Geotechnical Investigations & Foundation Design

Some level of geotechnical knowledge is necessary for all structural design jobs as all structures involve loads being transmitted to the ground. Ideally it is desirable to carry out some level of investigation to determine ground conditions where any change in loading on the ground takes place, but often this work can be omitted in a desire to save money, especially during times of economic slowdown.

Our experience has given us detailed knowledge of local ground conditions. We can often use this together with the use of local geological and soil maps to give clients initial advice on likely ground conditions for a site and the likely foundation types that will be required, and when for areas of likely poor ground conditions and for all larger schemes, geotechnical ground investigations are essential we can arrange the following:-

In house hand auger / mackintosh probe investigations which are ideal for small scale schemes and give low cost information about a limited area with minimal disruption (ideal for extensions, garden severance plots etc.) In some ground conditions e.g. made ground with bricks or dense gravel this equipment cannot be used.

JCB excavated trail pits are ideal for larger sites where shallow footings are anticipated and can give good information on ground up to 4 metres deep. Many pits can be excavated rapidly though there will be some disruption to the site simply getting the excavator into position and further disruption from the excavation of the pits.

Specialist shell and auger boreholes and other techniques e.g. window sampling and dynamic probing can give information about the ground at depth required for the design of bored piles or ground improvement techniques.

Foundation Design

This short article gives an overview of the common factors governing foundation design of residential or small commercial buildings in East Anglia and a summary of commonly used foundation types.

Most of East Anglia is relatively flat and is composed of either sands or clays with adequate capacity to withstand the loads imposed on them by low rise buildings, so on the face of it most foundation design in this part of the world should not be particularly onerous and this is indeed the case with the simple approach of digging a trench 1 metre deep and filling it with concrete, sufficing in most cases. However in the following cases this is not sufficient and special foundations can be needed.

  • On clay sites the roots of trees (or even large shrubs) can rip off by osmosis water molecules that are chemically adsorbed to the surface of the molecules of the clay causing large volume changes (shrinkage / heave) of the clay which can cause movement of foundations bearing onto the clay. Conventional foundations can be deepened to get below the level of the clay affected by the roots or piled foundations can be used instead. The recent increases in Landfill Tax have favoured the use of piled foundations or reinforced rafts for small stand alone buildings.
  • Made Ground. Unlike the North of England, East Anglia does not have a history of widespread mining, but small pits and quarries for the extraction of sand, gravel or clay were common and these were often subsequently infilled with soil or refuse. These pits are not always picked up by site investigations unless the site is peppered with boreholes or trial holes though they are sometimes recorded on historic Ordinance Survey maps. Old wells are often found during work on domestic extensions and they were, unsurprisingly placed as near as possible to the back of the original building and deep sewers or surface water drains can also be problematical.
  • Alluvium. Recently deposited river bed material is often so soft that it is unsuitable for taking the loads from conventional foundations or contains large amounts of peat which undergoes large volume changes with changes of loading.
  • On sandy soil the presence of a high water table means that the sides of excavations for conventional foundations can fall in before the concrete can be poured into a footing so other solutions need to be found.

With all of these, alternatives to conventional foundations can be required and we can provide designs for a variety of special foundation types to cover these situations.