A novel relationship between elastic modulus and void ratio associated with principal stress for coral calcareous sand

Elastic moduli,e.g.shear modulus G and bulk modulus K,are important parameters of geotechnical materials,which are not only the indices for the evaluation of the deformation ability of soils but also the important basic parameters for the development of the constitutive models of geotechnical materials.In this study,a series of triaxial loading-unloading-reloading shear tests and isotropic loading-unloadingreloading tests are conducted to study several typical mechanical properties of coral calcareous sand(CCS),and the void ratio evolution during loading,unloading and reloading.The test results show that the stress-strain curves during multiple unloading processes are almost parallel,and their slopes are much greater than the deformation modulus at the initial stage of loading.The relationship between the confining pressure and the volumetric strain can be defined approximately by a hyperbolic equation under the condition of monotonic loading of confining pressure.Under the condition of confining pressure unloading,the evolution of void ratio is linear in the e-lnp0 plane,and these lines are a series of almost parallel lines if there are multiple processes of unloading.Based on the experimental results,it is found that the modified Hardin formulae for the elastic modulus estimation have a significant deviation from the tested values for CCS.Based on the experimental results,it is proposed that the elastic modulus of soils should be determined by the intersection line of two spatial surfaces in the G/K-e-p’/pa space(pa:atmosphere pressure).“Ye formulation”is further proposed for the estimation of the elastic modulus of CCS.This new estimation formulation for soil elastic modulus would provide a new method to accurately describe the mechanical behavior of granular soils.

High-velocity projectile penetration test and theoretical calculation of pseudo fluid penetration of calcareous sand

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.

Compressibility characteristics of bio-cemented calcareous sand treated through the bio-stimulation approach

Calcareous sand is widely present in coastal areas around the world and is usually considered as a weak and unstable material due to its high compressibility and low strength.Microbial-induced calcium carbonate precipitation(MICP)is a promising technique for soil improvement.However,the commonly adopted bio-augmented MICP approach is in general less compatible with the natural soil environment.Thus,this study focuses on the bio-stimulated MICP approach,which is likely to enhance the dominance of ureolytic bacteria for longer period and thus is deemed more efficient.The main objective of this paper is to investigate the compressibility of calcareous sand treated by bio-stimulated MICP approach.In the current study,a series of one-dimension compression tests was conducted on bio-cemented sand pre-pared via bio-stimulation with different initial relative densities(D r).Based on the obtained compression curves and particle size distribution(PSD)curves,the parameters including cementation content,the coefficient of compressibility(a v),PSD,relative breakage(B r),and relative agglomeration(A r)were discussed.The results showed that a v decreased with the increasing cementation content.The bio-cemented sand prepared with higher initial D r had smaller(approximately 20%e70%)a v values than that with lower initial D r.The specimen with higher initial D r and higher cementation content resulted in smaller B r but larger A r.Finally,a conceptual framework featuring multiple contact and damage modes was proposed.

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 26e48 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 R2¼0.993.The daily periodically varying in situ deformation of calcareous sand primarily occurs between the highest and lowest groundwater tables,i.e.4.2e6.2 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.

Shear Strength and Dilatancy of Calcareous Sand in the South China Sea

The shear strength and dilatancy of typical uncemented calcareous sand from the South China Sea are investigated by soil lab tests.According to drained triaxial tests at various relative densities and confining stresses,it is found that the constant volume friction angle is approximated as 39°and the traditional Bolton’s equations can be modified to estimate the peak friction angle and dilation angle.The reliability of the equation proposed for the peak friction angle is verified in terms of calcareous sands from more onshore and offshore sites worldwide,while the errors of the predicted dilation angles scatter in a relatively large range.Totally,the dilation angles of sands in the South China Sea are estimated by the equation presented with an error of±30%.The peak friction angle measured by the undrained is similar to that by the drained tests as the relative density smaller than 60%,while the former is slightly lower for denser samples.

Mechanical property of calcareous sand under action of compaction

Calcareous sand is a kind of special medium which is composed of calcium carbonate and other difficult soluble carbonate substances. Because of its rich in inner pore space and easy crashed,the mechanical property is very different from conventional quartz sand. Based on the compaction test and direct shear test of calcareous sand,by means of data fitting,the coupling relationship between compaction density and mechanical property under different water contents was obtained; meanwhile,the shear strength expression was built on the basis of the relationship between water content and dry density.

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

Calcareous sand is typically known as a problematic marine sediment because of its diverse morphology and complex inner pore structure.However,the coupling effects of morphology and inner pores on the mechanical properties of calcareous sand particles have rarely been investigated and understood.In this study,apparent contours and internal pore distributions of calcareous sand particles were obtained by three-dimensional(3D)scanning imaging and X-ray micro-computed tomography(X-mCT),respectively.It was revealed that calcareous sand particles with different outer morphologies have different porosities and inner pore distributions because of their original sources and particle transport processes.In addition,a total of 120 photo-related compression tests and 4923D discrete element simulations of four specific shaped particles,i.e.bulky,angular,dendritic and flaky,with variations in the inner pore distribution were conducted.The macroscopic particle strength and Weibull modulus obtained from the physical tests are not positively correlated with the porosity or regularity in shape,indicating the existence of coupling effect of particle shape and pore distribution.The shape effect on the particle strength first increases with the porosity and then decreases.The particle crushing of relatively regular particles is governed by the porosity,but that of extremely irregular particles is governed by the particle shape.The particle strength increases with the uniformity of the pore distribution.Particle fragmentation is mainly dependant on tensile bond strength,and the degree of tensile failure is considerably impacted by the particle shape but limited by the pore distribution.

