When buying a building lot, we often limit ourselves to the visible aspect. Its exposure, its location or its surface will influence your choice. But what you don't see can be just as important (if not more so). A soil analysis will consolidate your project.
The episodes of drought followed by heavy rains have tended to multiply in recent decades. These climatic phenomena lead to settling and other ground movements that can jeopardize the solidity of buildings. In order to build on good foundations, in regions particularly exposed to these risks, the Elan law requires, since January 1st 2020, a preliminary geotechnical study, provided by the seller of the land. A soil survey will allow you to protect yourself against major damages and losses. Not to mention the cost of putting everything back in order.
Prevention is better than cure
Due to climate change, bad weather, often violent, is becoming more and more similar. Periods of drought follow those of soil rehydration. The grounds, especially those with a clayey tendency, suffer the consequences. And the buildings built on them too. The appearance of cracks and fissures, weakened foundations, possible rise in the water table, the presence of underground cavities... Nearly 20,000 claims are recorded each year for damages of between 30,000 and 40,000 €. When it is not the obligation to destroy the construction in case of proven danger. So it's better to know what you're up against before buying a piece of land to build your house on! In order to identify and limit the risks of damage, the Elan law requires the seller to carry out a soil study before selling the land. But this obligation is not general and is limited to certain geographical areas defined as being moderately or strongly exposed to the phenomenon of clay shrinkage and swelling (RGA).
The geotechnical study (called G1-PGC) must be annexed to the preliminary sales agreement or to thedeed of sale. It will allow to evaluate the risks at the scale of the land considering the geotechnical constraints and to define the general principles for the construction.
If you go through a builder via a Contract of Construction of Individual House (CCMI), he will have to provide you with a geotechnical study called "of conception". Indeed, the law specifies that a professional builder cannot build a house without taking into account the good or bad quality of the soil. This study will take into account the precise design of the house to be built (layout and characteristics) to define the provisions and construction rules to be respected. The cost of the soil study is the responsibility of the builder and he must ensure the material and financial burden.
Are you concerned?
To know if you are in a zone subject to the obligation of a geotechnical study, consult the map available on the site georisques.gouv.fr
It allows you to know if your land is in a risk zone, according to three levels: low, medium or high exposure.
Whatever the zone where you are located, you are not required to carry out a geotechnical study forextensions (garage, veranda....) whose surface area is less than 20 m² or work that does not affect the foundations of the house or the water flow system.
Thorough analyses
The soil study, carried out by geotechnical experts, follows a well-established protocol, analyzing the local context in which the house will be built.
The geotechnician will begin by studying the geological maps at his disposal and the "history" of previous construction sites. Then, by going to the site, he will make a visual reconnaissance of the place. The ground will be dug to know the homogeneity of the ground and possibly detect the presence of water.
The expert will then measure its dynamic resistance. The samples collected will be analyzed in the laboratory to determine the clay and water content of the soil.
Once the study is finalized, a report will be given to you within two to three weeks. It will contain all the geotechnical prescriptions to build safely. The file will usually be illustrated with maps and will specify the description of the existing risk on the site, the type of foundations recommended, the possible drainage needs... You can then easily estimate the cost of the foundations, if the ground requires particular works to obtain a stable building.
If you sign a CCMI, you will have to give this document to your builder who will then check the feasibility of your project and adapt the foundations to the nature of the ground if necessary. The building site will be carried out in complete safety, from the structural work to the finishing touches, without fearing that the ground will not support the foundations or that it will slip away as soon as the first pickaxe is turned.
How much does it cost?
The cost of a soil survey before construction varies according to several criteria (surface area and percentage of slope of the land, nature of the project, presence of an existing building...). Generally, a soil survey represents less than 1% of your total construction budget.
Marie-Christine Ménoire