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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.

Spanning Over a Soft Spot

Some sites have occasional soft spots in otherwise good soil. You usually discover such spots when you're driving stakes for the footing forms -- you hit a stake and it just about disappears with one blow. Maybe there's a layer of soft clay that rises from an old lake bottom at an angle and just intersects your trench in one or two places. If a stake sinks in easily under hand pressure, there's cause for concern.

You may have to excavate down past the soft spot and place a deeper footing, then pour a taller wall. Or you may have to pier down through the soft material to get a bearing on good material. Another option is to excavate out the soft soil and replace it with compacted gravel or low-strength concrete, also called "lean fill" (see "A Quick Cure for Problem Soils," 1/00).

But in many cases, widening the footing is the simplest solution. If you've got a 16-inch footing, increasing that to 32 inches doubles your bearing area, making the footing suitable for soil with half the capacity.

If you increase the footing width, the code requires an increased thickness as well. That's because a footing that's too wide and not thick enough will experience a bending force that could crack the concrete. The projection of the footing on either side of the wall is supposed to be no greater than the depth of the footing.

So, for example, a 32-inch-wide footing under an 8-inch wall would need to be at least 12 inches thick. Instead, however, you could reinforce the footing with transverse steel (running in the crosswise direction, not along the footing). In most residential situations, #4 rod at 12 inches o.c. will be plenty for 8-inch-thick footings up to 4 feet wide. The steel should be placed about 3 inches up from the bottom of the footing.

Even though a lot of contractors do it, one thing that will not help you span a soft spot in the soil is to add more steel along the long dimension of the footing. Throwing more longitudinal steel into a footing in this situation is just a waste of time and money. If you're going to add lengthwise steel, put it where it will do some good: in the wall, not the footing. Just as a 2x12 on edge is way stronger than a 2x4 on the flat, steel at the top and bottom of an 8-foot or 9-foot wall does much more work than steel placed into a skinny little footing. A wall with two #4 bars at the top and two at the bottom can span over a small soft area with no problem.