Different foundations are built for different structures. The foundation is the basis of the future structure; the service life of the entire structure depends on its design features. However, this basis also requires its own sub-basis. The integrity of the foundation of the future house depends on the type of soil on which it is placed. And in the case when strong stable soils are located under the future building, and when there are loams under the house, a pillow under the foundation is organized.
It is recommended to lay it under any foundation, regardless of its size and depth. It is very important to choose your own foundation for a specific type of foundation, so that it fully fulfills the tasks assigned to it.
A foundation pillow is an artificial foundation made of gravel, sand, concrete, reinforced concrete blocks, which is not included in the foundation structure, but replaces part of the soil under it. The pillow has a number of functions.
Functions performed by the pillow under the foundation.
|Function to be performed||Description|
|Align||Should level the bottom of the trench. It is necessary to set up a foundation for prefabricated and slab foundations When laying a shallow belt under low buildings, this function is performed by a layer of gravel or even compacted sand|
|Protection against ground movements|| Protects the foundation from seasonal ground movements ... |
When backfilling a layer of sand or gravel on weak and heaving soils, the bearing capacity of the soil increases, which ensures a reliable foundation
|Drainage||Prevents moisture from rising from the soil into the foundation and drains rainwater to the side|
|Protection against frost heaving||Preventing the foundation from being pushed out of the ground by the action of expanding water|
|Reduction of building shrinkage processes||Creates a stronger support under the object|
|Increases the stability of the structure||Especially appreciated in seismologically troubled regions|
Separately, I would like to focus on the phenomenon frost heaving , from which the denser one protects, than the ground, the base. This phenomenon is especially typical for clay soils . Clay soils freeze unevenly in winter, increasing in volume to 9% .
Moreover, during the entire winter, clay sucks moisture from the lower horizons of the soil, increasing even more in volume. Moreover, if the groundwater level is high, then the clay can swell up to tens of centimeters and raise a house, a road, and even a railroad bed.
As a result of this effect of frost heaving, the foundation can easily crack in the spring. Will save from the impact of destructive forces a sub-base made of coarse sand, crushed stone or sand-gravel mixture, which performs a shock-absorbing and drainage function.
Let's take a look at the main types of foundation pillows.
Sand cushion is a very common type of backing for tape, especially for one-story buildings due to the availability and low cost of building material. For its device, coarse river or quarry sand is taken.
When calculating the volume of building material, add about 15% for tamping. Sand has its merits:
The sand base is chosen most often in the following cases :
The dimensions and thickness of the sand base are selected based on the type of soil, number of storeys and building material, soil characteristics. The thickness of the sand base for the tape depends on its width.
A layer is poured under the slab with a thickness of up to 15 cm , under piles and pillars up to 30 cm , in addition, it should be wider than the size of the pillars by 15-20 cm around the circumference. On clayey soils, the thickness of the pillow can reach half a meter . In this case, sand is poured in layers of 15-20 cm and each layer is poured and compacted.
Tip! Experts do not recommend making a very thick baseline, since it is difficult to predict how it will behave in extreme conditions.
In addition, for the device of a high-quality sub-base in a place with high GWL you will need waterproofing material , for example, geotextile or roofing material .
The fact is that sand very well absorbs water, which, without waterproofing, will rise through the pores of the concrete of the foundation up into the walls. Geotextiles are laid under the sand layer and on top. The layers laid on it are pre-compacted with a rammer. For better compaction, it is better to water the sand.
The crushed stone cushion is also popular due to the uncomplicated backfill technology and greater strength than the sand cushion.
Crushed stone for these purposes can be taken of two types: gravel and granite . Limestone for such tasks is not suitable because of its low quality .
Gravel is a type of crushed stone obtained by screening quarry or river sand. It is gravel crushed stone that is used in 90% cases for backfilling the basement.It differs by more affordable price compared to granite.
Crushed gravel has a rounded grain shape.
Gravel absorbs less water than sand and withstands heavy loads . Thanks to this feature, the crushed stone pillow can serve as a basement for the foundation of two- and even three-story buildings. In terms of cost, this option will cost more expensive than sand backfill.
Granite crushed stone
For the manufacture of a pillow, a crushed stone fraction 20-40 mm with strength M1200 is taken.
This is a frost-resistant building material, which is especially important. Crushed stone is poured onto the sand bed and rammed. If a thick layer of gravel is required, then it is poured and rammed, like sand, in layers. As waterproofing , liquid bitumen compositions are used, since roll and fabric materials can be damaged by the sharp edges of crushed stone.
Pay attention! A number of experts believe that the use of waterproofing is not justified and can only lead to rise in the cost of construction with a low GWL.
Experience is given as an argument of the method. If you take 2 glass containers and fill one third with water, and then gently pour sand into one and gravel into the second, then the moisture along the sand will rise much higher from the water level, while in the container with gravel it will remain in the same place. This is because gravel does not draw in water as much as sand does.
Quite often builders fill up a combined cushion of sand and gravel. Requirements for building materials remain the same: coarse river or open pit sand without clay and gravel 20-40 mm .
Building materials are poured in layers in the form of a cake in the following order:
Step 1. Geotextiles are laid on the ground with a margin around the edges for wrapping the ends of the sand layer.
Step 2. A layer of sand is rammed.
The layer of sand is covered with geotextiles from above ...
Step 4. The geotextile is poured another thin layer of sand to avoid damage to the fabric waterproofing by the sharp edges of the gravel.
Step 5. 10-20 cm crushed stone, which is rammed up to 5-6 times . The cake is spilled with water and rammed.
