On measuring methods and influencing factors of air permeability of soils: An overview and a preliminary database

Air permeability of soils reflects the ability to allow the passage of air, which is significantly important to explore related issues in geotechnical, environmental, agricultural, and petroleum engineering. The current under-standing of soil air permeability is based on a limited number of influencing factors with the strict control of soil properties. It would be beneficial for future investigations if a clear knowledge base about the air permeability of soils has formed. This paper presents an overview of the measuring methods and influencing factors of air permeability of soils, as well as a database of soil air permeability based on a large collection of literature. The test methods for the air permeability of soils can be generally divided into steady and transient state methods, and corresponding equations are necessary to determine air permeability. Both characteristics and limitations of the two methods and various test devices are discussed. The relationships between air permeability and soil parameters including density, porosity, and water content of soils, are summarized and discussed, and some empirical formulas for quantifying their relations are compared. The research challenges in the study of soil air permeability are discussed and perspectives on the challenging topic are offered. It has been found that the air permeability of soil shows a wide test range, i.e., from 1E-7 to 1E-19 m2, due to the influence of various soil properties. The specimens used in the laboratory are cylindrical and the sample size is mainly determined by the test apparatus, with a height between 1 and 25 cm and a diameter in a range of 3.91 to 30 cm. The results also indicate that test devices that are capable of considering air slippage effects and inertial effects, should be designed and developed. The influence of the pore structure indexes of soils on air permeability and their quantitative relationships should be further investigated.