What should be the depth of the strip foundation?

Builders always strive to make their work easier and cheaper, to reduce wasted time. The complexity and laboriousness of foundation work when choosing a strip type of foundation turns out to be optimal, which has led to its great popularity. However, it is important to take into account all the subtleties and avoid common technical mistakes.

The strip foundation is supposed to be arranged around the perimeter of the house, including under the internal load-bearing walls. Often, such a foundation is built under heavy houses made of natural stone, brick or concrete blocks. But it is also compatible with buildings with reinforced concrete floors. Another advantage of the tape is its suitability for the placement of basements and cellars. It is much more difficult to equip slab structures with such premises, and sometimes it is completely impossible.

Even a general description shows that the depth of the tapes is usually quite large. However, the simplicity of the technology used justifies its use in low-rise buildings and in the construction of ancillary facilities. In addition, tape bases work well even where there is a risk of uneven building shrinkage. This is usually due to the heterogeneous composition of the soil, which has different mechanical characteristics. When constructing a basement, you can use foundation structures in the form of ready-made main walls.

The service life is highly dependent on the material used. So, concrete and rubble stone can work up to two centuries in a row. But a lot depends on:

• the exerted load and its changes;
• the quality of the materials used;
• characteristics of the solution;
• soil properties and climatic parameters of the area.

For the manufacture of the foundation, in addition to concrete and rubble stone, sometimes their mixture or brickwork is used. The tape is made both in the form of a straight contour and with breaks, the geometric shape is a rectangle or a trapezoid. In any case, the width is taken no less than that of the supported wall, and ideally more by 100-150 mm. The wide variety of types of strip foundations does not mean that they can be chosen arbitrarily, there are quite strict building standards.

The construction of a shallow foundation belt under a one-story house is possible even on a cushion of sand and gravel, this helps to save money and speed up work without any risk. But such work can only be done on certain soils:

• not inclined to heaving;
• completely dry;
• characterized by uniform freezing.

Reinforced concrete tape with a shallow deepening under a small private house is made 0.3-0.5 m wide underground, the height of the basement is at least 0.3 m. For the greatest accuracy, work begins with marking, then trenches are dug, the walls of which should be vertically even. The shallow laying makes it possible to do with trenches with a depth of 0.5 and a width of 0.6 to 0.8 m. When the excavations are dug and leveled, a sand cushion of 200-400 mm is made. It is supposed to be rammed, since the denser the base, the less the subsidence of the whole house will over time.

Sand is filled in layers, 150 mm each, it must be moistened before tamping. For the highest mechanical strength, gravel is poured from above with watering with liquid concrete.

To form the formwork, use 2 cm thick boards sanded on one side. Instead, you can take more:

• slate in the form of flat sheets;
• sheet metal;
• plywood.

The reinforcement of the formwork is carried out using spacers and support stakes, it must be verified vertically and horizontally. From the inside, the structure is laid with a dense waterproofing material. In order for the required thickness of this material to be less, it is necessary to select the depth of the bookmark, focusing on the level and movement of groundwater.

The foundation in the form of a tape for a two-story brick house is laid in a pit filled with 0.3 m of sand. Since the house will have to be equipped with bathrooms, it is recommended to add a cement and sand screed up to 0.1 m thick over the water and sewer pipes.

Waterproofing is placed on the frozen screed, but a heat-insulating layer is not always needed. Then comes the frame, created from the reinforcing steel network, then the formwork. Only then can the tape be poured as such. The sole of the base under the house must necessarily go 200-250 mm deeper than the freezing line. Houses made of foam blocks are lighter than brick buildings of similar size.

But this does not automatically mean that you can lay the foundation closer to the surface. We will have to analyze all the parameters that characterize the geological structure of the site. Additionally, we take into account the severity of the floors provided by the project, furniture products, snow load, which may be present on the roof even for a short time. Among the different options for bookmarking in depth, you should choose the one that you can only afford, for material reasons. The soil in different areas freezes by 100-180 cm, and in most cases, they choose a laying of up to 150 cm.

It should be borne in mind that even when using geological prospecting information and SNiP norms in calculations, it allows you to find only the minimum required values.

Trenches are thought out and dug out immediately with a reserve for all the necessary layers of bedding, screeds and additional structures. A relatively light house on a ground that is not prone to heaving is allowed to be placed on a base 600 mm deep, made in the format of a floating tape. Such a structure must be carefully calculated, only this allows one to avoid destruction during movements of soil masses.

A tape for aerated concrete should be calculated no less carefully than for a brick or other heavy material. The lightness of aboveground structures is deceiving; without careful calculations of the strength and bearing capacity of the support, they will turn out to be unreliable. The foundation project should be prepared with maximum buoyancy. For heavier wall materials, it is insignificant, but lightweight aerated concrete blocks are easily pushed out of the soil.

If, nevertheless, the choice is made in favor of the filler support, when calculating, they are guided first of all by:

• the mass of the walls and the pressure exerted by them by 1 linear meter. m;
• the mass of all floors;
• the severity of roofing materials and underlying structures.

The burial depth mentioned in various sources and special literature is by no means the depth of the trench being torn off. By this term, experts understand the gap that separates the soil surface from the lowest plane of the foundation. A tape without deepening is used extremely rarely, since its bearing capacity is extremely low. The minimum deepening occurs more often than deep, but at the same time it is capricious. We'll have to calculate the action of the soil heaving forces.

The depth of laying cannot be less than 50% of the depth of soil freezing.If the level of the ground liquid is high, a depth of 100-200 mm is usually made under the freezing line. An exception is made for rocky soils, gravel or coarse-grained sand. In swampy soil, on peat and similar substrates, the tape will have to be laid below the problematic layers. Sometimes only a trench is enough to a solid mass filled with sand; but such a decision can only be made by trained professionals.

Insulation of the foundation and adjacent soil will help to significantly reduce the required excavation. The organization of high-quality drainage has an important role, it helps to protect against freezing. The sand cushion should be placed both under the belt itself and to the side of it. The best way to solve the problem is a combined approach - a combination of a pillow, insulation and drainage structures.

The midpoint of the bookmark changes depending on whether the house is heated or not, whether it is planned to make a basement. For unheated buildings, a 10% burial reserve is sufficient, and if the building is to be heated, 30% is needed.

Digging a pit for a simple concrete tape placed under sheds, poultry houses and small outbuildings can be from 0.5 to 1 m deep. For most of such structures, except for the most massive, 80 cm is enough. But a residential building, even a relatively small (one-story) one, should be fixed lower, its root is 2 meters. However, the differences are not limited to this. In residential construction, the tape is supposed to be reinforced, which immediately increases its width.

The formwork necessarily contains a lattice made of reinforcing bar. A bundle of rods is achieved through the use of a knitting wire. Strength after pouring is achieved in 28 - 42 days on average. Walls can only be placed on the hardened tape. When building a house with a basement, the trench technique is not suitable, a foundation pit becomes mandatory. If you plan to build a two-story and higher dwelling, you will have to use standard blocks of increased strength; their height is certainly taken into account.

It is not recommended to prepare trenches wider than 400 mm. A total of 0.2 m is laid on the backing and backfilling.Only such layers give a real guarantee against subsidence.

In order for the design to justify itself, the solution is formed only from pure water, the absence of clay and soil impurities in inert materials is achieved, the proportions are strictly observed.

The depth of the strip foundation under the house, the calculation and construction of the strip foundation with your own hands are shown in the following video