Does the sand belong to heaving soils? Recommendations for the design of foundations and foundations on heaving soils
Heaviness of soils, caused by the ability of the soil to retain water in its structure, is a serious enemy of strip foundations. The uneven heaving of the underlying soils is especially critical, leading to uneven loads on the foundation. Most often, uneven heaving of soils can be caused by the presence of heterogeneous underlying soils under a shallow strip foundation. Also, uneven heaving can be caused by uneven heating of the soil from the sun, the difference in soil insulation (including when the soil is unevenly covered with snow near the house), the presence of heated and unheated rooms on the same foundation. In addition to clayey soils, heaving soils include silty and fine sands, as well as coarse-grained soils with clay filler, which have a moisture content above a certain level by the beginning of the freezing season.
The list of heaving soils according to GOST 25100-95 is given in the table:
Table. Heaviness of soils.
The degree of heaving of the soil (GOST 25100-95) /% expansion |
An example of soil requires research to decide on a classification) |
---|---|
Practically non-porous soils< 1% |
Hard clayey soils, poorly saturated gravelly, coarse and medium sands, fine and silty sands, as well as fine and silty sands, containing less than 15% by mass of particles finer than 0.05 mm. Coarse soils with aggregate up to 10% |
Weak soils<1-3,5 % |
Semi-hard clayey soils, medium water-saturated silty and fine sands, coarse-grained soils with aggregate (clay, fine and silty sand) from 10 to 30% by weight |
Medium-porous soils< 3,5-7 % |
Highly plastic clayey soils. Silty and fine sands saturated with water. Coarse soils with aggregate (clay, dusty and fine sand) over 30% by weight |
Heavily and excessively heaving soils\u003e 7% |
Soft-plastic clay soils. |
For an overview of the most important properties of soils and their suitability for construction, we suggest referring to the summary table:
Table. Soil characteristics(Table adapted from R406.1 section of the International Residential Code - 2006)
Priming |
Drainage capabilities of soils |
Potential for ground level rise when freezing. (Vertical and tangential components of frost heaving forces) |
Potential for soil expansion during freezing. (Horizontal components of frost heaving forces) |
---|---|---|---|
Boulder, pebble, crushed stone, gravel, grit. The sand is gravelly and coarse. |
Minor |
Minor |
|
Silty gravel, silty sand |
Minor |
||
Clay gravel, sandy-clay gravel mixture, clay sands |
Minor |
||
Silty and fine sand, fine clay sand, inorganic silt, clay loam with moderate plasticity |
Minor |
||
Low and medium plastic clays, gravelly clays, silty clays, sandy clays, skinny clays |
Minor to Medium |
||
Plastic and greasy clays |
|||
Inorganic silty soils, fine micaceous sands |
|||
Organic non-plastic silty soils, silty refractory clay |
|||
Clay and silty clay of medium and high plasticity, plastic silty soils, peat, sapropel. |
Unsatisfactory |
The heaving of the soil is determined by its composition, porosity, as well as the level of groundwater (GWL). The higher the water table is, the more the soil will expand when it freezes. The ability to retain and "suck" water from the underlying layers is ensured by the presence of a capillary in the soil structure and water suction by them. When expanding with freezing water (ice), the soil begins to increase in volume.
This happens due to the fact that water increases in volume when freezing by 9-12%. Therefore, the more water in the soil, the more heaving it is. Also, heaving is higher in soils with poor drainage characteristics. When the soil freezes from above (from the ground level or the layout), still unfrozen water is squeezed out by ice into the underlying soil layers.
If the drainage properties of the soil are insufficient, then the water is retained and quickly freezes, causing additional expansion of the soil. At the interface between positive and negative temperatures, ice lenses can freeze, causing additional soil uplift. The higher the density of the soil, the fewer capillaries and voids (pores) in it where water can be retained and, therefore, the less expansion potential during freezing.
By definition, a shallow strip foundation is laid to the depths of the seasonally freezing soil layer. When the soil freezes and begins to move, a force begins to act on the foundation, the vector of which is applied perpendicular to the base of the foundation (provided that the base lies in the horizon).
