Catastrophic failure in engineering structures of island reefs would occur when the tertiary creep initiates in coral reef limestone with a transition from short- to long-term load. Due to the complexity of biological structures, the underlying micro-behaviors involving time-dependent deformation are poorly understood. For this, an abnormal phenomenon was observed where the axial and lateral creep deformations were mutually independent by a series of triaxial tests under constant stress and strain rate conditions. The significantly large lateral creep deformation implies that the creep process cannot be described in continuum mechanics regime. Herein, it is hypothesized that sliding mechanism of crystal cleavages dominates the lateral creep deformation in coral reef limestone. Then, approaches of polarizing microscope (PM) and scanning electronic microscope (SEM) are utilized to validate the hypothesis. It shows that the sliding behavior of crystal cleavages combats with conventional creep micro-mechanisms at certain condition. The former is sensitive to time and strain rate, and is merely activated in the creep regime.

Loose and uncemented calcareous sand slopes are prone to collapse under rainstorm erosion. In order to improve the erosion resistance stability of slopes, it is crucial to enhance the erosion resistance of calcareous sand. In this study, a new method of cementing calcareous sand with zinc sulfate solution (ZSS) is proposed. The ZSS reinforcement technique can effectively cement calcareous sand, enhance the mechanical properties and help reduce erosion on calcareous sand slopes. A series of laboratory experiments were conducted, including uniaxial compression tests, Brazilian splitting tests, surface penetration tests, microscopic tests, and rainfall scouring tests. The test results show that the uniaxial compressive strength of calcareous sand achieved 8.3 MPa reinforced with ZSS. Microscopic analyses revealed the mechanism of reinforcement, discovering the formation of environmentally friendly compounds such as ZnCO3 and CaSO4 & sdot;2 H2O between calcareous sand particles, which enhanced the soil mechanical properties. The calcareous sand slope reinforced with ZSS forms a hard shell on the slope surface, which effectively improves the erosion resistance of the slope. After being reinforced with a ZSS concentration of 1.0 mol/L, the slope remained stable after 20 min of scouring at a rainfall intensity of 80 mm/h. This method provides a quick solution for reinforcing calcareous sand slopes and holds promising potential for practical engineering applications.

In order to address the issues of settlement and collapse of coral sand foundations on island reefs caused by rainfall infiltration,this study conducted top-down unidirectional penetration tests on gap-graded coral sand with varying D15/d85 ratios(D15 represents the particle size corresponding to a cumulative percentage of 15%for soil particles smaller than a certain size in the coarse-grained group,while d85 represents the particle size corresponding to a cumulative percentage of 85%for soil particles smaller than a certain size in the fine-grained group.)and fine particle contents using a self-designed permeameter.It identified the migration characteristics and fundamental conditions for fine particle loss,revealed the patterns of permeability change in coral sand,and analyzed the underlying mechanisms through microscopic methods.The results show that:(1)For coral sand with a soil skeleton particle size ranging from 2 mm to 10 mm,particles measuring 0.25 mm to 0.50 mm are critical for ensuring stable seepage erosion.(2)Coral sand foundations with a D15/d85 ratio below 10 and a fine particle content between 20%and 30%exhibit better stability against infiltration erosion.(3)Compared to quartz sand,the unique mineral composition and particle morphology of coral sand contribute to its increased resistance to particle migration and stronger resilience against seepage erosion.(4)In engineering applications,measures such as improving relative density,optimizing particle gradation,or implementing grouting consolidation can enhance the stability of foundations against seepage erosion.The research findings provide a scientific basis for the design of coral reef foundations with resistance to seepage erosion.

Foundation settlement and collapse disasters resulting from seepage deformation in hydraulic-filled islands and reefs have been observed in the South China Sea, but the underlying failure mechanism and characteristic remain unclear. This study aims to investigate the influence of compactness and fine particle content on the seepage deformation of gap-graded coral sand and revel the characteristics and mechanism of seepage deformation of gap-graded coral sand through laboratory seepage deformation tests. The results indicate that the seepage deformation failure mode of gap-graded coral sand is influenced by the content of fine particles which undergo an evolution process from continuous piping to discontinuous piping to boiling. Particle loss is affected by the constraints between coarse particles, and the ability of different particle contact forms to restrict the loss of fine particles is different. Moreover, irregular particle morphology increases intergranular constraints, enhancing the coral sand's resistance to seepage deformation compared to standard quartz sand. Based on these findings, the instability coefficient was used to consider the influence of particle morphology and inter-particle contact on the seepage deformation. A hydraulic criterion for the internal stability of coral sand was established, demonstrating its versatility. Furthermore, the applicability of existing geometric criteria in evaluating coral sand was analyzed. The existing methods were found to be inaccurate in evaluating the internal stability of coral sand specimens with a fine particle content below 20 %.