Study on Triaxial Mechanical Properties and Micro Mechanism of Fly Ash Reinforced Cement Calcareous Sand

In order to study the mechanical properties and micro-mechanism of industrial waste fly ash-reinforced cement calcareous sand(FCS),the triaxial unconsolidated undrained(UU)test and scanning electron microscope tests(SEM)were carried out on it.The results of UU test show that the peak stress and energy dissipation of the FCS sample first increase and then decrease with the increase in fly ash content.Fly ash enhances the cement calcareous sand by increasing both the cohesion and internal friction angle,and adding 10%content of fly ash gives the largest values.The SEM test results shows that the hydration products of cement and fly ash filled the pores and cracks on the surface of the calcareous sand,which increased the compactness and structure of the FCS samples.The porosity of cement calcareous sand can be reduced from 27.6%to 12.8%by adding 10%fly ash.A brittleness evaluation index based on energy dissipation is proposed to quantitatively characterize the brittleness of FCS samples.The results show that when the content of fly ash is 5%,the brittleness of FCS samples is the lowest.This study shows that the mechanical properties of cement calcareous sand can be effectively enhanced by adding the appropriate amount of fly ash.

Durability of MICP-reinforced calcareous sand in marine environments:Laboratory and field experimental study

As eco-friendly methods,microbial induced carbonate precipitation(MICP)method was used to reinforce the calcareous sand in the South China Sea in this paper.The durability characteristics and deterioration mechanism of MICP-reinforced calcareous sand under various environment factors were investigated synthetically based on the unconfined compressive strength,mass loss rate and microscopic morphology in laboratory and field experimental study.Results show that,the unconfined compressive strength value of the sample is only 35.19%of the initial strength,while the mass loss rate is about 6.69%after 30-days of field marine environment erosion.MICP-reinforced calcareous sand shows the strongest resistance to temperature cycles,followed by dry-wet cycles,coupling effect of temperature and dry-wet cycle and salt spraying with drying cycles.MICP-reinforced calcareous sand exhibits the worst resistance to the field marine conditions,but the integrity of the sample could still be maintained after 30-days of field tests.The deterioration mechanism of MICP-reinforced calcareous sand is consistent under the various environmental cycles.First,the weakly cemented calcium carbonate crystals on the sample surface fall off,and then the hard-shell layer on the sample surface became weaker under various erosion.Finally,the internal cemented structure of the sample was gradually destroyed.The results indicated the utilization value of the MICP method in ocean engineering,but it is necessary to enhance the performance of the MICP-reinforced calcareous sand to ensure its protective effect after a certain environmental impact cycle.

Experimental study on permeability and strength characteristics of MICP-treated calcareous sand

Calcareous sand is the main fill material for island reclamation projects,but untreated calcareous sand might not be used as a reclamation fill due to its poor mechanical properties.Microbial induced calcite precipitation(MICP)was directly used to consolidate calcareous sands.One-dimensional sand column tests were conducted to identify the optimized solutions and to investigate the effects of cement solution concentration,relative density,and consolidation frequencies on the permeability and mechanical properties of MICP-treated calcareous sands.Finally,three-dimensional model tests were carried out to investigate the effective consolidation range of microbially treated calcareous sands.The results show that the MICP-treated calcareous sand shows a reduction in the permeability of the sample,while the calcium carbonate cementation and its filling effect improves the mechanical properties of the soil.The one-dimentional test results show that the effective values for cement solution concentration,relative density,and consolidation frequencies range from 0.5 mol/L to 1.5 mol/L,30%-70%,and 5-15 times.The consolidation frequencies have the greatest influence on the permeability and strength properties of the treated calcareous sand.A quadratic polynomial regression model for permeability and strength was established through response surface analysis,and the regression model proved to be highly accurate and reliable through testing.In three-dimentional tests,the consolidation range tends to move downwards in a trapezoidal shape,showing a"big bottom and small top"pattern,with a consolidation range of approximately 34 times the diameter of the pipe.This study serves as a reference for selecting consolidation parameters for subsequent tests and applications of MICP-treated calcareous sands.

Effect of stress path on the mechanical properties of calcareous sand

Calcareous sand is widely observed in the foundation of off-shore infrastructure.Although a lot of research has been carried out on the mechanical properties of calcareous sand,study into the influence of the stress–strain path on the mechanical behaviour of calcareous sand is very limited.In this study,a series of triaxial tests were performed on calcareous sand under three different stress paths.The particle morphology of calcareous sand before and after the tests,the stress–strain relationship under different stress paths,and the char-acteristics of shear strength and deformation were investigated.The results show that the consolidation pressure and stress path have significant effects on the volume strain,strength,and particle breakage of calcareous sand.In addition,the underlying mechanisms of the different behaviours of calcareous sand observed in this study were discussed.