Under the influence of this force, the application of which is often uneven along the length of the foundation, the foundation and the building itself can also undergo uneven movements. In addition to upward pressure, when freezing, heaving soil can exert pressure both horizontally and tangentially to the vertical plane of the foundation tape.
The strength of frost heaving depends both on the magnitude of negative temperatures and on the duration of their action. The maximum frost heaving of soil in Russia occurs at the end of February-March. If you are building a strip shallow foundation on a deeply buried soil, you will have to think about how to reduce the impact of not only the tangential components of frost heaving forces, but also their horizontal components. The soil that freezes to the foundation is capable of not only providing lateral compression of the foundation, but also its pinching by lateral adhesion forces and lifting, which can cause deformation of the foundation (especially critical for prefabricated strip foundations made of blocks).
Therefore, if you decide to build a shallow strip foundation on a highly or excessively heapy soil, it is better for you to choose a rigid monolithic reinforced concrete frame as a foundation, and not a prefabricated strip foundation made of blocks. In addition, a number of measures will have to be taken to reduce the friction force between the foundation and the ground, and heat engineering measures to reduce the forces of frost heaving.
Table. Standard depth of seasonal soil freezing, m
Town |
Loam, clay |
Fine sands |
Medium to coarse sands |
Stony ground |
|
---|---|---|---|---|---|
Vladimir |
|||||
Kaluga, Tula |
|||||
Yaroslavl |
|||||
Nizhny Novgorod, Samara |
|||||
Saint Petersburg... Pskov |
|||||
Novgorod |
|||||
Izhevsk, Kazan, Ulyanovsk |
|||||
Tobolsk, Petropavlovsk |
|||||
Ufa, Orenburg |
|||||
Rostov-on-Don, Astrakhan |
|||||
Bryansk, Orel |
|||||
Yekaterinburg |
|||||
Novosibirsk |
|||||
What can be done to reduce the effect of frost heaving forces on the foundation:
- Arrange good drainage of seasonally freezing soil near the foundation.
- Provide drainage of storm and melt water using a hard or soft blind area.
- Insulate the surface of the freezing soil near the foundation.
- Consider the possibility of soil salinization with substances that do not cause corrosion of concrete and reinforcement.
The simplest and most inexpensive way is to horizontally insulate the soil around the building (which we will discuss in detail below) and vertically insulate the strip foundation. In addition to reducing heat loss at home (from 10 to 20%), insulation of the underground part of the foundation with expanded polystyrene also plays an important role in reducing friction between the soil and the foundation during heaving and compensating for the expansion of the soil.
Proper drainage plays an important role in reducing soil heaving. To reduce the forces of frost heaving, it is required to dehydrate the soil as much as possible in the immediate vicinity of the shallow strip foundation. For this, the trenches for the strip foundation are laid out with geotextiles, after casting the foundation and performing waterproofing and insulation of the foundation, the drainage pipes of the ring drainage around the entire house are laid on the bottom, and covered with a drainage mixture of sand and expanded clay, or just sand. The wall drainage membrane also helps channel water deeper into the drain pipes.
In especially difficult ground conditions you can resort to complete or partial replacement of the soil underlying and adjacent to the shallow strip foundation.
In the domestic construction literature, the role of large deciduous trees in the movement of heaving soils is not considered at all. Meanwhile
The problem of building buildings on heaving soils often arises in damp regions located in a temperate climatic zone. To date, many different methods of dealing with frost heaving have been developed and tested in practice.
The main thing is to choose the most suitable one for your construction conditions, and then the building will serve you without destruction and deformation for many years. Let us consider in more detail the issue of such construction and practical methods of its solution.
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, a change in its volume occurs due to the different density of ice and water: water has a much higher density than ice. Accordingly, when freezing, water, gradually turning into ice, expands, occupying a larger volume.
If such water freezes while in the ground, then the ground will expand with it. In this case, the forces expanding the soil will be called frost heaving forces, and such a 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 ground directly next to the building. In winter, with the onset of frost, the water freezes and expands, turning into ice. Together with it, the soil containing it begins to expand. The forces of frost heaving appear.
