FOUNDATIONS

INTRODUCTION TO FOUNDATION DESIGN

The main role of foundations is to structurally support the building by transferring the loads of the building through the walls into the surrounding soil. In terms of a timber frame structure, the foundations must also protect the timber from moisture ingress by lifting the members above the ground.

The foundation must transmit the combined, dead and imposed loads on a building to the ground safely. Building Regulations in the UK have led to standard forms of concrete foundations along with rigorous investigations into the bearing capacity of soils and bedrock.

IMPLICATIONS ON FOUNDATION DESIGN

The type of soil and bedrock on site will have a strong implication to the foundation design. The size and depth of the foundation is determined by the size and scale of the structure it must support, along with the bearing capacity of the ground that surrounds it.

Soil Top soil usually consists of a mixture of solid particles, water and air. It usually contains organic remains of decayed vegetation close to the surface. Top soil is unsuitable for supporting foundations, and should be stripped from the immediate site, and retained for landscaping the surrounding site.

Sub-Soil Sub-soils range from rocks, such as granite or sandstone, all the way through to soft clay and silt. This subsoil sits below the top soil and supports the load of the building. Different types of sub-soil can require specialist foundation design due to their poor bearing capacity. The following table shows the types of sub-soil and its suitability to supporting house foundations. Sub soils and their suitability for house foundations

Ground Type Notes Granite Limestone Sandstone Slate Hard chalk These sub-soils provide good support to founda5ons. They generally require pneuma5c or hydraulic tools for excava5on

Compact sands Sub-soils provide good support. They do not require special tools for Gravels excava5on, only standard machinery Firm and s<ff clays Wider founda5ons may be required than sands and gravels. Excava5on can Sandy clays be done by hand or machine Loose sand So> silt So> clay Easy to excavate but generally will require specially designed founda5ons

Figure 4.1 - Soil types and suitability

Before detailed foundation design can begin various site and ground investigation studies need to be carried out in order to ensure accurate design. Site investigations determine: • The nature, past use and condition of the site • Whether this has any implications for the proposed building and its foundations

Ground investigations can gather information on: • The nature and thickness of top soil • The nature, thickness and stratum depth of sub-soil • Assessment of bearing pressure • Ground water levels • Presence of chemicals in the ground • Any existing hazards in the ground

Approved Document Part A of the Building Regulations states that a building must be constructed so that any ground movement caused by shrinkage, swelling or freezing of the subsoil or any other landslip or subsidence, will not impair the stability of the building.

Frost Heave If the water table is high, and close to the surface, some soils will expand when frozen. This is due to ice crystals forming in the soil and causing it to expand - creating frost heave. In the UK the ground is rarely frozen at depths of more than 600-700mm which should be sufficiently deep for most strip foundations in these types of soil.

Change in volume Sub-soils than contain clay can often suffer shrinkage caused by drying and expansion on wetting, due to the changes in the season. These changes in volume can be further impacted by proximity to trees and shrubs. The more vigorous the growth of shrubs and trees in firm clay soils, the greater the depth below surface the volume change will occur. Likewise, when trees or shrubs are removed from site to enable building work, there can be movement in the clay soil for some years after clearance, as the clay recovers moisture previously taken up by the trees. The same can be said when planting new trees and shrubs in existing clay soils, as the trees will change the existing moisture in the clay which can result in shrinkage. In these types of scenarios, specialist foundations must be designed to ensure suitable resistance to the changes in the soil volumes.

TYPES OF FOUNDATION

There are four main types of foundations:

• Strip - the preferred and most common choice for low rise housing. Strip of concrete under all load bearing walls. • Pile- Long concrete members take the load of the building through weak soils to load bearing strata. • Pad - More commonly used under point loads, such as columns, but can be used under ground beams to transfer loads. • Raft - Concrete raft which spreads the loads over the whole ground floor, used where building loads are high, or ground conditions poor.

STRIP FOUNDATIONS

Strip foundations are the common foundation design in residential construction. They consist of a strip of reinforced concrete formed centrally under a load bearing wall. The width of the concrete strip is determined by the structure it will be supporting, along with the bearing capacity of the soil in order for the load to be safely transmitted to the ground.

The greater the bearing capacity of the soil, the less the width of the foundation. The following table (based on Approved Document Part A) suggests recommended minimum width of concrete strip foundations.

Type of ground 20kN/m (equivalent of single storey dwelling) 40kN/m (equivalent of two storey dwelling) 60kN/m (equivalent of three storey dwelling)

Gravel or sand (medium dense)

min 250mm min 400mm min. 600mm

Firm clay, firm sandy clay

min. 300mm min 450mm min 750mm

Loose clay or sand

min. 400mm Foundation to be specially designed to suit conditions and specific requirements Figure 4.2 - Minimum width of concrete strip foundations

Trenches are excavated to a level of undisturbed compacted soil, where the concrete strip will be spread. The width of the excavated trench must allow for the bricklayer to build the wall off the strip, usually a minimum of 600mm. A trench depth of 450mm is the minimum set out in the Building Regulation requirements, however, this is increased to 700mm if there is a danger of frost heave.

Given that most sands and gravels can support low rise housing with a concrete strip width of only 400500mm a cost effective alternative option is a trench fill method. This avoids the need for working space and can be a more economic option. The trench fill is a deeper concrete fill bringing it closer to ground level, where the wall is then constructed.

Figure 4.3 - Examples of strip foundation

PILE FOUNDATIONS

When strip foundations are not suitable, a pile foundation system can be used. Pile foundations can be seen where trees or shrubs are in close proximity to the proposed structure, or where a site has been cleared of existing trees and could be susceptible to volume changes. Pile foundations transfer the load of the building to a depth that is capable of supporting the full loads of the structure. It is an expensive process, due to the requirement of specialist subcontractors.

Once the piles have been driven into the ground in the correct position, the tops are cut to the required level in order to have a concrete beam cast over them forming the base for the walls. The beam spans from pile to pile, therefore not requiring support from the soil below.

PAD FOUNDATIONS

Pad foundations generally consist of a concrete square pad which supports ground beams onto which the structural walls can be built. Pad foundations differ from pile foundations as they do not extend to such depths as piles, and the width of the pad varies in order to distribute the loads to a greater area. Pits are excavated to the required depth, where the concrete is then cast. Brick or concrete piers are then built or cast on the pad foundations up to the underside of the concrete beams that support the walls.

RAFT FOUNDATION

Raft foundations may be used where soils consist of very soft clay, or other situations where strip, pad or pile foundations are not suitable. A reinforced concrete raft is designed to distribute the loads of the building over the whole area under the raft, which results in little if any settlement. Two main types of raft foundation, the flat slab raft and the wide toe raft.

The flat slab raft is used for smaller buildings where the loads are comparatively small. It is of uniform thickness, and consists of reinforcing to the top and bottom of the slab. Where loads on the foundations would require a thick slab, the wide toe raft is used. The wide toe has a reinforced stiffening edge beam, with a toe at the edge to provide a base for the outer leaf of a cavity wall.

Pile Pad

Figure 4.4 - Examples of pile, pad and raft foundations

Raft