Dynamics of heaving soils

Heaving phenomena are insidious and unceremonious processes that occur in wet clayey, fine sandy and dusty soils during their seasonal freezing. It is impossible not to take them into account, which is clear to anyone, even a poorly versed in construction developer. Many understood this when they discovered a crack in the brick wall of a country house in the spring, saw skewed door and window openings of a frame summer cottage, and noticed 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 soil phenomena on buildings lies in their unpredictability due to the simultaneous impact of several processes. To better understand this, we will describe some concepts related to this phenomenon.

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

This happens due to the fact that water increases in volume when it freezes by 12% (which is why ice floats on water). Therefore, the more water in the soil, the more heaving it is. So, the forest near Moscow, standing on heavily heaving soils, in winter rises by 5 ... 10 cm relative to its summer level. Outwardly, it is imperceptible. But if a pile is driven into the ground 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 soil in the forest could be 1.5 times greater if there was no snow cover in it, covering the soil from freezing.

Soils according to the degree of heaving are divided into:

- strongly fluffy - heaving 12%;

- medium heaving - heaving 8%;

- slightly puffy - heaving 4%.

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

The heaving of the soil is determined by its composition, porosity, as well as the level of groundwater (GWL). So clay soils, fine and silty sands are classified as heaving soils, and coarse-grained sandy and gravel soils are non-rocky.

Let's consider what it is connected with.

Firstly.

In clays or fine sands, moisture, as if by a blotter, rises quite high from the GWL due to the capillary effect and is well retained in such soil. Here, wetting forces between water and the surface of dust particles are manifested. In coarse-grained sands, moisture does not rise, and the soil becomes wet 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.

The rise in 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.

Slightly heaving soil
- by 0.5 m - in silty sands;
- per 1 m - in sandy loam;
- by 1.5 m - in loams;
- 2 m - in clay.

Medium soil- when GWL is located below the estimated freezing depth:
- by 0.5 m - in sandy loam;
- per 1 m - in loams;
- by 1.5 m - in clays.

Strongly gravelly soil- when GWL is located below the estimated freezing depth:
- by 0.3 m - in sandy loam;
- by 0.7 m - in loams;
– per 1.0 m – in clays.

Excessively heaving soil- if the GWL is higher than for strongly 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% of the silt-clay component in the coarse-grained soil, the soil will also be referred to as heaving.

Secondly.

The process of soil freezing occurs from top to bottom, while the boundary between wet and frozen soil falls at a certain rate, 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 soil structure or not, whether the degree of soil filtration is enough for this process to take place with or without heaving. If coarse sand does not create any resistance to moisture, and it leaves unhindered, then such soil does not expand when it freezes (Figure 23).

Figure 23. Soil at the freezing boundary:
1 - sand; 2 - ice; 3 - freezing boundary; 4 - water

As for clay, moisture does not have time to escape 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 closed volume - heaving:
1 - clay; 2 - groundwater level; 3 - freezing boundary; 4 - sand + water; 5 – ice + sand; 6 - sand

That is why the trench under shallow foundations is filled with coarse-grained sand, which makes it possible to even out the degree of humidity along its entire perimeter, to smooth out the unevenness of heaving phenomena. A trench with sand, if possible, should be connected to a drainage system that diverts top 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 sole of the foundation is strongly compacted, then its degree of heaving will decrease. Moreover, the greater the pressure itself per unit area of ​​the base, the greater the volume of compacted soil under the base of the foundation and the smaller the amount of heaving.

Example

B Moscow region (freezing depth 1.4 m) on medium heaving soil on a shallow strip foundation with a laying depth of 0.7 m, a relatively light timber house was erected. 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 laying 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 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 cushion; 2 - freezing boundary; 3 - compacted soil; 4 - strip foundation; 5 - columnar foundation

A strong compaction of heaving soil under a strip shallow foundation can occur if a stone house is erected on it with a height of at least three floors. 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, the stone walls of the house on such a foundation should be erected with mandatory 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 insidiousness of heaving soils is uneven heaving under one building

Soil freezing depth- this is not the estimated freezing depth and not the depth of the foundation, it is the actual freezing depth in a specific place, at a specific time and under specific weather conditions.