Forces begin to act on a nearby building, or rather on its foundation, lifting it up. In spring, when the temperature rises, the opposite process occurs: the building sinks 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 the action of 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 capable of retaining a sufficiently large amount of water in their volume. The more water is in a unit of soil volume, the more inclined this 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 amount of clay inclusions. Such soils have a high water retention capacity.
The least heaving in this case will be the following types of soils: all soils that do not contain or contain a minimum amount of clay particles, coarse or medium-grained sand, detrital rocks.
All these soils do not retain, they easily pass water through themselves into the underlying layers of the soil, since they consist of large particles that do not have the ability to stick together like clay.
Factors affecting the strength of heaving
1. The depth of the first aquifer.
The closer the water is to the surface, the more heaving it will obviously be. At the same time, even replacing, for example, clay with gravelly sand is ineffective, since the water will simply have nowhere to go through such soil - there will be an aquifer below.
2. The depth of soil freezing in winter, typical for the region.
At the latitude of Moscow, the soil freezes by 1.5 m. Obviously, heaving forces can act only in those regions where the temperature drops below 0 degrees in winter. C. The deeper the soil freezes, the stronger the heaving forces will act on the building, all other things being equal.
3. Types of soils.
The most susceptible to heaving are soils with small particles, which are able to retain water for a long period due to its poor passage through small particles.
Clay soils also retain water strongly. Water passes through large particles easily because there is enough space between the large particles for water to pass.
Methods for solving the problem of frost heaving during building construction
Currently, there are many methods for reducing heaving, which have proven themselves well in practice. Let's consider the most important ones.
1. Complete replacement of soil at the building site.
This method radically solves the problem of heaving, however, leads to increased construction costs due to the large amount of required earthworks.
The idea of \u200b\u200bthe method is as follows: the soil located at the site of the future construction of the building is completely removed and in its place, non-heaving soil, usually coarse sand, is placed.
2. The location of the base of the building foundation is below the mark, to which the soil usually freezes.
This method is widespread. In this case, choose suitable foundation... The most common types of foundations are pile foundations for large, heavy buildings and pile-screw foundations for cottages, summer cottages, and other relatively light, small structures.
Read also: How to put corrugated board on the roof with your own hands?
The pile is deepened up to the occurrence of a solid layer of soil and below the mark of its freezing. In this case, only the tangential forces of frost heaving will act on the building, more precisely on the walls of the foundation.
The action of the main, vertical forces will be neutralized, since the support of the building will be in non-porous soil.
3. Year-round heating of the building.
It is well known that the temperature in the area of \u200b\u200bthe foundation under a heated building is always about 20% higher than the temperature under an unheated building.
Accordingly, the soil under the house with year-round heating will freeze considerably 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 living.
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 cannot 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 action of frost heaving forces in winter.
Therefore, it is more profitable to build heavy buildings with a large mass on heaving soils, 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.
The slab foundation is a single reinforced concrete monolithic slab on which all other building elements rest.
The building itself, in this case, 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 the shallow deepening, will be subject to both tangential and vertical forces of frost heaving: it will simply rise in winter during frosts and lower in spring during thaws.
The peculiarity of this foundation is precisely a single monolithic structure, thanks to which, despite the frequent change in the height of the rise of the house, it does not collapse and does not crack.
6. Drainage of soil.
The idea of \u200b\u200bthe method is to reduce the water content in the soil by draining it directly from the foundation, after which the heaving capacity of this soil is correspondingly reduced. Water is drained from under the house and the area of \u200b\u200bits location and the soil in this place becomes less wet. To implement this method, at some distance from the house, a drainage well is being dug, designed to collect the water discharged from the building. A drainage system is being built around the house: a trench is dug and pipes are laid into it, containing small-diameter holes along their entire lateral surface; pipes are then connected to the well, thereby forming unified system plum.
Today, such a branch 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 foundation 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 must carefully study the hydrogeological features of a particular land plot... Indicators such as the depth of soil freezing, soil moisture, and the level of groundwater are of great importance.
Such a property of the soil as heaving depends on these indicators. Building is pretty dangerous. 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 lands. Let us consider in more detail what features a non-porous soil has, what it relates to, what measures can be taken in order to secure the foundation and the building itself. In addition, here you can learn about the use of a non-porous soil foundation.
