Physical and mechanical properties and microstructures of submarine soils in the Yellow Sea
In recent years, the exploration of seabed has been intensified, but the submarine soils of silt and sand in the Yellow Sea area have not been well investigated so far. In this study, the physical and mechanical properties of silt and sand from the Yellow Sea were measured using a direct shear apparatus and their microstructures were observed using a scanning electron microscope. The test results suggest that the shear strength of silt and sand increases linearly with the increase of normal stress. Based on the direct shear test, the scanning electron microscope was used to observe the section surface of sand. It is observed that the section surface becomes rough, with many “V”-shaped cracks. Many particles appear on the surface of the silt structure and tend to be disintegrated. The X-ray diffraction experiment reveals that the sand and silt have different compositions. The shear strength of sand is slightly greater than that of silt under high stress, which is related to the shape of soil particles and the mineral composition. These results can be a reference for further study of other soils in the Yellow Sea; meanwhile, they can serve as soil parameters for the stability and durability analyses of offshore infrastructure construction.
The physical and mechanical properties of silt and sand from the Yellow Sea are measured and microscopically explored.
After the direct shear test, it is found that the section surface of sand becomes rough, with many “V”-shaped cracks. Many particles of the silt structure appear on the surface and tend to disintegrate.
The X-ray diffraction experiment reveals that the compositions of sandy and silty soils are different.
The physical properties of submarine soils are increasingly important parameters for ocean exploration. They affect the occurrence of geohazards such as liquefaction, landslides, and other geological disasters, thus directly relating to the quality and safety of submarine structures and geoenvironments. Therefore, study of the physical and mechanical properties of submarine soil and its microstructures is conducive to the safety of marine engineering and marine infrastructure projects. The Yellow Sea area is a favorable area for evaluating strategic marine oil and gas resources, and thus it is of great value for the study of the soil in the Yellow Sea.
Research on the physical and mechanical properties of submarine soil began in the 1960s and has yielded useful results. In 1966, the Institute of Oceanology, the Chinese Academy of Sciences, in cooperation with Dagang Oilfield and other institutions, conducted a detailed investigation of the engineering properties and geological conditions of submarine soil in the western Bohai Sea for the first time. Sun (2000) studied the physical properties of coral sand in Nansha Islands. Zheng et al. (2004) studied the physical properties and characteristics of the submarine soil in the South China Sea by measuring its water content, density, and main compositions. Based on the analysis of data of soil samples from the Zhongsha gas hydrate prospect area in the South China Sea, Wei (2006) comprehensively studied the engineering geological characteristics of submarine soils and concluded that they have high water content, high porosity ratio, low density, and high plasticity. Liu et al. (2017) conducted laboratory geotechnical experiments and compared the physical and mechanical properties of submarine soils. Jing et al. (2021) carried out the dynamic triaxial liquefaction experiment to simulate wave loads of different strengths.
The physical properties and mechanical properties of submarine soil are closely related to the microstructure. Many studies have been conducted in the China sea. Using the fall cone method, Ren et al. (2021) tested the recovery characteristics of the thixotropic strength of deep-sea soft clay and explored the microscopic changes in the thixotropic process by scanning electron microscopy (SEM). Ding et al. (2019) studied the dynamic characteristics of frozen–thawed soil under cyclic loading using the dynamic triaxial test. Liu et al. (2021) measured the unconfined compressive strength of submarine soft soil and analyzed the microstructure of the soil stabilized using the SEM technique. Using SEM, Watabe and Saitoh (2015) found that the high porosity of natural clay soils was not only due to the high initial water content but also the sedimentation of flocculation, which played an important role in the formation of the microfabric of natural clay soils. Li (2017) analyzed the components of reticulated laterite through the X-ray diffraction (XRD) pattern. Lai et al. (2018) determined the pore characteristics and their coupling effect on oil-bearing properties by analyzing the size and distribution of the entire pore throat structure in E-type sandstone using SEM and XRD. The SEM results showed that the soil mass on the northeastern slope of the South China Sea presents an open flocculation structure with a certain directional distribution of particles. However, it is generally scattered and less directional (Jiang et al., 2017; Nian et al., 2018), and mostly fine granular clusters in sheet and pellet forms, with high porosity and large size (Nian et al., 2018). Recent studies revealed that the shear stress and apparent viscosity of submarine soil at low temperatures can be significantly increased compared with those at room temperature (Guo et al., 2020). All these previous studies focused on the submarine soils in other seas in China. However, the soil in the Yellow Sea might have different properties.
