Preparation for foundation on clay soil. What foundation to make on clay soil. Pile foundation on clay soils

13.11.2021

The foundation on clay, which one is better to use? This question arises before the developer when it is discovered that there is clay soil on his site. The strength of the foundation depends not only on the quality of building materials. The characteristics of the soil on which the house rests have a great influence on the reliability of the structure.

Therefore, it is extremely important at the design stage to choose the best option for the foundation, corresponding to the geological structure of the building site. This article will help private builders who are going to build a house on clay: which one is best for such a case?

The main problems when building a foundation on clay soils is the likelihood of subsidence, breakage and swelling of the base of the building. This can happen under the influence of too much pressure on the foundation, or due to insufficient depth of its laying.

In the zone of particular risk are houses whose walls are built of lightweight building materials - foam blocks, for example (). The heaving forces in clays can be balanced only by the heavy above-ground part - brick, stone or reinforced concrete.

Types of clay soils

Clay soils include:

sandy loam;
loam;
clay.

Clay is a collection of small (up to 0.01 mm) particles with a small admixture of sand and dust. It is characterized by flowability and high plasticity. It is possible to build on clay if the soil is homogeneous, and the groundwater is deep. But in any case, such a case is not at all suitable for self-construction.

Loam contains up to 10% clay. Depending on its specific volume, the soil can be light, medium and heavy. Loams are prone to frost heaving, so the construction conditions on them are classified as difficult.

The sandy loam contains no more than 5 percent clay. These soils have another name - quicksands, as they have great mobility under the influence of groundwater. It is undesirable to build a foundation on sandy loam soils, but if there is no other way out, pile structures with a crushed stone base are chosen.

Choosing the type of foundation

When building a foundation on clay soil, regardless of the type chosen, the main rule must be followed: the width of the base should be 25 percent or more larger than the upper part of the structure (). A pillow made of sand and gravel (or crushed stone) is mandatory.

Strip foundation

Clay strip foundations are suitable for heavy buildings and light timber structures ().

But if, in the first case, a buried structure is set up, the bottom of which is below the freezing line, then in the second case, preference should be given to a shallow foundation: on clay soil, the light aboveground part cannot become a counterweight to the heaving forces.

Monolithic tape is used only if the groundwater is below the soil freezing line. On loamy soils, a foundation of this type can be built if geological exploration has confirmed the uniformity of the layers throughout the construction site.

Only in this case, uneven deformations during heaving of the soil will not threaten the structure.

Slab foundation

Clay slab foundations are the most reliable, but also the most expensive). Therefore, they are practiced only in the case of the construction of luxurious mansions.

Another name for this type of foundation on clay soil is floating. With any movement of the soil, the slab moves with it, maintaining the uniformity of the load.

The slab is poured practically on the surface of the earth. But buried slabs are also practiced - in houses with basements. In this case, the walls of the underground part of the house and the slab form a single monolithic structure.

If you need to build a foundation on loam, there is no better solution than a monolithic slab (). Since the sole of such a foundation has a maximum area, even the calculated soil resistance of 1 kg / cm2 is sufficient to ensure the stability of the structure.

Pile foundations

With the help of piles, a foundation is being built on sandy loam and other types of clay soils. Pile fields are arranged under buildings of a large area: under the walls, supports are installed in the form of rows, and under the columns - with bushes.

When choosing the type of foundation on clay soil with a high level of groundwater, the most correct solution is to build a house on piles (). In this case, it is necessary to adhere to the basic rule - the support should be carried out with a widening downward.

Screw piles have proven themselves well - structures equipped with screw blades in their lower part). When the screw is screwed in, the density of the soil around the support is maintained - the blades do not loosen the soil, but cut off individual layers. The design depth of the screw is below the aquifer and the freezing point.

Scheme of the pile - screw foundation on clay soil.

Conclusion

The construction of a foundation on clayey soils is a complex business, but quite feasible. The main thing is to know all the advantages and disadvantages of such soils and use the former correctly, avoiding the latter.

Video about what clay is dangerous for the foundation.

The operational life of all structures depends on the foundation. Its estimated cost is 30% of the total amount required for construction.

But the quality of the foundation directly depends on the soil - the place where the structure was erected.

Types of clay soil

Such soil differs in different places and according to its characteristics can be classified into 3 types: clay, loam and sandy loam. As a result, the properties of the construction site are determined on clay soil before the foundation is erected.

In the presence of clay, the composition of which is small particles up to 0.01 mm with admixtures of sand and dust, the foundation can be laid if the soil is homogeneous and the level of its freezing is higher than the layer of underground sources.

Clay soil is highly free-flowing and plastic. The construction of the foundation in this case should be carried out by professional builders who have the skill and necessary knowledge.

