Is the loam heaving or not? Heaving soils Sandy loam heaving soil or not

Heaving soils cause many problems for builders. In winter, they can greatly increase in volume, putting increased pressure on the foundations of the building. At the same time, the structure rises unevenly from the ground, and serious cracks appear on the walls. Before you fight a phenomenon, you need to understand what it is.

A difficult issue during independent construction is determining what kind of soil is available: heaving or non-heaving. According to GOST 25100-2011, all bases are divided into five groups according to the degree of frost heaving:

• excessively heaving;
• highly heaving;
• medium heaving;
• slightly heaving;
• not heaving.

The last group can be called conditional. There are practically no types of soil in which frost heaving forces will never arise. The category of safe foundations includes only coarse rocks and granite, the occurrence of which on the surface is extremely rare.

Soil type does not have as much influence on the likelihood of frost heave forces occurring. The factor causing this phenomenon is not the soil, but moisture and negative temperatures. If certain conditions are met, negative phenomena can occur in almost any area.

The susceptibility of soil to heave is influenced by such properties as:

• capillary activity;
• filtration ability.

According to these indicators, clayey soils become the most dangerous types of soil. These include clay, loam and sandy loam. These soils do not filter water well, retain it and do not allow it to pass into deeper layers. The liquid remains dangerously close to foundations.

Types of soils.

At the same time, clays are characterized by high capillary activity. For comparison, sandy types of soil are capable of drawing water up to about 30 cm. This property is relevant when precipitation falls or snow melts. Moisture extends only 30 cm from the source. In this case, the foundations are protected from frost heaving by a blind area of ​​standard meter width. Clay can attract moisture to a distance of 1.5 m; to protect it from atmospheric moisture, you will need to build a very wide blind area to prevent damage.

If the groundwater level is high, even conditionally non-heaving soil types (coarse and medium sand) can lead to problems. The danger of frost heaving in sand can also appear under the influence of other factors (for example, a house is located on a site with a slope, even a slight one).

Why is frost heaving dangerous?

The combined effect of moisture and low temperatures on the soil leads to an increase in its volume. For any building, uneven deformations that are characteristic of frost heaving pose a particular danger. This is due to the fact that the soil under the outer walls is slightly heated by the building, and in the middle of the house the temperature is above zero.

A crack caused by heaving.

External walls, and especially corners, can rise relative to the initial level by 15 cm. In this case, deformations under the internal walls do not occur or they are small. Uneven lifting leads to the appearance of inclined cracks in the walls.

Frost heaving also has a negative impact on the side surface of the foundation.

Ways to fight

To prevent heaving soils from causing problems during operation, it is necessary to combat the causes of frost heaving of clays and other types of soils at the stage of foundation construction. Methods of control depend on the scale of the problem and the type of supporting part of the house. Most often, activities are provided in a complex.

Buried foundations

Every builder knows that to effectively combat frost heaving, it is necessary to lay the building supports below the depth of soil freezing. This value is found using special tables and maps or calculated using the formula from the SP “Foundations of buildings and structures”. But taking such measures is not always enough. When laid deeply, it is possible to avoid impacts on the base of the foundation, but tangential forces remain that act on its lateral surface. They can be decomposed into:

• vertical, which in some cases are capable of lifting structures;
• horizontal, bending foundations.

The strength of frost heaving depending on the depth of its occurrence.

Control methods depend on the type of structure and foundations. For massive buildings with deep foundations, one or more of the following measures can be recommended:

• coating waterproofing, which not only protects the foundation material from getting wet, but also impairs the adhesion of the soil to it (prevents structures from being lifted);
• insulation is carried out for the same purpose; extruded polystyrene foam is often used, which also takes on the function of protecting against moisture;
• drainage and filling the sinuses with coarse or medium sand allow moisture to be removed from the building;
• an insulated blind area prevents freezing of the soil in the immediate vicinity of the house, which means it eliminates one of the factors necessary for the occurrence of heaving;
• competent calculation and execution of reinforcement will allow the elements to withstand horizontal influences.

If the building is made of lightweight materials or has only one floor, it is recommended to use foundations using TISE technology. Such supporting elements are piles that widen towards the bottom. Due to the increased cross-section, it becomes almost impossible to pull the element out of the soil.

