Coral Sand Research Articles

Dynamic mechanical behaviors of coral sand under drop weight impact

Dynamic mechanical behaviors of coral sand under drop weight impact

Discrete element numerical analysis for bearing characteristics of coral sand foundation considering particle breakage

Owing to the construction demands of island and reef engineering, studying the bearing and deformation characteristics of coral sand foundations is significant. This study used the discrete element method to simulate the plate loading test of coral sand foundations based on the actual experiments of coral sand foundations. The particle breakage of coral sand was considered by the fragment replacement method, and the influence of relative compactness on the particle breakage behaviour and compression deformation characteristics of coral sand foundations was analysed. The results show that the compression process of the coral sand foundation undergoes three stages: compaction, shear deformation, and failure. The bearing capacity, deformation modulus, and particle breakage rate of the coral sand foundation increase with the increasing relative compactness, but the growth trends are different significantly at different relative compactness. The numerical analysis indicates when the relative compactness of the foundation increases, the contact force and coordination number would increase, the relative displacement would decrease, causing more particle breakage. This microscopic phenomenon explains why the increasing trend of bearing capacity and deformation modulus of the foundation decreases with the increasing relative compactness.

Micromechanical analysis of the particle corner breakage effect on pile load performance in coral sand

Micromechanical analysis of the particle corner breakage effect on pile load performance in coral sand

Study on the anti-corrosion properties of hydrophobic cement mortar containing coral sand

Study on the anti-corrosion properties of hydrophobic cement mortar containing coral sand

Effect of coral sand on the mechanical properties and hydration mechanism of magnesium potassium phosphate cement mortar

Effect of coral sand on the mechanical properties and hydration mechanism of magnesium potassium phosphate cement mortar

The Strain Rate Effects of Coral Sand at Different Relative Densities and Moisture Contents.

A 37-mm-diameter split Hopkinson pressure bar (SHPB) apparatus was used for impact loading tests to determine the effects of the relative density and moisture content on the dynamic properties of coral sand. The stress-strain curves in the uniaxial strain compression state were obtained for different relative densities and moisture contents under strain rates between 460 s-1 and 900 s-1. The results indicated that with an increase in the relative density, the strain rate becomes more insensitive to the stiffness of the coral sand. This was attributed to the variable breakage-energy efficiency at different compactness levels. Water affected the initial stiffening response of the coral sand, and the softening was correlated with the strain rate. Strength softening due to water lubrication was more significant at higher strain rates due to the higher frictional dissipation. The volumetric compressive response of the coral sand was investigated by determining the yielding characteristics. The form of the constitutive model has to be changed to the exponential form, and different stress-strain responses should be considered. We discuss the effects of the relative density and water content on the dynamic mechanical properties of coral sand and clarify the correlation with the strain rate.

Nonlinear seismic response analysis of reef-coral sand site in the South China Sea

To investigate the nonlinear seismic response of the reef-coral sand site in the South China Sea, a visco-elasto-plastic dynamic constitutive model and corresponding subroutine for coral sand material are developed in this study. The Matasovic skeleton curve and the derived hysteretic curve are refined in this model by introducing a critical shear modulus. Hysteresis criteria under irregular cyclic loads are established based on the improved Masing criteria. Moreover, increments of the octahedral strain and stress are utilized to construct the mapping rules between the three-dimensional stress-strain relationship and the one-dimensional stress-strain space. On that basis, a nonlinear finite element model of the reef-coral sand site is established, and the distribution pattern of the ground motion field and key ground motion parameters, such as the seismic amplification ratio and response spectrum of the reef-coral sand site, are investigated. The findings suggest that the coral sand layer significantly increases the site seismic responses, especially near the edge of the reef flat, thus posing a threat to the seismic safety of the buildings and construction in this area. Furthermore, the deformation of the reef-coral sand site tends to be concentrated on one side, resulting in a certain residual displacement after the earthquake.

