Backfilling of the foundation

Backfilling of the foundation

One of the conditions for normal stability and durability of the foundation of reinforced concrete is the correct docking of the side walls and the bottom of the excavation with the foundation box of the building. Despite the external simplicity of the procedure, the backfilling of the foundation has a sufficiently large number of nuances that must be taken into account, which is to achieve the design characteristics of the foundation. Moreover, the problem of the arrangement of the sinuses of the pit is so important for the foundation that a separate SNIP № 3.02.01-87.

What is so important is the backfilling operation

With any construction of the foundation, with the exception of casting concrete directly into the trench of the tape system, the technology of its construction requires the arrangement of a technological space between the wall and the formwork of the concrete wall or sinuses. A small, at first glance, increase in the size of the excavation makes it possible:

  • Remove from the space of the sinuses external shields of the foundation formwork without risking collapse of the pit wall. In addition, the presence of an external cavity 15-20 cm wide for a trench or a trench, just a meter deep in the sinus, accelerates the setting process of concrete by 15-20%;
  • Waterproofing and insulation of the concrete surface of the walls of the strip foundation with the help of coating or roll waterproofing. Even a small size of the 40 cm sinuses allows using gas burners or roller equipment and performing insulation works with high quality;
  • To equip the drainage system, which removes most of the groundwater in the base of the bearing and the heel of the foundation.
For your information! The construction of a foundation in a trench with sinuses simplifies the technology. In some cases, such as the arrangement of an extended support surface of the foundation tape, it is impossible to build it without excavation in the soil of the sinuses.

But the use of sinuses as a technological device leads to a significant deterioration in the stability of the foundation and the entire building box in the future. The ability of the building to withstand lateral and shearing loads depends on how hard and stable are the layers of soil that compress the foundation skeleton. It follows that, to the extent that the backfilling of the trenches of the excavation is carried out correctly, the foundation and supporting elements of the building will be so stable.

In fact, the backfilling of the trench of the foundation is a forced measure aimed at arranging a bundle of concrete tape with the parent soil of natural density. In addition, the backfilling of the soil can effectively protect against watering the layers on which the base of the foundation rests. If this is not done, or to make the filling of the sinuses the first material that comes under the arm, wetting the support on loam or clay soils will lead to the cracking of the foundation even without the participation of the punching forces.

How the reverse filling of the sinus cavity is performed correctly

SNiP 3.02.01-87 defines several main requirements for performing the procedure for backfilling the excavation. To ensure maximum compaction of the material laid in the sinuses of the excavation, it is necessary to control the most important parameters of the soil mixture:

  1. The moisture content of the material and the design density of the soil mixture used for loading into the sinus cavity. For each soil used for backfilling the sinuses, there is a "fork" of optimal moisture and density;
  2. The ability of a soil mixture thrown into the sinuses of the excavation, to compaction with the help of special equipment. This is one of the most important characteristics of the backfill material.
Important! In addition to the actual characteristics of the soil mixture, it is important to follow the technology and the sequence of operations when filling the sinuses of the excavation.

For proper compaction of soil materials, SNiP established the type of equipment, the number of passes, the thickness and the order of development of a single bed of the backfill mixture. It is important not only to throw out or sketch the soil in the cavity of the trench of the excavation, it will be necessary to use the vibration equipment of the required capacity. A weak and lightweight vibrator simply can not compact the material to the required level of density, and heavy rollers and slabs are not able to come close to the edge of the basement. Excessively heavy or powerful vibrators when working on viscous mixtures can tear off a layer of insulation from the vertical surface of concrete walls.

Technologically, backfilling of the sinuses does not pose a particular problem, but there are specific features that the SNiP pays special attention to:

  1. Preparation or preparation of a soil mixture for backfilling should be carried out until the material is loaded into the space between the foundation box and the walls of the excavation;
  2. The material used for backfilling should be as homogeneous as possible, sifted and cleaned of large stones, wood, fertile soil masses, plant organics;
  3. The cavity of the sinuses of the excavation is also freed from debris and excess moisture, while the humidification of the walls of the excavation and the lower layers of the soil can not be performed backfilling.
Advice! The best way to reduce the watering of the pit walls and foundation of the foundation is the construction of a contour drainage channel.

Such a solution will require additional capital expenditure, but effective drainage is the only way to reduce the punching and sagging of the concrete foundation structure under the influence of groundwater.

The perforated drainage pipe is covered with a layer of barrier material, most often a layer of coarse crushed stone with a sublayer of sand-gravel mixture separated by a geotextile canvas. This will make it possible to maintain the efficiency of the channel, even if the material for backfilling is predominantly fatty clay.

Backfilling of the foundation does not begin earlier than 14 days after the last layer of the foundation is poured and the basement part of the building is erected. Often, a concrete box is not at risk of being loaded with a backfill because of the weakness of the concrete. Vibratory loads of rammer hammers and hand tampers can lead to the development of a grid of tiny cracks through which air and moisture easily penetrates to the body of steel reinforcement, causing it to rust.

With the backfilling of the sinuses of the foundation pit try not to tarry only if the weather forecast involves rains, and the walls of the foundation pit are not reinforced with polyethylene film. For sandy soils, there is a certain hope for the rainwater rainfall in the process of natural loss. If the construction is carried out on clay soils, the backfilling of the sinuses of the pit should be performed before the rains, with the obligatory arrangement of a "rough" blind area. If there is not enough time, the surface of the backfilling of the excavation should be planned under the drainage slope and simply covered with a thick polyethylene film.

Soil compaction technology

The provisions of SNiPa allow the use as a material for backfilling the excavation of most soil mixtures, provided that their characteristics make it possible to effectively seal the layer with a seal coefficient of 0.95-0.98. It is quite difficult to obtain such a dense structure, so the technology recommends that the mixture is compacted with relatively thin layers.

