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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]

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]

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]

The resistivity of calcareous sand is one of its inherent physical characteristic parameters, and the study of the resistivity characteristics of calcareous sand is of great significance in the subsequent study of the relationship between the resistivity of calcareous sand under different loads and its mechanical properties. This study uses alternating current (AC) electrical resistance testing equipment based on the two-electrode method to study the effects of pore water, temperature, relative density, saturation, and other factors on the resistivity of calcareous sand. We used the experimental results to analyze in detail the effects of various influencing factors on the resistivity of calcareous sand and established a relationship between them. Research has shown that as the temperature increases, the conductivity of the medium in the solution increases and the resistivity decreases. The resistivity of the calcareous sand of the three size fractions decreases with increasing saturation, and the relationship between them is a power function. The resistivity of saturated calcareous sand with three size fractions increases with increasing density, and the two show a good linear relationship. The results of this research can provide support for the study of the relationship between the resistivity and mechanical properties of calcareous sand under different loads. [ABSTRACT FROM AUTHOR]

Microbially induced carbonate precipitation (MICP) technology holds great potential in enhancing soil properties. MICP can be employed to enhance the stability and strength of diverse sandy soil, but it has the shortcoming of low curing efficiency. In response to the identified problem, this study aims to investigate an optimized treatment protocol that involves formulating a cementing solution in a hydrochloric acid (HCl) solution to enhance the solidification rate in the MICP reaction and evaluate its effectiveness. The results indicate that when preparing a 1 M cementing solution in a 0.2 M HCl solution, it promotes the rapid bonding of calcareous sand particles, resulting in an unconfined compressive strength (UCS) of 1312.6 kPa in the sand column after five treatments. Compared to the conventional test group, the experimental group containing HCl exhibited an approximately 1357% increase in UCS. The analysis unveiled the pivotal role of metal ions dissolved from calcareous sand by HCl in enhancing the UCS of MICP-treated calcareous sand. The proposed experimental methodology serves as a valuable tool for designing treatment strategies for MICP-cemented calcareous sand in practical engineering applications. [ABSTRACT FROM AUTHOR]

Calcareous sand, a type of marine sediment formed from the skeletal remains of marine life, exhibits unique characteristics such as high porosity and fragility due to its biological origin. Particle breakage is a key attribute of calcareous sand. Given that foundations on calcareous sand islands encounter various types of loads, including pile driving, aircraft loading, earthquakes, and tsunamis, it is imperative to investigate its mechanical properties and particle breakage under high strain rates. This study focuses on assessing the dynamic mechanical properties of calcareous sand under quasi-one-dimensional impact loads using split Hopkinson pressure bar (SHPB) tests. Three particle sizes of calcareous sand with different water contents, strain rates, and relative densities were examined. The particle fragmentation degree of each sand sample was also analyzed quantitatively. The results indicated that stress–strain curves progress through an elastic phase with rapid elevation, followed by a plastic stage with a slower increase under various factors. Within the plastic phase, there are multiple instances of stress drops and recoveries. The stress–strain curves generally decrease as particle size increases, concurrent with an increase in particle breakage. Moisture content has minimal impact on the stress–strain curve; a higher moisture content does correspond to reduced particle breakage. Both the maximum strain and peak stress increase as the strain rate increases, resulting in a higher relative crushing rate. The difference between stress–strain curves under different relative densities diminishes as particle size increases, and greater relative density leads to reduced particle breakage. Functional relationships among peak stress and strain rate, relative fragmentation rate and water content, strain rate and relative density, as well as relative density and peak stress are also established. [ABSTRACT FROM AUTHOR]

