The Institution of Structural Engineers
David Ryland
I.Eng. A.M.I.Struct.E.
Structural Engineer
TRADA member

Limited Guidance Notes on Extensions

Section D - Foundation Engineering


The design of foundations for any structure is fundamental to the success of the project as a whole and for most domestic extensions un-reinforced trench fill concrete footings are perfectly adequate to support the loads applied in normal soil conditions.

The type of ground varies widely around the country from granite rock in Cornwall to chalk in the South Downs and other varieties such as sand, gravel or clay in other areas. There can also be variations locally within a generally well known area, which may not become obvious until work has begun on site.

One such common problem soil is shrinkable clay, which changes its volume with changes in moisture content, shrinking during dry summers and expanding again when the winter rain returns. This may cause subsidence and/or heave problems generating cracks in the building, which sometimes require expensive remedial work.
There is an added problem with shrinkable soils, caused by tree roots which can ingress below shallow foundations. To avoid this risk, the Building Control Officer, or supervising Engineer may instruct the builder to dig deeper foundations than originally expected. This may involve additional costs. There are recommendations laid down by the NHBC regarding the suitable depths in relation to the tree species and the distance of the tree from the proposed building.
In areas where the ground conditions are unsuitable for founding at a depth reasonable for trench excavations, it becomes necessary to consider alternatives such as piling. By comparison with deep excavations, which involve large volumes of spoil to be removed and also of replacement concrete, the option of piling and ground beams may even prove to be the cheaper alternative.
There are two main sources of information which can assist in determining the type of ground on which your proposed structure will be built. One is to consult the official geological maps that show the solid and drift soil types for a locality and the other is to seek local knowledge from the Building Control Officer for the area concerned.
Both of these options are useful, but by no means foolproof. If there is any likely possibility of 'difficult' ground conditions being encountered, requiring a more specialist design, it is best to have a soils report prepared by a laboratory with recommendations from the soils engineer as to the best form of foundations to employ. Whilst there is a cost implication to this option, it most often proves overall to be very cost effective.
David Ryland will consult the local Building Control Officer for advice and should he consider that special precautions should be taken, he will advise you at the earliest opportunity. It must be brought to your attention that, even with such local knowledge, sometimes bad ground is encountered after the builder has started work and remedial action may have to be taken at the time. You should allow a contingency within your budget for such a situation.

Contact details:
Tel: 01953 853040
Mobile: 07802 183823

 Guidance Notes on Extensions 

 Permitted Development 
 Planning Permission 
 Building Control 
 Foundation Engineering 
 The Builder 

Sample drawings
Planning:   Drawing1   Drawing2 
   Drawing3   Drawing4 
Building Control:   Drawing1   Drawing2