It is convenient to think of a building as consisting of three major parts: the superstructure, which is the above-ground portion of the building; the substructure, which is the habitable below-ground portion; and the foundations, which are the components of the building that transfer its loads into the soil (Figure 1). There are two basic types of foundations: shallow and deep.
Shallow foundations are those that transfer the load to the earth at the base of the column or wall of the substructure. Deep foundations, either piles or caissons, penetrate through upper layers of incompetent soil in order to transfer the load to competent bearing soil or rock deeper within the earth.
Shallow foundations are generally less expensive than deep ones and can be used where suitable soil is found at the level of the bottom of the substructure, whether this be several feet or several stories below the ground surface. The primary factors that affect the choice of a foundation type for a building are:
- Subsurface soil and groundwater conditions
- Structural requirements, including foundation loads, building configurations, and depth
Secondary factors that may be important include:
- Construction methods, including access and working space
- Environmental factors, including noise, traffic, and disposal of earth and water
- Building codes and regulations
- Proximity of adjacent property and potential impacts on that property
- Time available for construction
- Construction risks
The foundation engineer is responsible for assessing these factors and, working together with other members of the design and construction team, selecting the most suitable foundation system.
Most shallow foundations are simple concrete footings. A column footing is a square block of concrete, with or without steel reinforcing, that accepts the concentrated load placed on it from above by a building column and spreads this load across an area of soil large enough that the allowable bearing stress of the soil is not exceeded.
A wall footing or strip footing is a continuous strip of concrete that serves the same function for a load bearing wall (Figure 2). To minimize settlement, footings are usually placed on undisturbed soil.
Under some circumstances, footings may be constructed over engineered fill, which is earth that has been deposited under the supervision of a soils engineer.
The engineer, working from the results of laboratory compaction tests on samples taken from the soil used for filling, makes sure that the soil is deposited in thin layers at a controlled moisture content and compacted in accordance with detailed procedures that ensure a known loadbearing capacity and long-term stability.
Footings appear in many forms in different foundation systems. In climates with little or no ground frost, a concrete slab on grade with thickened edges is the least expensive foundation and floor system that one can use and is applicable to one- and two-story buildings of any type of construction.
Or, in colder regions the edges of a slab on grade may be supported with deeper wall footings that bear on soil below the frost line. For floors raised above the ground, either over a crawlspace or a basement, support is provided by concrete or masonry foundation walls supported on concrete strip footings (Figure 3).
When building on slopes, it is often necessary to step the footings to maintain the required depth of footing at all points around the building (Figure 4). If soil conditions or earthquake precautions require it, column footings on steep slopes may be linked together with reinforced concrete tie beams to avoid possible differential slippage between footings.
Footings cannot legally extend beyond a property line, even for a building built tightly against it. If the outer toe of the footing were simply cut off at the property line, the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail.
Combined footings and cantilever footings solve this problem by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized (Figure 5).
In situations where the allowable bearing capacity of the soil is low in relation to the weight of the building, column footings may become large enough that it is more economical to merge them into a single mat or raft foundation that supports the entire building.
Mats for very tall buildings may be 6 feet (1.8 m) thick or more and are heavily reinforced. Where the bearing capacity of the soil is low and settlement must be carefully controlled, a floating foundation is sometimes used.
A floating foundation is similar to a mat foundation, but is placed beneath a building at a depth such that the weight of the soil removed from the excavation is equal to the weight of the building above.
One story of excavated soil weighs about the same as five to eight stories of superstructure, depending on the density of the soil and the construction of the building (Figure 6).
Caissons: A caisson, or drilled pier (Figure 7), is similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded. It differs from a column footing in that it extends through strata of unsatisfactory soil beneath the substructure of a building until it reaches a more suitable stratum.
A caisson is constructed by drilling or hand-digging a hole, belling (flaring) the hole out at the bottom as necessary to achieve the required bearing area, and filling the hole with concrete. Large auger drills are used for drilling caissons; hand excavation is used only if the soil is too full of boulders for the drill. A temporary cylindrical steel casing is usually lowered around the drill as it progresses to support the soil around the hole.
