Effect Of Density And Sand Content On The Shrinkage Characteristics Of Compacted Montmorillonite

The swelling and shrinkage characteristics are one of the three characteristics of expansive soil.However,at present,there are many researches on the swelling properties of expansive soil in China,but few on the shrinkage properties.The soil shrinkage also has a significant impact on its physical and mechanical properties.The swelling and shrinkage characteristics of expansive soils are mainly affected by the hydrophilic clay minerals montmorillonite and illite.In this paper,sodium montmorillonite is taken as the main research object.Then the effects of initial dry density and sand content on soil shrinkage under constant humidity were studied.Finally,a simplified model of the relationship between porosity and moisture content is proposed,and the mechanism of soil shrinkage is simply analyzed.The main contents of the paper are as follows:(1)Proposed and implemented a set of conditions for constant humidity and temperature shrinkage of soil test method and device,namely the moisture at the bottom of the cylinder to join saturated salt solution,to control the relative humidity of the moisture inside the cylinder,protect wet cylinder controlling temperature in air conditioning room,to reduce the influence of temperature change on the solubility of salt solution that cause the moisture humidity in cylinder change.The experimental results show that the humidity in the humidifier remains constant,and the humidity of the soil sample will increase in the initial stage,but will remain constant in the later stage.(2)The effect of initial dry density on the shrinkage characteristics of montmorillonite was studied.The results show that under the condition of constant relative humidity,the smaller the initial dry density is,the higher the relative position of the shrinkage curve is,and the larger the final porosity ratio is.When using the three-line model to analyze the shrinkage curve,it is found that the lower the initial dry density is,the higher the slope of the sample in the basic shrinkage stage is;the slope value in the residual shrinkage stage does not change much with the dry density;and the slope in the structural shrinkage stage is discrete.The main reason for the above results is that the structure shrinkage stage is at the early stage of the test,when the sample volume changes rapidly and is not uniform,but the observed data points are few and the slope fitting data is discrete.In the basic shrinkage stage,the larger the initial dry density of the sample is,the larger the proportion of soil particle volume,the smaller the proportion of pore volume,the smaller the soil sample's shrinkage space and the larger the friction resistance between soil particles will be during the shrinkage,thus the slope will gradually decrease with the increase of dry density.In the stage of residual shrinkage,the volume change from sample shrinkage to final volume is relatively small,so the slope value obtained by fitting is basically the same.(3)With the same initial dry density,the lower the relative humidity of the environment,the higher the relative position of the shrinkage curve and the greater the dispersion.It is found from the analysis of the shrinkage curve that the higher the relative humidity is,the higher the slope value is in the stage of structural shrinkage and residual shrinkage,and the lower the slope value is in the stage of basic shrinkage.It can be seen that the lower the ambient humidity is,the more obvious the effect is on the shrinkage characteristics of the sample.This is mainly due to the fact that in the environment with low relative humidity,the soil loses water quickly,and the soil particles re-arranged at a relatively slow speed,thus causing the soil to be cracked to make the measured volume larger;and the relative humidity is higher.The water loss rate of the sample is compatible with the soil particle re-arrangement speed.At this time,the soil is not cracked,and the measured volume is the actual volume.Therefore,the lower the relative humidity of the environment,the smaller the slope of the structural shrinkage phase is fitted;The basic shrinkage stage is due to the fact that the cracks generated in the initial stage of the test in the low humidity environment are closed at this stage,and the soil continuously shrinks to shrink the internal pores;while the sample in the high humidity environment still shrinks at the normal speed,thereby Lead to this phenomenon.The residual shrinkage stage is due to the fact that the shrinkage of the sample under low humidity conditions has been substantially completed in the previous stage,and the shrinkage of the sample under high humidity conditions at this stage is relatively large.In summary,the lower the relative humidity,the greater the influence on the shrinkage characteristics of the soil;when the relative humidity is higher,the influence of the humidity change on the shrinkage characteristics of the soil is relatively small.(4)The influence of sand-mixing ratio on the shrinkage characteristics of montmorillonite was studied.The results showed that when the mass ratio of sandy soil was less than 15%,with the increase of sand content,the vertical shrinkage rate,radial shrinkage rate and volume shrinkage rate of samples would gradually decrease.When the mass ratio of sandy soil is greater than 15%,with the increase of sand content,the vertical shrinkage rate,radial shrinkage rate and volume shrinkage rate of samples will first have a small increase,and finally become a stable value.