Mixing Soil with Bentonite to Amend Its Microstructure and Permeability

【Background and objective】 Soil permeability is an important engineering parameter modulated by its microscopic structure. The southern section of Yongding River has been drying out of water for a long time with the riverbed deserted. To improve hydro-ecological properties of the riverbed, this paper studied the feasibility of amending the riverbed soil with bentonite. 【Method】 Sandy soil samples taken from the riverbed were amended with bentonite at ratios ranging from 3% to 15%. Following an incubation, each sample was scanned using the scanning electron microscope (SEM). Penetration test was conducted in a laboratory to measure the permeability, from which we analyzed the relationship between soil permeability and soil microstructure. 【Result】 Amending the soil with bentonite at ratio of 6% to 15% reduced the soil permeability from 10-5 cm/s to 10-7 cm/s. Microstructure analysis of the samples amended with bentonite at different ratios showed that with the increase in bentonite content, the microstructure changed gradually from granular structure to agglomerate structure, resulting in a decrease in permeability. The aerial percentage of macropores was positively correlated with permeability. Increasing bentonite content improved the orientation of the pore area distribution, with the percentage of pore area within 80o to 100o decreasing and the ratio of pore area around 20o to 40o and 150o increasing. These combined to reduce the soil permeability. Two- and three-dimensional analysis of the microscopic structure revealed that the pore ratio and the logarithm of the permeability were linearly correlated, with R2 being 0.98 for 2D and 0.96 for 3D, respectively. 【Conclusion】 Adding bentonite to the sandy soils taken from the riverbed of Yongding river showed that the addition improved the pore area, changing the average pore diameter as a result. These combined to reduce soil permeability significantly.