Large-Scale Triaxial Test on Mechanical Behavior of Coral Sand Gravel Layered Samples

Abstract

Layered structures comprising coral sand and gravel have been observed in hydraulic filled foundations in the coral reefs in the South China Sea, leading to anisotropy in their physical and mechanical properties. However, the effect of a layered structure on the strength and deformation of the coral soil foundation remains unclear. In this study, a series of large-scale triaxial compression tests and step-loading tests were carried out on four types of samples, i.e., clean coral sand, clean coral gravel, sand-over-gravel layered sample, and gravel-over-sand layered sample, to investigate the impact of confining pressure and the layered structure on the strength and failure modes of these soils. The results indicate that the stress–strain relationships of all samples predominantly exhibit strain hardening under drained conditions. Under identical confining pressures, the peak strength of clean coral sand is the lowest, while that of coral gravel is the highest. The peak strengths of the two layered samples fall between these extremes, with the gravel-over-sand layered sample exhibiting higher strength. All four samples have similar peak friction angles, slightly exceeding 40°. The difference in peak strength among the four types of samples is attributed to the variations in cohesion, with the cohesion of clean coral gravel being up to four times that of clean sand, and the cohesion of layered samples falling between these two. Both clean sand and clean gravel samples exhibit a bulging phenomenon in the middle, while the layered samples primarily exhibit bulging near the coral gravel layer. In the step-loading tests, the bearing capacity of the layered samples falls between those of clean coral sand and coral gravel, with the gravel-over-sand layered samples demonstrating higher strength. Moreover, the p-s curve of the gravel-over-sand layered samples obtained from the large-scale triaxial apparatus under a confining pressure of 400 kPa resembles that from the plate load tests on the same samples.

1. Introduction

In recent years, extensive land reclamation projects have been undertaken in the South China Sea. Due to the considerable distance from the mainland, coral sand is the only available hydraulic filling material. As a result, coral sand and debris from lagoons and outer reef flat were excavated by cutter suction dredgers or hopper suction dredgers and then pumped through pipelines onto the inner reef flat [1,2,3,4,5,6]. During the transportation and placement of the sand–water mixture, segregation occurred in both the horizontal and vertical directions, leading to heterogeneity in the foundation. In the horizontal direction, coarse particles are deposited near the pipeline, while finer particles tend to accumulate downstream [7]. Vertically, coarse particles settle rapidly at the bottom, whereas finer particles are deposited at the top, as shown in Figure 1. Consequently, well-sorted gravel and sand layers are observed in the hydraulic filling areas in the coral reefs [8].

In nature, this kind of layered deposit is widely observed, particularly in river basins [9], reclaimed land areas [10], and tailing dams built using the up-stream method [11]. Yoshimine and Koike [12] found that the liquefaction resistance of stratified samples is higher than that of uniform sand samples. To date, limited research has focused on the monotonic mechanical behavior of sand with stratified structures. Naeini and Baziar [13] investigated the influence of fines content on the residual strength of layered sand samples and found that the normalized shear strength decreases with an increasing fines content up to 35%. Zhang et al. [14] examined the strength and deformation of tailings with fine-grained interlayers, discovering that the shear strength of the layered structures is significantly lower than that of coarse-grained tailings. Moreover, particle movement varied across different layers, with more pronounced movement observed in the upper layer. The thickness of the fine-grained soil layer significantly affected the stress–strain behavior and failure mode of the samples. Shen et al. [15] studied the undrained shearing behavior of artificial clay and silt layered samples and found that the friction angle of the layered samples lies between those of sand and clay. Therefore, the monotonic and dynamic mechanical behavior of layered samples may be either under-estimated or over-estimated when examining homogeneously reconstituted samples.

