By foundation one can mean the soil or rock under the house, the base upon which the house rests, or one can mean those parts of the house that are below grade, that's , the substructure of the house. It is the second meaning that we will use in this article. This article will discuss the design of substructures (that is, the design of footings and foundation walls) insofar as their design is dependent on soil conditions. For the actual construction of footings and foundation walls see BRICK-MASONRY CONSTRUCTION; CONCRETE; CONCRETE-BLOCK CONSTRUCTION.
The structural integrity of any house depends in large part on the adequacy of the soil on which it's built; yet there is no part of a house that builders and homeowners take so much for granted. Unless, a house is resting on bedrock or on a very stable, permeable soil, the possibility always exists (however remote for most dwellings) that one day the soil will become less capable of supporting the house than it had been in the past.
If a house has a full basement, the substructure consists of the foundation walls, which support those parts of the house that are above grade (i.e., its superstructure), and the footings, upon which the foundation walls rest. The footings carry the entire weight of the house and transmit this weight to the soil below. The footings are usually thought of as being merely the base of the foundation walls, and in a sense they are, of course, but they have an entirely different structural function than foundation walls and they are designed on different principles.
If a house lacks a full basement, it may still have foundation walls and footings, but these will be built much shallower. Instead of foundation walls, a basementless house is very often supported by piers, which are short columns that rest, in turn, on individual footings. Or a basementless house may rest on pilings, which are long columns made either from tree trunks, precast or poured-in-place concrete, or steel beams. Pilings, whatever material is used, are driven deep into a stable stratum of soil.
A basementless house may also be supported by grade beams, which are beams made of poured-in-place or pre-cast reinforced concrete that appear to rest on the soil but are actually supported by piers or posts. Grade beams support the weight of the superstructure just as foundation walls do but they transmit this weight to their supporting piers or posts instead of to footings.
Whatever the method by which a house is supported on its foundations, it's the soil beneath that Counts. A house rarely fails structurally because of a defect or weakness in its substructure. Most apparent defects in a substructure can be traced back to an inadequate supporting foundation. That is, for one reason or another, the soil is, or has become, incapable of supporting the loads placed upon it, a fact that's , or was, unknown to the architect, engineer, or builder at the time the house was designed and built.
For the purposes of this article, we are concerned mainly with the particle size, or grain size, of soils, since it's the size of the particles that to a large extent determines a soil’s load-bearing characteristics. To simplify the discussion, since there exists an enormous variety of soils, ranging from very coarse gravel types to extremely fine clay types, and every mixture in- between (refer to Table 1), we will concentrate on two very opposite kinds of soil that have very different characteristics—that is, soils that are either very sandy or very clayey.
It should be intuitively obvious that the particles of matter making up a sandy soil don't make very close contact with each other. There are comparatively large voids, or spaces, between the particles that give the soil a very porous, or permeable, quality. These voids may contain air or they may be more or less filled with water. If the voids are filled with water, this water tends to drain away fairly quickly, assuming there is some place this water can drain to.
What happens when we place a heavy concentrated load upon a sandy soil? The load will compact the particles of sand together more closely, but not much more closely, and the load will settle into the soil a slight amount. Because of the porosity of the soil, this settlement takes place as quickly as the load is applied and , once this settlement has occurred, it's unlikely that any further settlement will take place.
If any water has collected in the voids, this water will be squeezed out of the soil by the load until the particles are packed together as closely as if the soil were completely dry. But if there should happen to be a considerable hydrostatic pressure behind the water, it may happen that the load will settle into the soil just to that point where it and the hydrostatic pressure balance each other, If this should happen, the load will, to some extent, be floating in the soil. (Many lightweight summer cottages located close to the ocean will rise and fall very slightly with the changes of the tides.)
As for clay soils, it's much less obvious that voids also exist between the extremely small particles of matter that make up the soil, especially since in some clays the particles are too small to be seen even with a low-powered microscope. Nevertheless, voids do exist between the particles. In fact, these voids occupy a much greater proportion of the total volume of the soil than they occupy in an equivalent volume of sandy soil. Because of this, a clay soil can absorb a much greater volume of water than a sandy soil, but because of the small size of the voids, the soil is very impermeable. That is, the flow of water into and out of the soil takes place very, very slowly. Clay soils tend to hold tenaciously onto their water content.
If we were to place the same heavy load on a clay soil that we placed on the sandy soil, the rate of settlement would be much, much slower. It would also continue over a few months certainly, and perhaps for as long as a few years. As long as the load was applied upon the soil, it would continue to force water out of the voids, settling deeper and deeper into the soil with the passing of time until either all the water had been removed or some sort of balance had been established between the load and any hydrostatic pressure acting on the water. In the long run, also, the total amount of settlement would probably be greater than in a sandy soil.
