Foundation 

How should backfill the foundation? Backfilling of the sinuses of the foundation

Backfill and soil compaction under cramped conditions are largely determined by the technological specifics of the work: the limited front of the work and the features of the geometric elements of the earthwork, which practically makes it difficult and sometimes excludes the possibility of using conventional machines. Soils in the axils of foundations, pipelines, collectors, manholes, under-floor bases inside buildings, at the intersections of various kinds of communications are most often subject to compaction (Fig. 4.13).

The conditions for performing backfill of the sinuses of the foundations, especially when erecting industrial facilities with a developed underground economy, are very diverse. The outer sinuses of pits, which are mostly wedge-shaped, in combination with the sinuses between underground structures and foundations inside buildings and structures often form a system of enclosed cavities and corridors with limited dimensions in the light and present serious difficulties when performing backfill.

For backfilling, small objects are handed over as a whole, and large ones in large parts after the completion of work on underground structures or after completion of work on a dedicated tier.

The most typical examples of backfill and soil compaction under cramped conditions are discussed below.

Soil compaction in the axils of the foundations for columns. At a column pitch of 6 m or more, when the foundations do not impede the movement of vehicles, soil is dredged from the farthest point on the work map “towards you”. In this case, dump trucks move along the base on which the soil layer is laid.

The dump truck unloading scheme is set depending on the distance between the axles of the columns. Ground filling is carried out in strips along the spans between the columns in order to reduce the complexity of layer-by-layer leveling of the soil.

At a column pitch of 6 m and the location of foundations that impede the movement of dump trucks, soil is poured into the lower layers of the backfill with the dump trucks traveling along the dumped soil covering the protruding parts of the foundations with a layer of at least 0.3 m thick to avoid damage.

The dump truck is selected taking into account the blade of the dozer and the maneuvering conditions at the work site.

Layer-by-layer soil leveling is carried out by D-159B, D-271M bulldozers, etc., and in less accessible places - by a small-sized bulldozer based on the T-54V tractor and the M-B-4 micro-bulldozer. When the clearance between the foundations of the columns is less than 0.8 m, where it is impossible to use bulldozers, the soil is leveled manually.

The work is carried out in two stages: Stage I - compaction of the soil between the foundations of the columns; Stage II - compaction of the soil above the foundations of the columns.

Soil compaction between the foundations of the columns is carried out in more cramped conditions than above the foundations. Compaction of the soil with heavy rammers, which have a large dynamic effect on the compacted soil, in this case is not allowed in order to avoid horizontal displacement of the columns.

To compact the soil in the area adjacent to the stand-alone foundations or other underground structures, compacting, vibration ramming or a combined effect on the soil is performed (vibro-rolling, viscous compaction with a load). For this, depending on the degree of tightness of the conditions of work and on the properties of the soil, use: self-propelled rollers with smooth rollers with cam bandages on them, vibratory rollers, self-moving vibrating plates manufactured by the GDR (type SVP), hydromechanical vibro-seals, electric self-moving vibratory rammers and electric rammers.

First of all, excavator penetrations should be carried out in the immediate vicinity of the foundations, then in the area between the foundations. After filling the foundations of the columns with soil, when there is a soil layer of at least 0.3 m above the upper edge of the foundation, they begin the work of stage II.

As an example in fig. 4.14 shows the technology of soil compaction in the trench with a number of separate foundations installed in increments of 12 m.

Before starting to fill the trench, the following work must be done: the construction of the foundations is completely completed and their design position is checked; waterproofing of foundations is made and checked; all auxiliary materials, equipment, mechanisms were removed from the trench; acts for covert work were drawn up and customer permission for backfilling was obtained.

Backfilling is carried out with imported soil, which is delivered to the place of work by dump trucks, filling and leveling of the soil, with the exception of the last layer, is carried out by an excavator-planner equipped with an extension of the boom and equipped with a loading bucket. The excavator-planner moves along the upper edge along the trench. Areas 40 cm wide around the foundations and kneecaps, as well as “dead zones” that are not accessible for leveling by the excavator-planner, are manually leveled.



The last layer is filled up and leveled by a bulldozer with a rotary dump.

