Fracture patterns and failure characteristics of loess–red clay composite under low-temperature conditions

The “double-layer heterogeneous soils” composed of loess and red clay in the Loess Plateau of China is prone to interface-type, overall-type, and spalling-type composite landslides under coupled variations in temperature and moisture conditions. In this study, three-point bending tests were conducted on loess–red clay composites with varying water contents under freezing conditions, with the entire fracture process recorded by high-speed camera and smartphone, to investigate fracture patterns, failure characteristics, and the response mechanisms to water content and temperature. The results show that increasing water content significantly enhances apparent Mode I fracture toughness. At low water content (8 %), interparticle cementation dominates the soil strength, leading to brittle single-peak with low peak load and small displacement. At medium water content (11–17 %), the ice-crystal skeleton develops and red clay aggregation enhances double-layer heterogeneity, resulting in a pronounced double-peak response inferred from the sequential failure of the red clay and loess layers. At high water content (20 %), the strong ice-crystal skeleton dominants load-bearing structure, leading to single-peak with high peak load and large displacement. In addition, decreasing freezing temperature strengthens the water freezing and the ice-crystal skeleton, further increasing the apparent fracture toughness. Meanwhile, the crack pattern evolves from multiple irregular secondary cracks (−5 °C) to a more brittle and stable single dominant crack (−10 to −20 °C). These findings provide a theoretical basis for stability evaluation and disaster prevention in loess–red clay composite strata.