Footings and foundations

Contents - Previous - Next

A foundation is necessary to support the building and the loads that are within or on the building. The combination of footing and foundation distributes the load on the bearing surface and keeps the building level and plumb and reduces settling to a minimum. When properly designed, there should be little or no cracking in the foundation and no water leaks. The footing and foundation should be made of a material that will not fail in the presence of ground or surface water. Before the footing for the foundation can be designed, it is necessary to determine the total load to be supported.

If for some reason the load is concentrated in one or more areas, that will need to be taken into consideration. Once the load is determined, the soil bearing characteristics of the site must be studied.

Soil Bearing

The topmost layer of soil is seldom suitable for a footing. The soil is likely to be loose, unstable and contain organic material. Consequently, the topsoil should be removed and the footing trench deepened to provide a level, undisturbed surface for the entire building foundation. If this is not feasible because of a sloping site, the footing will need to be stepped. This procedure is described in later and illustrated in Figure 5.5. The footing should never be placed on a filled area unless there has been sufficient time for consolidation. This usually takes at least one year with a normal amount of rainfall. The bearing capacity of soil is related to the soil type and the expected moisture level. Table 5.6 provides typical allowable soil-bearing values.

Table 5.6 Soil Bearing Capacities

Soil Type kN/m
Soft, wet, pasty or muddy soil 27 - 35
Alluvial soil, loam, sandy loam (clay +40 to 70% sand) 80 - 160
Sandy clay loam (clay +30% sand), moist clay 215 - 270
Compact clay, nearly dry 215 - 270
Solid clay with very fine sand - 430
Dry compact clay (thick layer) 320 - 540
Loose sand 160 - 270
Compact sand 215 - 320
Red earth - 320
Murram - 430
Compact gravel 750 - 970
Rock - 1700

An extensive investigation of the soil is not usually necessary for small-scale buildings. Foundation and pier footings can easily be designed to keep within the safe bearing capacity of the soil found on the building site.

Site Drainage

It is desirable to site any building on well-drained land However, other considerations such as access roads, water supply, existing services or a shortage of land may dictate a poorly drained area.

If a building site with poor natural drainage must be used, it may be improved by the use of contour interceptor drains or subsurface drains in order to cut off the flow of surface water or to lower the water-table level. Aparn from protecting the building against damage from moisture, drainage will also improve the stability of the ground and lower the humidity of the site. Figures 5.1 and 5.2 illustrate these methods.

Subsurface drains are usually laid 0.6 to 1.5m deep and the pipe layout arranged to follow the slope of the land. The spacing between drains will vary between 10m for clay soils to 50m for sand. Subsurface drains are usually formed from buttjoined clay pipes laid in narrow trenches. In cases where it is desirable to catch water running on the surface, the trench is back-filled nearly to the top with rubble either continuously along the trench or in pockets. A trench filled with rubble or broken stone will provide passage for water and is effective in dealing with flows on the surface. Pipes and trenches belonging to the main site drainage system may cause uneven settling if allowed to pass close to or under buildings. Where needed a separate drain, that surround the building and installed not deeper than the footing, is used to drain the foundation trench.

Figure 5.1 Contour interceptor drain.

Figure 5.2 Subsurface site drains.

Foundation Footings

A footing is an enlarged base for a foundation designed to distribute the building load over a larger area of soil and to provide a firm, level surface for constructing the foundation wall.

A foundation wall, regardless of the material used for its construction, should be built on a continuous footing of poured concrete. Although the footing will be covered and lean mixes of concrete are considered satisfactory, a footing that is strong enough to resist cracking also helps to keep the foundation from cracking. A 1 :3:5 ratio of cement - sand - gravel is suggested with 311 of water per 5O kg sack of cement. The amount of water assumes dry aggregates. If the sand is damp, the water should be reduced by 4 to 5l.

The total area of the footing is determined by dividing the total load, including an estimated mass for the footing itself, by the bearing by dividing the area by the length. In many cases the width required for light farm buildings will be equal to or less than the foundation wall planned. In that case, a footing that is somewhat wider than the foundation is still recommended for at least two reasons. The footings conform-to small variations in the trench and bridge small areas of loose soil making a good surface on which to begin a foundation wall of any kind. The footings are easily made level and this makes it easier to install the forms for a poured concrete wall or to start the first course of a block or brick wall.

Even when loading does not require it, it is common practice to pour a concrete footing that is as deep as the wall is thick and twice as wide. The foundation footings for large heavy buildings require reinforcing. However, this is seldom necessary for light-weight farm buildings. Once a firm footing is in place, a number of different materials are suitable for building a foundation. Figure 5.3 shows footing proportions for walls, piers and columns.

Figure 5.3 Footing proportions.

Although continuous wall footings are frequently loaded very lightly, that is not the case with column and pier footings. It is important therefore, to carefully estimate the proportion of the building load to be carried by each pier or column. Figure 5.4 illustrates the load distribution on a building with a gable roof and a suspended floor.

