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Sedimentary processes often produce natural and blown calcareous sands that are deposited in inclined and layered formations. These calcareous sands frequently exist in a complex stress state with rotating principal stress axes that result in intricate mechanical properties. To investigate the mechanical behaviors of calcareous sands with inclined sedimentary surfaces, we developed a device to prepare hollow cylindrical specimens from artificially crushed calcareous sands. By combining this device with a hollow cylindrical torsional shear apparatus, undrained monotonic loading and pure principal stress rotation tests were conducted to investigate the static and dynamic properties of the inherently anisotropic calcareous sands. The results indicate that the shear dilatancy property is related to the principal stress direction; the shear dilatancy of calcareous sand decreases as the direction of the principal stress increases, and the peak stress ratio decreases with increasing principal stress direction and increases with increasing deposition direction. In the precyclic loading period, increases in the deposition direction led to increases in the stabilities of the specimens and decreases in excess pore pressure accumulation, which further affect the dynamic shear modulus and damping ratio of the calcareous sand. In the late stage of cyclic loading, the inherent anisotropies of the specimens are destroyed, so that the excess pressure and hysteresis loop characteristics start to converge. [ABSTRACT FROM AUTHOR]

The construction of island reef projects using natural seawater, tailored to local conditions, holds significant importance for the advancement of environmentally-friendly geotechnical engineering practices. Previous studies on the application of MICP technology in a seawater environment have been scarce. In this study, Sporosarcina pasteurii were acclimatized to a seawater environment and employed for MICP mineralization reinforcement of calcareous sand, resulting in an enhanced mineralization effect compared to previous attempts under similar conditions. Both direct domestication and gradient domestication methods proved successful in enabling Sporosarcina pasteurii to adapt effectively within a seawater environment. Following domestication, the amount of CaCO3 generated through MICP solidification increased from 2.30% to 4.47%, resulting in a corresponding increase in the Ca2+ conversion rate from 5.19% to 10.02%. Additionally, after domestication, the unconfined compressive strength (UCS) reached up to 309kPa – representing an impressive improvement of 35.53% compared to pre-domestication levels, however, it should be noted that its reinforcement effect still falls short when compared with pure water conditions. The CaCO3 minerals formed after MICP reinforcement using seawater nutrient solution in domesticated Sporosarcina pasteurii predominantly consist of needle-shaped aragonite and granular anorthite minerals, as opposed to the previously observed calcite-type CaCO3 minerals. [ABSTRACT FROM AUTHOR]

Stress path is a key factor affecting the particle breakage of calcareous sand. In this study, the effects of stress path variations on calcareous sand particle breakage were investigated through triaxial compression tests across four distinct stress paths. Additionally, the gradation evolution of calcareous sand during particle breakage was analyzed. Furthermore, the correlation between the total input energy and characteristic particle size was investigated through energy dissipation analysis. The results indicated that the relative breakage index increases gradually with an increase in the maximum deviatoric stress and final volumetric strain, irrespective of the stress path. However, the dilatancy of calcareous sand is related to the relative breakage index as well as the stress path. Notably, the relationship between the relative breakage index and the total input energy can be represented using a power function. A gradation evolution model was formulated based on the results of energy dissipation analysis, and its validity was verified. The results confirmed the model's effectiveness in predicting the particle breakage evolution in both single-gradation and continuous-gradation calcareous sand specimens, accounting for the effects of the various stress paths. [ABSTRACT FROM AUTHOR]

AbstractTo address the durability of post-grouted piles in calcareous sand within marine environments, a series of tests including micro-cone penetration, unconfined compressive strength (UCS), scanning electron microscope, energy dispersive spectrometry, and X-ray diffraction was conducted to study the strength changes of cement-stabilized calcareous sand under different curing times and cement ratios. The relationship between UCS and penetration resistance was established and compared with similar tests on cement-stabilized siliceous sand, revealing the micro-erosion mechanisms specific to cement-stabilized calcareous sand. The results show that based on the distribution of penetration resistance, the cement-stabilized soil in a seawater environment can be divided into eroded and non-eroded layers, with erosion depth increasing as curing time extends and cement ratio decreases. After 90 days of curing, the eroded layer exhibited increased porosity, a 34.4% reduction in peak heights of hydration products, and a 16.4% decrease in Ca2+ content compared to the non-eroded layer. These findings highlight that strength changes under seawater erosion result from a balance between enhanced strength due to hydration and weakened strength due to erosion. The study provides insights into evaluating the durability and long-term stability of post-grouted piles in calcareous sand. [ABSTRACT FROM AUTHOR]

Due to the hydraulic sorting effect in the hydraulic filling process, a fine-grained aggregate layer dominated by silty fine sand with uneven distribution is easily formed in reclamation projects, which triggers issues with the bearing capacity and nonuniform settlement of calcareous sand foundations. In this study, a series of one-dimensional compression tests were conducted to investigate the effect of different fines contents (fc) on the compression behavior of calcareous sand. The results show that at the same relative density (medium-density, Dr = 50%), the addition of fine particles leads to a reduction in the initial void ratio (for fc ≤ 40%). Furthermore, while the compressibility of the soil samples increases with the rising of fines content, it begins to decrease with further addition of fine particles beyond a threshold value of fines content (fc-th). Additionally, particle crushing contributes to the compressive deformation of calcareous sand, and the particle relative breakage of calcareous sand increases at the initial stage of adding fine particles. Moreover, a comparison of the compression test results between calcareous silty sand (fc = 10%) and clean sand reveals that the addition of fine particles accentuates the compressibility differences among calcareous sands with different relative densities. These findings provide valuable insights for addressing the challenges posed by fine-grained layers in calcareous sand foundations. [ABSTRACT FROM AUTHOR]

