A Laboratory Study Of The Effect Of Soluble Salt On Loess Collapsibility In Heifangtai Area

With many people migrating to the Heifangtai area in 1960 s,water pumping irrigation projects were built to solve the people's production and living water,changing the Heifangtai geological conditions and resulting in the loess collapsibility frequently in this area.This constitutes a threat to the local people's life and property safety,and restricts the local economic development.To study the effect of soluble salt on the collapsibility of Loess,undisturbed loess samples were taken at depths of 5m,20 m,and 30 m from the Xinyuan village of Yongjing county,laboratory leaching system was used to simulate the effect of irrigation on the soluble salt to obtain the undisturbed specimens with a variety of specified salt contents.Wetting collapse tests were then carried out on these disturbed loess specimens with different salt content to investigate the effect of soluble salt content on collapsibility of loess.The scanning electron microscopy was finally used to study the microstructure of the specimens after collapse and prior to collapse to further explore the mechanism of collapsibility of loess.The main contents and conclusions are as follows:First,leaching tests were adopted to simulate the effects of agricultural irrigation on the content of soluble salt in loess,and it was found that the rate the soluble salt leaching was fast and then slow;and the total amount of salt leaching was 1.3547 g for the specimen taken at a depth of 5m,and 2.9422 g for the specimen at 20 m,and 3.8547 g for the specimen at 30 m,showing a trend of increasing soluble salt content in soil with increasing depth.Second,loess collapse tests were performed to study thee effect of the soluble salt content on the loess collapsibility,and it was found that at the same depth,the compression of loess increases and the collapsibility decreased with a decrease in salt content;and with a decrease in salt content,the variation of collapsibility becomes less sensitive;self-weight collapsibility coefficient increases first and then decreases with an increase in depth.Third,scanning electron microscope was used to detect the microstructural change in the specimens that was leaching for a variety of duration and that collapsed due to wetting and did not collapse,and it was found thatwith increasing leaching intensity,skeleton grain shape became rounded,small particles increased and adhered to large particles around;some small particles aggregated to play a role of skeleton support with a small amount of salt soluble cementation between grains.for the collapsed specimens,the number of macro pores reduced or even disappeared with an increase in the number of small pores and micro pores filled with a lot of small particles,with more closely connected the particles.For the specimens with low salinity,the particles closely contacted,particle deformation is severe,the overall structure varied from the mosaic structure to the flocculation structure,even clot structure.Four,the mechanism of loess collapsibility was explained by the change of microstructure of the collapsed and not-collapsed loess specimens.dissolution of water reduced the volume of soluble salt cemented between the particles and weakened connection between the particles,resulting in unable to resist external pressure and internal particle displacement.Some particles were thus moved into the nearby pores,making pores smaller,and increasing the number of pores.Finally,the effect of soluble salt content on the collapsibility of loess is analyzed,and it is found that the factors governing collapsibility and compression are not the same.Wetting collapse is of brittle failure,compared with compression.With a decrease in soluble salt content,the compressibility increases and the collapsibility decreases,indicating that soluble salt content is an important factor controlling the collapsibility of loess.