Abstract:

Atmospheric precipitation infiltration into rock and soil is a common natural phenomenon. However, understanding the response of different materials to infiltration remains limited, particularly for sandstone where in-situ moisture monitoring is challenging due to significant differences in hydrological parameters between materials like sandstone and loess. This study, conducted at the Yungang Grottoes in northern China, employed frequency domain reflectometry (FDR) to monitor moisture content at various depths. Monitoring sites included exposed sandstone atop Cave 4, where infiltration potentially causes internal seepage, and loess atop Cave 9, where infiltration does not lead to seepage inside the cave. The responses of these two materials to rainfall events were compared. Monitoring results indicated that shallow sandstone (10 cm depth) responded rapidly to rainfall events as small as 10 mm, with moisture content increasing significantly, nearly reaching saturation. In contrast, similar rainfall intensity only increased the saturation of shallow loess (10 cm depth) to approximately 0.75, while the deep loess (220 cm depth) showed almost no response. Mineral composition analysis using X-ray diffraction (XRD) on weathering products from the cave walls revealed that decades of acid rain infiltration into Cave 4 led to ${\mathrm{SO}}_{4}^{2-}$ accumulation and the formation of efflorescent salts, which dominate salt weathering. Analysis of weathering product samples collected in the 1960s, alongside recent samples from Caves 5-10, suggests that the overlying loess layer has prevented acid rain infiltration into these caves. This study demonstrates that the loess cover layer exhibits low permeability, a finding that can inform future decision-making for the conservation of the cultural relics within the caves.

Key words: rock weathering, rock moisture, water-salt transport, rainfall infiltration, XRD analysis