The adsorption characteristics of calcareous sand for heavy metals Pb(II), Cu(II) and Cd(II) have been studied by batch testing in this study. The influence of the solid–liquid ratio, initial pH0 value, ionic strength, reaction time, temperature and initial concentration on adsorption has been investigated. Test results indicate that the initial pH and the solid–liquid ratio have a significant influence on the removal efficiency. At T = 30 °C, r = 1.0 g/L, and C0 = 1000 mg/L and for 12 h of reaction, the removal efficiencies of Pb(II), Cu(II) and Cd(II) are 97.6%, 88.15% and 65.72%, respectively. The adsorption quantity is more than 80% of the maximum adsorption quantity within 60 min, and the equilibrium adsorption can be reached within 120 min. The pseudo-second-order kinetic model is suitable to simulate the dynamic adsorption process of calcareous sand, and the isothermal process is found to obey the Langmuir model. Calcareous sand has a very high adsorption capacity for Pb(II), Cu(II) and Cd(II), with a maximum adsorption quantity Qm reached 1052.95 mg/g, 1329.84 mg/g and 1050.56 mg/g, respectively. Thermodynamic test results indicate that the adsorption process is spontaneously exothermic and that low temperature is favorable to the adsorption reaction. [ABSTRACT FROM AUTHOR]

Calcareous sand is one of the main building materials in the construction of islands and reefs, and its shear property is very important for predicting their strength and deformation. However, the correlation research on the shear properties of calcareous sand is limited. In this paper, a series of the triaxial consolidation drainage shear tests of calcareous sand with relative densities (Dr) of 70% and 90% under confining pressures of 100, 200, 400 and 800 kPa were carried out by a triaxial testing apparatus, and the effects of relative density and confining pressure on the deformation and strength characteristics of calcareous sand were analyzed. The results show that the stress–strain curves of calcareous sand show a strain softening characteristic, and both peak deviatoric stress and failure strain increase with confining pressure, but the increase in failure strain is restrained when the confining pressure is larger than 400 kPa. The initial shear modulus of calcareous sand is positively correlated with confining pressure. Additionally, the molar circular envelope of calcareous sand is linear in the range of 100~400 kPa, but it deviates from linear when confining pressure exceeds 400 kPa. The critical state line (CSL) of calcareous sand is nonlinear, with almost the same exponent for calcareous sand with different relative densities. The research results have important reference value for the foundation construction of islands and reefs. [ABSTRACT FROM AUTHOR]

Calcareous sand is a special soil formed by the accumulation of carbonate fragments. Its compressibility is caused by a high void ratio and breakable particles. Because of its high carbonate content and weak cementation, its load-bearing capacity is limited. In this study, the optimal stimulation solution was obtained with response surface methodology. Then, the effect of reinforcing calcareous sand was analysed with unconfined compressive strength (UCS) tests, calcium carbonate content tests, microscopy and microbial community analyses. The components and concentrations of the optimal stimulation solution were as follows: sodium acetate (38.00 mM), ammonium chloride (124.24 mM), yeast extract (0.46 g/L), urea (333 mM), and nickel chloride (0.01 mM), and the pH was 8.75. After the calcareous sand was treated with the optimal stimulation scheme, the urease activity was 6.1891 mM urea/min, the calcium carbonate production was 8.40%, and the UCS was 770 kPa, which constituted increases of 71.41%, 35.40%, and 83.33%, respectively, compared with the initial scheme. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses showed that calcium carbonate crystals were formed between the particles of the calcareous sand after the reaction, and the calcium carbonate crystals were mainly calcite. Urease-producing microorganisms became the dominant species in calcareous sand after treatment. This study showed that biostimulation-induced mineralization is feasible for reinforcing calcareous sand. [ABSTRACT FROM AUTHOR]

Calcareous sand is a special marine soil rich in calcium carbonate minerals, characterized by brittle particles. It is, therefore, widely used as a filling material in the construction of islands and reefs. In this study, a series of cyclic tri-axial tests were conducted on calcareous sand taken from a reef in the South China Sea under different confining pressures and cyclic stress ratio (CSR). Then, applying the shakedown theory, the cumulative deformation of calcareous sand under a long-term cyclic load of aircraft was evaluated. Results showed that with the increase in the effective confining pressure, the stress–strain curves of calcareous sand showed a change from the strain-softening to the strain-hardening state; the volumetric strain of calcareous sand showed a change from shear shrinkage and then shear expansion to continuous shear shrinkage. Calcareous sand showed three different response behaviors under cyclic load: plastic shakedown, plastic creep and incremental plastic failure. With the plastic strain rate as the defining index, this study determined the critical CSR of calcareous sand under different shakedown response statuses and found them to increase with the effective confining pressure. The empirical formula for critical stress was established based on the fitting analysis of critical CSR under different confining pressures, taking the confining pressure as the variable. At the early stage of the cyclic load, calcareous sand samples were under compression. When the resilient modulus grew rapidly and the number of loading cycles continued to increase, the particles of calcareous sand samples were crushed, causing the fine particles to fill the voids among coarse particles, further compacting the samples and increasing the resilient modulus of calcareous sand samples. Hardin's breakage potential model was adopted to quantitatively describe the particle breakage of calcareous sand samples before and after tests. The results indicated that calcareous sand samples produced obvious particle breakage when the CSR was small. As the CSR increased, the extent of the breakage of the sample particles first increased and thereafter stabilized. This study provides a theoretical reference for the assessment of the dynamic stability of calcareous sand subgrade subjected to traffic loads. [ABSTRACT FROM AUTHOR]

