越江地铁循环荷载下软黏土累积应变特性及微观结构特征

Strain accumulation and microstructure characteristics of soft clay under cross-river subway loading

越江通道往往需要穿越江底深厚软黏土层,而这些土层容易发生过大的变形沉降,从而对地铁运营安全造成不利影响。通过室内双向动三轴试验模拟越江地铁循环荷载对软黏土地基的作用,得到软黏土应变发展规律曲线,建立越江地铁循环荷载作用下软黏土的累积应变模型。结合核磁共振扫描试验,分析循环加载前后的土体微观孔隙结构,研究越江地铁荷载作用下有效围压以及径向循环应力比对孔隙分布的影响,并从微观角度研究孔隙结构与宏观动力特性的相关性。基于试验结果,提出了越江地铁荷载作用下软黏土应变累积的显式关系式,且综合考虑了有效围压和径向循环应力比的影响。研究结果表明:在一定情况下,土体产生的累积应变与围压、径向循环应力比呈反比;循环加载破坏土单元之间的连接导致孔径分布发生显著变化;在越江地铁荷载作用下,土体孔隙结构具有振密趋势,土颗粒破碎程度加剧,土体大孔隙减少,小孔隙增多;在越江地铁循环荷载下,平均孔径与轴向累积应变之间具有负相关性。

The cross-river subway often passed through deep soft clay layers at the bottom of the river,and these soil layers were prone to excessive deformation and settlement,which will adversely affect the safety of subway operations.Through the indoor bidirectional dynamic triaxial test,the effect of the cross-river subway cyclic loading on the soft clay foundation was simulated,the curves of the soft clay strain development law were obtained,and the cumulative strain model of the soft clay under the cyclic load of the cross-river subway was established.Combined with the NMR scanning test,the soil microscopic pore structure before and after cyclic loading was analyzed.The effects of effective confining pressure and radial cyclic stress ratio on pore distribution under the load of the cross-river subway were analyzed,and the correlation between pore structure and macroscopic dynamic characteristics was studied from a microscopic point of view.Based on the experimental data,an explicit relationship of strain accumulation of soft clay under the load of the cross-river subway was proposed,and the effects of effective confining pressure and radial cyclic stress ratio were comprehensively considered.The results show that under certain circumstances,the accumulative strain generated by the soil body is inversely proportional to the confining pressure and radial cyclic stress ratio.The cyclic loading process will destroy the connection between the soil units,resulting in a significant change in the pore size distribution.Under the load of the cross-river subway,the pore structure of the soil has a tendency to vibrate,the degree of soil particle fragmentation increases,the large pores of the soil decrease,and the small pores increase.There is a negative correlation between the average pore diameter and the axial cumulative strain under cyclic loading of cross-river subway.