Foundations – also known as “footings” – are the part of a house, or other building, which is in direct contact with the ground, and transmits the load from the house to the soil, or underlying strata. Foundations need to be laid in such a way that settlement – any form of downward movement, due to the shifting, or compaction, of underlying soil – is reduced to an absolute minimum. Excessive settlement can cause damage not only to the structural components of a home, but also to drains, and other service pipes connected to it. An appropriate type of foundation can therefore be determined by calculating the structural load to be supported, and an examination of subsoil conditions – that is, the type, and quality, of the soil – including groundwater conditions, to an appropriate depth. This depth obviously varies from structure to structure, but, in most cases, is likely to be equivalent to, at least, twice the width of a building. Construction on previously disturbed soil, or fresh landfill, is likely to present problems, so these factors must be taken into account, along with typical climatic conditions in relation the nature of the soil, itself. A phenomenon known as “frost heave” is common in sandy soils, for example. Water in the soil freezes, and expands, pushing upwards through the soil, and reducing the effectiveness of foundations. Clay soils, on the other hand, are susceptible to shrinkage in very dry conditions, and, in the worst cases, can seriously damage foundations. Seasonal changes typically extend to 3, or 4, feet beneath areas with a covering of grass, at this may increase to 12 feet, or more, beneath tall trees. The proximity of surrounding vegetation, the contours of the landscape, and the depth, and width, to which it is possible to excavate are other factors that may influence your choice of the most appropriate, and economical, type of foundation.
Shallow foundations are suitable in situations where topsoil is sufficiently strong, and stable, to withstand the load imposed upon it. The most common type of shallow foundation – or “spread footing”, as it also sometimes known – is a “strip” foundation. Traditional, shallow strip foundations are straightforward, and inexpensive, to implement, and are used to support a line, or strip, of loads, such as those applied by a load-bearing wall, for example. A trench – deep enough to avoid potential damage from changes in topsoil conditions, or moisture content – is dug, its bottom is covered with a layer of concrete, to a depth of 6 inches, or so, and brickwork, or block work, is completed up to ground level.
So-called “raft” foundations – the name implies that a structure is, in some way, “floating” on the ground beneath – on the other hand, are best suited to soft, or loose, soil types, with low load bearing capacity, such as clay, or peat. A raft foundation usually consists of single, concrete slab – possibly reinforced by beams, or ribs – which spreads the load from a structure over a large area, often the same area as the structure, itself. The nature of a raft foundation is such that it is resistant to differential movement – more movement in one area, than another – due, for example, to differences in moisture content in clay soils. The raft should ideally, however, be extended beyond the load-bearing area, or provided with a protective apron, so that it cannot slide. Raft foundations are most frequently employed where “pad” foundations are impractical, because column, or structural, loads are very close together.
Pad foundations, themselves, are another form of shallow foundation, and usually consist of a block, or slab – which may be circular, square, or rectangular – and are used to support individual point loads, such as the structural columns in a house. The walls of the house, between the columns, rest on a beam – known as a “ground beam” – and the load is transferred, via the beam, to the pad foundations.
Deep foundations are necessary in situations where, for practical, or economic, reasons, it is impossible to transfer the load from a structure to soil near the surface with conventional, shallow foundation methods. Reasons may include clay subsoil, landfill, or poor drainage. Deep foundations usually penetrate to the surface of the Earth to a depth of 10 feet, or more, where the load can be transferred to deeper soil, or rock, with a greater load-bearing capacity.
In the case of extreme loads, deep foundations may take the form of cylindrical holes, filled with concrete, known as “caissons”. More typically, however, loads are transferred to deeper strata by a series of slender members – constructed from wood, steel, or concrete, and driven into, the ground with a hydraulic hammer, or actually cast in the ground, itself – known as “piles”. If piles are to be cast in the ground, holes are drilled using a machine, known as an “auger”. The drilling process allows samples of subsoil to be extracted, so that its quality, and a load-bearing, capacity can be assessed. Piles are usually situated at the corners, and intersections of load-bearing walls, and at regular intervals in between. Those excavated to a depth of 12 feet, or less, are referred to as “short bore”, to differentiate them from the much longer piles used in the construction of larger buildings. Do bear in mind that, although self build projects have been completed successfully with piles extending 30 feet, or more, underground, the cost does start to become rather prohibitive beyond 12 feet, or so.
Another form of deep foundation is the so-called “diaphragm wall”. This is constructed using the “slurry trench” technique, which involves the excavation of a deep, but fairly narrow, trench – perhaps up to 5 feet wide – which is filled with slurry, and a support fluid, to maintain its stability. The trench is excavated in discontinuous sections, or “panels” – typically varying in length between 10, and 25 feet – a steel reinforcement cage is placed in the center of each panel, and concrete is poured in. Concrete guide walls may be used at the top of each trench, to increase the accuracy of construction, and these may extend downwards to provide additional support.