Calcareous sand has been widely used as a construction material for offshore projects; however, the problem of foundation settlement caused by particle crushing cannot be ignored. Although many methods for reinforcing calcareous sands have been proposed, they are difficult to apply on-site. In this study, a permeable polyurethane polymer adhesive (PPA) was used to reinforce calcareous sands, and its mechanical properties after reinforcement were investigated through compression creep, direct shear, and triaxial shear tests. The reinforcement mechanism was analyzed using optical microscopy, CT tomography, and mercury intrusion porosimetry. The experimental results indicate that there is a critical time during the compression creep process. Once the critical time is surpassed, creep accelerates again, causing failure of the traditional Burgers and Murayama models. The direct shear strength of the fiber- and geogrid-reinforced calcareous sand reinforced by PPA was approximately nine times greater than that without PPA. The influence of normal stress was not significant when the moisture content was less than 10%, but when the moisture content was more than 10%, the shear strength increased with an increase in vertical normal stress. Strain-softening features can be observed in triaxial shear tests under conditions of low confining pressure, and the relationship between the deviatoric stress and strain can be described using the Duncan–Chang model before softening occurs. The moisture content also has a significant influence on the peak strength and cohesive force but has little influence on the internal friction angle and Poisson's ratio. This influence is caused by the different PPA structures among the particles. The higher the moisture content, the greater the number of pores left after grouting PPA. [ABSTRACT FROM AUTHOR]

The effect of breakable particle corners is often overlooked in research on pile–soil interaction, which hinders the understanding of the uplift pile behaviors in calcareous sand. This research examines the breakable corner effects on uplift pile–soil interaction in calcareous sand from macro to micro, through model tests and coupled discrete element method–finite difference method. Results revealed that compared to that in silica sand, the higher bearing capacity and relatively abrupt failure behavior of uplift piles in calcareous sands were attributed to the corner interlocking effect and corner breakage effect, respectively. The unstable load transmission along piles in calcareous sand was thoroughly explained by a coupled effect of corner interlocking and breakage. Furthermore, the reduction in effective contacts and alterations in soil skeletons were identified as critical factors contributing to the distinctive soil behaviors in calcareous sand. Moreover, the relative sliding distance of particles was found to be the key factor in determining the amount of corner breakages due to stress concentration at corners. Lastly, a positive feedback loop involving corner breakage effects was proposed, successfully explaining the distinctive phenomenon of uplift piles in calcareous sand. This study provides new perspectives to clarify distinctive pile–soil interaction behaviors in calcareous sand. [ABSTRACT FROM AUTHOR]

A large part of ocean surface is covered by calcareous deposits. The Low stiffness and weak particles of calcareous soils deposited by marine organisms showed significant particle breakage under applied loads, which resulted in different behavior compared to terrigenous soils. A series of compression drained triaxial tests were carried out to evaluate the monotonic shear behavior of crushable and non-crushable soils. Three different soils, including Firoozkuh siliceous sand (as a non-crushable soil) and Hormuz and Bushehr calcareous sands (as crushable soils) which were different in mineralogy compositions were obtained from northern and southern Iran. The specimens were prepared in different relative densities (i.e., loose, medium and dense conditions) and consolidated under confining pressures ranging from 100 to 600 kPa. The monotonic shear and dilatancy behaviors of the soils have been presented in detail and the effect of particle breakage has been discussed. The results showed that particle breakage has a significant effect on the mobilized friction and dilatancy angles of marine crushable soils. There is a linear relationship between the amount of particle breakage occurring during a triaxial test and breakage angle. In addition, the mineralogical difference influences the breakage properties of calcareous deposits. [ABSTRACT FROM AUTHOR]

To address the reduced bearing capacity of rubber-reinforced calcareous sand while also alleviating pollution from discarded tires, a combined reinforcement method using rubber and geogrid is studied. This approach utilizes the geogrid’s capability to enhance material bearing capacity through the netting effect, complementing the rubber’s reinforcement of calcareous sand. By cyclic direct shear tests, this study investigates the dynamic shear characteristics and particle fragmentation mechanism of calcareous sand reinforced with rubber and geogrid. The analysis focuses on the shear characteristics of reinforced calcareous sand and the corresponding patterns of particle crushing. The findings indicate that: (I) Geogrid reinforcement effectively restrains volume deformation in the soil induced by rubber particles. (II) Reinforcement with rubber and geogrid compensates to a certain extent for the reduction in shear stiffness attributed to the rubber mixture. (III) The relative crushing rate of unreinforced calcareous sand post-test is 25.9%, which increases to 29.8% with geogrid reinforcement, decreases to 19.1% with rubber reinforcement, and reaches 22.6% with combined rubber and geogrid reinforcement. The geogrid’s particle crushing rate disadvantage is offset by the advantage provided by rubber reinforcement. The collaboration of rubber and geogrid reinforcement effectively addresses the limitations of rubber sand as a foundation material, rendering it more suitable for practical applications. [ABSTRACT FROM AUTHOR]

The explosion in foundation poses a significant threat to people and buildings. Currently, a unified empirical prediction formula for crater in calcareous foundation has not been established. In this paper, analyzed the types and sizes of explosion crater with different scaled burial depths through field tests and numerical simulation. In field tests, revealed the influence of scaled burial depth on the type and size of explosion crater and obtained the critical scaled burial depth for three different types of explosion craters, namely ejecta-type crater, collapse-type crater and covert explosion. Through the Smooth Particle Hydrodynamic-Finite Element Method (SPH-FEM) coupling algorithm, studied the movement trajectory of sand particles around the explosive at the moment of explosion in detail. Based on the field tests and numerical simulation results, it was found that calcareous sand has a smaller specific gravity due to its own characteristics, and the size of the explosion crater is larger than that of quartz sand at the same scaled burial depth. Obtained an empirical formula for crater in calcareous sand. Which can quickly predict the size of explosion crater and provide calculation basis for explosion resistant design in calcareous sand foundations. [ABSTRACT FROM AUTHOR]

