Thawing process of high-ice-content frozen soil subjected to saturated steam

Engineering measures based on permafrost protection to inhibit high-ice-content permafrost thawing and subsidence in some areas of the Qinghai-Tibetan Plateau (QTP) are challenging owing to global warming. Hence, early permafrost thawing is an effective method for reducing the later settlement of the upper infrastructure. Saturated steam can thaw permafrost rapidly, but the research on the process and mechanisms of thawing permafrost with steam is lacking. In this study, steam-thawing tests were conducted on four types of frozen soil with high ice contents (20 %, 30 %, 40 %, and 50 % by volume). The temporal and spatial migration patterns of temperature and moisture in permafrost subjected to steam and the resulting thawing rates were analyzed by real-time monitoring of temperature and the volumetric water content (VWC) during the thawing process. Based on this, the mechanism of steam thawing permafrost was analyzed. The results showed that the continuous injection of saturated steam into the permafrost for 2 h resulted in complete thawing of permafrost with all four ice-contents, but the thawing rates were significantly different. The injection of steam led to an increase in VWC of thawed soil within a 5 cm radius by 1.26–1.68 times. The thawing of permafrost at different ranges was dominated by two different modes, pressure-driven water steam migration leading to permafrost thawing at a radius of about 10 cm and thawing of permafrost at the rest of the range by hydrothermal migration induced by temperature and humidity gradient. Compared to other conventional thawing methods, steam thawing is more suitable for targeted pre-thawing of deeply permafrost layers with high ice content.