Effect of entrapped air on hydraulic conductivity in quasi-saturated porous media

Key words: 
• quasi-saturated hydraulic conductivity /
• entrapment air /
• effective porosity /
• Kozeny-Carman equation /
• Faybishenko formula

Abstract: The groundwater flow with entrapped air was termed as quasi-saturated flow. The entrapped air in quasi-saturated flow of aquifer had a significant influence on hydraulic conductivity of porous media. In this paper,the relation between entrapped air saturation and hydraulic conductivity was studied by the column experiments with respect to four medium,silt,fine sand,medium sand and coarse sand,respectively. The results illustrated that the entrapped air saturation in a quasi-saturated aquifer was related to the particle size of the medium,and the entrapped air saturation in the fine-grained media was greater than that in the coarse-grained. Within a saturation range of 0-15% of entrapped air saturation,the hydraulic conductivity decreased by 32.82%-56.38% compared with that at entrapped saturation,and the hydraulic conductivity can be expressed as a negative linear correlation with entrapped air saturation. The results also showed that the variation of the quasi-saturated hydraulic conductivity could be generalized as the entrapped air occupied the original effective poresand resulted in a decrease in the original effective porosity and hence in hydraulic conductivity. According to this air entrapment theory,the Kozeny-Carman equation could accurately exhibit the variation of the hydraulic conductivity with entrapped air saturation in the quasi-saturated aquifer,and furthermore,the results illustrated the hydraulic conductivity formula based on the Hagen-Poiseuille equation was not suitable for quasi-saturated hydraulic conductivity. In Addition,the experimental results also showed it was necessary to remove entrapped air in laboratory measurement of hydraulic conductivity.