Characterizing thermo-hydraulic behaviors of seasonally frozen loess via a combined opto-electronic sensing system: Field monitoring and assessment

•A combined opto-electronic sensing approach was proposed for soil multivariables.•Loess thermo-hydraulic behaviors in freezing-thawing were deciphered by field data.•The laboratory-obtained soil freezing point is applicable to field investigations.•Importance of in-situ soil-freezing characteristic curve acquisition is emphasized.•Frozen soil matric suction considering ice is the driving force of water migration. Frost damage in mid-latitude loess regions is mainly caused by the coupled soil water and heat transfer during freezing-thawing (FT) processes. In this paper, to better understand the thermo-hydraulic (TH) behaviors of seasonally frozen loess, field monitoring and assessment were performed on the Loess Plateau, China with a quasi-distributed opto-electronic sensing system. This system successfully captured the spatiotemporal variations in multivariables of seasonally frozen soil in near real-time, including in-situ soil temperature, unfrozen water content, and ice content. The monitoring datasets collected during the 2020/2021 winter offer unique insight into the TH behaviors of freezing point (FP) depression, in-situ soil-freezing characteristic curve (SFCC), and cryosuction. The findings highlight the importance of acquiring SFCCs under natural boundary conditions and confirm the applicability of laboratory-obtained FP in determining in-situ soil freezing depth. Furthermore, the study reveals that soil matric suction, considering the presence of ice, is the primary driving force of water migration during FT. Finally, errors in field monitoring are discussed, which arise from two aspects: the special features of loess and imperfections in monitoring schemes. This assessment of the coupled TH responses to climate changes in loess offers valuable theoretical guidance for optimizing engineering construction schemes and mitigating geohazards in mid-latitude loess regions.