The set of rules for foundations and earthworks SP 45.13330 regulates the backfilling of the foundation. The technical norms TR 73-98 contain the rules for sealing the materials used. General principle is to apply inert materials outside, any inside.
The question of how to fill the foundation inside is relevant, both for floors on logs and for floors on the ground. The choice of material and sealing technology depends on the following factors:
- type of operation - in buildings permanent residence heating is year-round, the soil under the sole of the house does not freeze, so you can even sprinkle it with clay, which cannot swell in the absence of moisture and freezing;
- ceiling / floor construction - if the project includes a ceiling on beams, it is cheapest to fall asleep inner part clay, for footing a floating floor on the ground, sand is necessary to level the base at least at the upper level (10 cm minimum layer);
- the height of the basement - for large volumes, it is more rational to use the soil from the building spot, taken out of the trenches, to fill the footing, the very top must be covered with sand;
- GWL level - at high ground water ah, crushed stone filling is preferable, if the aquifer (“perch water”) is 1 m away from the base of the foundation, sand must be used to save the construction budget.
Attention: Clay castles are prohibited outside, although the technology was described in the old SNiP. Clay does not let moisture through, but intensively absorbs, which is dangerous due to uneven swelling, not only when freezing, but also when swelling.
When backfilling, a layer-by-layer (20 cm) compaction of the soil is required.
Do not neglect filling inner space strip foundation cells in the manufacture of floors along the joists:
- the underground is too low for normal operation;
- fumes are inevitably emitted from the soil, harmful to the power structures of the building;
- ventilation products are required, heat losses through the floor of the lower floor increase;
- harmful radon is often emitted from the pits, from which the dwelling will have to be protected with foil materials.
Backfilling before overlapping will solve all problems in a comprehensive manner and improve the quality of operation.
materials
Almost any soil is allowed in the internal backfill, however, large stones (more than 25 cm) must be excluded from them, if necessary, longitudinal drains should be laid, including them in the general contour located around the perimeter of the house (only when high GWL). Depending on the heating mode, GWL, freezing depth, the width of the shell of non-metallic material around concrete structures is:
- heating is permanent - no restrictions, with proper compaction, backfilling with clay is allowed;
- heating is periodic - 20 cm of a layer of sand or ASG is enough for internal walls foundation;
- freezing 1 m, no regular heating - sinuses 20 cm filled with inert material;
- freezing 1.5 m, without heating - 30 cm layer of non-metallic material near the tape;
- freezing 2.5 m - sinus width 50 cm minimum.
The depth of backfilling of the sinuses is considered from the planning mark (usually the blind area), it is ¾ of the depth of the sole of the strip foundation.
Technologies
Compaction of clay, sand, sandy loam and other materials is not required inside the foundation tape in the only case - when making floors along the logs. If it is planned to pour a screed, any of the indicated materials must be compacted to a density of 0.95 units. To do this, you need to use manual rammers or vibrating plates.
You can determine the quality of the rammer visually - as soon as the traces are no longer imprinted on the soil, you can pour a 5-10 cm screed "concrete". It is not recommended to shed sands, sandy loams, loams with water, so as not to saturate the underlying horizons. Instead, the backfill material is moistened to values:
- 15 - 23% heavy soils (including dusty), waterlogging is not more than 1%;
- 12 - 16% - light loams, waterlogging coefficient Kp 1.15%;
- 9 - 14% - light sandy loam, Kp 1.25%;
- 7 - 12% - coarse sandy loam, Kp 1.35%.
You can pour the screed after the soil has completely dried. When filling any monolithic foundation footing is used. This structural element allows:
- reduce the height of the protective layer;
- prevent leakage of cement laitance into the lower layer with high drainage characteristics;
- protect the waterproofing layer of the foundation sole.
Therefore, a small layer of sand on top of loam, sandy loam, crushed stone or clay will further level the base and reduce concrete consumption.
backfill outside
Unlike the inner perimeter, which cannot freeze (with a heated building), the soil adjacent to the outer edges of the foundation is not protected from the cold. It swells unevenly and tends to pull out the concrete structure with tangential forces. The problem is solved by the following methods:
- backfilling of the sinuses of the foundation with non-metallic material (minimum 20 cm sand, gravel shell);
- blind area insulation - 60 - 1.2 m tape around the building pushes the freezing zone;
- sliding and crushing thermal insulation - rigid fixation of high-density extruded polystyrene foam EPS to the outer walls of the foundation, covering with two layers of polyethylene film fixed at the base level, installation of PSB 25 sheets (minimum density of polystyrene) vertically close to the film without fastening (held by sand powder).
