1. Introduction
The coral reefs of the Nansha Islands are the most valuable land resource in the South China Sea. The reefs are the foothold and forward base for the exploitation of marine resources. Coral reefs are the preferred construction sites for island airports. Compared with general airport runway pavements, the runways of island airports often need to be reclaimed due to the limitations of island sites. A coral reef island reclamation airport runway is characterized by high water content, loose structure, a large void ratio, high compressibility, and low natural strength [
1,
2]. This runway can easily produce a large or uneven settlement under large loads, short-acting time, and low frequency of aircraft takeoff and landing. Therefore, the dynamic response of coral sand in island reclamation airport runways under aircraft load must be understood to construct airports on coral reefs.
Coral sand is mainly composed of coral debris and other marine biological debris, and the calcium carbonate content reaches 90%. The unique properties of coral sand are due to its special material composition, structure, and development environment [
3,
4,
5]. Scholars have conducted studies on the characteristics of coral sand. Some have studied the microscopic structure of coral sand through electron microscopy, CT scanning, and mercury intrusion tests [
6,
7,
8]. They found that coral sand particles are mainly composed of coral reef debris with different particle sizes and original biological textures. The roundness of coral sand is different from that of inland sand with an extremely irregular shape and low hardness, causing the coral sand particles to easily break at the edges and corners. Coral sand is more prone to particle breakage than ordinary sand, and continuous particle breakage causes the grading of coral sand to constantly change, thus affecting its properties, such as shear expansion, compressibility, and critical state, and ultimately threatening project safety [
9,
10,
11,
12]. Sharma et al. [
13] studied the static characteristics of coral sand by using triaxial compression equipment, and their results showed that the dynamic strength of calcareous sand is related to its relative density and effective confining pressure, and it is particularly sensitive to confining pressure. Wang et al. [
14] found that the hydraulic conductivity of coral sand increases linearly with particle size. Rui et al. [
15] reported that the compression modulus of a sample changes with the increase in coral sand’s rod-like or lamellar particles, but the irregular property of coral sand can improve the sample’s resistance to soil liquefaction. Aissa et al. [
16] studied the fatigue characteristics of coral aggregate concrete under static compaction and revealed the good adhesion between the calcareous tuff and cement matrix. Sandoval et al. [
17] compared the normalized pore pressure development curves of calcrete and quartz sand under the same confining pressure and relative density through an equal consolidation triaxial compression test and found that calcrete sand presents a large pore pressure fluctuation, and its pore pressure development curve is convex at the top, whereas that of quartz sand is concave at the bottom. Shahnazari et al. [
18] systematically studied the evolution law of coral sand particle breakage during triaxial shear from the perspectives of particle size, compactness, confining pressure, shear strain, drainage conditions, and energy and found that input energy plays an important role in particle breakage behavior. Coop et al. [
19] investigated the particle breakage degree of coral sand under different shear strains through a ring shear test and discovered that coral sand no longer breaks after reaching a certain shear strain; the initial gradation and normal stress of the sample are the main factors that determine the final gradation. Wang et al. [
20] conducted a direct shear test on calcareous sand in the South China Sea and found that the cohesion of calcareous sand is much higher than that of quartz sand and increases considerably with the average particle size. Rezvani et al. [
21] compared the triaxial shear behaviors of two kinds of coral sand with different particle sizes under the same conditions and found significant differences in their stress–strain curves. Notably, the abovementioned research only studied the mechanical properties of coral sand from a static or quasistatic perspective.
The dynamic characteristics of coral sand must be explored to ensure that buildings with a coral sand foundation can withstand strong dynamic loads, such as earthquakes, strong impacts, accidents, or man-made explosions. Dong et al. [
22] conducted an SHPB impact experiment and found that coral sand exerts an obvious strain rate effect, and the strain rate sensitivity is closely related to the inner pores and the intergranular friction. Coral sand behaves differently from ordinary terrestrial sand under dynamic loads, such as earthquakes, tsunamis, and waves. Many scholars have studied the characteristics of coral sand under the dynamic cyclic load of earthquakes, tsunamis, and waves by using a triaxial apparatus [
23,
24]. Airey et al. [
25] examined the cycling performance of coral calcareous sand in Northwest Australia and found that the frequency of loading affects the pore pressure response measured at the ends of samples. Sadrekarimi et al. [
26] conducted a triaxial drainage cyclic loading test on coral sand and discovered that the particle breakage degree increases with the number of cycles. Wang et al. [
27] performed multiple consolidation and drainage triaxial shear tests to explore the effects of particle size and confining pressure on the shear characteristics of coral sand.
The structure of coral sand is characterized by nonuniformity, discontinuity, and anisotropy. The conventional finite element method has certain limitations when used to study coral sand’s mechanical properties, and the mechanical behavior of coral sand is difficult to determine from the microscopic perspective. The discrete element method (DEM) proposed by Cundall can determine the crushing characteristics of granular materials [
28]. The particle flow code (PFC) developed based on DEM has become an important simulation method and has been widely used in the numerical simulation of sand particles. Xu et al. [
29] studied the impact of particle size distribution on the impact resistance of cemented coral sand by using DEM. Their results showed that the impact resistance of cemented coral sand increases sharply at first then gradually decreases with increasing average particle size and uniformity coefficient. Alshibli et al. [
30] applied DEM to simulate a direct shear test on granular materials from the perspective of microscopic mechanics, with focus on the influence of shear sample size on the macroscopic properties of materials. Chen et al. [
31] used PFC to conduct a numerical simulation of the triaxial compression test on sand, and the results showed that the microscopic parameters of sand markedly affect the macroscopic response of sand. Zhang et al. [
32] used DEM to study the influence of sampling factors on the triaxial shear performance of cemented sand and found that peak strength and pre-peak stiffness are enhanced by decreasing the initial void ratio. Shamy et al. [
33] used the fluid–particle coupling method to study the liquefaction of saturated sand under dynamic loads and provided valuable information on a number of salient microscale response mechanisms. Jiang et al. [
34] conducted biaxial tests on structural loess with different water contents with the help of PFC to explore the collapsibility characteristics of loess and loess-like materials under complex stress paths.
Research on the mechanical properties of coral sand under static and dynamic loads has achieved fruitful results, but research on the mechanical characteristics of coral sand under dynamic cyclic loads is limited, and research on the dynamic response of coral sand under dynamic aircraft loads is even rarer. Additionally, the dynamic characteristics of coral sand are affected by the loading mode and coral sand’s own microstructure, particle size, porosity, and other factors. To fully explore the dynamic response of coral sand under cyclic dynamic loading, this study combined the dynamic triaxial compression test and particle discrete element analysis to study the dynamic response of coral sand under different dynamic stress amplitudes, vibration frequencies, particle sizes, and porosities.