The volume of soil to be removed when excavating for a basement is actually smaller than the volume of the soil after it has been removed. This is because the untouched soil, measured in bank yards, becomes less compacted when it is disturbed. The less compacted soil occupies more space and is then measured in swelled yards. When discussing the excavations with your local authorities and builders, it is important to specify that your calculations are in bank yards.
Things You'll Need
- Architectural plans
Establish, from the basement plans, the external depth, width and length of the finished basement. Record these as D, W and L. For example, if the depth is 10 feet, the width is 20 feet and the length is 30 feet, you would have D = 10, W = 20 and L = 30.
Consult with your local authorities and builder to establish how much working space is required around the basement area. Call this distance S. For example, if the builder requires 10 feet of clear space all around the base of the excavation then S = 10.
Consult with your local authorities and builder to establish the angle at which the sides of the excavation will slope. The angle is measured from a horizontal line at the base of the excavation. Call this value a. For example, if the sides slope at 45 degrees, then a = 45.
Substitute the values into the equation: Volume = ((W + 2S + D/tan a) x D x L) / 27. The result is the volume of undisturbed material to be removed, expressed in cubic yards. For example, if D = 10, W = 20, L = 30, S = 10 and a = 45, the volume to excavate equals 555.56 cubic yards because ((20 + 2x10 + 10/1) x 10 x 30) / 27 = 555.56