In the set of rules for grounds and earthworks SP 45.13330 regulates backfilling of foundations. Technical standards TR 73-98 provide rules for compacting the materials used. General principle consists in using inert materials from the outside, any from the inside.

The question of how to fill the foundation inside is relevant both for floors on joists and for floors on the ground. The choice of material and compaction technology depends on the following factors:

• type of operation - in buildings permanent residence heating is year-round, the soil under the base of the house does not freeze, so you can even fill it with clay, which cannot swell in the absence of moisture and freezing;
• ceiling/floor construction - if the project includes a ceiling on beams, the cheapest way is to fill the inside with clay; for the footing of a floating floor on the ground, sand is needed to level the base at least in the upper level (minimum 10 cm layer);
• the height of the base part - for large volumes it is more rational to use soil from the building site, taken out of the trenches; to fill the concrete footing, the very top should be filled with sand;
• GWL level – at high groundwater It is preferable to fill with crushed stone, if the aquifer (“upper water”) is 1 m from the base of the foundation, sand should be used to save the construction budget.

Attention: Clay castles prohibited outside, although the technology was described in the old SNiP. Clay does not allow moisture to pass through, but absorbs intensely, which is dangerous due to uneven swelling, not only when freezing, but also when swelling.

When backfilling, layer-by-layer (20 cm) soil compaction is required.

Don't neglect filling out internal space strip foundation cells in the manufacture of floor joists:

• the underground is too low for normal operation;
• Vapors inevitably release from the soil, which are harmful to the power structures of the building;
• ventilation is required, heat loss through the floor of the lower floor increases;
• Harmful radon is often released from pits, from which the home will have to be protected with foil materials.

Backfilling before the ceiling will allow you to solve all the problems comprehensively and improve the quality of operation.

Materials

Almost any soil is allowed in the internal backfill, but large stones (more than 25 cm) must be excluded from them, and if necessary, lay longitudinal drains, including them in general outline, located around the perimeter of the house (only with high groundwater level). Depending on the heating mode, groundwater level, freezing depth, the width of the shell of non-metallic material around concrete structures is:

• heating is constant - there are no restrictions; with proper compaction, backfilling with clay is allowed;
• periodic heating - a 20 cm layer of sand or ASG is enough interior 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 – width of the sinuses 50 cm minimum.

The depth of filling the sinuses is calculated from the planning mark (usually the blind area), and is ¾ of the depth of the base 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 on joists. If you plan to pour a screed, any of these 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 compaction visually - as soon as traces stop being imprinted on the soil, you can pour 5 - 10 cm of concrete screed. It is not recommended to spill sand, sandy loam, or loam with water, so as not to saturate the underlying horizons. Instead the material backfill moisturize to the following values:

• 15 – 23% heavy soils (including dusty ones), waterlogging no 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 fill the screed after completely dry soil. When filling any monolithic foundation footing is used. This structural element allows:

• reduce the height of the protective layer;
• prevent the leakage of cement laitance into the lower layer with high drainage characteristics;
• protect the waterproofing layer of the foundation base.

Therefore small sand layer on top of loam, sandy loam, crushed stone or clay will further level the base and reduce concrete consumption.

Backfilling outside

Unlike the internal perimeter, which cannot freeze (in 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 by tangential forces. The problem is solved by the following methods:

• backfilling of the foundation cavities with non-metallic material (minimum 20 cm sand, crushed stone shell);
• insulation of the blind area - 60 - 1.2 m tape around the building pushes back the freezing zone;
• sliding-creased thermal insulation – rigid fixation of extruded polystyrene foam high density EPPS to the outer walls of the foundation, covering with two layers of a plinth fixed at the level polyethylene film, installation of sheets of PSB 25 (minimum density of polystyrene) vertically close to the film without fastening (held by sand powder).

When heaving forces occur, soft polystyrene crumples and rises up a perfectly smooth film without causing damage to the underlying layer of thermal insulation. In the spring, structural elements arrive original appearance after reducing the soil volume.

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, so 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 placing it in the sinuses. Natural shrinkage takes time, so it is better to rent or make a vibrating plate yourself, reducing the tamping time to a minimum.

If the groundwater level is high or there is a possibility of its seasonal rise, crushed stone should be used. The 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

Filling the sinuses from the outside with sand or crushed stone 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 foundation base are prerequisite 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 only possible by compacting materials with vibrating plates and hand rammers. The maximum effect is observed when preventing mutual penetration of inert materials and neighboring soils. The technology looks like:

• laying geotextiles or dormite on the walls of the sinus;
• backfilling the outside with sand or crushed stone in a 10–20 cm layer;
• compaction with a tamper or vibrating plate.

If a deep foundation strip is being poured, horizontal thermal insulation (5 cm sheets of high-density extruded polystyrene foam) must be laid at an area of ​​30–40 cm from the surface, after which the work must continue.

