One of the important issues at the stage of designing a house is the calculation of the foundation. The durability and reliability of the building will depend on how well it is made. When calculating it, they take into account the type of construction, the material of the future home, the characteristics of the soil, and climatic conditions.
The most reliable thing is to entrust the calculation of the foundation (for a house with three floors) to a design firm. However, if the building is planned no higher than two floors, then it is quite possible to do the project yourself.
How to choose the right foundation for your home?
There are several types of foundations that are used in private construction. They depend on what material the house will be made of, as well as on the type of soil in the area. The material of a building determines its weight. The larger it is, the stronger the base should be.
Besides the weight of the house, the bearing capacity of the soil is the decisive factor. The higher its density, the greater the load it can carry without deformation. Therefore, determining the type of soil will help to correctly approach the choice of design and foundation material. ATthe calculation also takes into account soil properties such as granularity, mobility, moisture.
The third factor influencing the type of foundation will be climatic conditions. They are associated with seasonal loads from snow on the roof, which increase the mass of the house, as well as the depth of freezing. The latter determines which design, as well as the amount of penetration, should be applied.
How does the foundation depend on the soil?
Before starting the calculation of the foundation of the house, a study of the soil is carried out. Its purpose is to determine the maximum load that can be applied to a unit area. The degree of heaving is also considered. This indicator indicates how much the foundation will be pushed out of the ground as the soil freezes. Also - the higher this indicator, the longer shrinkage will occur. Some types of foundations cannot be used on moving soils. The wrong choice will lead to the destruction of the house.
Soil types
There are the following types of soils:
- Rocky and stony soils. These varieties are the most durable. They are able to withstand any load with minimal shrinkage and are not subject to heaving, due to the lack of water in them. Ideal for any type of foundation.
- Cartilaginous soils. They consist of a combination of sand rock, stones and clay. Water also does not linger in this type of soil, so heaving does not occur in winter.
- Sandy soils. Well suited for construction. Do not accumulate excess moisture, butfreeze in winter to a depth of 1 meter, depending on the region.
- Clay. It accumulates moisture and is poorly released from it. Therefore, it freezes heavily in winter. When calculating the cost of the foundation, you need to include additional costs for a sand cushion under the foundation. Excess water will drain through it. If this is not done, then the bottom of the foundation will get wet, and when freezing, the concrete will begin to collapse. Freezing of clay soils reaches 1.5 meters.
- Loam and sandy loam. This is a kind of mixture of sand and clay. Depending on the ratio of clay to sand, the soil can both retain moisture inside and pass it through itself. Therefore, how strong the heaving will be in this case, you need to look at the ratio, as well as how cold the winters are in this climate zone.
- Peatlands and swampy soils. The calculation of foundations for this category begins with a mandatory geodetic survey. Such soils are characterized by a close occurrence of water and have a low bearing capacity. In addition, they are heterogeneous, which increases the risks of construction. During the construction of the foundation, a lot of additional work is carried out in the form of compaction of the soil with partial replacement of peat with sand backfill, arrangement of drainage slots. The most justified on such soils is the construction of monolithic tiled and pile foundations.
Pile foundations
This type of foundation is used for various purposes. Piles are used for the construction of light frame houses. There is no heavy load on the foundation, soalong the perimeter of the future house and in place of wall partitions, poles are screwed into the ground, which will serve as the basis.
Another option in which piles are used is construction on moving soils. For example, on marshy soils. Reinforced concrete piles are used here, which, by driving or vibrating, are sunk into the ground to more stable and durable layers.
How to calculate the pile foundation?
When calculating a pile foundation, the section of piles, the step between them, their length are determined. Of these three components, only the cross section is known in advance. The remaining indicators are calculated based on the following parameters:
- Load on grillage. This parameter includes the total weight of all possible loads, including the weight of the house, furniture, equipment, snow load, living people.
- Load on a single pile. It will depend on the bearing capacity of one element without destroying it.
The weight that will fall on one element of the pile foundation can be found by the formula:
P=(0, 7 • R • S) + (u • 0, 8 • fin • li), where:
- P - allowable load that will fall on one pile;
- R - soil strength, which is determined after a geodetic survey;
- S - cross-sectional area of that part of the pile with which it rests on the ground;
- u - the perimeter of the pile itself (found by the circumference formula);
- fin - friction force that occurs from the side of the pile walls (taken from tabular data);
- li -the thickness of the soil layer on which the pile will be twisted (selected from the table, depending on the bearing capacity of the soil);
- 0, 8 is the ratio.
