The strip foundation is the most popular type of foundation for the construction of buildings for various purposes. But there is another version 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 freezing zone of the soil.
The main feature of this foundation is that such a base allows you to level the frost heap of the soil. This is due to the fact that the structure is quite rigid, but together with the weight of the building it moves up and down depending on the time of year. Such a foundation does not deepen too much, but shifts evenly, therefore, destruction from vibrations does not occur.
Before laying a shallow ribbon foundation, it is necessary to familiarize yourself with the diagram of its structure. The first is a tape, then there is a layer of waterproofing and vertical waterproofing, which can be coated. Next is the reinforcement, the diameter of the rods of which can be 8-12 mm. Another part is the base, after which comes the wall.
Area of use
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 bottom of the base. Among such structures, it should be noted:
- log cabins;
- lightweight brickwork;
- cellular concrete;
- frame-panel buildings.
As for cellular concrete, they are also called lightweight and are represented by gas silicate blocks and foam concrete. If the foundation will have an impressive width, then it will be possible to erect a heavy building from a log or timber on it. In this case, the soil will freeze to a small depth, therefore, there is a possibility of deformation of the structure.
If the construction will be represented by a monumental building, 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 cap, reduces the depth of freezing of the soil in the space under the floor.
What to consider
Before laying a shallow ribbon foundation, the type of soil should be considered. Such a base can be equipped on heaving soil. But biogenic organic lands are not suitable for this purpose, clay, sapropelic soils, as well as peat soils should be included here. In the second case, we are talking about deposits of freshwater bodies of water.
Before starting work, you must also take into account the level of groundwater. The closer they are to the surface, the more unstable the foundation will turn out. It is necessary to pay attention to the load on the base, as well as the height difference. If the relief is characterized by a significant difference, 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 as an alternative solution, the leveling of the area under a shallow structure is advocated. If you take into account the cost of money and time, then both options will be equivalent.
A shallow ribbon foundation is built only after assessing the depth of its laying. It is also important to take into account the climate, or rather the depth of soil freezing. Professionals advise laying at a height that 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
The depth is determined by the height of the groundwater and the freezing line. To calculate the shallow depth of the strip foundation, it is necessary to determine the height above the surface of the soil, which is equal to four times the width. The height above the ground should be less, sometimes it is equal to the depth.
The width is determined by the formula: D = q / R, where D is the width of the sole, and q is the calculated load on the base. The calculated soil resistance is indicated by the letter R.
Cost 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 linear meter. The cost depends on several factors, among them it should be distinguished: height, width and number of jumpers.
For example, we should give a house whose dimensions are 6 x 6 m. The foundation for such a building will cost about 80,000 rubles. If the size of the house increases to 10 x 10 m, then the price may increase to 150,000 rubles.
The main varieties of shallow base
Before you start building a shallow strip foundation, you should ask what types of similar foundations exist today. Among them, one can distinguish a monolithic structure, which is poured on the site, which allows you to get a seamless tape.
You can build a block base, which consists of individual products assembled together at a construction site. The fixing material in this case is cement mortar.
Shallow foundation construction: preparation
Before you build a shallow strip of foundation for the house, you must make room 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 excavation of the trench. The reference point will be the rope.
Use when marking a better laser level. Beacons are clogging in the corners of the future foundation, it is important to check the diagonal. If necessary, beacons are moved. Having departed from them about a meter, it is necessary to make a blind area. A rope is fixed to 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, in-depth strip foundation under a house usually has a depth of 300 mm. The pillow is laid in 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 should start pouring the foundation immediately, otherwise the walls of the trench may crumble, and then part of the work will have to be repeated.
Pillow and formwork
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 well compacted. For the reason that the pillow will be porous, it should be separated from the foundation with a waterproofing film. The basis for this design can be natural soil, but its bearing capacity is lower than sand and gravel preparation.
A shallow foundation strip for the house is poured into the formwork, the installation of which can be done in the next step. It consists of planks that are fixed with props installed in increments of 600 mm. They exclude the destruction of formwork under the weight of concrete. The boards should fit together as tightly as possible, there should not be any differences. This will avoid the work of aligning the surface of the foundation under the cladding.
Reinforcement and filling
Before pouring, it is recommended to pour the formwork with water, then the concrete will lie more evenly along the edges. After every 50 mm of the height of the future base, the solution 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, the reinforcement step can be skipped. But with reinforcement, the base will be more durable.
For the correct conduct 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 metal and protect it from moisture. Next, the reinforcement is knitted, which is then laid in a trench. Then the concrete is poured to the desired height.
A 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, 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. Thirdly, welding should not be used for fittings with a diameter of more than 20 mm.
The main condition is an increase in the rigidity of the reinforcing cage. A shallow base will move depending on the season, so increased rigidity can cause it to collapse. The consequence will be the need for repairs or a complete replacement of the base. In order to eliminate the drying out of concrete and the formation of cracks, the solution is covered with a film and periodically moistened with water.
Thermal insulation
Insulation of a shallow tape foundation can be carried out with expanded polystyrene, which is a universal method. This material has excellent heat-saving abilities and increased moisture resistance. Installation is quite simple, polystyrene foam is fireproof, in addition, it is environmentally friendly and has a low cost.
At the first stage, it is necessary to carry out preparation, 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 apply any other waterproofing material in the form of rolls based on bitumen. It is glued on the side that needs thermal insulation. It is necessary to use bitumen mastic. The resulting layer on top is covered with a dense film or geotextile. This is necessary so that the surface is smooth, and the soil during expansion did not damage it.
At the last stage, you can start laying drainage pipes. After this, the trench is covered with a mixture of sand and gravel. If you build a shallow strip foundation with your own hands, then you can lay the polystyrene foam slabs 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 bitumen coating, which is used as a waterproofing. A great example is bituminous mastic.
How to avoid mistakes
The most common mistake in the construction of a shallow tape is an increase in the height of the structure, which entails an increase in stiffness, while having to increase the number of reinforcement. These costs are unjustified. Timber frame buildings are quite flexible. If you combine them with the same shallow bases, this allows you to get a balanced design, which is not afraid of severe deformation of the soil due to frost heaving.
Whereas if instead of a tape of 40 cm a structure of 80 cm is lined up, this entails obtaining a rigid foundation, which, under the influence of the load and frosty heaving forces, cracks. If, for a basement, a shallow strip foundation for aerated concrete 40 cm is not enough, then the height can be supplemented with brickwork or another monolithic tape, applying a separation in the form of a waterproofing layer. This will ensure that the two tapes slip during bending. The design in this case will turn out to be high, and its flexibility will not be reduced. Reinforcing an additional tape can be much less than the main one.
What substrate to use
A shallow foundation strip for a house made of aerated concrete may have a pillow of non-metallic materials. But it is important to observe the thickness, which is not the same in the documents. If you familiarize yourself with the norms of BCH-29-85, then you will need 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 take advantage of experience when only sand laid in a thickness of 40 cm is used. Sometimes, only crushed stone arranged with the same thickness is used.
Finally
If you plan to build a house of small-format materials like a brick or block, then a shallow foundation will be the best solution. And for a house or a bath from a bar, this solution is very profitable. In addition, construction work can be done on complex 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, a multiple margin of safety can be provided and an exploitable base level can be made.