Coral deposits are mainly composed of coral, shells, and other marine biological remains, with calcium carbonate content higher than 90% [16]. So far, many studies have investigated the physical and mechanical behavior of coral sand. Due to its biological origin, coral sand displays distinct physical and mechanical behavior compared to terrigenous sediments such as quartz sand. The particle shape of coral sand is notably irregular, and the surface is quite rough due to the existence of intraparticle pores [17,18]. Yao and Li [19] found that the compressibility of coral sand at low stress levels is even lower than that of quartz sand. Under high stress levels, coral sand demonstrates high compressibility primarily due to particle fragmentation [20,21]. The shear strength of coral sand mainly depends on the cohesion and the internal friction angle. Due to the irregular shape of the coral sand particles, the internal friction angle is typically higher than that of quartz sand [22]. The cohesion of coral sand, resulting from particle interlocking, is a significant factor that cannot be ignored, contributing to a substantially higher shear strength compared to quartz sand. The magnitude of the cohesion is influenced by the particle shape, particle size, and stress level [23]. The bearing capacity of coral sand is a crucial indicator to evaluate its engineering performance. Compared to quartz sand, coral sand exhibits a significantly higher bearing capacity [24]. Consequently, the dredged coral sand could provide satisfactory bearing capacity, with post-construction settlement being found to be insignificant [25].

Nevertheless, the quantity and quality of studies on the mechanical behavior of coral gravels, which are widely distributed in hydraulic filling areas in coral reefs, remain quite limited. Liu et al. [26] studied the single-particle strength of coral gravels with sizes ranging from 5 mm to 20 mm and found that the single-particle strength decreases with increasing particle size, while the apparent cohesion of coral gravel increases with increasing particle size [27]. Wu et al. [28] conducted large-scale dynamic triaxial tests on coral sand–gravel mixtures and discovered that the liquefaction resistance of the mixture is much higher than that of clean coral sand, as the addition of coral gravel forms a more stable structure. The mechanical properties of the layered samples differ from those of naturally deposited homogenous sand. Considering the segregation of coral sand and gravels, Fu et al. [29] examined the bearing characteristics of coral sand and gravel-layered samples through plate-loading tests. It was discovered that the bearing capacity of the layered sample increases with the thickness of the upper gravel layer. However, the effect of the anisotropy of the layered structure of coral gravel and sand on the shearing behavior of coral soil remains unknown.

In the current work, a series of monotonic large-scale triaxial tests was carried out on both homogenous samples and layered samples of coral sand and gravel to investigate the effect of the layered structure on the shearing behavior of coral deposits. The stress–strain relationship and the shear strength of the samples were carefully analyzed and compared. In addition to the triaxial tests, step-loading tests were performed using the large-scale triaxial apparatus to study the bearing capacity of the layered samples. Both the coral sand and gravel were collected from a dredged area in a coral reef in the South China Sea. The maximum particle size of the gravels used in this study was 60 mm, due to the size limitations of the triaxial apparatus.

2. Experimental Programs

2.1. Test Materials and Apparatus

In this study, the tested coral sand and coral gravel were sourced from a coral reef in the South China Sea. As shown in Figure 2, the particle shape of both the coral gravel and sand is quite irregular, and the surfaces are fairly rough with abundant intraparticle pores.

To quantitatively characterize the particle morphology of both materials, a Microtrac PartAn^3D Maxi (Montgomeryville, PA, USA) dynamic image analysis apparatus was employed. This system employs a high-speed, high-resolution camera to capture multiple images of each particle from arbitrary orientation. The images are then digitized and processed by the PartAn^3D (Version: PartDP-GC-20151002T143858) software, which measures particle length, width, thickness, perimeter, and area. Based on these measurements, morphological parameters, including aspect ratio, roundness, and sphericity, are calculated. This method allows for noncontact measurements of dry particles ranging in size from 0.16 to 135 mm.

Figure 3a presents multiple digital images of individual coral sand and gravel particles, offering a visual comparison of their morphological properties. In the current work, sphericity and roundness are employed to quantitatively assess the particle shape, which are defined as follows:

(1)$\mathrm{Roundness} \mathrm{R}=4\mathrm{A}/\mathsf{\pi }{FL}^2$

(2)$\mathrm{Sphericity} \mathrm{S}=4\mathsf{\pi }\mathrm{A}/P_{\mathrm{p}}^2$

where A represents the particle’s projected area; FL is the largest Feret diameter (the maximum distance between two paralleled planes that bound the particle perpendicularly), and P_p is the particle perimeter. In general, higher values of roundness (R) and sphericity (S) indicate that the particle is more rounded and closer to a sphere shape. As shown in Figure 3a, the roundness of branch-shaped particle 3 is only 0.29, which is much lower than that of rodlike particle 5 (0.84).

