Study On Behavior Of Shengzhou Diatomite Considering Structural Effects

As a special soil,diatomite has the characteristics of low density,high porosity ratio,strong structure and high compressibility.The engineering properties are highly susceptible to change under construction disturbance or dry-wet cycle,which can cause foundation pit collapse,landslide and bring security risks to engineering construction.Carrying out research on the engineering properties of diatomite,especially involving major projects of diatomite foundation and finding out its mechanical properties and strength deformation law have important guiding significance for engineering construction and disaster prevention in China diatomite distribution area.In this paper,Shengzhou diatomite is taken as an example to study its physical properties and mechanical properties through site-specific investigations,laboratory unit tests and microscopic tests.The influence of the structure of the Shengzhou diatomite on the strength and deformation characteristics of the soil is analyzed..Structural failure mechanism of Shengzhou diatomite is revealed from the microscopic point of view.And thus the relationship between microstructural evolution and macroscopic mechanical properties during structural damage are established.The main research results of this paper are as follows:(1)According to the on-site sampling problem feedback and disintegration test,the sampling method applicable to the undisturbed diatomite is further clarified,that is,the block sampling method.Shengzhou diatomite is rich in a large number of light diatom particles.The multi-porosity structure formed by diatom particles and illite/smectite formation is the internal cause of high porosity ratio,high water content,low density and high plasticity through work X-ray diffraction and scanning electron microscope.The strong structural and high-strength characteristics related to the rich siliceous cementation in Shengzhou diatomite.(2)The structural evaluation index of Shengzhou diatomite,that is,the structural stress ratio is as high as 32,which belongs to strong structural soil.According to the trend of the undisturbed soil compression curve,the compression deformation process can be divided into three stages.Structural yield stress is an important control index of compressive deformation.The compression curve,compression index and consolidation coefficient of undisturbed diatomite change significantly with structural damage,and gradually approach to the remolded soil properties.Microstructural observations indicates:With the consolidation pressure increases,the structure of diatomite will be destroyed,the diatom particles are broken,the intergranular cementations are lost and the pore structure are adjusted accordingly,and the influence of structural properties are weakened or even disappeared.The compression deformation law of diatomite is the result of the continuous evolution of its microstructure.(3)The unconfined compressive strength of natural Shengzhou diatomite is higher than that of the common soils,and its value is between 253.9kPa and 405.6kPa.Due to the structural damage of the remolded soil,the unconfined compressive strength decreased sharply,with a decrease of 5.58%?7.53%.Under unconfined conditions,the undisturbed soil is crusted and the axial stress decreases rapidly after the peak point,and the remolded soil is inflated without obvious signs of cracking.(4)The conventional triaxial test shows that the stress-strain relationship curves of the undisturbed Shengzhou diatomite under different confining pressures shows strain softening characteristics.After the peak point,shear failure occurs along with the shear band.The remolded soil hardens with the increase of axial strain until it reaches a level,and the soil sample shows obvious bulging deformation.Structure is an important cause of strain localization(the generation of shear bands)of Shengzhou diatomite.With the structural damage,the effective cohesion c' value decreased to about 1/20 of the undisturbed state,and the effective internal friction angle ?' did not change significantly.The shear band inclination of diatomite is consistent with the predicted value of the Mohr Coulomb formula.(5)Mercury intrusion tests indicate that the pore structure of the diatomite is mainly composed of small pores of 0.1 ?m to 1.0?m.The pore volume of this component accounts for 75.62%of the total pore volume.During the compression process,the amount of change in each pore size component is small before the consolidation pressure is less than the structural yield stress.The volume of medium pores(1?m?10?m)and small pores(0.1 ?m?1 ?m)decrease simultaneously,the pore size distribution shifts to the small and micro-aperture range,and the boundaries of large and medium pores become inconspicuous.The pores of Shengzhou diatomite have multiple fractal features.The law of pore variation in diatomite confirms the correlation between the microstructure and compression characteristics of diatomite.