As a rule, specialists begin designing a pile-screw foundation based on the number of piles, choosing their parameters and placing them on the diagram from the outside and inside of the house. Also, to guarantee quality work, it is necessary to carefully measure the distance between the piles. The whole process can be performed independently, especially since there is a simplified calculation of the foundation using piles.
Calculation Methods
The main elements of this type of base are screw piles made of metal, the length and diameter of which may vary depending on the technological requirements and the expected load that they will have to withstand. In specialized companies and design offices, you can order a professional complex calculation of a pile-screw base, in which various parameters and design features will be entered, as well as the bearing capacity of the soil in the area allocated for the construction of the future house. We suggest using a simplified version and calculating screw piles, based on practical experience in the construction of such structures.
Determination of the diameter of the supports
Screw piles, which are used when installing the foundation for residential and utility buildings, have a diameter of 57, 76, 89 and 108 mm. This parameter is selected depending on how much the finished structure will have:
- 57 mm - used for the construction of the simplest and lightest structures (fences and fences from the netting netting);
- 76 mm - choose as the basis for lightweight outbuildings or fences made of wood or corrugated board. The bearing capacity of such elements does not exceed 3000 kg;
- 89 mm - used where the bearing load does not exceed 5000 kg. This is an excellent solution for the construction of a one-story building (frame or panel), baths, summer kitchens, sheds and massive fences;
- 108 mm - the site is under construction for the construction of a house from foam blocks, a wooden beam, a frame (1-2 floors) with a small weight. The bearing capacity of screw piles of this diameter reaches 7000 kg.
Factors affecting the length of the supports
The strength of the future structure depends on the correct determination of the length of piles, and if these important elements of the foundation are short, the house can sag under its own weight after it is put into operation. The length of piles is determined taking into account the analysis of soil and landscape, namely:
- Soil density.
- The difference in height between different points of the site.
Soil density
Soil analysis is best done based on geological studies of the area. If studies of soil characteristics have not been conducted in a given territory, then a simplified method for determining its density can be used.
So, you need to dig a shallow ditch (up to 1 m) at the bottom of the site. If you see clay mass or sand at such a depth, then the choice is better in favor of piles up to 2.5 m long. If you find rocks with low density (peat), quicksand or groundwater, you will have to continue deepening until you get to hard rocks. Piles are installed here, the length of which is equal to the length of the drill.
Here is a table of the density and bearing capacity of various soils.
Type of soil | Dense soil | Medium density soil |
---|---|---|
Sand (large fraction) | 6 | 5 |
Sand (middle fraction) | 5 | 4 |
Sandy loam (dry) | 3 | 2.5 |
Plastic sandy loam (wet) | 2.5 | 2 |
Sand (fine fraction) | 4 | 3 |
Wet sand (fine fraction) | 3 | 2 |
Clay | 6 | 2.5 |
Wet clay | 4 | 1 |
Loam | 3 | 2 |
Wet loam | 3 | 1 |
The difference between the heights of the site
As mentioned earlier, in order not to make mistakes with piles and correctly calculate their length, it is necessary to determine the difference in height of different points of the site. If you are convinced that such a difference exists and, according to the density of the soil, piles 2.5 m long are suitable, then they must be mounted in the very top row.
Those supports that will be fixed in the lowlands should be longer by the difference in height between the points of their installation. The difference is calculated using a water level or level using a plumb line and a tape measure. With a significant difference in height (more than 0.5 m), it is recommended to add 50 cm to the obtained length of piles for the foundation, since in the lowest places it may not reach up to 20 cm of their height.
Calculation Method
The calculation of the number of screw piles is carried out taking into account the dimensions and weight of the house, which will be installed on the foundation. As a rule, the distance between piles can be:
- up to 2 m, if a structure of aerated concrete and foam concrete blocks or slabs is being built;
- up to 3 m, if it is planned to build a wooden house from timber, logs, etc .;
- up to 2.5 m - also choose for wooden structures. They also work with such piles in regions where there is a large wind load;
- up to 3.5 m - for the construction of lightweight fences and fences.
In order to correctly determine the number of supports for a pile-screw foundation, the following operations should be carried out:
- to draft a future foundation or first level of construction;
- place screw supports on every corner of the future building;
- install piles where the load-bearing partitions of the house intersect;
- between the piles located now it is necessary to install additional piles around the perimeter of the bearing walls so that the distance from one to the other element does not exceed what was fixed earlier (taking into account the weight and type of construction);
- the remaining space for the foundation is filled with piles so that the distance between adjacent supports does not exceed the specified in the calculations (2 - 3 m);
- where the stove or fireplace will be installed, provide for at least a pair of screw supports, again, given the size of the heating structure, otherwise the critical load on the foundation cannot be avoided;
- in case a terrace or any other extension will be equipped, the fixation points of the supporting elements are designated according to the previously agreed principle, taking into account the optimal step distance;
- Now that the distance between the piles is determined, it remains to count all the screw supports applied to the plan.
We calculate the grillage
The pile foundation can be constructed from one support, along which the lower strapping of the structure is laid.
In order for the load on the supports from the weight of the structure to be distributed more evenly, resort to the manufacture of grillage.
A grillage is called a beam or reinforced concrete slab, horizontally connecting the tops of each screw element. Pile and grillage foundations are equally well suited for the construction of wooden and foam block buildings. The tape grillage can be monolithic or prefabricated, the main thing is that it be poured out of concrete, whose brand is not lower than 150.
In order for the grillage to be correctly constructed and create a strong bond between the screw elements, it is necessary to correctly calculate its dimensions. There are a number of special calculations, but we will limit ourselves to the minimum dimensions of the adhesive tape:
- Height - 30 cm.
- Width - 40 cm.
To give the grillage the necessary rigidity, it must be strengthened with longitudinal and transverse reinforcement (in diameter 10-12 mm). The rods are connected using wire according to the principle of armopoyas. The distance from the reinforcement to the edge of the grillage should be at least 2.5 cm so that the metal rods are completely sealed with concrete mortar and not subjected to corrosive processes.
The connection of the grillage with the supports can be rigid when its reinforcement is connected with the piles of the piles, or loose when the grillage without additional garter rests on the supports of the foundation. In both cases, the load between piles is distributed evenly.