Experimental investigation on dynamic characteristics of hydrophobic modified silt after freezing-thawing under cyclic loading

Silt subgrades in seasonally frozen regions are subjected to progressive deterioration under the combined influence of freeze-thaw cycles and traffic loads, resulting in pavement cracking and uneven settlement. This study introduces nano-type hydrophobic material (NT-HM) into silt to enhance silt freeze-thaw resistance. The dynamic stability and freeze-thaw resistance of NT-HM modified silt were studied by dynamic triaxial tests. Based on the shakedown theory, the critical dynamic stress equation of hydrophobic silt under the shakedown limit state is established. The experimental results show that NT-HM wraps the surface of silt particles to form a covalent bond structure to give them hydrophobicity. When the content of NT-HM in silt reaches 0.5 %, the hydrophobic performance reaches super hydrophobic state. After 7 freeze-thaw cycles, the contact angle of the silt with 0.5 % NT-HM content surface only decreased by 2.3 %. The modification effect provided by NT-HM and the confinement effect provided by confining pressure have a coupled superposition effect. Moreover, the addition of NT-HM to silt reduces the sensitivity of the accumulated plastic strain to the cyclic stress amplitude. After seven freeze-thaw cycles, the cumulative plastic strain of the silt with 0.5 % NT-HM content was reduced by 47.7 %–53.1 % compared with the unmodified silt. Hydrophobic modification effectively extends both the plastic shakedown and plastic creep boundaries, with increases of approximately 1.7-fold and 1.4-fold, respectively, compared to the unmodified silt. A cumulative plastic strain model considering the number of freeze-thaw cycles, NT-HM content and stress state was established, which can accurately reproduce the test results. The silt with 0.5 % NT-HM content can still maintain a good skeleton structure after 7 freeze-thaw cycles. This effectively slows down the silt volume changes during the freeze-thaw cycle. This study providing a theoretical basis for hydrophobic material application in seasonally frozen road engineering.