In the presence of loam, sand and clay, the level of heaving of such soil is high. In this case, the clay is no more than a third of the volume of the soil. The amount of clay affects the type of loam: light, medium and heavy.

Sandy loam comes with a clay content of no more than 10%. Due to the ability to move under the influence of ground sources, they are also called quicksands. It is not recommended to build on such a site. But due to the hopelessness of the situation, a pile foundation based on rubble will be used.

Only after determining the type of soil, the levels of groundwater and its freezing, the foundation is chosen between two options: the belt is suitable for homogeneous soil or pile in the presence of stones in the soil. The choice will also be influenced by the material from which the structure will be erected.

When building a foam block or brick structure, a strip foundation is best suited to withstand the load exerted on it.

With insignificant loads exerted by a shed or greenhouse, the construction of a pile foundation is justified, which will ensure the monumentality of such a structure.

Despite the design of various buildings, the soil will be the same. In order to save money, it is possible to use sawdust or sand as substitutes for gravel or crushed stone. But clay is an insidious substance and requires compliance with all building rules and regulations.

Regardless of which foundation is better to build on clay, there is always the possibility of its breaking off, swelling and subsidence. The reasons for this may be insufficient depth of the foundation, or a lot of pressure on it.

In order to avoid the pressure of the freezing soil, it is recommended to treat the base with machine oil or cover it with PVC film, which will not let water through during a thaw. The upper ground layer should be insulated with expanded clay or crushed stone.

Choice Dependency Factors

On clay, it is possible to erect a slab or strip base, on poles or piles. It all depends on the following factors:

Price. In terms of stability and reliability, as well as in terms of price, the slab foundation is among the leaders. It is ideal for heavy structures.

But in the presence of a basement, a recessed strip foundation is more expensive than a slab foundation. Pile and columnar foundations are excluded for buildings with a basement on clay, such foundations are suitable for light and without basement buildings. At the same time, on piles is the best and inexpensive solution.

Uniform or intermittent arrangement of clay formations. The study of the site is carried out by drilling wells to the depth of soil freezing. With a different composition of the soil in different places, the base is covered with crushed stone and sand. When installing the base on piles, it must be brought to a dense clay layer.

With strong heaving of the soil, the determining factor is the weight of the structure. For high loads and a large sole area, a reinforced plate is preferred. With a light load, a tape or pile-grillage base is suitable.

Depth of underground sources. Clay is a natural waterproofing agent, but at depth it is constantly wet. With deep burial, the choice falls on a monolithic base or on piles. The tape type is excluded.

The depth of soil freezing. Labor intensity and cost of construction depend on the depth of water occurrence and freezing. A pile foundation for an uneven area is an opportunity to reduce costs, labor intensity and increase reliability. Piles are preferred for difficult ground where clay and stone are mixed.

The leveling of the disadvantages of clay soil and the use of its advantages to strengthen the structure depends on the correct choice of the foundation.

Construction process. As an example, we will describe the creation of a foundation on clay soil for a log bath:

Marking for the base;
Digging a trench 1.5 m below the fertile layer and 3 times wider than the proposed foundation;
Installation of metal or wooden formwork;
Making cement and pouring it into the formwork with its subsequent ramming;
Time for cement drying and formwork dismantling;
Heat treatment of the foundation by firing;
Backfilling the trench after the firewood has completely burned out.

Despite the disadvantages of clay soil, it has several advantages. Such a foundation is able to withstand a strong load with a deep location of underground sources. As a result, the supporting structure is reliable and can serve as an example of savings.

With a shallow location of soil, preference is given to the foundation on piles, which will be more expensive, but it does not take up reliability and quality.

Here's some more extra stuff

Foundation construction is the initial and most important part of construction work. The reliability and durability of the building largely depends on the strength and high-quality performance of this structural element. An improperly designed and installed foundation without taking into account the design of the building and the characteristics of the soil on the site can cause the destruction of the house. That is why this operation must be approached with full responsibility.

Particular attention should be paid to soils. On different types of them, the same structural solution of the foundation will have different strength. For example, the belt base will stand for centuries without problems on stony soil or on black soil, and on clay soils it may well “float away” with a high groundwater table. It is about clay soils and the peculiarities of building foundations on them - below.

Features of clay soils

The main feature of clay soil is that it does not hold its shape at all. A few "drops" of water are enough and the clay spreads. We all have some time to deal with this material. Its extreme plasticity makes it possible to make a large number of different products from it, however, in construction this is not an advantage, but a disadvantage. Due to the increased plasticity, the clay soil cannot act as a full-fledged support for the building, which, when the soil is washed out from under the foundation, can contribute to the formation of voids. They, in turn, can cause the base to sink. Which will lead to the destruction of the building. That is why the construction of a house on such soils should be accompanied by their in-depth analysis by specialists, who must issue their opinion on the possibility of erecting a structure.

