Reducing the depth of soil freezing. How deep should the foundation be. Ways to neutralize heaving forces

Information about the depth of the foundation, calculating the depth of the foundation, SNIP, you will learn how to determine the depth of the foundation, shallow and strip foundation and the depth of their foundation. Question from a client:

We decided to give a detailed answer to the client's question, and offer him a whole informational article on this topic.

Determining the depth of the foundation is the primary stage in the design of all types of reinforced concrete foundations.

From this article, you will learn what needs to be considered when determining the depth of the foundation, to what depth it is customary to bury tape bases of different types and how to independently calculate the GZF according to the requirements of the current "Building codes and regulations".

Figure: 1.1

What to consider when calculating the depth of the foundation

In construction practice, the depth of the reinforced concrete base - tape, slab and columnar, is calculated based on three determining factors:

• Geological conditions at the construction site;
• Design features of the building being built;

The depth calculation is made for each of the 3 above-mentioned factors, and the largest of the obtained GZF values \u200b\u200bis taken as the design depth.

Geological conditions at the construction site

An analysis of the geological conditions of the construction site is necessary to determine the depth of the bearing layer of the soil, on which the base base should rest.

• The sole of the base should be buried at least 20 centimeters into the bearing layer of soil;
• The total depth of the foundation, under any conditions, should not be less than 50 centimeters;

The determination of the groundwater level is also carried out. Ideally, the foundation should be laid above this level, however, situations are often encountered when the depth of freezing of the soil and groundwater level are the same, or the groundwater generally rises above the freezing level.

Figure: 1.2

If it is impossible to lay the foundation above the groundwater level, a drainage system is arranged around the base of pipes encircling the perimeter of the foundation. The presence of a drainage system allows water to be diverted from the soil located next to the foundation, due to which the forces of frost heaving of the soil that occur during the cold season are reduced.

The depth of soil freezing

The key factor affecting the GZF value is the depth of soil freezing. This factor becomes especially important in conditions of construction on soil prone to heaving, which includes:

• Moisture-saturated sandy soil;
• Dusty and fine sandy soil;
• Highly plastic clay soil;
• Clay loam.

Figure: 1.3

In the cold season, when the soil freezes, the moisture with which it is saturated turns into ice, increasing its volume by 3-9%.

Due to the huge density of the lower soil layers, the increased soil cannot expand downward, and it begins to press upward, exerting buoyancy loads of vertical and tangential impact on the base.

The result of heaving is deformation of the bases - strip and slab foundations are warped, walls are covered with cracks, window and door frames protrude.

Design features of the building being built

The depth of the foundation is determined taking into account the following design features of the structure being erected:

• The presence of a basement or basement;
• Availability of bases for free-standing equipment;
• The nature and strength of the loads that the building will exert on the bearing foundation (wind, snow and from the mass of the structure);

Figure: 1.4

Question from a client: "Good afternoon, specialists of the SK" Installation of Piles ". My brother and I are engaged in the construction of a cottage made of foam concrete in the Moscow region. We plan to build it on a shallow foundation of a tape type, but we doubt whether such a foundation is applicable in the conditions of local soils. Tell me how to choose the right depth laying the foundation. Best regards, Victor Romanovich "

This page contains information about the depth of the reinforced concrete foundations and the method for determining it. We will consider the requirements of SNiP, which normalize this process, and the typical depth of placement of buried-type bases and MLF.

Things to Consider When Calculating Depth

Based on the depth of the foundation, all reinforced concrete bases are classified into three groups:

• Shallow - the support sole is placed on the surface of the soil (applicable only in conditions of high-density, rocky rocks);
• Shallow-buried (MZF) - lowered into the soil by 30-80 cm (used in soil that is not inclined to heaving);
• Deep-laid - lowered into the soil by 80-180 cm (the only possible option for a strip foundation in problem soil).

Figure: 1.1

According to the provisions of the current SNiP, the following factors influence the depth of the foundation:

• Geological characteristics of the building plot;
• Design features and dimensions of the building to be equipped;
• The depth of soil freezing.