To protect this type of foundation from horizontal influences, you will have to consider the following points:

• competent calculation of the working reinforcement of the pile;
• rigid coupling of the pile to the grillage using reinforcement;
• calculation of the grillage for increased soil pressure on the side surface.

With a large freezing depth, installing a buried foundation with insulation, waterproofing, drainage and a warm blind area is not economically profitable. It will be easier to build shallow supports. Deepening will be justified only if:

• the need for a basement or ground floor;
• poor soil strength indicators closer to the surface.

Shallow foundations

Such designs have several advantages. They reduce the cost of building foundations and reduce the time required to complete the work. Shallow foundations can be used when the groundwater level is sufficiently high (at least 1.5 m).

The following measures used in the complex will help protect these types of building support elements:

1. . This design will reduce the depth of freezing of the base. The exact mark for safe laying of the sole depends on the climate, the thickness of the insulation and the width of the blind area. In most cases, it is advisable to use a protective strip 1 m wide with insulation 5-10 cm thick. The depth of the foundation will be 0.7 - 1 m.
2. . If you forget about the thermal insulation of the base, the foundation of the house will become an excellent conductor of cold under its own sole. For work, it is recommended to use extruded polystyrene foam (penoplex). It is fixed to the entire height of the supporting part of the house: from the sole to the base. The thickness of the insulation above the blind area is on average 100 mm, and below you can use penoplex with a thickness of 50 mm. Additionally, the material protects foundations from moisture, increasing their service life.
3. . The system eliminates the second factor in the occurrence of frost heaving: moisture. For drainage to work effectively, it must be positioned correctly. The pipe is laid next to the building site, but not under it. Drainage should be located below freezing or in a place where it does not occur (within the range of the insulated blind area). If pipes are laid in frozen ground, they may break in winter. You will also need to observe the recommended slopes of the drainage pipes, which depend on the cross-sectional diameter.

If it is not possible to install drainage (the work is highly complex, there is nowhere to drain it, etc.), you can only get by with a blind area. In this case, the protective strip around the perimeter of the building is made wide. It should completely prevent access of atmospheric moisture to the foundations. For clays, the width should be more than 1.5 m. Landscaping around the building is done so that the slope of the site is in the direction from the house.

The method is applicable if the following conditions are simultaneously met:

• good strength characteristics of the base under the black soil layer;
• low natural soil moisture;
• deep occurrence of groundwater;
• absence of slopes towards the building on the site.

With proper selection of the type of foundation and timely adoption of measures to combat frost heaving, serious problems during the operation of the house can be avoided. A careful approach to the issue will allow you to find an effective option that requires the least labor and financial costs.

Today, such a sector of the national economy as private construction is developing very actively. A special place in this area is occupied by the construction of the foundation. The foundation is the basis of any building and structure, which ensures the stability and strength of the entire building. Without knowledge of the nature of the soil, it is practically impossible to build a foundation correctly and safely. To build a foundation with your own hands, you need to carefully study the hydrogeological features of a particular land plot. Indicators such as soil freezing depth, soil moisture, and groundwater level are of great importance.

The property of soil, such as heaving, depends on these indicators. It's quite dangerous to build. Subsequently, this can cause distortion of the foundation and the entire building. The latter can cause cracks and defects in the walls. In order for the foundation to be protected from heaving forces, it is required to build it on dry and non-heaving soils. Let us consider in more detail what features non-heaving soil has, what relates to it, what measures can be taken to protect the foundation and the building itself. In addition, here you can learn about the use of non-heaving soil foundations.

Non-heaving soil type

Testing the soil is an important stage in a builder’s entire work. Before you directly build the foundation for a house, you need to know what heaving is. So, non-heaving soil is a soil that is not subject to frost heaving. Heaving includes such a concept as the degree of heaving. It shows how much soil can expand in volume as a result of freezing at low temperatures.

Non-heaving soils are soils that have a heaving degree of less than 0.01.

This indicates that when the ground freezes to a depth of 1 m, the soil increases in size by less than 1 cm.

Why does this phenomenon occur? It's quite simple. During the cold season (autumn or winter), the water that is located directly in the soil begins to freeze, turning into ice. According to the laws of physics, ice has a lower density than water, so its volume increases. This is called heaving. The soil, increased in comparison with the initial state, can exert great pressure on the foundation and change its location, the same applies to the entire building. In addition, moisture that gets directly into the foundation itself can gradually destroy it and render it unusable. All this is typical for heaving soil. For non-heaving soil, everything is different.