Calcareous sand deposits are widespread along the shoreline in tropical and subtropical regions. Microbially induced calcite precipitation (MICP) treatment is a new method for improving the soil's stiffness and strength. The small-strain shear modulus and damping ratio of MICP-treated calcareous sand, two critical parameters for predicting the dynamic behavior of soil, are little known. This study conducts a series of resonant column tests to investigate the dynamic characteristics of MICP-treated calcareous sand, emphasizing the influence of treatment duration and confining stress on the stiffness and damping characteristics. It analyzes the relationship between the initial dynamic shear modulus and unconfined compressive strength. In addition, empirical relationships between the reference shear strain and treatment duration or confining stress are provided. The results show that the normalized shear modulus G/G0 of MICP-cemented calcareous sand has a higher strain sensitivity than that of untreated sand, and the Hardin–Drnevich model can describe its attenuation pattern. The effective confining stress σc affects the degradation characteristics of the dynamic shear modulus of MICP-treated calcareous sand with a low cementation level; however, its impact decreases as the treatment duration increases. There is a linear relationship between the reference shear strain and confining stress. While the relationship between the reference shear stain and treatment duration is a power law. [ABSTRACT FROM AUTHOR]

A long-term cyclic loading test of calcareous sand from an island reef in the South China Sea was conducted under conditions of unequal consolidation and drainage. The axial cumulative strain and dynamic characteristics of coral sand samples with different physical and mechanical properties were studied under different stress levels, yielding a model prediction formula. The results show that (1) the axial cumulative strain and dynamic elastic modulus increase with increasing vibration times of the cyclic load, whereas the dynamic damping ratio decreases with increasing cyclic vibration time; (2) the axial cumulative strain, elastic modulus, and damping ratio are affected by dynamic stress amplitude, load frequency, confining pressure, consolidation stress, compactness and moisture content; and (3) based on dynamic triaxial test results, the axial cumulative strain model of calcareous sand under cyclic loading and its permanent deformation prediction formula are established by introducing the cyclic stress ratio. Two parameters, the elastic modulus evolution parameter and the damping ratio evolution parameter, are introduced, and the axial cumulative strain is normalized. The results of this research have significance for understanding the long-term deformation and dynamic response of coral sand subgrade soil under cyclic vibration loads. [ABSTRACT FROM AUTHOR]

In the construction of artificial islands in distant seas, calcareous sand has been widely used as a foundation filler due to its excellent mechanical properties and extensive availability in the marine environment. How to store more fresh water on the artificial islands by reducing its permeability is currently a great challenge. Microbial-induced carbonate precipitation (MICP) has always been considered as a great potential method to improve the cemented properties of calcareous sand, but the effect of grain gradation on the permeability of MICP-improved calcareous sand remains unclear. In this research, a self-made device was developed to conduct MICP grouting and permeability tests, where the permeability coefficient (k) under different grain gradations (curvature coefficient (Cc) and uniformity coefficient (Cu)) was measured. A CT scan was conducted to investigate the variation in the porosity (n) of sand samples before and after MICP treatment. The weighting method was adopted to measure the content of induced calcium carbonate (M). A scanning electron microscopy (SEM) technique was used to further study the micromechanism of the MICP treatment. Finally, the correlations between the k of MICP-treated sand and Cu, as well as Cc, were semiquantitively analyzed. The results show that the magnitude of M, k and n changes are closely related to Cc and Cu. The reduction amount of k and n increased with the rise in Cc and Cu, and the increased amount of M increased with the rise in Cc and Cu. The SEM results show that the particle surface became rough due to the coating effect of CaCO3 crystals, and the pore spaces were reduced because of the partially filling effect of the crystals, which was responsible for the decrease in permeability and porosity. Furthermore, k fitted well with Cu and Cc, respectively, and the fitting curve reveals that larger Cu (Cu ≥ 6.0) and smaller Cc (2.0 > Cc > 0.5) were more suitable for MICP treatments and lead to a large reduction in permeability. The above results indicate that the grain gradation of calcareous sand had a significant influence on its permeability improved by MICP. [ABSTRACT FROM AUTHOR]