If wall footings are very lightly loaded, it is advisable to design any pier or column footings required for the building with approximately the same load per unit of area. Then if any settling occurs, it should be uniform throughout. For the same reason, if part of the footing or foundation is built on rock, the balance of the footing should be twice as wide as usual for the soil and loading. Footings must be loaded evenly as eccentric loading may cause tipping and failure.

If a foundation is installed on a sloping site, it may be necessary to dig a stepped trench and install a stepped footing and foundation. It is important that all sections are level and that each horizontal section of the footing is at least twice as long as the vertical drop from the previous section. Reinforcing in the wall as shown in Figure 5.5 is desirable.

Figure 5.4 The division of loads on footings.

Each pier footing must carry t/8 of the floor load. The wall must carry 5/8 of the floor load and all of the roof and wall load.

Figure 5.5 Stepped footing and foundation.

The procedure for finding an appropriate footing may be illustrated using Figure 5.4. Assume a building is 16m long and 8m wide. The roof framing plus the expected wind load totals 130kN. The wall above the foundation is 0.9kN/ m. The floor will be used for grain storage and will support as much as 7.3kN/m. The floor structure is an additional 0.5kN/m. The foundation wall and piers are each 1 m high above the footing. The wall is 200mm thick and the piers 300mm square. The soil on the site is judged to be a compact clay in a well-drained area. Find the size of the foundation and pier footing that will safely support the loads. Assume that the weight of the mass 1 kg approximately equals 10N. The mass of concrete is 2400 kg/m.

1 The division of the load on each wall is as follows:

a Roof load - 50% on each wall, 130kN 65kN
b Wall load - for each side 16 x 0.9kN 14.4kN
c Floor load - each side carries 7/32 x 998kN 218.4kN
d Foundation load - each side, 16 x 0.2 x 24kN 76.8kN
e Estimated footing 0.4 x 0.2 x 16 x 24kN 30.7kN
f Total on one side 405.3kN
g Force per unit of length 405.3/ 16 25.3kN/ m
h Using for practical reasons,and assumed width of 0.4, 25.3/0.4 63.3kN/m
i Compact clay at 215 - 217kN/m easily carries the load.  
2 The division of the load on each pier is:
Floor load - 1/8 x 998kN 124.8
Pier 0.3 x 0.3 x 1 x 24kN 2.2
Footing estimate 0.8 x 0.8 x 0 5 x 24kN 7.7
Total 134.7kN
Load/ m 210kN/ m
O.K. but 1 x 1 x 0 7 gives more equality to wall loading 144kN/m

The most logical action to take would be to add one or more additional piers which would allow both smaller footings and smaller floor support members.

Footing Trenches

The trench must be dug deep enough to reach firm, undisturbed soil. For light buildings in warm climates, this may be as little as 30cm. However, for large, heavy buildings footing trenches may need to be up to 1m deep.

Pockets of soft material should be dug out and filled with concrete, stones or gravel. The trenches should be free of standing water when the concrete is poured for the footing.

A level trench of the correct depth can be insured by stretching lines between the setting-out profiles (batter boards) and then using a boning rod to check the depth of the trench as it is dug out.

The footing forms should be carefully leveled so that the foundation forms may be easily installed, or a brick or block wall begun. If the foundation walls are to be made of bricks or concrete blocks, it is important that the footings be a whole number of courses below the top of the finished foundation level.

Alternatively the footing can be cast directly in the trench. While this saves the cost of footing forms, care must be taken so that no soil from the sides is mixed in the concrete. Proper thickness for the footing can be ensured by installing guiding pegs, whose tops are set level and at correct depth, at the center of the foundation trench.

Types of Foundation

Foundations may be divided into several categories which are suitable for specific situations.

Continuous wall foundations may be used either as basement walls or as curtain walls. A continuous wall for a basement of a building must not only support the building but it must be a waterproof barrier capable of resisting the lateral force of the soil on the outside. However, because of the structural problems and the difficulties to exclude water it is recommended to avoid basement constructions in all, but a few special circumstances. Curtain walls are also continuous in nature but being installed in a trench in the soil, they are not usually subjected to appreciable lateral forces and they do not need to be waterproof. Curtain walls may be constructed and then the earth filled back on both sides, or they may be made of concrete poured directly into a narrow trench. Only that portion above ground level requires a form when the concrete is poured. See Figure 5.9. Curtain walls are strong, relatively watertight and give good protection against rodents and other vermin.

Pier foundations are often used to support the timber frames of light buildings with no suspended floors. They require much less excavation and building material. The stone or concrete piers are usually set on footings. However, for very light buildings the pier may take the form of a precast concrete block set on firm soil a few centimeters below ground level. The size of the piers is often given by the weight required to resist wind uplift of the whole building.

Pad and pole foundation consists of small concrete pads poured in the bottom of holes which support pressure treated poles. The poles are long enough to extend and support the roof structure. This is probably the least expensive type of foundation and is very satisfactory for light buildings with no floor loads and where pressure treated poles are available.

