A new method of assessing the collapse sensitivity of loess

Abstract

The damage caused by loess collapse after wetting is related not only to the magnitude of the collapse, but also to the resistance to and speed of the collapse. These factors are encapsulated in the concept of collapse sensitivity. However, there is no scientific and rational method available to evaluate it. To introduce this concept and develop a scientific evaluation method, four representative loess sites were selected in four different cities in China: Lanzhou, Xi’an, Taiyuan and Luoyang. Collapsibility tests were performed with loess samples from these sites. The concept of the collapse sensitivity of loess was introduced through the experiments, which involves the collapse magnitude, collapse speed and resistance to collapse. According to the physical significance of the concept, only two indexes, the time required to reach 90% of the collapse and the collapse potential under an appropriate pressure, were proposed to evaluate the collapse sensitivity of loess at a given site. The Chou Huang channel, which has collapsed, was presented as a case study to demonstrate the application and reliability of this method.

Appendix

Different structural and material characteristics of loess in different regions is the essential reason for the different collapse sensitivities of loess. The formation of Chinese loess is presented as an example to further introduce the concept of collapse sensitivity of loess.

The cold wind current from Siberia, influenced by Asian continental high pressure, blows to low-pressure areas in southeastern China. When the current passes through desert areas, it picks up sand grains in the air. They are further corroded into smaller particles of silt and clay by heat exchange with air and severe cold wind, then regularly deposited from northwestern to southeastern China, depending on the particle size (Fig. 1).

Due to the low relative humidity and low rainfall in arid and semi-arid areas of northwestern China (Lanzhou, Xi’an, Taiyuan), only a small amount of soluble salts (CaCO₃ as the main content) in the loess are leached. A higher content of CaCO₃ indicates that the soil particles are more rigid and that the structural strength of the loess is stronger under normal conditions. Therefore, shallow loess layers, specifically loess deposited during the Holocene (Q₄) and late Pleistocene (Q₃) periods, are difficult to fully compress. Insect holes and roots are abundant in these loess layers. Vertical and subvertical joints are also well developed. The microstructure of these loess layers is characterised by numerous macropores. The macrostructure of these loess layers is characterised by large e or small ρ_d values. After soaking, this shallow loess can rapidly and violently collapse under its own weight or an external load (Gao 1981; Gao and Gao 1980). In contrast, the deep loess layers, such as the Lishi loess and Wucheng loess, deposited during the Middle and Early Pleistocene (Q₂, Q₁), respectively, are fully compacted because of the high pressure exerted by the upper soil layers. The CaCO₃ content in these loesses is relatively low, and the microstructure of these loess layers is characterised by the interlocking mode. The macrostructure of these loess deposits is characterised by small e or large ρ_d values. This loess is considered a non-collapsible loess, which practically exhibits no collapsibility.

The greater content of soluble salts in the loess of southeastern China (Luoyang), which features a humid climate and abundant rainfall, can be dissolved during long infiltration. The salts are then deposited at the bottom of the loess layer or drained directly beyond the borders. Due to the low CaCO₃ content, the soil particles are not very rigid, and the structural strength of this loess is comparatively low and poorly supports the load of the thick upper soil. Therefore, the pore structure is gradually compacted into an interlocked structure, and the point contacts gradually change to cementation contacts. This type of loess, in both shallow and deep layers, has better compaction, as characterised by a higher ρ_d or lower e, and is difficult to further collapse, even when soaked (Gao 1981; Gao and Gao 1980).