However, an individual particle or a few particles is not representative of the entire sample. To provide a more comprehensive view, the cumulative distributions of roundness and sphericity of both coral gravel and sand are shown in Figure 3b. The mean values of sphericity of coral sand and gravel are similar, with both falling approximately at 0.83, which is similar to those found by Wei et al. [30]. The mean value of roundness of coral sand is 0.42, which is much higher than that of coral gravel (0.30), indicating that the particle shape of coral gravel is much more angular.

Mineral composition analysis of both materials was performed using a diffractometer (D8 Advance, Bruker, Karlsruhe, German) in accordance with the SY/T5163-201 standard [31]. Figure 4 shows the X-ray diffraction test results, revealing that the coral gravel and the coral sand are predominantly composed of aragonite and calcite, with calcium carbonate (CaCO₃) content in both materials exceeding 95%. The maximum particle size allowed by the large-scale triaxial apparatus used in this study is 60 mm; therefore, gravels coarser than 60 mm were removed prior to the test. Figure 5 shows the particle size distributions of coral sand and gravel. The particle size distribution curve of the coral sand is notably uneven, with a coefficient of uniformity (C_u) of 4.53 and a coefficient of curvature (C_c) of 0.9. According to the Unified Soil Classification System (ASTM D2487-00 2016) [32], the conditions of C_u > 5 and 1 < C_c < 5 are not met, indicating poor gradation of coral sand. Similarly, the coral gravel sample also exhibits poor gradation. The minimum and maximum dry densities of the coral gravel were determined using the large-diameter cylinder method and surface vibration compaction method, respectively, in accordance with the Chinese National Standard of Soil Test Method (GB/T50123–2019) [33]. The basic physical parameters of the tested coral sand and gravel are summarized in Table 1.

The experiments were conducted using a large-scale static and dynamic triaxial apparatus (TAJ-2000, Tianshui Hongshan Test Machine Co., Tianshui, China), as depicted in Figure 6. This device comprises a confining pressure control system, an axial pressure control system, and a data acquisition system. It is capable of performing tests on coarse-grained soil with sizes finer than 60 mm, as the sample size is 300 mm in diameter and 600 mm in height. The maximum allowable confining pressure of the apparatus is 10 MPa, and the maximum axial loading force is 2000 kN. A linear variable displacement transducer (LVDT) (Sino Co., Guangzhou, China) with a resolution of 0.1 mm is located on the top of the chamber to measure the axial strain of the samples during shearing, and the maximum displacement range is 300 mm.

2.2. Test Procedure

In this study, a series of large-scale triaxial consolidation drained shearing tests was conducted on four types of samples, which are clean coral sand, clean coral gravel, a sand-over-gravel layered sample, and a gravel-over-sand layered sample, as shown in Figure 7. These tests aimed to understand the mechanical behavior of different foundation structures formed by hydraulic filling. During this process, hydraulic sorting results in a layered foundation structure with an alternating distribution of coral sand and gravel. Hence, two kinds of layered samples were tested: one sample with a sand layer on the top and the other one with a gravel layer on the top, as shown in Figure 7b,c.

To compare the differences in the stress–strain relationship and peak strength between homogenous samples and layered samples, the relative densities of all samples were controlled as 80% according to the findings by Wang et al. [9] and Yao and Li [34]. The samples were directly prepared on the large-scale triaxial apparatus pedestal using the dry tamping method, where pre-determined quantities of oven-dried coral sand or gravel were compacted in 10 layers using a cylindrical tamper. To ensure the uniformity of the sample, the undercompaction technique introduced by Ladd [35] was adopted. Prior to sample reconstruction, the membrane was carefully marked at every 60 mm using an oil marker to strictly control the height of each layer. For the layered samples, both the thickness of the gravel and sand layers were 300 mm. To avoid particle breakage during sample preparation, care was taken during compaction based on the experience of Yao and Li [19]. After preparation, both the height and diameter of the sample were accurately measured to facilitate the calculation of the initial void ratio. The samples were flushed with carbon dioxide and then with de-aired water for approximately 6 h, and subsequently saturated with a back pressure higher than 200 kPa until the B value exceeded 0.95. After saturation, the samples experienced isotropic consolidation to the desired confining pressure and were subjected to shearing under drained conditions at a shearing rate of 0.5 mm/min until the axial strain stabilized or reached 15%. The confining pressures were 200 kPa, 400 kPa, 600 kPa, and 800 kPa; therefore, 16 tests were conducted in total. The testing program is summarized in Table 2.