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Classification of soil according to the degree of heaving

Before you do it yourself, you need to know the type of soil depending on its ability to increase in size at low temperatures. There are 4 types of soil: non-heaving, weakly, medium and highly heaving. The classification is based on the value of indicators such as water saturation coefficient and soil fluidity index. Non-heaving soils include those whose degree of heaving is less than 0.01. Slightly heaving soils include clay with a fluidity value from 0 to 0.25, silty and fine sands with a water saturation coefficient from 0.6 to 0.8. This group also includes coarse soils with filler. The latter may be fine and dusty sand.

In this case, its amount should be in the range from 10 to 30% in the mass ratio. The group of medium heaving soil includes soils with a degree of heaving from 0.035 to 0.7. These include clay with a fluidity from 0.25 to 0.5; sands are fine and dusty with water saturation from 0.8 to 0.95; coarse soils with filler content of more than 30% by weight. The greatest danger is posed by highly heaving soil. It is represented by the following indicators: degree of heaving more than 0.07; clay fluidity is more than 0.5; fine sands with a water saturation of more than 0.95.

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Characteristics of non-heaving soil and features of foundation construction

As mentioned above, it is best to build the foundation on safe soils. Non-heaving soil includes rocky and clastic soil. The latter is formed as a result of the destruction of rocks. This includes gravel and crushed stone. For the most part these are coarse-grained materials. They are often used in construction. This soil group includes medium and coarse-grained sand. There is some relationship between and the size of its particles. The larger they are, the safer this layer of soil is and the less impact it has on the foundation.

The foundation is laid for this type of soil using the following technology. Regardless of the depth of soil freezing and its moisture content, it is built shallowly, that is, not deep. This saves time and effort on excavation work. If there is rock, the foundation may not be built at all. In some European countries, for example in Montenegro, certain regions of Germany and Finland, houses are built without a foundation due to precisely these terrain features. In the presence of coarse sandy soil, the thickness of the concrete foundation is only about 20 cm.

Undoubtedly, these calculations are relevant only for small houses, and not for multi-story structures. After pouring the concrete, when it hardens, you can immediately erect the building’s base or walls. In other cases, when the nature of the soil is different, a trench 50-70 cm deep is dug. After this, it is covered with several layers of coarse sand, each 15-20 cm thick. It is important that all layers are thoroughly watered. As for what kind of foundation can be built, there are no restrictions. It can be monolithic (slab), columnar or strip. For heaving soil, a columnar foundation or an anchor-type foundation is most optimal, since in this case the load, including the action of tangential forces, on the foundation will be minimal.

Heaving soil is a soil mass that expands in the winter and puts strong pressure on the foundation walls. It leads to the destruction of the structure, its “pushing” out of the pit.

There are types of structures for construction in such conditions and a list of rules for work: from to reinforcement.

Calculation of heaving intensity in the area

To calculate the degree of soil heaving at a construction site with your own hands, you need use the formula: E = (H - h) / h, wherein:

• E – corresponds to the degree of soil heaving;
• h – height of the soil mass before freezing;
• H – height of the soil mass after freezing.

To calculate the degree, it is necessary to take appropriate measurements in summer and winter. The soil can be considered heaving whose height has changed by 1 cm when freezing by 1 m. In this case, “E” will be equal to the coefficient 0.01.

Soils that have a high moisture content are more susceptible to heaving processes. When it freezes, it expands to the state of ice and thereby raises the ground level. Heaving soils are considered to be: clayey soils, loams and sandy loams. Clay, due to the presence of a large number of pores, retains water well.

What is heaving soil and why is it dangerous? (video)

How to remove the effects of heaving on the ground?

There are simple ways to remove heaving around the foundation with your own hands:

1. Replacing the layer of soil under and around the base with a non-heaving layer.
2. Laying the foundation on a soil mass below the freezing layer.
3. Insulation of the structure to prevent soil freezing.
4. Drainage

The first method is the most labor-intensive. To do this, it is necessary to remove the heaving soil at a depth below the freezing level of the ground, and fill in its place with heavily compacted sand.

