases.The influence of confining pressure and radial cyclic stress ratio on the loss ratio of permeability coefficient was not obvious.The permeability coefficient loss ratio has a positive correlation with the axial cumulative strain of the soil mass.The permeability coefficient loss ratio caused by the cyclic load increases with the accumulation of axial strain.(5)The distribution of pore size in soft clay changes under the cross-river subway loading,which is manifested by the significant reduction of macropores and the increase of small pores;the distributions of pores in original remolded soil samples are"double peak",and distributions of pores in the samples after loading are "single peak".Decreasing of the effective confining pressure leads to the reduction of big pores and the increase of small pores;the proportion of small pores relatively increase with the radial cyclic stress ratio.The correlation between the pore size and the macroscopic dynamic characteristics after the cross-river load is analyzed.There is a negative correlation between the average pore diameter and the axial cumulative strain under the cyclic loading of the cross-river subway,indicating that the original connection of the soil is destroyed under the cyclic loading and the pores become more broken,resulting in axial cumulative strain.There is no significant correlation between the loss ratio of soil permeability coefficient and the average pore diameter.