In practical engineering projects, the bearing capacity of foundations is typically assessed using on-site plate-load tests. However, discrepancies exist between the stress states experienced by the soil under plate load and those under actual building foundations. Studies have shown that both the bearing capacity and the deformation modulus of the foundation increases with increasing confining pressure [36]. Moreover, the filling material is often non-homogenous, especially in reclaimed land areas. Nevertheless, the influence of the confining pressure and anisotropy on the bearing capacity of coral deposits foundation remains poorly understood. In addition to the conventional triaxial compression tests, step-loading tests using the large-scale triaxial apparatus were also conducted on the four types of samples. During the step-loading tests, an axial load was applied to the samples in increments of 7 KN, and the vertical strain was recorded at each loading level. The resulting p-s curves of the gravel-over-sand layered samples were then compared with the plate load tests on the identical layered samples performed by Fu et al. [29].

3. Test Results

3.1. Stress–Strain Relationship

Figure 8 depicts the stress–strain relationship of four types of samples under drained conditions with increasing confining pressure. As can be seen from Figure 8a, the stress–strain relationship of coral sand exhibits strain hardening under different confining pressures without an obvious peak of the deviatoric stress (σ₁−σ₃). At low stress levels (≤200 kPa), the stress increases rapidly with axial strain, reaching a peak at a relatively low axial strain (3% to 8%) and then keeps constant at large strains. According to studies on the shearing behavior of uniform coral sand finer than 2.0 mm, the strain-hardening behavior usually occurs at high stress levels due to significant particle breakage [34,37]. In the current work, the tested coral sand contained more than 20% particles coarser than 5.0 mm. Under shearing, the inter-particle voids of the samples are compressed, and the volume of the samples decreases. As a result, the samples display strain hardening even under relatively low confining pressure. When the confining pressure increases up to 400 kPa, even the samples undergo strain hardening, the stresses keep on increasing with axial strain, and a less obvious stabilization can be observed at the end of the test. This behavior is possibly due to the particle breakage under high stress levels in coral sand and that the finer particles generated from the breakage fill the pores between larger particles, causing continued volumetric contraction and a consequent increase in deviatoric stress with axial strain [38]. In the current work, quantifying the particle breakage of coral sand or gravel is challenging due to the large sample mass (50–70 kg). However, information on the particle breakage of coral sand can be found in the literature [39,40,41].

Figure 8b shows the stress–strain relationship of coral gravel under drained conditions. Similar to those of clean coral sand, the coral gravel samples also displayed strain hardening under different stress levels. Differently from that of coral sand, under high stress levels (≥400 kPa), the stress of clean coral gravel tends to stabilize at the axial strain around 8–10%. The maximum particle size of the gravels is up to 60 mm. According to the findings of Liu and Li [42], the single-particle strength of coral gravel increases with increasing particle size. Additionally, the particle shape of gravels is quite angular compared to coral sand as can be seen from Figure 2 and Figure 3. During shearing, the inter-particle friction of coral gravels is much higher than those of coral sand. Consequently, it is hypothesized that the particle breakage of coral gravel is much lower than that of coral sand, resulting in less volumetric change under shearing.

In this study, two kinds of coral sand and gravel-layered samples were tested. The test results of the sand-over-gravel layered samples are illustrated in Figure 8c. Similar to those of clean coral sand and gravels, the sand-over-gravel layered samples also exhibited strain hardening even under low confining pressures. When the samples sheared at 200 kPa, a much lower peak strength (734.4 kPa) can be observed in sand-over-gravel layered samples compared to the clean coral gravel (1071.5 kPa). The peak strength of clean coral sand (477.4 kPa) is slightly lower than that of clean coral gravel, resulting in a lower strength of sand-over-gravel layered samples. Figure 8d displays the stress–strain relationship of gravel-over-sand layered samples tested under drained conditions. It can be observed that although the stress–strain curves of the gravel-over-sand layered samples under different confining pressures also show strain hardening, the stresses of the samples at high confining pressures tend to stabilize at relatively lower axial strains. This indicates that for the layered samples, the position of the coral sand and gravel layer affects the stress–strain relationship of the samples, with the curves primarily influenced by the material in the upper layer.

