*3.3. Molecular Di*ff*usion and Mechanical Dispersion Tests*

When the pore flow velocity is greater than 0, molecular diffusion and mechanical dispersion processes coexist. With an increase in average pore flow velocity, the weight of molecular diffusion versus mechanical dispersion changes. In order to further study the relationship between the weight and the flow velocity, a custom-designed molecular diffusion and mechanical dispersion test device was employed in this study, as shown in Figure 6. The device consisted of three components, namely, a dispersion body, a water supply part, and a data acquisition system. The dispersion body was made up of organic glass tubes and samples, with each tube being 8 cm in inner diameter and 80 cm in length. The samples were coral sands with particle sizes of 0.25–0.5 mm and a dry density of 1.3 g/cm<sup>3</sup> . The data acquisition system was equipped with CS655 multi-parameter sensors; the tracer was a 20 g *J. Mar. Sci. Eng.*  /cm<sup>3</sup> NaCl solution. **2019**, *7*, x FOR PEER REVIEW 9 of 21

**Figure 6.** Molecular diffusion and mechanical dispersion test device. (a) Schematic Diagram (b) Real Product(01: dispersion body, 02: water supply part, 03: data acquisition system). **Figure 6.** Molecular diffusion and mechanical dispersion test device. (**a**) Schematic Diagram (**b**) Real Product (01: dispersion body, 02: water supply part, 03: data acquisition system).

**4. Test Results and Analyses**  *4.1. Effect of Particle Size on the Dispersion Coefficient*  In order to uncover the effect of particle size on the one-dimensional dispersion coefficient, The required coral sand samples were first prepared in the dispersion tubes. The samples were then saturated with fresh water, after which the inlet and outlet valves of the dispersion tube were shut. The NaCl solution was then injected into the tracer injection port and the injection port was shut. This was completed while simultaneously starting data acquisition, with the pore flow velocity in the device being 0. The above steps were repeated using different concentrations of NaCl solution.

dispersion tests were performed on a total of six groups of single-grained coral sand samples, with the sample groups having a particle sizes of < 0.1 mm, 0.1–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm, and 2–5 mm; the groups had particle size ranges within the categories of silt, fine sand, medium sand, coarse sand, gravelly sand, and crushed stone (angular gravel), respectively. These groupings were determined according to the soil classification method in the Code for the Investigation of Geotechnical Engineering (GB50021-2001) (Ministry of Construction of the People's Republic of Next, dispersion tests with pore flow velocities greater than 0 were conducted according to the above steps, with the exception that the inlet and outlet valves of the dispersion tube were reopened after the injection of the tracer to allow the pore fluid at a certain flow velocity to pass through the samples. The pore flow velocity was controlled by adjusting the output power of the water pump. By repeating the above test procedure, dispersion tests were performed at several different pore flow velocities.

Figure 7 presents a one-dimensional dispersion curve for single-grained coral sands, which indicates that when the particle size was 0–0.1 mm, the dispersion process consisted of three stages: a drainage stage, a displacement stage, and a stabilization stage. In the drainage stage, the original

China, 2009) [16]. The dry density of all samples was 1.3 g/cm3.

value and tending to flatten.

the tracer, the tracer underwent diffusion in the original saturated solution, in addition to producing the displacement effect. The diffusion "interface" existed in the form of a concentration transition zone between the original saturated fluid and the tracer. When the edge of the transition zone reached the sensor position, the measured concentration of the discharged fluid began to increase, indicative of the onset of the displacement stage, as manifested by the curve slope starting to increase. In the stabilization stage, when the tracer had completely displaced the original saturated fluid, the discharged fluid had a concentration similar to that of the tracer, with the curve reaching the highest