In tropical islands, calcareous sand with poor engineering properties usually needs to be treated before it can be used as building materials. Considering the scarcity of freshwater in these areas, this study proposes seawater-based enzyme induced carbonate precipitation (EICP) technology to enhance the properties of calcareous sand. It is to induce calcium carbonate crystals to bond calcareous sand particles together using the seawater-based crude soybean enzyme and cementation solution (i.e., urea and calcium chloride). In this study, the crude soybean urease extraction test was firstly carried out using seawater and it was also investigated what components of seawater had a greater effect on the soybean urease extraction. Afterwards, the solution test was conducted to explore the ability of the extracted urease in inducing calcium carbonate through analyzing the variation of concentration of calcium ions and pH of the solution. Finally, the biocementation effect of EICP treated calcareous sand using the seawater extracted urease solution was evaluated by the unconfined compressive strength (quc) and microscopic analysis. Test results show that the turbidity of the seawater-extracted soybean urease solution can be reduced by 66.7% compared to that of deionised water extracted urease, with only a slight reduction in urease activity. Among all the components of seawater, NaCl, MgCl2, CaCl2, NaHCO3 and KBr can significantly reduce the turbidity of soybean urease solution. The lower turbidity can effectively avoid bioclogging and contribute to the homogeneity of the EICP-treated calcareous sands, and thus improve the biomineralization efficiency and strength enhancement. Seawater-based EICP treatment will be a great promising technology in freshwater-scarce tropical islands, because it can directly use seawater for biomineralization treatment of calcareous sand, and meanwhile effectively avoid local clogging of biocementation. [ABSTRACT FROM AUTHOR]

Microbially induced calcium carbonate precipitation (MICP) technology is an emerging and environmentally sustainable method for improving the strength and stiffness of soil. Specifically, this innovative approach has gained favor in marine engineering due to the advantaged compatibility between precipitated calcium carbonate induced by MICP and coral sand. Sand containing fines is susceptible to liquefy. Whereas, the impact of fines contents on cyclic behavior of MICP-treated calcareous sand remains uncertain. Consequently, this technical note aims to investigate the liquefaction behavior of biocemented calcareous silty sand by conducting undrained cyclic triaxial shear tests and microscopic analysis. The results revealed the patterns of the excess pore water pressure curves and cyclic deformation characteristics as the fines contents increased. The liquefaction resistance of biocemented sand initially decreases with the addition of fines but subsequently exhibits an increasing trend. Microscopic analysis showed that at the cementation level with the cementation solution concentration of 1 mol/L, the calcium carbonate crystals are mainly attached to the surface of sand grains and this pattern does not directly affect the force chain. [ABSTRACT FROM AUTHOR]

Although ~ 20% of global carbonate production occurs on extra-tropical carbonate depositional systems, our understanding of these environments still lags behind that of tropical ones. The Maltese shelf in the central Mediterranean offers an opportunity to study in situ facies distribution and the factors controlling it in a light-dominated setting. The investigated region of the Maltese shelf visually exhibits three main depositional environments: seagrass meadows, sand flats and rhodolith and maerl beds. While visually distinctive, the grain composition of the sediments does not provide a clear differentiation of the three environments but rather a gradient. This gradient is marked by increasing grain size with water depth, a transition from green to red calcareous algae and an increase in the fraction of low magnesium calcite of total carboantes. While some of these features can be explained by changes in light availability, other factors are also in play. Baffling by seafloor vegetation and currents, storms and internal waves inducing sediment reworking appear to play important roles in governing the sediment texture and composition across the Maltese shelf. The role of seagrass meadows in regulating production and accumulation rates of carbonates appears to be of greater importance in Mediterranean C-type carbonate factories than in southern Atlantic ones and this could be an important marker to identify them in the geological record. [ABSTRACT FROM AUTHOR]

In order to protect and develop marine resources, it is essential to reinforce the calcareous sand foundation on coral reefs during engineering construction. This study proposes a novel approach that combines enzyme-induced carbonate precipitation (EICP) with casein to solidify calcareous sand, which overcomes the issues of low calcium carbonate crystallization and insufficient cementation strength encountered in the pure EICP method. This study investigated the reinforcement effects and mechanisms of the combined method across various scales. Sonic time testing revealed an initial decline in sample density followed by a gradual increase, with the lowest density observed at 10% casein content. Sample strength depends on casein dosage and curing time. Overall, the trend in sample strength showed an initial decrease followed by a gradual increase, with 10% casein content again resulting in the lowest strength. The measured experimental groups achieved a minimum strength of approximately 1.17 MPa and a maximum strength of around 6.34 MPa. As casein concentration rises, the optimal curing time increases from 12–48 h. Microscopic experiments reveal casein expansion and dissolution in the EICP liquid, explaining UCS value changes with casein dosage and curing time. Combining scanning electron microscopy and microscopic experiments highlights the cementitious material's vital role in strengthening samples. In conclusion, combining EICP and casein for solidifying calcareous sand is technically feasible and highly effective with significant reinforcement effects. [ABSTRACT FROM AUTHOR]

To explore the penetration resistance of calcareous sand media, penetration tests have been conducted in the velocity range of 200–1000 m/s using conical-nosed projectiles with a diameter of 14.5 mm. Further, a pseudo fluid penetration model applicable to the penetration of rigid projectiles in sand media is established according to the approximate flow of compacted sand in the adjacent zone of penetration. The correlation between the impedance function of projectile-target interaction and the internal friction features of pseudo fluid is clarified, and the effects of sand density, cone angle of nose-shaped projectile, and dynamic hardness on the penetration depth are investigated. The results verify the feasibility, wide applicability, and much lower error (with respect to the experimental data) of the proposed model as compared to the Slepyan hydrodynamic model.

Abstract The explosion in foundation poses a significant threat to people and buildings. Currently, a unified empirical prediction formula for crater in calcareous foundation has not been established. In this paper, analyzed the types and sizes of explosion crater with different scaled burial depths through field tests and numerical simulation. In field tests, revealed the influence of scaled burial depth on the type and size of explosion crater and obtained the critical scaled burial depth for three different types of explosion craters, namely ejecta-type crater, collapse-type crater and covert explosion. Through the Smooth Particle Hydrodynamic-Finite Element Method (SPH-FEM) coupling algorithm, studied the movement trajectory of sand particles around the explosive at the moment of explosion in detail. Based on the field tests and numerical simulation results, it was found that calcareous sand has a smaller specific gravity due to its own characteristics, and the size of the explosion crater is larger than that of quartz sand at the same scaled burial depth. Obtained an empirical formula for crater in calcareous sand. Which can quickly predict the size of explosion crater and provide calculation basis for explosion resistant design in calcareous sand foundations.