When heaving forces occur, soft polystyrene is crushed, rises up along a perfectly smooth film, without harming the underlying layer of thermal insulation. In the spring, structural elements come to original view after the reduction of the soil in volumes.
materials
The possibility of freezing of the soil adjacent to the foundation from the outside is always present. Therefore, despite the insulation of the blind area, the sinuses of the trenches are filled from the outside with sand, ASG or crushed stone, depending on the groundwater level. For normal operation of concrete structures, a shell density of 0.95 units is required, therefore, non-metallic materials are poured in layers of 10–20 cm, compacted with a vibrating plate, hand tools. Spilling sand is not recommended, because. there is a danger of erosion of the lower layers (relevant for silty soils).
Therefore, when backfilling with sand, it is necessary to moisten the material abundantly before laying it in the sinuses. Natural shrinkage takes time, so it is better to rent or make your own vibrating plate, reducing ramming time to a minimum.
With a high GWL or the possibility of its seasonal rise, crushed stone should be used. Gravel material is inferior to this non-metallic product in terms of its main characteristic - flakiness. Therefore, shrinkage during operation is possible, leading to deformation of the blind area.
Technologies
Backfilling the sinuses from the outside with sand or gravel allows you to completely eliminate swelling of the layer adjacent to the foundation. However, all non-metallic materials have excellent drainage properties. Therefore, ring drains at the level of the base of the foundation are a prerequisite for normal operation.
Scheme of ring drainage around the base of the foundation.
When filling the sinuses, it is necessary to ensure that there is no shrinkage during operation. This is possible only by compacting materials with vibrating plates, manual rammers. The maximum effect is observed when preventing the mutual penetration of inert materials and neighboring soils. The technology looks like:
- laying geotextile or dornite on the walls of the sinus;
- backfilling outside with sand or gravel 10 - 20 cm layer;
- compaction with a rammer or vibrating plate.
If a deep foundation tape is being filled up, at a distance of 30 - 40 cm from the surface, horizontal thermal insulation should be laid (5 cm sheets of high-density extruded polystyrene foam), after which work should be continued.
In MZLF tapes, the depth of occurrence usually does not exceed the specified level, therefore, thermal insulation is laid outside along the bottom of the trench by default. Backfilling is done on top of it.
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Laying the foundation of the house is the first and most important stage of construction, but this is preceded by the preparation of the foundation under the entire building spot on the site.
The type of foundation, its design and characteristics depend on the bearing capacity of the soil at the construction site, as well as the strength of the house that will be built subsequently.
Even at the design stage, it is necessary to determine the properties of the soils and determine which sand or crushed stone bedding under the foundation will be relevant as a preparation.
It is somewhat incorrect to raise the question of choosing a backfill for a foundation of sand or gravel. The key to a strong and sustainable home is a solid and reliable foundation, which must meet a number of requirements:
- High strength and density of the soil, capable of withstanding the distributed load of further development;
- Ground water should not linger under the base, therefore the high drainage capacity of the soil is important;
- When wet or dry, the base should not lose its basic characteristics.
- It should not contain organically active components;
- The presence of combustible or plant residues capable of decay is not allowed.
- Cold heaving of the soil is not allowed;
- Uneven shrinkage or deformation is not allowed.
In the course of construction work, the base must not be deformed even under the load of the involved construction equipment or the activity of the builders.
The strength of the surface layer should be sufficient to accommodate all the necessary elements, such as a reinforcing frame, formwork, etc.
bedding device
Since it will not be possible to select the type of soil on the site in advance, you should work with what is in fact. If the soil does not meet the specified requirements, a bedding under the foundation is used from:
- sand;
- gravel;
- sand and gravel mixture (SGM);
- gruss (crushed rock fragment type);
- crushed stone;
- lean concrete.
Since the properties of each of the listed materials are different, as well as the methods of their use, the choice of bedding should be carried out based on the final requirements for the base under the foundation.
The main conclusion: backfilling under the foundation of sand or gravel is needed to adjust the properties of the soil on which the house will be built. It is part of the foundation preparation activities and is not an unconditional component.
Anyway according to the type of soil, the optimal type of foundation is first determined(tape, pile, monolithic slab, etc.) and after that, if necessary, the type of bedding that is required is selected.
Quality preparation for strip foundation or a monolithic slab is to fill the bottom of the pit with lean concrete to level the level and prepare a solid foundation. Sand or gravel is for the most part a cheap alternative to lower overall construction costs.
