Microstructural insight into the characteristics and mechanisms of compaction during natural sedimentation and man-made filling on the Loess Plateau

Loess is widely distributed in northern China on the Loess Plateau, which is well known for its serious soil erosion and shortage of urban land. Undertaking filling construction under the conditions of an optimum water content is very difficult for so many mega-engineering projects in loess regions because of the (1) moisture loss that occurs over a certain transport distance, (2) water shortage, and (3) construction costs. Consequently, the inadequate compaction behaviors of such compacted loess without improvement may have significant effects on the safety and reliability of man-made structures. This study focuses on the compaction mechanism of large-scale and high-fill compacted soil under complex construction conditions by comparing intact loess with in-situ compacted loess from a microstructural perspective. The morphological characteristics of the particles and pores of (1) intact soil obtained from a typical 75-m high loess–paleosol section, and (2) compacted soil obtained from a 30-m-deep filling section are described. The main experimental comparisons between the intact soil and compacted soil are discussed with respect to the pore size distribution (PSD), microstructural micrographs, and characteristics of the particle/pore structure. The results showed that both the intact soil and compacted soil exhibited trimodal characteristics in their PSDs; hence, a new pore classification is presented to agree with the pore name. The compaction mechanism of compacted loess was mainly attributed to two aspects: one is the progressive and ordered transformation of different pore sizes, and the other is the gradual rearrangement of order and close cementation of particles under dynamic loading. The results of the present study are very useful for better understanding the properties of loess soil and improving engineering construction safety in loess areas.