The low bearing capacity and high erosion potential of calcareous soils are major concerns in marine environments. Lime stabilization is one of the earliest and most widely used methods for improving the mechanical properties of these weak deposits. Nonetheless, the significant amount of air pollution and high energy consumption associated with lime production have led researchers to the exploration of alternative strategies, such as the utilization of supplementary materials to partially replace lime in the stabilization process. In this study, the mechanical behavior of calcareous sand specimens stabilized with 4%, 6%, and 8% of hydrated lime and zeolite-to-lime replacement proportions of 0%, 15%, 30%, 45%, 60%, and 75% was examined through a comprehensive set of unconfined compressive strength (UCS) and ultrasonic pulse velocity (UPV) tests. The specimens were also subjected to consecutive wetting and drying cycles so that the effects of hydrated lime and zeolite proportions on the durability characteristics of treated calcareous sands were discussed. Results indicated that, in all lime contents, the UCS and constrained modulus (D) of treated samples reached their peak values when lime was substituted with zeolite at an optimum percentage of 60%. Additionally, it was observed that after four and eight wet–dry cycles, the optimum zeolite replacement ratio decreased to 45% and 30%, respectively. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were also conducted to achieve a better understanding of the microstructural changes in calcareous sands due to the stabilization with hydrated lime and zeolite. Practical Applications: The stabilization of noncohesive soils such as marine calcareous sands with lime has been a subject of interest among geotechnical engineers in recent decades. Due to the significant air pollution and high energy consumption associated with lime production, alternative strategies, such as using supplementary materials to partially replace lime, are deemed an attractive approach. In this study, the mechanical behavior of a calcareous sand (obtained from Hormuz Island as a strategic island in the Persian Gulf) stabilized with 4%, 6%, and 8% of hydrated lime and zeolite-to-lime replacement proportions of 0%, 15%, 30%, 45%, 60%, and 75% was evaluated through a series of UCS and UPV tests. Additionally, the influence of wetting and drying cycles on the strength and stiffness characteristics of treated Hormuz Island soil has also been studied; the topic which can be very significant due to the effects of waves and sea tides on the soil and structures of marine regions. Based on the experimental results, replacing lime with zeolite could efficiently improve the mechanical behavior of calcareous sand specimens, specifically when subjected to consecutive wet–dry cycles. In particular, 45% and 30% of zeolite replacement were shown to be the best substitutes for lime after four and eight cycles of wetting and drying, respectively. [ABSTRACT FROM AUTHOR]

Abstract Particle geometric is a key parameter that defines the eometric attributes of calcareous sand particles and is intricately related to their mechanical traits, such as compression and shear. The scanning electron microscopy and digital imaging were applied to capture the microscopic properties and geometric projections of calcareous sand. The qualitative analysis, conventional statistical methods and fractal theory were employed to describe the geometric morphology of sand particles. Additionally, we analyzed the structural and physical traits of calcareous sand based on its unique biological genesis. We developed a hypothetical structural-physical model for calcareous sand. Our findings revealed the interwoven reticulation on the surface of calcareous gravel particles, along with an uneven distribution of pores on the external surface. As the particle size increased, the global profile factor decreased and the angularity increased. The critical threshold for the variations in flatness, surface roughness, and circularity was observed at a particle size of 5 mm, with the particle size having a relatively minor effect on these characteristics for particles smaller than 5 mm. The shape of the calcareous sand particles exhibited fractal characteristics, with fractal dimension serving as a measure of surface smoothness, particle breakage, and strength. These experimental results could significantly enhance our understanding of the mechanical behavior of calcareous sand.

Calcareous sand is easily broken under external force, which brings great difficulties to island reef engineering. Based on the particle flow program, a discrete element model that can reproduce the results of laboratory tests is established, the large principal stress method is introduced to identify the particle force chain, and the bond strength between particles is increased to obtain an unbreakable model with the same initial conditions, and different confining pressures are compared and analyzed. The evolution law of the force chain of the following two models establishes a macro-meso cross-scale analysis in the deformation process of calcareous sand, explores the internal mechanism of the crushing of calcareous sand particles. The results show that particle breakage plays an important role in the evolution of the force chain. Particle breakage will reduce the probability of the force chain on both sides of the axis, forcing the probability of the axial force chain to rise steadily. The macroscopic deviatoric stress is the external manifestation of the probability of the axial force chain on the meso level. The faster the probability of the force chain in the direction of the potential shear band increases, the more obvious the shear band is.

AbstractThe generation mechanism of pore pressure plays an essential role in understanding the liquefaction behavior of sand under cyclic loading. Extensive undrained simple shear tests were undertaken to study the pore pressure and shear strain development characteristics of calcareous sand reinforced by fibers. The results show that the deformation patterns of the tested calcareous sand gradually shift from brittle to ductile failure as fiber content increases. The mechanism of pore pressure generation in calcareous sand subjected to cyclic loading is quite distinct from that of siliceous sand, exhibiting more pronounced accumulation in the initial stage of cyclic loading. Fiber reinforced calcareous sand exhibits reverse shear contraction behavior when liquefaction is imminent. A remarkable finding is the establishment of a unique correlation between pore pressure ratio and shear strain, irrespective of the fiber reinforcement. Consequently, a shear strain-based pore pressure generation model of reinforced calcareous sand is then developed to predict the pore pressure built-up trend under varying fiber content and length conditions. This model is also applicable to various testing conditions and soil types. [ABSTRACT FROM AUTHOR]

