Backfilling of the foundation: what is the best way to do it and how?

The principle and technology of compaction of the material for backfilling the foundation

According to the provisions of the SNIP, a variety of soil mixtures can act as a material for backfilling, which we will discuss below. But, there is one condition - the characteristics (moisture and compaction parameter) will contribute to the effective compaction of the bulk layer. The recommended compaction factor is 0.95–0.98. The structure is dense, you just can't achieve it. That is why the technology provides for a stage-by-stage compaction of the material, layer by layer of small thickness.

Advice! If you simply pour the mixture into the sinuses and perform compaction, then the lower layers will not have the desired compaction coefficient. Therefore, all work may be in vain.

Backfilling of a trench or foundation pit begins with the first layer of material. It is poured onto a prepared surface of a sand and gravel mixture, the thickness of which is from 15 to 20 cm... After that, the soil must be leveled. An entrenching tool and a hand rammer are suitable for this purpose. It is necessary to go along the edge that is adjacent to the wall of the strip base. In this case, the first manipulations are performed with an effort of no more than 70% of the standard value of the material used. Subsequent passes through the backfill should be performed with an overlap of the previous track by a third or a quarter of the width. This is necessary in order to allow the vibrator plate to compact the space above the working element for at least two minutes.

Important! In the case when the width of the filling cavity is small, about 15–20 cm, then the edge adjacent to the concrete is knocked out with a foot. At the same time, it is necessary to achieve ideal soil compaction in critical areas.

Although outwardly this method is primitive, thanks to it it is possible to solve the problem of sealing in areas where heavy manual ramming or electrical equipment will easily damage the thermal insulation layer of the foundation walls.

As for the upper layers of the reverse sinuses, they must be placed under maximum load and pressure. If the building is large, a two-story cottage or a house, then a manual and motorized vibratory roller can be used. It will help to perfectly compact sand and clay in the right places.

The last stage of backfilling the trench, creating a foundation blind area, made with a slope to drain moisture from the base. In this case, the arrangement of this part of the structure needs to be done the faster. The thing is that the soil surface adjacent to the building will be vulnerable to moisture and water.

Note! Professionals advise against engaging in foundation construction and backfilling in winter. Why? There is a lot of ice and snow in the ground

This means that the material has poor sealing qualities. It will be very difficult to achieve effective ramming and the required coefficient of compaction of the pit sinuses.

So, if we divide the whole process of backfilling the foundation, then it consists of the following steps:

Checking the soil, cleaning the sinuses of the excavation or trench.
Check soil moisture. The soil should be neither too dry nor too wet. For heaving soil, this is 12-15% moisture, and for heavy soil - 20%. If necessary, the soil is moistened or dried.
A sand and gravel layer is being built.
The first small layer of the selected material is filled in. It is equal to 30 to 50 cm. It should not include fertile soil, stones and other inclusions. The base is filled.
It remains to compact each layer.

You will learn how to compact trenches that are intended for utilities in this video.

You can see the features and process of backfilling in this video.

How to sleep: the question is not easy

In most cases, for these purposes, the same soil is used that was removed to form the foundation. But there are universal rocks: clay and sand. There are two main components: they are refillable. Each of the soils is used in specific cases, with its positive and negative sides.

Backfilling is best done with a mixture of soil combinations.

Backfilling the foundation with clay acts as a barrier (clay castle) for water to prevent it from entering the foundation area. In this capacity, you can combine: pour not pure clay, but loam or other combination of soils with a density higher than the main soil near the foundation. As an example - a log house built on loamy soils. In this case, the sinuses and the internal space are filled with the same loam or clay extracted during the construction of the foundation. If the house is built on clay, then the backfill is made of clay. Less dense sandy loam as the main soil should be covered with loam or clay.
On heaving soils that freeze to a considerable depth, backfilling of the foundation with rubble mixed with sand is best suited. The crushed stone-sand mixture does not retain water, does not allow it to freeze between fractional particles, which excludes an increase in the volume (swelling) of the backfill. Such a composition will not put pressure on the foundation in cold weather, creating an additional load from the forces of pushing it out. But there is also the other side of the coin. The same loose sand, passing moisture through itself, creates its accumulation at the base of the foundation. With poorly executed or poor-quality insulation, a threat to the base is created, despite even the blind area arranged around it. It is practically impossible to make it absolutely impenetrable. Additional drainage is needed to drain stormwater and groundwater.
It is not recommended to use pure sand. If, nevertheless, a "sandy" decision is made, then the degree of density of the filler should be the same or higher than the degree of compaction of the main soil in its normal state. Compaction is performed at optimal density and moisture content with a coefficient of 0.95. You can find out the degree of soil compaction in a particular area from the geological data located in special institutions.

