The Research On The Influence Of Salt On Soil Desiccation Cracking

With the increasing scope and scale of the construction of our country,the harm of the saline soil is becoming more and more important.As the soil salt by the pore water flow and temperature and other factors,the occurrence of physical and chemical changes,resulting in the engineering nature of the soil is very unstable,often bring a series of engineering geological problems,resulting in huge economic losses.Therefore,it is important to understand the engineering properties of saline soil,especially the effect of salt concentration and composition on engineering properties,and it is of great significance to the engineering practice of saline soil.In this paper,the effects of NaCl,KCI and CaCl2 concentration on the shrinkage and shrinkage properties of soil were studied.The dry climatic conditions were simulated under the conditions of controlling temperature and relative humidity,and the samples were evaporated and dehydrated.During the process,the moisture content of the sample was measured and the fissure generation and evolution process were recorded by the camera.The fissure image obtained by digital image processing technique was used to quantitatively analyze the surface fissure rate,the number of nodes,the number of blocks,Dimension and other fissure measurement index,and the evolution of the above-mentioned shrinkage and cracking characteristics of saline soil under dry and wet cycle conditions is discussed.In addition,the microstructure of the soil under the influence of salt was analyzed by scanning electron microscopy(SEM)and mercury intrusion test(MIP).Finally,the microstructure of the soil was analyzed from the salt-clay particles The interaction mechanism and the change of the double layer,the mechanism of evaporation and the mechanical mechanism are explained.It is very important to further understand the mechanism of shrinking and cracking of saline soil and to guide the engineering practice in saline-alkali area.The paper has achieved the following main achievements:(1)Evaporation:In general,evaporation can be divided into three stages,namely constant rate stage,deceleration rate stage and residual rate stage.Under the same salt condition,with the increase of salt concentration,the evaporation duration of the soil gradually increased,and the residual moisture content of the sample increased.In the low concentration condition,the effect of salt species on the evaporation rate was small,and the evaporation rate of NaCl,KCl and CaCl2 was similar.When the concentration increased,the effect of salt species on the evaporation rate and residual moisture content was obvious.The CaCl2-containing sample achieves a stable evaporation rate for the longest time,followed by KC1 and NaCl samples.At the end of evaporation,the final moisture content of NaCl samples was the lowest,followed by KCl samples,but the difference was not significant,and the highest moisture content was CaCl2-containing samples.(2)Fracture development:The application of particle image velocity measurement(PIV)to the process of soil shrinkage and shrinkage analysis shows that the shrinkage motion of the soil around the fracture is basically perpendicular to the development direction of the fracture.Tensioning.After the formation of block area,shrinkage deformation from the four weeks to the center of development,closer to the central area,the smaller the amount of shrinkage deformation,or even no defonnation,there is "shrink center" phenomenon.When the salt species is the same,increasing the salt concentration will result in a decrease in the surface fissure rate(Rsc),a decrease in the final Rsc,an average fissure width(Wav),a number of soil blocks per unit area(Nc)and a fractal dimension(Lav),indicating that the increase of salt concentration had a certain inhibitory effect on the dry and cracking of soil.At the same concentration,the effects of NaCl and KCI on soil shrinkage fissure were similar.In CaCl2 The surface fissure rate,the average width of fissures,the number of soil clumps and the fractal dimension of the soil containing CaCl2 are lower than those of NaCl and KCI,and the average fissure length is higher than that of NaCl and KCI Sample(3)Drying-wetting cycle:With the increase in the number of wet and dry cycles,the end of the dry,smooth surface,angular cracks gradually become twisted and rough jagged,soil fissure connectivity gradually increased,micro-cracks increased,The shape of the clumps becomes smaller and more irregular by the larger rules.For soils with the same kind of salt,the surface fissure is higher than that of the lower concentration soil after the same number of times.In the process of wetting,the effect of CaCl2 on the fracture morphology of the sample after wet and dry cycle has some similarities and differences compared with NaCl.When the CaCl2 concentration is low,with the soil without salt soil and soil similar to the soil,the soil will collapse immediately after collapse collapse,become very loose,and the water is quickly absorbed by the soil,but when the soil contains more When the concentration of CaCl2 in the high concentration of CaCl2,the soil slowly after the wet swelling,the fissure gradually reduced,the surface of the residual liquid level,indicating that CaCl2 sample water absorption has been suppressed,and after repeated wet and dry cycle after the high concentration of the sample pattern is still And maintain a high similarity to the samples after the first drying.(4)Microstructure:salt changes the original structure of the soil,from the original dense become more loose,and when the salt concentration is high,the emergence of the crystallization of the cluster structure;salt concentration on the sample pore The proportion of large pores in the sample increases obviously,and the apparent pore ratio also increases with the increase of salt concentration,which indicates that the increase of salt leads to the loosening of soil structure.(5)Mechanism explained:When the solute through the diffusion into the soil pore water,the appearance of negative charge of clay particles and aggregates will attract salt in the positive charge ions,so that adjacent clay particles and aggregates between the double The thickness of the layer decreases,resulting in the reduction of the pore volume.The high concentration of salt and high-valent cation will lead to the decrease of the thickness of the double layer,and the shrinkage of the soil will be weakened and the degree of fracture development will be reduced.Under certain temperature conditions,The relative humidity of the saturated NaCl,KCl and CaCl2 solutions is different,and the relative humidity in the solution and the air will cause a pressure gradient at the soil-air interface.Compared with the sample containing NaCl and KCl solution,the relative humidity of the sample mixed with CaCI2 solution is lower than that of the surrounding environment,so that the soil containing CaCl2 solution has high soil suction and less water evaporation.At the same salt concentration,a stronger crack pattern on the surface of the sample containing NaCl and KCl solution results in a stronger weakening effect of Na+and K+ on the swelling pressure;increased salt concentration in the pore water,Also increased,the ability to enhance the water molecules,under the same conditions,lower water evaporation rate.In addition,the increase in permeability will further strengthen the effective stress between the particles to improve the tensile strength of the soil,thus delaying the emergence of cracks,corresponding to the critical cracking water content is also reduced accordingly.When the salt concentration is high,the evaporation of water will lead to salt precipitation in the soil surface of the crystal,cement particles,even in the surface of the formation of crystalline hard shell.blocking fissure development.