3.1 Basic physical properties

In this experiment, the cutting ring method was used to measure the density of soil samples. The density of the collected soil in this study was 1.92 g/cm3 for silt and 1.56 g/cm3 for sand (Table 1). The moisture content of the soil sample was determined using the drying method, with 23.08% for the silt and 13.42% for the sand. The sieving method was adopted to determine the grain distribution of soil samples. The particle size of the silt was mainly within 0.075–0.300 mm, and the median particle size of D50 was 0.13 mm. The nonuniformity coefficient was 2.12, smaller than 5. This indicated that the soil had a uniform nature. The curvature coefficient was 0.048, and the gradation was poor (Figure 1). The particle size of sand was mainly within 0.075–0.600 mm, and the median particle size D50 was 0.25 mm. The nonuniformity coefficient was 2.68, thus indicating that the soil had a uniform nature. The curvature coefficient was 0.102, and the gradation was poor (Figure 1).

               

3.2 Direct shear test

As an important parameter to evaluate the mechanical properties of soil, soil shear strength includes the cohesion and the angle of internal friction. The essence of soil shear failure is that part of the soil overcomes the bonding and friction action of the direct shear of soil particles. The relative displacement between soil particles causes the relative displacement between soil bodies. In this experiment, the consolidation fast shear method was used in the geotechnical laboratory of China University of Mining and Technology. An electric constant strain direct shear instrument (model: GJY-Ⅱ; Nanjing Soil Instrument Factory) and a consolidation instrument were used to measure the shear process of the samples (Figures 2 and 3). The specific operation for soil sample treatment was as follows: the disturbed soil sample was cut and stirred evenly to determine the density, water content, and particle distribution. Then, the experiment was carried out using the following procedures.

               

               

First, the soil samples were consolidated using the consolidation instrument for 4 h. Then, the strain-controlled direct shear instrument was used for the direct shear experiment. The samples were remolded into the following size: diameter: 6.18 cm, height: 2 cm, and volume: 60 cm     3. The shear rate in the direct shear test was 0.02 mm/min. Different normal stresses were applied for the same sample, that is, 50, 100, 200, and 400 kPa for silt and sand (Table     2). The shear displacement and corresponding variations of the stressed steel ring were recorded. Based on the deformation of the stressed steel ring, the shear stress was calculated by

    (1)    

where     τ denotes the shear stress, kPa;     is the deformation coefficient of the stressed steel ring,     ; and     R is the deformation value of the stressed steel ring.

The shear strength of silt and sand can be determined from the curves of the relationship between their shear stress and shear displacement at different normal pressures (Figure 4), that is, the peak shear stress is the shear strength. The shear strengths of silt and sand at different normal stresses are shown in Table 2.

               

The relation curves of silt and sand between the shear strength and normal stress are presented in Figure 5. It is observed that the shear strength of silt and sand increases linearly with the increase of normal stress. According to the available data, a linear function was obtained by fitting the curve (Figure 5). This linear function can be used to predict the shear strength of silt and sand at different normal pressures. The cohesive force and internal friction angle of silt are 0.89 kPa and 3.7°, respectively. The cohesive force and internal friction angle of sand are 0° and 4.1°, respectively.

               

An intersection point was also observed between the shear strength and normal stress of silt and sand. The corresponding normal stress at the intersection point is the threshold of normal stress. At this threshold of normal stress, the shear strengths of silt and sand are the same. When the normal stress value exceeds the threshold, the shear strength of sand is greater than that of silt.

3.3 Observation of microstructures with SEM

The macroscopic physical and mechanical properties of soil are closely related to its internal microstructure, and thus the microstructure plays a decisive role in the mechanical properties and changes of submarine soil. SEM is an important research tool to explore the microscopic world at the molecular scale. SEM images are obtained through the signals of secondary electrons, Auger electrons, and backscattered electrons.

