Mechanical Properties And Micro-Mechanism Of Cement-Stabilized Organic-Matter-Disseminated Sand In Hainan

Organic-matter–disseminated sand（OMDS）is a widely distributed problematic soil in the northern belt areas,Wenchang areas and western coastal areas of Hainan province.Due to the organic matter,its engineering performance is poor,and its existence makes the installation of piles difficult and has the risk of insufficient bearing capacity.Therefore,it is necessary to reinforce the OMDS by using cement and other materials.The mechanical characteristics of the cement-stabilized OMDS should be researched in order to meet the needs of different engineering construction.In this paper,a systematic experiment,including macro curing tests and micro scanning electron microscopy tests was carried out on the OMDS for being found out the stabilized mechanism.Then,the unconfined compressive strength test and comventional static triaxial test were implemented to discuss the macro and micro strength and deformation characteristics of the cement-stabilized OMDS.Furthermore,the small strain dynamic characteristics of the cement-stabilized OMDS were investigated through dynamic triaxial tests.Finally,on the basement of the Discrete Element Method,the mechanical properties of the interface between the sand particles and the cementitious soil and the development of the interface damage behavior are analyzed based on the discrete element simulation.The major contributions and conclusions of the study are as follows:（1）There are four forms of organic matter in the OMDS,such as free,aggregated,adsorbed and disseminated.The presence of organic matter can inhibit the strength of conventional cement-stabilized OMDS,which is mainly manifested in delaying cement hydration,decomposing the formed hydration products,and hindering the combination of hydration products and sand particles.It can be found that the better effect is achieved by using slaked lime and cement to reinforce the OMDS.And there is a threshold for the mixing ratio of slaked lime.Moreover,the strength formation mechanism of slaked lime-cement-stabilized OMDS is to increase the p H value,remove the organic matter membrane wrapped on the surface of the OMDS particles,and increase the concentration of Ca²⁺and OH^-.（2）The relationship between the unconfined compressive strength（UCS）of cement-stabilized OMDS and curing time is exponential growth.Meanwhile,the peak deviatoric stress and failure strain of the cement-stabilized OMDS are significantly affected by increasing the initial comfining pressure and curing time.And the average deformation modulus,E₅₀,is slightly increased,showing stronger toughness.The UCS and shear strength of the cement-stabilized OMDS have the strongest correlation with the proportion of<1μm pore diameters and the fractal dimension of pore morphology.The stress-strain relationship of cement-stabilized OMDS under uniaxial compression is a strain softening type.On this basis,the static constitutive model of cement-stabilized OMDS is expressed by modifying the Duncan-Zhang model.That can better reflect the strain-softening phenomenon and more accurately simulate the static triaxial stress-strain relationship.（3）Under the same axial strain,the higher the confining pressure,the larger the cement content or the longer the curing time,the larger the dynamic axial stress of cement-stabilized OMDS.As well as the normalized dynamic elastic modulus of cement-stabilized OMDS is nearly independent of confining pressure and is mainly affected by cement content and curing time.Under the same conditions,the damping ratio decreases with the increase of confining pressure,cement content and curing time,respectively.The relationship between the damping ratio and the normalized dynamic modulus of elasticity is a power function.In addition,the dynamic elastic modulus of cement-stabilized OMDS is higher than remolded soil,but the damping ratio is lower.Especially,the strong cementation can significantly improve the resistance of OMDS to dynamic load.（4）A new discrete element sample preparation method is proposed,namely two elements,three types of contacts,and five sets of bearing forces.When the content ratio of slaked lime is higher than 7.5%,the smooth interface can be significantly ameliorated after the organic matter being dissmeninated on the surface of the sand particles.So that the friction of the contact interface and the cohesive force of the cementation interface can be improved.Obervously,the cementation bond between the cementation materials is more likely to be broken.With high confining pressure,the cementation contact fracture is accelerated.And then,the friction bearing between sand particles is enhanced.Furthermore,in the early stage of cyclic loading,there are little amount of cementation contacts failured.After several cycles,the cementation bonds are suddenly and massively broke,with higher failure rate than one-way loading.As a consequence,the load-bearing force of each component is transformed into being borne by the friction and rotation energy between sand particles and sand particles or cementation materials.