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Footing Fundamentals

Footing Fundamentals

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    If you know your soil bearing capacity, following these practical guidelines will ensure strong footings.

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    As the load under a footing spreads out, pressure on the soil diminishes. Soil directly under the footing takes the greatest load, and therefore should be thoroughly compacted.

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    The author checks soil density in a footing trench using a penetrometer. Soil strength directly under the footing, where loads are concentrated, is crucial to foundation performance.

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    This incorrectly placed footing caused the foundation wall to be off-center. If the soil is very strong, this may not lead to problems. If the footing is on a weaker soil however, the author will recommend that it be fixed.

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    In strong soils, a mistake in footing layout can be corrected by placing gravel to support the wall (top). In weaker soils, the author recommends casting an augmented footing alongside the existing footing (above), connected by dowels epoxied into the side of the existing footing. Be sure to fill any notches in the footing, and cut off any existing steel dowels that will miss the wall.

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    If a form stake sinks in too easily, the soil may be too soft. For localized soft spots, the author recommends widening the footing. In wet, mucky areas, he recommends compacting large cobbles into the mud to provide bearing.

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    When a footing must be widened to boost bearing ability, it should also be reinforced or deepened. An unreinforced footing that is too wide may crack close to the wall, overloading the soil beneath. Without reinforcement, codes say the thickness of the footing should be at least as great as the distance it projects next to the wall. As an alternative, the author recommends transverse (crosswise) #4 bar at 12 inches o.c.

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    Steel in the wall has a greater effect than steel placed in the footing. In the wall, steel bars are almost 8 feet apart, while in a footing, the bars are only a few inches apart; the greater the spacing, the better the effect.

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    When water is pooled in the trench, the author recommends placing large cobbles in the form bottom and compacting them down into the mud. Muck and water may fill the spaces between stones, but contact between the stones will provide bearing. Be sure to use a stiff concrete mix when you cast the footings.

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    Stepped footings are used at changes in elevation in masonry foundations, but may not be necessary for poured concrete foundations.

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    A short reinforced concrete wall has been formed and cast to span the distance from its footing to the adjoining wall (the trench will be backfilled as usual).

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    Discontinuous footings work fine for concrete walls, which can be reinforced to take the loads. A typical situation where a garage stemwall abuts a main basement wall can be handled by reinforcing the short section of wall that spans the opening with two #4 bars at the top and bottom, extending 3 feet into each adjoining section of wall above the footing. This solution is limited to a 4-foot maximum span and a 5-foot maximum change in elevation. If the walls are at right angles, the rebar has to be bent accordingly.

Water in the Excavation

When you're working in an area with a perched water table during the wet season, you sometimes find ground water moving into your trench. If the flow is slow enough so you can pump the water out without it flowing right back in, then that's the best solution. You can place concrete in up to 1 inch of water -- concrete is 2 1/2 times heavier than water, and it will displace the water. You might want to thicken the footings in that case, because the bottom of the concrete may absorb some water and be a little weaker than normal.

But if the soil is loose and porous, and water and soil keep coming back into the trench as you pump the water out, use large aggregate to build up the trench. For this, large stone or cobbles -- 2 1/2-inch- or 3-inch-diameter rock -- are best.

When you form the footings, place enough large stone into the wet, mucky zone to get up above the water table. Compact the stone down into the mud, then pour your footing. The large aggregate allows the muck to fill into the pore space, but as long as all the pieces of stone are in contact with each other, the stone can still transfer the load.

If the stone is piled so high in the forms that your footing becomes too thin (less than 4 inches thick), place transverse rebar to reinforce it (be sure that the footings are thick enough to cover the steel by at least 3 inches).

Changes in Elevation

It's pretty common for a short wall to tie into a tall wall, especially in the North, where most houses have full basements but garages just have short frost walls. The code calls for continuous footings at all points. But that part of the code dates from the days when foundations were made mostly with concrete block, not poured concrete. Masonry foundation walls have no real spanning capability, so they have to be stepped down when elevations change.

Concrete walls, on the other hand, can be reinforced with steel to span openings. That means the footings can be discontinuous, jumping from the 4-foot to the 8-foot or 9-foot elevation. The shorter wall can span the distance.

The concrete has to be appropriately reinforced. A typical house situation, where a 4-foot garage frost wall has to span 4 feet or less and tie into the main foundation, calls for two #4 bars at the top of the wall and two #4 bars at the bottom. The steel has to extend 3 feet into the main wall and 3 feet into the shorter wall beyond the point where the footing starts.

For this detail, the footings are formed and cast as usual. When you form the walls, the bottom of the forms must be capped with a piece of wood where the forms pass over empty space. In termite country, that wood must be stripped when the forms come off.

Brent Anderson is a consulting engineer and concrete contractor who serves on the American Concrete Institute Committee 332, Residential Concrete.