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

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

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

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

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

Figure 27. Dependence of the depth of freezing on the thickness of the snow cover

Frost duration for each region is an average parameter, which is very difficult for an individual developer to evaluate, 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 \u003d 100 24 15 \u003d 36000), then with a snow cover 15 cm thick, the freezing depth will be 1 m, and with a thickness of 50 cm - 0.35 m.

If a thick layer of snow covers the ground like a blanket, then the freezing boundary rises; at the same time, both during the day and at night, its level does not change much. In the absence of snow cover at night, the freezing boundary drops strongly down, and during the day, with solar heating, it rises. 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 very wet. 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 underground vents are closed for the winter).

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

For example, medium-heavy soil along the outer perimeter of the house, when freezing 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 on the south side warms up better during the day and freezes more strongly at night.

From experience

In the spring, in mid-March, I decided to check how the soil "walks" under the built house. At the corners of the foundation (on the inside), bars were concreted into paving slabs, along which I checked the subsidence of the foundation from the weight of the house. On the north side, the ground 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 on the site manifests itself not only in space, but also in time. The depth of freezing is subject to seasonal and daily changes to a very large extent and can vary greatly even in small areas, especially in built-up areas.

Clearing large areas of snow in one place of the site, and creating snowdrifts in another place, you can create a 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 seen on the graph (Figure 27).

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

Side grip forces frozen soil with side walls of the foundation - the other side of the manifestation of heaving phenomena. These forces are very high and can reach 5 ... 7 tons per square meter of the side surface of the foundation. Similar forces arise if the surface of the column is uneven and does not have a waterproofing coating. With such a strong adhesion of frozen soil to concrete, a vertical buoyant force of up to 8 tons will act on a pole 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 carried out to the estimated freezing depth (Figure 28, a). With a small weight of the structure itself, the forces of frost heaving can lift it, and in the most unpredictable way.

Figure 28. Raising the foundation by lateral cohesive forces:
A - columnar foundation; B - column-strip foundation according to TISE technology;
1 - foundation support; 2 - frozen ground; 3 - freezing boundary; 4 - air cavity

In early winter, the freezing line begins to descend. Frozen solid ground grabs the top of the post with powerful cohesive forces. But in addition to the increase in cohesive 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 pillar in the ground do not allow this to be done, as long as the layer of frozen soil is thin and the area of ​​adhesion of the pillar to it is small. As the freezing boundary moves down, the area of ​​adhesion of the frozen soil to the column 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. Frozen soil pulls out the pillar, leaving a cavity below, which immediately begins to fill with water and clay particles. During the season, on heavily heaving soils, such a pillar can rise by 5–10 cm. The rise of the foundation supports under one house, as a rule, occurs unevenly. After thawing of the frozen soil, the foundation pillar does not, as a rule, return to its original place on its own. With each season, the unevenness of the exit of supports from the ground increases, the house leans, coming into an emergency state. "Treatment" of such a foundation is a difficult and expensive job.

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

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 (Figure 4).

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

The foundation pillar, made according to the TISE technology (Figure 28, b), is not lifted by the cohesion forces of heaving frozen soil due to the lower expansion of the pillar. However, if it is not expected to load it with a house in the same season, then such a pole must have reliable reinforcement (4 bars with a diameter of 10 ... 12 mm), excluding the separation of the expanded part of the pole 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 force of frost heaving will lift it.

Lateral cohesion forces can play a sad joke with developers who make a columnar foundation with a large margin in terms of bearing capacity. Extra foundation pillars may indeed be superfluous.

From practice

A wooden house with a large glazed veranda was installed on foundation pillars. The clay and high water table required foundations to be laid below freezing depth. The floor of the wide veranda required an intermediate support. Almost everything was done right. However, during the winter the floor was raised by almost 10 cm (Figure 29).