Compaction of the cohesive soil is provided by manual electric tampers, and of the incoherent one by self-propelled vibrating plates, the lower layers being compacted with smaller vibrating plates, and the upper layers large. Soil compaction should begin with the areas around the foundations (sub-columns), and then in the area between the foundations (sub-columns). Each subsequent passage of the compaction machine should overlap the track of the previous layer by 0.1-0.2 m. The thickness of the compaction layer depends on the type of soil and the type of compaction machine (varies from 0.2 to 0.6 m).

Soil compaction in pits with complex foundations and underground structures. When erecting objects with complex foundations and underground structures that form a plan of a system of closed cavities, dead ends and narrow passages, the movement of large-sized machines along them is excluded. Backfilling of soil is carried out immediately after the construction of the underground part of the building or structure (dismantling and waterproofing of the surfaces of underground utilities) before starting work on the aboveground part.

The soil delivered by dump trucks using an excavator equipped with a grab (or conveyor system) is fed to the work card within the area limited by underground structures. Depending on the nature and size of the sinus, leveling the soil is carried out by a small-sized bulldozer type UZBT-54V or a micro-bulldozer MB-4. Soil compaction is carried out by electric rammers or a suspended vibratory rammer PVT-3. For this purpose, VP-1 or VPP-1 pile vibratory drivers mounted on a metal pallet can be used.

For compaction of soil in closed cavities, soil compactors suspended to the crane are more convenient, and when working in sinuses connected by passages, self-moving vibrating plates and manual rammers are used.

To fill in the sinuses that communicate with each other, use the flow method of work. At the same time, machines that perform layer-by-layer laying of the soil, after filling the soil layer in the zone of operation of the feed unit, are moved to the next work card, and soil compacting machines take their place.

When backfilling the sinuses of foundations with different depths of laying, at first they perform work within the area with lowered marks until they reach a common level, then work is carried out throughout the foundation pit.

As an example in fig. 4.15 shows the technology of the work on backfilling, leveling and compaction of soils in pits with complex foundations for technological equipment in industrial building   with a step of columns of 12 m.


The backfill soil is delivered by ZIL-MMZ-555 dump trucks, supplied by an excavator with a grab bucket with a capacity of 1 m3.

With a pit depth of 4 m, leveling the soil to a mark of 2 m is carried out manually. From the level of 2 m to ± 0.0 m, the soil is leveled with a DZ-14A bulldozer (shaded area in Fig. 4.15), and manually in hard-to-reach places. The soil is manually leveled also around structures within a radius of 40 cm. Compaction of incoherent soil of group I is carried out by means of vibrating plates of type SVP, of cohesive soil of group II by electric rammers of type IE.



If there is a bridge crane in the reconstructed workshop, the latter can be equipped with a clamshell bucket for feeding soil into the working area (Fig. 4.16). The soil is leveled with the MB-4 micro-bulldozer or manually in narrow sinuses between the foundations and walls, followed by compaction with a rammer and electric rammers. -

Soil compaction in narrow and deep sinuses. Narrow sinuses are generally considered to be sinuses with a width of less than 1.4 m (the maximum size of the sinuses that allows the operation of a small bulldozer). In sinuses with a width of 0.7 to 1.4 m, a worker can work, a width of less than 0.7 m is not possible for a worker to access.

The soil delivered to the place of work by dump trucks or loaders in the amount necessary for laying the compacted layer is poured on the edge of the pit and then fed with an excavator, an excavator-planner or pushed into the bosom by a bulldozer (Fig. 4.17). It is necessary to feed and push the soil dispersed to reduce the amount of work to level and move the soil at the bottom of the sinus.

Layer-by-layer soil compaction in the lower (narrowest) part of the sinus is carried out with PVT-3, VTM-2 suspended vibratory ramming plates or pile vibration dampers on metal pallets suspended from a crane installed on the pit edge.

In the middle (wider) part of the sinus, for leveling and compaction of the soil with layers of a given thickness, BM-4 micro-bulldozers and small-sized rollers are used, which are fed to the sinus with a crane. Then, as the sinus expands (more than 1.4 m), a small bulldozer based on the T-54V tractor is used.