3.2. Shear Strength

According to the Chinese National Standard of Soil Test Method (GB/T 50123-2019) [33], if there is no distinct peak in the stress–strain curve of a sample under drained conditions, the deviatoric stress at 15% axial strain is generally taken as the peak strength. For samples where the deviatoric stress stabilizes before reaching 15% axial strain, the stabilized value is considered as the peak strength. If the deviatoric stress stabilizes only at 15% axial strain, the strength at 15% axial strain is taken as the peak strength [33]. The peak deviatoric stress of the samples under increasing confining pressures is shown in Figure 9. In the figure, the four variables in the X-axis represent the four types of the samples, i.e., clean coral sand, clean coral gravel, sand-over-gravel layered sample, gravel-over-sand layered sample. The peak strength of all the samples increases with increasing confining pressure. Under identical confining pressure, the peak strength of the layered samples falls between those of clean coral sand and clean coral gravel. Furthermore, for layered samples with the gravel layer on top, the peak strength is higher. This finding suggests that, in layered coral sand and gravel samples, the peak strength is predominantly influenced by the material in the upper layer.

In geotechnical engineering, the Mohr–Coulomb criterion is a fundamental principle used to describe the failure criterion of materials under shear stress. The Mohr–Coulomb failure criterion is mathematically expressed by the following equation:

τ_f = c + σ tan φ(3)

where τ_f is the shear strength; c denotes the cohesion of the material; σ is the normal stress, and φ is the internal friction angle of the soil.

Thus, the shear strength of soil is composed of cohesion and inter-particle friction. In general, cohesion reflects the inherent strength of the soil due to inter-particle bonding. Sand or gravel, being non-cohesive soil, are typically assumed to be cohesionless. However, in the shear test results of coral sand samples, significant cohesion is observed in the Mohr–Coulomb envelopes [43]. This cohesion primarily arises from the interlocking induced by the irregular shape of coral sand particles.

The Mohr–Coulomb theory is visually represented using Mohr’s circle, a graphical method illustrating the relationship between normal stress and shear stress on different planes within the material. Figure 10 shows Mohr’s circles for clean coral sand under different confining pressures. In this figure, the envelope tangent to Mohr’s circles represents the failure criterion. The slope of the envelope corresponds to the friction angle (tan φ), and the intercept on the Y-axis represents the apparent cohesion (c). The internal friction angle of clean coral sand can be as high as 40°. This value is consistent with research findings on coral sand from other regions, which typically report peak friction angles ranging from 37.8° to 50° [44,45,46]. Although the single-particle strength of coral sand or gravel is considerably lower than that of quartz sand, its peak friction angle is significantly higher (28–29°) [47], which is attributed to the irregular shape of coral sand particles. The apparent cohesion of clean coral sand calculated from the envelope is around 8 kPa and is believed to result from particle interlocking.

Based on the triaxial compression data under drained conditions for four types of samples, the shear strength parameters, notably the apparent cohesion and internal friction angle values, were calculated following the Mohr–Coulomb criterion and summarized in Table 3. The difference in the friction angle among the four types of samples is insignificant, while the apparent cohesion varies. Among these samples, coral gravel exhibits the highest apparent cohesion at 35.2 kPa, nearly four times that of clean coral gravel (8.4 kPa). Consequently, clean coral gravel exhibits the highest peak strength under identical confining pressure. Since the layered samples consist of both a coral sand layer and a coral gravel layer, their apparent cohesion falls between that of coral sand and gravel. Notably, the apparent cohesion is significantly higher in the layered samples with the gravel layer on top.