How to fill the foundation inside: the choice of material

The owners of the plots at the beginning of construction are interested in the issue of arranging the foundation. Do I need to fill it up from the inside, what materials are the work performed and what affects the choice of bulk materials.

When performing work on a pile foundation, it is necessary to take into account minor features.

To finally decide on the selection of material and the thickness of the dump, you should find out at what level the groundwater is located, how much the soil and the financial condition of the owner can sink.

It's also a good idea to think about what is more profitable in terms of finance:

dumping under the foundation when using bulk materials;
arranged basement with ventilation and high-quality thermal insulation of the flooring.

Materials used for internal backfill

An excavator is most often used to dig a hole for the foundation. The construction project is the main document. Therefore, when digging a recess, a reconciliation with it is necessary. It has calculations based on the amount of excavated land.

The pit should first be sprinkled with sand, the thickness of which should be no more than fifteen centimeters. The sandy coating is well leveled and compacted.

In order not to make a mistake in choosing the best way to fill up the internal foundation, special attention should be paid to the type of soil. Construction soils are divided into the following types:

black soil, which is not used for backfilling inside the foundation;
loam;
rocky soil;
clay;
crushed stone.

Clay and loam have the most benefits.They are characterized by excellent waterproofing properties and can resist impact loads. It is not recommended to use clay with other impurities. Because it loses its waterproofing qualities.

The inside of the foundation is sprinkled with it no more than thirty centimeters. Do not forget about the intensive compaction, which must be done when backfilling. This helps to exclude sinuses. Mixed gravel and sand are not used for the pile foundation.

Good raw materials for backfilling the foundation are sand and crushed stone. Coming into contact with water, their qualities do not change. The sand is well compacted with periodic watering. After that, backfilling is performed, which balances the pressure on the foundation.

It is produced evenly throughout the entire part of the future home.

Internal filling under the screed

It fits and tamped down to a thickness of thirty centimeters. The clay mound provides complete protection from groundwater. The next ball is covered with sand, which must be tamped and soaked well with water. After that, it is necessary to waterproof with roofing material.

From above, to form a heat-insulating layer, foam is laid or sand is poured in order to make a concrete screed.

In addition to these actions, other work is carried out to provide additional waterproofing. In particular, drainage is being set up, the basement is waterproofed, a blind area is made throughout the entire territory of the house.

External backfill of the foundation

The outer foundation can quickly give in to swelling due to the fact that its edges are not protected from the cold. The soil freezes and begins to pull out the concrete structures. To avoid this, it is also necessary to sprinkle the foundation from the outside. This problem is solved using the following methods:

the sinuses of the foundation during backfilling are filled with non-metallic materials: a sandy or crushed stone shell of at least twenty centimeters;
the blind area is insulated;
thermal insulation is performed using expanded polystyrene, polyethylene film, PSB sheets.

The main layer of thermal insulation does not get damaged due to the soft polystyrene. In the spring, the structural elements return to their original state, after the soil is reduced in volume. Of the materials for sprinkling the outer part, sand and crushed stone are used.

Normal operation requires a certain density of the shell, therefore backfilling is performed in layers and must be rammed. The sand does not need to be watered, so that there is no danger of erosion of the lower layers. It is moisturized before styling.

In the case of a high groundwater table, it is recommended to use crushed stone.

The better to fall asleep

Having determined the term, they choose the material. Used: sand, clay, soil removed during the construction of the base. The materials are available, the latter is completely free.