In this study, the VEGA COMPACT Tungsten Filament SEM (TESCAN CHINA) was used. This device uses the latest Essence electron microscopy control software, so that SEM imaging and real-time component analysis can be integrated in the same window. The instrument uses a tungsten lamp as the electron gun; the magnification can reach 2–1 000 000 times, and the field of vision is 7.7 mm (WD = 10 mm), >50 mm (maximum WD). The experiment was conducted in the compact SEM room of the Modern Analysis & Computing Center, China University of Mining and Technology. Before the experiment, the soil samples were treated with ultrasonic dispersion in an absolute alcohol environment.

Due to the large changes in the particle sizes of soil samples, the field of view and resolution requirements often cannot be satisfied simultaneously for SEM images obtained using the same magnification. Thus, the sand particles at the same position were amplified 2500, 5000, and 10 000 times. The following figures (Figures 6 and 7) present the images of sand and silt particles obtained by SEM before the direct shear experiment. It is found that the sand particles have an open flocculation structure with distributed small particles and flaky soil particles, and the surface is smooth (Figure 6). The silt particles also have an open flocculation structure in an agglomeration shape, and a large number of flaky soil particles are distributed on the surface, with a concentrated distribution (Figure 7). After magnification, it is found that the surface topography of silt particles is lamellar, more complex, and coarser than that of sand. Moreover, the geometric shapes of fine particles in sand and silt have self-similar characteristics, that is, fractal characteristics.

               

               

3.4 Composition of silt and sand measured by an X-ray diffraction experiment

The physical and mechanical properties and microstructure of soils are related to their mineral composition. XRD analysis is an important method for studying the phase and crystal structure of a substance. The XRD method is a research tool that is used to obtain the composition of materials and the structure or morphology of atoms or molecules within materials. As a modern scientific method used to analyze the structure and composition of materials, XRD analysis has been widely used in the research and production of various disciplines. Since each mineral crystal has a unique XRD, and its corresponding characteristics do not change after mixing with various substances, the X-ray micro-diffraction test was conducted in this study to determine the mineral composition of the target sample.

The experimental instrument is a powder crystal X-ray diffractometer (D8 Advance; Bruker). The measurement accuracy is angle reproducibility ±0.0001°. The XRD patterns of sand and silt (Figure 8) show that the phase composition of the sample can be roughly obtained by comparing the curve with the standard line spectrum. The mineral composition of the sample can be roughly inferred from the composition of the compound. The mineral composition of sand is mainly quartz, with a small amount of albite and microcline (Table 3). The mineral composition of silt is mainly quartz and albite, with a small amount of boron nitride and potassium nepheline.

               

4.1 Shear strength characteristics of submarine soil

In this experiment, remolded samples were used for direct shear tests on silt and sand in the Yellow Sea. The results showed that the shear strength of silt and sand increases with the increase of normal stress. When the normal stress is 50 kPa, the shear strength of silt is 4.155 kPa and that of sand is 4.423 kPa (Figure 5). Sun (2000) studied the strength characteristics of coral sand in Nansha and found that when the normal stress was 50 kPa, the shear strength of coral sand samples was about 50 kPa, which is much higher than that of the soil samples tested in this study. According to the study on the shear strength characteristics of calcareous sand in the South China Sea by Chen et al. (2020), the shear strength of calcareous sand samples of different grades was all above 10 kPa when the normal stress was 50 kPa, which is also higher than that of the soil samples tested in this study. According to the study on the mechanical properties of Nansha soft soil by Wu (2020), when the normal stress was 50 kPa, the shear strength of the original soil sample was 13.04 kPa, but that of the soil sample reconstructed using the mud settlement method was 6.10 kPa, which is close to the results measured in this study. The above comparison reveals that the shear strength of silt and sand in the Yellow Sea measured in this study is relatively small because the grain gradation is poor and the sample is remodeled.

Generally, the Mohs hardness of quartz is about 7.5 and that of albite is 6.0–6.5. Their hardness and physical and chemical properties are different. According to Wang et al. (2020), in the crystal–chemical genetic characteristics of albite and its floatability prediction, there are many unstable chemical bonds in albite that readily produce water-soluble Na+ when encountering water. The albite content in silt is significantly higher than that in sand. The direct shear results show that the shear strength of silt is lower than that of sand at low vertical stress. It is inferred that the low hardness and instability of albite may adversely affect the shear strength of the soil. Boron nitride has a high Mohs hardness, second only to diamond. In addition, the physical and chemical properties of boron nitride are relatively stable, and it does not easily react with external substances in the normal state, which may explain why the shear strength of silt is higher than that of sand in the high-stress state.

