Laboratory investigation on strengthening behavior of frozen China standard sand

In this paper, a series of triaxial cyclic loading–unloading (L–U-C), triaxial cyclic loading–unloading–holding (L–U–H-C) and triaxial cyclic stress relaxation tests (L–R-C) were performed for frozen China standard sand (FCSS) under laboratory conditions varying with confining pressure, temperature and holding time. A composite system synchronizing the real-time computed tomography (CT) device and the triaxial testing device was used to explore the internal structure change of FCSS during L–U–H-C. The strengthening behavior was clearly observed by comparing the L–U-C and L–U–H-C tests. While this behavior is generally caused by particle compaction as occurred in common soil, pressure melting of ice crystal and recrystallization followed may additionally contribute to strengthening behavior of frozen soil. The increase in mean CT number during the holding stage further confirms the effects of pressure melting of ice crystal and recrystallization on the strengthening of FCSS from an internal structure aspect.