Deep coral reef limestone (CRL) exhibits strong heterogeneity due to complex pore and crystal morphology, which significantly affect its strength and damage mechanisms. The scarcity of deep CRL samples limits a comprehensive understanding of its pore characteristics. The computed tomography scan and pore analysis for 112 samples and 3 m drilling cores of deep CRL indicate that the Pearson correlation coefficient of pore principal axes a and b reaches 0.928. The distribution of pore parameters was analyzed by kernel density estimation, and new pores consistent with the characteristics of deep CRL pores were generated by resampling the inverse cumulative distribution function. Subsequently, a porous grain-based modeling strategy was proposed, and the novel finite-discrete element method was adopted to study the effects of pore structure, crystal morphology, and strain rate on the mechanical parameters and macro-micro damage mechanisms of the CRL. The results indicate that the crack propagation under quasi-static and intermediate strain rates is affected by crystal arrangement and pore weak zones, while it is not affected under high strain rates. Strength increases with increasing crystal sphericity, and large-sized pores increase the tendency of sample volume dilatancy. Additionally, the pore size effect was revealed to ensure the mechanical test reliability of deep CRL. This study realized the refined modeling for deep CRL and solved the sample-dependent issue of reflecting CRL pore characteristics in numerical models.

Submarine cables are crucial components of infrastructure supporting offshore renewable energy projects and global data transmission. However, their safety is threatened by seabed scour resulting from ocean waves and currents. While seabed scour under waves and currents has been studied around other marine structures, few studies have focused on the combined wave-current effect on the seabed surface and pore water pressure response around the submarine cable. This study presents flume experimental results and compares seabed scour evolution and pore water pressure responses around a submarine cable under current-only, wave-only, and combined wave-current loading conditions. The results demonstrate that scour has only a small influence on the excess pore water pressure under current-only loading. However, waves induce a significant, periodic excess pore water pressure response but cause no obvious cable exposure. The coupling effect of wave and current loading amplifies the scour intensity more than either alone does, leading to extensive cable exposure. Furthermore, the pore pressure is consistently higher in the upstream region and significantly decreases immediately downstream of the cable. These findings underscore that ignoring wave-current coupling effects may overestimate cable stability in burial design.

Coral reef limestone (CRL) serves as a critical foundation material for reef and island engineering, whose strength is controlled by physical properties and pore structure. However, the interaction influence of these characteristics on CRL strength remains insufficiently understood. Thus, confronting severe data limitations in current research, this study established a databset through original laboratory tests and computed tomography (CT) scans. This dataset includes dry density (ρd), porosity, P-wave velocity, isotropic component of the pore tensor, anisotropy coefficient (log10(Γ)) and fractal dimension. Subsequently, an optimal ensemble learning model, Stacking-XRB combining extreme gradient boosting (XGBoost), random forest (RF) and Back propagation neural network (BPNN), was developed for predicting CRL strength, achieving high accuracy (R2 = 0.93 on the test set). Furthermore, interpretable machine learning was employed to analyze the influence of physical properties and pore morphology on strength. SHapley Additive exPlanations (SHAP) and Individual Conditional Expectation (ICE) result reveal a nonlinear interaction between ρd and log10(Γ), which quantifies the competition between rock compaction and pore-structure in effecting UCS. The threshold (log10(Γ) ≈ -2.25) of model applicability was determined. CT-based analysis confirms that this value demarcates a structural transition from nearly isotropic to oriented pore channels, which correlates with a decrease in strength. This work advances rock mechanics by quantitatively interpreting the interacting control mechanism of physical properties and pore structure, offering meso‑structural insights into the pore-scale mechanisms that controls the strength of CRL.

Coral reef limestone (CRL) has undergone complex diagenetic processes driven by marine sedimentation and biochemical activity, producing multiscale pore structures that introduce considerable inherent defects and significantly influence its mechanical behavior. To address the disturbance issues within multi-strain rates during the construction of island reef underground, this study adopted X-ray computed tomography to capture two-dimensional CRL morphologies and proposed two strategies for resolving the intersection issues of complex pore boundaries. Four representative CRL numerical models were developed using the finite-discrete element method, and quasi-static and high strain rate simulations were performed to explore how geological diagenesis affects dynamic mechanical response and damage mechanisms. The results indicate that pores formed through coral growth lines, bio-boring, and mechanical scouring enhance the strain-rate sensitivity of CRL. Furthermore, the study identifies a stress-wave shielding effect induced by densely distributed pores generated by diagenetic processes, including bio-boring, erosion, and dissolution in gravel and calcarenite limestones. Based on analyses of energy dissipation and failure modes, a blasting optimization scheme tailored to island reef strata was proposed, providing theoretical guidance for underground engineering applications.

