Experimental Study On Permeability Characteristics Of Calcareous Soft Soil In Dredging Coral Island Reef

Calcareous soft soil is a product formed by dredger-filling in coral islands and reefs.It has a milky white color,finer particles,soft texture,silt-like when the water content is high,poor permeability,and high calcium content.This article focuses on the permeability of calcareous soft soil.Permeability is an important property of soil.It is related to many engineering issues such as foundation treatment,cofferdam waterproofing,and water-soil stability.It is of guiding significance for the construction of island reef project in the South China Sea.It is necessary to study the permeability of soft soil.Up to now,no systematic or deep research has been carried out for the permeability of the calcareous soft soil in the South China Sea.This paper will conduct laboratory geotechnical tests,XRD diffraction experiments and scanning electron microscope experiments on representative soil samples of calcareous soft soil collected from a dredger-filled coral island reef in the South China Sea.The corresponding basic physical property indicators,mineral composition and microstructure characteristics are obtained.Subsequently,a series of consolidation and permeability combined tests were carried out on calcareous soft soils through the GDSPERM environmental rock and soil permeability test system,and the effects of void ratio,consolidation history,consolidation pressure,hydraulic gradient,void ratio,ion concentration and coarse particle content on the permeability of calcareous soft soil were explored.Finally,the obtained test parameters are applied to the numerical simulation of the seepage problem of coral-soil cofferdams on islands and reefs,and the seepage characteristics of two cofferdams made of calcareous soft soil under different seawater levels is analyzed.The main work and conclusions of this paper are as follows:（1）Laboratory tests on soil samples taken from the site show that:calcareous soft soil has a high water content and a high void ratio,which can be defined as silty soil;the soil particles are fine,with a particle size of less than 0.075mm accounting for 70%,and contains mainly powder particles with less clay particles,which is well equipped.The mineral composition of calcareous soft soil mainly includes aragonite,calcite and magnesia calcite.The calcium content is very high,but it nearly does not contain clay minerals.The soil particles are irregular in shape,mainly strips,flakes,loosely arranged,and rich in intergranular pores.（2）Permeability tests are carried out on calcareous soft soil,and the results indicate that:the permeability coefficient of calcareous soft soil basically does not change with the change of hydraulic gradient.The relationship between its seepage velocity and head difference is a straight line that does not pass through the origin.There is an obvious initial hydraulic gradient,which deviates from Darcy’s law.The coefficient is basically stable on the order of 10^-6,and the test of consolidation and penetration at the same time has a lower permeability coefficient than the test of first consolidation and then penetration.（3）The main influencing factors of the permeability of calcareous soft soil are analyzed,the permeability coefficient of calcareous soft soil decreases with the increase of the consolidation pressure;it increases with the increase of the void ratio,and the change trend is nonlinear.The lg[k _v（1（10）e）]-lge model is found to be more suitable after fitting analysis.The permeability coefficient of calcareous soft soil decreases with the increase of the Na Cl concentration of the permeate;it gradually increases with the increase of the coarse particle content,and the whole change law can be divided into two stages.When the coarse particle content is less than 50%,the increase is not obvious;after more than 50%,the permeability coefficient increases sharply with orders of magnitude change.（4）A numerical analysis model for the seepage problem of coral soil cofferdams has been established.The analysis of the infiltration surface,seepage point,flow field,seepage velocity and seepage flow shows that calcareous soft soil performances well as an anti-seepage material in the seepage problem of the cofferdam.In addition,the calcareous sand-calcareous soft soil mixed cofferdam has the extra advantages of less material spending for the core wall,controllable seepage point,linear law and high stability.