Heaving phenomena are insidious and unceremonious processes that occur in wet clayey, fine sandy and dusty soils during their seasonal freezing. They cannot be ignored, which is clear to anyone, even a developer with little knowledge of construction. Many realized this when they discovered a crack in the spring brick wall country house, seeing the skewed door and window openings of the frame country house building, noticing a dangerously tilted fence.

Heaving phenomena are not only large deformations of the soil, but also enormous forces - tens of tons, which can lead to great destruction.

The difficulty in assessing the impact of heaving soil phenomena on buildings lies in some of their unpredictability, due to the simultaneous impact of several processes. To better understand this, let's describe some concepts associated with this phenomenon.

Frost heaving, as experts call this phenomenon, is due to the fact that during the freezing process, wet soil increases in volume.

This happens because water increases in volume by 12% when it freezes (which is why ice floats on water). Therefore, the more water in the soil, the more heaving it is. Thus, a forest near Moscow, standing on very heaving soils, rises in winter by 5...10 cm relative to its summer level. Outwardly it is invisible. But if a pile is driven more than 3 m into the ground, then the rise of the soil in winter can be tracked by the marks made on this pile. The rise of soil in the forest could be 1.5 times greater if there were no snow cover to cover the soil from freezing.

Soils according to the degree of heaving are divided into:

– highly heaving – heaving 12%;

– medium heaving – heaving 8%;

– slightly heaving – heaving 4%.

With a freezing depth of 1.5 m, severe heaving soil is 18 cm.

The heaving of soil is determined by its composition, porosity, and groundwater level (GWL). Likewise, clayey soils, fine and silty sands are classified as heaving soils, and coarse sandy and gravel soils are classified as non-heaving soils.

Let's look at what this is connected with.

Firstly.

In clays or fine sands, moisture, like a blotter, rises quite high from the groundwater level due to the capillary effect and is well retained in such soil. Here wetting forces between water and the surface of dust particles appear. In coarse-grained sands, moisture does not rise, and the soil becomes wet only according to the groundwater level. That is, the thinner the soil structure, the higher the moisture rises, the more logical it is to classify it as more heaving soil.

The water rise can reach:
– 4...5 m in loams;
– 1...1.5 m in sandy loam;
– 0.5...1 m in dusty sands.

In this regard, the degree of soil heaving depends both on its grain composition and on the level of groundwater or flood waters.

Slightly heaving soil
– 0.5 m – in dusty sands;
– at 1 m – in sandy loams;
– 1.5 m – in loams;
– at 2 m – in clays.

Medium heaving soil– when the groundwater level is located below the calculated freezing depth:
– 0.5 m – in sandy loams;
– at 1 m – in loams;
– 1.5 m – in clays.

Heavily heaving soil– when the groundwater level is located below the calculated freezing depth:
– by 0.3 m – in sandy loams;
– by 0.7 m – in loams;
– by 1.0 m – in clays.

Excessively heaving soil– if the groundwater level is higher than for highly heaving soils.

Please note that mixtures of coarse sand or gravel with silty sand or clay will fully apply to heaving soils. If there is more than 30% silt-clay component in coarse soil, the soil will also be classified as heaving.

Secondly.

The process of soil freezing occurs from top to bottom, with the boundary between wet and frozen soil falling at a certain speed, determined mainly by weather conditions. Moisture, turning into ice, increases in volume, displacing itself into the lower layers of the soil, through its structure. The heaving of the soil is also determined by whether the moisture squeezed out from above will have time to seep through the soil structure or not, and whether the degree of soil filtration is sufficient for this process to take place with or without heaving. If coarse sand does not create any resistance to moisture, and it flows away unhindered, then such soil does not expand when frozen (Figure 23).

Figure 23. Soil at the frost line:
1 – sand; 2 – ice; 3 – freezing limit; 4 – water

As for clay, moisture does not have time to escape through it, and such soil becomes heaving. By the way, soil made of coarse sand, placed in a closed volume, which may be a well in clay, will behave like heaving (Figure 24).

Figure 24. Sand in a closed volume is heaving:
1 – clay; 2 – groundwater level; 3 – freezing limit; 4 – sand + water; 5 – ice + sand; 6 – sand

That is why the trench under shallow foundations is filled with coarse-grained sand, which makes it possible to equalize the degree of humidity along its entire perimeter and smooth out the unevenness of heaving phenomena. The trench with sand, if possible, should be connected to a drainage system that drains the perched water from under the foundation.

Third.

The presence of pressure from the weight of the structure also affects the manifestation of heaving phenomena. If the soil layer under the base of the foundation is strongly compacted, then the degree of heaving will decrease. Moreover, the greater the pressure per unit area of ​​the base, the greater the volume of compacted soil under the base of the foundation and the less the amount of heaving.

Example

B Moscow region (freezing depth 1.4 m) a relatively light timber house was erected on medium-heaving soil on a shallow strip foundation with a laying depth of 0.7 m. When the soil completely freezes, the outer walls of the house can rise by almost 6 cm (Figure 25, a). If the foundation under the same house with the same depth is made columnar, then the pressure on the soil will be greater, its compaction will be stronger, which is why the rise of the walls due to soil freezing will not exceed 2...3 cm (Figure 25, b).

Figure 25. The degree of soil heaving depends on the pressure on the base:
A – under the strip foundation; B – under a columnar foundation;
1 – sand cushion; 2 – freezing limit; 3 – compacted soil; 4 - strip foundation; 5 – columnar foundation

Strong compaction of heaving soil under a shallow strip foundation can occur if a stone house of at least three floors in height is erected on it. In this case, we can say that the heaving phenomena will simply be crushed by the weight of the house. But even in this case, they will still remain and can cause cracks to appear in the walls. Therefore, the stone walls of a house on such a foundation should be erected with mandatory horizontal reinforcement.