Clay soil is different, depending on the content of clay in it:

1. Beginning at 30%, soil is considered clay. Due to the high moisture content, such soil is practically unsuitable for construction. The complexity and high cost of the foundation built on it makes the building very expensive. The fact is that during freezing, the moisture contained in such soil tends to expand, which in turn can damage the foundation.

2. A mixture of sand, clay and stone. Of all clay soils, the latter is the most suitable for construction. Due to the stone, it perfectly keeps its shape, does not float, which makes construction on it not very difficult and rather strong.

3. Other types of soil. Distinguish also:

Sandy loam - a type of soil that contains sand and clay, the latter no more than 5%;

Loam is a soil consisting of sand and clay, in which the latter is not more than 10%.

Sandy loam and loam, similarly to clay, belong to the category of unreliable soils, the behavior of which depends on many factors. In each individual case, there is a need for a thorough analysis of the state of the soil in order to make a decision on the structure of the foundation.

There are two main types of clay:

1.Red clay. This material contains a large amount of sand. It is because of this that it perfectly absorbs moisture and does not hold its shape well. Soils containing red clay are extremely unstable and very often subject to destruction and structural changes.

2.Blue clay. Has higher quality strength characteristics. Virtually impervious to moisture, which can accumulate on its surface.

Clay soil. What foundation is right for her

The construction of foundations on clayey soils cannot be carried out without special preparation. If possible, it is better to move it to another place, if not - to attract specialists to help. It is worth realizing that such construction will be not only difficult, but also costly.

Strip foundation on clay soil

On clay soil, it is allowed to build a strip foundation, however, it will have a slightly different structure in comparison with the same type for other types of soils. The advantages of the tape basis are:

1.High reliability in compliance with all technology requirements.

2.Excellent strength and waterproofing.

3.Suitable for almost any type of building and materials from which they are built.

4. The ability to build a foundation on clay soil with your own hands in a short time.

The disadvantages of this type of foundation are as follows:

1. High cost of time and money for earthworks.

2.Large weight and high cost of materials.

Formwork device for strip foundations

There are several options for arranging such a foundation:

The first scheme for the construction of a strip foundation on clay soils involves the construction of a special cushion. Several layers of sand and gravel are placed in a trench dug under the base tape, alternately. The thickness of each of them should be at least 18-22 cm. After filling the next layer, it is watered and carefully tamped. This pillow is poured down to the zero level. Then a base is built on it from a large stone or brick. From above, the base is covered with several layers of PVC film or roofing material.

The second option for the construction of the foundation involves filling only half the depth of the pit with a sand-gravel mixture. The pillow is carefully compacted and leveled. Then one layer of brickwork is laid on its surface. After the mortar has dried, a formwork is constructed for pouring concrete to the required height. Formwork boards must be treated with a special antiseptic and covered with tar. The formwork is set to the required foundation height and poured with concrete. After that, the foundation is leveled and a layer of waterproofing is laid on top. For the latter, you can use a dense PVC film or roofing material.

The third version of the basis involves the use of flat slate for formwork. A formwork assembled from sheets of roofing material is immersed in the dug pit. The distance between the ground and the formwork is filled with gravel. Inside, half of the container is filled with a mixture of sand and gravel and carefully compacted. Lastly, concrete is poured in layers of no more than 15 cm, which must be compacted with a special construction vibrator until cement milk appears on its surface. After cooling down, the foundation must be covered with waterproofing.

If possible, it is advisable, before filling the distance between the ground and the base, to open its outer planes with bituminous mastic and cover it with waterproofing. The filling is done with gravel.

Pile foundation on clay soil

A pile or column foundation is the simplest and most balanced solution for building on clay soils. Due to the need to drill special wells and the corresponding use of special tools and equipment, this type of base can hardly be called the cheapest. However, its durability on unstable ground makes it the most acceptable.

Among the advantages of such a foundation are the following:

1. The minimum required amount of materials and their availability.

2.Minimum shrinkage of materials.

3. Simplicity in execution.

4. Possibility of installation in areas with the most difficult soil composition.

He has only one drawback - the need to use special equipment.

At first glance, the structure of such a foundation is not too complicated. The order of work:

1.Depending on the project of the building, special holes with a diameter of 200-300 mm are drilled under each load-bearing slab. The depth in each individual case is determined by a specialist. It should be below the freezing point of the soil.

2. A layer of gravel is poured at the bottom of the well, after which an asbestos-cement pipe is lowered into it. Concrete is poured inside it.

3. Before pouring into the pipe, several reinforcing rods are inserted to reinforce the structure.

4. Columns are installed at the intersection of load-bearing walls and in the corners of the building. The step of installing the supports should not exceed 2 meters.