Reviewing literature revealed that the studies on the bearing characteristics of pile foundations mainly focuses on clay, ordinary sand, loess, saline soil, and other areas. However, few studies on the bearing characteristics of the pile foundation in calcareous sand were conducted. Besides, existing traditional studies ignored the variation of soil compression modulus with depth, and the effect of void ratio on the transverse bearing characteristics of the pile foundation in a calcareous sand area were not well understood. In response of these problems, this study conducted a theoretical investigation on the transverse bearing characteristics of the pile foundation in a calcareous sand area. The transverse bearing characteristics of the pile foundation were derived based on the Pasternak foundation model and the Winkler foundation model, incorporating the heterogeneous distribution of compressive modulus with buried depth. The calculation results of the Pasternak foundation model are closer to the observed results than the Winkler foundation model. Therefore, the following research on the transverse bearing characteristics of the pile foundation in the calcareous sand area adopts the Pasternak foundation model. Then, the effects of the pile length, pile diameter, pile elastic modulus, horizontal load, bending moment, and void ratio on the transverse bearing characteristics of the pile foundation in a calcareous sand area were thoroughly analyzed. Furthermore, the difference between the transverse bearing characteristics of the pile foundation in a calcareous sand area and a quartz sand area was discussed. Results show that the horizontal displacement of the pile top in a calcareous sand area is greater than the quartz sand area under the same conditions. [ABSTRACT FROM AUTHOR]

For crushable calcareous sand, the stress-dilatancy curve has a significant turning hook around the peak stress ratio, the hook contains the main features of the loading process, including the phase transformation point and the peak stress ratio point. However, more than half of this turning hook, i.e., the line after the peak stress ratio point, is usually ignored by known stress-dilatancy models. It is difficult to directly establish the stress-dilatancy model with such turning hook characteristics, since such turning hook demonstrates that the dilatancy is not a single-valued function of the stress ratio. Based on the first law of thermodynamic, we related dilatancy to breakage energy. Then, we mapped breakage energy from the stress-energy plane to the strain-energy plane to avoid the non-single-valued function problem. Then, the stress-dilatancy model was conveniently established. Compared with the other four existing stress-dilatancy models, the benefit of our modeling process is that it can easily capture the turning hook of the stress-dilatancy curve. Our model is also verified by simulating colloidal-silica-stabilized and MICP-stabilized calcareous sands, as well as three types of calcareous sands, respectively. [ABSTRACT FROM AUTHOR]

To study the influence of the fractal distribution of particle size on the critical state characteristics of calcareous sand, a type of calcareous sand from a certain reef of the South China Sea was used in this study. For comparison, standard quartz sand was also used. A series of drained shear tests on the two sands were then conducted to investigate their critical state characteristics. It was demonstrated that the fractal dimension is suitable for characterizing the particle size distribution (PSD) of calcareous sand with different fine sand content. The critical state equation of sand proposed by Li and Wang (1998) is suitable for fitting the critical state line of calcareous sand. In the plane of deviatoric stress versus the effective confining pressure (q–p′ plane) and the plane of void ratio versus (p′/pa)α, the critical state lines of calcareous sand are always above those of quartz sand. The critical state lines of calcareous sand with different fractal dimensions in the q–p′ plane are unique. However, in the e–(p′/pa)α plane, the critical state lines appear to rotate anticlockwise as the fractal dimension increases. In addition, there is an "intersection" in the e–(p′/pa)α plane. Considering the influence of the fractal distribution of particle size, an expression for the critical state line of calcareous sand in the e–(p′/pa)α plane was proposed. The related constitutive model was also revised, where a complete set of model parameters suitable for modeling calcareous sand was provided. [ABSTRACT FROM AUTHOR]