Important: when designing the depth of the foundation, the calculation is carried out for each factor individually, and the maximum obtained depth is used as a final indicator.

Geological characteristics of the object

To determine at what depth the bearing layer of soil is located at the site, geodetic surveys are carried out, during which wells are drilled and a core sample is taken for laboratory analysis. A soil layer is considered as a bearing layer of soil, the actual resistance of which is equal to or greater than 150 kPa.

Requirements for the depth of laying the foundation for geological conditions are as follows:

• The supporting heel of the foundation should deepen into the bearing layer of soil by 20 cm or more;
• In the surface strata of high-density rocks (clay, sandy, sandy loam), the MLF should be deepened by at least 30 cm.

Figure: 1.2

If such an arrangement is not applicable (the GWL is high, and the foundation needs to be laid to a depth of 1.5-2 m), dewatering is carried out during construction, or drainage channels are created around the foundation.

Building design features

• Weight and size characteristics;
• The magnitude of the loads to which the foundation will be subjected during operation (effects from the weight of the building, snow and useful pressure);
• The nature of the distribution of loads (the need to strengthen the foundation in certain places - when installing heavy production equipment, etc.);
• The presence or absence of a basement or basement.

Figure: 1.3

Important: when arranging the basement floor, the deepening of the columnar foundations is performed 1.5 m below the floor slab, the strip foundations - 0.5 m below.

The depth of soil freezing

Important: heaving is the property of a water-saturated soil to increase its volume during freezing (due to the transition of moisture from a liquid to a solid state), which leads to destructive pushing loads on the foundation tape, which can cause deformation of the bases, cracks on walls and ceilings.

The following types of soil are considered to be soil that has a high tendency to heaving:

• Sands saturated with groundwater;
• Sandy soil with a lot of dusty particles;
• Plastic clay soil;
• Loam.

Figure: 1.4

In soils with a medium and high tendency to heaving, the foundation should always be laid below the freezing depth - with this arrangement, loads from vertical heaving do not act on the foundation.

How and how to determine the depth of laying

K0 - coefficient individual for each type of soil:

• 0.24 - for clays, loams;
• 0.28 - for sands and sandy loams;
• 0.3 - for large sandy rocks;
• 0.35 - for hard rocky soil.

We give the average annual temperatures for the Moscow region:

Figure: 1.5: Average monthly temperatures in Moscow region

Based on the table (only the numbers highlighted in red are used), the root of minus temperatures will be - 4.79 degrees.

Having received the required initial data, you can use the basic formula (we take the coefficient for the clay soil prevailing in the Moscow region): Kfn \u003d K0 \u003d 0.23 x 4.79 \u003d 110 cm

Knowing the calculated level of soil freezing in the region, you can calculate the depth of freezing under a certain building. The calculation is carried out using the formula: Df \u003d Кh x Kfnwhere:

• Kfn - the estimated level of freezing;
• Kh - coeff. freezing.

Important: the Kh value is different for unheated and heated buildings. If the structure is not heated, but is located in a region with an average annual temperature above zero, the coefficient. is 1.1.

The value of the freezing factor for heated buildings is given in the table:

Figure: 1.6

Based on the coefficient and the total depth of freezing of the earth, you can calculate the level of freezing under a certain structure and set the required depth of foundation.

Figure: 1.7

Laying depth - SNIP

The depth of the strip foundation

Despite the fact that the depth of the strip foundation device is not the only indicator of reliability and durability, it plays a huge role in the integrity of the entire house during its operation. Reinforced concrete tape of any size and grade of concrete can burst over time if it is incorrectly placed in the ground, not taking into account its features.

In order not to get confused in all types of foundations and soils, let's try to figure everything out in order. First, we will analyze the types of monolithic tapes, and then, specifically for each type of strip foundation, we will determine the depth of laying.