Sand
simple and enough effective option bedding under the foundation. A well-compacted sand pad is able to take on the same strength and density as the underlying soil, while being easily molded to match all the unevenness of the bottom of the excavation. 
Benefits of backfill sand:
- With high-quality ramming, it is possible to achieve a base strength equal to the initial value for the soil;
- Well fills all the irregularities of the pit and transfers the load evenly;
- Sand retains drainage properties;
- Easily molded and leveled;
- Does not require the involvement of heavy construction equipment.
Flaws:
- Weak mechanical strength of transverse point load.
- Sand is washed away by groundwater over time.
Sand for backfilling is ideal in the case of using precast concrete blocks and slabs, allowing the load to be transferred evenly throughout the base.
Sand for backfilling is selected large and medium fractions without clay inclusions. Even with full ramming, the drainage properties of the base are preserved, and cold heaving has almost no effect on the strength of the base.
The thickness of the bedding can be from 10 to 60-70 cm depending on soil properties. The depth of soil freezing in many regions of the country exceeds 30 cm, and cold heaving can occur even under a well-insulated foundation during prolonged winter cold snaps.
The optimal height for adding sand is considered to be 45-60 cm. It is difficult to compact such a layer of sand at a time, therefore the material is gradually covered with layers of 5 cm thick and gradual tamping and mandatory moistening.
It is quite difficult to determine the required amount of water to moisten the sand. A common mistake is excessive wetting of the sand, from which the entire mass acquires plasticity and diverges more on the sides of the rammer than it compacts.
The volume of liquid should be determined individually so that the sand is easily crushed in the hands, keeping the shape of the cake. On the other hand, with mechanical compaction, water should not protrude over the sand.
The degree of compaction of sand is quite simply determined. If no traces remain on the prepared sand cushion when walking on it, then the base is ready for further work.
Gravel
For backfilling, gravel of medium and coarse fractions is used in those situations when under the base of the foundation it is necessary to ensure the maximum throughput of the drainage layer in combination with the distributed drainage system, focused on the removal of groundwater from the base of the foundation.
Gravel is often used as a cheap substitute for lean concrete in preparing and strengthening soil for foundations. To do this, it is tamped and mixed with the soil mechanically or manually.
However, this is not the best solution, since without a binder, which can be cement, such a base is subject to erosion by groundwater, followed by loss of bearing strength.
More often, ASG is in demand - a sand and gravel mixture for the formation of a flat area under the foundation. In combination with sand, the mixture is easier to impart density and strength comparable to that of the mother soil at the construction site, while maintaining the drainage capacity of the backfill.
Benefits of gravel bedding:
- The low water capacity of the substrate, the liquid is poorly retained in it, and the surface area of the gravel for wetting is much lower than that of sand;
- Backfill strength and high load capacity and resistance to scouring or side loading.
Flaws:
- Under heavy loads, even distributed, gravel bed can "drown", reducing its own strength and strength of the parent soil;
- It is difficult to level the surface of the backfill;
- When concrete is poured, part of the laitance sinks aimlessly through the bedding, weakening the main body of the foundation.
If gravel is used for backfilling under a strip foundation or a monolithic slab, then, of course, it should be pre-insulated to prevent weakening of the concrete. However, this often incurs more costs than using initially lean concrete.
What is better sand or gravel
The requirements for preparing the base of the foundation pit require strict guidelines in the construction project based on an analysis of the bearing capacity and properties of the parent soil.
The best preparation for a strip foundation or a monolithic slab is lean concrete and only in some cases is it allowed to replace concrete with sand, gravel or ASG to reduce overall costs. At the same time, sand has a large set of advantages and is more practical. 
Gravel is suitable only in cases where high throughput if necessary, the arrangement of a drainage layer with low water capacity. At the same time, it is difficult to isolate the bedding from the volume in which the foundation will be poured.
Gravel bedding goes well with pile foundations, where it is enough to remove excess moisture from under the base of the house, and at the same time, there will not be a significant load on the bedding itself.
Under the strip foundation
Sand filling is needed by definition only in the case of using ready-made reinforced concrete slabs and blocks in order to distribute the load evenly over the base plane.
With the help of sand, it is easier to level the bottom of the pit, and tamping gives the sand the necessary density and bearing capacity.
However, this is only relevant if it is possible to place a massive vibrating plate in the trench for mechanical sand compaction. In most cases, it is safer to use lean concrete footing to level the base and prepare.
