Factor of safety in ground freezing design

Design of frozen earth structures for shafts, tunnels, and cross passages has evolved since the publication of guidelines in 2002 by the Working Committee of the International Symposium on Ground Freezing. This evolution is due to the development and implementation of numerical modeling methods, specifically the finite element method (FEM). The design was previously based on determining a thickness of the frozen earth structure with an average frozen temperature. The factor of safety was incorporated by dividing the unconfined compression strength of the frozen soil by two, and thus F.S. = 2. The finite element model allows more specific an analysis of a frozen structure cross section and adjusting strength parameters based on temperature and time-dependent strength reduction. Analyses results permit the evaluation of the internal stress regime of the frozen earth structure. This paper discusses the procedure and reports on key recent projects.