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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.

Under every house is a foundation, and under most foundations are footings. Most of the time we take footings for granted, and usually we can: For typical soils, a common 16- or 20-inch-wide footing can more than handle the relatively light weight of an ordinary house.

On the other hand, if you build on soft clay soil or if there's a soft zone under part of your foundation, there can be trouble. A footing that performs well in good soil may not do so well in weak bearing conditions. We don't often see outright failure, but it's not uncommon to see excessive settlement when soil bearing capacity is low.

If the whole house settles slowly and evenly, some additional settlement is no big deal; but if settlement is uneven (differential settlement), there could be damage. A frame house with wood siding and drywall interiors can probably handle up to 1/2 inch of differential foundation movement, but even 1/4 inch of uneven settling is enough to cause cracks in masonry, tile, or plaster.

It's the unusual situations that cause the most trouble. When the footing is laid out off-center so the wall misses its bearing, when you encounter a soft zone on site, or when the footing is undersized, the builder faces a judgment call. If you think there's a problem ahead, you know you should stop and call an engineer. But if the risk is low, you'd like to keep the job moving.

In these tough cases, it's helpful to understand the bearing strength of soil and the reasons behind footing design rules. In very strong soils, minor mistakes probably aren't a big deal. In weak or marginal soils, however, it's best to be very cautious -- some of the solutions contractors think up may not really work.

I'm a consulting engineer as well as a contractor, and I get called in to a lot of problem situations. I find that people understand the problems better if they have some background knowledge. For the benefit of builders in the field and at the risk of oversimplifying, I'm going to use non-technical language in this article to briefly explain a little about how footings work and to present some ideas for dealing with special situations. As you look at the solutions I recommend, however, keep in mind that high-bearing-capacity soil is assumed. Any time you're in doubt about the soil under your foundation, you'd be wise to get professional help.

Why Soils Matter

In addition to providing a level platform for forms or masonry, footings spread out the weight of the house so the soil can carry the load. The load spreads out within the footing itself at about a 45-degree angle, and then spreads out in the soil at a steeper angle, more like 60 degrees from the horizontal.

Because the load spreads out, the pressure on the soil is greatest right beneath the footing. By the time we get down below the footing a distance equal to the footing's width, the unit soil pressure has dropped by about half. Go down the same distance again, and the pressure has dropped by two-thirds. So it's the soil right under the footing that is the most critical -- and also, typically, the most abused.

When we excavate for the footings, the teeth on the bucket stir up the soil and mix air into it, decreasing its density. Also, soil from the embankment may fall into the trench. That loose soil has much less bearing capacity than the original soil. That's why it is so important to compact the trench bottom (use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay). If you don't compact that soil, you could get 1/2 inch of settlement in just the first 6 inches of soil.

If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it won't settle to any significant degree.

Soil types and bearing. The type and density of the native soil is also important. The International Building Code, like the CABO code before it, lists presumed bearing strengths for different types of soils (see "Soil Bearing Capacities," below). Very fine soils (clays and silts) typically have lower capacities than coarse granular soils (sands and gravels).

Soil Bearing Capacities

Class of Materials
Load-Bearing Pressure (pounds per square foot)
Crystalline bedrock
12,000
Sedimentary rock
6,000
Sandy gravel or gravel
5,000
Sand, silty sand, clayey sand, silty gravel, and clayey gravel
3,000
Clay, sandy clay, silty clay, and clayey silt
2,000
Source: Table 401.4.1; CABO One- and Two- Family Dwelling Code; 1995.

However, some clays or silts have higher bearing capacity than the values in the code tables. If you have a soil test done, you could discover that you have a denser clay with a much higher bearing strength. Mechanically compacting the soil can also raise its bearing capacity.

You can get a pretty good idea of the soil bearing capacity in the trench bottom using a hand penetrometer. This pocket-sized device is a spring-loaded probe that estimates you the pressure the soil can resist and is calibrated to give readings in tons per square foot. In my opinion, every contractor and building inspector should have one of these -- it can help you avoid a lot of trouble.

Footing Dimensions

So, how does soil bearing capacity relate to the size of footings? The footing transmits the load into the soil. The lower the bearing capacity of the soil, the wider the footing needs to be. If the soil is very strong, the footing isn't even strictly necessary -- just the soil under the wall would be enough to hold the building up.

Minimum Width of Concrete or Masonry Footings (inches)

 

Load-Bearing Value of Soil (psf)

 

1,500
2,000
2,500
3,000
3,500
4,000

Conventional Wood Frame Construction

1-story

16
12
10
8
7
6

2-story

19
15
12
10
8
7

3-story

22
17
14
11
10
9

4-Inch Brick Veneer Over Wood Frame or 8-Inch Hollow Concrete Masonry

1-story

19
15
12
10
8
7

2-story

25
19
15
13
11
10

3-story

31
23
19
16
13
12

8-Inch Solid or Fully Grouted Masonry

1-story

22
17
13
11
10
9

2-story

31
23
19
16
13
12

3-story

40
30
24
20
17
15

Source: Table 403.1; CABO One- and Two- Family Dwelling Code; 1995.


You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. As you can see, heavy houses on weak soil need footings 2 feet wide or more. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. Under an 8-inch-thick wall, that's the same as saying you have no footing.