The choice of material is influenced by many factors. The main one is the condition of the soil. Puffy requires a different type of reinforcement, in contrast to persistent, durable. The depth of the groundwater (the presence of a drainage system), the foundation affects. Climate and temperature factors are considered in the last turn.

Sand

Sand filling is considered common. The reason for the strip foundation is affordability.

Benefits of sand:

Moisture conductivity. The sprinkling material requires moisture permeability so that rotting, mold, and soil leaching do not begin. Proper arrangement of drainage under the tape base of the house makes it possible to sprinkle with sand. Water is not retained, it is discharged in a natural way.The backfill remains dry and does not allow ice to form. Ice is dangerous due to its ability to expand, exerting pressure, deforms the foundation.
Create a thick pillow. Laying sand in the sinuses of the foundation creates a sealing effect, holds the foundation. The sand cushion acts on the ground, preventing it from heaving.
Handmade. Compaction of sand in sinuses does not require equipment. Manual ramming is relevant for a small size.

Negative sides of sand:

Moisture conductivity. The item refers to both pluses and minuses. Sand backfill is a direct way to excess pressure on the waterproofing of the foundation part. The condition of the drainage affects - if the drainage system is equipped with poor quality, the sand of the backfill does not retain moisture, it passes to the base of the foundation. Water washes away the soil, forcing the foundation to settle.
Limited choice. Building sand is required. Ordinary river water permeates moisture, is washed out, the backfill is destroyed. Sand with clay elements in the composition enters into chemical, physical reactions with water, strengthening more densely in the pledged place.

Working with sand requires following the recommendations.

Drainage lining. The drainage system is laid along the perimeter of the strip foundation. This is an additional protection of the site from flooding.
High quality tamping. A layer is rammed, maximum 70 centimeters high.

Clay

The clay mixture is also used for back treatment of the foundation. Of the advantages, the actual strengthening of the base, the passage of the water mass, stands out. The clay layer is a barrier that does not allow the external environment to destroy the belt structure.

The material has more negative sides than positive ones. Cons of clay:

Heaviness. There is a property of humidification, saturation with moisture, which is reflected in the state of the foundation in winter.
The requirement for a special composition. They buy clay that absorbs a small amount of water under normal conditions. About 5-7 percent of the sand is kneaded into it. Everything is stirred, moistened. It turns out an elastic, correct mass.

The maximum clay layer for ramming is 50 centimeters. Clay is used when the water table and annual rainfall are low.

Initial soil

In the budgetary plan, the use of the soil base removed from the trench is optimal. No spending is required, money is saved on the delivery of material to the construction site.

Another plus of the option is the same permeability to the ground on which the base is placed. There will be no problems with erosion. Natural material is good because it does not require additional decoration.

The minus that stands out is weightless. Problems arise with the organization of a place for storing the removed soil at the time of laying the foundation. The place is required close to the natural microflora, but protected from precipitation, exposure to moisture.

Sometimes, soil is mixed with sand. The mixture of materials is optimal for strengthening the strip base of the building. Less sand is required than for sprinkling with one material. The mixing method is optimal when the resulting soil is small (most of it is soaked, turned out to be unsuitable for tamping in the sinuses).

Foundation technology

All modern technologies provide for the production of work on the development of the pit before the foundation. An exception may be when performing a pile foundation, with a grillage. The pit for the future house is carried out on the basis of construction drawings, which indicate the depth, dimensions in the plan and the required slope angle along the edge of the pit. Technologically, for the construction of the foundation, after the development of the pit, the following work is performed:

leveling the base under the foundation, sand filling device 100 mm;
installation of formwork from the inner and outer sides of the strip foundation to the height and width according to the design documentation;
installation of the reinforcement cage;
pouring the foundation with concrete.

After completing all these stages and holding for 5–7 days, the formwork is disassembled. The next thing to do is to waterproof the foundation surface. Waterproofing is carried out on completely dry concrete no earlier than 25–28 days after concreting at positive temperatures and dry weather.

Backfilling technology outside the foundation

It is worth saying that when digging a foundation pit or trench under the foundation, it is necessary to achieve the smallest possible size of the sinuses. Optimally, in the lower part of the foundation - 300-400 mm to the slope of the pit, in the upper part - 500–800 mm - this is quite enough to install and remove the formwork panels. After concreting, you need to wait 5-7 days before dismantling the formwork. The optimum holding time is 28 days, when the concrete will have sufficient strength.