Why are heaving soils dangerous? What processes take place in them that frighten developers with their unpredictability?

What is the nature of these phenomena, how to deal with them, how to avoid them, can be understood by studying the very nature of the ongoing processes.

The main reason for the insidiousness of heaving soils is uneven heaving under one building

Soil freezing depth- this is not the calculated freezing depth and not the foundation depth, this is the real Freezing Depth in a specific place, at a specific time and under specific weather conditions.

As already noted, the depth of freezing is determined by the balance of the power of heat coming from the bowels of the earth with the power of cold penetrating into the soil from above during the cold season.

If the intensity of the earth's heat does not depend on the time of year and day, then the supply of cold is affected by the air temperature and soil humidity, the thickness of the snow cover, its density, humidity, pollution and degree of heating by the sun, the development of the site, the architecture of the structure and the nature of its seasonal use(Figure 26).

Figure 26. Freezing of the building site:
1 – foundation slab; 2 – estimated freezing depth; 3 – daytime freezing limit; 4 – night freezing limit

The unevenness of the thickness of the snow cover most significantly affects the difference in soil heaving. Obviously, the depth of freezing will be higher, the thinner the layer of snow blanket, the lower the air temperature and the longer its effect lasts.

If we introduce such a concept as frost duration (time in hours multiplied by the average daily subzero air temperature), then the freezing depth of clay soil of average humidity can be shown on the graph (Figure 27).

Figure 27. Dependence of freezing depth on snow cover thickness

Frost duration for each region is an average statistical parameter, which is very difficult for an individual developer to assess, because this will require hourly monitoring of air temperature throughout the cold season. However, in an extremely approximate calculation this can be done.

Example

If the average daily winter temperature is about -15 °C, and its duration is 100 days (frost duration = 100 24 15 = 36000), then with a snow cover 15 cm thick the freezing depth will be 1 m, and with a thickness of 50 cm - 0 .35 m.

If a thick layer of snow cover covers the ground like a blanket, then the freezing line rises; at the same time, both day and night its level does not change much. In the absence of snow cover at night, the frost line drops significantly, and during the day, when the sun warms up, it rises. The difference between the night and day levels of the soil freezing limit is especially noticeable where there is little or no snow cover and where the soil is very moist. The presence of a house also affects the depth of freezing, because the house is a kind of thermal insulation, even if no one lives in it (the underground vents are closed for the winter).

The site on which the house stands may have a very complex pattern of soil freezing and rising.

For example, medium heaving soil along the outer perimeter of a house, when frozen to a depth of 1.4 m, can rise by almost 10 cm, while drier and warmer soil under the middle part of the house will remain almost at the summer level.

Uneven freezing also exists around the perimeter of the house. Closer to spring, the soil on the south side of the building is often wetter, and the layer of snow above it is thinner than on the north side. Therefore, unlike the north side of the house, the soil on the south side warms up better during the day and freezes more strongly at night.

From experience

In the spring, in mid-March, I decided to check how the soil “walks” under the built house. At the corners of the foundation (on the inside) rods were concreted into paving slabs, along which I checked the subsidence of the foundation from the weight of the house. On the northern side the soil rose by 2 and 1.5 cm, and on the southern side by 7 and 10 cm. The water level in the well at that time was 4 m below the ground.

Thus, the unevenness of freezing in the area manifests itself not only in space, but also in time. The depth of freezing is subject to seasonal and daily changes within very large limits and can vary greatly even in small areas, especially in built-up areas.

By clearing large areas of snow in one place of the site and creating snowdrifts in another place, you can create noticeable uneven freezing of the soil. It is known that planting shrubs around the house retains snow, reducing the freezing depth by 2–3 times, which is clearly visible in the graph (Figure 27).

Clearing snow from narrow paths does not have much effect on the degree of soil freezing. If you decide to fill a skating rink near your house or clear an area for your car, you can expect greater unevenness in the freezing of the soil under the foundation of the house in this area.

Lateral adhesion forces frozen soil with the side walls of the foundation is the other side of the manifestation of heaving phenomena. These forces are very high and can reach 5...7 tons per square meter side surface of the foundation. Similar forces arise if the surface of the pillar is uneven and does not have a waterproofing coating. With such strong adhesion of frozen soil to concrete, a vertical buoyancy force of up to 8 tons will act on a pillar with a diameter of 25 cm, laid to a depth of 1.5 m.

How do these forces arise and act, how do they manifest themselves in the real life of the foundation?

Let's take, for example, the support of a columnar foundation under a light house. On heaving soil, the depth of the supports is set to the calculated freezing depth (Figure 28, a). Given the light weight of the structure itself, the forces of frost heaving can lift it, and in the most unpredictable way.

Figure 28. Raising the foundation by lateral adhesion forces:
A – columnar foundation; B – columnar-strip foundation using TISE technology;
1 – foundation support; 2 – frozen soil; 3 – freezing limit; 4 – air cavity

In early winter, the frost line begins to drop down. Frozen, strong soil grabs the top of the pillar with powerful adhesion forces. But in addition to increasing the adhesion forces, the frozen soil also increases in volume, causing the upper layers of the soil to rise, trying to pull the supports out of the ground. But the weight of the house and the forces of embedding the pillar in the ground do not allow this to be done while the layer of frozen soil is thin and the adhesion area of ​​the pillar with it is small. As the freezing line moves downwards, the area of ​​adhesion between the frozen soil and the pillar increases. There comes a moment when the adhesion forces of frozen soil to the side walls of the foundation exceed the weight of the house. The frozen soil pulls out the pillar, leaving a cavity below, which immediately begins to fill with water and clay particles. Over the course of a season, on heavily heaving soils, such a pillar can rise by 5–10 cm. The rise of the foundation supports under one house, as a rule, occurs unevenly. After the frozen soil thaws, the foundation pillar, as a rule, does not return to its original place on its own. With each season, the unevenness of the supports coming out of the ground increases, the house tilts, falling into disrepair. “Treatment” of such a foundation is a difficult and expensive job.

