Experimental Study On Permeability And Seepage Deformation Characteristics Of Coarse-grained Calcareous Sand

In recent years,marine territories have occupied an increasingly important position in the national strategy,offshore island-building projects have been carried out rapidly,and calcareous sand as a marine geotechnical medium has been widely used in offshore projects.However,calcareous sand material exhibits different physical properties from conventional land-based sand in application,such as extremely irregular particle shape,fragility,and cementability.Calcareous sands are usually in the marine environment and are subject to complex hydrodynamic environments.Therefore,the permeability of calcareous sands has attracted widespread attention.In order to study the seepage problem of the coarse-grained calcareous sands frequently encountered in engineering,model tests were carried out using the vertical seepage model box and the horizontal seepage model box designed in this study.The study compared the differences in the shape of calcareous sand particles and terrestrial sand particles,and explored the effects of particle gradation,soil compactness and soil skeleton on the permeability and deformation characteristics of calcareous sand soil under different seepage directions.Through the combination of macro and micro approaches,the mechanism relationship between soil permeability and permeability deformation is expounded.The main work contents and conclusions are as follows:?1?Four shape parameters of aspect ratio S₁,roundness S₂,convexity S₃,and sphericity S₄ are used to quantitatively describe and compare the shape of calcareous sand and river sand particles,respectively.The results show that the shape parameters of calcareous sand and river sand are both negatively skewed.The shape parameters of calcareous sand particles are highly discrete,and the average value of the parameters is lower than that of river sand.The degree of regulation is significantly lower than that of terrestrial river sand.Based on the statistical results of the shape parameters,a method for classifying the particle shape according to the numerical values of the width-to-length ratio S₁'and the thickness-to-length ratio S₁'in the aspect ratio S1 is proposed.The particle groups of this kind of material all showed a linear decrease in the content of massive particles,a linear increase in the content of flat and strip-shaped particles,and the irregularity of the particles increased with the increase of the particle size.?2?The vertical seepage of calcareous sand was analyze based on Darcy's law of seepage,and the horizontal seepage of calcareous sand was analyze based on Forchheimer's nonlinear seepage theory in high-speed seepage.According to the change of soil permeability,it is proposed to divide vertical seepage into three stages:stable seepage stage,hydraulic adjustment stage and seepage failure stage,and divide horizontal seepage into two stages:seepage stage and hydraulic adjustment stage.According to the change of the seepage stage,the critical hydraulic gradient i_cr and the destructive hydraulic gradient i_d of the soil are determined?3?Through the fitting method,the permeability coefficients of the calcareous sand in two different seepage directions are obtained.The results show that the vertical inherent permeability coefficient of the calcareous sand body is between 0.048?0.930 cm/s and the horizontal inherent permeability coefficient is between 0.094?9.259 cm/s.The horizontal permeability coefficient of the soil is significantly higher than the vertical permeability coefficient.The size of the permeability coefficient and the changes in different seepage stages are affected by the soil gradation,compactness and soil skeleton.After the uneven coefficient C_u exceeds 12.6,the permeability coefficient increases with the increase of C_u,and decreases linearly with the density D_r.The permeability coefficient of good graded soil with a curvature coefficient C_c between 1 and 3 is much lower than that of bad graded soil.Under different seepage directions,the permeability coefficient of the soil with calcareous sand as the skeleton is higher than that with regular river sand as the skeleton.The difference is 1.1 to 2.4 times in vertical seepage and 1.05 to 6.04 times in horizontal seepage.?4?The integrity of calcareous sand is strong and the skeleton is strongly constrained to fine particles.The vertical seepage is mainly caused by fluid soil deformation,but pipe surge phenomenon may occur in poor graded calcareous soil with low fine particle content under a low hydraulic gradient.However,the calcareous sand in the horizontal seepage is affected by the lateral confinement of the surrounding soil,and only a slight pipe surge occurs under a high hydraulic gradient.Comparing the seepage deformation of the calcareous sand and river sand as the skeleton soil respectively,it is found that the soil with the river sand as the skeleton under the same conditions will have pipe failure under a lower gradient,indicating the calcareous sand framework with lower regularity and complicated structure is more restrictive to fine particles than the river sand framework.?5?Through image analysis of Image J binarization processing and PIV,the surface porosity and soil displacement field of the soil at different hydraulic gradient stages are obtained.The reason for the change in permeability of the soil at different stages is explained from the perspective of soil deformation.The results show that in the stage of vertical stable seepage,fine particles migrate in the initial large voids with small amplitude,and the surface porosity changes between 0.1%and 0.5%,which has little effect on permeability.At all levels of the hydraulic adjustment stage,the displacement area of the soil particles develops from the void to the non-void area and develops globally to the soil.The surface porosity changes from 1.1%to 4.5%,and the permeability of the soil is improved.In the stage of seepage failure,the areas where the soil particles are displaced are connected in a global range,the soil deformation occurs,the surface porosity increases greatly,and the soil permeability increases suddenly.In the horizontal seepage,the surface porosity gradually increases with the increase of the average hydraulic gradient i_a'.After reaching i_cr,a pipe channel is developed inside the calcareous sand body,and the fluid is concentrated to seep along the channel.The growth of the surface porosity curve tends to be numerically stable,and the permeability increases slightly.