Cementing loose coral sand to reduce the permeability of island reef formations is one of the key approaches to enhancing freshwater retention in ecological islands. To this end, a permeability test was conducted on zinc sulfate-bonded coral sand, analyzing the effects of varying immersion durations, zinc sulfate solution concentrations, and initial dry densities on its permeability. The durability was verified through dry-wet cycling tests, while the microscopic cementation mechanism was investigated using XRD(X-ray diffraction), SEM(scanning electron microscope), EDS(energy dispersive spectrometer) and CT(computed tomography) scanning. The test results show that: (1) the permeability coefficient of coral sand is reduced by 70.17% to 95.79% after cementation with zinc sulfate. (2) After undergoing 16 dry-wet cycles and 36 hours of cementation, the mass loss rate of the cemented coral sand samples does not exceed 4%, and the variation of permeability coefficient is less than 1.0x10-3 cm/s, indicating excellent durability of the samples. (3) Additionally, the reaction between coral sand and zinc sulfate produces dihydrate gypsum and smithsonite, filling the pores of the samples, resulting in a decrease in the average pore-throat radius and coordination number, and a significant reduction in pore connectivity. This technology holds significant engineering application value, as it can be applied to sand fixation and erosion prevention in the initial stage of island reef reclamation, improving foundation bearing capacity and reducing permeability to promote groundwater conservation and eco-island construction.

The damage characteristics of coral limestone are of great significance for guiding the design and construction of offshore engineering such as submarine tunnels and pipelines. The mechanical properties and damage behavior of coral limestone collected from the Zhongsha Islands, South China Sea were investigated based on the results of uniaxial compression tests. A theoretical porosity calculation method was put forward and applied to the coral limestone with a large number of closed internal pores. The failure characteristics of the coral limestone are very different from those of terrestrial rocks because the strength decreases gradually after failure. The threshold stresses of coral limestone were calculated based on the uniaxial compression tests and compared with that of terrigenous rocks. The crack initiation stress is 39–56

Biological and biochemical geneses bring complex structures to coral reef limestone (CRL), leading to pronounced anisotropy and heterogeneity. The stress field concentration induced by pore structure introduces a considerable dispersion in mechanical parameters and reduce the strength of CRL significantly. The CRL-C specimens were made using coral reef limestone to conduct uniaxial compression tests with loading rate of 0.002 mm/s, revealing the controlling mechanism of interface inclination angle for the strength of CRL-C specimens. The experiment results shows that the peak strength, peak strain, and modulus of elasticity of the combination all tend to lessen and then grow with increasing the interfacial inclination angle. The achieved results indicate that the tensile damage occurs in specimens with 0 degrees, 15 degrees, and 90 degrees inclination angles, whereas the tension-shear stress damage takes place for specimens with 30 degrees inclination angle. In turn, the shear damage occurs along the interface for specimens with 45 degrees, 60 degrees, and 75 degrees inclination angles. The strength of combination specimens underwent the shear failure is noticeably smaller than that of the combination specimens damaged across the interface. Additionally, the cement mortar at the interface has a remarkable reinforcing effect on the interface strength, and the CRL-C interface possesses the highest cohesion and a lower internal friction angle compared to terrestrial rocks. Simultaneously, the parameters of the CRL-C specimens in the single-interface model are appropriately determined based on the interface strength. The inclination range of the combination specimen damaged along the interface is 35.58 degrees<alpha< 85.83 degrees. The predicted strength parameters based on the Coulomb model and the single-interface model provide a solid reference for the engineering design of CRL-C.

膨胀土裂缝灌浆效果评价是工程界的一大难题.采用微米CT扫描对膨胀土试样裂缝的灌浆充填效果进行了定量评价和分析,提出采用整体孔隙率、逐层面孔隙率、孔隙等效直径与灌入指数等指标对膨胀土裂缝灌浆充填效果进行定量综合分析.采用超细水泥和膨胀土配置4种浆液对膨胀土裂缝进行灌浆试验,在灌浆前、后对试样进行CT扫描获取内部切片图像后再进行三维重建和后处理分析.采用整体孔隙率、逐层面孔隙率、孔隙等效直径和灌入指数定量分析了膨胀土裂缝灌浆效果和最小可灌入裂缝宽度.结果表明:浆液的可灌性随水固比增大而提高;灌入水固比为1.2的浆液可使膨胀土试样的整体孔隙率降低幅度可达到99％以上,其中超细水泥含量50％的浆液可灌入的最小裂缝宽度为0.6 mm.该方法可为定量评估各类浆液在膨胀土裂缝中的灌浆充填效果提供参考.