Post-grouted piles, as a foundation form for large-span and large-scale structures on calcareous sand, are expected to provide a high bearing capacity, but research on the response of post-grouted piles subjected to axial load in calcareous sand is still in the exploratory stage. In this paper, a model test is constructed for static pressure piles in calcareous sand under axial loading. The response of axial compressive piles, with and without post-grouting, in calcareous sand were investigated, and the test results were compared with those of axial compressive piles, with and without post-grouting, in siliceous sand. The influence of post-side-grouting on the response of a single pile subjected to axial compressive load in calcareous sand and its bearing mechanism were further analyzed. The results show that the change in shaft resistance, caused by the lateral extrusion of calcareous sand, is less than the negative effect caused by particle breakage during pile driving, so single piles without post-grouting in calcareous sand exhibit weaker axial bearing behavior than that in siliceous sand. A single pile with post-side-grouting in calcareous sand can provide a higher bearing capacity by increasing the shaft resistance and tip resistance compared with a single pile without post-side-grouting, and the increased ratio of the bearing capacity of piles, after grouting in calcareous sand, is better than that of piles in siliceous sand. Post-side-grouting can not only strengthen the surrounding soil by the solidification effect of injected cement grout, but it can also have a strengthening effect on the tip resistance. In addition, ideal-geometry grouting has more obvious advantages in improving the bearing behavior of pile foundations than annular point grouting, and higher stability in improving the bearing properties of pile foundations is evident for ideal-geometry grouting. Therefore, it is suggested that a directional grouting device should be adopted in actual projects in the future to form a more stable pile-soil interaction system and to expand the application prospect of pile foundations in calcareous sand. [ABSTRACT FROM AUTHOR]

Calcareous sand is a special soil formed by the accumulation of carbonate fragments. Its compressibility is caused by a high void ratio and breakable particles. Because of its high carbonate content and weak cementation, its load-bearing capacity is limited. In this study, the optimal stimulation solution was obtained with response surface methodology. Then, the effect of reinforcing calcareous sand was analysed with unconfined compressive strength (UCS) tests, calcium carbonate content tests, microscopy and microbial community analyses. The components and concentrations of the optimal stimulation solution were as follows: sodium acetate (38.00 mM), ammonium chloride (124.24 mM), yeast extract (0.46 g/L), urea (333 mM), and nickel chloride (0.01 mM), and the pH was 8.75. After the calcareous sand was treated with the optimal stimulation scheme, the urease activity was 6.1891 mM urea/min, the calcium carbonate production was 8.40%, and the UCS was 770 kPa, which constituted increases of 71.41%, 35.40%, and 83.33%, respectively, compared with the initial scheme. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses showed that calcium carbonate crystals were formed between the particles of the calcareous sand after the reaction, and the calcium carbonate crystals were mainly calcite. Urease-producing microorganisms became the dominant species in calcareous sand after treatment. This study showed that biostimulation-induced mineralization is feasible for reinforcing calcareous sand.

In this study, a series of nuclear magnetic resonance (NMR) tests was conducted on calcareous sand, quartz sand, and glass bead with a wide range of grain sizes, to understand the effect of grain size on the micro-pore structure and fractal characteristics of the carbonate-based sand and silicate-based sand. The pore size distribution (PSD) of the tested materials were obtained from the NMR T2 spectra, and fractal theory was introduced to describe the fractal properties of PSD. Results demonstrate that grain size has a significant effect on the PSD of carbonate-based sand and silicate-based sand. As grain size increases, the PSD of sands evolves from a binary structure with two peaks to a ternary structure with three peaks. The increase in the grain size can cause a remarkable increase in the maximum pore size. It is also found that the more irregular the particle shape, the better the continuity between the large and medium pores. In addition, grain size has a considerable effect on the fractal dimension of the micro-pore structure. The increase of grain size can lead to a significant increase in the heterogeneity and fractal dimension in PSD for calcareous sand, quartz sand and glass bead. [ABSTRACT FROM AUTHOR]

