The stability of a building depends primarily on the foundation it is built on. The construction of the foundation is in turn dependent on the type of building and, above all, on the loadbearing capacity of the ground. Soft soils, or those that become soft when wet, require more sophisticated and expensive foundations than hard soils. Natural hazards, such as earthquakes, hurricanes, floods, etc., also have an influence on foundation construction.
On account of the numerous requirements and constraints, there is a large variety of foundations. With regards to low-cost constructions, five main types are briefly dealt with here.
Types of Foundations
Linear or strip foundation
This is the most common type of foundation, consisting of a continuous strip, which supports a load-bearing wall along its full length. It is also used to bridge or cantilever over soft portions of the ground, in which case, it must be reinforced.
Spot or pad foundation
This is the common foundation for columns or poles (skeleton constructions), and mainly comprises a square (sometimes rectangular) footing, which is thicker than the width or diameter of the column or pole, the length and breadth each being at least three times the thickness.
Slab or raft foundation
This type of foundation is often used for small buildings or structures with uniformly distributed loads (eg water tanks). Slabs on homogeneous ground can do without reinforcement, but over large areas, reinforcement is advisable, as non-uniform ground conditions lead to differential stresses.
Building on poor soils or under water calls for this type of foundation. Holes are dug down through the weak soil up to the loadbearing layer, and filled with stable foundation material (either placed in situ or precast). The piles carry a reinforced concrete slab or are connected at the top by beams, which act like strip foundations. Lateral stability is achieved by placing some of the piles at a slant.
Building on sloping ground makes a stepped foundation necessary. It is a special form of strip foundation, designed to save material, and to provide horizontal surfaces at intervals along the slope.
Most other types of foundation are variations of the ones presented above, or are of special types, which are less relevant in low-cost constructions.
Basic parts of a foundation
A linear or strip foundation is built up as follows:
• The footing serves three main purposes: 1. to provide a solid, level base for the foundation walls; 2. to transmit the weight of the house evenly to the soil; 3. to resist the lifting forces of hurricanes.
• The foundation wall also serves three main purposes: 1. to provide a level base for the wall; 2. to provide the necessary bending and torsion strength for the construction of the house; 3. to prevent underground moisture from moving up into the walls.
• The footing must be deep enough to reach good solid earth free from plants, roots, filled-up materials, etc. Average depths are generally 50 to 100 cm, but should be considerably deeper, if washing out or shifting due to rain or flooding is expected.
• The "easy method" of determining the depth of footing is by asking neighbours, whose houses have shown good stability (without cracks or other damage). In case of doubt, deeper footings are advisable.
• Sizes of footing depend on the strength of the soil and weight of the house. The height should preferably exceed the wall or column thickness and the base should be wide enough to permit a 60° angle of load distribution. Average footing widths lie between 30 and 60 cm.
• Foundation walls should preferably be thicker than the walls they support, and high enough above ground to protect the wall from rain splash. Heights of 20 to 50 cm above the ground are common, but depend on rainfall intensity and roof overhang.
• Foundation trenches should be carefully dug to provide a hard, level bottom surface and side walls at right angles to it. Rounded edges must be avoided.
• The excavated soil should be retained for backfilling, when the foundation wall is ready. The backfilling should have the same characteristics (soil type, moisture, density) as the surrounding, undisturbed soil.
• Foundations can be made of several materials with differing qualities. A good reinforced concrete foundation is the strongest and best foundation for any type of residential building. Where cement is too expensive or scarce, other materials can provide satisfactory results.
Protection of foundations
• Penetration of rainwater and ground moisture is largely prevented by good waterproof concrete, natural stone, waterproof burnt bricks, but also with a waterproof coating or membrane, and protective roof overhang. Drainage tubes laid in a gravel bed alongside the footing are also effective.
• For protection against termites. see section on Biological Agents.
The foundation can be attacked by:
1. Rain and wind
2. Hillside surface water
3. Hillside underground water
4. Seepage water
5. Ground water
• Soft clayey soils, which are unsuitable to build on, can be consolidated by providing vertical drains which draw out the water. These can be rigid sand drains or flexible drains. Cheap and effective flexible drains using coir and jute fabric have been developed at the University of Singapore and the Central Building Research Institute in India.
• The water from fresh concrete or from the mortar in masonry foundations is quickly absorbed by the soil, if it is very dry. Therefore, foundation trenches should be properly watered before placing the foundation material, so that absorption is reduced.
• In highland regions, in which temperatures can also fall below 0° C, the water in the soil can freeze and expand, damaging the foundation and consequently the whole building. This problem, called frost heave, occurs mainly in silty soils. The problem is avoided by placing the footing below the frost line, which can lie between 50 and 100 cm, or much lower in colder climates.
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