Calcareous Sand Particles Research Articles

Single-Particle Crushing Test of Coated Calcareous Sand Based on MICP

Calcareous sand is a crucial construction material for island and reef development and reinforcing it using Microbially Induced Calcite Precipitation (MICP) technology is a promising new method. This study employed 3D scanning technology to assess changes in the particle size and morphology of MICP-treated, coated calcareous sand particles. Single-particle crushing tests were conducted to analyze their crushing strength, crushing energy, crushing modes, and fragment fractal dimensions. The results indicated that MICP treatment significantly increased particle size, surface area, and volume, while reducing flatness. At a cementation solution concentration of 1 mol/L, both crushing strength and crushing energy were optimized. The coated particles exhibited three crushing modes: explosive crushing, mixed crushing, and splitting crushing. Thicker coatings led to a tendency for particles to break into larger fragments through the mixed and splitting crushing modes. Fractal analysis revealed that coating thickness directly affects the local crushing characteristics of the particles.

Improving erosion resistance of calcareous sand slope with zinc sulfate solution

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

Particle shape analysis of calcareous sand based on digital images

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.

Evolution characteristics of calcareous sand force chain based on particle breakage

Abstract 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.

Effect of rubber particles on mechanical properties of calcareous sand under large displacement shear

To 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.

Study on microscopic mechanisms of slurry infiltration in calcareous sand based on CT scanning

A novel detachable microslurry infiltration device tailored for computed tomography (CT) scanning was developed. Using this device, a series of slurry infiltration tests was conducted on calcareous and Fujian sand columns with various bentonite slurry concentrations. CT scanning technology nondestructively captured the cross-sectional image slices of each specimen for analysis. A comprehensive image processing methodology was deployed to precisely differentiate the three phases—calcareous sand, slurry, and air—enabling 3D reconstruction of the infiltrated sand column. This approach combines three techniques: threshold segmentation, the watershed algorithm, and deep learning. Distinctive particle morphologies inherent to the calcareous and Fujian sands were observed. Notably, CT scans revealed a markedly greater angularity and irregularity of the calcareous sand compared with the Fujian sand, leading to different pore characteristics. Given an identical permeability coefficient, the calcareous sand porosity exceeded that of the Fujian sands. Furthermore, image analyses revealed distinct features of filter cake formation in these two soil columns. In calcareous sands with finer grain sizes, the slurry particles were more prone to clogging because of constricted and irregular seepage pathways. Conversely, with increasing particle size, the internal pores of the calcareous sand particles augmented channels for slurry infiltration, hindering filter cake formation.

Experimental Study on Bio-Reinforcement of Calcareous Sand through Hydrochloric Acid Solution Precipitation into Cementing Solution.

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.

Coupling effects of morphology and inner pore distribution on the mechanical response of calcareous sand particles

Coupling effects of morphology and inner pore distribution on the mechanical response of calcareous sand particles

Strength mechanism and electrochemical characterization of cement-bonded calcareous sand in different water environments

Cement-bonded calcareous sand (C-BCS) can effectively enhance the strength of calcareous sand, which is widely in infrastructure construction on islands in the South China Sea, and can fulfil the engineering requirements of island pavement subbase. However, the enhancement mechanisms and meso-generalized structural characteristics of C-BCS under different water environments and cement dosages are unclear. In this study, we consider the unconfined compressive strength as an evaluation index to quantify the degree of hydration reaction, change of hydration products, and meso-generalized structure of C-BCS in freshwater and seawater. We performed analyses using electrochemical impedance spectroscopy, X-ray diffraction, and scanning electron microscopy. The results demonstrate that C-BCS can satisfy the strength requirements of highway and airport runway subbases with 10% and 15% cement admixture, respectively. Furthermore, the corrosion deterioration phenomenon occurs and spreads in C-BCS in seawater. The internal hydration process of C-BCS can be continuously monitored using the electrochemical impedance spectroscopy test, and a correlation between the electrochemical and mechanical parameters at different ages is proposed. The meso-generalized structure inside the C-BCS can be densified by the bridging and pore filling effects of hydration products on calcareous sand particles, thereby improving the strength of calcareous sand. Additionally, we provide a concrete mechanistic explanation underlying the forming strength of C-BCS, and an optimized design approach of C-BCS considering several complex engineering properties.