Non-porous soil type
Soil testing is a critical stage in all the builder's work. Before directly building the foundation for a house, you need to know what heaving is. So, non-heaving is a soil that is not exposed to frost heaving. Heaving includes such a concept as the degree of heaving. It shows how much the soil can expand in volume as a result of freezing at low temperatures.
Non-heaving - these 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 is this phenomenon happening? It's pretty simple. In the cold season (autumn or winter), the water that is 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 increased soil compared to the initial state is able to exert great pressure on the foundation and change its location, the same applies to the entire building. In addition, moisture that has got directly into the foundation itself is capable of gradually destroying it and rendering it unusable. All this is typical for heaving soil. For non-porous soil, everything is different.
Back to the table of contents
Classification of soil according to the degree of heaving
Before doing 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-porous, weak, medium and strong. The classification is based on the magnitude of indicators such as water saturation coefficient and soil fluidity index. Non-heaving soils include those with a heaving degree of less than 0.01. Low-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-grained earths with filler. The latter can be fine and silty sand.
Moreover, its amount should be in the range from 10 to 30% in mass ratio. The group of medium-heaving soil includes soils with a heaving degree of 0.035 to 0.7. These include clay with a fluidity of 0.25 to 0.5; sands are fine and silty with a water saturation of 0.8 to 0.95; coarse soils with filler over 30% by weight. The greatest danger is posed by highly heaving soil. It is represented by the following indicators: the 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-porous soil and features of the construction of the foundation
As mentioned above, it is most optimal to build the foundation on safe soils. Non-rocky soil includes rocky and detrital soil. The latter is formed as a result of the destruction of rocks. it includes gravel and crushed stone. For the most part, these are coarse-grained materials. They are often used in construction. This group of soil includes both 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 effect it has on the foundation.
The foundation is laid with this type of soil using the following technology. Regardless of the depth of soil freezing and its moisture content, it is erected shallowly, that is, not deep. This saves time and effort on earthworks. In the presence of rock, the foundation does not need to be equipped at all. In some European countries, for example, in Montenegro, in some regions of Germany and Finland, houses are built without a foundation due to precisely these features of the area. In the presence of coarse sandy soil, the thickness concrete foundation is only about 20 cm.
Undoubtedly, these calculations are relevant only for small houses, and not for multi-storey structures. After pouring the concrete, when it hardens, you can immediately erect the basement of the building or the walls. In other cases, when the nature of the soil is different, a trench 50-70 cm deep is pulled out. After that, 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 you can build, there are no restrictions. It can be monolithic (slab), columnar or tape. For heaving soil, the most optimal columnar foundation or an anchor-type foundation, since in this case the load, including the action of shear forces, on the foundation will be minimal.
Heaving soil is a soil that is prone to frost heaving. The value that shows how inclined the soil is to heave is the degree of frost heaving, which is defined as the relative change in the volume of the soil during freezing:
E \u003d (H - h) / h,
Where E is the degree of heaving, H is the height of the frozen (swollen) soil, h is the height of the soil before freezing.
The degree of heaving shows how much the volume of the soil changes during freezing. Heaving soils are called soils in which the degree of heaving is greater than 0.01, i.e. this is a soil that, when frozen to a depth of 1 m, increases in volume by more than 1 cm.
What heaving soils?
Swelling occurs due to the fact that the moisture contained in the soil freezes, and, as you know, ice has a lower density than water, and therefore takes up a larger volume. An increase in the volume of water during freezing leads to heaving, therefore which soils are heaving and which are not depends on the water content in them: the more it is in the soil, the more it swells. All belong to heaving: clay, loam and sandy loam. Unlike sand, clay has many pores and retains moisture well; water does not seep between the smallest particles of clay and does not go into deeper layers of the earth. Therefore, the higher the clay content, the more heaving the soil is.
