Experimental behaviour of frozen coarse-matrix soils with variable fine content

Artificial Ground Freezing (AGF) is an eco-friendly consolidation technique, commonly adopted for the construction of shallow tunnels and wells below groundwater level. The application consists of the insertion of freezing pipes around the excavation area. A coolant is circulated inside the pipes, thus provoking the surrounding soil to freeze. When a continuous frozen wall is formed, the area with lower permeability and higher strength allows for a safer and easier building phase. AGF is often applied in soils characterized by intermediate or well-graded granulometry (e.g., along the new Line C of Rome Underground). Consequently, the influence of heterogeneous and variegate grain size distribution on the thermo-hydro-mechanical (THM) behavior of soils must be systematically investigated. In literature different experimental studies focus on site materials. Although they are important for the effectiveness of individual projects, they are rarely generalizable. The presented research consists of an experimental campaign on gap-graded materials aimed at standardizing the behavior of intermediate soils. A sandy matrix is mixed with kaolin at percentages in mass from 0 to 15%, then the mixtures are subjected to a freezing process to investigate the influence of fine content on the THM behaviour observed. To these aims, a temperature-controlled triaxial apparatus, FROZEN, is used. Its setup allows for the reproduction of on-site freezing conditions around a freezing pipe. After a brief description of the materials and experimental device, the paper contains the main results of the experimental campaign aimed at understanding the THM behavior of the mixtures upon freezing. In particular, the performance of pure sand is compared with that observed when kaolin is added to the matrix. Additionally, the influence of increasing confining pressure (from 50 to 800 kPa) on freezing behavior is investigated.