When a firm bearing stratum is reached, the bell, if required, is created at the bottom of the shaft either by hand excavation or by a special belling bucket on the drill. The bearing surface of the soil at the bottom of the hole is then inspected to be sure it is of the anticipated quality, and the hole is filled with concrete, withdrawing the casing as the concrete rises. Reinforcing is seldom used in the concrete except near the top of the caisson, where it joins the columns of the superstructure.
Caissons are large, heavy-duty foundation components. Their shaft diameters range from 18 inches (460 mm) up to 8 feet (2.4 m) or more. Belled caissons are practical only where the bell can be excavated in a cohesive soil (such as clay) that can retain its shape until concrete is poured. Where groundwater is present, the temporary steel casing can prevent flooding of the caisson hole during its construction.
But where the bearing stratum is permeable, water may be able to fill the hole from below and caisson construction may not be practical. A socketed caisson (Figure 7) is drilled into rock at the bottom rather than belled. Its bearing capacity comes not only from its end bearing, but from the frictional forces between the sides of the caisson and the rock as well.
Piles: A pile (Figure 7) is distinguished from a caisson by being forcibly driven into the earth rather than drilled and poured. It may be used where noncohesive soils, subsurface water conditions, or excessive depth of bearing strata make caissons impractical. The simplest kind of pile is a timber pile, a tree trunk with its branches and bark removed; it is held small end down in a piledriver and beaten into the earth with repeated blows of a heavy mechanical hammer. If a pile is driven until its tip encounters firm resistance from a suitable bearing stratum such as rock, dense sands, or gravels, it is an end bearing pile. If it is driven only into softer material, without encountering a firm bearing layer, it may still develop a considerable load-carrying capacity through frictional resistance between the sides of the pile and the soil through which it is driven; in this case, it is known as a friction pile. (Some piles rely on a combination of end bearing and friction for their strength.)
Piles are usually driven closely together in clusters that contain 2 to 25 piles each. The piles in each cluster are later joined at the top by a reinforced concrete pile cap, which distributes the load of the column or wall above among the piles (Figures 8 and 9).
End bearing piles work essentially the same as caissons and are used on sites where a firm bearing stratum can be reached by the piles, sometimes at depths of 150 feet (45 m) or more. Each pile is driven “to refusal,” the point at which little additional penetration is made with continuing blows of the hammer, indicating that the pile is firmly embedded in the bearing layer.
Friction piles work best in silty, clayey, and sandy soils. They are driven either to a predetermined depth or until a certain level of resistance to hammer blows is encountered, rather than to refusal as with end bearing piles. Clusters of friction piles have the effect of distributing a concentrated load from the structure above into a large volume of soil around and below the cluster, at stresses that lie safely within the capability of the soil (Figure 10).
The loadbearing capacities of piles are calculated in advance based on soil test results and the properties of the piles and piledriver. To verify the correctness of the calculation, test piles are often driven and loaded on the building site before foundation work begins.
Where piles are used to support loadbearing walls, reinforced concrete grade beams are constructed between the pile caps to transmit the wall loads to the piles (Figure 11). Grade beams are also used with caisson foundations for the same purpose.
Pile Driving: Pile hammers are massive weights lifted by the energy of steam, compressed air, compressed hydraulic fluid, or a diesel explosion, then dropped against a block that is in firm contact with the top of the pile. Single-acting hammers fall by gravity alone, while double-acting hammers are forced downward by reverse application of the energy source that lifts the hammer.
The hammer travels on tall vertical rails called leads (pronounced “leeds”) at the front of a piledriver. It is first hoisted up the leads to the top of each pile as driving commences, then follows the pile down as it penetrates the earth. The piledriver mechanism includes lifting machinery to raise each pile into position before driving.
In certain types of soil, piles can be driven more efficiently by vibration than by hammer blows alone, using a vibratory hammer mechanism. Where vibrations from hammering could be a risk to nearby existing structures, some lightweight pile systems can also be installed by rotary drilling or hydraulic pressing.
