Invisible but… fundamental, the foundations support the weight of your house and ensure its adaptation to the terrain. But beware: they must be carried out in the rules of the art to ensure the sustainability of the work.
Foundations are fundamental! And for good reason: they ensure the solidity of the house. They guarantee the stability of the construction over time by distributing the loads in a coherent manner. Poorly designed and poorly made, they can generate damage (cracks for example), the most serious of which can make the house uninhabitable. Fortunately, there are many ways to avoid these problems, which are rare in construction. From the field visit to the different types of foundations, including the soil study and the guarantees that are due to you, we give you the main keys to building on solid foundations.
1- Foundations: where to start?
Your objective: to obtain the maximum of information at the research stage. Certain indices can indeed inform you about the nature of the ground, which conditions the type of foundations therefore the price of the house. Cracked walls or tilted trees, for example, may indicate that the plot is unstable. A survey of neighbors or the town hall can give you information.
The opinion of the pros of the house. You can also consult the manufacturers. “They know their area of activity and can usefully inform individuals,” says Jean-Michel Bournerias, who runs Terre et Demeure, in Ile-de-France. Another track: the georisques.gouv.fr site. Thanks to its interactive tools, you will know if your electoral district is exposed to geological risks. Other information concerning the land is available on the websites www.construiresamaison.com and www.qualiteconstruction.com.
© Yvelines tradition
2- Field: what are the risks?
If your soil is clay, it is exposed to the risk of shrinking during periods of drought and swelling in the event of heavy rain. This can destabilize the foundations and cause the walls to crack. Some soils have low resistance; this is particularly the case if they are backfilled. You will also have to take the slope into account, knowing that the more steep it is, the more expensive the adaptation work will be. Also pay attention to natural cavities (limestone soils for example) or artificial ones (underground galleries). In seismic zones, specific building rules must be followed to reinforce the foundations.
3- Foundations: how to identify risks?
You can have a preliminary geotechnical study called G1 carried out. It determines the characteristics of the site, identifies possible problems and makes it possible to draw up a first draft of the foundations. For more precise data, it is better to carry out a soil study called G2 Mission Avant-Projet (G2 AVP, standard NF 94-500). “Carried out at the preliminary project stage, it studies the possible construction and adaptation principles of the project on the ground and provides the dimensional outline of a typical profile for each geotechnical structure”, indicates the office of Fondasol studies.
A precise study. At the project stage, a G2 PRO soil study may be carried out. “It provides an updated summary of the site, the execution methods for geotechnical structures and the associated threshold values, as well as the optimized design calculation notes for all geotechnical structures and for all construction phases. It allows an approach of the quantities / deadlines / costs of execution of these works. If necessary, additional investigations are carried out, ”specifies Fondasol.
4- Soil study: how to proceed?
Once owner of the plot (or with the written agreement of the seller if you have not yet signed), you will call on a geotechnician. He will carry out core drilling where the house will be located, which allows him to know precisely the nature of the soil. You will communicate this soil study to the builder. The latter will then have a concrete study carried out by a specialized design office. And the type of foundations to use? so the cost of this work ?? can be fixed. Count three to four weeks of delay and a price of 1,000 to 2,500 ?? depending on the slope of the land, the area of the house, the proximity of the road.
5- Is the soil study compulsory?
Yes, but only in clay areas subject to swelling shrinkage phenomena. On these sites, the seller of a piece of land must provide a soil study to his buyer. This measure which reinforces the security of buyers is taken within the framework of the Elan law of November 2018. Please note, however: its implementing decrees have not yet been published. They must be published before the end of 2019.
Soil study: why do it. At the time of writing, it must be recognized that the soil study is not compulsory. But it remains highly recommended. With it, the risks concerning the solidity and the durability of the structural work are particularly reduced. Also, don’t forget that in the event of a claim, the insurer will ask you for the soil study. Another advantage: if you do it, you will get a discount on your damage insurance. Mandatory, the latter facilitates the implementation of the ten-year guarantee (see at the end of this article).
6- What type of foundations?
If your land does not present any problems, the house will rest on shallow foundations (80 cm in general) on continuous footings. Trenches are dug, depending on the shape of the house. Reinforced concrete is poured onto it, on which concrete blocks will be placed. It is also possible to install a raft. This concrete slab, to be used on a lower quality floor but without major problems, distributes the loads evenly. Once these works are ready, the low floor is laid.