3.3. Failure Mode

Due to the large size of the samples, the confining pressure chamber of the large-scale triaxial apparatus is made up of steel. To determine the failure mode of the samples, the chamber was removed, and photographs of the samples were taken. Figure 11 presents the failure modes of the four types of samples sheared at a confining pressure of 200 kPa. All samples exhibit bulging failure modes without the occurrence of strain localization (shear bands). However, the lateral deformation of the samples varies. The coral sand and coral gravel samples present bulging deformation in the middle, while in the layered samples, bulging deformation is mainly observed in the gravel layer. It is also found that the clean coral sand samples exhibit less lateral deformation, whereas the samples containing a gravel layer show more pronounced lateral deformation by visual observation. Figure 12 shows the failure mode of the gravel-over-sand layered sample sheared at a confining pressure of 400 kPa. A much more pronounced bulging phenomenon has been observed in the sample under higher confining pressure. At low confining pressures, the particles in the sample first experience slippage and rearrangement, followed by axial compaction, and then lateral expansion during shearing [48].

4. Step-Loading Test by Large-Scale Triaxial Apparatus

Figure 13 presents the p-s curves of four types of samples under a confining pressure of 400 kPa, where s represents the axial displacement of the sample under loading, and p illustrates the vertical stress, calculated as the axial force over the cross-sectional area of the samples. The p-s curves of the four samples could be divided into three segments. Using the sand-over-gravel layered sample as an example, these segments can be described as: (I) the initial linear deformation stage, representing the elastic behavior of the soil in which stress and displacement are directly proportional; (II) the non-linear transition stage, indicating the onset of plastic deformation; (III) the inflection point, characterized by a sharp increase in displacement, which signifies the ultimate bearing capacity of the foundation, and reaching the peak stress before failure. Generally, the ultimate bearing capacity of the foundation is defined by the vertical stress at this inflection point.

In Figure 13, the clean coral sand sample exhibits elastic deformation within a load range of 0–792.64 kPa, reaching a settlement of 13.3 mm. By contrast, the layered sample and clean coral gravel sample under the same settlement can bear a load close to 1200 kPa. By comparing the p-s curves of the four types of samples, it is found that the bearing capacity of clean coral sand is the lowest, with an ultimate strength of 1684.36 kPa, while the bearing capacity of clean coral gravel is the highest among the four kinds of samples, with an ultimate strength of 2080.68 kPa. The bearing capacities of the layered samples fall between these two extremes, which is 1755.13 kPa for the sand-over-gravel layered sample, and 1981.6 kPa for the gravel-over-sand layered sample. Under the same vertical load, coral gravel exhibit less vertical deformation compared to clean coral sand, indicating higher stability. However, when the load exceeds 1500 kPa, the layered sample with the gravel layer on the top exhibits less axial deformation. In practical reclamation projects, arranging coral gravel on the surface is beneficial for enhancing the bearing capacity of the coral soil foundation.

Figure 14 presents the p-s curves of the gravel-over-sand layered samples under different confining pressures, compared with the results from the indoor plate load model tests conducted by Fu et al. [29]. The bearing capacity of the gravel-over-sand layered samples increases significantly with increasing confining pressure, while the deformation shows a decreasing trend. During the indoor plate load model tests, considering the maximum output of the hydraulic jack and the safety of the reaction frame, the axial load did not exceed 2000 kPa. However, a comparison reveals that within the indoor plate load model tests, the p-s relation curve of the gravel-over-sand layered samples closely resembles the results from the triaxial stress staged loading tests under a confining pressure of 400 kPa. This demonstrates that the lateral confining pressure experienced by the soil beneath the loading plate in the indoor plate model tests is similar to 400 kPa.

5. Conclusions

In reclaimed foundations within coral reefs, layered structures composed of coral gravel and sand are frequently observed. This study conducted a series of consolidation drained shear test and step-loading test using a large-scale triaxial apparatus on four types of samples: clean coral sand, clean coral gravel, sand-over-gravel layered samples, and gravel-over-sand layered samples. The effect of the layered samples on the internal friction angle and cohesion of coral soil were examined, as well as the impact of confining pressure and the layered structure on the bearing capacity of coral soil foundations.

1. The stress–strain relationships of the four types of samples primarily exhibit strain hardening under drained conditions. Under identical confining pressure, the peak strength of coral sand is the lowest, while coral gravel has the highest peak strength. The strength of the layered samples falls between those two, with the layered samples having coral gravel on the top layer displaying much higher peak strength than those with a sand layer on top.

2. Based on the Mohr–Coulomb calculation, the four samples have similar peak friction angles, at slightly higher than 40°. However, the difference in cohesion is more significant. The cohesion of clean coral gravel can be four times that of clean sand. The layered samples’ cohesion falls between these two, while the cohesion of the gravel-over-sand sample is relatively higher.