It shows high load-bearing capacity and does not retain moisture. The large volume of excavation work makes it the least popular, although it is an effective way to combat heaving. This technique is effective for laying low-rise buildings, shallow recesses, for example, a barn.

A feature of the second method is the removal of the influence of heaving on the base of the foundation, but its preservation when exposed to the walls of the foundation. On average, the lateral pressure on the walls is 5 t/1 m2. With its help you can build brick houses.

The third method allows you to make a shallow foundation for a private house with your own hands in heaving conditions. The essence of the method is to lay insulation along the perimeter of the foundation to its entire depth. The calculation of the material is done as follows: if its height is 1 m, then the width of the insulation should be 1 m.

To drain water around a house or barn, you need to build drainage. It is a ditch at a distance of 50 cm from the building, the depth of which is the same as the level of the structure. A perforated pipe is laid in the drainage trench at a technical slope and wrapped in geotextiles, and then filled with gravel and coarse sand.

Below we will consider the types of bases that can be used on soil prone to heaving.

Shallow strip foundation on heaving soils

An effective way to make a strong foundation for a house or barn is a shallow (low depth) strip foundation on heaving soils. This is a concrete strip with reinforcement elements, arranged around the entire perimeter of the building and in places where load-bearing walls lie.. To build a shallow foundation with your own hands, you must follow these steps:

1. , 50-70 cm deep. The width is calculated based on the width of the base itself plus formwork, insulation or waterproofing, as well as decor.
2. Lay the slopes of the open trench with waterproofing. For this purpose, roofing felt and film are used.
3. Fill the excavation with layers of compacted sand, 20-30 cm each. To compact the material, it is periodically moistened with water.
4. Place formwork from any available material (board,).
5. Lay a hydroprotective barrier on the sand.
6. Make a reinforcing belt with a rod diameter of 12 mm.
7. Fill the shallow foundation with concrete mortar.
8. Lay the second layer of the reinforcing belt into the shallow foundation using a liquid mortar (a feature that is required only by the shallow type of foundation)

Welding is not used to connect reinforcement. To make the non-buried foundation more rigid, a 20 cm long wire is used.

Columnar foundation on heaving soils

The structure can be used to lay a house or barn on heaving soils, the freezing level of which does not exceed one and a half meters. The columnar foundation was based on ready-made piles. Their height reaches 3-4 m.

If you plan to build a small building, then such types of piles as driven ones made of wood or reinforced concrete, as well as screw ones, are effective. Wood is a less durable material for foundation purposes.

The columnar foundation is laid below the soil freezing level, so only lateral heaving pressure is maintained. Compared to buried strip structures, it is insignificant, since the area of ​​the pile is smaller.

Among all types of foundation pillars, screw piles for foundations are the most convenient. To make a columnar foundation with their help, you do not need to drill wells. The screw blades will do all the work.

All watery types of soil are accessible to the pile structure: swampy, damp areas. To give the building rigidity, the pillars are connected with support-anchor platforms. To do this, the pillars are screwed into the ground.

On their surface you need to make formwork, lay out a reinforcement frame stitched with metal wire and fill it with concrete mixture. Calculation of the level of the concrete strip is equal to the soil surface or slightly below.

TISE technology is a new way to combat heaving

For laying a foundation with your own hands, the most affordable design is TISE. It is a structure whose piles are connected by a grillage. Chise can be used for brick, frame or stone construction.

Slab foundation under heaving conditions

There are other ways to construct a foundation on heaving soils. In addition to TISE, shallow and columnar foundations, slab foundations are used. This is one that resists heaving due to the large area of ​​the sole.

It is effective with a simple building design, when the foundation is a square or rectangle. Calculation of materials shows that this is the most expensive, but no less reliable type of structure. Made from concrete or reinforced concrete.

A monolithic foundation requires a low base. The calculation of the width of the monolithic slab is made depending on what material is used to build the walls.

The average indicator corresponds to parameters from 15 to 35 cm. 15 cm is suitable, for example, for wooden structures, and 20 cm for brick ones. To lay utility lines in the slab, holes of the appropriate diameter are made in advance.

What type of foundation to choose - shallow, columnar, slab or TISE - depends on the ability to use technology, the size of the house, its configuration and the financial capabilities of the developer.

The problem of building buildings on heaving soils often arises in damp regions located in the temperate climate zone. To date, many different methods of combating frost heaving have been developed and tested in practice.