Factors affecting the depth of the strip foundations

Probably, it is worth starting with the fact that the strip foundations themselves are divided into three main types:

1. Not buried
2. Shallow
3. Recessed

Each of these types is laid at a certain depth, which depends on several main factors:

• The depth of soil freezing
• Soil type
• Ground water level

It should be noted that the depth of the strip foundation is the distance from the soil surface to the base of the foundation, and not the depth to which the trench is dug. In the trench, in addition to the foundation, a pillow may be present.

Now let's see how these factors affect each type of strip footing separately.

Shallow strip foundation

An unburied strip foundation is used extremely rarely in the construction of private houses, because it is a very weak support for the future structure. As a rule, it is all located on top of the ground, and inside there is only a sand or sand and gravel cushion.

I will not write a lot about the unburied strip foundation, especially since an entire article has already been devoted to it. And in general, the very concept of the depth of the foundation is absent from such a foundation.

Calculation of the depth of laying strip shallow foundations

This is the most capricious foundation in terms of the depth of the foundation. Firstly, it is not as reliable as a buried one, and secondly, in order for such a strip foundation to withstand the load of the structure, and also to restrain all heaving forces transmitted from the ground, its calculation must be approached with special responsibility.

I have already described in detail how to fill in one of the previous articles. Therefore, we will not go into details.

Such a strip foundation is laid at a depth that is much higher than the depth of soil freezing, therefore it is called shallow. On it, in contrast to the buried, the forces of heaving of the soil can largely act.

Also, an important difference between shallow foundations is that it must be made monolithic not only below the ground level, but also immediately, having exposed the formwork, fill the above-ground part of the foundation - the basement. This will greatly strengthen the entire strip foundation.

The depth of the shallow foundation is directly dependent on all three factors described above. In order not to get confused, let's look at the table.

Table 1: The depth of the strip shallow foundation (minimum), depending on the type and depth of soil freezing

Table number 2: The depth of soil freezing in some regions

Note: In addition to the fact that the depth of freezing and the type of soil affects the depth of the strip foundation, it is also not worth discarding another very important factor - the level of groundwater, which we will talk about further.

Dependence of the depth of the strip foundation on the groundwater level (GWL)

There are two options for the location of groundwater - when they are located below the depth of soil freezing, and when - above.

Groundwater level below the depth of soil freezing

This can be considered a good indicator, and in this case, groundwater in most types of soils does not have a particular effect on the depth of the monolithic reinforced concrete tape.

The only limitation, in this case, is that in soils such as loams, clays and the like, the tape must be laid at least half the depth of freezing of such soil. In other, "good" soils, this factor does not affect the foundation.

In other words, if the freezing depth in your region is, say, 1.5 meters, then the shallow strip foundation must be arranged at least 0.75 meters.

The groundwater level is above the depth of soil freezing

If the groundwater is high, then the depth of digging a trench for strip foundation does not depend on their level only on rocky soils, sandy coarse-grained, gravel and the like.

On any other types of soils, with a high GWL, the monolithic tape will have to be buried below the freezing depth by 10-20 cm (table No. 2). In this case, it will become a buried foundation.

Recessed strip foundation

Recessed strip foundation is considered the most reliable of all belts. It is laid below the freezing depth of the soil by 10-20 cm. Another condition for its construction is that the soil under its sole should be more or less hard.

In the case of marshy soils, peat bogs and the like, the strip foundation is laid to a depth below these layers. In some cases, it is enough to dig a trench to hard ground, and then arrange a sand or sand-gravel cushion to a level that is just below the depth of soil freezing in your area.

When the soil at the construction site is very poor for laying a strip foundation, or its construction requires huge costs, you can try to calculate another type of foundation, for example, slab. Perhaps it will be both cheaper and more reliable.

How to reduce the depth of the strip foundation

After carrying out all the calculations for the depth of the strip foundation, it often happens that, taking into account the soil and the region, it must be laid very deeply. This raises the question of how to cut costs and reduce depth.