Particle geometric is a key parameter that defines the eometric attributes of calcareous sand particles and is intricately related to their mechanical traits, such as compression and shear. The scanning electron microscopy and digital imaging were applied to capture the microscopic properties and geometric projections of calcareous sand. The qualitative analysis, conventional statistical methods and fractal theory were employed to describe the geometric morphology of sand particles. Additionally, we analyzed the structural and physical traits of calcareous sand based on its unique biological genesis. We developed a hypothetical structural-physical model for calcareous sand. Our findings revealed the interwoven reticulation on the surface of calcareous gravel particles, along with an uneven distribution of pores on the external surface. As the particle size increased, the global profile factor decreased and the angularity increased. The critical threshold for the variations in flatness, surface roughness, and circularity was observed at a particle size of 5 mm, with the particle size having a relatively minor effect on these characteristics for particles smaller than 5 mm. The shape of the calcareous sand particles exhibited fractal characteristics, with fractal dimension serving as a measure of surface smoothness, particle breakage, and strength. These experimental results could significantly enhance our understanding of the mechanical behavior of calcareous sand. [ABSTRACT FROM AUTHOR]

Reinforcing calcareous sands with geogrids is a potentially effective method for large-scale geotechnical constructions in coastal lands. The breakable nature of polygonal calcareous sands determines the complex particle-geogrid interactions. A three-dimensional numerical model of geogrid reinforced calcareous sand (GRCS) was established to investigate the potential mechanical laws based on the discrete element method (DEM), and the reasonableness of the numerical model was verified by comparing with the indoor triaxial test. It follows that the macro-microscopic mechanical behavior of GRCS under the influence of aperture size and tensile resistance of geogrids was further investigated via effective DEM simulations. The presented results show that the decreased aperture size and increased tensile resistance are beneficial to enhance the macro-mechanical properties of GRCS, including strength, internal friction angle and pseudo cohesion. Particle crushing is mainly affected by shear strain and confining pressure. The bulging deformation of GRCS is partially suppressed due to the confining effect of geogrids. Besides, the source of strength enhancement of GRCS is revealed based on the microscopic particle-geogrid interactions, and the calculation method of horizontal and vertical additional stresses in the reinforced soil element considering the effects of tensile resistance and aperture size is further established. • A DEM model is established for geogrid reinforced calcareous sand. • Aperture size and tensile resistance effects are considered. • The distribution and evolution of particle crushing are characterized. • The macro-micro mechanical behavior of GRCS is revealed. • An additional stress calculation method is developed. [ABSTRACT FROM AUTHOR]

Particle breakage plays a pivotal role in shaping the deformation behavior of granular soils when subjected to compression. To comprehensively assess the influence of particle breakage on the evolution of particle size distribution and the compression characteristics of calcareous sand, this study conducted 63 sets of confined compression tests on calcareous sand, specifically under high-pressure conditions. The results indicated that the compression behavior of calcareous sand can be classified into three distinct stages within the logarithmic coordinate system of void ratio (e) and vertical pressure (σv): (1) σv < 2.18 MPa, (2) 2.18 MPa < σv < 4.35 MPa, and (3) σv > 4.35 MPa, with the first and third stages representing linear relationships. Furthermore, the concept of yield stress ascertained within the e–logσv coordinate system proves invaluable in assessing the effects of the initial void ratio (e0) and particle size (d) on particle breakage. Notably, when σv exceeds 2.18 MPa, the relative breakage index experiences a substantial increase. The characteristic particle sizes (d10, d30, d50, and d60) of calcareous sands exhibit a consistent decrease with an increase in the relative breakage index (Br); however, this relationship is independent of e0 but is closely related to the particle size (d) of calcareous sands. The one-dimensional compression characteristics of calcareous sand are effectively represented using a power function relationship, and a formula for calculating the compression modulus (Es) of calcareous sand is theoretically derived. The deformation (Δs) of calcareous sand can be calculated using the index parameters obtained through the Es–σv curve method. In addition, a method for computing the preconsolidation pressure (pc) of calcareous sand is introduced. This method uses the parameter (eb) as an indicator of particle breakage and has been experimentally validated. [ABSTRACT FROM AUTHOR]

AbstractTo reveal the influence of rubber particles on the mechanical properties of calcareous sand under the large-displacement shear effects of earthquakes and waves, indoor ring shear tests were conducted on rubber particle-calcium sand mixtures. The effects of rubber particle size and content on the mechanical properties of calcareous sand were studied via indoor ring shear test. Then, based on PFC3D, a numerical model of a ring shear test that can restore the shape of calcareous sand particles is established, and a mesomechanical analysis of mixed sand is carried out. The results showed that the effect of the rubber particle content on the crushing characteristics and strength of calcareous sand was significant and that the effect of the particle size was small. The rubber content causes a gradual decrease in the peak strength and particle crushing rate and increases the deformation stability of the mixed sand. The softening coefficient and relative crushing rate were significantly reduced to 0.01–0.06 and 0.28–1.2%, respectively, at 40% rubber content. The numerical simulations and experimental results were in good agreement. An increase in the rubber content significantly influences chain development, which explains the macroscopic mechanical properties of mixed sand at the fine level. [ABSTRACT FROM AUTHOR]

This study conducts a series of large-scale pullout tests to explore the pullout behavior of the biaxial geogrid-calcareous sand interface. It investigates the effects of normal stress, relative density, and embedded length on the interface pullout behavior of geogrid reinforcement in calcareous sand. In addition, this research examines changes in interface shear strength and the interface friction coefficient. The interface interaction mechanism of geogrid reinforcement in calcareous sand and the evolution of pullout force-clamp displacement curves are determined. The results indicated that the pullout force-clamp displacement curve primarily exhibits softening and hardening behaviors. The passive resistance of the geogrid mesh significantly influences the softening behavior, while shear friction predominantly affects the hardening behavior. The interface friction coefficient ranged from 0.28 to 0.88 for embedded lengths of 300 − 460 mm. The effects of relative density and embedded length on interface cohesion are more pronounced than those on the interface friction angle. When the embedded length increased from 300 to 460 mm, the rise in interface cohesion ranged from 42.0% to 47.7%, whereas the increase in internal friction angle ranged from 9.5% to 14.5%. [ABSTRACT FROM AUTHOR]

