Foundation pillows: what are they and what are they needed for?

The foundation of a building is one of the main structural elements responsible for the overall stability of the structure and the duration of its service life. The foundation of any structure experiences serious weight and vibration loads, which are associated with the mobility of the soil, the number of storeys and the peculiarities of the operation of the structure. In order to increase the strength and durability of the structure, a pillow is laid under the foundation, which multiplies the operational characteristics of the object.

The foundation cushion is an artificial foundation that is laid on top of the sand dump of the excavation and is formed from various materials. A foundation cushion has a number of important functions.

1. Alignment. This purpose is one of the main ones and consists in the elimination of defects in the bottom of the excavation, arising after the work of construction equipment. For such purposes, a sand cushion is usually used. In the case of laying a strip cast foundation, the equipment of the cushion is not a prerequisite: it is enough to pour in a concrete solution, which will effectively fill all existing voids and cavities, and will level it without using additional measures.
2. Haveless load on the ground. The foundation cushion protects the soil from movement and subsidence, acting as a compensator and taking the weight load onto itself. Through its formation, weak and heaving soils are replaced with sand, as a result of which the bearing capacity of the soil is significantly increased and the strength of the base is ensured.
3. Drainage function. When arranging a sand pillow 30 cm thick, the capillary permeability of the soil is disturbed. This leads to the impossibility of raising moisture from the ground to the foundation, as well as to the removal of excess moisture from precipitation to a lower, relative to the foundation, level.

The device of the foundation cushion is regulated by SNiP and HPES, therefore, when forming it, it is important to take into account all the requirements and standards. So, when building a reinforced concrete cushion, they are guided by GOST 13580, adopted back in 1985, but still relevant. The document regulates the standards of arrangement and the assortment of concrete cushions for the foundations of industrial and civil facilities, involving the presence of heavy walls.

Such blocks must strictly comply with all GOST requirements, and have high strength, resistance to seismic loads, extremely low temperatures and corrosion. The concrete pad used in the strip foundation significantly increases the width of the support base, which, in turn, leads to a decrease in the specific load per unit area. This makes it possible to consider the foundation pad as the main structural element that takes over the entire weight load. That is why, when building a foundation, it is important to strictly observe all the necessary norms and standards.

Foundation pillows can be made of various materials, the choice of which depends on the number of storeys, the state of the soil, seismicity and the purpose of the structure.

The most inexpensive building material used when arranging pillows for light frame or wooden one-story buildings. With a layer thickness of 25–30 cm, sand reliably protects the base of the house and acts as a compensator in the event of soil shrinkage. When making a pillow, both river and quarry type of sand can be used. The main advantages of using this inexpensive material are its availability, ease of installation, low thermal conductivity, which significantly reduces the heat loss of the building and high rates of compaction.

The disadvantages of sand include the impossibility of its use in the construction of multi-storey buildings and industrial enterprises, as well as the irrationality of its use on soils with a high occurrence of groundwater. In cases where the aquifer level is unstable and experiencing seasonal fluctuations, a drainage system should be provided before building a pad.

This option is the most common for the formation of pillows, it is widely used in the construction of private houses and summer cottages. Such a pillow is formed on weak soils and needs careful compaction. The mixture must have a medium-grained structure, the use of sand dust or fine-grained sand is not allowed.

Quite durable material that evenly distributes the weight load over the entire area of ​​the base. It is used to form foundation cushions in houses not exceeding two floors. To form a crushed stone layer, sand and gravel are used as auxiliary components, the proportion of which must be at least 30% of the total thickness.

It is the most expensive and reliable material used in the construction of multi-storey residential buildings and industrial buildings. The concrete pad acts as a compensator for the reaction of problem heaving soils and takes on the entire weight of the structure. A concrete pad is a block of reinforced concrete marked FL, which means a tape foundation

The use of this material as a pillow in the construction of multi-storey buildings presupposes the presence of large-fraction elements in the composition, the size of which varies from 2 to 4 cm.