This force can be reduced by 4...6 times by smoothing the surface of the well with a roofing felt jacket inserted into the well before filling it with concrete mixture.

A recessed strip foundation can rise in the same way if it does not have a smooth side surface and is not loaded on top with a heavy house or concrete floors(Figure 4).

The basic rule for recessed strip and column foundations (without expansion at the bottom): The construction of the foundation and loading it with the weight of the house should be completed in one season.

The foundation pillar, made using TISE technology (Figure 28, b), does not rise due to the lower expansion of the pillar due to the adhesion forces of heaving frozen soil. However, if it is not expected to be loaded with a house during the same season, then such a pillar must have reliable reinforcement (4 rods with a diameter of 10...12 mm), which prevents the extended part of the pillar from being separated from the cylindrical one. The undoubted advantages of the TISE support are its high load-bearing capacity and the fact that it can be left for the winter without loading from above. No amount of frost heaving will lift it.

Lateral adhesion forces can play a sad joke on developers making columnar foundations with large supply according to bearing capacity. Extra foundation pillars may indeed be unnecessary.

From practice

A wooden house with a large glassed-in veranda was installed on foundation pillars. Clay and high level groundwater required laying the foundation below the freezing depth. The floor of the wide veranda required an intermediate support. Almost everything was done correctly. However, over the winter the floor rose by almost 10 cm (Figure 29).

Figure 29. Destruction of the veranda ceiling due to the adhesion forces of frozen soil to the support

The reason for this destruction is clear. If the walls of the house and veranda were able to compensate with their weight the adhesion forces of the foundation pillars with frozen soil, then light floor beams were unable to do this

What should have been done?

Significantly reduce either the number of central foundation pillars or their diameter. The adhesive forces could be reduced by wrapping the foundation pillars with several layers of waterproofing (tar paper, roofing felt) or by creating a layer of coarse sand around the pillar. Destruction could also be avoided by creating a massive grillage tape connecting these supports. Another way to reduce the rise of such supports is to replace them with a shallow columnar foundation.

Extrusion– the most tangible cause of deformation and destruction of the foundation laid above the freezing depth.

How can this be explained?

Extrusion is required daily allowance the passage of the freezing boundary past the lower supporting plane of the foundation, which occurs much more often than the lifting of supports from lateral adhesion forces having seasonal character.

To better understand the nature of these forces, let’s imagine frozen soil in the form of a slab. In winter, a house or any other structure becomes securely frozen into this stone-like slab.

The main manifestations of this process are visible in the spring. The side of the house facing south is quite warm during the day (you can even sunbathe when there is no wind). The snow cover melted, and the soil was moistened with spring drops. Dark soil absorbs sunlight well and warms up.

On a starry night in early spring especially cold (Figure 30). The soil under the roof overhang freezes heavily. A ledge grows from below a slab of frozen soil, which, with the power of the slab itself, strongly compacts the soil underneath due to the fact that wet soil expands when it freezes. The forces of such soil compaction are enormous.

Figure 30. Slab of frozen soil at night:
1 – slab of frozen soil; 2 – freezing limit; 3 – direction of soil compaction

A 1.5 m thick slab of frozen soil measuring 10x10 m will weigh more than 200 tons. The soil under the ledge will be compacted with approximately the same force. After such exposure, the clay under the protrusion of the “slab” becomes very dense and practically waterproof.

The day has come. The dark soil near the house is especially heated by the sun (Figure 31). As humidity increases, its thermal conductivity also increases. The freezing line rises (under the ledge this happens especially quickly). As the soil thaws, its volume also decreases; the soil under the support loosens and, as it thaws, falls under its own weight in layers. Many cracks form in the soil, which are filled from above with water and a suspension of clay particles. At the same time, the house is held by the forces of adhesion between the foundation and the slab of frozen soil and the support along the rest of the perimeter.

Figure 31. Slab of frozen soil during the day:
1 – slab of frozen soil; 2 – freezing limit (night); 3 – freezing limit (day); 4 – defrosting cavity

As night falls cavities filled with water freeze, increasing in volume and turning into so-called “ice lenses”. If the amplitude of the rise and fall of the freezing boundary in one day is 30–40 cm, the thickness of the cavity will increase by 3–4 cm. Along with the increase in the volume of the lens, our support will also rise. Over several such days and nights, the support, if it is not heavily loaded, sometimes rises by 10–15 cm, like a jack, resting on very strongly compacted soil under the slab.

Returning to our slab, we note that the strip foundation violates the integrity of the slab itself. It is cut along the side surface of the foundation, because the bitumen coating with which it is covered does not create good adhesion between the foundation and the frozen soil. The slab of frozen soil, creating pressure on the ground with its protrusion, begins to rise itself, and the fracture zone of the slab begins to open up and fill with moisture and clay particles. If the tape is buried below the freezing depth, then the slab rises without disturbing the house itself. If the depth of the foundation is higher than the freezing depth, then the pressure of the frozen soil raises the foundation, and then its destruction is inevitable (Figure 32).

