Characterizing hydro-mechanical behaviours of compacted subgrade soils considering effects of freeze-thaw cycles

Abstract

Abstract This paper presents a series of experimental studies for evaluating the effects of closed-system freeze–thaw (FT) cycles on the hydro-mechanical behaviours of two subgrade soils (a low plastic lean clay, SS and a lean clay with higher plasticity, HC). Investigated hydro-mechanical behaviours include the soil–water characteristic curve (SWCC) obtained from the filter paper method, resilient modulus (MR) determined from cyclic triaxial tests, unconfined compression strength (qu) and reloading tangent modulus (E1%) and stress (Su1%) at 1% strain measured from unconfined compression tests, with emphasis on the SWCC and MR. Specimens compacted at the maximum dry density (ρdmax) and optimum moisture content (wopt) were firstly subjected to multiple FT cycles (number of FT cycles NFT = 0, 1, 3, 6 and 10) and then dried or wetted to different moisture contents before determining hydro-mechanical behaviours. Experimental results revealed that (i) FT cycles reduce the magnitude of volumetric strain upon moisture variation for the HC but have little impact on the SS; (ii) FT cycles reduce the water retention capacity of both soils. For each soil, the void ratio (e)-moisture content (w)-suction (s) relationships after different FT cycles are possibly distributed on a unique surface; (iii) Reductions in the mechanical properties (i.e. MR, qu, E1% and Su1%) are more significant at NFT = 1 and vary with the post-FT cycle moisture content. Reductions in the MR are most serious at a threshold w level on the wet side of wopt; (iv) FT cycles reduce the sensitivity of the mechanical properties to moisture content for the HC but exert minor influence on that of the SS; (v) Relationships of the MR to the qu, E1% and Su1% are not influenced by the NFT and moisture content for both soils. They are non-linear and can be well described by quadratic polynomials. Soils with higher plasticity such as the HC is, in general, more vulnerable to effects of closed-system FT cycles at wopt than low plastic soils such as the SS.