Effect Of Montmorillonite Content And Sodium Chloride Solution On The Residual Swelling And Soil Water Characteristics Of An Expansive Clay

Expansive soil is mainly composed of strongly hydrophilic clay mineral(i.e.montmorillonite).It is a high plastic clay with some engineering properties such as multi-fissure,strong expansion and contraction,over-consolidation and strength attenuation,which has a great impact on the long-term stability and safety of engineering construction.In this paper,the compacted and remolded Nanyang expansive soil was taken as the research object,and the effects of montmorillonite content and NaCl solution on the residual swelling pressure and residual swelling deformation of expansive soil were studied through swelling tests,combined with scanning electron microscopy(SEM).Swelling-time history curve,swelling dynamic parameters and swelling index were studied through a series of humidifying swelling tests under overlying load conditions.At the same time,a model was established to predict the maximum swelling strain and the wetting-induced swelling deformation.The effects of NaCl solution on the pore structure characteristics of expansive soil were studied by mercury intrusion test(MIP)from the aspects of cumulative mercury intrusion curve,pore size distribution curve,specific surface area,pore tortuosity and pore fractal characteristics.The effect of NaCl solution on the soil water characteristic of expansive soil was preliminarily explored by filter paper test.At the same time,the pore distribution curve was used to predict the soil water characteristics and made a comparative analysis.The main contents and conclusions of this paper are as follows:(1)Residual swelling pressure and residual swelling strain increase with the increase of montmorillonite content and initial dry density.Soil samples with higher montmorillonite content have stronger hydration capacity and water adsorption capacity.NaCl solution limits the expansion of montmorillonite crystal,reduces the thickness of double layer,and therefore reduces the residual swelling pressure and the residual swelling strain.A mathematical model of effective montmorillonite density and residual swelling pressure is established considering montmorillonite content and dry density,which can be used to predict the swelling pressure of compacted expansive soil.(2)The hyperbolic function relationship proposed in this paper can be used to predict the relationship between the moisture content and the wetting-induced swelling strain of specimens under various overlying loads,and the applicability of the model is verified by eight soil samples in the literature.Swelling index decreases with the increase of initial dry density and remains unchanged with the increase of initial water content.To obtain the swelling index accurately,the effects of initial dry density,liquid limit and plastic index should be considered comprehensively.(3)NaCl solution and initial dry density have an effect on pore distribution curve,the most probable pore size,critical pore size,average pore size and D50 pore size,which are related to the micro-scale effect of NaCl solution and the macro-scale effect of initial dry density.Accumulated mercury intrusion and specific surface area are negatively correlated with NaCl solution concentration,and pore tortuosity is positively correlated with NaCl solution concentration.The pore structure has obvious fractal characteristics.The total fractal dimension and the fractal dimension of each pore section vary with the concentration of NaCl solution and the initial dry density,which show a certain regularity.(4)With the increase of NaCl solution concentration,the soil water characteristic curves of total suction and osmotic suction move to the right as a whole.The adsorption of soil particles and the blocking effect of soil particles make the actual osmotic suction greater than the theoretical osmotic suction.The influence of NaCl solution on soil-water characteristic curve of matrix suction is not obvious,but in general,NaCl solution can enhance the water holding capacity of soil.The pore size distribution obtained from mercury intrusion test is used to calculate the soil-water characteristic curve.The data points of the relationship between the measured saturation degree and matrix suction are all distributed above the calculated curve.