Air Content

on . Posted in Geotechnical Engineering

partially saturated soil 1

Air content, abbreviated as A, refers to the volume or proportion of air voids within a given volume of soil.  It represents the amount of space or voids that are filled with air rather than water or solids.  Air content is an important parameter in soil engineering as it directly affects the soil's properties and behavior.  The presence of air voids in soil affects its density, compressibility, permeability, and shear strength.  It can also influence the soil's ability to retain and transmit water.  Air content is typically expressed as a percentage of the total volume of soil being considered.  It is determined by conducting laboratory tests on soil samples using techniques such as the gravimetric method or the volumetric method.

In the gravimetric method, a soil sample is collected and dried to remove all moisture content.  The weight of the dry soil sample is then measured.  Subsequently, the soil sample is heated or subjected to a vacuum to remove all the air, and its weight is measured again.  The difference in weight before and after air removal represents the weight of the air in the soil sample.  Dividing this weight by the weight of the dry soil sample gives the air content as a percentage.

In the volumetric method, a soil sample is collected and placed in a container of known volume.  The container is then saturated with water, and any excess water is removed.  The container with the saturated soil is weighed to determine its wet weight.  Next, the soil is dried to remove all moisture, and the container is weighed again to obtain the dry weight.  The difference in weight between the wet and dry states represents the weight of the water.  Finally, the air content is calculated by subtracting the water content from the total volume of the container.

By knowing the air content of a soil sample, engineers and geotechnical professionals can make informed decisions regarding the soil's suitability for various engineering applications, such as construction, foundation design, and slope stability analysis.


air content formula

\( A =  V_a \;/\; V_v  \)     (Air Content)

\( V_a =  A \; V_v  \)

\( V_v =  V_a \;/\; A  \)

Symbol English Metric
\( A \) = air content \(dimensionless\)
\( V_a \) = volume of air \(ft^3\) \(m^3\)
\( V_v \) = volume of void \(ft^3\) \(m^3\)


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Tags: Volume Air Soil