Study On Compression Characteristics And Gradation Evolution Of Calcareous Sand Considering Strain Rate Effect

Calcareous sand is the main building material of islands and reefs in the South China Sea.Due to its special physical properties,the research on calcareous sand is gradually deepened.Because a series of problems such as uneven settlement of foundation,pile driving,aircraft landing,etc.During the construction of islands and reefs in the South China Sea can be classified as rate effect problems,the research on rate effect of calcareous sand is of great significance.The compression characteristics of calcareous sand have attracted extensive attention of scholars at home and abroad,and have achieved fruitful results.However,there are few reports on the strain rate effect and gradation evolution in the compression process.In this paper,the calcareous sand of an island reef in the South China Sea is taken as the research object,and the one dimensional compression test is carried out to study the rate effect in the compression process.Secondly,the gradation evolution equation is established to calculate the gradation change in the compression process.Based on this,the main research results of this paper are as follows:(1)Carry out the indoor geotechnical test to obtain the corresponding basic physical parameters of calcareous sand.The measured specific gravity is 2.79,the maximum dry density is 1.44 g/cm~3,and the minimum dry density is 1.02 g/cm~3.Determine the initial gradation of the sample through the screening test.(2)Three kinds of calcareous sand with relative compactness(D_r=30%,D_r=60%,D_r=80%)were selected for compression test under five loading rates(v=20 mm/min,10mm/min,5 mm/min,1 mm/min,0.1 mm/min).The compression curve and stress-strain curve of the sample during loading were measured.The test results show that the stress-strain curve and compression curve show typical nonlinear characteristics,and the maximum termination stress of the curve is between 16-18 MPa.In addition,it can be seen from the curve that the test has good repeatability.(3)Based on the results of one dimensional compression experiment,the degree of particle breakage during loading is analyzed.From the results,it can be seen that during compression,the particle breakage gradually intensifies with the gradual increase of termination strain,and the fractal dimension gradually increases,with a value range of 2.6-2.8.In addition,from the mass percentage content of each particle group after the test,the percentage content of particles of 0.25-0.075 mm and lt<0.075 mm in the sample is higher,and the particle crushing capacity is introduced to characterize the particle crushing state during the test.(4)Based on the research results of this research group,the size effect and strain rate effect of single particle of calcareous sand are analyzed.The results show that single particle has obvious size effect and strain rate effect.The particle strength decreases with the increase of size and increases with the increase of strain rate.In addition,the relationship between macro and micro stress states of particles is established,and the relationship between macro stress and particle strength is derived.It is found that the particle strength has nothing to do with the particle size,mainly because the small particles produced by particle crushing increase the coordination number of large particles,thus offsetting the particle size effect.(5)The contact stress distribution characteristics of particle aggregates are described by statistical distribution model,and a complete grading evolution model is established by introducing Weibull distribution and fractal model.The relevant evaluation indexes RMSE and NSE are introduced to evaluate the performance of the model.The results show that the performance of the model is excellent.In addition,the model is used to predict the grading evolution in the whole process of compression.The results show that the prediction results match the actual situation.