The cost of constructing foundations is one of the biggest variables in a self

Build or extension project

damage due to expansion (heave) by lining the trenches with compressible material such as clayboard.

FIRM CLAY OVER SOFT CLAY

In cases where clay becomes softer as you dig down, a traditional strip foundation is sometimes acceptable, but it is important not to over dig as this may increase the stress on the softer clay beneath. A common solution is to dig wide strip foundations with embedded steel reinforcement, however an engineered foundation such as a reinforced raft or piled foundation may be necessary in some cases.

PEAT

Peat and loose waterlogged sand are very poor subsoils. If stripping back the peat can uncover suitable loadbearing ground of at least 1.5m depth, strip foundations may be suitable. Otherwise a reinforced raft foundation will likely prove to be the most economical solution.

FILLED GROUND

Where ground has previously been excavated and filled, it is generally necessary to dig down to a level beneath the area of the fill. It is important therefore to find out the depth of ‘made up ground’. As with all previously developed sites, it is vital to check for possible contamination before disturbing any material.

Types of foundation systems: Strip and trenchfill foundations

An engineer’s design based on calculations of the building’s loads and the loadbearing capacity of the ground will indicate the foundation solution to be used. Excess spoil for all foundations is usually carted away to landfill.

The standard foundation solution in most parts of the UK is the strip foundation, also known as strip footings. The topsoil is scraped back (and usually stored for reuse) and trenches dug down to a depth at least 450mm, which is enough to put the foundations below frost action. The trenches are then filled with concrete to a minimum 150mm below the surface ground level.

The walls are then built up to just below finished ground level in masonry (concrete blockwork or engineering bricks) with the outer leaf switching to the selected external facing material for the walls or plinth, typically brick or stone, just below ground level.

Approved Document A of the Building Regulations (England and Wales) defines the minimum widths for strip footings based on the type of ground and loadbearing wall — see page 103.

As such, there are several ‘types’ of strip foundation:

Where strip foundations need to be at a lower level to reach soil with suitable loadbearing capacity, a wider trench can be dug to work within, and the strip foundations dug and poured to the required width starting at this lower level. Masonry walls can then be built up to just

DEEP STRIP

below ground level before the trenches are backfilled in layers to the finished ground level using clean subsoil or other material as specified.

WIDE STRIP

Where the soil is soft or of a low loadbearing capacity, wide strip foundations can be used to spread the load over a larger area, reinforced with steel so that the loading per square metre is reduced.

OFFSET STRIP

Where it is not possible to construct conventional strip footings because the new building is hard up against an existing structure, or access to the adjoining land is not available, it may be possible to use an offset strip foundation rather than a more expensive piled foundation or a reinforced raft foundation. Typically 750mm wide and 450mm deep with a layer of A193 mesh placed in the bottom of the concrete with at least 50mm cover to the steel.the solution is generally suitable for single-storey structures.

TRENCHFILL

A widely used alternative to strip footings is the trenchfill foundation, where the trenches are filled with concrete (typically ready-mix concrete for speed) to a depth just below ground level.this allows the first course of the external facing material (typically brick or stone) to be laid up to damp proof course.this is a quick solution compared to laying masonry but is generally more expensive due to the amount of concrete required to fill the full width of the trench. Steel reinforcement may be added in areas close to trees.

Engineered foundation systems

Where the required depth of foundations is more than 2.5m it becomes impractical to use conventional strip or trenchfill foundations unless a basement storey is planned. In these cases it is more costeffective to look at alternatives such as a concrete raft or piled foundations.

RAFT

A raft foundation is a reinforced concrete slab cast over a compacted hardcore sub-base that spreads the load of the building over a larger area of ground to overcome ground conditions with a lower load-bearing capacity.

Raft foundations are usually designed by a structural engineer, taking into account the ground conditions assessed in a geotechnical survey or at least inspection of trial holes by the engineer.