SUBSOIL

 

SUBSOIL

As with climate, soil type has important structural implications and will be taken into account by an engineer if you are employing one in the design of your hut. The hut foundations must rest upon natural ground comprising one of the following soil types:

 

  • Sound Rock
     

  • Compact gravel or sand
     

  • Stiff or firm clay or sandy clay

This guide does not cover ground conditions where fill material, peat or highly weathered rock is present. Engineering advice must be sought in these cases. If you are at all uncertain about what soil type your site exhibits, seek professional help.

 

Preliminary information on soil and rock types at your site can viewed on the British Geological Survey map website.  Use the Geology key to find the types.

http://mapapps.bgs.ac.uk/geologyofbritain/home.html

 

Mining activity should also be checked. If your site is in an area where a coal mining report is recommended you should seek engineering advice.

http://mapapps2.bgs.ac.uk/coalauthority/home.html

 

BEARING CAPACITY

There are simple ways to assess the ground bearing capacity of your subsoil by digging and examining a trial pit.  Hand digging or a mini excavator can be used, but you must be certain that there are no underground services that might be damaged if you use an excavator.  Record the different types of soil you can see in the pit, and the difficulty or otherwise of excavating, and test with wooden peg or fingers.  Note also if water is present in the pit as this will have an effect on the foundations.

 

In most soil types it is safe to hand dig a pit up to 1m deep. A typical size on plan is 600 mm x 600 mm. Allow a batter on the side (i.e. ensure the wall of the pit has an inward slope to support it) if the soil is particularly unstable.

The table below, based on Building Standard 8103-1:2011, shows typical observations to help identify soil types.  Many soils are a mixture of gravel, sand, silt and clay and exact identification is not critical.  The difference in all these terms is one of decreasing grain size, below what can be seen with the naked eye for the last two in the list.  Larger stones (boulders or cobbles) can also be present.  The important factor is to identify the condition shown in bold. The looser or softer materials may still be suitable for the foundations, but engineering design would be required to ensure structural soundness.

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