Such a sand-crushed stone sub-base is perfect for low-bearing soil. It should be borne in mind that sand cushions themselves are subject to slight shrinkage .
Therefore, special attention should be paid to ramming.
Concrete sub-base is rarely used in private housing construction because of its high cost and labor intensity in pouring. However, it is she who has the greatest strength among all presented. The concrete sub-base is made in the same way as the foundation arrangement, only thinner. In general, the filling can be divided into the following stages:
This makes the strongest cushions under the foundation.
Option support for prefabricated foundations - special finished trapezoidal products. There is a steel reinforcement cage inside them. It is the foundation cushions that are used for the construction of large buildings.
They are designed as a support for the finished foundation blocks.
On the upper part of the product there are eye-loops , designed for easy gripping by a crane. additional items are produced for the main blocks. Advantages of reinforced concrete pillows: versatility , durability and quick installation . As disadvantages, we can note the need to use special equipment to work with them.
In private house-building, blocks are used for the construction of manor-type houses, as well as in the construction of luxury houses.
Before starting work on their installation, the bottom of the pit or trench is leveled or rammed. For leveling, unstable soils can be covered with a sandy substrate.
On the leveled and compacted bottom, blocks are placed close to the end so that a line is formed. The formed sinuses are sealed with a cement-sand mortar. Concrete foundation blocks with dressing are installed on the upper plane.
Unlike sand and gravel cushions, in many cases, concrete is an extension of the foundation. Broadening is necessary when the width of the foundation wall is clearly not enough to absorb the weight from the house under construction. The broadening is performed as an armored belt under the foundation wall or as a shoe under the post.
Such concrete widening is necessary to increase the area of the base on which the structure will rest with all your weight.The larger the area, the more the base can support more weight.
With regard to dimensions, the thickness of such broadening should be not less than 300 mm , but not more than 400 mm .
How such a pillow is arranged:
However, if you add Penetron Admix to concrete, waterproofing is not needed. However, a layer of thick construction polyethylene (or 2, if the base was gravel) should be laid so that water does not leave the concrete.
Pouring concrete is best done once to avoid the formation of seams. Otherwise, you need to perform vertical cuts. The surface is covered with polyethylene to ensure high moisture content of the concrete during hardening.
On sale today you can find a huge selection of ready-made reinforced concrete foundation pillows. Structurally, they are trapezoidal products made of heavy concrete reinforced with a mesh frame. Such blocks optimize the load on the bottom of the foundation and reduce the pressure of the structure on the ground.
In addition, the installation of ready-made blocks reduces labor costs during construction. The foundation cushions are placed on a sand bed and connected with cement mortar.
In the future, foundation blocks are already installed on them.
Experts advise placing a pillow under any tape, regardless of its size and depth ... Any sub-base can be laid under the tape: concrete, reinforced concrete, sand, gravel, combined.
What specific material can be used depends only on the properties of the soil in the construction spot, groundwater level and the budgetary possibilities of the future homeowner.
The most common cushioning method is sand technology , which is most suitable for stable soils with sufficiently low GWL. Thorough layer-by-layer compaction of the sandy base will prevent it from subsidence. Sand is covered in layers of 10-20cm and compacted with a roller or rammer to an approximate density of 1.6t per cubic meter.
Such density cannot be achieved by trampling, therefore it is better to rent special equipment. Sand is usually rolled 5-6 times .
In addition to a carefully compacted sand layer, you can use a mixed material ...
The mixture of sand and gravel should be in the following percentage: 40% sand , 60% gravel with fraction 20-40 mm . Such a substrate does not need tamping. This is an excellent option for a one-story brick and any houses built from light building materials: timber, aerated concrete, etc. In capital construction, it is useless to increase the size of the pillow under the tape. Definitely, concrete should be used as a substrate.
What dimensions will be suitable:
Since a sand cushion is most often covered under the belt, let's consider the stages of organizing work during its construction:
A slab is the most reliable type of foundation, but in most cases a sub-foundation is also needed under it. The choice of material depends on the conditions already listed above: soil composition, percentage of water filling of the soil, etc. The sand pad will help level the bottom of the pit and reduce the load on the base .
In addition, sand is good for drainage - it will drain water quickly.
If there is a high GWL at the site of the construction site, it is worth thinking about arranging a gravel base. It should be borne in mind that the pillow must also perform a thermal insulation function.
We advise you to read our article on the topic - Insulation of the foundation of a house outside with expanded polystyrene, where the advantages of the material are considered, as well as the methods of its independent installation.
There are different pillow cakes under the stove. A waterproofing layer must be placed between the pad and the slab.
In this case, the waterproofing should not be damaged subsequently by the reinforcing parts of the foundation.Therefore, in the complex, the pie under the slab may look like this:
As for the choice of crushed stone or sand, in the case under the slab it is not particularly important.
Crushed stone is poured where there is a lot of it and where it is possible to buy it cheaply. In the Moscow region and the Moscow region, for example, there is a lot of cheap sand and there is no particular need to raise the cost of construction processes. The slab itself is a strong base, and the pillow only serves as a preparatory function.
Thus, a cushion under the base of the house is needed to increase strength of the structure and extension of the term of its operation . For private one-story construction the best option is - sand cushion .
If the house is built with heavy materials, is large and will have more than one floor, the best choice is gravel-sand bed . The most strong type of support is a concrete base cushion or a ready-made reinforced concrete structure for a prefabricated foundation.
Posted By: Work Style |04, Nov 2020
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