The main thing is to choose the most suitable one specifically for your construction conditions, and then the building will serve you without destruction or deformation for many years. Let us consider in more detail the issue of such construction and practical methods for solving it.

Watch a video about construction on heaving soils

What is heaving soil?

As you know, water turns into ice when it freezes. In this case, its volume changes due to the different densities of ice and water: water has a significantly higher density than ice. Accordingly, when water freezes, gradually turning into ice, it expands, occupying a larger volume.

If such water freezes while in the ground, then the soil will expand along with it. In this case, the forces expanding the soil will be called the forces of frost heaving, and such water-saturated soil itself will be called heaving.

What is the danger of heaving soil for a building?

Let's see what happens to the heaving soil directly next to the building. In winter, when frost sets in, water freezes and expands, turning into ice. Along with it, the soil containing it begins to expand. Frost heaving forces arise.

Forces begin to act on a nearby building, more precisely on its foundation, lifting it. In the spring, when the temperature rises, the opposite process occurs: the building lowers due to the fact that the ice melts, turning into water and, accordingly, shrinking, increasing its density and reducing its own occupied volume.

If the foundation is not protected from heaving forces, then the building may shift, which sooner or later will lead to the formation of cracks in the walls of the building and the foundation, and then to the destruction of the building.

Features of heaving soils

Heaving soils can be understood as any soils that can retain a sufficiently large amount of water in their volume. The more water there is in a unit volume of soil, the more prone the soil is to heaving.

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The most striking representatives of heaving soils are clay and yellow (quarry) fine sand containing a large number of clay inclusions. Such soils have a high ability to retain water.

The least heaving in this case will be the following types of soils: all soils that do not contain or contain a minimal amount of clay particles, coarse or medium-grained sand, clastic rocks.

All these soils do not retain water and easily pass through them into the underlying layers of soil, since they consist of large particles that do not have the ability to stick together like clay.

Factors influencing the strength of heaving

1. Depth of the first aquifer.

The closer to the surface the water is, the more heaving it will obviously be. At the same time, even replacing, for example, clay with gravelly sand is ineffective, since water will simply have nowhere to go through such soil - there will be an aquifer below.

2. The depth of soil freezing in winter, characteristic of a given region.

At the latitude of Moscow, the ground freezes to 1.5 m. Obviously, heaving forces can only act in those regions where the temperature drops below 0 degrees in winter. C. The greater the depth the soil freezes, the stronger the heaving forces will act on the building, all other things being equal.

3. Soil types.

The most susceptible to heaving are soils with small particles that can retain water for a long period due to its poor passage through small particles.

Clay soils also retain water greatly. Water passes through large particles easily because there is sufficient space between large particles for water to pass through.

Methods for solving the problem of frost heaving during building construction

Currently, there are many methods for reducing heaving that have proven themselves in practice. Let's look at the most important ones.

1. Complete replacement of soil at the building construction site.

This method radically solves the problem of heaving, but leads to increased construction costs due to the large volume of excavation work required.

The idea of ​​the method is as follows: the soil located at the site of the future construction of the building is completely removed and non-heaving soil, usually coarse sand, is placed in its place.

2. The location of the base of the building’s foundation is below the level to which the soil usually freezes.

This method is widespread. In this case, choose a suitable foundation. The most common types of foundations are pile foundations for large, heavy buildings and pile-screw foundations for cottages, dachas, and other relatively light, small buildings.

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The pile is buried until a solid layer of soil occurs and below its freezing mark. In this case, only the tangential forces of frost heaving will act on the building, or more precisely on the foundation walls.

The action of the main, vertical forces will be neutralized, since the support of the building will be in non-heaving soil.

3. Year-round heating of the building.

It is well known that the temperature in the foundation area under a heated building is always about 20% higher than the temperature under an unheated building.

Accordingly, the soil under a house with year-round heating will freeze much less and the effect of heaving forces will be weak.

When planning and designing a building, it is important to take this factor into account: it will be more profitable to use the building for year-round use.

4. General weighting of the building.

The forces of frost heaving are capable of lifting a building that has a relatively small mass. If the building is heavy, then such forces will not be able to significantly affect the position of the building.

Hence the conclusion: the heavier the building, the greater its mass, the more successfully such a building, other things being equal, will be able to withstand the forces of frost heaving in winter.