Building a foundation on heaving soil
Clay soil is soil that is more than half composed of very small particles less than 0.01 mm in size, which are in the form of flakes or plates. Clay soils include sandy loam, loam and clay. Heavy phenomena are insidious and unceremonious processes that occur in moist clay, fine sandy and silty soils during their seasonal freezing. They cannot be ignored, which is understandable to anyone, even a developer who is poorly versed in construction. Many realized this, having discovered in the spring a crack in the brick wall of a country house, having seen the skewed door and window openings of the frame suburban buildingnoticing a dangerously tilted fence.
Heaving phenomena are not only large deformations of the soil, but also huge efforts - tens of tons, which can lead to great destruction.
The difficulty in assessing the impact of heaving phenomena of the soil on buildings is in some of their unpredictability due to the simultaneous impact of several processes. To better understand this, we will describe some of the concepts associated with this phenomenon.
Frosty heavingAs experts call this phenomenon, it is due to the fact that in the process of freezing the wet soil increases in volume.
This happens due to the fact that water increases in volume when freezing by 12% (which is why ice floats on water). Therefore, the more water in the soil, the more heaving it is. So, a forest near Moscow, standing on very heaving soils, in winter rises by 5 ... 10 cm relative to its summer level. Outwardly, this is imperceptible. But if a pile is driven into the soil by more than 3 m, then the rise of the soil in winter can be tracked by the marks made on this pile. The rise of the soil in the forest could have been 1.5 times more, if there was no snow cover in it, covering the soil from freezing.
According to the degree of heaving, soils are divided into:
- strongly heaving - heaving 12%;
- medium heaving - heaving 8%;
- slightly heaving - heaving 4%.
With a freezing depth of 1.5 m, the heavily heaving soil is 18 cm.
The heaving of the soil is determined by its composition, porosity, as well as the level of groundwater (GWL). So clayey soils, fine and silty sands belong to heaving soils, and coarse-grained sandy and gravel soils - to non-heaving soils.
Let's consider what this is connected with.
Firstly.
In clays or fine sands, moisture, like a blotter, rises high enough from the groundwater level due to the capillary effect and is well retained in such a soil. Here, the forces of wetting between water and the surface of dust particles are manifested. In coarse sands, moisture does not rise, and the soil becomes moist only at the level of groundwater. That is, the thinner the soil structure, the higher the moisture rises, the more logical it is to attribute it to more heaving soils.
Raising water can reach:
- 4 ... 5 m in loams;
- 1 ... 1.5 m in sandy loam;
- 0.5 ... 1 m in silty sands.
In this regard, the degree of heaving of the soil depends both on its grain composition and on the level of groundwater or flood waters.
Weakly loose soil
- by 0.5 m - in silty sands;
- 1 m - in sandy loam;
- by 1.5 m - in loams;
- 2 m - in clays.
Medium porous soil - when the groundwater level is located below the calculated freezing depth:
- by 0.5 m - in sandy loam;
- by 1 m - in loams;
- by 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 loam;
- by 0.7 m - in loams;
- by 1.0 m - in clays.
Excessive heaving soil - if the groundwater level is higher than for highly heaving soils.
Please note that mixtures of coarse sand or gravel with dusty sand or clay will fully apply to heaving soils. If there is more than 30% of the dusty-clay component in the coarse soil, the soil will also be classified as heaving.
Secondly.
The process of soil freezing occurs from top to bottom, while the boundary between wet and frozen soil descends 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 has time to seep through the structure of the soil or not, 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 leaves without hindrance, then such soil does not expand when freezing (Figure 23).
Figure 23. Soil at the border of freezing:
1 - sand; 2 - ice; 3 - border of freezing; 4 - water
As for the clay, moisture does not have time to leave through it, and such soil becomes heaving. By the way, coarse sand soil placed in a closed volume, which may be a well in clay, will behave like heaving (Figure 24).
Figure 24. Sand in a confined space - heaving:
1 - clay; 2 - groundwater level; 3 - border of freezing; 4 - sand + water; 5 - ice + sand; 6 - sand
That is why the trench under the shallow foundations is filled with coarse-grained sand, which makes it possible to equalize the degree of moisture along its entire perimeter, smooth out the unevenness of heaving phenomena. Sand trench, if possible, should be connected to drainage systemdiverting the top water from under the foundation.
Thirdly.