Next, you should waterproof the foundation with bitumen mastic. After that, to a depth of 1200 mm, the surface of the foundation is pasted with high-density polystyrene foam.

After that, you need to clean the entire cavity around the foundation from debris. Before laying, the sand is moistened with water from a hose outside the pit at the place of its unloading. Now sand is poured in layers of 200-300 mm and tamped with manual rammers or mechanical vibrating plate, if the width of the trench and the size of the vibrating plate allows. Thus, layer by layer, the entire trench is filled up to the very surface. Considering that the process of filling and compaction is not difficult, you can do it yourself. After thorough compaction of the outer surface of the sand, they proceed to the construction of a concrete blind area 50-100 mm thick with a slope from the building to the outside. With proper sand placement and compaction, you can be sure that the blind area will not sag over time. Instead of sand, you can use sandy loam or loam.

Ramming with a vibrating plate

If in a given area there is a high level of groundwater and, in addition, a basement is provided in the house, it is necessary to carry out a drainage system at the level of laying the foundation before backfilling. Usually this is all included in the construction project for the building, if, of course, it is executed correctly and all geological nuances are taken into account in it. In this case, after laying the drainage system from perforated pipes, crushed stone and geotextiles, sand is poured in the same way, as in the first case. When entering the house through the foundation of various engineering pipe communications, it is necessary to equip this passage with high quality by installing concrete trays and sprinkling them with soft soil on all sides.

Backfilling the strip foundation: how to do it right

Backfilling is a complex and time-consuming process that must be carried out with great care. The technology of filling the sinuses when performing backfilling should be observed as much as possible - this concerns the timing of the start of the process; the choice of soil mixture for work, the correct implementation of soil compaction and other aspects.

The timing of the start of work on the backfilling of the sinuses

After pouring the strip foundation, the house cannot be continued insulation work, facing, it is necessary to wait until the foundation base gains its design strength. To gain strength, the foundation structure must stand for at least 28 days, only after that can work on the construction of the house be continued.

Backfilled foundation

The existing technology for the construction of the strip foundation of the house does not allow the dismantling of the formwork earlier than 14 days after the completion of the concreting of the supporting elements of the base, but even after the removal of the formwork panels, it is not allowed to backfill the foundation.

If it is planned to arrange a basement at a facility under construction, work on the construction of the basement floor must be completely completed before the backfilling of the foundation sinuses begins.

Backfill soil: material requirements

When starting work on the reverse filling of the sinuses of the strip foundation, it is worth using natural soil, which was extracted when digging trenches or a foundation pit. This land is stored in heaps near the construction site to be used as foundation backfill. According to SNiP, it is required that the soil intended for filling the sinuses of the strip foundation has the same structure and moisture content with the soil at the construction site.

Backfilling with sand

If the backfilling of the sinuses of the trenches (foundation pit) is still done with a sand mixture, it is required to check the compaction coefficient, which should be close in value to the compaction coefficient of natural soil on the building site in its natural state.

It is very important to remember that it is not allowed to use soils containing black soil and the top fertile layer for backfilling.

Why soil moisture is important

Soil moisture is the main indicator that has a huge impact on the quality of the backfilling of the walls of trenches or excavation. Soil previously removed from the pit, which is stored near the construction site, can change its moisture content. This comes from the absorption of moisture from atmospheric precipitation (rainwater, fog, dew) or weathering.

Natural moisture soil

For backfilling, it is allowed to use soil of natural moisture, therefore, dry or water-filled soil should be additionally prepared for work. Heavy and dry soils are pre-soaked, but not with ordinary water.

To carry out the procedure for soaking the soil, a special cement milk is prepared, blocking the water with a small portion of Portland cement.

The moisture content of the soil should be from 12% (fine sands, clayey rocks) to 20% (heavy soils). If the soil moisture indicators are higher than these values, the soil is pre-dried, stirring evenly several times.