But once a balance had been established between the load exerted on a clay soil and this hydrostatic pressure, the soil would thereafter be as stable as a sandy soil. If the hydrostatic pressure increased for any reason, water would again be forced into the soil. Indeed, clay soils are noted for their ability to absorb large quantities of water, even under load, and to expand in volume as their content of water increases. During a very rainy season, for example, if a clay soil should become soaked with water to a depth of 8 ft or more, the surface of the clay may expand upward by as much as 1 in. The deeper into the soil one goes, the less the expansion will be, of course. Even though the 1-in, expansion may not be obvious to the eye or appear to be of great consequence, it would certainly be sufficient to have potentially serious consequences on a house having a shallow substructure, if the expansion were uneven. And as the soil dried out, it would slowly shrink back down to its original volume, again with potentially serious consequences for the substructure of the house.
The Bearing Capacity of Soils
Table 2 shows the number of tons per sq ft that different types of soil can support. (Looking through the table, one might wonder why anyone would worry about the ability of a foundation to support a load at all; we will take up this point later.)
As Table 2 shows, different soils have different bearing capacities. That is, they vary in their ability to resist the pressures exerted on them by an exterior load. If a soil has no bearing capacity at all, any load applied to the soil would simply sink into it until it reached a stratum of soil that could support it. The outstanding example of a soil with an exceptionally low bearing capacity is quicksand.
Table 2. Presumptive Surface Bearing Values of Foundation Materials
The actual bearing capacity of any particular soil can be determined only by suitable tests at the site itself and by laboratory examination of soil samples taken from the site, procedures that are both expensive and unnecessary for the great majority of houses.
As far as most communities are concerned, there is a back ground of experience that local builders can fall back on insofar as the kinds of soil in the neighborhood and their bearing capacities are concerned. The local buildings department will probably have a file of records regarding subsoil investigations for particular building lots, information that can be extrapolated for nearby building sites. In addition, most local buildings departments also publish as part of the local building code conservative rule-of-thumb tables of permissible footing sizes and foundation-wall thicknesses that are based on local experience and that will almost positively guarantee that a house built in that community will be safely supported on its foundation. Local builders must, of course, follow the recommendations of these tables if they wish to get their building plans approved.
Why, then, undertake any subsoil investigation at all? For several reasons. (1) Because the possibility always exists, given the great variability of most soil strata, that any given building site may have an underlying Stratum of soft clay or silt, or the land may at one time have been filled in; if so, the substructure of the house will have to be carried down to more solid strata since neither soft clay, silt, nor landfill has much bearing capacity; (2) because the possibility also exists that groundwater may run through the site, which will not only greatly increase the cost of excavating the soil and building the sub structure but may also make the site unusable; and (3) because there may be large boulders or an outcrop of rock just below the surface that will make the digging of the basement or foundation walls prohibitively expensive. If no problem shows up, the builder can go ahead with a deep sense of relief that construction costs will not exceed original estimates.
Subsoil investigation usually means drilling a hole into the ground to a depth of about 1 1/2 times the depth of the substructure in order to bring up samples of soil for examination. The hole is drilled this deep to check whether there is groundwater present at this depth, since if there is any water it might travel upward through the soil and flood the excavation.
The drilling is done by a firm that specializes in this work. Usually an on-the-spot examination of the soil samples will be sufficient to determine whether or not the soil has the necessary bearing capacity. If the soil samples look suspiciously weak or are ambiguous, a test pit may be dug into the soil as deep as the bottom of the substructure so that the soil can be examined in situ.
The Settlement of Houses
All houses settle, except when they are built on bedrock. The amount of settlement is, for the great majority of houses, very slight, amounting at most to perhaps 1/4 to 1/2 in. If the settlement is even throughout, no harm is done and the homeowner will probably not even be aware that any settlement at all has occurred. Wood-frame houses are, after all, very lightweight structures, and very few will exert a pressure exceeding 2000 lb per sq ft on their foundation. A two-story house with masonry walls may exert a pressure in excess of 3000 lb per sq ft, but even this load isn't considered especially heavy for a foundation, as Table 2 shows.
If all this is true, the reader may be wondering by now what all the fuss is about, since most houses will settle only a fraction of an inch at most, even in what seem to be the most unsatisfactory of soils.
The problem isn't one of settlement, since all houses settle, but uneven settlement, which may place very great stresses on the structural framework of any house, especially if the settlement occurs across the corner walls of a house. The amount of differential settlement we are talking about is quite small -- 1/2 to 3/4 in. A differential settlement as little as this may result in plaster cracks and jammed doors and windows. Any differential settlement greater than in. may open up large cracks in the foundation walls of the house.