Figure 29. Destruction of the veranda ceiling by the forces of adhesion of 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 with their weight for the forces of adhesion of the foundation pillars to the frozen ground, then the light floor beams could not do it.

What should have been done?

Significantly reduce either the number of central foundation pillars or their diameter. Cohesion forces could be reduced by wrapping the foundation pillars with several layers of waterproofing (roofing, roofing felt) or creating a layer of coarse sand around the pillar. It would be possible to avoid destruction through the creation of a massive grillage tape connecting these supports. Another way to reduce the rise of such supports is to replace them with a shallow columnar foundation.

extrusion- the most tangible cause of deformation and destruction of the foundation, laid above the freezing depth.

How can it be explained?

extrusion is bound daily the passage of the freezing boundary past the lower support plane of the foundation, which occurs much more often than the lifting of supports from lateral cohesion forces that have seasonal character.

To better understand the nature of these forces, we represent the frozen ground as a plate. A house or any other structure in winter is reliably frozen into this stone-like slab.

The main manifestations of this process are visible in the spring. On the side of the house facing south, it is quite warm during the day (you can even sunbathe in calm weather). The snow cover melted, and the ground was moistened with a spring drop. The dark soil absorbs the sun's rays well and warms up.

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

Figure 30. Frozen ground slab at night:
1 - plate of frozen soil; 2 - freezing boundary; 3 - direction of soil compaction

A slab of frozen soil 1.5 m thick, 10x10 m in size, will weigh more than 200 tons. With approximately this force, the soil under the ledge will be compacted. After such an impact, the clay under the ledge of the "slab" becomes very dense and almost waterproof.

The day has come. The dark soil near the house is especially strongly warmed by the sun (Figure 31). With increasing humidity, its thermal conductivity also increases. The freezing boundary rises (under the ledge, this happens especially quickly). With the thawing of the soil, its volume also decreases, the soil under the support loosens 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 to the frozen ground slab and the support along the rest of the perimeter.

Figure 31. Frozen soil slab during the day:
1 - plate of frozen soil; 2 - freezing boundary (night); 3 - freezing boundary (day); 4 - thawing cavity

As night falls cavities filled with water freeze, increasing in volume and turning into so-called "ice lenses". With an 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 an 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 a 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 bituminous coating with which it is covered does not create a good adhesion of the foundation to the frozen ground. A plate of frozen soil, creating pressure on the ground with its protrusion, begins to rise by itself, and the fault zone of the plate opens up, fills 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. Frozen ground slab with a fracture along the foundation strip:
1 - plate; 2 - fault

It is interesting to imagine a slab of frozen soil turned upside down. This is a relatively flat surface, on which at night in some places (where there is no snow) hills grow, which turn into lakes during the day. If now we return the plate to its original position, then just where the hills were, 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 uneven ground heating and snow cover thickness. It is according to this scheme that ice lenses appear in clayey soils, which are well known to specialists. The origin 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 with frozen soil is carried out during the daily passage of the freezing border past its sole. Here is how this process happens.

Until the moment when the boundary of soil freezing does not fall below the supporting surface of the column, the support itself is motionless (Figure 33, a). As soon as the freezing boundary falls below the base of the foundation, the "jack" of heaving processes immediately starts working. The layer of frozen soil under the support, having increased 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 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 frozen ground slab. Water with soil particles seeps into the gap formed under the sole 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 supports by adhesion forces with frozen soil is seasonal (once per season).

With a large vertical load on the column, the soil under the support, strongly compacted by pressure from above, becomes slightly heaving, and the water from under the support itself is squeezed out through its thin structure during the thawing of the frozen soil. Raising the support in this case practically does not occur.

Figure 33. Lifting the foundation column 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 soil; 4 - the upper position of the freezing boundary; 5 - the lower position of the freezing boundary; 6 – mixture of water and clay; 7 - a mixture of ice and clay