Sedimentary processes often produce natural and blown calcareous sands that are deposited in inclined and layered formations. These calcareous sands frequently exist in a complex stress state with rotating principal stress axes that result in intricate mechanical properties. To investigate the mechanical behaviors of calcareous sands with inclined sedimentary surfaces, we developed a device to prepare hollow cylindrical specimens from artificially crushed calcareous sands. By combining this device with a hollow cylindrical torsional shear apparatus, undrained monotonic loading and pure principal stress rotation tests were conducted to investigate the static and dynamic properties of the inherently anisotropic calcareous sands. The results indicate that the shear dilatancy property is related to the principal stress direction; the shear dilatancy of calcareous sand decreases as the direction of the principal stress increases, and the peak stress ratio decreases with increasing principal stress direction and increases with increasing deposition direction. In the precyclic loading period, increases in the deposition direction led to increases in the stabilities of the specimens and decreases in excess pore pressure accumulation, which further affect the dynamic shear modulus and damping ratio of the calcareous sand. In the late stage of cyclic loading, the inherent anisotropies of the specimens are destroyed, so that the excess pressure and hysteresis loop characteristics start to converge.

To explore the feasibility of geogrid reinforcement as a promising countermeasure to improve the liquefaction and post-liquefaction resistance of calcareous sand, extensive undrained monotonic and multi-stage triaxial tests were performed on unreinforced and geogrid reinforced calcareous sand with different relative densities. The test results illustrate that pore pressure generation curves of unreinforced and reinforced calcareous sand gradually evolve from S-shaped to hyperbolic-shaped with the increase in relative density, cyclic stress ratio, and effective confining pressure. Following this, a pore pressure model applicable to both unreinforced and reinforced calcareous sand is proposed. The liquefaction resistance of calcareous sand increases with the increase in relative density, whereas an elevated cyclic stress ratio increases its liquefaction susceptibility. A virtually unique relationship can be observed between the liquefaction resistance normalized to the product of phase transformation strength ratio and relative density against the number of cycles for triggering liquefaction, providing an effective means of early assessing sand liquefaction resistance. Moreover, the geogrid exhibits excellent reinforcement efficiency in enhancing the liquefaction resistance of calcareous sand at relative densities of 50% and 70%. During the post-liquefaction stage, increasing relative density and geogrid reinforcement can accelerate the recovery of stiffness and strength for liquefied calcareous sand and improve the post-liquefaction strength. In general, geogrid reinforcement is considered a good alternative to densification for improving the engineering properties of calcareous sand and offers great application prospects in marine engineering construction. • The effects of geogrid reinforcement and relative density on the liquefaction and post-liquefaction behaviors of calcareous sand are investigated. • A pore pressure model applicable to both unreinforced and reinforced calcareous sand is proposed. • The inclusion of geogrid enhances the liquefaction resistance of calcareous sand. • Increasing relative density and geogrid reinforcement accelerate the recovery of stiffness and strength for liquefied calcareous sand and improve the shear strength during the post-liquefaction stage. • Geogrid reinforcement is considered a good alternative to densification for improving the engineering properties of calcareous sand and offers great application prospects in marine engineering construction. [ABSTRACT FROM AUTHOR]

This study presents a novel method for seepage control in calcareous sand using microbially induced Ca-Alginate polymeric sealant, which was achieved by the reaction between supplied alginate and microbially induced Ca2+ ions directly derived from calcareous sand. This approach includes two steps: (i) microbially induced soluble Ca2+ generated inside the pores of calcareous sand through a bio-acidification process using activated sludge and (ii) reacting with supplied alginate to form Ca-alginate polymeric sealant via the in situ gelation. With the effect of Ca-alginate polymeric sealant, the seepage rate of the calcareous sand was intensely decreased to the level of 10–9 m/s from an initial 10–4 m/s when 5 g/L of alginate was applied. In this way, a low seepage rate of calcareous sand was achieved which was able to remove heavy metal ions and suspended solids from the wastewater sludge. A model test was also carried out to assess the erosion resistance of Ca-alginate treated calcareous sand dam subjected to wave attack. Built on the current research, the proposed polymeric sealant is a promising method for the reduction of seepage, formation barriers for coastal construction on the island, and remediation of specific contaminants. [ABSTRACT FROM AUTHOR]

To date, the dynamic behavior of slopes in calcareous sand is unclear owing to the marine biological origin of calcareous sand. In this study, the internal deformation and the liquefaction mechanism of calcareous sand slopes were investigated by shaking table tests and numerical simulations. The influence of the presence of initial shear stress due to site inclination on the dynamic response of the slope was considered. Moreover, based on the microscopic particle properties of calcareous sand, the special dynamic response of the calcareous-sand slope during earthquakes was explored through the parallel test with the quartz sand field. The results reveal that calcareous-sand slopes exhibit higher dynamic stability during earthquakes than quartz-sand slopes, which is related to the larger occlusion caused by the irregular shapes and porosities of calcareous sand particles. However, the calcareous-sand slope still demonstrates a horizontal displacement of 0.32 times the slope height under strong earthquakes. Moreover, the settlement of the calcareous-sand slope suddenly increased 60 cm from the slope top, while that of the quartz-sand slope decreased with an increase in the distance from the slope top. The seismic protection strategies involving the use of quartz-sand slopes in the calcareous-sand slopes are inapplicable. [ABSTRACT FROM AUTHOR]