Thermal-induced metamorphism of coral reef limestone (CRL) leads to the deterioration of its engineering properties, causing a threat to the safety of reef caverns. The physical, mechanical, and fracture characteristics of CRL at high temperatures were studied in this paper utilizing the acoustic emission (AE) technology. The results reveal that 400 °C is the threshold temperature for thermal damage to CRL. The porosity significantly increases after temperatures exceed 400 °C, indicating the development of thermal-induced cracks within the rock. Thermal-induced cracks and mineral decomposition severely degrade the strength and elastic modulus of CRL. The frequency distribution of AE waveforms represents the transition of failure mode from tensile–shear composite failure to shear failure as the temperature evolves. The increase of the b value with rising temperature indicates a transition in the crack pattern from large-sized crack dominance to small-crack network dominance. The results of this study fill a research gap concerning thermal damage in CRL. The key findings provide theoretical guidance for the design and construction of underground chambers in islands and reefs.

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.

The marine depositional environment and biological genesis result in complex pore structures in coral reef limestone (CRL), which introduce pronounced heterogeneity and dispersion. Understanding the influence of pore structure on the mechanical behavior of deep CRL under dynamic triaxial impact is crucial for blasting excavation in reef island underground engineering. In this study, dynamic triaxial impact tests were conducted under varying pre-hydrostatic and pre-deviatoric stresses using a modified split Hopkinson pressure bar system. Results indicate that confining pressure significantly enhances the dynamic fracture resistance of CRL, whose strain rate sensitivity is notably weaker compared to sandstone and granite. Axial prestress in the elastic regime reduces initial damage, thereby increasing specimen dynamic fracture resistance. Additionally, a numerical model of CRL based on the finite-discrete element method was established to simulate dynamic impacts under hydrostatic pressure. The numerical results show that pores critically influence stress concentration and crack tip extension during initial stress-wave loading, while pores, cracks, and fragments primarily contribute to tensile crack propagation due to post-peak harmonic diffraction. It is necessary to consider the harmonic diffraction effect during blasting excavation. These findings provide essential references for predicting failure modes in reef island rock masses, analyzing stress wave propagation in CRL strata, and identifying potential failure zones.

The content of coral powder in coral aggregates significantly impacts the quality of coral mortars using coral aggregates, ultimately affecting various properties of coral mortars. Hence, to elucidate the role of coral powder in coral mortars, this paper explores its impact on the properties of coral mortars. Additionally, mercury intrusion porosimetry, thermogravimetric analysis, and Fourier transform infrared spectroscopy are employed to unveil the underlying mechanisms through which coral powder enhances or diminishes the properties of coral mortars. The results illustrate that coral powder reduces the fluidity and increases the dynamic yield stress of coral mortars. Furthermore, due to its filling effect, coral powder facilitates enhancements in both the flexural and compressive strength of coral mortars. However, the refinement of pore size by coral powder, in turn, influences the shrinkage behavior of coral mortars. Simultaneously, coral powder diminishes the hydration and content of hydration products in coral mortar to a certain extent. The pore surface fractal dimension (Ds) was found to serve as a crucial indicator of strength. Notably, it displays a strong positive linear correlation with both compressive strength and flexural strength. Moreover, the composite exponential model proposed in this paper provides a more accurate simulation of the shrinkage behavior of coral mortars.

Ground reinforced embankment (GRE) is a common and efficient rockfall mitigation measure. However, due to the diversity of geometric dimensions and composite components of the embankments worldwide, the design methods have not yet been unified. This article proposes a DEM-based framework for modeling the GREs impacted by rockfalls, and to optimize the structural design by comparing the block-intercepting performance. The numerical model based on MatDEM is validated by restoring the Peila's field tests, and the simulated materials are calibrated by comparing the laboratory test results. The design elements can be determined through simulated impact tests, with the site topography and rockfall trajectory as prerequisite information. The simulation test results show that the structural positions and cross-sectional shapes alter the interaction between rockfalls and embankments, thereby affecting the block-intercepting capacity. Under the impact of high-energy blocks, the characteristic of structural failure is that the extrusion of the downhill face is greater than the displacement of the uphill face, which can be used as a criteria to determine the reasonable design elements. The proposed framework can be applied to an actual site and maximize the cost-benefit performance of design depending on the site space and budget conditions.

