Study On Microstructure And Mechanical Properties Of Granite Residual Soil Considering The Free Iron Oxide

Granite residual soil is the debris of granite remaining in place under the action of physical and chemical weathering.It has the characteristics of high void ratio and low compressibility.Its physical properties are poor,but it has high structural strength.One of the important reason is that there is free iron oxide between the soil particles.Granite is widely distributed in the southeastern region of our country.The granite residual soil formed by weathering is a common geotechnical material in the local area.The different climates in different regions make the content of free iron oxide in the granite residual soil vary greatly,which directly affects the microstructure and the mechanical properties of the granite residual soil.In order to explore how free iron oxide affects the microstructure and macro-mechanical properties of granite residual soil,a series of experimental studies were conducted on granite residual soil collected from Bozhi County,Guangdong Province.First,the DCB method was used to remove free iron oxide in the soil sample.The test results show that the color and touch of the soil sample have changed greatly after iron removal.Then,the samples before and after iron removal were subjected to X-ray fluorescence spectroscopy,chemical analysis and X-ray diffraction.The test results show that the content of kaolinite in the granite residual soil is greater than 50%,and it contains no feldspar,which is the primary minerals.Granite residual soil has a high degree of weathering,and free iron oxide mainly exists in the form of goethite.After iron removal,the average content of free iron oxide in the granite residual soil has dropped from 6.5%to2.5%.Secondly,SEM scanning test and CT scanning test were carried out.The test results show that the granite residual soil is mainly composed of flocculent structure,composed of randomly distributed aggregated minerals and massive minerals.The surface of the minerals is rough and uneven,the edges of clay minerals are not clear,and the clay masses wrapped by cement can be seen locally.Free iron oxide exists in the form of coatings and bridges in the granite residual soil.The coatings are the main one,followed by the bridges.Its specific function is that the clay minerals are wrapped,the pores of the soil are reduced,and a denser structure is formed.The image processing results show that the free iron oxide is unevenly distributed in the granite residual soil.After iron removal,the cross-sectional density was significantly reduced,the porosity increased from less than10%to 24.4%,and the content of particles with an equivalent particle size in the range of0.2-0.6 mm increased from less than 40%to 53%,which was much higher than the sample without iron removal.Consolidation and undrained triaxial compression tests were carried out on the granite residual soil samples before and after iron removal.The test results show that the presence of free iron oxide increases the friction angle and cohesive force of the granite residual soil.The deviatoric stress-strain relationship curves of the samples before and after iron removal are all strain hardening.The greater the confining pressure,the greater the peak deviator stress of the sample.And under the same conditions,the deviator stress of the sample without iron removal is smaller than that of the sample without iron removal.The excess pore water pressure of the samples before and after iron removal showed a trend of first increasing and then decreasing with the increase of the axial strain.Under the same conditions,the peak value of the excess pore water pressure of the samples without iron removal was larger.After reaching the peak,there is a clear tendency to dissipate.The presence of free iron oxide improves the strength of the soil.Finally,a granite residual soil disturbance model considering the free iron oxide content under triaxial undrained shear conditions was established,and the relationship between the free iron oxide content and the model parameters was studied.The parameters K,G and Z_dare positively correlated with the content of free iron oxide,and the parameter A_dis negatively correlated with the content of free iron oxide.