Therefore, on heaving soils it is more profitable to build heavy buildings with a large mass, although this naturally leads to large financial and time costs both for the construction of such a building and for its subsequent maintenance during operation.

5. Construction of a slab foundation for a house.

A slab foundation is a single reinforced concrete monolithic slab on which all other elements of the building rest.

In this case, the building itself, together with the foundation, is a single structure. The foundation itself is built either directly on the surface of the earth, or at a shallow depth.

In any case, it turns out that the foundation, due to its slight depth, will be subject to both tangential and vertical forces of frost heaving: it will simply rise in winter during frosts and fall in spring during thaws.

The peculiarity of this foundation is precisely its single monolithic structure, thanks to which, despite frequent changes in the height of the house, it does not collapse or crack.

6. Soil drainage.

The idea of ​​the method is to reduce the water content in the soil by draining it directly from the foundation, after which the heaving capacity of the soil is correspondingly reduced. Water is drained from under the house and the area where it is located and the soil in this place becomes less wet. To implement this method, a drainage well is dug at some distance from the house to collect water drained from the building. A drainage system is built around the house: a trench is dug and pipes containing small diameter holes along its entire side surface are laid in it; the pipes are then connected to the well, thereby forming a single drainage system.

Heaving phenomena- processes that occur in wet clayey, fine sandy and dusty soils during their seasonal freezing (heaving soils).

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, it is necessary to understand some of the processes involved in this phenomenon.

Frost heaving 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.

Degree of soil heaving

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, the rise of highly heaving soil can be 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-grained sandy and gravel soils are classified as non-heaving soils.

What does this mean:

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 - when the groundwater level is located below the calculated freezing depth:

• at 0.5 m - in dusty sands;
• at 1 m - in sandy loam;
• at 1.5 m - in loams;
• at 2 m - in clays.

Medium heaving soil - when the groundwater level is located below the calculated freezing depth:

• by 0.5 m - in sandy loam;
• at 1 m - in loams;
• by 1.5 m - in clays.

Strongly heaving soil - when the groundwater level is located below the calculated freezing depth:

• by 0.3 m - in sandy loam;
• at 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% silty-clayey component in coarse soil, the soil will also be classified as heaving.

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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 without hindrance, then such soil does not expand when frozen (Fig. 1).

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 (Fig. 2).

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:
In the 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 (Fig. 3, 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 (Fig. 3, b).

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 the building.
Soil freezing depth

The depth of soil freezing is not the calculated depth of freezing and not the depth of laying the foundation, it is the real Depth of freezing 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 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 (Fig. . 4).

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 sub-zero air temperature), then the freezing depth of clay soil of average humidity can be shown on the graph (Fig. 5).

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 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 nighttime and long-term levels of the soil freezing limit is especially noticeable where there is little or no snow cover and where the soil is highly 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.

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 (Fig. 5).

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

The forces of lateral adhesion of frozen soil to the side walls of the foundation are another side of the manifestation of heaving phenomena. These forces are very high and can reach 5...7 tons per square meter of the lateral 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 (Fig. 6, a). Given the light weight of the structure itself, the forces of frost heaving can lift it, and in the most unpredictable way.

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 very 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. “Treating” such a foundation is difficult and expensive work.

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 buried 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.

The basic rule for buried 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 the TISE technology (Fig. 6, 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 intended 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 who make a columnar foundation with a large margin of load-bearing capacity. Extra foundation pillars may indeed be unnecessary.

A wooden house with a large glassed-in veranda was installed on foundation pillars. Clay and high groundwater levels required the foundation to be laid below the frost 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 (Fig. 7).

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

Soil extrusion

Extrusion is the most noticeable cause of deformation and destruction of the foundation laid above the freezing depth.

How can this be explained?

Extrusion is due to the daily passage of the freezing line past the lower supporting plane of the foundation, which occurs much more often than the lifting of supports from lateral adhesion forces, which are seasonal.

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 it is especially cold (Fig. 8). 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.

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 (Fig. 9). 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.

As night falls, the 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 (Fig. 10).

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 boundary drops below the supporting surface of the pillar, the support itself is motionless (Fig. 11, 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 (Fig. 11, 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 (Fig. 11, 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 (Fig. 11, 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.