Coral reef limestone (CRL) strata inevitably encounter coupled static-dynamic tension during the process of underground space development of island reefs. This paper conducted a series of dynamic tension tests with different pre-static loads by a modified split Hopkinson pressure bar (SHPB) with the high-speed camera module. Moreover, we used CT scans to reveal the effect of pore structure on crack propagation path. The results demonstrate that the total tensile strength of CRL is independent of pre-static loads. An empirical formula is proposed to predict the normalized total tensile strength. The impedance difference between the CRL samples and incident bar leads to the proportion difference between reflected and absorbed energies in the experiment. As the impact velocity further increases, the reflected energy becomes significantly greater than the absorbed energy. Furthermore, with an increase in pre-static load, the absorbed energy of CRL decreases at the same impact velocity, this decreasing trend is more pronounced at higher impact velocities. Cracking induced by large macropores near the diameter results in a "crescent" shape macrocrack. The conclusions provide a promising reference for explosion excavation of underground space of island reefs.

The cohesive zone model is an important model for describing the fracture process zone (FPZ) of brittle materials, with a constitutive relationship corresponding to the force-displacement behavior in the non-elastic deformation zone, providing clear physical significance. Coral reef limestone (CRL), a heterogeneous sedimentary rock, has complex pore structure that significantly affects its fracture behavior. Given this, dynamic impact tests were conducted on CRL notched semi-circular bending (CRL-NSCB) specimens using a modified Split Hopkinson Pressure Bar. The study comparatively analyzed the differences in calculations of dynamic fracture toughness using different methods, such as the empirical formula recommended by the International Society for Rock Mechanics and Rock Engineering, the J-integral method, and method of linear elastic fracture mechanics. Additionally, a high-speed camera was adopted to capture the dynamic fracture process of the specimens, and the FPZ length of CRL was accurately measured based on the digital image correlation method. A numerical model of the CRL-NSCB specimen was also established, and a numerical case was conducted by hybrid finite-discrete element method to explore the effect of pore structure on the fracture process. The results indicate that the Jintegral method accounts for energy release in the FPZ, allowing for a more accurate consideration of the heterogeneity effects. Loading rate primarily influences the crack propagation mode. At low loading rates, large pores near the expected crack path induce deviation from the expected crack path. Additionally, FPZ parameters exhibit a clear rate effect. These research conclusions provide new insights into the fracture process of porous CRL.

Understanding the fracture characteristics of coral reef limestone (CRL) at elevated temperatures is fundamental to the safety and long-term stability of island engineering in the marine environment. The fracturing behaviors of CRL specimens exposed to various temperatures were investigated through notched semi-circular bending tests. SEM was employed to analyze the fractographic features and crack propagation patterns. Thermal analysis tests examined the physical and chemical changes occurring in CRL at high temperatures. The results indicated that the fracture toughness increased with temperature in the range of 25-100 degrees C. However, beyond 100 degrees C, the fracture toughness decreases with increasing temperature, accompanied by an increase in the rate of decline. SEM observations revealed that 400 degrees C serves as a threshold temperature for developing thermal-induced cracks. Thermal analysis suggested that the critical temperature range for mineral thermal decomposition reactions is 510-760 degrees C. Based on the fitted relationship between fracture toughness and temperature, a damage threshold temperature of 400 degrees C for the surrounding rock in coral island engineering was determined. The findings of this study not only fill a significant gap in the research on the high-temperature fracture characteristics of CRL but also provide a theoretical reference for the safe design of underground island engineering.

With the continuous advancement of marine engineering and coral reef construction, the construction process of using coral reefs and sand as aggregate to make concrete or mortar came into being. Therefore, this study delves into the impact of aggregate type, water-cement ratio (W/C), and sand-cement ratio (S/C) on the workability, shrinkage, and mechanical properties of mortar. Simultaneously, it uses microscopic techniques to elucidate the mechanisms underlying the impact of aggregate type, W/C, and S/C on mortar performance. The results demonstrate that the S/C of coral mortar should be controlled at 1.5-2. The self-shrinkage and drying shrinkage values of coral mortar decrease progressively with higher S/C and increase with increasing W/C. Coral mortar exhibits a smaller self-shrinkage than ordinary mortar. Unlike self-shrinkage, the early drying shrinkage of coral mortar is less than that of ordinary mortar. However, as time progresses, the drying shrinkage of coral mortar becomes notably greater than that of ordinary mortar. The flexural and compressive strengths of coral mortar are inferior to those of ordinary mortar. Coral mortar experiences transcrystalline fracture, in contrast to the intergranular failure observed in ordinary mortar. Moreover, strength exhibits a quadratic power function correlation with the S/C and a negative linear correlation with the W/C. In addition, the construction technology for coral mortar differs from that of ordinary mortar.

