## 1 Introduction  
Shear band is defined as a band-shaped zone where the strain inside the zone is significantly greater than that outside the zone when the soil is subjected to external forces. The formation of a shear band significantly affects the failure of soil [1, 2, 3, 4]; when the external load of the soil is higher than the strength of the soil, shear failure will occur along the shear band. Naturally structured clays are founded in many countries around the world, such as Ariake clay in Japan [5] and Batiscan clay in Canada [6], and they are also widely distributed in coastal areas of China. Naturally structured clay possesses complicated mechanical properties owing to soil structure. Researchers conducted extensive research on this mechanical behavior, including its compressibility and shear strength [7, 8, 9]. The mechanical properties of specimens under consolidation stress lower and higher than yield stress differ significantly. For example, in oedometer tests, when the effective vertical stress is lower than the yield stress, undisturbed specimens show low compressibility; when the effective vertical stress is higher than the yield stress, undisturbed specimens show high compressibility. With the rapid economic development in China's coastal areas, many infrastructures, such as buildings and highways, are built on naturally structured clay layers. In the construction of these infrastructures, excavation works are inevitable, such as foundation pits and ditches. Therefore, artificial slopes are often produced, so it is necessary to ensure the safety of slopes to avoid safety accidents. Unfortunately, in recent years, a large number of landslides have occurred on naturally structured clay layers in China's coastal areas, such as Shanghai, Nanjing, Hangzhou, and other major cities [10], which have caused great difficulties for infrastructure construction. Because the formation and development of shear bands are associated with failure of soil slopes [11; 12; 13], the evolution of shear band is crucial for investigating the mechanics of naturally structured clay and ensuring the safety of infrastructures [14; 15; 16]. The study of shear bands can be traced back to the research on strain localization theory by [PERSON] [17]. Owing to the development of testing technology in recent years, many effective testing methods have been applied to investigate shear bands. [PERSON] et al. [18] studied the shear band inclination in Shanghai clay using local displacement sensors. [PERSON] et al. [19] took advantage of local pore water pressure sensors to investigate the shear bands formation of Shanghai silty clay in plane strain compression. [PERSON] et al. [20] investigated the characteristics of shear bands of coral sand in the interface shear tests based on the digital image correlation (DIC) technique. [PERSON] et al. [21] analyzed the inner structure of shear bands in sensitive clays using the computed tomography (CT) measurements. Among the testing methods mentioned above, the most effective testing methods for investigating shear bands are the DIC technique and CT measurements [22; 23]. Compared with DIC, which can only measure the surface deformation of specimens, CT can measure the inner deformation of specimens non-destructively, rendering its superior over DIC. Only limited heterogeneous geotechnical materials are suitable for CT measurements, such as granular materials with high quartz contents, as high speckle quality for local deformation calculation can be obtained owing to the density difference between quartz and air [24; 25]. However, for natural clay, almost no density difference occurs inside it owing to its homogeneity. Hence, its internal speckle quality is insufficient for local deformation calculations in CT measurements. The DIC technique is a better method for natural clay because it can yield information pertaining to the formation and development of shear bands by measuring the deformation distribution of the surface of the specimens.