The presence of pressure from the weight of the structure also affects the manifestation of heaving phenomena. If the layer of soil under the base of the foundation is strongly compacted, then the degree of heaving will decrease. Moreover, the greater the pressure itself per unit area of \u200b\u200bthe base, the greater the volume of compacted soil under the base of the foundation and the smaller the amount of heaving.
Example
In the Moscow region (freezing depth 1.4 m), a relatively light log house was erected on a medium-heapy soil on a shallow strip foundation with a depth of 0.7 m. With complete freezing of the soil, 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 ground will be greater, its compaction will be stronger, which is why the rise of the walls from freezing of the soil will not exceed 2 ... 3 cm (Figure 25, b).
Figure 25. The degree of heaving of the soil depends on the pressure on the base:
A - under the strip foundation; B - under the columnar foundation;
1 - sand pillow; 2 - border of freezing; 3 - compacted soil; 4 - strip foundation; 5 - columnar foundation
Strong compaction of heaving soil under the belt shallow foundation may arise if a stone house with a height of at least three floors is erected on it. In this case, we can say that 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 in the walls. Therefore, stone walls of a house on a similar foundation should be erected with obligatory horizontal reinforcement.
Why are heaving soils dangerous? What processes that frighten developers with their unpredictability take place in them?
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 treachery of heaving soils is uneven heaving under one structure
The depth of soil freezing - this is not the estimated freezing depth and not the depth of the foundation, this 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 the heat coming from the bowels of the earth, with the power of the cold that penetrates into the ground from above in the cold season.
If the intensity of the earth's heat does not depend on the time of year and day, then the air temperature and soil moisture, the thickness of the snow cover, its density, humidity, pollution and the degree of warming 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 line; 4 - border of freezing at night
The unevenness of the thickness of the snow cover most noticeably affects the difference in the heaving of the soil. Obviously, the depth of freezing will be the higher, the thinner the layer of the snow blanket is, 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 minus air temperature), then the depth of freezing of clayey soil of average humidity can be shown on the graph (Figure 27).
Figure 27. Dependence of the freezing depth on the thickness of the snow cover
Frost duration for each region is an average statistical parameter, which is very difficult for an individual developer to estimate, because this will require hourly temperature monitoring throughout the cold season. Nevertheless, 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 \u003d 100 24 15 \u003d 36000), then with a 15 cm thick snow cover the freezing depth will be 1 m, and with a thickness of 50 cm - 0 , 35 m.
If a thick layer of snow cover, like a blanket, covers the ground, then the freezing line rises up; at the same time, both day and night its level does not change much. In the absence of snow cover at night, the freezing boundary drops strongly down, and in the daytime, with solar heating, it rises up. The difference between the night and day levels of the soil freezing boundary is especially noticeable where there is little or no snow cover and where the soil is highly moistened. The presence of a house also affects the depth of freezing, because the house is a kind of thermal insulation, even if they do not live in it (the air vents of the underground are closed for the winter).
The site on which the house is located can have a very complex picture of freezing and soil rising.
For example, medium-gravelly soil along the outer perimeter of the house, when frozen to a depth of 1.4 m, can rise by almost 10 cm, while the drier and warmer soil under the middle part of the house will remain almost at the summer mark.
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, the layer of snow above it is thinner than on the north side. Therefore, unlike the north side of the house, the soil from the south side warms up better during the day and freezes more at night.
From experience
In the spring, in mid-March, I decided to check how the ground "walks" under the constructed house. At the corners of the foundation (from the inside), rods were concreted into the paving slabs, by which I checked the foundation subsidence from the weight of the house. From the north side, the soil rose by 2 and 1.5 cm, and from the south - by 7 and 10 cm. The water level in the well at that time was 4 m below the ground.
Thus, the uneven freezing in the area is manifested not only in space, but also in time. The freezing depth is subject to seasonal and daily changes within very large limits and can vary greatly even in small areas, especially in built-up areas.
Clearing large areas from snow in one place of the site, and creating snowdrifts in another place, you can create a noticeable unevenness of soil freezing. It is known that planting shrubs around the house traps snow, reducing the freezing depth by 2 - 3 times, which is clearly seen on the graph (Figure 27).