The engineering properties of soils in the Yellow Sea have been studied because this area is an important oil and gas resource area in China. Li et al. (2012) studied the microstructure characteristics of clay in the central South Yellow Sea based on the microstructure theory and soil property technology. The relationship between physical and mechanical properties and particle contact, particle accumulation, and porosity was explored based on the parameters of these properties. Cheng et al. (2011) investigated the physical properties of submarine soils by analyzing columnar soil samples from the South Yellow Sea by laboratory tests and found the characteristics of compression wave velocity, high porosity ratio, and high plasticity. Zhou (1991), Lu et al. (2006), and Li et al. (2001, 2005) discussed the distribution characteristics of water content and natural bulk density of surface soil in the Yellow Sea and studied the physical and mechanical properties of soil mass. Meng et al. (2015) measured the physical properties of various soils in the Yellow Sea and discussed the characteristics of submarine deposition, which is an indispensable preliminary work for ocean development. Li et al. (2005) provided a more detailed understanding of the physical and mechanical properties of submarine soils by exploring the properties of clay soils in the South Yellow Sea resource area as well as the size and characteristics of particles. On the basis of measured data on the shear wave velocity of submarine soil in the Yellow Sea, Kan et al. (2014) found some correlation between soil shear wave velocity and physical and mechanical parameters. In addition, in situ testing on the mechanical properties of submarine soils has been widely carried out (Zhang et al., 2019). However, most of the studies on the soil in the Yellow Sea focused on the basic physical properties. The mechanical properties and microstructure of the marine soil in the Yellow Sea are less studied. The physical properties, mechanical properties, and microstructures of submarine soil are interconnected. For the oil- and gas-rich Yellow Sea, it is critical to understand the physical and mechanical properties, composition, and microstructures of the submarine soil in the Yellow Sea area.
On this basis, the physical and mechanical properties and microstructure of the soil mass in the Yellow Sea were analyzed in this study through physical and mechanical experiments, SEM, and XRD. First, the basic physical properties of silt and sand were measured, including density, water content, and particle size of soil. Second, the shear strength of the silt and sand was compared by a direct shear experiment with loads of 50, 100, 200, and 400 kPa. The microstructures of soil samples before and after the shear experiment were observed and compared using SEM. The surface morphological changes of particles were explored to unveil the physical properties of the soil at the microscopic level. Then, the composition of the submarine soil, the material inside the atomic and molecular structures, and its shape were obtained through an XRD experiment analysis of the diffraction pattern.
2 OVERVIEW OF THE STUDY AREA
Due to the different geological and sedimentary environments of the seafloor in the Yellow Sea, the physical and mechanical properties and microstructures of the submarine soil in the Yellow Sea might also be quite different. Hence, the experimental study on the submarine soil collected in the Yellow Sea has certain characteristics. The sea area under study is near Lianyungang City, northern Jiangsu Province, China. The latitude and longitude of the sampling point are 119.38°E and 34.77°N, respectively; 200 L of silt and sand were collected from the beach of Lianyungang City, respectively. The sampling point was submerged during the high tide. After the soil was collected, the soil samples were sealed and stored in a thermostatic sealed environment to minimize the disturbance to the soil samples. The coast in northern Jiangsu Province can be divided into gravel coast, bedrock coast, and silty coast according to their components (Li, 1996). The silt and sand studied in this paper are mainly collected from the sandy and gravel coast in the coastal area.
3 LABORATORY EXPERIMENTS AND TEST RESULTS
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).
Table 1. Physical properties of silt and sand.
Water content (%)
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
τ denotes the shear stress, kPa;
is the deformation coefficient of the stressed steel ring,
R is the deformation value of the stressed steel ring.
Table 2. Shear strength of silt and sand at different normal stresses.
Normal pressure (kPa)
Shear strength of silt (kPa)
Shear strength of sand (kPa)
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.
Table 3. Mineral composition of sand and silt.
BN (boron nitride)
VF3 (vanadium monoammonium trifluoride)
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.
Although submarine soils in different seabeds have been studied in terms of their physical and mechanical properties, there are few studies on the physical and mechanical properties and microstructures of soils in the Yellow Sea. In this study, the physical properties, shear strength, and microstructure of sand and silt collected from the Yellow Sea area were investigated and the following conclusions were drawn.
In this study, the low shear strength of sand and silt in the Yellow Sea is related to the poor grain gradation and the remolded samples. The shear strength of sand is slightly greater than that of silt under high stress, which is related to the shape of soil particles and the mineral composition. SEM showed that the section of sand became rough, and many “V”-shaped cracks appeared after the direct shear test. After the direct shear tests, the structure of silt is dominated by fine particles and tends to disintegrate. The results indicate that soil particles are destroyed by shear to a certain extent. The physical properties, mechanical properties, and the microstructure of submarine soil are interactional.
Many factors should be considered in the construction of offshore infrastructure, including the corrosion effect of seawater and the stability of building construction. This study provides an excellent reference for the stability of offshore construction projects, which can have a far-reaching influence on the prevention of marine accidents.
This research was funded by the Natural Science Foundation of Jiangsu Province (Grant No. BK20210527), the National Natural Science Foundation of China (Grant No. 42107158), and the undergraduate Training Program for Innovation and Entrepreneurship, China University of Mining and Technology.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
Zhuangcai Tian, professor (assistant), is from the State Key Laboratory of Deep Rock and Soil Mechanics and Underground Engineering (Research Center for Deep Ocean Science and Underwater Engineering), China University of Mining and Technology. He obtained his PhD in environmental geological engineering from the Ocean University of China in 2020. Currently, his focus is on prevention and control of marine engineering geological environment and disaster, including the interaction between deep-sea internal waves and the seabed or structures, the mechanical properties and engineering characteristics of marine soil, and application of artificial intelligence in geological disasters.
Jianhua Yue, professor, is from the School of Resources and Geosciences (Research Center for Deep Ocean Science and Underwater Engineering), China University of Mining and Technology. Currently, his main research areas are electrical exploration, mine geophysics, engineering and environmental geophysics.