Pile Materials: Piles may be made of timber, steel, concrete, and various combinations of these materials (Figure 12). Timber piles have been used since Roman times, when they were driven by large mechanical hammers hoisted by muscle power. Their main advantage is that they are economical for lightly loaded foundations. On the minus side, they cannot be spliced during driving and are, therefore, limited to the length of available tree trunks, approximately 65 feet (20 m).
Unless pressure treated with a wood preservative or completely submerged below the water table, they will decay (the lack of free oxygen in the water prohibits organic growth). Relatively small hammers must be used in driving timber piles to avoid splitting them. Capacities of individual timber piles lie in the range of 10 to 55 tons (9000 to 50,000 kg).
Two forms of steel piles are used, H-piles and pipe piles. H-piles are special hot-rolled, wide-flange sections, 8 to 14 inches (200 to 355 mm) deep, which are approximately square in cross section. They are used mostly in end bearing applications. H-piles displace relatively little soil during driving. This minimizes the upward displacement of adjacent soil, called heaving, that sometimes occurs when many piles are driven close together. Heaving can be a particular problem on urban sites, where it can lift adjacent buildings.
H-piles can be brought to the site in any convenient lengths, welded together as driving progresses to form any necessary length of pile, and cut off with an oxyacetylene torch when the required depth is reached. The cutoff ends can then be welded onto other piles to avoid waste.
Corrosion can be a problem in some soils, however, and unlike closed pipe piles and hollow precast concrete piles, H-piles cannot be inspected after driving to be sure they are straight and undamaged. Allowable loads on H-piles run from 30 to 225 tons (27,000 to 204,000 kg).
Steel pipe piles have diameters of 8 to 16 inches (200 to 400 mm). They may be driven with the lower end either open or closed with a heavy steel plate. An open pile is easier to drive than a closed one, but its interior must be cleaned of soil and inspected before being filled with concrete, whereas a closed pile can be inspected and concreted immediately after driving.
Pipe piles are stiff and can carry loads from 40 to 300 tons (36,000 to 270,000 kg). They displace relatively large amounts of soil during driving, which can lead to upward heaving of nearby soil and buildings. The larger sizes of pipe piles require a very heavy hammer for driving.
Minipiles, also called pin piles or micropiles, are a lightweight form of steel piles made from steel bar or pipe 2 to 12 inches (50 to 300 mm) in diameter. Minipiles are inserted into holes drilled in the soil and grouted in place. When installed within existing buildings, they may also be forced into the soil by hydraulic jacks pushing downward on the pile and upward on the building structure.
Since no hammering is required, they are a good choice for repair or improvement of existing foundations where vibrations from the hammering of conventional piles could damage the existing structure or disrupt ongoing activities within the building.
Where vertical space is limited, such as when working in the basement of an existing building, minipiles can be installed in individual sections as short as 3 feet (1 m) that are threaded end-to-end as driving progresses. Minipiles can reach depths as great as 200 feet (60 m) and have working capacities as great as 200 to 300 tons (180,000 to 270,000 kg).
Precast concrete piles are square, octagonal, or round in section, and in large sizes often have open cores to allow inspection. Most are prestressed, but some for smaller buildings are merely reinforced (for an explanation of prestressing.
Typical cross-sectional dimensions range from 10 to 16 inches (250 to 400 mm) and bearing capacities from 45 to 500 tons (40,000 to 450,000 kg). Advantages of precast piles include high load capacity, an absence of corrosion or decay problems, and, in most situations, a relative economy of cost. Precast piles must be handled carefully to avoid bending and cracking before installation.
Splices between lengths of precast piling can be made effectively with mechanical fastening devices that are cast into the ends of the sections. A sitecast concrete pile is made by driving a hollow steel shell into the ground and filling it with concrete. The shell is sometimes corrugated to increase its stiffness; if the corrugations are circumferential, a heavy steel mandrel (a stiff, tight-fitting liner) is inserted in the shell during driving to protect the shell from collapse, then withdrawn before concreting. Some shells with longitudinal corrugations are stiff enough that they do not require mandrels.
