Strip foundation is the most popular type of foundation for the construction of buildings for various purposes. But there is another kind of it, which is called a shallow foundation. The depth of its laying does not exceed 700 mm, it is designed for arrangement on heaving soils and is located above the soil freezing zone.
The main feature of this foundation is that such a base allows you to level the frost heaving of the soil. This is due to the fact that the structure is quite rigid, but along with the weight of the building it moves up and down depending on the season. Such a foundation does not deepen too much, but shifts evenly, so the destruction from vibrations does not occur.
Before you lay a shallow strip foundation, you need to familiarize yourself with the scheme of its device. The first is the tape, then there is a layer of waterproofing and vertical waterproofing, which can be coated. Next comes the reinforcement, the diameter of the rodswhich can be equal to 8-12 mm. Another part is the plinth, followed by the wall.
Use area
Shallow strip foundation is used in the construction of low-rise buildings for various purposes from materials that will not be able to create pressure on the sole of the base. Among such structures it is worth highlighting:
- log cabins;
- lightweight brickwork;
- cellular concrete;
- frame-panel buildings.
As for cellular concrete, they are also called light and are represented by gas silicate blocks and foam concrete. If the foundation has an impressive width, then it will be possible to build a heavy building from a log or timber on it. In this case, the soil will freeze to an insignificant depth, so there is a possibility of deformation of the structure.
If the building will be represented by a monumental building, then it is better to prefer a monolithic strip foundation. Increasing the width of the tape allows you to build heavier houses, which are complemented by an attic. The impressive width of the tape, hence the plinth, reduces the depth of soil freezing in the space under the floor.
Things to consider
Before you lay a shallow strip foundation, you should consider the type of soil. Such a base can be equipped on heaving soil. But biogenic organic lands are not suitable for this purpose, clay should be included here,sapropelic soils, as well as peat soils. In the second case, we are talking about sediments from freshwater reservoirs.
Before starting work, you also need to take into account the level of groundwater. The closer they are to the surface, the more unstable the foundation will be. It is necessary to pay attention to the load on the base, as well as the height difference. If the terrain is characterized by a significant drop, which is true when the house is built on a slope, the foundation will be problematic. At the same time, a traditional strip foundation is laid, and an alternative solution is leveling the area for a shallow structure. If we take into account the cost of money and time, then both options will be equivalent.
A shallow strip foundation is built only after assessing the depth of its foundation. It is also important to take into account the climate, or rather the depth of soil freezing. Professionals advise to bookmark at a height, which is determined by subtracting 20% from the freezing depth. In this case, you can be sure that the foundation will rise with the building. The minimum depth is regulated by SNiP II-B.1-62.
Calculation
Depth is determined by the height of groundwater and the freezing line. To calculate a shallow strip foundation, it is necessary to determine the height above the soil surface, which is equal to four times the width. The height above the ground should be less, sometimes it equals the depth.
Widthdetermined by the formula: D \u003d q / R, where D is the width of the sole, and q is the calculated load on the base. The design resistance of the soil is indicated by the letter R.
Pricing information
If you want to install a shallow strip foundation on heaving soils, you should ask about its cost. The price will vary from 4,000 to 6,000 rubles. per running metre. The cost depends on several factors, among them we should highlight: height, width and number of jumpers.
For example, a house should be given, the dimensions of which are 6 x 6 m. The foundation for such a building will cost about 80,000 rubles. If the size of the house is increased to 10 x 10 m, then the price can increase to 150,000 rubles.
Main varieties of shallow base
Before you start building a shallow strip foundation, you should ask what types of such a foundation exist today. Among them, one can single out a monolithic structure, which is poured on the site, which makes it possible to obtain a seamless tape.
You can build a block base, which consists of individual products that are assembled together at the construction site. The fixing material in this case is a cement mortar.
Building a shallow foundation: preparation
Before you buildshallow strip foundation for the house, you must free up space for work from all that is superfluous, such as trees and roots. Pegs for marking are installed on the territory. They will allow you to determine the line of digging a trench. The rope will serve as a guide.
It is better to use a laser level when marking. Beacons are clogged in the corners of the future foundation, it is important to check the diagonal. If necessary, the beacons are moved. Stepping back from them about a meter, you need to make a blind area. A rope is fixed on the blind area boards, which will point to the edges of the foundation.
The marking process is completed by digging a trench. It will be a strip of excavated soil. The depth of the pit is determined by the depth of the structure and the thickness of the pillow. A shallow strip foundation for a house usually has a depth of 300 mm. The pillow is laid with a thickness of 200 mm, which depends on the quality of the soil. The depth of the trench in this case will be 500 mm. Given the type of soil, you must start pouring the foundation immediately, otherwise the walls of the trench may crumble, and then part of the work will have to be repeated.
