The most commonly used type of foundation is a shallow foundation; this is typically used for smaller buildings such as single-family homes or small garage structures. It involves digging down into the ground to create a firm base that can support the weight of the structure above. This type of foundation requires less excavation than others and has lower material costs, making it an ideal choice for those on a budget.
Shallow foundations consist of strip, trench fill, pad and raft foundation depending upon the soil condition and load requirements. Strip footings are most commonly used in residential construction. Early strip foundations were constructed of brick or stone, widened at the base of the wall to distribute the load of the wall over a wider area. This was a technique used in Victorian and Edwardian properties. The foundations were typically constructed at a shallow depth, possibly no more than 450–500 mm below ground level. The wall would thicken below the ground surface, corbelling the stone or brickwork to a width that would ensure the load could be sustained by the receiving soil. Strip foundations are used to sustain line loads and are shallow. Since the 1900s concrete has been used and typically a foundation will be a minimum of 150 mm deep (usually 225 mm) and 150 mm wider on each side of the wall.
The depth from ground level to the bottom of the foundation is important to ensure that the foundation does not experience movements in the soil due to climatic changes such as heat, frost and moisture changes. Except where strip foundations are founded on rock, they should have a minimum depth of 450 millimetre to their underside to avoid the action of frost – except in clay soils where this is increased to 750 millimetres to avoid the influence of vegetation. However, these are minimum values and will normally need to be increased to transfer the loads to suitable subsoil.
Trench-fill foundations are trenches with a minimum width of 450 mm. When excavated the sides are trimmed, and prepared and then mass concrete is poured into the trench, levelled at the top to receive the wall construction. This type of construction can be quicker than strip foundations, since there is a labour sabing in the construction of subsurface walls. Concrete mixes for strip and trench-fill foundations are generally ST2 or Gen 1 mix. ST2 is a standard mix consisting of 4–20 mm stone, graded sand and water. BS 5328: 1 ST2 has a compressive strength of 10 Newton/square millimetre. Gen 1 is a designated mix. BS 5328:1 Section 5 specifies the mix. The mix strength is also Newton/square millimetre.
Pad footings can be used for columns that carry heavy loads while raft footings provide full bearing capacity on weak soils or there is a likelihood of differential settlement due to changes in the soil or moisture. Raft foundations spread the load over the area of the slab, thus reducing the bearing pressure to the ground. This is achieved through reinforcement of the slab.
The extraction of moisture in clay soils can lead to volumetric changes in the soil, thus a raft foundation can be used to ensure any depressions or downward movement of the subsoil can be accommodated, or the voids spanned by spreading the load of the supported structure over a wider area. Ground heave is caused when clay soils rehydrate, causing the clay to swell and push on the underside of the foundation. In these circumstances a reinforced concrete slab can be employed which is suspended from a boot or edge beam, thus creating a void beneath the slab. The void is high enough to accommodate any movements in the ground. Products are available which overcome the effects of heave and can comprise a moulded hexagonal cellular polystyrene void former which is placed on the prepared ground. The reinforced concrete is poured over the product, and this supports the concrete until it sets. The product is available in different grades and each grade is designed to support a safe working load for a period of approximately 16 hours. After this time the material disintegrates, and the slab remains suspended; thus, the ground beneath can rise or heave without adversely affecting the structural integrity of the slab.
Different types of raft foundation, slab raft and boot raft, will depend on the line load along the edges and the spans required.
Slab Raft This comprises a flat slab which can be reinforced at the top only or the top and bottom depending on the tension forces anticipated in the foundation and the particular conditions the slab has been designed for.
Boot raft the boot design will allow a step to hide the outer skin of brickwork in the ground. Boot rafts can be tapered or square on the underside and usually require reinforcement shear links.
The strength of the concrete used will depend on exposure and loading, but generally an RC35 mix is suitable. This is a ready-mixed concrete specified in accordance with BS 5328:1 Section 5 and is used for reinforced concrete. The numbers relate to the strength of the mix and in this case the mix will have a compressive strength of 35 Newton/square millimetre.