Coral reef limestone is a kind of porous anisotropic geo-material. Understanding its dynamic mechanical properties subjected to high loading rates is of great significance to the blasting excavation and military attack defense of the reef underground space. In this paper, the Split Hopkinson Pressure Bar tests were carried out on the specimens of deep coral reef limestone in the South China Sea, and the difference of dynamic response between deep reef limestone and shallow calcified coral skeleton was revealed. The results show that the pore structure affects the dynamic deformation response of coral reef limestone, which makes the strain rate of the specimen independent of other mechanical parameters. However, the high loading rate overcomes the influence of the primary pore structure on the strength, so that the dynamic compressive strength and the degree of breakage are linearly related to the stress rate. In addition, under high loading rate, the coral framework limestone is mainly damaged by tension, which is significantly different from the calcified coral skeleton along the coral growth line. At the same strain rate, the fractal dimension of the dry specimen is greater than that of the saturated one, and the self-similarity characteristics of the specimen fragments that reach the crushing level at higher strain rate are poor. The viscous effect of water hinders the propagation of cracks in the coral framework limestone, but free water reduces the surface energy of the crystal boundary. These two kinds of mechanisms compete with each other, and the key factors controlling the competition mechanism are water content and loading rate.

The buried diagenesis and dolomitization in the deep stratum of coral reef island enable the Coral reef limestone (CRL) to exhibit a significant distinction compared with the calcified coral skeleton in mechanical properties. This paper utilized the Split Hopkinson Pressure Bar and X-ray computed tomography to investigatethe dynamic mechanical characteristics of deep CRL, revealing the effect of free water on stress–strain behaviors and the energy evolution law of CRL under high-rate impact. Furthermore, the concept of pore tensor proposed in our previous investigation was introduced to modify the dynamic constitutive model. The results indicate that the dynamic compressive strength (DCS) of CRL specimens is strongly related to stress rate, whereas the dynamic elastic modulus and DCS are independent of strain rate. The peak strains of dry specimens are significantly greater than those of saturated ones at comparative loading rates, and the viscosity effect of free water at a high loading rate impedes the deformation of CRL specimens. Moreover, the energy absorption efficiency of CRL is significantly higher than that of Bohus granite and Solnhofen limestone. Dry specimens have higher energy absorption efficiency under the same energy input, but the dynamic increase factor (DIF) of saturated specimens is larger under the same energy absorption efficiency. For the determination of DIF, this paper adopted the Back Propagation Neural Network model trained by pore tensor to predict the quasi-static compressive strength of CRL specimens, which solved the problem of considerable discreteness in mechanical parameters of CRL. The critical transition value of input energy proportion that the DIF increases rapidly is about 30

Coral reefs are abundant in waste coral powder, and how to utilize coral powder efficiently to construct far-sea projects has become a major focus of research and engineering field, which is conducive to the development of green economy. However, existing research lacks a comprehensive and systematic investigation of the cement-coral powder-metakaolin ternary system. In this study, the hydration and rheological behavior, micromechanical and mechanical properties, and pore structure evolution of the ternary system are systematically investigated to uncover the synergistic mechanism between metakaolin and coral powder. The results indicated that, based on hydration, rheology, and mechanical properties, the recommended proportions of cement, coral powder, and metakaolin are 75 %, 10 %, and 15 %, respectively. The combination of coral powder and metakaolin effectively promotes hydration. Furthermore, it accelerates the aluminate reaction, resulting in a stronger aluminate exothermic peak immediately. In addition, metakaolin reacts with coral powder to form carboaluminate, activating the activity of the coral powder, which in turn improves the mechanical properties and pore structure of the ternary system. Moreover, metakaolin undergoes a pozzolanic reaction with calcium hydroxide, which contributes to optimizing the pore structure of the ternary system and enhancing its mechanical properties. Additionally, coral powder promotes the conversion of monosulfate to carboaluminate while inhibiting the transformation of ettringite into monosulfate. The Herschel-Bulkley model accurately simulates the dynamic rheological behavior of the ternary system. This study is highly significant and practically valuable for transforming waste coral powder into a supplementary cementitious material for the construction of the far-sea projects, providing a scientific basis for future related studies.