Crack initiation stress ( σ_ci ) can be used to estimate the spalling strength of rock mass for engineering applications. Its accurate identification is of great significance for analyzing the long-term stability and brittle failure mechanism of rocks. This paper presented an error analysis of various crack initiation stress determination methods and proposed a new method for the identification of crack initiation stress by the expansion rate of axial crack strain. The comparison of the crack initiation stresses calculated by this method with those calculated by the crack volumetric strain method, lateral strain response method, and lateral strain interval response method through tests on three types of rocks showed that the results for marble obtained by all four methods were similar. However, the crack initiation stresses of granite and sandstone calculated by the lateral strain response method were significantly smaller, While the results obtained by the other three methods were similar. The results showed that the σ_ci determined by the method proposed by this paper is accurate. This method can accurately identify the crack initiation stress and its application in engineering practice is convenient.

The dredged coral sand ground contains fine-grained calcareous silt interlayer with high water content, making it impossible for ground vibroflotation to achieve the expected effect. Taking the vibroflotation of the reclaimed coral sand ground in the South China Sea as a case analysis, this study improved the vibroflotation method with regard to the problem of weak silt interlayers, and proposed the approach of encrypting vibroflotation holes and adding coarse-grained coral gravels in vibroflotation holes, respectively. By analyzing the results of in-situ tests, the effectiveness of the improved vibroflotation method for improving the silt interlayers was verified. The results show that, after the improved vibroflotation, the average dynamic penetration blow count (N-63.5) is greater than 20, the ground bearing capacity is greater than 250 kPa, and the average ground settlement is 4.3 mm. Adding coarse aggregate can effectively improves the strength of coral sand ground containing silt interlayer. The deformation modulus (E-0) of coral sand ground has a good linear relationship with N-63.5. The research results can guide the engineering practice of coral reef sand ground treatment in the world.

Atoll reef flats in the South China Sea are ideal sites for land reclamation and engineering construction. Calcareous soil deposited in atoll lagoons is an excellent filling material for land reclamation. This brittle ma-terial with irregular particle shapes is prone to obvious particle breakage under impact or vibration loads. First, this manuscript introduced the geomorphology and geological conditions of coral reefs, and explored the se-lection of reclamation sites and the source of filling materials. Then, compaction characteristics of calcareous soil were studied, and particle breakage was analyzed. On this basis, the optimal compaction test method was proposed based on the test results. Finally, a new method of "impact rolling + water spray" for coarse-grained calcareous soil, which proved effective in engineering practice in the South China Sea, was put forward. Ac-cording to in-situ test results, after 20 passes of "impact rolling + water spray" on calcareous soil foundation, the compactness within the depth range of 1.0 m exceeded 80%; the foundation compactness at 0.3 m, 0.7 m, and 1.0 m deep increased by 8.4%, 4.8%, and 4.1%, respectively. This method significantly improved foundation compaction efficiency, offering important references for the compaction treatment of coarse-grained calcareous soil in coral reef regions.

Calcareous soil is a peculiar geotechnical medium which is utilized as land filler for engineering activities, such as the Maldives, Middle East, and South China Sea (SCS). Since the hydraulic dredged calcareous soil has a wide grain size distribution, irregular particle shape, crushable particles, high porosity, and poor uniformity, ground-improvement was conducted to improve the bearing capacity and reduce foundation settlement. In this paper, the geological conditions, material characteristics, and mechanical behavior of dredger fill site on a coral reef in the SCS were studied. The densification of soil using deep vibro-compaction combined with impact rolling was investigated. An innovative method of sprinkling water during impact rolling was put forward to improve rolling efficiency and soil compactness. A set of modified in-situ measurement and calculation methods suitable for the compactness of calcareous soil were put forward. Based on in-situ test results, the relationship between the soil bearing capacity and blow count of dynamic penetration test was established. It was suggested that the two-point parallel and arranged in equilateral triangles vibro-flotation method is effective for the reinforcement of hydraulic dredged calcareous soil, and after vibro-flotation, the loose soil on the ground surface should be compacted with a 25 kJ three-side impact roller for 20 passes and then compacted with 25 t smooth vibratory roller until there are no visible wheel tracks. In addition, the methods of ground improvement, in-situ test, and foundation evaluation were proposed to contribute to coral reef engineering designers. This paper also provides new insights for coral reef foundation filling and improvement around the world.