How to Estimate the Volume of an Underground Void
Calculating an underground void can be challenging as the void dimensions can vary widely depending on the cause of the void. Often the measure is more of an estimation or best guess than an exact number.
Side Access Voids
If you can visually see the void from the side (side access), your best option is to use a tape measure or measuring stick to collect the dimensions.
Blind Access Voids
A blind access void exists when you know there is a hollow cavity under concrete or decking but you can’t visually inspect it. In these cases we recommend drilling several holes across the void to measure depth. Length and width are approximated using the hammer sounding method (go to minute 1:21 in this video). Another way to estimate the size of a blind access void is to use a borescope/endoscope camera. We recommend choosing a model that has dual-lens technology providing multiple viewing angles.
Using the Calculator
For Void Filling:
When using spray foam to only fill a void, the accuracy of your measurements will determine the cubic feet of spray foam needed. Our highest volume kit yields 16.6 cubic feet of foam under ideal conditions. If your calculations are close to 16.2 cubic feet, plus or minus 10%, you’ll want to double check your measurements to avoid coming up short.
For Concrete Lifting: As you inject spray foam into the cavity, it will first fill the void. After it is hardened and you begin injecting foam again, it MAY compact the soil underneath and require additional foam. Consider the type of soil in your area. If the soil is loose and spongy, it will compress as the foam expands. Heavy and dense clay type soils will compress much less if at all. We have seen situations where soil compression required twice the amount of foam to ultimately lift the concrete slab.