There are several ways to reduce the depth of the strip foundations, all of them are based on reducing the importance of the main factors affecting the foundation.

Reducing the depth of soil freezing

Of course, we will not be able to change the climate in the region, but we will be able to change the depth of freezing, specifically under the base of the foundation, by insulating the foundation itself and the soil adjacent to it from the outside.

Thus, we can reduce the depth of the foundation, as well as reduce the cost of it.

Drainage of groundwater from the strip foundation

Another effective way to reduce the depth of the strip foundation is to drain water from it.

This is done with the help of a good drainage system, which will divert a significant part of the water from the foundation and prevent it from adversely affecting it.

Sand or sand and gravel cushion under the foundation

In the case when heaving soil layers lie deep enough on the site, strip foundation you will also have to lay at great depth. It can be reduced by replacing the heaving soil with a sandy or sandy-gravel pillow.

In other words, it is necessary to dig a deep trench to solid soil, and then arrange a massive sand and gravel cushion there, which will distribute the load from the foundation and the house to the ground evenly and prevent the heaving forces from adversely affecting the foundation.

It is advisable to make a pillow not only under the sole of the foundation, but also next to it, as shown in the diagram.

It should be noted that the most reliable method for reducing the depth of the strip foundation is the combined method, i.e. and the device of a pillow, and insulation, as well as a drainage device, if necessary.

In this article we will consider the calculation of the foundation depth for a private house, according to the instructions of the joint venture "Foundations of buildings and structures".

The depth of the foundations depends on many factors, such as the surface topography, the engineering and geological conditions of the construction site, the design features of the house, the depth of soil freezing, the depth of the groundwater, and more.

The importance of geotechnical surveys is undeniable, but for many private developers, this procedure is expensive. Our articles will be aimed at people who, for whatever reason, cannot afford to hire geologists and designers, but who want to use ready-made examples to figure out the calculations of the foundations, as well as other elements of their future home.

So let's get started.

Determine the depth of the foundation in Moscow. Consider several options: unheated house; a heated house without a basement with a room temperature of 20 ° C and a heated house with an unheated basement.

1. First of all, we need to determine the normative depth of seasonal soil freezing (d fn), in meters, which is determined by the formula:

where d 0 is the value, in meters, for:

Clay and loam - 0.23

Fine and dusty sands, sandy loam - 0.28

Gravelly, coarse and medium-sized sands - 0.3

Coarse soils - 0.34

For non-uniform soil composition, d 0 is determined as a weighted average within the freezing depth.

M t is a coefficient equal to the sum of the absolute values \u200b\u200bof monthly average negative temperatures for the winter in a given area, taken according to table 5.1 JV "Construction climatology"

For Moscow:

Determine M t:

M t \u003d 7.8 + 7.1 + 1.3 + 1.1 + 5.6 \u003d 22.9

Then the standard freezing depth for Moscow, where clays and loams predominate, will be:

d fn \u003d 0.23 √22.9 \u003d 1.1m

If you do not know what kind of soil lies on your site, then take an ordinary hand drill, which is sold in hardware stores, and drill 1 hole in the center, and preferably 4 in the corners of the future building. Basically, on the territory of the Russian Federation, it is heaving loams and clays that are found. In SNiP of 1962 there was no value d 0, instead of it there was one value 23 cm, i.e. 0.23 meters, so it won't be a gross mistake if you accept it.

2. After the standard freezing depth has been determined, it is necessary to calculate the estimated freezing depth (d f).

For this, the formula is used:

k h for external and internal foundations of unheated buildings is 1.1, except for areas with negative average annual temperatures. In our case, the annual temperature is +5.4 o. If you have a negative annual temperature, then the estimated freezing depth for unheated buildings must be determined according to SNiP "Foundations and foundations on permafrost."

k h for heated buildings is determined from the table:

Note: In heated buildings with a cold basement with a negative average winter temperature k h \u003d 1

We consider the estimated freezing depth:

Building unheated in winter d f \u003d 1.1 * 1.1 \u003d 1.21m. We round up and take d f \u003d 1.25m

Heated building without a basement, with floors on the insulated basement floor: d f \u003d 0.7 * 1.1 \u003d 0.77m. We accept d f \u003d 0.8m

Heated building with a cold basement with a negative temperature d f \u003d 1 * 1.1 \u003d 1.1m. We accept 1.1m.