Sand is also relevant in case of significant height differences along the bottom of the prepared trench. To reduce costs and reduce the volume of mortar for footing, sand or crushed stone is used with layer-by-layer tamping and moistening.
Under a monolithic slab
It is important to strictly bring out the plane of the base of the pit and prepare the soil for the installation of the reinforcing frame and pouring. Either lean concrete or compacted sand is used in layers.
stages of construction of a monolithic foundation
Sand is mainly used in cases where it is necessary to significantly raise the bottom of the foundation pit after sampling the entire fertile soil layer to the base of the parent soil.
When forming the backfill, it is important to pre-distribute the trays for water drainage, the supply lines of communications that will pass through the foundation slab, and also to mark the required planes of the future base.
According to the requirements, a base is formed under a monolithic slab not strictly in one plane, but with a slight elevation in the center of the building and with a slope of 2-3% in all directions, for effective removal of moisture from the substrate of the future foundation.
Particular attention is paid to the quality of sand compaction. So the density of the bedding under the foundation should be from 1.65 t/m3 and preferably not less than the density of the parent soil with an error within 0.05 t/m3.
The height of the bedding is determined as the difference between the level of the bare soil base after the removal of the fertile layer and the design level of the foundation base.
Under the pile foundation
The backfill primarily performs the function of drainage for the removal of groundwater, and also acts as a substitute for the fertile soil layer in order to remove the volume of material containing organic or combustible inclusions from under the foundation.
pile foundation bedding device
For these purposes, it is best to use large and medium gravel, crushed stone. Expanded clay pads are often used, which further increase the thermal insulation properties of the base.
Sand and gravel is one of the most common inorganic materials used in the construction industry. The composition of the material and the size of the fractions of its elements determine which variety the extracted mixture belongs to, what are its main functions, where it is more suitable for use.
Sand and gravel mixture is used in construction for filling in the lower layers of various bases, for example, asphalt or other road surface, and for the manufacture of various mortars, such as concrete with the addition of water.
Peculiarities
This material is a universal ingredient, that is, it can be used in different types activities. Since its main components are natural materials(sand and gravel), this indicates that the sand and gravel mixture is an environmentally friendly product. Also, PGS can be stored for a long time - there is no expiration date for the material.
The main condition for storage is the presence of the mixture in a dry place.
If moisture still gets into the ASG, then when it is used, a smaller amount of water is added (for example, in the manufacture of concrete or cement), and when the sand and gravel mixture is needed only in dry form, it will first have to be thoroughly dried.
A high-quality sand and gravel mixture, due to the presence of gravel in the composition, should have good resistance to temperature extremes and not lose its strength. Another interesting feature of this material is that the remains of the used mixture cannot be disposed of, but can later be used for its intended purpose (for example, when laying a path to a house or when making concrete).
Natural sand and gravel mix is low cost, while the enriched ASG has a high price, but this is offset by the durability and quality of buildings from such an environmentally friendly material.
Specifications
When purchasing a sand-gravel mixture, it is necessary to pay attention to the following technical indicators:
- grain composition;
- the volume of content in the mixture of sand and gravel;
- grain size;
- impurity content;
- density;
- characteristics of sand and gravel.
Technical characteristics of sand and gravel mixtures must comply with accepted state standards. General information about sand and gravel mixtures can be found in GOST 23735-79, but there are also other regulatory documents governing specifications sand and gravel, for example, GOST 8736-93 and GOST 8267-93.
The minimum size of sand fractions in ASG is 0.16 mm, and gravel - 5 mm. The maximum value for sand according to the standards is 5 mm, and for gravel this value is 70 mm. It is also possible to order a mixture with a gravel size of 150 mm, but not more than this value.
In the enriched ASG, the average content of gravel is 65%, the clay content is minimal - 0.5%.
According to the percentage of gravel content in enriched AGM, materials are classified into the following types:
- 15-25%;
- 35-50%;
- 50-65%;
- 65-75%.
Important characteristics of the material are also indicators of strength and frost resistance. On average, ASG should withstand 300-400 freeze-thaw cycles. Also, the sand and gravel composition cannot lose more than 10% of its mass. The strength of the material is affected by the number of weak elements in the composition.
Gravel is divided into categories according to strength:
- M400;
- M600;
- M800;
- M1000.
Gravel category M400 is characterized by low strength, and M1000 - high strength. The average level of strength is present in gravel categories M600 and M800. Also, the number of weak elements in the gravel of category M1000 should contain no more than 5%, and in all others - no more than 10%.