This study examines the cyclic behavior of calcareous sand obtained from the South China Sea. A series of undrained cyclic triaxial tests were performed on the calcareous sand with various relative densities, cyclic stress ratios, and effective confining pressures. The test results show that whereas the effective confining pressure exhibited a small influence on the cyclic behavior of the calcareous sand, the effect of changes in CSR and relative density was notable. Due to its angular nature, the liquefaction resistance of calcareous sand was much higher than that of silica sand. In addition, differences between calcareous and silica sands in terms of pore pressure generation characteristics and deformation responses were also observed. [ABSTRACT FROM AUTHOR]

Calcareous sand, as a blow-fill or construction material, is widely used in island and reef construction projects in marine environments after treatment. When microorganism-induced mineralization is used to strengthen calcareous sand, salinity and other conditions in the marine environment will adversely affect microorganisms or their mineralization process. For this reason, the two environmental conditions created by deionized water and simulated seawater were introduced to explore their effects on the growth and urease activity of Sporosarcina pasteurii. Then, the changes in the permeability and mechanical strength of calcareous sand under different mineralization methods were compared by one-dimensional sand column tests. Finally, the reinforcement mechanism was compared and analyzed based on the results of scanning electron microscopy and X-ray diffraction tests. The results show that Sporosarcina pasteurii can induce carbonate and phosphate precipitation and mineralization to strengthen calcareous sand in simulated seawater. The mineralized products greatly reduce the permeability of calcareous sand and significantly improve the mechanical strength by wrapping calcareous sand particles, filling water seepage channels and cementing adjacent particles. The reinforcement effect of carbonate mineralization is better than that of phosphate mineralization, but phosphate mineralization has less impact on the environment during the treatment process. [ABSTRACT FROM AUTHOR]

The erosion of limestone and calcarenite ridges that existed parallel to the Mediterranean shoreline forms the calcareous sand (CS) formation at the surface layer of Egypt's northern coast. The CS is often combined with broken shells which are considered geotechnically problematic due to their possible crushability and relatively high compressibility. In this research, CS samples collected from a site along the northern coast of Egypt are studied to better understand its behavior under normal and shear stresses. Reconstituted CS specimens with different ratios of broken shells (BS) are also investigated to study the effect of BS ratios on the soil mixture strength behavior. The strength is evaluated using laboratory direct-shear and one-dimensional compression tests (oedometer test). The CS specimens are not exposed to significant crushability even under relatively high-stress levels. In addition, a 3D finite element analysis (FEA) is presented in this paper to study the degradation offshore pile capacity in CS having different percentages of BS. The stress–strain results using oedometer tests are compared with a numerical model, and it gave identical matching for most cases. The effects of pile diameter and embedment depth parameters are then studied for the case study on the northern coast. Three different mixing ratios of CS and BS have been used, CS + 10% BS, CS + 30% BS, and CS + 50% BS, which resulted in a decrease of the ultimate vertical compression pile load capacity by 8.8%, 15%, and 16%, respectively. [ABSTRACT FROM AUTHOR]

In the South China Sea, calcareous sand, as a natural foundation, has the features of low mechanical properties, including its compressive strength. With the development of South China Sea islands, the problems of calcareous sand foundation are encountered in the process. However, the experience of traditional pile foundation engineering could not be applied to calcareous sand. In this study, different proportions of curing agents were added to calcareous sand to improve the compressive strength. The quantitative analysis of the relationship between the unconfined compressive strength and microstructure of solidified calcareous sand is discussed. The unconfined compressive strength was gauged from unconfined compressive strength tests. Microscopic images, acquired using a scanning electron microscope (SEM), were processed using the Image-Pro Plus (IPP) image processing software. The microscopic parameters, obtained using IPP, include the average equivalent particle size (Dp), the average equivalent aperture size (Db), and the plane pore ratio (e). This research demonstrates that the curing agent could improve the compressive strength, which has a relation with the three microstructure parameters. The curing agent, through hydration reaction, generates hydration products, i.e., calcium silicate hydrate, calcium hydroxide, and calcite crystals. They adhere to the surface of the particles or fill the space between the particles, which helps increase the compressive strength. In addition, there is a good linear relationship between the macroscopic mechanics and the microscopic parameters. Using the mathematical relation between the macroscopic and microscopic parameters, the correlation can be built for macro-microscopic research. [ABSTRACT FROM AUTHOR]