Clearing narrow paths from snow does not have a particular effect on the degree of soil freezing. If you decide to fill a skating rink near your house or clean the area for your car, then you can expect great unevenness in the freezing of the soil under the foundation of the house in this zone.
Side grip forces frozen soil with 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 the lateral surface of the foundation. Such forces arise if the surface of the pillar is uneven and does not have a waterproofing coating. With such a 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?
Take, for example, a pillar foundation support under a light house. On heaving soil, the depth of the supports is carried out at the calculated freezing depth (Figure 28, a). When light weight of the structure itself, the forces of frost heaving can raise it, and in the most unpredictable way.
Figure 28. Lifting the foundation by lateral adhesion forces:
A - columnar foundation; B - columnar-strip foundation using TISE technology;
1 - foundation support; 2 - frozen ground; 3 - border of freezing; 4 - air cavity
In early winter, the freezing line begins to go down. The frozen solid ground grabs the top of the pillar with powerful traction forces. But in addition to increasing the adhesion forces, the frozen soil also increases in volume, which is why the upper layers of the soil rise, trying to pull the supports out of the ground. But the weight of the house and the forces of embedding the column in the ground do not allow this to be done as long as the frozen soil layer is thin and the area of \u200b\u200badhesion of the column to it is small. As the freezing boundary moves down, the adhesion area of \u200b\u200bthe frozen soil to the pillar increases. There comes a moment when the adhesion forces of the frozen soil with 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. During the season, on highly heaving soils, such a pillar can rise by 5 - 10 cm. The rise of the foundation supports under one house, as a rule, is uneven. After the frozen ground thaws, the foundation pillar, as a rule, does not return to its original place on its own. With each season, the unevenness of the output of the supports from the ground increases, the house tilts, coming into an emergency condition. "Treatment" of such a foundation is a difficult and expensive job.
This force can be reduced by a factor of 4 ... 6 by smoothing the surface of the well with a tar paper jacket inserted into the well before filling it with concrete.
A recessed strip foundation can be lifted in the same way if it does not have a smooth side surface and is not loaded from above by a heavy house or concrete slabs (Figure 4).
The basic rule for buried strip and columnar foundations (no expansion at the bottom): the construction of the foundation and its loading with the weight of the house should be carried out in one season.
The foundation pillar, made using the TISE technology (Figure 28, b), does not rise by the cohesion forces of the heaving frozen soil due to the lower expansion of the pillar. However, if it is not supposed to load it with a house in the same season, then such a pillar must have reliable reinforcement (4 rods with a diameter of 10 ... 12 mm), excluding the separation of the expanded part of the pillar from the cylindrical one. The undoubted advantages of the TISE support are its high bearing capacity and the fact that it can be left for the winter without loading from above. No forces of frost heaving will lift it.
Lateral adhesion forces can play a sad joke with developers who are making a columnar foundation with a large margin of bearing capacity. Extra foundation pillars may indeed be superfluous.
From practice
A wooden house with a large glazed veranda was set on foundation pillars. Clay and high groundwater levels required the foundation to be laid below the freezing depth. The floor of the wide veranda required an intermediate support. Almost everything was done correctly. However, during the winter, the floor increased by almost 10 cm (Figure 29).
Figure 29. Destruction of the veranda ceiling by the forces of adhesion of the frozen soil to the support
The reason for this destruction is clear. If the walls of the house and the veranda were able to compensate for the adhesion forces of the foundation pillars with the frozen ground with their weight, then light floor beams were not able to do this.
What should have been done?
Significantly reduce either the number of central foundation pillars or their diameter. Adhesion forces could be reduced by wrapping the foundation pillars with several layers of waterproofing (roofing felt, roofing felt) or creating a layer of coarse sand around the pillar. Destruction could also be avoided through the creation of a massive grillage tape connecting these supports. Another way to reduce the lift of such supports is to replace them with a shallow columnar foundation.
Extrusion - the most tangible reason for the deformation and destruction of the foundation, laid above the freezing depth.
How can it be explained?
Extrusion is required daily the passage of the freezing border past the lower support plane of the foundation, which occurs much more often than the lifting of supports from lateral adhesion forces seasonal character.
