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Clay soils

In the province of Quebec, clay soils can be found in the more developed parts of the territory, in particular in the St. Lawrence Valley, the Ottawa Valley and the Saguenay – Lac-Saint-Jean region.

Houses are commonly built on clay soils. However, since clay soils may be highly compressible, it is important to take the necessary precautionary measures to reduce the risks of foundation settling.

If you must build in an location where soils are clayey, you must take all precautionary measures to reduce these risks of settling.

The items below will help you to learn more on this topic:

Features of clay soils

Clay soils include loams (or silts) and clays, which are defined as fine-grained soils with a dry weight made up of more than 50% of particles sized less than 0,075 mm. They may vary in consistency from hard to very soft.

These soils are characterised by a usually high water content, which generally renders them compressible, regardless of consistency. They are also sensitive to freezing and may undergo volume fluctuations during period of climatic changes such as dry spells.

The majority of houses in the province of Quebec are built with a basement. Their foundations are basically protected against frost. The issues related to settling are usually marginal since the load transmitted from the house to the soIt is largely compensated by the soil removed in way of the basement.

However, foundations built on clay soils having a low consistency, or on organic soils, or backfilled soils, as well as concentrated loads such as a masonry chimney or a foundation made up of a concrete slab poured directly on the soil surface (such as for garages), must always receive special attention.

It is therefore necessary to verify the soil conditions at the time of the construction and to take further appropriate steps to avoid or minimize any possible structural damage.

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Have an expert do a reconnaissance

Clay soils, because of their natural properties, can have a low bearing capacity. It is essential to properly assess the pressures which these soils can resist so that the loads from the building may be transferred efficiently to the soil and the effects of any future settling may be prevented.

A simple rule for the sake of caution

If undisturbed clay from a control trench wall gives easily under the pressure of your thumb, this soIt is plastic of soft clay.

As a precaution, we recommend that you refer to a geotechnical engineer (or any expert in soils laboratory), who shall establish the soil’s bearing capacity and express the necessary recommendations to correctly calculate the dimensions of foundations.

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Consider the features of the soil when designing the foundation footings

Hard clays (soil depressing of a few centimeters under a moderate pressure of the thumb) already make up for the serious issues encountered, and in this case it is definitely more cautious to call upon an expert to calculate the foundations. Within this bearing capacity, special, thicker and/or wider footings must be provided to ensure an efficient transfer of all building loads.

In some cases, steel rebars will also be required to strengthen the footings and the foundation. Thus, breakage which might be caused by deformations in the soil may be avoided. It is sometimes impossible to build conventional foundations in such cases, and the builder must incorporate structural cushions or opt for deep foundations, for instance, by using piles.

The width and thickness of the foundation footings are determined by the building’s weight and the nature of the soil. Reinforcement is also as critical for the foundations as the quality of the concrete itself. Omit no details: your care and vigilance will translate into foundations which are well adapted to the soils and a good durability of the building as a whole.

Minimize backfilling work

Backfilling work around a building is also an important aspect of its stability. If the soIt is of low bearing capacity, one must avoid creating major build-ups near the building. These create additional pressures on the soil, and thus may cause settling issues leading to subsidence of the foundations.

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Perform foundation work with care

It is of primary importance, in order to keep the bearing capacity of the soil unaltered, to not rework the bottom of the excavations. Furthermore, one must proceed immediately to the pouring of the foundations footings. It is important to ensure a uniform contact between the building and the soil so as to minimize foundation movements.

If it is unfeasible to immediately pour the footings, it is recommended to pour a concrete bed as soon as possible after excavating. This should keep the soil from being altered by treading, poor weather or some other cause.

In the case of surface concrete slabs, it is preferable to pour the slab on a supporting cushion made of sand or well compacted gravel, and of a thickness of at least 300 mm. One must also insulate the slab to protect it against frost.

If the foundation is frost-prone, such as in the case of a surface concrete slab, it is necessary to take the adequate steps to protect it.

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Respect the specified distances if you must build at the top of a bank

Clayey banks may undergo natural slides under the effect of erosion or during strong shower events. The situation may also be made worse by poor practices such as backfilling at the top of the bank, a concentration of water towards the grade, a load such as that of a building or an above-ground pool, excavations at the foot of the bank, etc.

It is necessary also to comply with the municipal regulations if you wish to build in areas which are prone to landslides. Furthermore, you must avoid any intervention or practice that could alter the original stability of the slopes.

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Consider landscaping to minimize evaporation

It is advisable to set out the landscaping of the lot in such a fashion as to not affect the water content of the soil. The planting of certain tree species may influence the water content of the soil and may cause heaving or contraction of the clay. This situation could be aggravated by longer episodes of dry spells.

It is sometimes advisable also to build a walkway or to install some anti-evaporation geomembrane of 1.5 m in width on the perimeter of the house.

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Opt for simple remedial measures

It is better from the start to prevent the risks of foundation settling when building in an area where the soIt is of a clayey nature, than later having to deal with stabilisation issues. These prevention steps include the following:

  • Foundations set a little deeper and built thicker;
  • Use of reinforcement;
  • Draining of runoff and surface waters further away from the foundations;
  • Installation of a geomembrane at the perimeter of the house, to minimize evaporation;
  • Respecting a minimum distance between house and surrounding trees.

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Your rights of recourse

The rights of recourse will vary according your situation.

If you are covered by the Guarantee plan for new residential buildings

We recommend that you immediately verify the protections and compensations you might be entitled to, and to learn about the claim procedures provided for in the Guarantee plan for new residential buildings.

The Régie du bâtiment du Québec (RBQ) wishes to remind you that the timeliness of the claim is highly important for you to assert your rights in this regard.

If you are not or no longer covered by the Guarantee plan for new residential buildings

The guarantee offered by the plan expires five (5) years after the acceptance of your house or condo.

If your house or condo has been built more than 5 years ago, it is no longer covered by the Guarantee plan for new residential buildings.

If you try to make a claim with the manager of the plan, he/she will likely give a decision based on the fact that the guarantee is expired.

Even though such a decision may be appealed in arbitration, you may also examine your possible rights of recourse with a legal advisor. Such rights include the possibility of appealing to civil courts.

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