## 1 Introduction  
Expansive soil is a kind of special soil with strong water sensitivity and structural instability, so it has problems of swelling-shrinkage, fissure and over consolidation [1]. A lot of studies have been carried out on the macroscopic behavior of expansive soil such as swelling-shrinkage and its treatment [2; 3; 4]. Nevertheless, more studies on the microscopic structure of expansive soil under different dehydration environments are needed before the treatment. The Pishihihang irrigation area in the Anhui province in China is located in the temperate monsoon region [115.4\({}^{\circ}\) E 32.7\({}^{\circ}\) N, 117.9\({}^{\circ}\) E 31.2\({}^{\circ}\) N], which spans a large expandable soil and has a distinct climate throughout the year. It is characterized by high temperature and humidity in summer, dry in autumn, and low temperature and little rain in spring and winter. The expansive soil located in the study area is in an environment of significant temperature and humidity change all year round, and the moisture dehydration is severe, which leads to the cracking and deformation of roadbed and slope in different degrees, and seriously restricts the local traffic and the operation of irrigation. Therefore, it is of great significance to carry out experimental research on the microscopic pore structure and mechanical properties of expansive soil under different dehydration conditions. Many studies showed that the occurrence of soil fissures was closely related to the rate of dehydration, and the secondary fissures generated by the dehydration became a new water transport channel [5; 6; 7]. In the process of dehydration, the water content of soil decreased, and the internal pore structure of soil was constantly adjusted, resulting in the change of mechanical properties of soil [8; 9; 10]. According to the experimental studies of mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), [PERSON] et al., found that temperature and humidity had significant effects on the pore size distribution and micro-structure of the large pore and medium pore of the expansive clay, but little effect on the small pore [11]. [PERSON] et al., adopted MIP, SEM, computed tomography (CT) and some other methods to study the evolution of soil micro-structure under wetting-drying cycles. It was found that the decrease of soil volume was mainly caused by the shrinkage of pores between soil aggregates in the drying process. The number and size of pores inside and between soil aggregates increased gradually during wetting, and the changes of soil structure were not completely reversible during wetting-drying cycles [12]. The change of small pore structure led to macro cracks in soil, which affected the bearing capacity of soil [13; 14]. [PERSON] et al., measured the characteristics of mechanical strength attenuation of expansive soil under freezing-thawing cycles through triaxial test [15]. [PERSON] et al., proposed the safety factor of expansive soil slope with different fracture forms by analyzing the hydraulic mechanical properties of expansive soil [16]. [PERSON] et al., studied the development process of dry-shrinkage cracks in soil during dehydration, and found that water evaporation is the premise of soil cracking, and the dry-shrinkage cracks were the result of internal stress of soil [17]. For expansive soil, the fracture morphology and pore structure caused by shrinkage were different under different dehydration, which results in different mechanical properties of soil, and the dehydration rate was controlled by the dehydration environment and the mineral composition of soil itself. Most of the existing studies considered the evolution of macroscopic expansion and contraction deformation and microstructure of soil under single environment when soil is hygroscopic or dehydrated, but the research on the evolution process and mechanical properties of microscopic pore structure of expansive soil under different dehydration environments is relatively rare.