Experimental Study on Structural Anisotropy of Subgrade Soil Induced by Freeze–Thaw Cycles

Studies on the induced anisotropy of soil by the shear wave velocity test with bender elements have been investigated for decades, but few considered the anisotropy of soil induced by freeze–thaw cycles. This study reports the shear wave velocities of subgrade soil specimens subjected to different numbers of freeze–thaw cycles measured using bender element tests. Taking shear modulus and anisotropy index as the evaluation indicators, the evolution law of structural anisotropy of subgrade soil after different numbers of freeze–thaw cycles was systematically analyzed. The results showed that the freeze–thaw cycles can induce new structural anisotropy of subgrade soil. For the specimen with optimal moisture content which had undergone freeze–thaw cycles in a closed system, the shear modulus exhibited a slight decrease in the vertical direction and a slight increase in the horizontal direction as the number of freeze–thaw cycles increased, and the anisotropy indices were between 1.28 and 1.62. For the saturated specimen, the shear modulus exhibited a decreasing trend and eventually reached a stable state as the number of freeze–thaw cycles increased, and the anisotropy indices were between 1.1 and 1.25. The anisotropy index of the subgrade soil increased as the number of freeze–thaw cycles increased, and its growth decreased and tended to be stable after the 7th freeze–thaw cycle. The relationship between the anisotropy index of subgrade soil specimen and the number of freeze–thaw cycles could be quantitatively represented by an exponential function.