Furthermore, the problem of differential settlement is usually of concern only in certain types of soil—clays, silts, and landfill, although any soil completely soaked with water is potentially unstable if for any reason its water content should drain away one day (but why build on a wet soil in the first place?).
The fact is, large numbers of houses are built on clays, silts, or landfill today because more suitable soils are either economically more useful for agriculture or for large commercial structures; because houses have already been built upon these more suitable soils in most communities; or because clay is the predominant soil in an area. In any expanding community, the last parcels of land that are considered for home construction (by commercial builders) are those in which the soil has a low bearing capacity, since the cost of excavating the soil and building a strong enough substructure to support the house adequately are greater in a poor soil than in a soil having a high bearing capacity. In short, in many parts of the country today, a builder has no choice except to build on a poor or inadequate soil.
Most foundation walls and piers rest on footings (refer to Fig. 1), the bases of which are in direct contact with the soil. The footings, therefore, transmit all the building loads to the foundation. The type of footing most often used in dwellings is the spread footing, which simply means that the footing is wider than the foundation wall or pier it's supporting.
The principle behind the use of spread footings is quite simple. Assume a building has a total weight that's equal to 4000 lb per linear ft. of foundation wall. If the foundation walls are in fact 12 in. wide, then each linear foot of wall will bear down upon the soil with a force of 4000 lb per sq ft. But when spread footings 24 in. wide are placed under the foundation walls, then each linear foot of foundation wall will bear down on the foundation with a force of only 2000 lb per sq ft. The weight of any house can, therefore, within limits, always be accommodated to the bearing capacity of the soil.
This principle can't be extended indefinitely (with spread footings, anyway) because the footing itself will crack if it's made too wide. The weight of the foundation walls or piers will shear or punch through the footing, unless the footing is heavily reinforced, which will increase its cost unduly.
To give the footings adequate rigidity, they are always made of concrete, even when the foundation walls or piers they support are made of masonry. The footings are usually made as deep as the foundation walls are thick, while the total width of the footings is twice the thickness of the foundation walls (refer to Fig. 1). Thus, for example, if a foundation wall is 8 in. wide, the footing will usually be 8 in. deep and 16 in. wide. If the footings are made much wider than twice the thickness of the foundation walls, they may crack because the load exerted upon them by the foundation wall will be concentrated too much at the center of the footing.
A footing should be reinforced with steel rods placed length wise within it wherever a tunnel is dug under it to lay a drain pipe or an electrical conduit. In addition, this excavation shouldn't be refilled with soil (which will consolidate too much and thus fail to support the footing) but with gravel tamped firmly into place.
For most houses, the footings are usually the same width all around the perimeter of the house. This may be satisfactory for box-like houses, under which the bearing pressures are the same everywhere, but if a house is built so that the weight upon the foundation is uneven, the house may settle into the soil unevenly. Assume, for example, that one half of a masonry house is two stories high and the other half is only one story high. If the footings are the same width under the entire foundation wall, the result will be that one half of the footing will be supporting a weight that's twice as great as that being supported by the other half of the footing. The result might very well be a differential settling of the house.
What is in fact done under these circumstances is to calculate the size of the footing according to the heaviest anticipated loads and to construct the entire footing to this load, even under the one-story section of the house. This is cheaper than attempting to calculate different sized footings and constructing different sized forms for the concrete.
The footing must be dug deeply enough that it's unlikely to be affected by any soil heaving caused by freezing soil or swelling clay. Whatever the depth at which the soil freezes in any particular locale, which may be as much as 8 ft below the surface in some northern sections of the United States, the footing must be located 11 ft below this level. Even in southern states where frost is extremely unlikely, it may still be necessary to place the footing 5 ft below the surface to avoid any soil heave caused by swelling clay. In short, footings must always rest upon solid, undisturbed soil, even when—or especially when—the soil happens to be a clay.
Like footings, foundation walls are usually sized according to some rule-of-thumb guide based on local experience. These wall thicknesses are very conservative and can be considered more than strong enough to resist any compressive loads that may ever be placed upon them.
If a house has a full basement, the foundation walls enclosing the basement must also be strong enough to resist the lateral loads imposed on them by the weight of the soil lying against the house. Included among the lateral loads are earth quake loads, as well as such above-ground loads as wind pressures, insofar as these wind pressures may be transmitted to the foundation walls.
There is very little point in constructing footings and foundation walls under a basementless house. Piers spaced at appropriate intervals along the perimeter of the house and under its center (see Fig. 2) are much cheaper to construct and equally capable of supporting the superstructure via the beams that run from pier to pier.
Building codes usually specify the minimum allowable size of a pier since, of course, the thicker the pier the more stable it's and the more capable it's of supporting a heavy load. Whatever the size of a pier, its height is usually required to be less than 10 times its minimum thickness. In the case of concrete-block piers, the piers can't be more than four times their minimum thickness.