Filling with soil

The screed is laid when the walls and foundation of the building have been tested for their ability to withstand the intended load without cracking. When the concrete has stood and gains strength, you can proceed to backfill. Until fully solid, the concrete should stand for about 15 days. It is imperative to withstand this period.

It is necessary to fill the sinuses of the foundation with soil that was dug out during its installation, or with sand. Before compacting the soil, moisten it, taking into account the density of the soil in the place where the construction is taking place. Information on soil density can be obtained from the geodetic service. The sand with which the sinuses of the foundation are poured is also good, as is the soil dug from the base of the house, after which it is well rammed.

Soil moisture standards, according to GOST:

sand - 8 - 12%;
sandy loam - 9 - 12%;
light soils and loam - 12 - 17%;
heavy soil - 16 - 23%.

These are approximate values, sometimes the humidity may be higher. More accurate data are obtained in the laboratory, after a series of soil tests.

In winter, the water under the foundation and in the immediate vicinity of it freezes, thereby expanding the soil and creating pressure on the screed, which is why it is literally squeezed out. This effect is especially pronounced in fine-dusty clay and peat soils. Therefore, the ditch for the foundation must be dug to a depth to which the soil freezes. You can fill the trenches with sand by one third, or with a mixture of gravel and crushed stone in a 1: 1 ratio.

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 (at least 20 cm sandy, crushed stone shell);
insulation of the blind area - 60 - 1.2 m tape around the building pushes the freezing zone;
sliding-crush insulation - rigid fixation of extruded high-density polystyrene foam EPS to the outer walls of the foundation, covering with two layers of a polyethylene film fixed at the basement level, installing PSB 25 sheets (minimum density of polystyrene) vertically close to the film without fastening (held by sand powder).

When heaving forces occur, soft polystyrene crumples, rises up along a perfectly smooth film, without harming the underlying layer of thermal insulation. In spring, structural elements return to their original form after reducing the soil in volumes.

Materials (edit)

The possibility of freezing outside the soil adjacent to the foundation 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 the normal operation of concrete structures, a shell density of 0.95 units is required, therefore nonmetallic materials are poured in layers of 10 - 20 cm, compacted with a vibrating plate, hand tools. Spilling sand is not recommended as there is a risk of blurring 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 the tamping time to a minimum.

With a high groundwater level or the possibility of its seasonal rise, it is necessary to use crushed stone. The gravel material is inferior to this nonmetallic product in terms of the main characteristic - flakiness. Therefore, shrinkage is possible during operation, leading to deformation of the blind area.

Technologies

Filling the sinuses from the outside with sand or crushed stone allows you to completely exclude the swelling of the layer adjacent to the foundation. However, all nonmetallic materials have excellent drainage properties. Therefore, ring drains at the level of the basement base are a prerequisite for normal operation.

Circular drainage scheme around the foot of the foundation.

When filling the sinuses, it is necessary to ensure that there is no shrinkage during operation. This is possible only by compaction of 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 geotextiles or dornite on the walls of the sinus;
backfilling outside with sand or crushed stone 10 - 20 cm of the layer;
compaction with a rammer or vibrating plate.

If a deep foundation tape is poured, horizontal thermal insulation should be laid at a distance of 30-40 cm from the surface (5 cm sheets of extruded high-density polystyrene foam), after which, continue work.

We recommend reading: Do-it-yourself insulation of the foundation and blind area.

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

We recommend reading: Insulation of a shallow strip foundation.

Good publicity

Terms of use

The cost of square meters of both land plots and directly residential premises is growing from year to year. In this regard, the most rational option for a developer is the fullest possible use of useful areas. To increase the useful area of the dwelling, a basement or basement is often arranged in the foundation part. However, there are times when this option becomes impractical. Most often this happens when the groundwater level at the construction site is elevated.

Before starting construction, already at the design stage of the building, a study of the type of soil is carried out

First of all, attention is paid to the bearing characteristics of the soil and the level of groundwater. The first factor affects the choice of the type of foundation, and depending on the height of the groundwater, a decision is made on the possibility of arranging a basement or basement floor

If this indicator is low, then you can use the inner space of the foundation for an auxiliary or living room without any problems.