Very narrow and cramped places saturated with communications, when the possibility of using mechanization means for layer-by-layer compaction is excluded, it should be covered with sandy soil. Sanding is carried out with concomitant abundant watering, creating the effect of hydraulic washing. This method is also suitable for backfilling in confined spaces, when sand is local soil and underground structures can be heavily moistened. This method is not applicable in winter conditions.

For compaction of backfills from loesslike loam in the axils of foundations with a thickness of more than 2 m, a deep method can be used. This method of compaction is based on the immersion of dies, which form wells with the displacement of the soil radially to the sides; while the soil is compacted around the well.

Work on the foundation is directly related to earthworkswhen digging a foundation pit, trenches and subsequently concrete mixture is poured into the formwork. The free space remaining around the foundation and called the sinus, it is necessary to fill up with soil and thereby strengthen the entire structure. This part of the work is called backfilling of the foundation sinuses with sand, and it is carried out only after filling and drying the foundation.

The external backfill of the foundation is carried out necessarily, and the internal backfill is done only if the house does not provide for the construction of a basement.

Than the sinuses of the foundation fall asleep:

  1. sand, gravel or sand-gravel mixture (ASG);
  2. clay.

Or for backfill, it should have ideal humidity for these works and be from 12 to 15%. In some heavy soils, this figure reaches 20%. Too wet sand is dried, and dry sand is added with cement milk. This mixture is a light cement binder and can be used as an adhesive. It can be easily made independently by mixing several handfuls of cement with water. The mortar will be ready for use when it changes color to a white hue.

The next step is the filling itself. It is important to note that filling the sinuses of the trench is done from the inside and outside at the same time, in layers. The rows should be soaked every 0.5 m with cement milk and compacted.

In no case does fertile soil be used for backfilling; a large amount of organic matter will decompose when decomposed.

Sand for backfill, gravel or sand-gravel mix do not belong to heaving soils, which is especially important in the cold season. This property does not allow the material to swell and does not create an additional load on the foundation. The negative point is that the sand is permeable, and all the water will drain exactly into the sand mixture inside the foundation. This functionality destroys the protective properties of waterproofing and reduces the bearing capacity of the soil under the foundation. To keep the foundation in perfect condition, during construction it is necessary to take care of the blind area and provide high tightness. In any case, to extend the durability of the foundation, it is important to prepare a good drainage system.

For filling the overall pits, specialized equipment is used: graders, escalators and bulldozers. Small volumes are easily filled in manually.

The clay for backfilling in the sinuses of the foundation will create an ideal barrier to water and provide natural protection against moisture. The practical properties of this material can protect the base from the accumulation of water and prevent the structure from collapsing.

Examples of soil and their filling

Sinus Ground backfilling
clay clay
loam loam, clay
sandy loam loam, clay, loam
sand sand, clay, loam, sandy loam

Step-by-step instructions for backfilling:

  1. clean the backfill from debris and check the condition of the soil. No pieces should remain in trenches building material, tools and foreign objects;
  2. check the humidity of the trench. It should not be too crude and comply with accepted standards in construction;
  3. If communications lie on the sinus of the foundation, then the pipes should be located at a depth of 30 cm from the bottom of the pit. If they are located above the specified distance, in this place for pipes, it is necessary to add soil and level the base;
  4. backfilling is carried out by specialized equipment - a bulldozer. Manually this process will take too much time. If tamping is not required for backfilling, then a small tuber from the ground is left on top, which will subsequently disappear. If ramming was used for backfill, then subsequently you should check the density of the layer, which may change and lose its shape.

Not only foundation pits but also narrow trenches for communications need backfill. The step-by-step process involves laying soil and sand cushions to preserve the integrity of the pipelines.

Construction work on the erection of a building of any type involves the process of laying the foundation for this structure. Depending on the type of foundation, such processes may consist of digging a recess or pit, installing a wooden structure (formwork), as well as installing a frame and reinforcing steel. After such organizational work, pouring a liquid solution into the formwork follows, which, after solidification, forms a reliable foundation for the home.