To better understand the nature of these forces, we represent the frozen ground in the form of a slab. A house or any other structure in winter is reliably frozen into this stone-like slab.
The main manifestations of this process are seen in spring. The side of the house, facing south, is warm enough during the day (you can even sunbathe in calm weather). The snow cover has melted, and the soil is moistened with a spring drop. Dark soil absorbs sunlight well and warms up.
On a starry night in early spring especially cold (Figure 30). The soil under the eaves of the roof freezes strongly. At the slab of frozen soil, a protrusion grows from below, which, with the power of the slab itself, strongly compacts the soil under it due to the fact that the moist soil expands during freezing. The forces of such soil compaction are enormous.
Figure 30. Frozen ground slab at night:
1 - slab of frozen soil; 2 - border of freezing; 3 - direction of soil compaction
A slab of frozen soil 1.5 m thick with dimensions of 10x10 m will weigh more than 200 tons. Approximately with this effort, the soil under the ledge will be compacted. After such exposure, the clay under the "slab" ledge becomes very dense and practically waterproof.
The day has come... The dark soil near the house is especially heated by the sun (Figure 31). With increasing humidity, its thermal conductivity also increases. The freezing line rises (this happens especially quickly under the ledge). With the thawing of the soil, its volume also decreases, the soil under the support is loosened and, as it thaws, falls under its own weight in layers. A lot of cracks are formed 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 of the foundation with 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 ground; 2 - border of freezing (night); 3 - border of freezing (day); 4 - thawing cavity
As the night falls cavities filled with water freeze, expanding and turning into so-called "ice lenses". With the amplitude of raising and lowering the freezing boundary in one day of 30 - 40 cm, the thickness of the cavity will increase by 3 - 4 cm. Together with the increase in the volume of the lens, our support will also rise. For several such days and nights, the support, if it is not heavily loaded, sometimes rises by 10 - 15 cm, like a jack, leaning 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 lateral surface of the foundation, since the bitumen coating with which it is covered does not create good adhesion of the foundation to the frozen ground. The frozen soil slab, creating pressure on the soil with its protrusion, itself begins to rise, and the fracture zone of the slab opens up, becomes filled 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. A slab of frozen soil with a break along the foundation strip:
1 - plate; 2 - fault
It is interesting to imagine a slab of frozen ground 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 now we return the slab to its original position, then just where there were hills, ice lenses are created in the ground. In these places, the soil below the freezing depth is strongly compacted, and above, on the contrary, it is loosened. This phenomenon occurs not only in building 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, arise in clay 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 rise of the foundation pillar by frozen soil is carried out with the daily passage of the freezing boundary past its bottom. This is how this process works.
Until the moment when the boundary of freezing of the soil has not dropped below the supporting surface of the column, the support itself is motionless (Figure 33, a). As soon as the border of freezing falls below the base of the foundation, the "jack" of heaving processes is immediately put into operation. The layer of frozen soil under the support, increasing in volume, raises it (Figure 33, b). The forces of frost heaving in water-saturated soils are very high and reach 10 ... 15 t / m². With the next heating, the frozen soil layer under the support thaws and decreases in volume by 10%. The support itself is held in the raised position by the forces of its adhesion to the frozen ground plate. Water with soil particles seeps into the gap formed under the foot of the support (Figure 33, c). With the next lowering of the freezing boundary, 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 of a daily (multiple) nature, and the extrusion of the supports by adhesion forces with frozen soil is seasonal (once a season).
With a large vertical load on the pole, the soil under the support, strongly compacted by pressure from above, becomes weakly porous, and water from under the support itself is squeezed out through its thin structure during the thawing process of the frozen soil. In this case, the lifting of the support practically does not occur.
Figure 33. The rise of the foundation pillar with heaving soil;
A, B - the upper level of the freezing boundary; B, D - the lower level of the freezing boundary;
1 - grillage tape; 2 - foundation pillar; 3 - frozen ground; 4 - upper position of the freezing boundary; 5 - lower position of the freezing boundary; 6 - a mixture of water and clay; 7 - a mixture of ice and clay
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