Experimental Study On Soil-water Characteristic Curve And Strength Test Of Granite Residual Soil Solidified By Bacillus Pasteur

Granite residual soil is a special type of soil with complex material composition and strong structural properties,often characterized by unloading and disintegration,and softening when encountering water.It is widely present in southern China and belongs to the category of poor roadbed filling.In order to enhance the engineering properties of granite residual soil and enable it to be used for filling the roadbed of high-grade roads,a microbial generated calcium carbonate precipitation(MICP)method was proposed to improve and solidify granite residual soil.This article is based on the study of the water holding and strength characteristics of granite residual soil solidified by the MICP method under unsaturated conditions,which is an important prerequisite for using the MICP method to solidify granite residual soil in practical engineering.Compared with the traditional granite residual soil solidification method,the MICP method used in this article is more economical and environmentally friendly,in line with the concept of green transportation.In this paper,the granitic residual soil in K117+200 of Guangfozhao Expressway in Guangdong province is studied,the experimental study and theoretical analysis of unsaturated granite residual soil before and after MICP solidification were carried out.The strengthening mechanism of Bacillus subtilis solidification of granite residual soil was explored through X-ray diffraction experiments and scanning electron microscopy experiments.Through soil water characteristic curve tests,the influence of cement concentration,compaction degree,and moisture content on the soil water characteristics of solidified granite residual soil was explored.By conducting triaxial tests,the influence of binder concentration,compaction degree,and moisture content on the strength characteristics of solidified granite residual soil was revealed,and the relationship between the shear strength of solidified granite residual soil and matrix suction was explored.Based on experimental research and theoretical analysis,the main research progress and innovative achievements are as follows.(1)The changes in mineral composition and microstructure of granite residual soil before and after solidification were analyzed through X-ray diffraction tests and scanning electron microscopy tests.The solidified granite residual soil has added calcite crystals,and there is obvious calcium carbonate precipitation on the surface of the soil particles.The calcium carbonate precipitation fills the pores between the soil particles,cements the soil particles,enhances the connection between the soil particles,and thus improves the strength of the soil.(2)The filter paper method was used to analyze the influence of cement concentration,compaction degree,and moisture content on the water holding characteristics of granite residual soil before and after solidification.After solidification by Bacillus subtilis,the water holding capacity of granite residual soil has been improved,and the best effect is achieved when the concentration of the binder is lmol/L.The water holding characteristics of granite residual soil before and after solidification increase with the increase of compaction degree.The water holding characteristics of granite residual soil before and after solidification are poor on the wet side of the optimal moisture content of the soil sample,and strong on the dry side of the optimal moisture content.Through comparative analysis,it was found that VG(2 parameters),VG(3 parameters),Gardner model,and Fredlund&Xing model can all better fit the soil water characteristic curve of granite residual soil before and after solidification,among which the Fredlund&Xing model has the best fitting effect.(3)Through mercury intrusion testing,the variation pattern of pore characteristics of granite residual soil before and after solidification was analyzed.Compared with uncured granite residual soil,the pores and distribution range of solidified granite residual soil become smaller,thereby improving the water holding characteristics of the soil.(4)Through triaxial tests,the influence of binder concentration,compaction degree,and moisture content on the strength characteristics of granite residual soil before and after solidification was analyzed.After solidification by Bacillus subtilis,the maximum deviatoric stress of granite residual soil has been increased,the cohesion has increased,and the internal friction angle has decreased.Among them,when the concentration of the binder is lmol/L,the above changes are the greatest.As the compaction degree increases,the maximum deviator stress,cohesion,and internal friction angle of solidified granite residual soil also increase.As the moisture content increases,the maximum deviator stress,cohesion,and internal friction angle of solidified granite residual soil decrease.The shear strength of solidified granite residual soil increases linearly with the matrix suction,which is consistent with the Fredlund bivariate theory.Based on this theory,a shear strength formula for solidified granite residual soil with a cementation concentration of 1mol/L has been established.The above research conclusions provide a theoretical basis for further research on the solidification of unsaturated granite residual soil using the MICP method and its application in engineering practice.They also provide a certain theoretical basis for improving the engineering technology of MICP method for solidification of granite residual soil.