The backfill of the foundation is done after the concrete mixture has finally strengthened, and the foundation will be ready for the subsequent construction of the building. In the process construction work   along the perimeter of the base so-called sinuses (voids) are formed, which must be filled with a variety of materials. It would seem that it can be difficult in an easy, at first glance, operation. However, when carrying out backfilling, a number of nuances must be taken into account.

When backfill is done


No matter how much you would like to speed up the process of building a new building, do not rush to backfill the foundation. Firstly, you should wait until the concrete mortar has completely hardened and regains its quality characteristics. Secondly, it is advisable to finish all the work associated with the organization of the basement basement.

With good and sunny weather, the foundation freezes on average in just a week. Already after such a short period of time, the base will gain the strength it needs, which is necessary for backfilling. The thing is that the rush with backfill can entail the formation of strong pressure from the side, which can harm the structure of the base. Nevertheless, think about giving your preference to the option in which the foundation (or rather the solution) solidifies for about 20 days.

We select material for backfilling


Due to its characteristics, one of the following materials is used to organize backfill of the foundation in the construction industry:

  • Sedimentary rocks (sand);
  • Ground rocks (clay);
  • Soil, originally located on the site of a dug trench under the foundation.

Each of the presented materials has its own positive and negative qualities:

  • When using sand as a material for backfilling the base, it is possible to avoid negative consequences as a result of soil heaving in the cold season. This is achieved due to the good water permeability of this material, especially if it was previously optionally mixed with gravel or crushed stone. However, the good fluid throughput makes one doubt the choice of sand as the material for our work. The fact is that the groundwater located in the construction area will drain into the filled trench. As a result, the foundation waterproofing will have to experience constant loads, which will lead to its premature wear, and the level bearing capacity   under the base will greatly decrease over time.

But, as they say, each action has its own opposition. The problem is solved with the help of an elementary blind area, built around the house directly close to the foundation. A waterproof strip of concrete will prevent water from entering the soil near the base and prevent the soil from being eroded by large flows of fluid. You can construct a blind area with your own hands using both a concrete mixture and a waterproofing layer with underlying material. The latter option does not guarantee full surface tightness. Water flowing along the blind areas should also be diverted, therefore it is better to immediately take care of the organization of the drainage system.

  • Having resorted to the operation of clay as a material for filling the foundation, you can create an excellent barrier between the base of the house and groundwater. As an alternative, loams with sandy loams are also suitable. The main thing is that they have a denser structure than the surrounding soil layer.

Nevertheless, due to its heaving nature, clay can absorb moisture under the influence of certain factors, which, as we have already found out, will negatively affect the overall construction of the foundation and its characteristics.

  • Soil extracted from the upper layers of the soil during the organization of trenches for the arrangement of the foundation is advantageous primarily because there is no need to remove it from the construction site. Well, and secondly, the rest of the material can later be used to decorate the landscape.

Backfill Algorithm


If you want to protect yourself from future problems, you must carefully observe the technology for the upcoming work. The process of filling the base consists of the following successive steps:

  1. Before starting work, you should carefully inspect the trenches for foreign objects. In the recesses there should not be pieces of wood that rot over time and become a conductor of water to the base of the building, construction tools and concrete.
  2. Further, the soil is examined for moisture content. It is best to know the percentage of water in the material is possible only in laboratory conditions. The indicator is allowed in the region from 12 to 15% for heaving soils. For heavy, it increases to 20%. If the requirements do not meet, it is necessary to carry out measures to dry it, or vice versa - to moisturize.

With the drying of the soil should not be a problem. It is simply layered in the street under the sun. Moisturize the material for filling with a special solution, which is prepared from water and a small amount of cement. Cement is poured into a container with a liquid until a milky mixture is formed, which is subsequently irrigated with soil.

  1. The next step is directly filling the base. Prepared material is laid at the bottom of a trench or foundation pit. It is fixed by layers with an average thickness of 40 cm. Note that in no case should fertile soil be used as soil for backfilling. Most likely, it will contain a large amount of organic substances, which after some time will begin to decompose, which in turn will lead to shrinkage.
  2. Next, the basement is filled in the absence of basement. The internal sinuses in this case must be fully or partially filled.