Investigation of the effect of microbial-induced calcite precipitation treatment on bio-cemented calcareous sands using discrete element method

Owing to the bio-geological origin, calcareous sands are characterized by irregular particle shapes and abundant internal pores. These weak soil structures undergo extensive particle breakage under specific stress conditions. In recent years, microbial-induced calcite precipitation (MICP), a green soil treatment technique, has been widely utilized to improve the mechanical properties of calcareous sands. The treatment effect of MICP on calcareous sands and the underlying mechanism are still relatively unclear owing to the complex microstructures of the sands. This study investigated the micromechanism of MICP treatment on calcareous sands via the discrete element method (DEM). First, a group of calcareous sand particles were individually treated via a proposed MICP process. Single-particle crushing tests were conducted on the MICP-treated calcareous particles and a control group of untreated calcareous particles. The experimental results were further used to verify and calibrate the DEM models and parameters. The DEM model of MICP-treated calcareous particles was established via a novel modeling technique using the commercial PFC3D platform, which considered the inter-particle bridging and intra-particle pore filling effects by MICP treatment. Through this method, a series of MICP-treated calcareous sand samples subjected to uniaxial compression were simulated. The effects of calcite content and its spatial non-uniformity on the compression behaviors of the bio-cemented calcareous sand samples were studied. The micromechanics in terms of crack propagation and distribution as obtained from DEM were investigated to elucidate the mechanism of the MICP treatment of calcareous sands.

Responses of calcareous sand foundations to variations of groundwater table and applied loads

The long-term settlement of calcareous sand foundations caused by daily periodic fluctuations has become a significant geological hazard, but effective monitoring tools to capture the deformation profiles are still rarely reported. In this study, a laboratory model test and an in situ monitoring test were conducted. An optical frequency domain reflectometer (OFDR) with high spatial resolution (1 mm) and high accuracy (±10-6) was used to record the soil strain responses to groundwater table and varied loads. The results indicated that the fiber-optic measurements can accurately locate the swelling and compressive zones. During the loading process, the interlock between calcareous sand particles was detected, which increased the internal friction angle of soil. The foundation deformation above the sliding surface was dominated by compression, and the soil was continuously compressed beneath the sliding surface. After 26–48 h, calcareous sand swelling occurred gradually above the water table, which was primarily dependent on capillary water. The swelling of the soil beneath the groundwater table was completed rapidly within less than 2 h. When the groundwater table and load remain constant, the compression creep behavior can be described by the Yasong-Wang model with R2 = 0.993. The daily periodically varying in situ deformation of calcareous sand primarily occurs between the highest and lowest groundwater tables, i.e. 4.2–6.2 m deep. The tuff interlayers with poor water absorption capacity do not swell or compress, but they produce compressive strain under the influence of deformed calcareous sand layers.

Insight into the mechanism of microbially induced carbonate precipitation treatment of bio-improved calcareous sand particles

Insight into the mechanism of microbially induced carbonate precipitation treatment of bio-improved calcareous sand particles

Mechanical Property of Biomodified Geogrid and Reinforced Calcareous Sand

The strength and deformation properties of maritime geotechnical structures made primarily of calcareous sand are critical for project safety. The geogrid reinforcement is developed as a promising approach to improve the mechanical properties of calcareous sand. This study investigates the mechanical property of biomodified geogrid via a microbially induced calcite precipitation (MICP) process to improve the effectiveness of geogrid for reinforcement of calcareous sand. A series of unconsolidated undrained triaxial experiments were conducted to evaluate the mechanical property and deformation behaviors of biomodified geogrid and reinforced calcareous sand (BGRCS), taking into consideration the impacts of the geogrid layer, times of biotreatment, and confining pressure. Compared to the untreated geogrid, the strength of the BGRCS is distinctly changed due to the increase roughness, and the deviatoric stress-strain curves are evidently hardening. Strength and pseudocohesive force can be further enhanced by raising the geogrid layer of the reinforced specimens, while internal friction angle also increases the amplitude of variation with the times of biotreatment. The geogrid, times of biotreatment, and confining pressure are all intimately related to the strength and the deformation of the reinforced specimens. The interactions of geogrid ribs and calcareous sand particles are analyzed and friction using scanning electron microscope tests that could provide a reference for revealing the mechanical mechanism of BGRCS.

Breakage effect of calcareous sand on pile tip resistance and the surrounding soil stress

Due to the irregular shape, high porosity and fragility of calcareous sand particles, the construction of reef engineering is seriously affected, and the application of pile foundation engineering technology in calcareous sand stratum is also faced with great challenges. Based on the discrete element method (DEM), the pile installation process in the fractured calcareous sand stratum and non-fractured silica sand stratum are simulated, respectively, and the influence of particle breakage of calcareous sand on soil stress around the pile and pile tip resistance are discussed. The results show that the particle contact has no obvious dominant direction, and the particle breakage has little contribution on the variation of the particle contact normal directions while has great influence on the number of contact normals and the magnitude of contact normal force which is more uniform after particle breakage. The vertical and horizontal stresses in calcareous sand stratum are obviously lower than that in siliceous sand stratum. With the increase of the pile penetration depth, the number of crushing particles increases gradually. The particle breakage only can be found within the range of 3 times of pile diameter at the bottom of the pile and also within the range of 1 time of pile diameter around the pile. The effect of particle breakage on pile tip resistance is mainly reflected in the later stage of pile sinking. In calcareous sand, when the pile tip resistance increases to a certain extent, particle breakage occurs and the stresses in the soil cannot continually increase. Therefore, the pile tip resistance fluctuates around a stable value (∼0.3MPa). This is the reason that the pile tip resistance in calcareous sand is far less than that in siliceous sand, which is prone to pile foundation failure.

