Monolithic and prefabricated tapes are the most popular types of foundation foundations in low-rise construction. This is easily explained by the most optimal ratio of cost and reliability of the design. However, not everyone knows that the degree of foundation reliability largely depends on the depth of its foundation. A monolithic tape can simply burst under the influence of loads if the builders do not take into account the features of the soil and some other factors. To prevent this from happening, let’s try to figure out what the sizes of the tape depend on and how to correctly determine them.
Parameters that determine the amount of laying
All foundation foundations are subdivided according to the level of support location into buried, shallow and shallow. By the name you can understand that each of them is laid at a certain depth, which is determined by:
- type of soil;
- soil freezing depth (hereinafter GP);
- groundwater level (hereinafter referred to as groundwater);
- weight load from the structure.
When determining the mass of a building, one should take into account not only building elements, but also snow cover, engineering equipment, finishing materials and everything inside.
Regulatory requirements
There are only two main documents that should be referred to when performing calculations:
- SNiP 2.02.01-83 - Foundations of buildings and structures (older edition of SNiP II-B.1.62);
- SNiP 23-01-99 - Construction climatology (or the modern, updated version of SP 131.13330.2012).
The first set out all the requirements for the construction of foundations, the second indicates the depth of soil freezing by climatic zones for most large cities.
Data on groundwater and soil type can be obtained only on the basis of hydrogeological surveys on the site. They are carried out by specialized organizations through trial drilling. But, if you are not the first developer, then such information can be obtained (bought, but much cheaper) from neighbors that have already been built up.
The effect of GWA and GP
The depth of the foundation tape directly depends on the level of groundwater and the freezing point of the soil. The regulations indicate that if water is:
- more than 2 meters below the GP - the foundation is buried at 0.6 m and deeper, without a sand cushion;
- from 2 m to GP - the depth of the tape from 0.6 m with the obligatory device of a sand and gravel pillow;
- water above GP - the foundation base with a pillow should be 0.1-0.3 m deeper than the GP.
It should be understood that the height of the sand cushion is not included in the size of the foundation. The exact data on the minimum depth of the foundation tape depending on the GP and soil type are given in table 1 SNIP 2.02.01-83. It can be seen from it that in those cases when the house is built on non-heaving soils and the outer surface of the flax has vertical and horizontal warming, you can not be tied to the freezing depth.
The second table of the same standards indicates the depth of the trench under the strip foundation in houses with cold basements, depending on the level of groundwater and the type of soil.
Calculation of the depth of the tape
At the next stage, you need to understand how all of the above factors affect the choice of the type of foundation. There are only three of them, as you remember.
Unfinished
This type of foundation tape is not able to withstand heavy weight loads and therefore is used very rarely. It is completely located on the surface of the soil, and only a sand and gravel pillow is buried. Therefore, the concept of the depth of laying to this type of foundations can only be applied conditionally. The main advantage of this design is that the heaving force does not act on the side concrete walls.
Unfinished foundation can only be mounted on firm, solid soils. For its device, the topsoil is removed to a depth of 30-40 cm. After that, a mixture of sand with gravel is poured into the trench and well compacted with spilling water. The formwork is installed, the reinforcing cage is assembled and the concrete mixture is poured. The height of the foundation, or rather the base - 50-60 cm, width 30-40 cm. Therefore, the width of the pillow should be 40-50 cm.
Shallow
The shallow foundation perceives loads from heaving and is not as reliable as the buried one. It is placed above the freezing point of the soil. Therefore, the depth of its depth depends on such factors as the type of soil, the depth of freezing and the level of groundwater. Where to get this data was indicated above. Tables of dependencies of the depth of the laying of a shallow strip foundation are given in SNIP 2.02.01-83 and above in the text of the article.
If the groundwater level is lower than the GP, the heaving process usually does not happen. An exception is the presence of a layer of clay, which can retain and accumulate rain and melt water. In any case, even for a one-story house, the rule applies here - the depth of the tape should be at least half the position of the freezing point. And if in your locality GP is at around 1.8 m, then the foundation should be deepened by 0.9 m.
If the water is above the freezing point, and the soil is not particularly dense and stable, then you will simply have to abandon the installation of a shallow base. With a high GW, the foundation tape will have to be deepened below the freezing point by 10-30 cm.
In-depth
This is the most reliable type of strip foundation. The depth of its laying is 100-300 mm below the freezing point. The main condition for its construction is that the tape should rest on dense layers of soil. In case of waterlogging or peat bogs it is necessary to dig a trench below these layers. After this, it is imperative to fill up and tamp the sand and gravel pillow and then put the formwork. The device technology of this type of foundation is described in this article.
Is it possible to reduce the depth of laying
It often happens that the calculations performed show the need for large sizes and the cost of erecting the tape becomes too significant. Especially if the depth of the strip foundation for a two-story house is calculated. Then the question immediately arises, how to reduce the overall dimensions of the foundation structure.
There are three options:
- reduce GP;
- lowering of GWL;
- buffer pad device.
It is impossible to change the climate, but using a heater, you can reduce the depth of freezing of the soil. To do this, the outer wall of the tape is covered with polystyrene plates. Also, the insulation must be laid under the blind area for its entire width. As a result, the GP will decrease and the foundation can be made smaller.
Groundwater discharge below the water level is carried out by laying an effective drainage system. It must be mounted below the freezing depth of the soil. This is a large amount of earthwork, a lot of materials and the construction of wells, but the effect will be great.
With a large layer of heaving or swampy soils, the foundation will have to be laid deep. To avoid this, unstable soil should simply be replaced with bulk materials. To do this, from the outside of the tape dig a trench about a meter wide and cover it with sand and gravel. A pillow under the foundation also remains mandatory.
Averaged values for different types of buildings
Another indicator that should be taken into account when calculating this is soil resistance. For the main types of soil, it is as follows
- dense dry clay - 1.6-3.0;
- sand of large fractions - 3.6-4.6;
- medium-grained - 2.5-3.6;
- fine-grained - 2.2-3.4;
- sandy loam - 2.6-3.6;
- loam - 1.6-3.0;
- gravel, crushed stone, pebbles - 5.1-6.5.
After that, you need to determine the weight pressure of the building. For basic building structures, these indicators are as follows (in kg / cm2):
- plank walls with insulation - 30-50;
- log - 80-120;
- expanded clay concrete - 460-580;
- brick - 570-970;
- overlapping wooden beams - 120-160;
- basement - 170-300;
- reinforced concrete - 320-550.
By calculating the weight of the building, and comparing the result with the calculated soil resistance, you can determine whether the strip foundation is suitable for your building. It also happens that you have to think about how to make the building easier or choose a different type of foundation. A complete example of calculating the strip foundation we gave here.