Introduction to Foundation Details
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.
The Building Regulations in the UK provide guidance on foundation design. You can find the Building Regulations here.
Types of foundation
There are four main types of foundations:
Strip foundation – the preferred and most common choice for low rise housing. Strip of concrete under all load bearing walls.
Pile foundation– Long concrete members take the load of the building through weak soils to load bearing strata.
Pad foundation – More commonly used under point loads, such as columns, but can be used under ground beams to transfer loads.
Raft foundation – Concrete raft which spreads the loads over the whole ground floor, used where building loads are high, or ground conditions poor.
We will briefly look at each type of foundation in turn and provide examples of each foundation detail.
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.
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 400- 500mm 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.
Simple Strip Foundation Detail
Trench Fill Foundation Detail
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.
Pile foundation detail example
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.
Pad foundation detail example
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.
Simple raft foundation detail with down stand beam
Foundation Detail Examples
The following images provide examples of foundation details.
Strip Foundation Detail with ground bearing concrete slab
Ground Bearing Concrete Raft Foundation Detail
Strip Foundation Detail wit Suspended Concrete Slab
Notes from the Building Regulations on Foundations
General Requirements – Foundations
The building shall be constructed so that:
the combined dead, imposed and wind loads are sustained and transmitted by it to the ground, safely and without causing any building deflection/deformation or ground movement that will affect the stability of any part of the building.
ground movement caused by swelling, shrinkage or freezing of the sub-soil, land slip or subsidence will not affect the stability of any part of the building.
(Approved Doc A)
The walls and floors of the building shall adequately protect the building and people who use the building from harmful effects caused by ground moisture, precipitation wind wind-driven spray, interstitial and surface condensation, and spillage of water from or associated with sanitary fittings or fixed appliances. All floors next to the ground, walls and roof shall not be damaged by moisture from the ground, rain or snow and shall not carry that moisture to any part of the building that it would damage.
(Approved Doc C2)
Site preparation and resistance to contaminants and water
The ground to be covered by the building shall be reasonably free from any material that might damage the building or affect its stability, including vegetable matter, topsoil and pre- existing foundations.
Reasonable precautions shall be taken to avoid danger to health and safety caused by contaminants on or in the ground covered, or to be covered, by the building and any land associated with the building.
Adequate subsoil drainage shall be provided if it is needed to avoid: passage of ground moisture to the interior of the building; damage to the building, including damage through the transport of water-borne contaminants to the foundations of the building
(Approved Doc C1)
Rainwater drainage systems shall ensure that rainwater soaking into the ground is distributed sufficiently so that it does not damage foundations of the proposed building or any adjacent structure.
(Approved Doc H3)