The breakage behavior of granular materials is prevalent across various fields and significantly impacts the mechanical properties of granular materials.A substantial amount of research on particle breakage behavior relies on reasonable calculation methods for breakage rate.This article proposes a calculation method for particle breakage rate based on particle volume as the fundamental parameter,addressing the current limitation of existing methods in accurately evaluating the breakage rate of irregular particles,exemplified by coral gravel.It improves upon the existing Hardin breakage rate calculation formula,which is based on particle size,to better assess the breakage rate of irregular particles.Breakage tests of coral gravel with different shapes were designed and conducted to conduct reliability validation of the aforementioned theoretical formulas,and the causes of errors generated by traditional methods were analyzed.Through experiments,the breakage rate values of particles with different shapes under different pressures and algorithms were obtained.Meanwhile,combining stress,deformation,and image observations from the experimental process,the fragmentation thresholds of coral gravel in three shapes—sheet,block,and rod—were determined,revealing the evolution characteristics of fragmentation modes exhibited by particles with different morphologies during the pressure change process.

Coral reef sand concrete (CRSC) plays an indispensable role in island reef projects, but limited knowledge of its damage poses safety and stability concerns for coral reef sand concrete structures. Therefore, the in-situ computed tomography (CT) is employed to investigate the strength and deformation, and pore structure evolution of coral reef sand concrete under uniaxial load. Additionally, the crack propagation and three-dimensional strain field evolution under different stress states are revealed by the digital volume correlation (DVC) techniques. The results demonstrate that when the stress is below 35.79 MPa, the porosity gradually decreases as the stress increases. However, when the stress exceeds 35.79 MPa, the local porosity increases with the rising stress, and the porosity at 39.89 MPa is higher than the initial porosity. Moreover, stresses evidently concentrate at areas of local maximum porosity, resulting in the initiation and development of cracks in those regions. During the failure process, CRSC experiences splitting damage firstly caused by longitudinal cracks, followed by the extension of these cracks into oblique cracks, resulting in shear damage. Coral aggregate is the weak phase in CRSC, exhibiting transgranular fractures. Additionally, CRSC exhibits significant strain in the pores and coral aggregates, leading to crack development along the direction of the pores or coral aggregates. Therefore, this study offers a deeper understanding of the damage mechanisms of CRSC, providing theoretical support for its further research and practical application in far-sea projects.

The subtropical hurricane, ocean circulation, and excavation disturbance by deep engineering of island reefs bring many challenges for the insight into the mechanical behaviors of coral reef limestone (CRL) under cyclic loads. This paper investigated the damage characteristics of coral gravel limestone (CGL) under triaxial cyclic loads based on the in-situ computed tomography (CT) technique and digital volume correlation method (DVC). The results suggested that the plastic strain of CGL still existed after unloading in the stage of linear elastic deformation. Crack initiation within the specimen occurred at the stress level of about 0.72, and accelerated damage occurred when the stress level exceeded 0.86. Subsequently, we utilized DVC technique to visualize the evolution process of 3-D strain field under cyclic loads. The strain field of the specimen generated strain release in the locality of crack initiation, and the heterogeneous degree of strain field firstly increase and then decrease with the increase of upper limit pressure. Moreover, the secant modulus exhibits a trend consistent with the heterogeneous degree of strain field. Therefore, we introduced the concept of self-information entropy to characterized the heterogeneous degree of strain field, attempting to describe the process of compression and damage of sample. The evolving relationship between secant modulus and axial strain of CGL under cyclic loads was established based on the heterogeneity of strain field. The proposed model provides promising way to predict secant modulus of porous rock materials based on-site CT testing and in-situ measurement in practical engineering.

Mechanical performance of coral reef limestone (CRL) is becoming an important topic in recent years due to the boom in offshore construction. However, as there is a lack of rock samples in specific regions, very limited progress has been made in terms of the compressive behaviour (both static and dynamic) of the CRL. Specially, proper descriptions and accurate quantifications of the strain-rate effect upon the compressive performance of the CRL remain to be performed, which will be the main emphasis of this paper. Four types of CRL, namely, coral framework (CF), coral boulder (CB), coral gravel (CG), and coral calcarenite (CC) limestone, were obtained from a human-made coral reef island in South China Sea. Static uniaxial and dynamic split Hopkinson pressure bar (SHPB) compression tests are conducted to obtain the strengths at very low and medium to high compressive strain rates, respectively. The strain-rate effect of the compressive strength is well described with the Herschel-Bulkley model, parameterising the static strength, the viscosity coefficient, the ‘shear-thinning’ index, and the reference strain rate. The overall quality of the CRL rock mass is then classified by the basic quality method, which is based on the uniaxial compressive strength and joint distributions of the rock mass. Variations of the Young’s modulus of the CRL rock mass under different strain rates are also discussed.