In MZLF tapes, the depth usually does not exceed the specified level, so thermal insulation is laid outside along the bottom of the trench by default. Backfilling is done on top of it.

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ASG is a high-quality, purified mixture of sand and gravel. Thus, the sand-gravel mixture refers to non-metallic materials used in the construction of residential and industrial use, creating high quality road surfaces, in railway construction, when laying drainage layers, for leveling areas in landscape design, for the production of reinforced concrete products used for the construction of panel buildings. Also adding ASG necessary to create a cushion for the construction of foundations, equipment of sites for the operation of special equipment and during road construction, for backfilling trenches and pits.

Can be natural and enriched. It is mined in quarries or from the bottom of reservoirs. Natural ASG contains at least fifteen percent gravel, and in an enriched mixture the gravel content can significantly exceed this figure. Enriched ASG differs in gravel content into five groups: 15-25%, 25-35%, 35-50%, 50-65%, 65-75%. The more gravel a mixture contains, the correspondingly harder it is. Gravel is added to the mixture artificially during production. The price of enriched ASG depends on the amount of gravel in it and its size. Both types of PGS are characterized by durability and resistance to physical influences, which contributes to their wide application. If you need such high quality materials, then construction company offers to buy and deliver all the necessary non-metallic construction materials: sand, crushed stone, gravel, bedding, ASG and others.

The backfill, as a rule, is made from non-heaving soil and instead of ASG, construction soil no finer than medium-grained is also used as backfill. The size of the bedding is determined by a special temperature calculation, and it is necessary to provide ventilation and thermal insulation so that it is not too large. ASG backfill is used for arranging a monolithic foundation, single- or two-story house. The installation of such a foundation begins with the development of a pit, compaction and creation of a cushion using ASG backfill, sand and gravel. Placed on top of it waterproofing material and poured with a small layer of concrete, then reinforcement is laid, and the pit is filled with a special concrete solution. The result is a single monolithic slab used for the basement floor.

Technology of construction of various structures on initial stage construction also almost always involves the arrangement of a site for work (this requires bedding). Filling sites and roads is an important stage of any construction. Thus, depending on the type of structure, climatic and geographical conditions, filtration properties of the material, the quality and service life of buildings largely depends on adding ASG, crushed stone, sand – High Quality. The technology consists of several stages:

• First, using a bulldozer or excavator, they remove upper layer soil to hard rock, this is necessary to insulate the base of the future structure.
• Then soil rollers are used to strengthen the soil and form a platform for subsequent work.
• The actual installation of the cushion and compaction is carried out using bedding with ASG, crushed stone, and sand.
• Additional backfilling of hard rock crushed stone and its compaction is possible.

Laying the foundation of a house is the first and most important stage of construction, but this is preceded by preparing the foundation under the entire building area on the site.

The type of foundation, its design and characteristics depend on bearing capacity soils 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 soil and determine which backfill for the foundation, sand or crushed stone, will be relevant as preparation.

It is somewhat incorrect to raise the question of choosing a backfill for a foundation made of sand or crushed stone. The key to a strong and stable home is a strong and reliable foundation, which must meet a number of requirements:

• High strength and density of soil that can withstand distributed load further development;
• Groundwater should not linger under the foundation, 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 flammable or plant residues capable of rotting.
• Cold heaving of the soil is not allowed;
• Not allowed uneven shrinkage or deformation.

In progress construction work the foundation should not deform even under the load of the involved construction equipment or the activity of builders.

The strength of the surface layer should be sufficient to accommodate all the necessary elements, such as reinforcing frame, formwork, etc.

bedding device

Since it is not possible to select the type of soil on the site in advance, you should work with what you actually have. If the soil does not meet the specified requirements, a foundation filler is used from:

• sand;
• gravel;
• sand and gravel mixture (SGM);
• debris (crushed fragmentary rock);
• crushed stone;
• skinny concrete.

Since the properties of each of the listed materials differ, as do the methods of their use, the choice of backfill should be made based on the final requirements for the base under the foundation.

The main conclusion: backfilling the foundation with 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 absolute component.

Anyway the type of soil is first determined optimal type foundation(tape, pile, monolithic slab, etc.) and after that, if necessary, select the type of bedding that will be required.

High-quality training under strip foundation or monolithic slab is pouring the bottom of the pit with lean concrete to level the level and prepare a solid foundation. Sand or gravel are for the most part a cheap alternative to reduce total costs for construction.

Sand

Simple and enough effective option foundation fillers. Well compacted mechanically the sand cushion is capable of taking on the same strength and density as the main soil, and at the same time is easily shaped to accommodate all the unevenness of the bottom of the pit.