Calcareous sand, which is characterized by irregular morphologies and abundant intraparticle pores, poses difficulties for ocean construction. This study investigated the effects of microbially induced carbonate precipitation (MICP) treatment methods and grain size on the biocementation of calcareous sand. Calcareous sand specimens with a wide range of grain sizes were first treated by MICP with immersion and mix methods. Scanning electron microscopy analysis revealed the presence of air bubbles inside the specimens treated by the mix method, which hindered biocement production, further promoting the emergence of a gradual failure pattern and reducing the strength and stiffness of the sand specimens. A series of unconfined compressive strength tests indicated that the strength and stiffness of the calcareous sand specimens treated by the immersion method were considerably greater than those of the specimens treated by the mix method in the suitable grain size range. In addition, the immersion method was not suitable for sand specimens with a median grain size of <0.5 mm. By contrast, the mix method was suitable for sand specimens of various sizes. As with the immersion-treated specimens, the specimens treated by the mix method displayed better mechanical properties at the early treatment stages, although the mix method-treated specimens exhibited much lower increase rates of strength and stiffness; moreover, the mix method-treated specimens exhibited lower strength and stiffness after long-term treatment than the immersion-treated specimens. [ABSTRACT FROM AUTHOR]

AbstractThe dynamic shear modulus G and its degradation tendency G/Gmax are essential parameters for characterizing the dynamic properties of soil. Coastal facilities constructed on calcareous sand foundations are subjected to multiple types of cyclic loadings. To investigate the effects of the uniformity coefficient Cu, effective confining pressure σ0′, relative density Dr, and median grain size d50 on the dynamic G and its degradation rule for calcareous sand, a set of resonant column tests is conducted on calcareous sand with different grain-size-distribution curves. The experimental results clearly show that the G degrades faster for calcareous sand with a lower σ0′ and a larger Cu. By contrast, the degradation of the G is affected less by Dr and d50. The G/Gmax values decreased slightly as γ increased but remained less than 10−5, whereas the G/Gmax curve descended rapidly as γ increased beyond 10−5. Based on the test results obtained in this study and previously published data, a new mathematical model characterizing the degradation tendency of G/Gmax with respect to the shear strain level of calcareous sands with different gradation conditions is proposed. The relevant parameters associated with the proposed model are correlated with Cu and σ0′. [ABSTRACT FROM AUTHOR]

Stress-induced anisotropy in calcareous sand can cause an uneven displacement in island reef engineering. In this study, stiffness, as a quantitative indicator, is explored to reveal the stress-induced anisotropy in calcareous sand. Based on the discrete element method, the stiffness anisotropic characteristics of calcareous sand during shearing, as well as the impact of particle breakage, are investigated by numerical simulations. Both the macro and micro responses, i.e., the maximum shear modulus, contact normal, strong and weak contact normal force, and the direction of particle breakage, are explored for calcareous sand with different particle breakage ratios. The results show that calcareous sand exhibits notable anisotropy during shearing, with the maximum shear modulus in the vertical direction (deviatoric stress direction) being significantly greater than that in the horizontal direction. Moreover, the higher the particle breakage rate, the lower the stiffness and its anisotropy. The micro-mechanism results indicate that the primary particle breakage during the shearing process occurs in the vertical direction. That is, the particle breakage weakens the strong contact force in the vertical direction, leading to a redistribution of the strong contact forces from the vertical direction to other directions. This redistribution mainly manifests in a decrease in the anisotropy of contact normal and contact vector within the sample, as well as a decrease in the proportion of strong contact forces in the overall contacts. This, in turn, reduces the shear strength and stiffness of calcareous sand, particularly in the vertical direction, and results in a decrease in the maximum shear modulus and its anisotropy. The maximum reduction can be up to 50% of the original value. These insights can provide a certain theoretical support for the uneven displacement and long-term stability of calcareous sand for islands and reefs. [ABSTRACT FROM AUTHOR]

Abstract The particle breakage effect and compression characteristics of calcareous sand are related to the water content in the sand material. However, the effects of water content on the particle breakage and compression characteristics of calcareous sand have rarely been investigated. In this work, 50 sets of confined compression tests were conducted on calcareous sand specimens, and the compression characteristics and particle breakage effects of two single-particle-size groups (particle size ranges of 1–0.5 mm and 0.5–0.25 mm) of calcareous sand were investigated under five different water contents. The test results showed that with the increase in the water content, the final compression deformation of calcareous sand was positively correlated with the water content. The final compression deformation decreased when the water content reached a certain value. The water content corresponding to the peak final compression deformation was related to the gradation of the calcareous sand; the specific values were 10% and 15% for particle size ranges of 1–0.5 mm and 0.5–0.25 mm, respectively. With the increase in the water content, the slope of the loading curve of calcareous sand appeared to increase and then decrease, reaching maximum when the water content was 10%. Moreover, the slope of the loading curve was close to twice that of the loading curve of dry sand, whereas the slope of the unloading curve changed little. Under the same water content, the initial gradation had no effect on the compression and unloading characteristics of the specimens beyond a vertical pressure of 1 MPa. The effects of the variation in the water content on the particle breakage of calcareous sand were mainly reflected in the softening effect of water on the specimen particles, which reduced the Mohr strength of the particles.

As a new type of deep-sea anchorage device, the torpedo anchor is widely used in deep-sea oil and gas exploitation projects, but few attentions have been paid to the torpedo anchor penetrating into coral deposits. Based on the discrete element method, the penetration process of torpedo anchor into calcareous sands is numerically analyzed. The reliability of the numerical model is verified by comparing with the existing indoor test results. The numerical simulation focuses on the evolution of particle breakage of soils around the anchor during the penetration process, and its macro and micro effects on the anchor penetration characteristics are also discussed. The results show that in the process of torpedo anchor penetration, the soil around the anchor tip is seriously broken, while the particle breakage at the anchor side is relatively weak. The particle breakage quantity increases with the increase of penetration depth, and the distribution of particle velocity field is basically symmetrical along the central axis of the anchor body. With the increase of penetration depth, the distribution range of particle velocity field expands, and the peak particle velocity first increases and then decreases during the penetration process. The peak value in the stress concentration area first increases and then decreases, when the penetration depth increases. After the flange starts to contact the soil, the stress within the soil reaches the maximum. The research results can provide references for the design of dynamic anchor penetration in island and reef engineering.

