Loess is a predominantly silt-sized Quaternary sediment which has a wide distribution in arid and semi-arid regions (Peng et al., 2018). It covers approximately 10% of the Earth’s land surface, and can be found over large areas of Eurasia, South and North America, and smaller areas of New Zealand, Australia, Africa, and the Middle East (Muhs et al., 2014). In China, the total area of loess and loess like soils is approximately 630,000 km2, being about 6.3% of the country’s land surface (Lin and Liang, 1980). Due to its special properties of high water sensitivity and collapsibility (Gao, 1988; Leng et al., 2018), loess is prone to uneven deformation under the action of external forces and water, which has caused many problems in infrastructure construction in the loess area (Grabowska-Olszewska, 1988; Lin and Wang, 1988; Sadeghi et al., 2019; Smalley, 1992; Zang et al., 2019). How to eliminate the collapsibility and ensure the stability of the projects has aroused extensive research in past decades (Evstatiev, 1988; Feng et al., 2015; Gaaver, 2012; RenéJacques, 1988; Tan, 1988).
According to Evstatiev (1988), the loess improvement methods have been classified into 8 groups, including compaction, the addition of coarser materials, injection of various adhesives, various cushions or jet grouting, geomembranes, etc. Based on the statistics provided by the Soviet Union, compaction is the most cost-effective way to improve loess. Its basic principle is to increase the density of loess by rolling or heavy tamping. As early as the 1930s, the Soviets firstly used compaction to improve the loess foundations (Abelev, 1939). Since World War II, it has been used in almost all countries for engineering construction on loess soils (Evstatiev, 1995; Fonte et al., 2018; Gao et al., 2004; Houston et al., 2001). The scale of these projects was generally not large, only involving a small amount of building foundations in most cases, and the thickness of compacted loess was generally no more than 10 meters. As a consequence, it was unlikely to cause widespread changes to the surrounding terrain.
However, with the continuous development of human society, some new projects begin to appear. Loess Plateau, the most concentrated and largest loess area on the earth, spans seven provinces in China, making up 20% of arable land and supporting 17% of its population (Zhang, 1993). Due to serious soil erosion and shortage of urban land, in recent years, local governments have promoted projects that remove the tops of mountains to fill in valleys to create lands for developments (Li et al., 2014). The project, called mountain bulldozing and city creation (MBCC), has become the largest loess project in the world. Dozens of hills 100–150 meters in height were being flattened over hundreds of kilometers, and the height of compacted loess filled in the gully could exceed 80 meters. Given the scale of such mega-projects, there is no experience to draw from. Whether this large-scale manual filling compacted loess can be kept stable for a long time to meet the urban construction demand needs to be studied urgently.
Different from previous projects, there is not only manual filling compacted loess but also natural sedimentary loess in the MBCC projects, which formed two types of sites, namely filled area and excavated area. Recent work by engineers has established that the foundations of the two areas have obvious differences (Ge et al., 2017; Zhang et al., 2019). In the excavated area, the natural sedimentary loess is uniform in nature and has good engineering geological conditions. By contrast, due to the differences in filled thickness, soil quality, tamping quality, and moisture content, the manual compacted loess in the filled area has a different degree of compressibility and collapsibility. These differences make the foundation in the filled area prone to uneven settlements, and ground fissure often occurs at the contact place of the excavated area and filled area. Hence, the comparative study of these two kinds of loess is of great value to the stability analysis of the site. Moreover, in the current evaluation system, the engineering quality of the filled body is mainly achieved by controlling the compaction degree and dry density of the loess after layered compaction (Wu, 2018). However, the backfill compacted loess is formed by human reconstruction in a short period, and it will take a long time to stabilize under the influence of various factors such as the change of external water environment and overburden pressure (Kong et al., 2018). Therefore, the stratified and batch detection results in the construction stage do not necessarily represent the overall pressure and change of the filled body (Zhang et al., 2019). As the basic physical properties of the backfill compacted loess directly reflect the construction quality and its strength and deformation characteristics are directly related to the long-term stability of the high filled body, the physical and mechanical properties of the backfill compacted loess after construction remain a major problem to be investigated.
For several decades, a large number of scholars have carried out research on the properties of natural and compacted loess from both micro macro scales. It has previously been observed that the compacted loess is quite different from the natural loess (Jiang et al., 2014; Ng et al., 2016; Sadeghi et al., 2016; Zhang et al., 2020). Even though the dry density of compacted loess is the same as that of natural loess, their micro-structure is still quite different (Rendell, 1988; Wang et al., 2018). Moreover, the compacted loess can still have water sensitivity and collapsibility, which can be even more serious than the natural loess (Chen and Sha, 2009; Jia, 2000; Ma et al., 2017; Wu et al., 1997; Zhang, 1986). Besides, several attempts have been made on the relationship between the physical and mechanical properties of compacted loess. Wang et al. (2006) studied the water diffusion ratio of unsaturated compacted loess considering the influence of density, and the results have shown that the permeability coefficient of the unsaturated dense loess is sensitive to soil density change compared with loose loess. When the dry density is unchanged, the cohesion and internal friction angle slightly decrease as the water content increases. Based on one-dimensional consolidation tests, Chen and Sha (2010) discussed the relationship between physical indices (water content, compaction degree) and deformation indices (compression deformation coefficient, compression coefficient) of the compacted loess. They have found that the compression deformation coefficient and compression coefficient increased with the increase of water content, and decreased with the increase of compaction degree. Kim and Kang (2013) investigated the mechanical characteristics of four kinds of compacted loess having different clay contents (10%, 20%, 25%, and 30%), and implies that the resilient behavior of compacted loess materials was considerably affected by the clay content. Li et al. (2019) used Mercury intrusion porosimetry (MIP), scanning electronic method (SEM), and filter paper method to explore the pore size distribution (PSD), the soil water characteristics (SWCC), and the microstructure of the compacted loess at different water contents. It is found that the moisture content can significantly affect the microstructure and soil water characteristics of compacted loess.
However, previous studies have mainly focused on the compacted loess manufactured under laboratory conditions, and the selection of physical indicators is sole and disperse, so that the test results may not accord with the laws of loess on site. More recently, because of the MBCC project, literature has emerged that offers data about the manual filling compacted loess in large-scale high-fill sites (Duan et al., 2018; Ge et al., 2017; Kong et al., 2018; Yin et al., 2016). Nonetheless, existing researches mainly focus on the monitoring parameters like soil moisture content, pore water pressure, and settlement, lacking systematic analysis of other physical and mechanical parameters. What’s more, there have been no controlled studies which compare differences in the layers at different depth of the natural and compacted loess.
In this paper, comprehensive comparison and analysis of the physical properties were carried out to study the exploration well profiles of natural loess and compacted loess from the Loess Plateau. The data were party from experiments and partly from the previously published literature. Statistical theories such as t-test and correlation coefficient checks were used to describe the difference between the two kinds of loess, and the degree of correlation among various indicators.
This study has complemented the current research data on MBCC projects which is helpful for further understanding the environment changing in large-scale compaction loess projects, and has important practical significance to ensure the safety and reliability of land creation or man-made infrastructures. In addition, it has provided a new idea to describe the difference and correlation of physical indices of loess.