After the possible load on one pile is calculated, knowing the total weight of the house, you can determine the required number of piles and the step between them: l=P/Q, where Q is the weight of the building per meter of foundation.
How to determine the weight of a house?
Before you make a calculation of the foundation of any type, the total weight of the house is determined. Knowing its design and the materials of which it consists, you can calculate the mass using the following table.
| Building element and material of manufacture | Specific gravity (kg/sq.m) |
| 1 brick wall thickness | 684 |
| Brick wall 1.5 bricks thick | 918 |
| Wooden wall 200mm | 100 |
| Wooden wall 300mm | 150 |
| Wall of a frame house with insulation thickness up to 150 mm | 30-50 |
| 80mm plasterboard walls | 27, 2 |
| Insulated drywall walls 80 mm | 33, 4 |
| Insulated wooden ceilings | 100-150 |
| Reinforced concreteslabs 220 mm thick | 500 |
| Roof cake coated with the following materials | Specific gravity (kg/sq.m) |
| Curfboard and metal tiles | 60 |
| Bitumen tiles | 70 |
| Ceramic tiles | 120 |
| Loads from equipment, furniture and people | 150 |
| Snow load | Depends on roof geometry, material, region. |
Pillar concrete foundation
The main material for the construction of the foundation is concrete. It allows you to make bases in the form of pillars cast in the ground. Before pouring, a hole is drilled in the ground. Then a frame of steel reinforcement is installed in it and poured with concrete. Formwork of the required height is made above ground level. Under the building, several similar pillars are installed in increments of 1.5–2 meters. Such a foundation is used on stable soils that are not subject to heaving.
Strip foundation
This variety, in turn, is divided into deep and shallow foundations. The difference between them is that the first one is used for heavy houses, as well as in cold regions, where there is a large depth of soil freezing. The shallow version can be used for low-rise buildings, as well as onsoils of high strength. It is not recommended to use on sandstones and loams. It may crack due to uneven shrinkage. The calculation of the strip foundation is carried out depending on its occurrence. The shallow one is located at a depth of 0.5–0.7 meters. The deep band is below the freezing depth.
Monolithic Foundation
This foundation is made in the form of a solid slab, which is cast over a sand cushion. This variety may be the only option for building on unstable soils. In addition, the monolith is not subject to heaving. The only thing that needs to be observed is a perfectly leveled surface on which a sand base is poured. If this is not done, then over time, the pillow will begin to slide down along with the house.
Calculation of the foundation of the house: step by step instructions
First of all, you need to find out what parameters you need to calculate. Firstly, it is the width (for a strip foundation), the number of piles (for a pile foundation). Secondly, the amount of concrete required for pouring.
For example, let's take the calculation of the strip foundation and determine: will the planned width of the strip allow it to withstand the weight of the house? For this you need:
- Find the perimeter of the foundation. Fold all the sides of the house and add the length of the internal load-bearing walls.
- We calculate the concrete for the foundation: we multiply the resulting value by the width and height. This is how we find the required volume.
- We multiply the volume by 2500 (specific gravity of concrete). ATthe result is the weight of the concrete structure.
- We perform the calculation of reinforcement for the foundation and add the result to the previous one.
- The weight of the house is added to the resulting value, then the result is divided by the area of the foundation. This is the pressure that the building will exert per unit area.
- Then, it is checked against the table with characteristics whether your type of soil can withstand such a load. If not, then the width increases and the calculation of the foundation is done again.
After determining the required volume of concrete, the cost of work and materials is calculated. The calculation of the cost of the foundation depends on the market price of the brand of concrete required for construction, on the required amount of reinforcement, as well as transportation costs for the delivery of materials to the construction site.
How much reinforcement is needed for the foundation
Concrete structures greatly increase their bending strength if a metal frame is used inside. Depending on the type of foundation, bars of different diameters are used. When calculating the reinforcement for the foundation, the weight of the house is taken into account. Dense soils reduce the deformation of concrete under the weight of the structure, which affects the reduction in the amount of reinforcement.
For the strip foundation, bars with a diameter of 10–16 mm are used. They are placed in the formwork in increments of 10-15 cm and are interconnected by transverse bars, which are fastened with a knitting wire. As the foundation experiencestransverse load, then the main impact falls on the longitudinal bars of the reinforcement. In this regard, transverse reinforcement can be used with a smaller diameter.
For a monolithic tiled foundation, thin reinforcement from 10 mm or a reinforcing mesh is used. If the plate is made on unstable ground or the weight of the house is large, then the diameter of the bars increases. Caracas is made in the form of a grid with a step of 20 cm. If high strength of the plate is not required, then the reinforcing bars can be replaced with a grid.