Figure 32. Slab of frozen soil with a fault along the foundation strip:
1 – plate; 2 – fault

It is interesting to imagine a slab of frozen soil turned upside down. This is a relatively flat surface, on which at night in some places (where there is no snow) hills grow, which turn into lakes during the day. If you now return the slab to its original position, then exactly where the hills were, ice lenses are created in the ground. In these places, the soil below the freezing depth is highly compacted, and above, on the contrary, it is loosened. This phenomenon occurs not only in built-up areas, but also in any other place where there is unevenness in the heating of the soil and in the thickness of the snow cover. It is according to this scheme that ice lenses, well known to specialists, appear in clayey soils. The nature of the formation of clay lenses in sandy soils is the same, but these processes take much longer.

Raising a shallow foundation pillar

The foundation column is lifted with frozen soil by passing the freezing line daily past its base. Here's how the process happens.

Until the moment the soil freezing line drops below the supporting surface of the pillar, the support itself is motionless (Figure 33, a). As soon as the freezing line drops below the base of the foundation, the “jack” of heaving processes immediately starts working. The layer of frozen soil located under the support, increasing in volume, lifts it (Figure 33, b). Frost heaving forces in water-saturated soils are very high and reach 10…15 t/m². With the next warming up, the layer of frozen soil under the support thaws and decreases in volume by 10%. The support itself is held in a raised position by the forces of its adhesion to the slab of frozen soil. Water with soil particles seeps into the gap formed under the sole of the support (Figure 33, c). With the next decrease in the freezing limit, the water in the cavity freezes, and the layer of frozen soil under the support, increasing in volume, continues to rise the foundation column (Figure 33, d).

It should be noted that this process of lifting the foundation supports is daily (multiple) in nature, and the extrusion of the supports by adhesion forces with frozen soil is seasonal (once per season).

With a large vertical load on the pillar, the soil under the support, strongly compacted by pressure from above, becomes slightly heaving, and water from under the support itself is squeezed out through its thin structure during the process of thawing the frozen soil. In this case, practically no lifting of the support occurs.

Figure 33. Raising the foundation pillar with heaving soil;
A, B – upper level of the frost line; B, D – lower level of the frost line;
1 – grillage tape; 2 – foundation pillar; 3 – frozen soil; 4 – upper position of the frost line; 5 – lower position of the frost line; 6 – mixture of water and clay; 7 – mixture of ice and clay

A properly calculated foundation can withstand significant loads and preserve the integrity of load-bearing walls and the entire house for a long period. The design of any structure begins with foundation calculations.

Influencing factors

The choice of foundation design is influenced by many factors, the main of which are considered to be indicators related to the soil on the site:

• Soil type.
• Height of rise of groundwater.
• The depth to which the soil freezes in winter.

In addition, such indicators of the future home as the number of storeys, the selected construction material and design features(with or without a basement).

The calculated depth of the foundation and the volume of excavation work depend on these factors.

Freezing depth and the need to take it into account

The level of soil freezing is decisive in calculating the depth of laying the foundation for a building. There are two levels of freezing:

• Good conditions for laying a foundation are considered if the groundwater is located below the freezing level of the soil.
• Difficult conditions for laying and operating the foundation of a house include freezing of the soil layer with groundwater. In this case, the soil swells in winter, which leads to increasing loads on the base of the building.

Regulations require that the foundation be located below the freezing depth of the soil. Let's look at why.

In winter, lateral loads caused by swelling of the soil are added to the existing vertical loads on the foundation (gravity of the house and soil resistance). As the ground freezes, these forces increase, having a colossal impact.

If the foundation is not laid deep enough, then the frozen ground begins to put pressure on the sole, “pushing out” the foundation. Such loads can reach 10 tons per square meter of area. In addition, this force is uneven in different areas, so there is a slight distortion of the building. This is clearly visible when cracks begin to appear along the walls of the house, increasing each spring after the soil under the house thaws and subsides.

With the correct calculation and choice of the depth of laying the foundation of the structure (below the soil freezing level), the influencing forces become less. There is no effect of “pushing” the house out of the ground. The foundation does not warp and will last for a long time without subsidence or distortion of the load-bearing walls.

Advice! If the groundwater on your site comes too close to the surface and significantly complicates the construction of the house, try laying several drainage ditches into the nearest ravine. This will drain the building site and reduce the heaving of the soil.

Calculation of soil freezing

The formula by which this parameter is manually calculated looks like this: h=vM*k. Using this formula, you need to multiply the sum of average monthly temperatures by a special coefficient that is used for each type of soil:

• clayey - 0.23;
• sandy - 0.28;
• gravel - 0.30;
• coarse clastic -0.34.

The square root is taken from the resulting value. This is long and you have to consult reference books. Therefore, it is easier to take ready-made average values ​​of soil freezing by region. An example of such a table with some major cities is given below.

Influencing factors

Separately, we note that such calculations are averaged and are made without taking into account some data that affects the freezing depth. Here are two factors:

1. Snow cover in the region. In addition to natural moisture, snow cover is considered an excellent heat insulator for the soil. It follows from this that the more snow on the site, the less the ground freezes.
2. Purpose of the building. When constructing a residential building or a heated building, the level of freezing decreases. If the structure is not heated in winter, then the ground freezes more than average.

Take these factors into account when planning and developing the foundation, since the difference with the tabular data is up to 30%, which is important in the calculations.

Quite often, after the end of the winter season, cracks appear on the facades and plinths of cottages, door frames warp, or cracks appear in window frames. The cause of these troubles in most cases is the movement of foundation foundations caused by the forces of frost heaving of the soil, which arise as a result of an increase in the volume of soil when it freezes.

Almost all soils (except rocky ones) can be subject to frost heaving, but to the greatest extent this disadvantage is inherent in clayey soils (loams, clays, sandy loams, fine and silty sands), as well as sands containing silty clay particles. Gravelly, coarse and medium-sized sands that do not contain silt-clay particles are considered non-heaving.

