Experimental Study On Mechanical Properties Of Sand-cement Mixture Based On Fine Particle Content And Plasticity Inde

In recent decades,the liquefaction damage of sand-clay mixtures in earthquakes has challenged the stereotype that damage occurs only in sand and’relatively pure’sand under earthquake.Previous studies on the mixture of non-plastic and low-plastic sand have found that when the fine content in the soil increases,the development trend of shear strength and cyclic strength of the soil will increase or decrease.Obviously,it is necessary to pay more attention to the sand-clay mixture and conduct in-depth research on its mechanical properties.In this paper,the sand-clay mixture(standard sand,two kinds of non-plastic fine particles,two kinds of plastic fine particles)is taken as the research object.Two groups of test groups are set up to change the fine content FC and keep the plasticity index I_P constant(FC=0%,10%,20%,30%,50%)and to change the plasticity index I_P and keep the fine content FC constant(I_P=7.98,22.02,50.09).Through physical property test,monotonic triaxial test,cyclic triaxial test and environmental scanning electron microscope microscopic test,the effects of fine content and plasticity index on the mechanical properties of sand-clay mixture are discussed from macro and micro perspectives.(1)Fine content has an important influence on the mechanical properties of sand-clay mixture.For static characteristics,the increase of fine content will promote the transformation of sand-clay mixture from strain softening type to strain hardening type.The volume strain develops from dilatancy to shear contraction;the critical state line index showed a wave-like development.For dynamic characteristics,the threshold fine content makes the liquefaction failure of the samples dominated by pore water pressure gradually change to strain-dominated,and the effective stress path curve becomes loose and then dense.The cyclic strength of the sand-clay mixture decreases first and then increases with the increase of fine content.(2)Plasticity index has an important influence on the mechanical properties of sand-clay mixture.The influence of fine-grained soil plasticity index on the mechanical properties of sand-clay mixture is different at different fine content stages.On the whole,the influence of plasticity index on sand-clay mixture with low fine content is significant in the range of low plasticity and medium plasticity,while it has a great influence on sand-clay mixture with high fine content in the range of low plasticity to medium plasticity to high plasticity.For example,in terms of static characteristics,the increase of plasticity index will promote the sand-clay mixture to strain hardening,and the influence on the critical state index will change when the fine content is 30%.In terms of dynamic characteristics,with the increase of fine content,the difference between strain development and cyclic strength of sand-clay mixture caused by the increase of plasticity index gradually decreases.(3)According to the results of environmental scanning electron microscope test,the difference of mechanical properties of sand-clay mixture is determined by its microstructure characteristics.The increase of fine content will change the connection between particles of sand-clay mixture from coarse-coarse contact to coarse-fine contact and fine-fine contact,which leads to the deflection of particle behavior from coarse particle to fine particle.The increase of plasticity index is essentially the increase of plastic fine particles.In the case of low fine content,the increase of plastic fine particles will further weaken the connection between particles,but when the fine content is high,the plastic fine particles will promote the contact between fine particles to form a closer flocculation structure.(4)The viscous energy dissipation ratio can be used as a normalized index of energy dissipation to study the energy dissipation law of sand-clay mixture.The development of VEDR of sand-clay mixture mainly goes through four stages of development.In the case of different fine contents and plasticity indexes,there are three development modes.By comparing the energy dissipation results with the dynamic effective stress path and the critical state line,it is found that the cycle number corresponding to the intersection of the critical state line and the dynamic effective stress path is very close to the peak point of the viscous energy dissipation ratio,while the increase of fine content will make the gap between the two develop in a wave-like manner,and the turning point occurs at FC10 and FC30.The increase of plasticity index will narrow the gap between the two.In general,the research in this paper deepens the understanding of sand-clay mixture on the mechanical properties,and provides an analytical method based on energy method for studying mechanical properties of this type of soil from multiple perspectives.