3. Determine the depth of the foundation according to the conditions for preventing frost heaving according to the table below, depending on the location of the groundwater level (UWG).

One of the main conditions for determining the depth of foundations on heaving soil is the depth of its freezing. In our country, seasonal soil freezing can reach a depth of 2.5 meters or more. In buildings without basements, the cost of foundations of such a height is unjustifiably high, so many people have questions: is it possible to install a foundation above the freezing depth and can the depth of soil freezing be reduced?

There are answers to these questions. Yes, foundations can be installed on freezing ground. These are foundations in the form of monolithic reinforced slabs or reinforced strip foundations on a deep underlying layer of non-porous soil. We will not consider them in this section, this is a separate large topic. The depth of soil freezing can also be influenced. This is what this article will be about.

Impact of air temperature on soil

The whole process will be considered in the Celsius scale, taking 0 ° С as the reference point.

Imagine that there is a steel ball on the ground with a temperature equal to that of the ambient air. Let us represent the temperature, which the ball will spread to the ground, in the form of vectors (Fig. 16).

Fig. 16. Temperature effect on the ground

Thus, during the winter, the ball will spread a negative temperature to the ground and freeze the ground around itself in a hemisphere on a scale that repeats the contour of the ball. The more cold days there are in winter, the further the frozen hemisphere will spread into the ground. Since winter is not eternal, one day the hemisphere will reach its maximum and will not increase any more. The maximum depth at which the soil turns from plastic to solid is called the depth of soil freezing.

In the spring, the ball heats up and begins to melt the frozen soil beneath it. That is, the same process occurs as during freezing, only the temperature vector changes its sign from minus to plus. If there are few warm days, then the soil will not have time to melt to the full depth to which it is frozen. Such soil is called permafrost. We will not consider it now. Further, we are only interested in the soil that fully warms up on summer days.

We examined the process of soil freezing from the action of one ball, in fact, billions of such conventional balls lie on the ground and act on it, forming a frozen or thawed field underneath. If you place any building structure on this field, it will cause an anomaly in it (Fig. 17). The disturbance of the frozen soil field will be different and depend on the thermal regime of the object placed on it. When placing an unheated building, the soil under the building will freeze to a shallower depth, since the temperature in the building will still be higher than in an open field. If the building is heated, the soil under it will not freeze at all or it will freeze slightly as it will be heated by the building. Therefore, the thermal regime of the building is taken into account by regulatory documents (Table 10) and affects the depth of the foundations.

Reducing the negative impact of frozen ground

Construction rules (SP 22.13330.2011) define the freezing depth “equal to the average of the annual maximum depths of seasonal freezing of soils (according to observations for a period of at least 10 years) on an open horizontal site, bare from snow, at a groundwater level below the depth of the seasonal freezing of soils. "

In this definition, every phrase is important:

• "Average of the annual", that is, the depth of freezing can be greater than or less than the specified value;
• “An open area, bare from snow,” means that the depth of freezing of the soil under the snow will be less (the thicker the snow, the less freezing);
• "With underground waters below the freezing depth", that is, dry soil is examined, if it is wet, the freezing depth will increase.

It is not in the building rules, but everyone knows that the compacted soil becomes more heat-conducting due to compaction and freezes deeper.

Thus, based only on the definition of the Building Rules, we see several ways to reduce the depth of freezing. The area around the building structure must be covered with snow, not compacted or damp. Ideally, this should be a plowed field and then the soil on it will definitely not freeze to the standard depth even in the most severe winter. But in reality, everything looks a little different. There are driveways to the house, snow from which is removed whenever possible, and autumn rainwater from the roof is diverted near the house.