Due to the hydraulic sorting effect in the hydraulic filling process, a fine-grained aggregate layer dominated by silty fine sand with uneven distribution is easily formed in reclamation projects, which triggers issues with the bearing capacity and nonuniform settlement of calcareous sand foundations. In this study, a series of one-dimensional compression tests were conducted to investigate the effect of different fines contents (fc) on the compression behavior of calcareous sand. The results show that at the same relative density (medium-density, Dr = 50%), the addition of fine particles leads to a reduction in the initial void ratio (for fc ≤ 40%). Furthermore, while the compressibility of the soil samples increases with the rising of fines content, it begins to decrease with further addition of fine particles beyond a threshold value of fines content (fc-th). Additionally, particle crushing contributes to the compressive deformation of calcareous sand, and the particle relative breakage of calcareous sand increases at the initial stage of adding fine particles. Moreover, a comparison of the compression test results between calcareous silty sand (fc = 10%) and clean sand reveals that the addition of fine particles accentuates the compressibility differences among calcareous sands with different relative densities. These findings provide valuable insights for addressing the challenges posed by fine-grained layers in calcareous sand foundations.

Due to the characteristics of geographical environment and many complicated factors, the seepage properties of calcareous sand are quite different from those of common continental and marine sediments. To investigate the change of permeability coefficient of calcareous sand under external load, a seepage test device equipped with an impact loading system was developed. The effects of particle size distribution, initial relative density and impact load on the permeability of calcareous sand were investigated with this device. The results showed that the larger the curvature coefficient and uniformity coefficient, the smaller the initial relative density lead to the greater the permeability coefficient of calcareous sand and the more obvious the change of permeability coefficient before and after loading. The permeability change of calcareous sand under impact load was greater than that of quartz sand. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Calcareous sand, ubiquitous in the geotechnical makeup of the South China Sea, exhibits both compressibility and vulnerability to fragmentation when subjected to external loading, spanning a spectrum from typical to extreme conditions. This investigation aims to quantitatively assess the compression and particle breakage characteristics of calcareous sand under varied parameters, including relative density, saturation, applied loads, and loading paths, specifically focusing on sustainable geotechnical methodologies. Through a series of confined compression tests, this evaluation employed the relative breakage ratio and fractal dimension as key evaluative metrics. The results indicated that employing this integrated approach offered a more comprehensive understanding of calcareous sand breakdown mechanisms than relying on a singular particle breakage index. Furthermore, an increase in relative density can induce a transition in particle contact behavior, shifting from point-to-point interactions to face-to-face contact, thereby reducing inter-particle stress and minimizing grain breakage, particularly under loads below 200 kPa. Increasing loads exacerbated particle breakage, with finer particles predominantly initiating this process. During reloading, pore ratios across various load levels surpass those observed during initial loading, except at 1600 kPa, where a decline in pore ratio was noted, coinciding with pore water extrusion and the onset of new particle fracturing. The lubricating effect of water reduces inter-particle friction, enhancing stress concentration at particle edges and localized particle breakage, thereby increasing the presence of finer particles without significantly altering the overall structure. Notably, the influence of pore water pressure is evident during the reloading phase. These findings contribute to a refined theoretical framework for predicting coastal erosion risks and devising effective environmental protection strategies for sustainable coastal engineering practices. [ABSTRACT FROM AUTHOR]

Due to coastal storm surge and wave or man-made activities, widespread coastal erosion events are emerging. In this study, the feasibility of bio-carbonation of reactive magnesium cement (RMC) to mitigate coastal erosion was investigated by conducting laboratory-scale wave erosion tests. The accumulative erosion volume (V), erosion resistance duration (Dre), and volume erosion rate (VIS) were calculated to evaluate the erosion resistance of sand dunes with the different cement thickness (T = 3 mm, 4 mm, and 5 mm) and RMC content (R = 1%, 1.5%, and 2%). The surface penetration resistance and the cementation product content (brucite/hydrated magnesia carbonates (HMCs)) were also measured to assess the effectiveness of bio-carbonation in sand dunes. Results show that in comparison with RMC hydration, the VIS decreased by 77%, Dre increased by 333%, and peak penetration resistance increased by 42% after the bio-carbonation treatment of sand dunes. Further improvements in these results were obtained with increasing T or R. Given varying T or R, we identify two types of erosion process responses under wave action. For bio-carbonation treatment with higher T or R, the erosion process response follows four phases. In contrast, for hydration treatment, or bio-carbonation treatment with lower T or R, Phase II does not exist because of small crust thickness or weak bond. The presentation of Phase II effectively delays the slope failure depending on larger erosion volume (V ≥ 93 cm3) and turns sudden into flexible failure. Scanned electron microscopy observations further revealed that HMCs formed by RMC bio-carbonation provided better filling and cementation effects than brucite formed by RMC hydration, however, massive amounts of HMCs concentrated in the shallow surface layer, causing hindered penetration and reducing the number of cementation products at higher depths. [ABSTRACT FROM AUTHOR]

Subgrade soils are exposed to simultaneous cyclic deviatoric stress and variable confining pressure (VCP) caused by traffic loading. However, the effect of VCP on cyclic behaviour is still debated and no attempt was made to investigate its effect on calcareous sand. In this study, high-cyclic triaxial tests with a wide range of mean effective stress, cyclic stress ratio (ζ), and VCP amplitude were conducted to understand the effect VCP on the long-term cyclic behaviour of calcareous sand. The results demonstrate that VCP has a considerable effect on the accumulated axial strain ( ε q acc ) and shakedown behaviour of calcareous sand, both of which depend greatly on the stress levels. At a low ζ, the effect of VCP on the ε q acc and plastic shakedown behaviour is insignificant, where the application of VCP tends to slightly promote the ε q acc . However, at a high ζ, ε q acc decreases significantly with an increase in the VCP amplitude. For different ζ, the damping ratio increases and resilient modulus decreases significantly as VCP amplitude increases. Moreover, microscopic observations of pore size distribution were obtained through nuclear magnetic resonance tests, and were analysed to discover the microstructural mechanism highlighting how VCP affects cyclic behaviour. [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)