Experimental Investigation of the Hydration Swelling Effect of Clay Minerals on Loess Collapsibility

Compositional characteristic in loess is a key factor controlling its collapsibility. Clay cementation between particles plays an important role in loess collapse. In this study, the type, content, properties, and occurrence mode of clay minerals in Malan loess were fully investigated by X-ray diffraction and scanning electron microscopy combined with energy-dispersive spectroscopy. The thickness of clay-bound water was determined by isothermal adsorption experiments and the pycnometer method. The results indicated that interstratified illite–montmorillonite and illite are two primary clay minerals in the studied loess. In particular, the illite has experienced serious degradation with low crystallinity and considerable swelling layers. Clay minerals, mixed with skeleton particles of various sizes, mostly exist in the form of aggregates and cementation between particles. The relation between the adsorbed water content of clay minerals and relative humidity (RH) was obtained; when the RH was 99%, the water content reached 5.74%, and the corresponding thickness of bound water was 10.51 Å, which is equivalent to the thickness of nearly 3.8 water molecules and as thick as montmorillonite itself, which demonstrates the high hydration swelling potential of clay minerals in the studied loess. Therefore, the hydration swelling of clay cementation was illustrated to be the primary trigger of loess collapse. This was further confirmed by single-oedometer-collapse tests using different wetting solutions with different polarities. This study highlights the important role of clay minerals in loess collapse, which contributes to further understanding of the collapse mechanism.

Loess collapse is a focus issue that needs to be considered in engineering construction and disaster prevention. A full interpretation of the collapse mechanism can help us to take effective measures to reduce the collapse amount or probability of collapse. In the study, hydration swelling of clay cementation is an important factor causing a decrease in strength between particles, based on a detailed and thorough investigation of clay minerals such as properties and hydration reactions; therefore, loess soil can be effectively improved by mixing with some materials or solutions that can restrain hydration swelling of clay cementation from reducing collapse amount. In addition, loess collapsibility may be quantitatively evaluated more accurately using mathematical methods by establishing a systematic database that contains information on clay minerals and other factors such as microstructural features in different loess regions.