Void Ratio of Soil

Void Ratio

• The void ratio of a soil is the volume of soil not occupied by solid particles. The greater the void ratio the looser the soil is. Increasing the void ratio of the soil is done by plowing. Organisms such as worms and ants create a significant amount of the void space in a soil. The higher the void ratio of a soil the more easily water can absorb into it. Soil with a high void ratio is easy for plants to grow in.

Soil Structure

• The fine mineral material found in the soil is a mixture of three different-sized materials. From largest to smallest, they are sand, silt and clay. Individual sand particles have a very weak attraction to each other. A soil with lots of sand has a very poor soil structure. When a soil has a lot of clay and silt, the three different sizes of particles adhere much more strongly. A sandy soil has lots of large pore spaces between sand particles, but the total volume of pore spaces will be less than that of a soil with lots of clay. Clay particles do not allow for many large pore spaces, but they do harbor many small spaces.

Compaction

• When the void ratio of the soil is reduced, it has been compacted. Human interaction with the soil is the usual cause of compaction. Most soil compaction comes from agricultural equipment. Foot traffic can also compact the soil by crushing the soil pores. Wet soil is especially susceptible to soil compaction.

Calculating the Void Ratio

• Finding the bulk density of the soil will allow you to calculate the void ratio. Dry a known volume of soil in an oven and weigh it. Divide the dry weight of the soil by the volume to get the bulk density. Divide the bulk density by a standard particle density for mineral materials of 2.65. Subtract the answer to this calculation from 1 and multiply by 100. This will be the porosity percentage of the soil, or void ratio.