As already noted, soils containing tiny dust and clay particles are subject to frost heaving. Compared to coarse and medium sands, these particles bind water very well. When freezing, the water-saturated mass increases significantly in volume and begins to put pressure on structures located in the ground and push them out of the ground.

Frost heaving deformations are the result of the influence of so-called normal and tangential forces on the structure. The former arise under the base of the foundation as a result of freezing and an increase in the volume of heaving soil, the latter - due to the vertical displacement of soil frozen to the side surfaces of the foundation or to the basement walls. In addition, the frozen soil, which has increased in volume, begins to press perpendicular to the surface of the basement walls, causing deformation of the foundations in the horizontal direction.

The heaving process intensifies with an increase in the humidity of heaving soils as a result of precipitation (in particular, heavy autumn rains), with capillary rise of moisture and an increase in groundwater levels.

In the Moscow region, 80% of all soils are classified as heaving, and their freezing depth is winter time can reach 1.4 m. Therefore, protecting foundations, pipes laid underground, areas covered with asphalt or tiles, as well as entrances to garages from deformations caused by frost heaving forces is an urgent need.To reduce the impact of frost heaving forces on underground structures during the construction and renovation of a house, it is recommended to take the following measures (Table 1).

Table 1.

Reasons causing deformation of structures	Design solution
The impact of normal forces of frost heaving on the base of the foundation	Installation of backfill (1) 100-200 mm thick under the base of the foundation from non-heaving soil: gravelly, coarse or medium-sized sand, gravel, crushed stone or sand-crushed stone mixture (sand 40%, crushed stone 60%)
The impact of tangential forces of frost heaving on side surfaces foundations and basement walls	installation of coating (2) on the side surface of foundations and basement walls, reducing their roughness and adhesion forces with frozen heaving soil to the freezing depth; backfilling (3) of the foundation cavities to the entire freezing depth with non-heaving soil; The width of the backfill at the bottom of the excavation must be at least 0.5 m.
Moistening heaving soil with precipitation	Construction of a blind area (4) with a slope of 3-5% away from the house, the width of which exceeds the width of the excavation for backfilling
Increase in the moisture content of heaving soil due to rising groundwater levels	Drainage device (5) to lower the groundwater level and drain it from the foundation
Siltation of non-heaving soils with silt-clay particles	Protection of sand bedding from the penetration of heaving soil particles into it using special filter materials (6)

Protection of foundations and basement walls from frost heaving deformations.

When constructing buildings on heaving soils, it is necessary to place a cushion of washed sand, gravel or gravel-crushed stone under the base of the foundation. A base made of these non-heaving materials will prevent the normal (pushing) forces of frost heave from affecting the base of the foundation.

It should be noted that when the groundwater level rises (in autumn, as well as during the melting of the snow cover), the backfill becomes surrounded by water saturated with particles of silty-clay soil. Migrating along with water, these particles penetrate into the bedding and clog it, gradually turning non-heaving soil into heaving one.

As a result, after several years of operation, the foundation again finds itself standing on soil that deforms when it freezes. The use of special filter materials (fiberglass, "Taipar", etc.) that allow water to pass through well, but prevent the penetration of the smallest silt-clay particles into the sand bed can prevent siltation of the bedding.

To reduce the impact of tangential forces on the foundation, it is recommended to replace heaving soil in contact with vertical surfaces of the foundation or with basement walls with non-heaving soil. Backfilling, which is carried out along the entire perimeter of the building, must (as in the previous case) be protected with a layer of filter material (Fig. 1).

Significant moistening of heaving soils leads to the fact that when they freeze, they increase in volume much more than soils with less moisture. This entails an increase in the level of deformation, and, as a consequence, the need for more serious protection of foundations from the effects of frost heaving forces. One of the ways to reduce the activity of heaving soils is to install drainage, which makes it possible to reduce soil moisture by lowering the groundwater level.

The traditional design is a system drainage pipes, placed in a layer of washed gravel that retains soil particles. The pipes are laid with a slight slope to ensure water flows into a special well or sewer.

Despite the presence of a gravel filter, during the operation of the drainage system, the drainage holes gradually become clogged with soil particles. Cleaning drainage is a rather labor-intensive process that requires the construction of special wells. Clogging of the system can be prevented by laying filter material ("Taipar" or fiberglass) around the drainage pipes, which does not allow the smallest particles to pass through and ensures effective work drainage system for a long time (Fig. 2).

If there is filter material, laying a layer of gravel around the drainage pipes is not necessary, but is recommended to increase the area of ​​water penetration into the drainage system.

Rice. 2

1. existing foundation;2. drainage tubes;3. filter material;4. washed gravel.

Insulation of foundation bases

The measures considered make it possible to reduce the impact of frost heaving forces, but not to eliminate their cause. Thermal insulation around the building can prevent frost heaving of the soil. The essence of this method is that the soil located near the building is protected from freezing by thermal insulation materials, thereby eliminating the cause of frost heaving.

To insulate the material, insulation materials are used that are capable of maintaining the necessary heat-protective qualities in a humid environment and absorbing loads from structures located above them. These requirements are best met by polyurethane foam (PPU) and extruded polystyrene foam (EPP) of various grades.

, is the most effective, both in terms of the required thickness of thermal insulation, since it has the lowest thermal conductivity coefficient, and in terms of service life, thanks to its unique chemical and biological resistance. PPU comes in slabs (in Lately due to its wide distribution, EPP is not widely used) and in the form of spraying.

has the greatest insulation efficiency when used in water-saturated soils, since, thanks to its seamless nature, it also provides additional waterproofing, which eliminates thermodynamic conventional moisture flows in cooling foundations and basement floors.