3. All samples exhibit a bulging failure mode after shearing, with bulging deformation observed in the middle of clean coral sand and gravel samples. In layered samples, bulging deformation is mainly observed in the gravel layer.

4. In the step-loading tests, under identical confining pressure, the bearing capacity of clean coral sand and clean coral gravel are the lowest and the highest, respectively, with the bearing capacity of the layered samples falling between of those two. The bearing capacity of the four types of samples increases with increasing confining pressure. The p-s curve obtained by the step-loading tests of the gravel-over-sand sample under 400 kPa confining pressure is close to that obtained in the plate load model tests.

Author Contributions

Investigation, X.T. and X.L.; Writing—original draft, X.T. and T.Y.; Methodology, X.T. and D.X.; Resources, D.X. and Q.M.; Funding acquisition, Q.M. and T.Y.; Data curation, X.T. and X.L.; Visualization, D.X.; Formal analysis, X.T. and X.L.; Writing—review and editing, T.Y. and Q.M., Supervision and Project Administration D.X. and Q.L. Validation, Q.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author Dongfeng Xin was employed by Huajin Aramco Petrochemical Company Limited, Panjin, China. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Layered coral sand and gravel structure in the hydraulic filling area in a coral reef in the South China Sea.

Figure 2. The tested coral sand and gravel: (a) coral sand; (b) coral gravel.

Figure 3. Quantification of particle shape of coral sand and gravel: (a) individual particles; (b) cumulative distributions of sphericity and roundness.

Figure 4. XRD test results of tested materials: (a) coral sand; (b) coral gravel.

Figure 5. Particle size distributions of coral sand and gravel.

Figure 6. Large-scale triaxial test apparatus.

Figure 7. Schematic diagram of four types of samples: (a) coral sand; (b) sand-over-gravel layered sample; (c) gravel-over-sand layered sample; (d) coral gravel.

Figure 8. Stress–strain relationships for four types of tested samples: (a) coral sand; (b) coral gravel; (c) sand-over-gravel layered sample; (d) gravel-over-sand layered sample.

Figure 8. Stress–strain relationships for four types of tested samples: (a) coral sand; (b) coral gravel; (c) sand-over-gravel layered sample; (d) gravel-over-sand layered sample.

Figure 9. Relationship between peak deviatoric stress and confining pressure of four types of samples with relative density of 80%.

Figure 10. Shear strength envelopes of coral sand.

Figure 11. Failure modes of the samples under 200 kPa confining pressure: (a) coral sand; (b) coral gravel; (c) sand-over-gravel layered sample; (d) gravel-over-sand layered sample.

Figure 12. Failure mode of the gravel-over-sand layered sample under 400 kPa confining pressure.

Figure 13. p-s curves of different samples under step loading with a confining pressure of 400 kPa.

Figure 14. p-s curve of gravel-over-sand layered sample under step loading with different confining pressures.

Table 1

Physical parameters of coral sand and gravel.

Sample	Maximumdry Densityρd_max (g/cm3)	MinimumDry Densityρd_min (g/cm3)	Specific GravityGs	WaterContentw (%)	Cu	Cc
Coral sand	1.80	1.25	2.79	0.5	4.53	0.9
Coral gravel	1.32	1.07	2.80	0.5	3.62	1.15

Table 2

Test program.

Type of Sample	Confining Pressure (kPa)	Type of Test
Clean coral sand	200, 400, 600, 800	large-scale triaxial test
Clean coral gravel	200, 400, 600, 800
Sand-over-gravel layered sample	200, 400, 600, 800
Gravel-over-sand layered sample	200, 400, 600, 800
Clean coral sand	400	step-loading tests
Clean coral gravel	400
Sand-over-gravel layered sample	400
Gravel-over-sand layered sample	200, 400, 600, 800
23 tests in total

Table 3

Peak strength parameters of tested materials.

Type of Sample	C (kPa)	φ (°)
Clean coral sand	8.4	43.2
Clean coral gravel	35.2	44.3
Sand-over-gravel layered sample	13.4	42.8
Gravel-over-sand layered sample	25.8	43.6

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