How To Build A Foundation On Heaving Soil - 1

2024 Author: Sebastian Paterson | [email protected]. Last modified: 2023-12-16 13:47

About the dangers of heaving soils - how to protect summer cottages from this harmful phenomenon

Arriving after winter at the summer cottage, carefully look around. And you will see that in some houses, cracks snake on the walls and glass of windows. In other areas, the gates were slanted (Figure 1), the woodshed or shed leaned strongly (Figure 2).

This is the result of such an extremely undesirable natural phenomenon as soil swelling. Especially badly, or rather destructively, heaving affects, first of all, that part of the foundations of buildings that is in the ground. This phenomenon is often not taken into account not only self-builders summer residents, but sometimes also professional builders.

Where does this malignant heaving of the soil come from and how is it formed? As you know from a school physics textbook, water in the process of freezing increases in volume by 10-15 percent. Because of this, the rise and fall of the soil in the North-West reaches 20 centimeters and more.

If the expansion of water occurs in moist, dense clays, in fine sandy and dusty soils, which are capable of dramatically changing the volume and deforming (that is, swelling) at negative temperatures, then these soils are considered heaving. And coarse-grained and gravel - non-porous. Provided that they have a free outflow of water.

What processes take place in them that make it possible to divide all soil soils into these categories? In heaving soils, moisture rises high enough from the groundwater level and, accumulating, is well retained in soils such as in a sponge.

In non-porous soils, moisture settles under its own weight, as if falling through, as if through a sieve, and therefore does not rise high. In other words: the finer (thinner) the structure of the soil, the higher the moisture rises along it, and the more it becomes heaving.

It is clear that the soil freezes from top to bottom. The moisture in the upper layers, turning into ice, increases in volume and goes down. And if it, without lingering, seeps through the structure of the surrounding soil, for example, through gravel, coarse sand, which practically do not create resistance, then the soil does not expand without moisture, which means that the heaving effect does not occur. And vice versa…

This is especially true for dense clay. From such clay, moisture not only does not have time to leave, but also accumulates. As a result, such soil will certainly become heaving. Heaving phenomena are not only significant completely unpredictable ground movements, but also colossal loads on the foundation, reaching a pressure of 6-10 tons per square meter.

Hence the immutable conclusion: before starting construction, it is imperative to find out what is the maximum freezing depth in a given place:

• in the coldest season;
• at the highest soil moisture;
• in the complete absence of snow cover.

In the Leningrad Region, the freezing depth is up to 1.5 meters. It is clear that the simultaneous combination of all these factors is unlikely, but this is a safety event that allows you to predict, and, therefore, avoid any natural disasters.

It is also essential that even if heaving, deforming the soil, does not directly affect the base of the foundation located below the freezing level, the stress at the border of the freezing zone can be so significant that it can squeeze out the foundation together with the frozen soil or tear off its upper part from the bottom. Such cases are most likely when constructing a foundation made of stone, brick or small blocks, especially under light buildings and structures.

This is the result of the so-called lateral grip forces. They arise when frozen soil adheres to the side walls of the foundation and, under certain conditions, reach a pressure of 5 to 7 tons per square meter of the side surface.

For example, a foundation pillar with a diameter of 20 centimeters with a freezing depth of 150 centimeters is affected by lateral adhesion forces of more than 9 tons. This is several times the load from the weight of the building. And so there is a heaving effect.

This is due to the fact that above the surface there is a constant collision of the cold above and the heat of the earth. If the heat of the earth is generally constant, then the degree of soil freezing depends on many factors: temperature and humidity of the surrounding air, soil moisture, density and thickness of snow, the degree of warming up by the sun.

Due to the temperature difference, the freezing line during the day is higher than at night. This difference increases especially where there is little or no snow cover. Closer to spring, the soil on the south side thaws faster than on the north, and therefore becomes wet, and, accordingly, the snow layer above it becomes thinner than on the north side.

Therefore, unlike the northern side of the house, the soil on the southern side warms up more intensively during the day and freezes more at night, thereby contributing to the emergence of lateral adhesion forces. The effect of these forces is especially enhanced if the surface of the foundation is uneven and does not have an appropriate waterproofing coating.

A recessed strip foundation can also be lifted by lateral forces if, again, it does not have a smooth, sliding side surface and is not sufficiently crushed from above by a house or concrete slabs.

How can we avoid such dangerous destructive and often just catastrophic troubles? One of these options, which allows you to avoid them, is shown in (Figure 3.) As we can see, there are no supports buried in the ground that could be subjected to heaving loads. In this case, the building rests on base plates. They are pressed by a force equal to part of the weight of the building, that is, a very small load.

The coarse sand (anti-rock) pillow will keep ice from forming and will ensure its balance. Such foundation slabs can be made at home (summer cottages) from concrete with the addition of gravel, laying metal reinforcement. It is best to use wire. The thickness of the slab must be at least 10 centimeters. Ready-made slabs can also be used. Before laying the slabs, the sand is moistened and tamped.

However, the so-called shallow foundations are much more widespread in suburban construction. This is when the depth of the foundation does not reach the depth of soil freezing (Figure 4). It is clear from the law of physics that the weight of a part of a building (VZ) must be balanced by the soil heaving force (GH) generated by the expansion of the freezing soil (ice) and lateral adhesion forces (BS), which push out the supports.

The force of heaving of the soil at low temperatures can significantly exceed the weight of the building, and then the foundation support will inevitably be pushed out. This is very noticeable in early spring, when the topsoil thaws completely and warms up well. In warm weather, the support will drop, but not by much, since the space under it is filled with water and flooded soil. After a while, such a support will shift, and the building will inevitably warp.

To avoid such an undesirable phenomenon, very often metal reinforcement is laid in the foundation and walls, and reinforcing belts are also constructed (Figure 5). Or, the base of the foundation is made expanded in the form of a support platform-anchor (Figure 6). In these cases, the stiffness of the walls and foundation increases, and, consequently, the resistance of the entire structure to loads from soil swelling increases sharply.

To be continued