Recommendations and code requirements regarding backfill of newly poured basements are one of the most ignored aspects of foundation construction. The International Residential Code, ACI 332 Standard, and the CFA Standard all state that foundation walls must be supported at the top and bottom before backfill is placed. Empirical tables presented in each are based on that fact. This means that either temporary bracing or a properly constructed and connected deck must be present at the top of the wall and either a keyway or dowels are in-place at the bottom of the wall. A foundation wall is designed as a simply supported beam with restraint at the top and bottom. If there is no top support, the wall becomes a cantilevered element requiring a different design, as well as reinforcement requirements for both the wall and footing.
The reality, however, is that most walls are backfilled without the stipulated support. The fact that the walls are much stronger than they need to be to resist lateral loads helps keep problems to a minimum but in many cases, backfilling without sufficient support is a problem waiting to happen.
There are several methods that foundation contractors can use to reduce the likelihood of a problem. First, keep the height of the backfill to no more than 4 feet except at the corners or offsets until the deck is in place. Four feet of unbalanced backfill usually will not exert sufficient pressure to damage the wall.
You generally can backfill to full height at corners and offsets. A wall extending at right angles to the backfilling is the best support you can have for a wall. In most cases, you can backfill short segments of the wall's full height (up to 12 feet in length). When you have closely spaced supports at right angles to the wall (such as corners or offsets), the wall actually can span horizontally as well as vertically. This recommendation should be used with caution unless an engineer has given specific design requirements for the method.
Closely related to offsets is the use of counterforts—thickened areas of walls or buttress walls—that, in effect, act the same way as offsets or corners. These can be on either the inside or outside of the wall and should be cast integrally with the wall. This concept is similar to the use of piers in masonry construction.
There are two other issues that can contribute to problems during and after the backfilling operation. The first of these is the material used to backfill. Most builders simply put the same soil back in the over dig because it is cheap and available. If the materials are not well drained, they could result in an under-designed wall. Poor soils can increase the pressure on the wall well above the design load requirements.
A second concern is the compaction of the backfill material. It is not acceptable to compact by driving heavy equipment next to the building—the force exerted will exceed even the largest of assumed soil pressures. Neither is it suitable to put a garden hose in the hole and let it run. If the soil is not well drained, this act in itself may cause a wall failure. Backfilling should be accomplished with light duty equipment, preferably a tamper, and should be performed in lifts. Finally, always leave the final grade with a minimum slope away from the foundation of 6 inches in the first 6 feet. More is better as the soil closer to the foundation will likely settle, reducing the slope.
Although the foundation is seldom seen, it is the key to what is typically the largest investment in most people's lives: their home. Treating it properly, particularly in the initial stages of construction, will pay dividends for the life of the house. For more information on residential foundations, visit the Concrete Foundations Association Web site at www.cfawalls.org.
— Jim Baty is technical director of the Concrete Foundations Association and the Concrete Homes Council, both in Mt. Vernon, Iowa. He can be reached at firstname.lastname@example.org.