Mechanical behavior analysis and bearing capacity calculation of CFG pile composite foundation on coral sand site

Coral sand foundation formed by hydraulic fill often faces the problem of poor bearing capacity. This paper proposed for the first time to apply CFG pile composite foundation to coral sand sites to verify the feasibility of this scheme and understand its mechanical characteristics. Firstly, taking on-site coral sand as the research object, a pile sand interface shear test was conducted to clarify the mechanism of pile side friction. At the same time, the ultimate bearing capacity of CFG pile and its composite foundation was measured through in-situ static load tests. Then, based on the strength parameters of the pile sand interface revealed by indoor tests, numerical simulations were conducted to analyze the bearing characteristics of CFG piles and their composite foundations. Finally, a method for calculating the vertical bearing capacity of rigid piles in composite foundation considering interface parameters was proposed. The results showed that the bearing capacity characteristic values of single pile and composite foundation meet the design requirements; The interface friction angle and cohesion together increased the ultimate side friction by 64.41%; The load is mainly borne by the pile tip resistance, and the increase of the interface friction angle will make the proportion of the side friction load first increase and then decrease more obviously; The pile soil stress ratio first increased and then tended to stabilize as the interface strength increased. Compared with the field static load test results, the rationality of the calculation method for composite foundation rigid piles was verified. This study may have reference significance for the design and construction of coral sand foundation treatment in offshore island and reef projects.

One-Dimensional Strain Research of Coral Mud Based on a Modified Burgers Model Considering Stress History

Coral mud is a special rock and sediment mass and is widely distributed in the South China Sea. Studying the deformation of coral mud is important for infrastructure development in the South China Sea. When choosing a model to describe the long-term deformation of coral mud, it is difficult for a simple nonlinear deformation model to accurately and universally describe the complex deformation processes of the sediment; a complex model is too time-consuming and difficult to apply to practical engineering. In this article, based on a classical element model, the Burgers model, certain elements are modified in combination with one-dimensional oedometer tests under normally consolidated situations. Then, combined with an unloading and reloading test, the modified Burgers model is further improved, and a modified Burgers model considering stress history is obtained. The modified Burgers model considering the stress history only has four parameters, all of which have practical physical significance, which makes the model easy to use. Different loading times and cyclic loading and unloading tests prove that the model has good stability and can, not only simulate the deformation characteristics of sediment, but can also provide good variation rules for the parameters.

Biomineralization of coral sand by Bacillus thuringiensis isolated from a travertine cave

Travertine is a typical product of microbial mineralization in the nature and its mineral composition is mainly calcite and aragonite. In this paper, Bacillus thuringiensis, a kind of mineralize bacterium is extracted from the travertine crystal to cenment coral sand, and the reinforcement effect of microbial induced carbonate precipitation (MICP) technology on coral sand under different cementation times is studied. Firstly, the culture conditions are optimized in nine pairs of trials, including urea content, microbial inoculation, shaker speed and incubation time. Under the optimal culture conditions, the coral sand is cemented by soaking method. With the increase of reinforcement times, the permeability coefficient of the sand sample is reduced to 10−4 cm/s, and the shear strength is increased by more than 130%. Compared with Sporosarcina pasteurii, the cohesion and internal friction angle of the coral sand column cemented by Bacillus thuringiensis are increased by more than 50% and 10%, respectively. The area distribution of T2 spectrum shows that with the increase of the number of cementation, the amplitude of the main peak decreases, indicating that the large pores are better filled, the number of medium and small pores are also reduced, and the pore area is significantly reduced, with the amplitude of about 44%. The above experiments verified that microorganism in travertine could also be used in MICP technology, and even achieve better reinforcement effect. It also provides a new way and idea for the selection of mineralized bacteria by MICP technology.

Synthesis of carboxymethyl cellulose-g-poly (acrylic acid-co-acrylamide)/polyvinyl alcohol sponge as a fast absorbent composite and its application in coral sand soil

To improve the rapid absorption capacity of coral sand soil for rainfall, a composite of carboxymethyl cellulose-g-poly (acrylic acid-co-acrylamide)/polyvinyl alcohol sponge (CMC-g-P(AA-co-AM)/PVA) was designed and synthesized by coupling CMC-g-P(AA-co-AM) granules with a PVA sponge. The results showed that the rapid water absorption of CMC-g-P (AA-co-AM)/PVA in distilled water in 1 h was 26.45 g/g, twice that of CMC-g-P(AA-co-AM) and the PVA sponge, which is suitable for short-term rainfall. In addition, the cation had a slight influence on the water absorption capacity of CMC-g-P (AA-co-AM)/PVA, which were 29.5 and 18.9 g/g in 0.9 wt% NaCl and CaCl2 solutions, respectively, indicating the great adaptability of CMC-g-P (AA-co-AM)/PVA to high‑calcium coral sand. With the addition of 2 wt% CMC-g-P (AA-co-AM)/PVA, the water interception ratio of the coral sand increased from 13.8 % to 23.7 %, and 54.6 % of the total interception water remained after 15-day evaporation. Moreover, pot experiments demonstrated that 2 wt% CMC-g-P(AA-co-AM)/PVA in coral sand enhanced plant development under water scarcity, suggesting that CMC-g-P (AA-co-AM)/PVA is a promising soil amendment for coral sand soils.