The density of ASG is determined in order to find out which component in the composition is contained in a larger amount, and to determine the scope of the material. Average specific gravity 1 m3 should be approximately 1.65 tons.
It has great importance not only the size of the sand, but also its mineralogical composition, as well as the particle size modulus.
The average compaction factor of ASG is 1.2. This parameter may vary depending on the amount of gravel content and the method of compacting the material.
Not the last role is played by the coefficient Aeff. It stands for the coefficient of the total specific efficiency of the activity of natural radionuclides and is available for enriched PGS. This coefficient means the rate of radioactivity.
Sand and gravel mixtures are divided into three safety classes:
- less than 370 Bq/kg;
- from 371 Bq/kg to 740 Bq/kg;
- from 741 Bq/kg to 1500 Bq/kg.
The safety class also determines which scope of application this or that CGM is suitable for. The first class is used for small construction activities, such as the manufacture of products or the repair of a building. The second class is used in the construction of automotive coatings in cities and villages, as well as for the construction of houses. The third class of safety is involved in the construction of various sites with a high load (these include sports and children's playgrounds) and large highways.
The enriched sand-gravel mixture is practically not subject to deformation.
Kinds
There are two main types of sand and gravel mixtures:
- natural (PGS);
- enriched (OPGS).
Their main difference is that the enriched sand and gravel mixture cannot be found in nature - it is obtained after artificial processing and the addition of a large amount of gravel.
Natural sand and gravel mixture is mined in quarries or from the bottom of rivers and seas. According to the place of origin, it is divided into three types:
- mountain ravine;
- lake-river;
- marine.
The difference between these types of mixtures lies not only in the place of its extraction, but also in the scope of further application, the amount of volumetric content of the main elements, their sizes and even shapes.
The main features of natural sand and gravel mixtures:
- the shape of gravel particles - the mountain-ravine mixture has the most pointed corners, and they are absent in the marine AGM (smooth rounded surface);
- composition - the minimum amount of clay, dust and other pollutants is contained in the marine mixture, and in the mountain-ravine mixture they prevail in large quantities.
The lake-river sand-gravel mixture is characterized by intermediate characteristics between the marine and mountain-ravine SGM. Silt or dust can also be found in its composition, but in small quantities, and its corners are slightly rounded.
In OPGS, gravel or sand can be excluded from the composition, and crushed gravel can be added instead. Gravel crushed stone is the same gravel, but in a processed form. This material is obtained by crushing more than half of the original component and has sharp corners and roughness.
Crushed gravel increases the adhesion of building compounds and is perfect for the construction of asphalt concrete.
Crushed stone compositions (sand-crushed stone mixtures - PShchS) are divided by particle fraction into the following varieties:
- C12 - up to 10 mm;
- C2 - up to 20 mm;
- C4 and C5 - up to 80 mm;
- C6 - up to 40 mm.
Crushed stone compositions have the same characteristics and features as gravel compositions. Most often, a sand-gravel mixture with a fraction of 80 mm (C4 and C5) is used in construction, since this type provides good strength and stability.
Scope of application
The most common types of construction in which sand and gravel mixtures are used are:
- road;
- housing;
- industrial.
Sand and gravel mixtures are widely used in construction for backfilling pits and trenches., leveling the surface, building roads and laying a drainage layer, producing concrete or cement, when laying communications, backfilling bases for various sites. They are also used in the construction of the base of the railway track and landscaping. This available natural material participates in the construction of one-story and multi-story buildings (up to five floors), laying the foundation.
The sand-gravel mixture as the main element of the highway pavement ensures the resistance of the road to mechanical stress and performs water-repellent functions.
In the manufacture of concrete (or reinforced concrete), in order to exclude the possibility of the formation of empty spaces in the structure, it is the enriched ASG that is used. Her factions various sizes perfectly fill the voids and this determine the reliability and stability of structures. Enriched sand-gravel mixture allows you to produce concrete of several grades.
The most common type of sand and gravel mixture is ASG with a gravel content of 70%. This mixture is highly durable and reliable, it is used in all types of construction. Natural ASG is used much less frequently, because due to the content of clay and impurities, its strength properties are underestimated, but it is ideal for backfilling trenches or pits due to its ability to absorb moisture.
Most often, natural ASG is used to equip the entrance to the garage, pipelines and other communications, the construction of a drainage layer, garden paths and arrangement of household territories. The enriched composition is involved in the construction of high-traffic highways and houses.
How to make a pillow for the foundation of a sand and gravel mixture, see below.