If the groundwater is too high, above the foot of the building foundation, the basement floor can be fraught with a number of problems. First of all, you will need wall cladding outside and inside, as well as finishing the basement floors with a reliable layer of waterproofing. In addition, an efficient drainage system will be required around the perimeter of the building. All this is necessary to prevent the penetration of moisture into the basement.

Foundation tape filled with soil

Excessive dampness can cause the development of fungus, mold, putrefactive bacteria - all this leads to the premature destruction of both the exterior decoration elements and the supporting structures of the building. All this significantly increases the estimated cost of construction, often making the arrangement of basement floors economically unprofitable. It is much easier and cheaper to perform a complete backfilling of the sinuses inside the house. This operation allows you to achieve a number of goals:

Protection against moisture penetration into the inner foundation space. Unventilated voids in the base of a building due to high humidity can become a place for mold and mildew to form, from where they spread to the rest of the building.
Strengthening the walls of the foundation. Backfilling the walls of the bearing base from the outside and from the inside makes it possible to increase their resistance to lateral deformations. Also, this procedure reduces the likelihood of its subsidence due to the increased coefficient of friction of the side walls against the soil backfill.
Thermal insulation. When filling the inner space of the foundation, a powerful heat-insulating cushion is created, which prevents the freezing of the concrete pour. This allows you to increase the life of the foundation due to a decrease in the number of cycles of its freezing and thawing during the year.
Ability to use the backfill as a basis for pouring subfloors on the first floor.

Pouring subfloors over the backfill

Internal filling under the screed

Very often the basement cannot be used. Owners whose plots are located on swampy soils fall into this category. Then backfill is made inside under the screed. The materials for work are the same.
Clay is one of the best screed bases. It fits and tamped down to a thickness of thirty centimeters. The clay mound provides complete protection from groundwater. The next ball is covered with sand, which must be tamped and soaked well with water. After that, it is necessary to waterproof with roofing material. From above, to form a heat-insulating layer, foam is laid or sand is poured in order to make a concrete screed.
In addition to these actions, other work is carried out to provide additional waterproofing. In particular, drainage is being set up, the basement is waterproofed, a blind area is made throughout the entire territory of the house.

Backfilling of the foundation sinuses

The process of adding soil or sand mixture is technically very simple: the soil or mixture is simply moved into the sinuses between the wall of the foundation and the wall of the excavation (trench). However, like any other business, the filling procedure is associated with a lot of nuances that turn a seemingly simple process into a full-fledged technological operation.

Therefore, in practice, backfilling the foundation with sand or soil looks like this:

At the very beginning, by means of laboratory tests, the moisture of the soil is checked.After all, no dry soil or wet, mud-like soil is needed for backfilling. The optimum moisture level for heaving soils (dusty, sandy, clayey varieties of soil) is 12-18 percent. Heavy soils can be soaked up to 15-20 percent.
Further, excessively moist soils are dried in the sun. And if the soil is too dry, then it is moistened. Moreover, not water is used for moistening, but "cement milk" - a weak solution of Portland cement in water. Moreover, milk can be made on your own. To do this, several handfuls of dry cement should be introduced into the water and when the water changes color to a milky white color, the solution is considered ready.
At the next stage, you can start filling the perimeter of the base, filling the space between the foundation and the walls of the pit. The filling process in this case occurs both outside and inside the base box. Moreover, both branches of the process are carried out in stages: that is, earth, silt, sand are poured onto the bottom of the pit, from the inside and outside, in layers of 30-50 centimeters. Each layer is first moistened with cement milk, and then compacted.

At the end of the technology review, it should be noted that oversized pits are filled up with the help of heavy construction equipment - excavators, dump trucks and graders. And shallow foundations can be covered with a shovel, stretcher and proper motivation of a couple of workers.

Moreover, with both mechanical and manual backfilling, special attention should be paid not only to the movement of the soil itself

An equally important procedure is the compaction of the backfill layers. After all, this operation not only restores the natural density of the soil, but also prevents soil subsidence around the perimeter of the foundation.

After all, this operation not only restores the natural density of the soil, but also prevents subsidence of the soil along the perimeter of the foundation.