The first layer of the backfill material is laid out on the surface of the PGS 15-20 cm thick and carefully leveled with a trench tool and a manual rammer. When the filling is compacted, the tool passes along the edge adjacent to the wall of the strip foundation. The first pass is performed with an effort of no more than 70% of the recommended value for a particular material. Each next pass along the backfill layer is performed with overlapping of the previous track by 1 / 3-1 / 4 of the width, so that the vibrator plate seals the spot under the working element for at least 2 minutes.

Tip! If the width of the axillary sinus does not exceed 15-20 cm, then the edge adjacent to the concrete is sometimes pummeled with a foot, achieving maximum compaction in the most important zone of the sinuses.

Despite the external primitiveness of this method, it allows you to easily solve the problem of padding in those places where a heavy manual tamping or an electric tool can damage the insulation of the foundation walls.

The upper layers of the back sinuses are rolled with the maximum load and pressure. For two-storeyed cottages and buildings, manual and motorized vibro cassettes are often used, giving a good compaction of sand and clay.

The final stage of the backfilling of the sinuses of the excavation is the arrangement of a full basement footprint. While the natural sediment of the ground mass, buried in the trenches of the excavation, does not end, at least simultaneously with the sedimentary processes of the entire foundation, the entire surface adjacent to the building's shell remains extremely vulnerable to rain and perch water. Therefore, the arrangement of the blind area should be carried out in the shortest possible time.

For your information! Experts do not recommend the backfilling of foundations of private houses in the winter. A large amount of snow and ice in the ground, poor compaction qualities of the material do not allow efficient cleaning of the sinuses of the excavation.

Materials for filling the sinuses of the excavation

One of the problems associated with the implementation of backfilling is the correct selection of the composition of the mixture to fill the sinuses of the excavation. Approximately an equal number of specialists recommend the use of sand or clay to fill cavities. In each of the two cases, there are numerous arguments and arguments, references to GOSTs and SNiPs. The practice of most construction organizations looks even worse. Sinuses are buried either raised from the excavation soil, or purchased material of very different composition, as in the video:

How to choose the right material for backfilling the sinus cavity

Sand backfilling

Numerous practices believe that the best option will be backfilling with sand. The opinion of one of them can be seen on the video:

A certain amount of common sense and logic in this solution exists:

  1. At first, backfilling of the foundation with sand with effective drainage and correctly planned slopes of the blind area makes it possible to make a mass embedded in the sinuses of the basement, rigid and maximally dry, and therefore forget about the problems of whipping and underflooding;
  2. Secondly, sand mixtures do not belong to subsidence ground, which provides for the foundation of the building a good additional "anchor" that keeps the box of construction in an unchanged position;
  3. Thirdly, sand is well trampled and "holds" the density, and accordingly, the strength of the sprinkling, but under one condition - the sinuses of the excavation are covered with a prepared sand mixture.

The latter factor is the most important condition for the backfill strength. To sand laid in the sinuses of the pit, was not a simple ballast, easily washed by soil water, it is necessary that after tamping large sand particles are bound by an intermediate substance.

It is best to use ordinary gully material, not quarry and not river sand, namely, mined in thin subsoil layers, sometimes with small inclusions of clay substance. The sand contains a large number of dissolved salts and concretions of lean clay saturated with aluminum and iron compounds. Such material for sandy filling of sinuses will do the best. It categorically can not be washed, most of the sand mass is cut on a blade or roll machine, but before that you must necessarily cleanse of roots and vegetation.

Sand moisturize and rammer in the sinuses. Over time, amphoteric aluminum salts, iron oxides and calcium concrete bind silicon sand in one mass. Its strength is low, but it is enough to reliably hold the foundation and at the same time to let water through, getting rid of the problems of punching. In a few thousand years, this mass will turn into sandstone.

More rational will be the use of ASG. A mixture of sand and fine gravel without any ligament makes it possible to make the filling of the sinuses very heavy and dense. Sand-gravel mixture, the ratio of 60:40, has good mechanical properties, is well trampled and passes water, but to make 60-70 tons of material by hand, it is almost very difficult.

Therefore, often resort to a more universal way - unload the machines sand and gravel along the edge of the axils of the foundation with two parallel ribbons, and then moisturize with a small amount of water. Using a tractor with a knife, push both strips of material together. When leveling with the help of shovels, more or less uniform mixing of sand and gravel is achieved. The backfilling of the backfill can be performed by layers 40-50 cm thick.

Application of clay materials for filling the sinuses

Half of the back sinuses of foundation pits are filled with clays and clay mixtures. The use of clay materials is much more complicated and requires careful preparation. First of all, for backfilling use lean grades of clay, which absorb little water.

To work with lumpy and hard lean clay it was easier, in order to give the bulk of the backfilling plasticity, a small amount of oily soaked and broken clay with sand can be added. Adding 5-7% of fatty clay to shrinkage and strength of the backfill will not affect, but will simplify and facilitate the ramming process. Such backfill can be performed for excavations on relatively heavy stony soils with low soil water levels.


Often, to reduce the watering of backfill sandy masses with rain and surface water, a so-called clay lock is placed under the pillow of the basement pavement-a layer of oily clay 15-20 cm thick. The clay mass is initially moistened and interrupted to a viscous test. Laying the lock out of the clay is carried out on a previously deposited layer of gravel and sand in three to four layers, 3-5 cm thick. The waterproofing is laid on the surface of the lock, the pillow is piled under the foundation of the footbridge. The clay is in a plastic state, practically does not freeze and maintains normal water-hardening qualities for tens of years.

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