A floating slab or raft foundation consists of a poured concrete floor in which the outer edges are thickened to 20 to 30cm and reinforced. This is a simple system for small buildings that must have a secure joint between the floor and the sidewalls.

A pier and ground level beam foundation is commonly used where extensive filling has been necessary and the foundation would have to be very deep in order to reach undisturbed soil. It consists of a reinforced concrete beam supported on piers. The piers need to be deep enough to reach undisturbed soil and the beam must be embedded in the soil deeply enough to prevent rodents from burrowing under it. For very light buildings such as greenhouses, timber ground level beams may be used.

Piles are long columns that are driven into soft ground where they support their load by friction with the soil rather than by a firm layer at their lower end. They are seldom used for farm buildings.

Foundation Materials

The foundation material should be at least as durable as the balance of the structure. Foundations are subject to attack by moisture, rodents, termites and to a limited extent, wind. The moisture may come from rain, surface water or ground water, and although a footing drain can reduce the problem, it is important to use a foundation material that will not be damaged by water or the lateral force created by saturated soil on the outside of the wall. In some cases the foundation must be watertight in order to keep water from penetrating into a basement or up through the foundation and into the building walls above. Any foundation should be continued for at least 150mm above ground level to give adequate protection to the base of the well from moisture, surface water, etc.

Stones

Stones are strong, durable and economical to use if they are available near the building site. Stones are suitable for low piers and curtain walls where they may be laid up without mortar if economy is a prime factor, it is difficult to make them water tight, even if laid with mortar. Also, it is difficult to exclude termites from buildings with stone foundations because of the numerous passages between the stones. However, laying the top course or two in good rich mortar and installing termite shields can largely overcome the termite problem.

Earth

The primary advantage of using earth as a foundation material is its low cost and availability. It is suitable only in very dry climates. Where rainfall and soil moisture are a little high for unprotected earth foundation, they may be faced with stones as shown in Figure 5.6 or shielded from the moisture with polythene sheet. See Figure 5.8.

Earth foundation faced with stones.

Poured Concrete

Concrete is one of the best foundation materials because it is hard, durable and strong in compression. It is not damaged by moisture and may be made nearly watertight for basement walls. It is easily cast into the unique shapes required for each foundation.

For example, curtain walls can be cast in a narrow trench with very little formwork required. The principle disadvantage is the relatively high cost of the cement required to make the concrete.

Concrete Blocks

Concrete blocks may be used to construct attractive and durable foundation walls. The forms required for poured concrete walls are unnecessary and because of their large size, concrete blocks will lay up faster than bricks. A block wall is more difficult to make watertight than a concrete wall and does not resist lateral forces as well as a poured concrete wall.

Bricks

Stabilized earth bricks or blocks or blocks have inherently the same restrictions as monolithic earth foundations. They are suitable only in very dry areas and even there they need protection from moisture. Adobe bricks are to easily damaged by water or ground moisture to be used for foundations. Locally made, burnt bricks can often be obtained at low cost, but only the best quality bricks are satisfactory for use in moist conditions. Factory made bricks are generally too expensive to be used for foundations.

Foundation Construction

Stone Foundation

If the stones available are relatively flat, they may be laid up dry (without mortar) starting on firm soil in the bottom of a trench. This makes a very low cost foundation suitable for a light building. If monolithic earth walls are to be constructed on top of the stone foundation, no binder is necessary for the stones. If masonry units of any type are to be used, it would be prudent to use mortar in the last two courses of stone in order to have a firm level base on which to start the masonry wall. If a timber frame is planned, then mortar for the top courses plus a metal termite shield is necessary both to provide a level surface and to exclude termites.

If the stones available are round or very irregular in shape, it is best to lay them up with mortar to obtain adequate stability. Figure 5.7 shows earth forms being used to hold stones of irregular shape around which a grout is poured to stabilize them. Stones to be laid in mortar or grout must be clean to bond well.

Figure 5.7a. shows a mortar cap on which a concrete block wall is constructed. A stone shield to protect the base of an earth block wall is shown in b., and in c. the embedding of poles in a stone foundation as well as a splash shield. Proper shielding may reduce the risk of a termite infestation.

Figure 5.7 Stone foundations.

Earth Foundation

Although more moisture resistant materials are generally recommended for foundations, circumstances may dictate the use of earth. Figure 5.6 shows an earth foundation that has been faced with fieldstones. The joints have been filled with a cement-lime mortar and the entire surface coated with bitumen. Figure 5.8 illustrates the use of sheet polythene to exclude moisture from a foundation wall. While either of these methods helps to seal out moisture, the use of earth for foundation walls should be limited to dry-land regions.

Place the polythene sheet on a thin layer of sand or on a concrete footing. Overlap the single sheets by at least 20cm. Construct a foundation wall from stabilized rammed earth or stabilized soil blocks. Once the wall has hardened and dried out, the polythene is unrolled and soil filled back in layers in the foundations trench. Fasten the ends of the sheet to the wall and protect with a drip deflection strip, a skirting or with malting and plaster.

Figure 5.8 An earth foundation protected from moisture with polythene sheet.


Contents - Previous - Next