Each of the materials used to create a foundation cushion has unique technical characteristics or combines several properties inherent in its constituent components. So, when forming a crushed stone layer, sand and gravel are mandatory components, without which the functioning of the pillow will not be complete. Therefore, when choosing the right material, it is necessary to take into account the totality of all factors that affect the strength and durability of the foundation.

Installation of some types of foundation pillows can be carried out independently without the use of expensive equipment and the involvement of specialists. The most effective, but at the same time inexpensive way of arranging the interlayer is the installation of a sand and gravel cushion, the installation technology of which involves several stages.

Initially, you should dig a trench, the depth of which will correspond to the beginning of the dense soil layer. Then, coarse river sand should be poured into the prepared ditch. Falling asleep should be done in small portions, gradually forming uniform layers 15 cm thick, and alternately spilling them with water and tamping them.

Layers of gravel are placed between the layers of sand. Their thickness can vary from 5 to 25 cm, and depends on the type of soil and the number of storeys of the house. It is recommended to equip a sand and gravel cushion under the entire structure: this will ensure uniform shrinkage of the building and prevent cracks on the walls. The width of the pillow should protrude 30 cm beyond the foundation.

An important condition for arranging this type of interlayer is a particularly careful compaction of each layer. The density of the newly poured foundation for the foundation should be 1.6 g / cm3 relative to the density of the soil.

A crushed stone pillow is made in the same way. In this case, the two lower layers are sand and gravel, on which a layer of crushed stone with a thickness of 25 to 30 cm is poured from above. The horizontalness of the laid layer should be checked using a building level. The dimensions of the pillow must exceed the dimensions of the foundation by 50 cm.

The arrangement of a sand cushion is quite simple. To install it, you first need to make a markup and extract the soil to the required depth. Then, at the bottom of the dug trench, you need to lay geotextiles, which will serve to protect the foundation from moisture. Then you can start filling the drainage layer, for the formation of which coarse sand is suitable.

The quality of the ramming can be checked as follows: Step on the pillow with both feet and then step off and check for footprints. Traces should not be visible on properly compacted sand. Otherwise, work should be resumed and a high density of the created pillow should be achieved.

The calculation of the thickness of the main layer is made individually and depends on the area of ​​the house and the material for building the walls. In any case, the total thickness of the sand cushion should be at least 15 cm. Upon completion of the installation work, you need to check the horizontal position again, after which you can start building the formwork and pouring the foundation.

Before proceeding with the choice of material for creating a pillow, it is necessary to consider a number of points.

• When building a strip foundation, the thickness of the interlayer should not be less than 25 cm, and the width should exceed the dimensions of the strip by 10 cm on both sides. The process of compaction of sand and gravel layers is best done with a vibrating plate. At the end of tamping, the pillow must be carefully leveled and a layer of waterproofing should be placed on it.

• During the construction of a monolithic foundation, the cushion is arranged over the entire area of ​​the excavated pit. To do this, its bottom is leveled and covered with rubble or gravel. The layer thickness should be 15 cm. 10 cm of sand is poured on top of the crushed stone, after which the whole cake is carefully compacted with a vibrating plate. Further, the layer is waterproofed and the installation of the foundation begins.

• For columnar and pile foundations, it is recommended to equip a sand and gravel cushion with a thickness of at least 30 cm.Its width should be greater than the pillar size by 20 cm. From above, the cushion must be covered with polyethylene or roofing felt, which will prevent it from saturating moisture from the poured concrete solution.

• For two- and three-story private houses, a homemade concrete pillow is sometimes used. To do this, gravel is poured into the bottom of the pit with a layer of 10 cm and tamped well. Then the formwork is installed, 30 cm high. To strengthen the structure, you can carry out reinforcement from rubble or reinforcing rods. Next, you should prepare a concrete solution and pour it into the prepared formwork.After the concrete has completely hardened, it is recommended to waterproof the pillow.

Foundation cushions are an effective solution to the problem of soft soils in the construction of buildings and structures. They help to form a reliable foundation, strengthen the base and significantly extend the life of the structure.

In more detail about how to make a foundation pillow with your own hands, the following video will tell.