Research on water migration and deformation characteristics of coastal composite strata under artificial ground freezing affected by seepage

When the artificial ground freezing (AGF) method is used to construct underground structures in the coastal composite strata, the differences between characteristics of soft clay and silty sand under seepage often increase the risk of freezing construction and the difficulty of deformation control. In order to find out water migration and deformation characteristics of costal composite strata under seepage condition in freezing process, a series of unidirectional freezing tests of silty sand and soft clay in Shanghai and AGF model experiments on composite strata of soft clay and underlying silty sand were carried out under seepage condition. The results of unidirectional freezing tests of single soil type showed that the layered deformation curve of silty sand and soft clay can be divided into two types that steep mode of silty sand and gradual mode of soft clay. It also indicates that the water migration is fully developed within soft clay during freezing; but most of water inside silty sand froze quickly, thus water migration was not developed as that much in soft clay. In AGF model experiments, the seepage contributed to the development of water migration inside composite strata. However, the deformation characteristics of composite strata under seepage condition were evidently different from that of unidirectional freezing tests. The frost heave force development of the lower silt sand in the area directly affected by seepage was gradual mode, while that of the upper soft clay in the area indirectly affected by seepage was steep mode. The above results demonstrated that influence on freezing process of seepage is greater than that of soil properties in freezing process. Through research results, some optimization strategies can be made for the relevant measures to deal with the adverse effects of seepage and frost heave in the practice of the freezing method for coastal composite strata.