Some types of mandreldriven piles are limited in length, and the larger diameters of sitecast piles (up to 16 inches, or 400 mm) can cause ground heaving. Load capacities range from 45 to 150 tons (40,000 to 136,000 kg). The primary reason to use sitecast concrete piles is their economy. There is a variety of proprietary sitecast concrete pile systems, each with various advantages and disadvantages (Figure 13).
Concrete pressure-injected footings share characteristics of piles, piers, and footings. They are highly resistant to uplift forces, a property that is useful for tall, slender buildings in which there is a potential for overturning of the building, and for tensile anchors for tent and pneumatic structures. Rammed aggregate piers and stone columns are similar to pressure-injected footings, but are constructed of crushed rock that has been densely compacted into holes created by drilling or the action of proprietary vibrating probes.
Seismic Base Isolation: In areas where strong earthquakes are common, buildings are sometimes placed on base isolators. When significant ground movement occurs, the base isolators flex or yield to absorb a significant portion of this movement; as a result, the building and its substructure move significantly less than they would otherwise, reducing the forces acting on the structure and lessening the potential for damage. A frequently used type of base isolator is a multilayered sandwich of rubber and steel plates (Figure 14).
The rubber layers deform in shear to allow the rectangular isolator to become a parallelogram in cross section in response to relative motion between the ground and the building. A lead core deforms enough to allow this motion to occur, provides damping action, and keeps the layers of the sandwich aligned.
Underpinning: Underpinning is the process of strengthening and stabilizing the foundations of an existing building. It may be required for any of several reasons: The existing foundations may never have been adequate to carry their loads, leading to excessive settlement of the building over time. A change in building use or additions to the building may overload the existing foundations. New construction near a building may disturb the soil around its foundations or require that its foundations be carried deeper.
Whatever the cause, underpinning is a highly specialized task that is seldom the same for any two buildings. Three different alternatives are available when foundation capacity needs to be increased:
The foundations may be enlarged; new, deep foundations can be inserted under shallow ones to carry the load to a deeper, stronger stratum of soil; or the soil itself can be strengthened by grouting or by chemical treatment. Figures 15 and 16 illustrate in diagrammatic form some selected concepts of underpinning.
Retaining Walls: A retaining wall holds soil back to create an abrupt change in the elevation of the ground. A retaining wall must resist the pressure of the earth that bears against it on the uphill side. Retaining walls may be made of masonry, preservative-treated wood, coated or galvanized steel, precast concrete, or, most commonly, sitecast concrete. The structural design of a retaining wall must take into account such factors as the height of the wall, the character of the soil behind the wall, the presence or absence of groundwater behind the wall, any structures whose foundations apply pressure to the soil behind the wall, and the character of the soil beneath the base of the wall, which must support the footing that keeps the wall in place.
The rate of structural failure in retaining walls is high relative to the rate of failure in other types of structures. Failure may occur through fracture of the wall, overturning of the wall due to soil failure, lateral sliding of the wall, or undermining of the wall by fl owing groundwater (Figure 17). Careful engineering design and site supervision are crucial to the success of a retaining wall. There are many ways of building retaining walls.
For walls less than 3 feet (900 mm) in height, simple, unreinforced walls of various types are often appropriate (Figure 18). For taller walls, and ones subjected to unusual loadings or groundwater, the type most frequently employed today is the cantilevered concrete retaining wall, two examples of which are shown in Figure 19. The shape and reinforcing of a cantilevered wall can be custom designed to suit almost any situation.
Proprietary systems of interlocking concrete blocks are also used to construct sloping segmental retaining walls that need no steel reinforcing. Earth reinforcing is an economical alternative to conventional retaining walls in many situations. Soil is compacted in thin layers, each containing strips or meshes of galvanized steel, polymer fibers, or glass fibers, which stabilize the soil in much the same manner as the roots of plants.
Gabions are another form of earth retention in which corrosion-resistant wire baskets are filled with cobble- or boulder-sized rocks and then stacked to form retaining walls, slope protection, and similar structures.