© Armature Valoise-Standarm
7- What if the terrain is difficult?
If the “good soil”, the one that will support the weight of the house, is located below an unstable layer (backfill), foundations on wells will be used. We dig holes 80 cm in diameter and 2 to 4 meters deep to reach the solid layer. It takes nine for a house of a hundred square meters. A metal structure (reinforcement) is introduced there and then, concrete is poured. Reinforced concrete beams, stringers, connect the wells. It is on these that the interjoists supporting the insulation will be installed, all covered with a self-compacting concrete slab.
Lift of the ground. Please note: Some plots, with loose soil, offer practically no lift. In this case, micropiles are used. It involves drilling holes 6 to 30 m deep depending on the case. They are filled with concrete and contain, in their top part, a steel reinforcement. This is where the sills are placed (concrete beams) intended to receive interjoists, insulating boxes, waterproofing membranes and other self-compacting concrete slabs.
8- Termite humidity, frost: what to do?
Some basements are damp. To protect the house from infiltration, the builder will take care to install a casing. If the premises are habitable (playroom, home cinema for example), an industrial membrane will be used ensuring the waterproofing of the foundations. For cellars and garages, a mortar and waterproof coating will be necessary. If the land is in a termite area, it will be subject to special treatment to eliminate these wood-eating insects.
Guard against frost. A soil is said to be frost-hard when the water it contains, rising by capillarity, freezes in winter. If your foundations rest on such a soil, the water, freezing, will increase in volume. With a risk of cracks in the key. This drawback is overcome by the frost guard. This term designates the depth level of the foundations from which they cannot be subjected to freezing and thawing. The frost guard varies from one region to another. In the plain, there is a minimum of 50 cm depending on temperate climate and up to 1 m in severe frost (mountains). The construction of the foundations is specified in the Unified Technical Documents 13.12 and 13.3 of the Scientific and Technical Center for Building (CSTB).
9- Is there a crawl space?
If the ground is solid, well drained and coherent from its surface, the house can rest on a paving on the median. It is a simple slab that covers the earth. But this technique is much less effective than the crawlspace. How it works ? “It is the raising of the ground floor floor,” says one at Maisons Pierre. From the foundations, a wall of three or four rows of concrete blocks is laid. The struts connected by chaining are placed there. Between the sleepers, the insulating interjoists are installed.
The advantages of the crawlspace. “This work leaves a volume of air between the land and the construction,” says Pierre. It effectively combats humidity in the house and acts on soil compaction. This will prevent cracking of tiles and partitions. »Other advantages of the crawlspace: it adapts to all terrain configurations, it allows pipes to pass easily. In short, it only has advantages. It is not for nothing that 80% of the houses built in 2018 use this solution according to an EDF study of 80,000 new houses.
10- Foundations: what guarantees?
If you use a professional working under the construction contract? 1990 law, you benefit from a very protective framework. If, after signing the contract, it turns out that special foundations are necessary, the additional costs are borne by the manufacturer. You will sign an amendment to the contract to specify the technical changes, but no additional amount may be required. With other legal frameworks (architect, project management, business or self-construction contract, it is the individual who bears the additional costs.
Ten-year warranty and damage insurance. Once the work is finished, the foundations are covered by the ten-year guarantee. To facilitate its implementation, you must necessarily take out damage-to-work insurance (article L 242-1 of the Insurance Code and L 111-30 of the Construction Code). When a disaster occurs, the insurer compensates you and then turns against those responsible. Individuals who do not take damage-work will have to prove the responsibility for the culprits. Which is as long as it is complicated. Even impossible if the offending company has closed shop. And then, not taking this insurance can completely block the project. Please note: the loan is very likely to be refused if the individual building the building does not take out the damage.
The price of foundations. Difficult to precisely quantify the foundations, each project falling on a case by case basis. Note, however, that special foundations can add to the house budget. The bill, in this case, can reach 15 or 20,000 ?? more than the original budget. To avoid this pitfall, it is better to carry out a soil study and sign a Construction-law contract of 1990, a legal framework which secures your project on the technical and budgetary plans (see elsewhere).