You can backfill using special equipment (bulldozer, glider, excavator) or manually. With the large dimensions of the building, 2-3 workers are enough. Lay the soil in the pit evenly over the entire area. This will help to avoid the occurrence of pressure differences in different parts of the foundation, which can lead to deformation of the base.

Soil compaction


  • When using sand as a material for filling the pit and trenches, the thickness of the laid layers varies within 0.7 m;
  • If sandy loam or loam plays the role of soil for backfilling - 0.6 m;
  • Clay - up to half a meter.

If there is no possibility to use special equipment, then the thickness of the layers should not exceed 30 cm. Start the manual seal from the edges of the foundation to the slopes. Next, a blind area is erected to protect the backfill from moisture.

Backfill of communication holes

When filling out these tranches, you need to take into account some nuances:

  1. Before laying the pipes, the bottom of the recesses is covered with a 10 cm layer of crushed stone. After that, a sand pillow 30-40 cm thick is laid.
  2. After laying and checking the performance of the pipeline, filling is carried out. First, trenches are filled with sand 0.3 m thick and compacted with improvised means. Then layer by layer the soil is laid. The thickness of the layers is 0.5 m.

Remember that backfilling the base of the room is one of the most important stages of the entire construction. It is necessary to produce it only after completion of all work on the organization of waterproofing and basement coating of the foundation. Special construction equipment will help to speed up the process of backfilling of the sinuses, however, you can do all the filling and tamping work with your own hands.

Let's clarify some points in the following video:

The eternal problem of choice - what to fall asleep with sand or clay?

How does this look right for us:

Backfill the foundation with earth - this is a technological operation, mandatory for objects whose foundation was installed in a trench or foundation pit. There is a special technology for performing work on filling the sinuses of the foundation, which is regulated by the norms of SNiP 3.02.01-87 “Earthworks, foundations and foundations”. How to properly fill up the foundation of a built house, what types of backfill are - we will try to figure this article out.

Filling the strip foundation: how to do it right

Backfill is a complex and time-consuming process that must be performed with great care. The technology for filling the sinuses when performing backfilling should be followed as much as possible - this applies to the timing of the start of the process; the choice of soil mixture for work, the proper implementation of the compaction of soil and other aspects.

Deadlines for starting sinus backfilling

After pouring strip foundation   at home it is impossible to continue work on insulation, cladding, it is necessary to wait until the foundation foundation gains design strength. To gain strength, the foundation structure must be left at least 28 days, only after this you can continue to work on building the house.



The existing technology of the device foundation strip of the house does not allow the dismantling of the formwork earlier than 14 days after completion of concreting of the supporting elements of the base, but even after removing the formwork panels, filling the foundation is not allowed.

If it is planned to arrange a basement at the building under construction, construction works ground floor it is necessary to finish completely before starting backfilling of the sinuses of the foundation.

Filling the foundation sinuses with soil after a set of installation works on laying floor slabs and waterproofing works.

Backfill: material requirements

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

Using a soil previously selected from trenches for a home can significantly save money on soil removal from a construction site. The use of sand mixtures or gravel for filling the sinuses is not recommended, but there is one exception - it is allowed to fill very narrow gaps with a gravel-sand mixture, and not with the ground.



If the backfilling of the sinuses of the trenches (foundation pit) is nevertheless carried out with a sand mixture, it is necessary to check the compaction coefficient, which should be close in value to the compaction coefficient of natural soil at the building site in a natural state.

It is very important to remember that it is not allowed to apply for backfilling soils containing chernozem and the upper fertile layer.

Why is soil moisture important?

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



It is allowed to use soil of natural humidity for backfilling, therefore, dried up 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 soil soaking procedure, a special cement milk is prepared by shutting the water with a small portion of Portland cement.

Soil moisture should be from 12% (thin sands, clay rocks) to 20% (heavy soils). If the soil moisture indicators are above these values, the soil is pre-dried, evenly mixing several times.

How to do the filling

The soil is added along the perimeter of the entire foundation belt of the house, with prepared soil of a certain moisture content, the free space is filled between the foundation and the walls of the trench (pit) inside and outside the foundation structures.

