Effects Of Fines Content And Confining Pressure On Shear Properties Of Calcareous Sand

With the continuous advancement of China’s "maritime power" strategy,the South China Sea Islands and Reef Project is rapidly under construction.Because the islands and reefs in the South China Sea are far away from the inland,the cost of transporting terrigenous sand is too high.The calcareous sand near the islands and reefs has become an ideal construction material,and has become the most widely used basic material in the construction of islands and reefs.In the construction process of the reef project,the foundation of the reef is generally treated by the reclamation technology.In the process of blowing and filling,due to the inhomogeneity of coarse and fine particles and hydraulic screening,it is easy to form a collection layer mainly composed of calcareous sand and fine particles in some foundation filling areas,which has become an important factor affecting the insufficient bearing capacity and uneven settlement of the reclaimed foundation.Understanding and mastering the effects of fine particle content and confining pressure on the shear characteristics of calcareous sand is a necessary condition to accurately measure the engineering performance of reef calcareous sand foundations.Therefore,it is of great practical engineering significance to carry out research on the effect and mechanism of fine particle content and confining pressure on the shear characteristics and particle breaking characteristics of calcareous sand.This paper takes the calcareous sand in the waters near an island reef in the South China Sea as the research object,the triaxial consolidation drainage shear test is mainly carried out on calcareous sand samples of three particle size groups under various control factors such as fine particle content and effective confining pressure,the evolution law of the shear characteristics and particle breakage characteristics exhibited by the calcareous sand during the shearing process is mainly studied.And based on the particle contact state theory,the mechanism that affects the particle breakage characteristics and shear characteristics expression of the sample is deeply analyzed and discussed.The main research contents and conclusions are as follows:(1)By carrying out triaxial consolidation drainage shear tests on calcareous sand samples of different particle size groups,The failure modes exhibited by the specimen during the test include four types: X-type shear failure,oblique shear failure,bulge failure and irregular shear failure,And the failure mode shows a high correlation with the particle size of the sample.(2)Based on the analysis of the particle curve and particle breakage index of the samples before and after the test,it can be seen that the relative particle breakage rate of the calcareous sand is jointly affected by the fine particle content and the confining pressure.Confining pressure is the dominant factor affecting particle breakage,and fine particle content is the changing trend that affects the degree of particle breakage within a certain range.(3)During the shearing process,the softening and dilatation characteristics of the sample are continuously weakened with the increase of fine particle content and confining pressure,and finally show the hardening and shearing characteristics slowly;There is a critical fine particle content and a critical confining pressure value,which determine the transformation of the strain softening properties and shear dilatancy properties of the specimen,respectively.The change of the critical value of the shear characteristics of the sample is inseparable from the fine particle content and confining pressure.(4)The apparent cohesion of the sample decreases greatly with the increase of fine particle content,the internal friction angle showed a trend of increasing first and then gradually decreasing with the continuous increase of fine particles,and peaked at10-20% fine particle content.Based on the analysis of particle contact state theory,it is believed that the increase of fine particle content changes the contact state between particles,and the force transfer mode between particles also changes immediately.The fines transitioned from their initial filling role to a lubricating role,which in turn resulted in a change in the shear properties of the calcareous sand samples.