## 1 Introduction  
Diatomite soil is a kind of porous silicous soil, which is formed by the accumulation of diatoms and other plant remains. Diatomite soil has different colors according to its mineral content, usually light or gray-black color. There are abundant diatomite soil resources in the world, it is estimated that the geological reserves of diatomic soil in the world are at least two billion tons. It is mainly distributed in China, the United States, Denmark, France, and other places [1, 2].  
Diatomite soil has special structural and chemical stability, which is characterized by soft texture, high adsorption, high porosity, and low density [3]. At present, it is often used as an adsorption material in the chemical industry [4, 5, 6]. The main component of diatomic soil is SiO\({}_{2}\), and it contains more clay minerals, detritus, and organic matter [7, 8, 9]. Considering the influence of the external environment and the difference in material composition, the change in diatomic soil microstructure will directly affect its macro and micromechanical properties [10, 11, 12]. Relevant tests show that different calcination temperatures can affect the microstructure of diatomic soil [13, 14]. The SiO\({}_{2}\) content of diatomic soil is the standard to judge the quality of diatomite soil, whether in the development and utilization of non-metallic materials or in geotechnical engineering [15]. The diatomite soil which is used as filling soil in Southern California, USA, has a density of 1.0-1.2 g/cm\({}^{3}\) and a water content of 38-53% and belongs to soil with low dry density and high natural water content [16, 17, 18]. For natural diatomite soil in Japan, research shows that the natural water content ratio and SiO\({}_{2}\) content of diatomite soil are very high [19]. Diatomite soil also has low shear strength, complex composition, different degrees of expansibility, and usually contains clay minerals, carbon, iron, carbonate, and other minerals [20; 21; 22; 23; 24; 25]. Moreover, it is easy to soften when encountering water and its engineering properties are poor [26; 27; 28; 29; 30; 31; 32].  
In recent years, infrastructure construction is increasing. From ports in Africa to the Hangzhou Shaoxing high-speed railway in China, a large number of roads, ports, and high-speed railways are being built in diatomite soil areas. Due to the lack of understanding of the characteristics of diatomite soil, many engineering problems are brought about. Through the analysis of engineering data, it is found that diatomite soil has different colors--usually white, yellow, black, and other colors. Currently, there are minimal studies on physical and mechanical strength changes in diatomite soil. In order to serve the construction needs of the national high-speed railway, it is necessary to study and analyze the diatomite soil from a macroscopic and microscopic perspective.  
In this paper, diatomite soil was taken from the Hangzhou-Taizhou high-speed railway project in Zhejiang Province, China, since this new high-speed railway project is built in a diatomite soil area. The properties of diatomite soil are very important. A series of basic tests such as consolidation tests, direct shear tests, and triaxial shear tests were carried out. Combined with SEM and XRD tests, the mechanisms of deformation and mechanical properties change in diatomite soil are analyzed from macroscopic and microscopic perspectives.