The soil is added along the outer and inner perimeter of the house, while the layer thickness should not exceed a height of 30-50 cm. The ground layer is spilled with cement milk and compacted. Work is carried out simultaneously inside and out. The sinuses of the pit are filled with soil in layers, each layer is shed with a solution of cement milk.



Does the house provide for a basement, basement or underground technical floor? In this case, it is allowed not to fill the foundation of the house internally, or to partially fill the sinuses with earth.

The depth of the internal backfill directly depends on the type of foundation and the constructed grillage. In the strip foundation, the height of the backfill inside the perimeter of the building is limited by the level of vents, or is performed to the height of the basement.

For columnar and pile foundations, the pit bosoms or trenches are poured inside to the full height. Is there a high base for the house? Then inside is an additional dusting of structures with a slope from the inner wall of the base.

The top layer of soil backfill in a compacted state should reach the level of the blind area.

Backfilling of the overall sinuses of the pit inside and out can be performed using special earthmoving equipment. With mechanical filling of the sinuses, special attention should be paid to layer-by-layer compaction of the soil. A poorly tamped backfill threatens with subsidence along the entire perimeter of the foundation belt of the house. Watch a video on how to choose material for backfill and make it during construction.

Why is it important to comply with work technology?

Compliance with the technology when performing backfilling of the sinuses guarantees the strength and durability of the entire constructed building:

  • Soil, filled in the sinuses without compulsory compaction, after the first wintering will give significant sediment, which is fraught with failures of the blind area.
  • The soil prepared for backfilling trenches should be carefully checked for sharp metal objects that, when tamping, can damage the layer of waterproofing materials.
  • Calcareous and organic inclusions are unacceptable in the soil - quickly decaying, they leave significant empty shells behind them, which causes an uneven subsidence of the blind area and its destruction.

The protective function of the blind area is to protect the walls of the building and its foundation structures from moisture. If the blind area sags, it will not be able to fully protect the erected building, this can lead to the destruction of the building.

One of the conditions for the normal stability and durability of the reinforced concrete foundation is the correct joining of the side walls and the bottom of the foundation pit with the foundation box to the building’s foundation box. Despite the outward simplicity of the procedure, backfilling of the foundation has a fairly large number of nuances that must be taken into account in order to achieve the calculated characteristics of the foundation. Moreover, the problem of equipping the pit sinuses is so important for the foundation that a separate

Why backfilling is so important

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 the technological space between the wall and the formwork of the concrete wall or sinuses. A small, at first glance, increase in the dimensions of the pit makes it possible:

  • Remove the outer shields of the foundation formwork from the sinus space without the risk of collapsing the pit wall. In addition, the presence of an external cavity of 15-20 cm wide for a pit or trench, only a meter deep in the sinus, accelerates the setting process of concrete by 15-20%;
  • To waterproof and warm the concrete surface of the walls of the strip foundation with plastering or roll waterproofing. Even the small size of the pit sinuses of 40 cm allows the use of gas burners or roller equipment and insulation work with high quality;
  • To establish a drainage system that removes most of the groundwater at the base of the supporting part and the heel of the foundation.

Note! The foundation in the pit with sinuses simplifies the technology. In some cases, such as arranging the extended supporting surface of the foundation tape, it is impossible to build it without excavating the sinuses in the ground.

But the use of the pit sinuses as a technological technique leads to a significant deterioration in the stability of the foundation and the entire building box in the future. The ability of a building to withstand lateral and shear loads depends on how rigid and stable the soil layers are, compressing the foundation frame. From this it follows that how correctly backfilling of the sinuses of the pit will be performed, the foundation and the supporting elements of the building will be so stable.


In fact, backfilling of the foundation trench is a necessary measure aimed at arranging a bundle of concrete tape with natural density mother soil. In addition, backfilling of the soil allows you to effectively protect from flooding the layers on which the base of the foundation rests. If this is not done, or backfilling of the sinuses is the first material that comes to hand, waterlogging of the support on loams or clay soils will lead to cracking of the foundation even without the participation of heaving forces.