Backfilling concrete is the process of pushing crushed rock and topsoil against a concrete foundation wall or slab to fill in the space left around the concrete that allowed for proper pouring of the material. While the backfilling process is a simple one, it’s heavily dependent on timing. If you backfill too soon, you risk cracking concrete that hasn’t quite cured yet by the weight of the soil. Done after the correct waiting period, however, backfilling not only fills in the space surrounding the concrete, but also provides additional support for the poured material. This enables the concrete to support more weight, without shifting around in the soil that surrounds the material.
Wait for the concrete slab on the lowest level of foundation walls to be poured and the first floor of the foundation to be built at least before you begin the backfilling process. Give the concrete adequate time to cure to design strength. At 70 degrees Fahrenheit, this can take 28 days, but the process can be longer in colder weather.
Complete any insulation placement on the exterior of the concrete, or the installation of pipes that later will be covered by the soil.
Remove any debris left from the building process surrounding the concrete foundation wall or slab. You don’t want anything to interfere with the placement of the soil surrounding the concrete.
Fill the trench area around the concrete with crushed rock to a level that’s about 124 inches below the surface of the surrounding landscape. Use rock with a diameter of about 1 inch, and layer it into the trench in 12-inch increments to avoid stressing the concrete with too much added weight at once, and to allow the rocks to settle into place. This also lessens the air contained between the rocks, adding insulative qualities to the backfill material, and serves as a drainage layer beneath the soil. Use a backhoe or shovel to fill the trench.
Cover the rocks with topsoil, filling the final 24 inches of trench height. Create a slight slope of 1 inch per 8 inches of soil away from the structure to aid in drainage.