Development characteristics of excess pore water pressure in saturated marine coral sand based on shear strain characteristics: An experimental study

Using a hollow-cylinder torsional shear apparatus, we experimentally investigated the development characteristics of excess pore water pressure (EPWP) in saturated marine coral sand. These coral sand specimens were tested under various values of nonplastic fines content (FC), relative density (Dr), and cyclic stress ratio (CSR). A laboratory cyclic torsional shear test under isotropic consolidation showed that the development rate of the EPWP ratio (Ru) versus the number of cycles (N) increased with increasing FC and CSR but decreased with increasing Dr. Additionally, the increase in FC significantly reduced the cyclic resistance ratio (CRR) of marine coral sand. For a given Dr and FC, Ru of the specimens under different CSR was uniquely related to the amplitude of the shear strain (γa). Moreover, a pore pressure evaluation model based on shear strain characteristics was established. The measurements showed that the EPWP model parameter A is a soil-specific constant, and the density-corrected EPWP model parameter B/(Dr)1.5 has a single negative-power relationship with the equivalent skeleton void ratio (esk*.

Experimental investigation of cyclic failure mechanism of saturated coral sand under various consolidation conditions

Experimental investigation of cyclic failure mechanism of saturated coral sand under various consolidation conditions

Liquefaction characteristics of saturated marine coral sand with different non-plastic fines content subjected to complex cyclic stress paths

Coral reefs thrive in complex marine geological environments and are likely to suffer from dynamic marine disasters. The liquefaction characteristics of saturated marine coral sand subjected to dynamic loading, such as ocean waves, submarine earthquakes, storm tides, volcanic activities, and hurricanes, are key factors affecting the safety of coral reefs. A systematic study is conducted through a series of hollow-cylinder torsional shear experiments on the undrained response of saturated marine coral sand with different non-plastic fines contents (FCs) under cyclic linear stress paths with various cyclic loading direction angles (αd). An interesting discovery is that the cyclic linear stress paths with angles αd and 90°−αd have the same stress effect on liquefaction characteristics under isotropic consolidation condition. The test results show that if the near-zero effective stress state is defined as the criterion for initial liquefaction, then the liquefaction resistance of saturated coral sand decreases with increasing αd or FC. The generation of excess pore water pressure (EPWP) presents three modes: (1) “rapid–stable–fast,” (2) “rapid–stable,” and (3) “fast linear-like.” Moreover, an energy-based EPWP prediction method is established. A novel discovery is that the generalized shear strain amplitude (γga) is uniquely related to the EPWP ratio of coral sand for a given relative density and FC. In addition, by introducing unit cyclic stress ratio (USR) as a new index of liquefaction resistance, a common correlation between equivalent skeleton void ratio esk* and USR15 (required for initial liquefaction in 15 cycles) is established for all test cases considered. This is confirmed by the experimental data of five types of terrestrial siliceous sands reported in the literature.

Effects of water saturation and salinity on particle crushing of single coral sand

To evaluate the effects of particle size, water saturation and salinity on the mechanical properties of coral particles, single particle crushing tests were performed on 366 dried and saturated coral particles with varying sizes under different solution salinities. The AE energy release during particle breakage was monitored along with the diametrical compression loading. The results indicate that the increase in particle size and solution salinity decreases the particle crushing strength and increases its variability. The descending tendency in size-dependent crushing strength is attenuated by water saturation, causing more reduction in tensile strength for smaller particles than for larger ones. The breakage of single coral particles may be categorized into four typical patterns, predominated by the “Angularity fracture” and “Uniform failure” patterns. The AE features are distinctly different in the progressive failure of coral particles. The majority of the AE activities originate from frictional sliding, crack initiation and propagation.

Experimental investigation on cyclic behavior of geogrid-reinforced coral sand from the South China Sea

The present study provides insight into the cyclic behavior of unreinforced and geogrid-reinforced coral sand. To achieve these goals, stress-controlled undrained cyclic triaxial tests were conducted on medium-dense coral sand with and without geogrid reinforcements. The influence of geogrid reinforcement, cyclic stress ratios, and confining pressure on the deformation, pore pressure, liquefaction resistance, stiffness degradation, and particle breakage characteristics of saturated coral sand was investigated. The results indicated that coral sand reinforced with geogrid increased the liquefaction resistance and decreased the rate of pore pressure accumulation. The effectiveness of the geogrid inclusion on coral sand’s liquefaction resistance was found to be more pronounced under large effective confining pressure and a smaller cyclic stress ratio. Increasing the effective confining pressure increases the liquefaction resistance for both unreinforced and geogrid-reinforced coral sand, whereas increasing the cyclic stress ratios decreases the liquefaction resistance of the unreinforced and reinforced coral sand. The particle breakage was observed to be increased with increasing the effective confining pressure and cyclic stress ratio, and the breakage was more in the reinforced coral sand than the unreinforced coral sand.