The greatest danger to the foundation is represented by temperature vectors located in a strip around the building with a width equal to the depth of soil freezing. If they are removed or somehow reduced, then the foundation can be installed above the depth of soil freezing (Fig. 18).

There are at least two ways to reduce the negative impacts from soil freezing:

1. changes in the physical and mechanical properties of the soil;
2. thermal insulation of the soil.

These are the simplest ways available to an amateur developer.

Changes in the physical and mechanical properties of soil

From the previous pages of this site topic, we know that different soils have different properties. Some of them, when frozen, do not change their structure, others increase in volume and push out the foundation, breaking it in different planes. Let us call such soils frost-susceptible and immune.

Frost-resistant soils consist of rock debris (coarse sands, gravel and pebble soils). They also need to replace the heaving soils around the perimeter of the building, in whole or by mixing with the old soil removed during the development of the foundation pit. To reduce the effect of atmospheric water on the properties of soils, it is diverted from the foundation. This is done in two ways. Surface rain and melt water is removed by means of a blind area around the building with slopes from 5 to 10%. Water can be diverted along the terrain or into a special drainage ditch filled with coarse-grained soil with a top layer designed in the form of beautiful paths. In construction areas with high snow and frequent rains, water seeping to the foundation is diverted from the foundation by means of underground drainage. Perforated pipes are laid around the building in a layer of coarse-grained drainage soil, covered with geotextiles in order to avoid siltation of the pipes and covered with drainage fine-grained soil. Further, pipes drain water from the foundation along the slope of the terrain or discharge water into drainage wells buried at a distance from barrels covered with stones. The soil around the foundation will not retain water, which means it will not swell in frost (Fig. 19).

The suction of groundwater into the body of the foundations and basement screeds is interrupted by the installation of coating and gluing waterproofing, as well as the installation of bedding from fine-grained drainage soils. Due to the relatively large distances (by molecular standards) between the particles, such a bedding cannot retain water in itself, and even more so it cannot suck its top and moisten the bottom of the foundation. Capillary suction can also be stopped by spreading a polyethylene film under the foundation (Fig. 21).

Thermal insulation of soil

If the replacement and drainage of soils around the house involves a large amount of earthwork in which we influence the thermal conductivity of the soil by simply replacing one type of soil with another, then the thermal insulation of the soil assumes leaving the same soil with a decrease in its thermal conductivity. This is done by installing thermal insulation. I have said more than once on other pages of the site and will repeat again that the common term "insulation" is used incorrectly. The correct name for the material is insulation. It is a partition between two materials that interrupts the flow of heat. Thermal insulation retains heat if the material it covers was warm or retains cold if the material was initially cold.

Laying a strip of thermal insulation along the perimeter of the building with a width equal to the freezing depth will weaken the flow of negative temperatures penetrating into the soil and it will freeze to a shallower depth. On such soil, it will be possible to install a foundation of a lower height (Fig. 22). Structurally, soil insulation is combined with a blind area and is called an insulated blind area. To prevent frost from passing to the base of the foundation through its body, the cold bridge is interrupted by thermal insulation of the basement base (Fig. 23).

If you come across drawings showing thermal insulation along the outer vertical wall of the foundation, then the basement is insulated, and not the ground. Such insulation retains heat in the basement, while the warmth of the house does not warm up the soil, and the depth of its freezing does not change. That is, the thermal insulation of the foundation walls has nothing to do with the thermal insulation of the ground. These are different design solutions that solve different problems.

Laying a strip of thermal insulation around the house can be done at the level of the basement sole and combined with the thermal insulation of the basement (Fig. 24). In this case, two tasks are solved simultaneously: basement insulation and thermal insulation of the soil. The strip of thermal insulation here will be narrower than on the ground surface and will depend on the depth of the foundation.

Insulated blind area is best used for buildings without a basement, and recessed thermal insulation for buildings with a basement.