Experimental Investigations On Mechanics Properties Of Calcareous Sand On Islands And Reefs

Reef calcareous sands are widely distributed in tropical waters,which are characterized by abundant internal pore,brittle,fragile and irregular shape.With the rapid development of island reefs in the South China Sea,abundant calcareous sand is often used as the main filling material and building foundation of roadbed slope in the construction process.The settlement and stability of roadbed slope and building foundation are paid more and more attention.The solution of these engineering problems is closely related to the mechanical properties of reef calcareous sand,such as stress,strain and strength.Especially,the creep properties of reef calcareous sand are closely related.However,the research on creep properties of reef calcareous sand has not been carried out at home and abroad.Therefore,it is of great theoretical significance and engineering application value to study the mechanical properties of reef calcareous sands,especially the creep properties of reef calcareous sands.In this paper,the strength and creep characteristics of calcareous sand in an island reef reclamation area in the South China Sea are studied and discussed on the basis of the preliminary research work of Tianjin Institute of Water Transportation Engineering,Ministry of Transportation,on the special project of "Research on the microcosmic mechanism of creep and viscous-sub-plastic constitutive model of calcareous sand".Firstly,the calcareous sands with 0.075~2mm grain size were sifted and distributed well.The basic physical and mechanical parameters of calcareous sands on reefs were determined by specific gravity test and maximum(small)dry density test.Then,the conventional triaxial consolidation drainage shear test(CD)was carried out on the saturated calcareous sand specimens with different relative compactness,confining pressure and loading rate to obtain the stress-strain relationship curve of calcareous sand under different influencing factors and the peak deviation stress under shear failure.According to the peak deviation stress obtained from shear test,the stress-controlled loading method was adopted.The creep properties of calcareous sand under different test conditions were analyzed by step loading creep tests under the same confining pressure and stress level.After shear and creep tests of each group of calcareous sand,the particle sieving test was carried out on calcareous sand samples by sieving method,and the particle breakage characteristics of calcareous sand under load were analyzed.Based on the study of mechanical properties of reef calcareous sand by laboratory tests,the Mesri creep model is used to fit the test results,and the applicability of the model to reef calcareous sand is explored.The main research results of this paper are as follows:Under different shear test conditions,the biased stress-strain relationship of calcareous sands in reefs is obviously hyperbolic,but their shear characteristics are different: at the same loading rate,the increase of confining pressure or relative compactness will increase the ultimate strength of sands;at the same confining pressure and compactness,the increase of loading rate will show the shear characteristics of sands with different compactness.The peak deviation stress of 75% compactness calcareous sand increases slightly with the increase of loading rate,while the ultimate strength of 30% compactness sand is negatively correlated with loading rate.The shear strength of calcareous sands can be significantly improved by increasing the compactness of sand samples,while the shear strength can only be slightly increased by increasing the loading rate of 75% compactness of sand samples.In the shear process,increasing confining pressure,compactness and loading rate will aggravate the fragmentation of calcareous sand particles.The creep-time of calcareous sands has a power function relationship and obvious attenuation.When the stress level does not exceed 0.6,the creep behavior is similar to consolidation,and the volume of the specimen is basically shear-shrinkage.When the ratio of deviating stress to peak deviating stress exceeds 0.6,the volume of the specimen begins to expand obviously,indicating that the specimen is about to undergo creep failure.The fragmentation characteristics of calcareous sand particles after creep failure are basically the same as those after shearing: the degree of fragmentation of calcareous sand particles is positively correlated with three factors: relative compactness,confining pressure and loading rate.According to the mechanical properties of calcareous sands obtained from shear and creep tests,it is found that the Mesri creep model can basically reflect the creep characteristics of calcareous sands at a lower stress level(less than 300 kPa).The creep deformation energy of calcareous sands under the test conditions in this paper is roughly predicted,but it has certain limitations.It can also be used to predict the long-term settlement of buildings in islands and reefs within a lower stress range.Downward prediction has certain reference significance.