Experimental study on the preparation and physical thermal performance of coral sand-based phase change material concrete composites

To promote the application of phase change materials (PCMs) in the field of green building, this research prepared coral sand-based PCM composites by vacuum adsorption method with coral sand as the porous carrier and polyethylene glycol 1000 as phase change material. The thermal and physical properties of the material, and its leakage characteristics after cement encapsulation were investigated through scanning electron microscope (SEM), differential scanning calorimetry (DSC), fourier transform infrared spectrometer (FT-IR), x-ray powder diffractometer (XRD), and thermal gravimetric analysis (TGA). The DSC results indicated that the coral sand-PCM composites had a melting point and latent heat capacity of 28.55℃ and 48.57 J/g, respectively. TGA and thermal cycling test demonstrated that coral sand-based PCM composites had good stability in terms of their chemical structure, thermal degradation, and cycling reliability in latent thermal energy capacity properties. The coral sand-based PCM composites were later prepared as coral sand-based PCM concrete composites by replacing coarse aggregates with different proportions. And the compressive strength of the PCM concrete composites was tested. In addition, the thermal insulation properties of coral sand-based PCM concrete composites were verified through model tests. The results of model test showed that the maximum indoor temperature differences between the Oridinary-0 and CS-50 concrete rooms were 7.45℃ during heating and 2.61℃ during cooling. The strength of coral sand-based concrete still could meet the requirements according to the structural design code at a mass substitution ratio of 50%. The CS-50 composite concrete exhibited acceptable mechanical properties and was able to regulate indoor temperatures and reduce energy consumption in buildings. Therefore, it is suitable for engineering applications.

Effect of particle size distributions on the mechanical behavior and particle breakage of coral sands

Effect of particle size distributions on the mechanical behavior and particle breakage of coral sands

Experimental study on the behavior of single pile foundation under vertical cyclic load in coral sand

The bearing behaviors of piles in coral sand are critical for project safety due to the unique physical and mechanical properties of coral sand, such as high void ratio, high compressibility, high friction angle and easy to break. A series of model experiments were carried out in this study to evaluate the vertical behavior of single pile in coral sand foundation under cyclic dynamic load with the consideration of the influence of the amplitude of cyclic load, the static biased load and the number of cyclic. Load-settlement relationships, settlement-cyclic number relationships, pile tip and pile side resistances have been thoroughly investigated. Results showed that the load in coral sand foundation will transfer to the pile tip more obviously compared with the pile foundation in quartz sand. Therefore, the pile side resistance will be reduced while the corresponding pile tip resistance will be increased. Particularly, the weakening of side resistance mainly occurs in the first 500 cycles. The development of cyclic cumulative settlement is affected by the combination of the magnitude of static biased load and cyclic amplitude. The existence of static biased load is conducive to restrain the cyclic cumulative settlement when the amplitude of cyclic load is less than static load.

Laboratory Experiments and Numerical Simulation on Dynamic Response of Island Reclamation Coral Sand under Aircraft Load.

The construction of island airports on coral reefs inevitably encounters the impact load of aircraft takeoff and landing. However, previous studies have not presented a detailed description of the dynamic response of the coral sand beneath the runways of island reclamation airports under aircraft load. In the current study, the coral sand of Mischief Reef Airport in the Nansha Islands, China, was selected as the background. The pore water pressure and strain characteristics of reshaped coral sand under aircraft loads with different dynamic stress amplitudes and vibration frequencies were studied using dynamic triaxial tests. Particle discrete element software was employed to study the deformation characteristics of coral sand with different particle sizes and porosities under aircraft loads. Results show that when the dynamic stress amplitude and vibration frequency were small, the pore water pressure and strain of the coral sand samples gradually increased with the number of load cycles, and the growth rate became increasingly small. When the dynamic stress amplitude and vibration frequency were large, the axial strain of the coral sand samples increased with the vibration frequency, and the growth rate exhibited an increasing trend. The deformation of the coral sand samples increased with porosity under aircraft loading. The larger the variation range of the coral sand particle size was, the larger the coral sand deformation caused by aircraft takeoff and landing load was. These results can provide a reference for the treatment and repair of the airstrip foundation of island airports.