Cushion arrangement and formwork installation
A sand cushion is laid under the foundation, which is made up of sand and gravel. These materials can be mixed, but it is easier to fill them in layers. Each layer is wetted with water and compacted well. For the reason that the pillow will turn out to be porous, it should be separated from the foundationwaterproofing film. The basis for such a construction can be natural soil, but its bearing capacity is lower than sand and gravel preparation.
Shallow strip foundation for the house is poured into the formwork, which can be installed in the next step. It consists of boards that are fixed with supports installed in increments of 600 mm. They will exclude the destruction of the formwork under the weight of concrete. The boards should fit as tightly as possible to each other, while there should be no differences. This will avoid the work of leveling the surface of the foundation for cladding.
Reinforcement and pouring
Before pouring, it is recommended to water the formwork with water, then the concrete will lie more evenly along the edges. Every 50 mm of the height of the future base, the mortar is beaten off, which allows air bubbles to be expelled, which can destroy the structure. If there is no heavy load on the shallow strip foundation for the bath, then the reinforcement stage can be skipped. But with reinforcement, the base will be more durable.
For the correct implementation of such work, you must first fill in the starting layer, which should be 30% of the total height. This will create a flat surface for laying the metal and protect it from moisture. Next, the reinforcement is knitted, which then fits into the trench. The concrete is then poured to the desired height.
Wire is used for knitting. A more familiar approach to this issue is the use of resistance welding. But this method has several disadvantages. First of all,you will need a welding machine and related skills. Secondly, the temperature of the welding arc contributes to the loss of strength of the hardened rod. Third, welding work should not be used for rebars with a diameter greater than 20 mm.
The main condition is an increase in the rigidity of the reinforcing cage. A shallow base will move seasonally, so increased rigidity can cause it to break. The result will be the need for repair or complete replacement of the base. In order to prevent overdrying of concrete and the formation of cracks, the solution is covered with a film and periodically moistened with water.
Insulation
Insulation of a shallow strip foundation can be carried out with polystyrene foam, which is a universal way. This material has excellent heat-saving abilities and increased moisture resistance. Installation is quite simple, Styrofoam is fireproof, in addition, it is environmentally friendly and has a low cost.
At the first stage, you need to prepare, which consists in the need to protect the base with a waterproofing layer. It can be represented by bitumen. It is applied to the wall of the foundation and basement. But you can use any other waterproofing material in the form of rolls based on bitumen. Its gluing is carried out on the side that needs thermal insulation. In this case, it is necessary to use bituminous mastic. The resulting layer is covered from above with a dense film or geotextile. This is necessary in order toso that the surface is even, and the soil, when heaving, cannot damage it.
At the last stage, you can start laying drainage pipes. The trench is then backfilled with a mixture of sand and gravel. If you are building a shallow strip foundation with your own hands, then you can lay polystyrene foam plates using a gas burner. The roll material is heated at several points, and then applied to the wall. Gluing can also be carried out on a layer of bituminous coating, which is used as a waterproofing. An excellent example is bituminous mastic.
How to avoid mistakes
The most common mistake in the construction of a shallow belt is an increase in the height of the structure, which leads to an increase in rigidity, while it is necessary to increase the amount of reinforcement. These costs are unjustified. Frame wooden buildings are quite flexible. If combined with the same shallow bases, this allows you to get a balanced design, which is not afraid of severe soil deformation due to frost heaving.
Whereas if instead of a 40 cm tape a 80 cm structure is built, this entails obtaining a rigid foundation, which cracks under the influence of load and frost heaving forces. If, however, 40 cm is not enough for the basement of a shallow strip foundation for aerated concrete, then the height can be supplemented with brickwork or another monolithic tape, using separation in the form of a waterproofing layer. This will ensure that the two tapes slide when bent. The structure in thiscase will be high, and its flexibility will not be reduced. You can reinforce an additional tape much less than the main one.
Which backing to use
Shallow strip foundation for aerated concrete house can have a cushion of non-metallic materials. But at the same time, it is important to observe the thickness, which is not the same in the documents. If you familiarize yourself with the norms of VSN-29-85, then you will have to lay 30 cm of sand and gravel. In practice, a technique is more often used in which the thickness of these two components is 20 cm. You can use the experience when only sand is used, laid with a thickness of 40 cm. Sometimes only crushed stone is used, arranged with the same thickness.
In conclusion
If you plan to build a house from small-format materials like a brick or block, then a shallow foundation will be the best solution. Yes, and for a house or a bath from a bar, this solution is very profitable. In addition, construction work can be carried out on difficult soils. It allows you to save the construction budget. Such a foundation is in third place after columnar and pile structures. With the help of such a foundation, it is possible to provide a multiple margin of safety and make an exploitable basement level.
