The new houses want to take root in the landscape. Not only do they respect urban planning regulations, but they are now based on a geotechnical study. A diagnosis to ensure that the soil offers the desired stability.
The phenomenon of climate change that we are witnessing can only reinforce the interest of carrying out a soil study before starting an individual construction. Intended to apprehend the phenomenon of shrinkage and swelling of clays, this "geotechnical study" diagnosis amounts to taking into account the combined effects of a strong drought and an excessive rain on clay soils.In the presence of high humidity, these soils are supple and malleable. In contrast, high temperatures make them hard and brittle.
While variations in the soil are slow, they can reach a large enough amplitude to damage buildings.
Since October 1, 2020, this soil study must be attached to the sales agreement for a building lot or carried out when the building permit is submitted.
Thisprecaution is all the more important as 48% of the French territory is located in a zone of medium or high exposure to the phenomenon of clay shrinkage and swelling, as shown on the map opposite.
What does the soil study include?
With its double purpose, the geotechnical study allows to identify the risks of swelling - shrinkage of claysin a first step and then to adapt the construction to the risks of ground movements. This results in two types of studies :
- the G1-PGC soil study (General Construction Principles) is made mandatory by the Elan law before the transfer of a building plot. It is the seller who is responsible for carrying out the study and for the cost. He must attach it to the deed of sale.
- The G2-AVP soil study (preliminary project) is carried out before the start of the works and allows to define the possible solutions for the foundations. It leads to the G2-PRO (project) to help design the house. The sensitive points identified during the previous phase are studied in order to find an adapted solution.
What are the grounds concerned?
A geotechnical study is required for all risky sites. However, it is important to know if they are located in a high or low risk area. In practice, clay soils concern a large part of the territory, except for Brittany, Vosges, Massif-Central, Pyrenees, Alps, Corsica...
The map opposite of exposure to the phenomenon of differential ground movement allows to identify three distinct zones:
- with high exposure ;
- medium exposure ;
- with low exposure.
GOOD TO KNOW
Mandatory since October 1, 2020, the geotechnical study is required when signing promises and compromises to sell land, as well as contracts for the construction of individual houses.
What is the budget required?
It is important to distinguish between the sale of land and a construction project. In the first case, the "preliminary" study to be provided in case of a transaction represents an average budget of 500 €. In the second case, the so-called "design" soil study to be provided for a construction project costs about 1,000 €. The geotechnical study is valid for up to 30 years if the soil has not been modified.
Christophe RAFFAILLAC