Zhao et al. (2020) pointed out that, as an essential parameter of the mesostructure of sand, particle shape would inevitably have an impact on its mechanical properties. Combined with the mineral composition and content of the samples in this experiment, the main mineral composition of both is quartz. Quartz is a kind of mineral with stable physical and chemical properties and usually has a regular particle shape; the crystal belongs to the oxide mineral of the tripartite crystal system. The quartz content of sand is much higher than that of silt. The grain size of sand is larger, and its crystal structure is more likely to be damaged under high vertical stress, so the shear strength of sand is slightly greater than that of silt under high stress.

Zhang et al. (2008) pointed out that calcareous sand particles are brittle, and a large number of particles will be broken under shear, thus affecting the mechanical properties of calcareous sand. Their results indicate that the mineral composition of soil affects the failure form of soil particles and the mechanical properties of soil (Zhang et al., 2008). The XRD results show that the mineral composition of silt is more complex than that of sand, which may be the reason for the large difference between the two direct shear test results.

4.2 Changes in the soil microstructure after shearing

In this experiment, direct shear tests were carried out on submarine silt and sand, and the soil microstructures before and after direct shear tests were analyzed by SEM. The surface of the sand particles without shear was smooth and flat, without apparent cracks (Figure 9). After consolidation and shear experiments, a large number of “V”-shaped cracks of different sizes appeared on the surface of the sand particles, indicating that the soil particles were broken and the microstructure was unstable (Figure 9). The silt particles were also amplified 2500, 5000, and 10 000 times by SEM. A large number of flake soil particles were distributed on the surface of the original silt. The distribution was relatively concentrated, and the flake particles were mostly clustered and distributed (Figure 10). After consolidation and shear tests, many scattered granular soil particles appeared on the surface, forming irregular voids. The results indicate that the shear can destroy the flake particles on the surface and transform them into granular particles.

               

               

Jiang et al. (2010) analyzed the shear zone of natural soft soil using SEM and found that the particles in the shear zone of the South China Sea soft soil were broken into finer particles with a more discrete distribution, and the large pores were crushed into small pores without obvious orientation. Ren et al. (2019) tested the strength characteristics of deep-sea soft clay using an improved full-flow penetration device and found that flocs in soil particles would be destroyed in the full-flow cycle, and large flocs would be dispersed into small flocs. The failure state of deep-sea soft clay is similar to that of sand and silt, but the cracks on the surface of sand basically show a “V” shape (Figure 9). After shear tests, the soil structure changed to a certain extent, and the surface of sand particles changed from smooth and flat to having more “V”-shaped cracks. It is inferred that the surface shear cracks appear because the hard sand particles cannot easily disperse into finer particles. After the direct shear experiment, many flaky particles on the surface of the silt particles were scattered in granular form (Figure 10). He et al. (2021) studied the distribution of soil pores using the fractal theory. The results show that the pore structure in soil shows fractal characteristics, which can be used to quantitatively describe the geometric characteristics of the pore structure in the medium. Kong et al. (2022) analyzed the microscopic pore structure and fractal characteristics of soil using SEM and concluded that the fractal dimension could characterize the complex characteristics of soil microstructure. The results indicate that shear damages the soil particles to a certain extent but will not disintegrate the main structure, providing a reference for the future advancement of ocean engineering.

Through comparative analysis, sand particles also showed certain fractal characteristics before and after direct shear tests (Figure 9). Before direct shear, the surface was uniform and smooth, while many “V”-shaped cracks appeared on the surface with obvious changes after direct shear. Therefore, the fractal dimension after shear is greater than that before shear. The change difference of silt particles before and after direct shear tests is small (Figure 10), lamellar particles become smaller after shear tests, and the fractal dimension changes little compared with that before shear tests.