| Calcareous sand is a special geotechnical medium composed of coral,seaweed,shells and other marine organisms.Calcareous sand has special physical characteristics such as fragile,multiangular and multi-internal pores,and its engineering mechanical properties are also different from ordinary terrestrial sand,which is widely distributed in the South China Sea.With the gradual development of construction on islands and reefs in the south China sea,there are more and more construction projects on islands and reefs,and the scale of construction is getting larger and larger,and the future engineering problems are becoming more and more complex.Calcareous sand is an important building foundation material,such as the ambiguity of the physical and mechanical properties of calcareous sand,which may cause engineering accidents and waste of resources.Therefore,the mechanical study on the rate effect of calcareous sand,the further development and improvement of the mechanical properties of calcareous sand,the guidance of the actual engineering design and construction and exploitation of resources and tourism in the south China sea,which have important theoretical value and practical engineering significance.The calcareous sand near a coral reef in the south China sea is taken as the main research object,which based on the investigation of its basic physical and chemical properties,the effects of different shear rates and normal stresses on the strength and deformation of dry calcareous sand were studied by direct shear test and discrete element numerical simulation.The main contents are as follows:(1)The preliminary physical properties of calcareous sand samples from South China Sea were analyzed.The particle analysis test,specific gravity test and scanning electron microscopy test were carried out to investigate the basic physical properties of calcareous sand.(2)In order to study the effect of shear rate on the strength and deformation characteristics of calcareous sand,direct shear tests were carried out on dry calcareous sand under different shear rates.The results show that with the increase of shear rate from 0.1 mm/min to 2.4 mm/min,the shear strength of calcareous sand decreases first and then increases,and a similar trend of change is found for its internal friction angle.Therefore,it is concluded that the critical shear rate is 1.6 mm/min.Under low normal stress condition,the shear dilatancy phenomenon in calcareous sand sample is more easy to present with an increase in the shear rate;under relatively high normal stress condition,the shear rate changes from 0.1 mm/min to 1.6 mm/min,and the overall shear shrinkage decreases gradually;when the shear rate continues to increase from 1.6 mm/min to 2.4 mm/min,the maximum shear shrinkage increases gradually.The micro-mechanism of loading rate effect of calcareous sand varies with the applied normal stress level,i.e.,the loading rate effect of calcareous sand is mainly caused by the dislocation,displacement and rearrangement of particles in the sample at lower stress levels,while the loading rate effect of calcareous sand is mainly resulted from particle breakage at higher stress levels.(3)Based on the open source discrete element analysis software Particle Flow Code,the mathematical model of calcareous sand particles was established by using the discrete element program on the basis of laboratory tests and the basic morphology and mechanical characteristics of calcareous sand particles.On this basis,the discrete element calculation program for the direct shear test of calcareous sand was prepared,and the correctness and rationality of the calculation program were verified by comparison with the indoor test results.On the basis of the above,the numerical calculation of calcareous sand direct shear test under different axial pressures and different shear rates was carried out,and the microscopic variable evolution process and morphology obtained from laboratory test results and numerical test were combined to discuss the meso-mechanism of rate effect.The results show that the particle breakage in calcareous sands is not significant at low pressure,and the rate effect is weak;and at higher stress level,the critical rate characteristics of calcareous sands are mainly caused by particle breakage. |
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