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

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 (M-R) determined from cyclic triaxial tests, unconfined compression strength (q(u)) and reloading tangent modulus (E-1%) and stress (S-u1%) at 1% strain measured from unconfined compression tests, with emphasis on the SWCC and M-R. Specimens compacted at the maximum dry density (rho(dmax)) and optimum moisture content (w(opt)) were firstly subjected to multiple FT cycles (number of FT cycles N-FT = 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. M-R, q(u), E(1% )and S-u1%) are more significant at N-FT = 1 and vary with the post-FT cycle moisture content. Reductions in the M-R are most serious at a threshold w level on the wet side of w(opt); (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 M-R to the q(u), E-1% and S-u1% are not influenced by the N-FT 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 w(opt) than low plastic soils such as the SS.