It has the best characteristics in terms of thermal conductivity, strength and durability, due to the highest quality microporous structure.Of no small importance is the fact that the proposed technology can be implemented both during the construction of new houses and during the operation of existing buildings, and the placement of thermal insulation material around the perimeter of the building allows not only to protect the soil from freezing, but also to insulate basements (Fig. 3 ).

The soil around the house is dug to a depth of 0.5-0.6 m. The dimensions of the excavation should ensure the laying of insulation with a width of at least 1.2 m. After this, a layer of washed sand with a thickness of at least 200 mm is poured onto the bottom of the trench, and a slight slope of the sand cushion is arranged in side away from the foundation and compacted thoroughly.

Thermal insulation boards made of extruded polystyrene foam are laid on the sand. The thickness of the slabs is taken depending on the thermal conductivity coefficient of the insulation (Table 2).

Table 2.

Insulation	PPU sprayed Foamglass	Other polyurethane foam sprayed	PPU slabs	EPP Styro-form, Stirodur	Other EEP	Polystyrene foam roll
Thermal conductivity coefficient of insulation / in the pie, taking into account the gaps W/m °C	0,02/ 0,02	0,035/ 0,035	0,03/ 0,045	0,03/ 0,045	0,036/ 0,054	0,04/ 0,065
Insulation thickness not less than, mm	40	70	90	90	100	120

We should not forget that heat losses through the outer corners of the building significantly exceed losses through the surface of the wall, therefore additional insulation must be provided in the corner area.

To do this, at a distance of 1.5-2 m from the corner, insulation is laid with a thickness 1.4-1.5 times greater than that shown in the table (Fig. 4).

Then the insulation is covered with a layer of sand or gravel at least 300 mm thick to the ground surface. Such insulation will prevent soil freezing and the appearance of frost heaving forces.

Insulating the base of the porch

Lots of trouble for owners country houses cause seasonal deformations of the porch and stairs at the entrance to the house.

The reason for this is frost heaving of the soil, causing bulging of the relatively lightweight design stairs. In addition, the base of the porch or staircase is located at a depth less than the base of the foundation, so frost heaving forces cause especially severe deformations of these structures.

The most radical way to protect the porch from bulging is to protect its base from freezing (Fig. 5).To do this, make a recess 700 mm deeper than the bottom of the porch or stairs. At the bottom of the excavation, a sand bedding with a thickness of at least 400 mm from washed sand or gravel is arranged. EPP or PPU slabs are laid on a compacted base, or the thickness of which is taken in accordance with the above table. A layer of sand of at least 50 mm is poured on top of the insulation, on which it is installed flight of stairs or porch. To protect the base from freezing, the insulation should protrude 1.2 m beyond the boundaries of the porch.

Protection of garage entrances from deformations, 
 caused by frost heaving of soils

At the entrance to the garage, as a result of frost heaving of the soil, unevenness may appear that prevents the normal opening of the gate.

The area in front of the garage is constantly cleared of snow, so the ground freezes to a greater depth, which entails an increase in the level of soil deformation caused by frost heaving forces. These phenomena can be prevented by installing thermal insulation under the road leading to the garage. To do this, a small pit about 400 mm deep is dug under the site or road. Its width on each side should be 1.2 m greater than the width of the road (Fig. 6).

At the bottom of the pit, a sand or gravel bedding with a thickness of at least 100-200 mm is arranged, on which slabs of extruded polystyrene foam of the required thickness are laid. It should be noted that, in addition to the ability to maintain high heat-protective characteristics in a soil environment, extruded polystyrene foam is a material that can withstand quite large loads, in particular from the asphalt surface of the road and the car standing on it.

The insulation material located under the road surface is covered with an additional layer of sand 200 mm thick, on which a slab or asphalt covering is laid. You can install a side stone on a sand bedding, burying it approximately 200 mm into the sand. The insulation located outside the coating being used is covered with a layer of sand (20-30 mm), after which the excavation is filled with soil and leveled.

Walking paths and areas in front of the house covered with tiles are insulated in the same way. We should not forget that the recess for the insulation should be 1.2 m wider than the platform or path on each side (Fig. 7).

Rice. 7		Rice. 8
sand or gravel bedding 200 mm thick; a layer of sand 30 mm thick; backfilling with sand and soil; site covering; sand bedding.		sand or gravel bedding 100 mm thick; insulated pipes; gravel-sand mixture 100 mm thick; extruded polystyrene foam; backfilling with sand, gravel or soil.

Protection of pipelines from freezing

Rice. 9

Typically, pipelines engineering communications(water supply and sewerage) are laid below the freezing level of the soil. However, at the entrance to the house, sections of pipelines rise closer to the surface and end up at the freezing depth, so this area must be insulated.

Construction of trenches 1.5-2 m deep for laying pipelines with subsequent backfilling takes a lot of time and is a rather labor-intensive process. The depth of installation of communications can be reduced by installing thermal insulation that protects the pipes and the adjacent area of ​​soil from freezing (Fig. 8). In addition, in heaving soils with a shallow burial depth, it will protect pipes from soil deformations caused by frost heaving forces.It should be noted that this work can be carried out not only during the construction of a new line, but also during the operation of the existing one.

Table 3.

At the bottom of the open trench, a compacted sand or gravel bedding about 100 mm thick is arranged, insulated pipes are laid on it and covered with a layer of sand or gravel (at least 100 mm), on which (after compaction) slabs of extruded polystyrene foam are placed or polyurethane foam is sprayed. The insulation is covered with sand or gravel (20-30 mm) on top, and then with soil.

Existing pipelines can be insulated by placing thermal insulation not only on top, but also on the sides (Fig. 10), and when laying new utilities, it is recommended to place them in a heat-protective channel made of polyurethane foam (pipes with polyurethane foam insulation are currently on sale) or sprayed ( Fig. 11).