Advantages of sand for backfilling:

• With high-quality compaction, you can achieve a foundation strength equal to the original value for the soil;
• Fills all the unevenness of the pit well and transfers the load evenly;
• Sand retains drainage properties;
• Easily shaped and leveled;
• Does not require the use of heavy construction equipment.

Flaws:

• Weak mechanical strength transverse point load.
• Sand is washed away by groundwater over time.

Sand for backfilling is ideal when using ready-made reinforced concrete blocks and slabs, allowing the load to be transferred evenly over the entire base.

Sand for backfilling is selected from coarse and medium fractions without clay inclusions. Even with complete compaction, 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 range from 10 to 60-70 cm depending on the properties of the soil. The depth of soil freezing in many regions of the country exceeds 30 cm, and cold heaving can appear even under a well-insulated foundation during prolonged winter cold spells.

The optimal height for adding sand is considered to be 45-60 cm. Such a layer of sand is difficult to compact at a time, so the material is gradually filled in layers 5 cm thick and gradually compacted and necessarily moistened.

It is quite difficult to determine the required amount of water to moisten the sand. A common mistake is excessive moistening of the sand, as a result of which the entire mass becomes plastic and diverges more to the sides of the tamper than becomes compacted.

The volume of liquid should be determined individually so that the sand is easily crushed in your hands, maintaining the shape of a cake. On the other hand, during mechanical compaction, water should not come out on top of the sand.

The degree of sand compaction is determined quite simply. If on a prepared sand cushion there are no traces left when walking on it, which means the base is ready for further work.

Gravel

Medium and coarse gravel is used for backfilling in situations where the maximum throughput of the drainage layer should be ensured under the base of the foundation in combination with distributed drainage system, aimed at draining groundwater from the base of the foundation.

Gravel is often used as a cheap substitute for lean concrete when preparing and strengthening the soil for a foundation. To do this, it is compacted and mixed with the soil mechanically or manually.

However this is not The best decision, since without a binder, which can be cement, such a foundation is susceptible to erosion by groundwater with subsequent loss of load-bearing strength.

More often in demand is ASG - a sand and gravel mixture to form a level area under the foundation. In combination with sand, the mixture is easier to impart density and strength comparable to that of the parent soil on construction site, while maintaining the drainage capabilities of the bedding.

Advantages of gravel backfill:

• Low water capacity of the substrate, liquid is poorly retained in it, and the surface area of ​​gravel for wetting is much lower than that of sand;
• The strength of the bedding and its high load-bearing capacity and resistance to erosion or lateral loads.

Flaws:

• Under heavy loads, even distributed, gravel bed can “sink”, reducing its own strength and the strength of the parent soil;
• It is difficult to level the bedding surface;
• When pouring concrete, part of the cement laitance falls aimlessly through the bedding, weakening the main body of the foundation.

If gravel is used for backfilling under a strip foundation or monolithic slab, then, of course, it should be pre-insulated to prevent the concrete from weakening. However, this often incurs more costs than using initially lean concrete.

Which is better sand or crushed stone

The requirements for preparing the foundation pit for the foundation require strict instructions 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 monolithic slab is lean concrete and only in some cases is it possible to replace concrete with sand, gravel or ASG to reduce overall costs. At the same time, sand has large set advantages and turns out to be more practical.

Gravel is suitable only in cases where high throughput is required when it is necessary to arrange 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.

The gravel backfill 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 strip foundation

By definition, sand bedding is needed only if ready-made reinforced concrete slabs and blocks to distribute the load evenly along 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 load-bearing capacity.

However, this is only relevant if it is possible to place a massive vibrating plate in the trench for mechanical compaction of sand. In most cases, it is safer to use a lean concrete footing to level the base and prepare it.

Sand is also relevant in case of significant differences in height along the bottom of the prepared trench. To reduce costs and reduce the volume of solution for the concrete base, sand or crushed stone is added with layer-by-layer compaction and moistening.

Under a monolithic slab

It is important to strictly level the base of the pit and prepare the soil for installation of the reinforcing frame and pouring. Either lean concrete or sand compacted in layers is used.

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 removing the entire fertile soil layer to the base of the parent soil.

When forming the bedding, it is important to pre-distribute trays for water drainage, communication lines that will pass through the foundation slab, and also mark the required planes of the future foundation.

According to the requirements under monolithic slab the base is formed 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 backfill for 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 backfill is determined as the difference between the level of the bare base of the soil after removal of the fertile layer and the design level of the foundation base.

Under a pile foundation

The backfill primarily performs the function of drainage to drain groundwater, and also acts as a substitute for the fertile layer of soil in order to remove the volume of material containing organic or combustible inclusions from under the foundation.

backfill device for pile foundation

For these purposes, it is best to use large and medium gravel and crushed stone. Expanded clay bedding is often used, which further increases thermal insulation properties grounds.