A key moment in basement construction occurs when the builders complete the process by filling in the excavated hole with soil or other material. Called backfilling, this tricky procedure requires a great deal of knowledge and skill to perform properly. If mistakes happen during backfilling, you might face expensive repairs later on.
Some soils make poor backfill. Heavy clay soils fare especially poorly as they exert too much pressure against the foundation, sometimes leading to structural damage. Sandy, granular soils are better. You can use crushed rock as backfill, although this costs more than soil. Using sand or gravel as backfill materials and combining them with crushed rock or soil are other options.
Backfill, regardless of the material used, puts considerable pressure on the foundation walls, especially if the process proceeds too quickly. Installing the basement concrete floor and the first floor subflooring material before backfilling will help support to the walls so that they don't crack or lean inward due to backfill pressure.
When completing a basement backfill, the contractor should ensure that the soil above the foundation slopes away from the house so it carries moisture in the ground away from the basement. Many local building codes require a minimum slope of 6 inches every 10 feet. For areas receiving large amounts of rainfall, an even steeper grade might be necessary. Also, when soil is used as backfill, it should never come in contact with the frame of the house. At least 8 inches of above-grade foundation wall should separate the wooden frame from the soil around the house.
Damage to foundation walls can easily result when contractors start the backfill process too soon. Concrete foundation walls that have not had sufficient time to cure properly have less resistance to the pressure the backfill exerts. It can take a significant amount of time before the concrete has enough solidity to withstand backfill pressure. Home improvement expert Tim Carter states concrete may need 28 days to reach 75 percent of its ultimate strength.
- Bougalis Construction Inc.: Basement Backfill Procedures
- University of Missouri Extension: Concrete Basement Construction
- Partnership for Advancing Technology in Housing: Foundation Drainage
- Ask the Builder: Foundation Drain Tile & Backfilling Tips
- Ask the Builder: Tips on Foundation Soil Grade Around Houses
- Concrete Network: Concrete Basement Construction Info
- Oak Ridge National Laboratory: Builder's Foundation Handbook
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