The depth of a sonotube depends on the purpose for which it is being used. Generally, it is recommended that a sonotube be installed at least two feet deeper than the deepest frost line in your area to ensure that the foundation remains protected from ground frost heave and shifting soils. For projects requiring an even more stable foundation, such as retaining walls or structures exposed to wind forces, some engineers may recommend going 4 feet deep or even more. However, other factors should also be taken into consideration when determining how deep a sonotube needs to go including local building codes and regulations governing excavation depths, the soil type surrounding the project site and potential water flow through or around the installation area. Ultimately it’s best to consult with an engineer who specializes in foundations before making any decisions about how deep you need to dig your sonotubes.