What is geotextile? What does it do? Is it necessary?

Sometimes called filter fabric, geotextile is made of plastic threads that allow water to pass around them, but not very small particles of soil beneath them. Geotextile separates and contains the base from the underlying soil subgrade. It allows the base to shed water, and prevents the soil around it from working its way into the base.

Without geotextile, the soil will work its way into the base and weaken it. This is a slow process that happens when the soil is saturated with water or during periods of thawing. Geotextile stops this process and extends the life of the base by many years. Geotextile is recommended for use over silt and clay soils. It is not essential in sandy soils.