When using slab insulation, to ensure the reliability of thermal insulation (minimizing gaps), it is advisable to connect the insulation slabs forming a heat-insulating channel to each other using screws, but it is still better to either purchase the pipelines in thermal insulation with PPU (pre-insulated pipes) or spray the existing ones with polyurethane foam.

Strip foundation - reinforced concrete structure with a rectangular cross-section. This type of building foundation is used for buildings made of various materials with a density of more than 1000-1300 kg/m 3. Its use is determined by the severity of the floors, the presence of a basement, and other factors.

It is not recommended to lay a strip foundation on deeply frozen and highly heaving soils.

It is generally accepted that the foundations of the main building and the adjacent extension are laid at the same depth. But if the difference in the loads of buildings on the foundations is large, the depth of their laying may be different. In this case, along the entire length of the foundation, ledges with oblique angles are made, connecting the multi-level parts of the structure. The height of the ledges should be from 300 to 600 mm, the angle does not matter.

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Factors influencing foundation depth

The higher it is located, the less concrete mixture will be required to fill it and, accordingly, the financial costs. But sometimes saving on this is unacceptable. The depth of laying the foundation of a structure depends on three main factors: the depth of soil freezing, the proximity of groundwater and the type of soil at the construction site.

Other factors that determine the degree of foundation deepening include the planned durability of the building (building class), the sensitivity of the house structures to uneven precipitation, and the topography of the site. Other characteristics of the object related to specific conditions are also of decisive importance.

Often the top layers of soil have strong compressibility and the ability to change their properties depending on weather conditions. The foundation in such areas must be buried on stable load-bearing soils, no matter how deep they are.

Based on their influence on the strength of the foundation, soils are divided into several groups:

• rocks, coarse rocks with sand, gravelly sands of large and medium size;
• fine and dusty sands;
• sandy loam;
• loams, clays, coarse rocks with clay filler.

There is an opinion that by deepening the foundation below the freezing layer, we solve everything possible problems with structural stability. But this method does not guarantee protection from the effects of frost heaving of the soil, especially for light buildings. When the pressure of the freezing layer on the base of the foundation is eliminated, its effect on the walls of the structure is preserved. This influence can be reduced in the following ways:

• a sliding layer is created on the side surface of the base from a material with a low coefficient of friction (construction film, coating or weld-on waterproofing, roofing felt);
• the foundation is poured in a trapezoidal shape with a narrowing upward;
• the soil near the foundation is protected using screens combined with devices against waterlogging ( storm drain, drainage);
• the foundation sinuses are filled up.

The primary task when designing a foundation is to determine the depth at which the load-bearing layer, together with the underlying layers, would ensure a uniform settlement of the structure, not exceeding the maximum permissible norm.

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Determining the depth of the foundation

To calculate the depth of the foundation of a building, simple studies of the soil of the site and calculation of significant parameters will be required.

Using the standard indicator, the depth of soil freezing on the site is calculated taking into account the heating mode of the building using the formula: Df=k×Dfn, where:

• Dfn - standard freezing depth;
• Df—estimated freezing depth;
• Kn is a coefficient that takes into account the heating mode of the building (SNiP 2.02.01-83).

The type of soil can be determined by kneading it in the palm of your hand and rolling it into a cord. Then try to shape the sample into a ring and notice its plasticity:

• if the ring remains intact, the soil is clayey;
• if it breaks up into fragments, it is loam;
• a ring that crumbles when rolled up - the soil consists of sandy loam.

If determining the type of soil is difficult, it is better to contact a specialist.

Then you need to determine what it is in the place where the strip foundation will be laid. A well is drilled to a depth of 2.5-3 m. A plastic or metal pipe is lowered into it so that soil does not fall into the well. Water levels are measured at different times of the year. Measurements are taken to determine whether groundwater rises above 2 m to the soil freezing depth.

Using the obtained data (calculated freezing depth, type of soil, groundwater level) and Table 2 of SNiP 2.02.01-83, the required one is determined.

If the groundwater level is more than 2 m below the soil freezing depth, the strip foundation is laid to a depth depending on the composition of the soil:

• gravelly, medium and coarse sands - 0.5 m;
• sandy loam and fine sand - at least 0.5 m;
• clays, loams, coarse soils - at least 0.5 Df.

When groundwater is closer than 2 m from the soil freezing depth (Df), the foundation is laid to a depth of at least Df.

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Ways to reduce the required foundation depth

To reduce the cost of laying foundations to great depths, measures are taken to reduce the impact of heaving soil on the foundation of the future structure.

The most radical way is to replace heaving soil with non-heaving soil. To do this, they dig a pit whose size exceeds the design parameters of the foundation to a depth below the freezing level. Instead of the selected soil, sand is poured and compacted. Sand has good load-bearing capacity and does not retain moisture in the structure. This method is the most reliable, but requires a large amount of excavation work.

The blind area equipment reduces the depth of freezing and waterlogging of the soil. They are concrete platforms with a slope of about 10°. The width of the platforms depends on the type of soil and the size of the roof overhang. On subsiding soils, the blind area is made about a meter wide.

To lower the groundwater level under construction site ditches are arranged to drain water along the slope of the terrain. Such structures are effective for drainage during rainfall and snow melting. For areas where the groundwater level is constantly elevated, thorough drainage systems are constructed.

There is another method that reduces the depth of soil freezing. It is relatively cheap and effective. It consists of laying polystyrene foam slabs under the foundation blind area. When using slabs up to 5 cm thick, soil freezing is reduced to a depth of 30 cm.

