In this section we will take a look at everything any self-builder needs to know about different types of footings and foundations
What you need to know Soil survey
First things first; you need a soil survey to identify the soil composition and ultimately, to ensure that your footings are constructed appropriately to support the investment in the house. The soil survey involves digging trial pits from which samples are taken at various depths. The samples are analysed in a laboratory to determine the actual make-up of the soil – percentage sand, clay, grit etc. It also takes into account the moisture content and checks are made as well for the presence of hazardous substances such as radon.
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This shows the layout of your site and its level relatively to a standard datum. A topographical survey isn’t always essential but can be very useful. Allow £1,000 to £1,500.
Structural calculations for footings
The technical specification for the footings is dependent on the soil survey and combined with the structural drawings, the depth of the footings, whether double or single steels are needed in the slab and the required depth of the concrete over-site, etc., would then be determined by the structural engineer who produces structural foundation and slab drawings. One thing’s for sure, it’s vitally important to get the structural calculations right to enable the groundworker to work to the precise drawings.
N.B. The structural calculations can cost £1000-£1800.
The depth of the footing is initially set in the technical specification and is based on the load the footings have to bear and the soil composition as identified by the soil survey. The normal depth for footings is 1 metre for estimation only, but the structural engineer will determine the depth according to soil type and density. Identification of pockets of poor soil structure, tree roots, etc., can result in the footings having to be made deeper. This problem is more likely to be encountered on “brown field” sites i.e. those where there has been construction in the past and where there’s a presence of trees.
Types of foundations
Depending on the type of ground, different types of foundation are available:
1. Strip foundation
This is a footing dug to depth issued by the structural engineer and checked by the building inspector. Strip foundation has a concreted footing block to the under slab, and then a mesh bottom with an anti-crack mesh over internal walls which is shuttered and concreted
This is the same as strip foundation but the footings are filled with concrete to a specified level thus cutting down the block depth.
Used if the soil is unstable, i.e. soft clay or where the ground is not consistent all the way around the footing and there’s very low compaction.
A reinforced concrete raft or mat is used on very weak or expansive soils such as clays or peat. They allow the building to ‘float’ on or in the soil. A raft is used where the soil requires such a large bearing area that wide strip foundations are spread too far, making it more economical to pour one large reinforced concrete slab. A raft is an alternative to piles as it can be less expensive.
Used when isolated loads need to be supported, for instance to support the columns of a steel or post and beam frame house. The load is concentrated on a small area. N.B. Some pre-manufactured housing can’t use block and beam as they need ground bearing or strip foundations due to their fixing bolts. On some of the foundations some pre manufacturers also require different tops. For example, some English timber companies require an upstanding on every internal and completely around the outer edge of the slab of a dolly block. This allows the timber frame to be above the wet screed that would be placed inside the house as a flooring
And finally beware of escalated costs
Removal of waste can incur an extra cost. An estimate is usually based on the assumption that soil excavated during the digging of the footings can be redistributed around the site. If that is impractical or undesirable though, the waste soil will need to be removed. The price for this varies widely throughout the country with costs varying between £180 and £250 for a single 16 ton load. The cost for disposal of plants and other green matter can be even higher, and it is worth considering the environmentally friendly alternative of chipping and composting on site. In some cases local bye-laws may permit burning of the organic material on site, but you need to consider the impact on your neighbours even where the practice is permissible. Remember also that although your house is free of VAT if it is your primary private residence, any services that you buy are not VAT exempt. So you will more than likely have to pay VAT on the soil and topographical services, the disconnection of services (reconnection or first time connection are normally exempt) and the hire of security fences, gates, signs and toilet