Three-Dimensional Discrete Element Analysis of Crushing Characteristics of Calcareous Sand Particles

Particle crushing is an important factor affecting the mechanical characteristics of calcareous sand, but at present, most of relative studying methods rely on physical experiments. In order to study the influence of particle crushing characteristics on the micromechanics of calcareous sand, the 14-fragment replacement method satisfying the Apollonian distribution is used to simulate the calcareous sand particles, and the fragment replacement method (FRM) is used to simulate the particle crushing. The three-dimensional discrete element model of calcareous sand particle crushing is established, and the development of relative crushing rate and the evolution laws of coordination number, porosity, and sliding contact ratio in triaxial consolidated drained shear test are analyzed. The results show that the three-dimensional model considering particle breakage can well reflect the micromechanical properties of the internal structure of the sample. The micromechanical response with and without particle crushing is quite different, which can be well reflected by the numerical test under high confining pressure. In addition, the modified relative crushing rate proposed by Einav based on the research of Hardin can better describe the relative crushing rate of calcareous sand under wide gradation and can provide a reference for the study of particle crushing characteristics of laboratory test of calcareous sand under wide gradation.

Mechanical property and deformation behavior of geogrid reinforced calcareous sand

The strength and deformation properties of maritime geotechnical structures made primarily of calcareous sand are critical for project safety, and geogrid reinforcement is a promising new approach. A series of consolidated drained triaxial experiments were conducted to evaluate the mechanical property and deformation behaviors of geogrid reinforced calcareous sand (GRCS), taking into consideration the impacts of the geogrid layer, relative density, particle size, and confining pressure. In comparison to the unreinforced calcareous sand, the strength of the GRCS is greatly enhanced, and the deviatoric stress-strain curves are altered from slightly softening to hardening, as well as the suppressed shearing dilatancy. The geogrid, relative compactness, particle size, and confining pressure are all intimately related to the volume changes and shearing dilatancy of reinforced specimens, but particle crushing is mostly impacted by the confining pressure. The interactions of geogrid ribs and calcareous sand particles are summarized as two types of constraint and friction using scanning electron microscope tests to establish a simplified calculation method of horizontal and vertical equivalent additional stresses that could provide a reference for revealing the mechanical mechanism of GRCS.

Impacts of organic matter and loading methods on one-dimensional compression behavior of calcareous sand

To explore the effects of organic matter (OM) and loading methods on compression and particle breakage behaviors of calcareous sand (CS), a set of one-dimensional (1D) compression experiments were completed with two loading methods of constant rate of strain and incremental loading. Particle size distribution (PSD) analysis and three-dimensional (3D) modelling tests were performed to assess particle breakage degree and OM distribution form. Test results demonstrated that there existed an OM content of 25% in which the total vertical strain (TVS) in loading process declined first and then grew after that. While in unloading process, TVS decreased linearly from 16.1% to 4.2% approximately when OM content increased from 0 to 100%. In addition, there existed two loading stress turning points of 800 and 1600 kPa corresponding to vertical strain variation for CS-OM (A) and CS-OM (B) mixtures, respectively. The rebound index of mixtures increased from 0.002 to 0.107 when OM content increased to 100% which was determined collectively by the percentage of PCS and OM. OM can restrain particle breakage of CS to some degree and relative breakage ratio decreased from 0.24 to 0.03. Under the same condition, the relative breakage ratio (0.24) of CS-OM (B) mixtures with IL loading method were larger than that of CS-OM (A) mixtures or CRS loading method. Moreover, SEM and 3D models of CS-OM mixtures suggested that the OM are flocculent and distributed in surface or pores of CS particles.

Effect of Different Mineralization Modes on Strengthening Calcareous Sand under Simulated Seawater Conditions

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.

The characteristics of internal and accumulated pores of calcareous sand

Calcareous sand is characterized by irregular shapes and high porosity, which makes its engineering properties more special. Based on the calcareous sand from a reef island in the South China Sea, the characteristics of the internal pores and the accumulated pores between the calcareous sand particles were explored by a microscopic system and image analysis techniques. The results show that for calcareous sand particles with different particle sizes, the distributions of internal pores and accumulated pores follow the exponential distribution. The probability distribution of internal pores is the same as that of accumulated pores, and the area of accumulated pores between particles is about 100 times that of internal pores of particle itself. The characteristics of accumulated pores are affected by the size and shape of the particles.

Effects of internal pores on the mechanical properties of marine calcareous sand particles

Effects of internal pores on the mechanical properties of marine calcareous sand particles