When building a non-massive wooden (frame, timber) house, you can save on deepening the foundation by installing it directly into the freezing layer at a shallow depth. But such a foundation must be well reinforced and laid above the groundwater level. The base, united along the perimeter of the building into a single rigid frame structure, redistributes uneven loads.

When the soil swells in one of the areas under the foundation, the structure does not crack, but rises, supporting the weight of the structure. At the same time, the plane of the base is maintained and no deformations occur in the structure of the house. To construct the foundation, sand and gravel must be added. The use of bedding makes it possible to smooth out uneven heaving of the soil, and the reinforced concrete frame distributes loads along the perimeter, preventing distortion of structures.

Frost heaving of soils poses a serious danger to all structures resting on the ground. Low-rise buildings, light structures, and roads are especially affected by swelling. Heaving occurs due to freezing of water. As the soil expands, it squeezes out structures, deforms them, and the soil level rises.

What forces act on buildings

Structures buried in the soil are subject to several multidirectional forces:

• normal - directed from bottom to top at the sole of the structure,
• perpendicular - act in a horizontal plane,
• tangents - frictional forces when raising or lowering soils.

The magnitude of the impact forces depends on the degree of soil moisture, their composition, and can vary greatly along the length of even one foundation. This only increases the danger, since uneven extrusion or bending of the structure occurs, which leads to its fracture.

What soils swell?

On the territory of Russia, up to 80% of the areas are heaving soils. Therefore, the problem of combating frost heaving is relevant for previously constructed buildings without proper insulation of the ground adjacent to the foundation.

All soils containing clay are prone to heaving - clays, loams, sandy loams, sands with silty clay particles. It is clay that contains cohesive water. Only coarse and medium-sized sands are classified as non-heaving.

Typical damage is cracks in foundations and walls, distortion of door and window openings, swelling of paths with the inability to open the door, distortion of light structures near the house. In the worst case, the walls collapse.

Soil insulation is the main method of combating heaving

The main method of combating frost heaving of the soil is to insulate the soil. Thermal insulation sheets create increased resistance to heat flow; as a result, the cold coming from the surface will not be able to freeze the layers under the insulation, since heat will constantly flow there from the ground, from the building through the foundation.

Previously used measures for backfilling structures with a sand cushion up to 0.5 meters thick, fencing it with canvas to prevent silting, and draining water by drainage, can be considered useful in addition to modern soil insulation.

The optimal insulation material that can be in the ground in an unprotected state is extruded polystyrene foam. It is quite strong and does not absorb water. Grades with a density of 35 kg/m3 are used. For insulation under the roads along which the car moves - 50 kg/m3.

Insulation dimensions

What thickness of insulation is required for effective soil insulation? According to the recommendations of specialists who carried out thermal calculations and based on the experience of operating insulated blind areas near houses, the minimum thickness of extruded polystyrene foam insulation is 50 mm. But around the corners of the building (within 2 m from the corner), where the cold accumulates, double thickness is needed.

It is recommended that the width of the insulation at the level of the soil surface be no less than the freezing depth. This will ensure sufficient band width with a positive temperature. But typical designs of shallow insulated foundations provide for the laying of horizontal thermal insulation at the level of the base of the foundation - 0.4 - 0.5 meters of depth, while the width of the insulation strip is much narrower and is determined by calculation. The wide pit is backfilled on top with non-heaving fine material.

Thermal insulation design

Sheets of extruded polystyrene foam insulation must be connected to each other in a groove; they must be laid close to the foundation insulation.

The strip is laid with a slope of 2 - 3% from the foundation to ensure water drainage from the house. Often, drainage is laid in the ground along the edge of the insulation, which removes water from the foundation.

A trench is made 0.5 - 0.6 meters deep. The bottom of the trench is filled with sand 10–20 cm thick, which also forms a slope away from the house.

Sheets of extruded polystyrene foam are laid on the sand and covered with waterproofing. The insulation is covered with a sand cushion at least 20 cm thick. Piece material for paths is laid on top of the cushion, which forms the blind area around the house. It is not recommended to concrete the blind area due to the unreliability of such finishing.

Insulation of soil under light buildings and roads

Very often it is necessary to insulate the soil under all kinds of extensions to the house - a veranda, a terrace, a staircase with a porch, a driveway to a garage, etc. All these buildings need protection from frost heaving. Soil insulation is carried out in the same way as near the foundation. But in in this case buildings are not heated and freeze in winter, so the soil under their entire area must be insulated.

A pit is made to a depth of up to 0.6 meters from the base of the structure and wider to the depth of freezing in each direction (calculated widening).

A sand bedding is placed at the bottom of the pit, which forms the flow of water in the desired direction (usually from the center of the structure). Sheets of insulation are laid on the bedding, covered with waterproofing material, and a sand and gravel bedding with a thickness of 300 mm or more is made on top, which forms a cushion to redistribute point pressures. Sometimes, for this purpose, ready-made reinforced concrete blocks are laid, or a light foundation is poured.

Thermal insulation of pipelines

Typically, pipelines are insulated with a shell made of extruded polystyrene foam. But this method is bad because if warm water (energy) stops flowing into the pipeline, it will still freeze in frozen soil, no matter how thick the shell is.

A pipeline laid not deep (below half the freezing depth) can be heated with the energy of the earth if an entire section of soil is insulated by analogy with the examples given above.
The insulation strip is laid at half the depth from the location of the pipeline, and the width of the sheets must be calculated. But the feasibility of such actions in comparison with the deep location of the pipeline should be determined by calculation; however, it is always more reliable to locate the pipeline below the depth of soil freezing. The width of the trench can be reduced slightly if you make a half-box from insulation - with side edges of small height.

Soil insulation has recently become very widespread and is the main way to prevent the effects of frost heaving on buildings.