There is one more foundation on clay soil, the scheme of which is similar to the previous one - the so-called pile-backfill. It is being built using a slightly different technology, and it can be used in places with water-saturated soil. It is worth noting that such a foundation will cost much less than a pile foundation.

Its construction technology is quite simple:

1.In places where, according to the project, the supports are to be installed, a layer of soil is removed and holes are dug 0.5 m deep and 0.4x0.6 m in size.

2.A mixture of sand and gravel is poured into its bottom, which is compacted and poured with water. The filling is carried out in layers to the ground level, constantly tamping.

3. Then concrete blocks are placed on the prepared cushion. For this purpose, products of a standard size 0.2 x 0.3 x 0.5 m or 0.3 x 0.3 x 0.5 m are used.

4. For the device of one support, as a rule, 2 concrete blocks are used, connected by a solution. The sides of the blocks are treated with an antiseptic, and a layer of roofing material or PVC film is laid on the top to ensure waterproofing.

The stability of such a basis lies in the fact that water cannot penetrate into the lower layers of the fill, since there is air in them. Although water penetrates into layers of clay soil during autumn rains or spring floods, the bottom fill remains completely dry. Due to this, during the onset of frost, the filling does not expand, which excludes the movement of the base and distortions in the geometry of the entire building.

If it is not possible to buy concrete blocks, you can cast them from concrete on site. For this, the formwork of the required height is installed and the solution is poured. This should be done in layers, constantly vibrating. After that, the finished pillars must be processed in the same way as concrete blocks.

Comments:

A monolithic foundation slab becomes the only way out when it becomes necessary to erect a capital structure on soil that has a weak bearing capacity. These types of soils include sandy and sandy loam, clayey, loamy, peaty, as well as places where groundwater is close to the surface. A monolithic slab foundation distributes the weight of the walls and floors of the house over its entire area, which prevents cracks from appearing on them. Thanks to him, the building acquires a closed volume and becomes less dependent on the movement of the ground below it. This is one of the most reliable types of foundations, and at the same time the most expensive.

Design features of a monolithic slab under the foundation

Such a foundation is also called floating due to the fact that the house built on it gets the opportunity to move in time with the movements of the ground masses on which it stands. The foundation covers the entire area of the building that is built on it. When the soil freezes during the winter frosts, it rises with it. At the moment of spring thawing, the soil subsides, and the whole building sinks with it. At the same time, it does not undergo any destruction. Since the soil on which the construction is being carried out cannot become a reliable support for the future structure, it makes no sense to lay the foundation at a considerable depth. To make a monolithic foundation in the form of a solid slab, it is enough to dig a bed 45-50 cm deep under it.

The foundation slab has several varieties:

Solid. It is constructed in the case when the building structure does not provide for the presence of a basement, and the upper plane of the foundation is at the same time the floor of the lower floor.
Ribbed. In this case, the ribs can be made both in the form of a single monolith with a foundation slab, and from reinforced concrete blocks. Their height is equal to the height of the base.
Box-shaped. In this case, the ribs represent the walls of the basement of the house. In this case, the ribs are interconnected by floor slabs, forming a monolithic box.

In areas of increased seismic hazard, vertical stiffeners are not only cast together with a horizontal solid foundation slab, but the reinforcing bars of their frames are also welded together. This gives the entire structure an additional margin of safety. It is worth remembering that this type of foundation is reinforced over the entire area in two layers, which are interconnected by vertical posts.

Back to the table of contents

How to make a solid slab foundation

This type of foundations is usually laid under houses, whose load-bearing walls are built from logs, wooden beams or various types of ready-made wall blocks. The thickness of the concrete slab should be calculated based on the estimated weight of the walls and floors, to which you must remember to add the weight of the residents and furniture. Moreover, its area should slightly exceed the living area of the first floor. A pit of the appropriate size and shape is dug under the foundation, up to 50 cm deep, its bottom is leveled and filled with concrete mortar with a layer of 10 cm thick.On top of it, after complete solidification, a layer of coarse sand or fine gravel with a thickness of 20 cm is poured, which must be tamped tightly. For this, it is best to use a vibrating plate.

A layer of waterproofing is laid on top of the supporting sand and gravel cushion, and if the house is planned to live all year round, then a layer of insulation. Best of all, expanded polystyrene plates play this role. After that, the lower layer of reinforcement is placed, which is recruited in the form of a lattice from a ribbed reinforcing bar with a thickness of at least 12 cm. The frame parts must be attached to each other by welding, and in no case should they be tied with wire. The width of the grid cells should be approximately 25 x 25 cm. Then vertical posts are attached to this grid, the length of which determines the future thickness of the slab.

Back to the table of contents

Pouring concrete to create a foundation slab

For an ordinary two-story house with a frame structure, the foundation slab should have a thickness of 20 cm.If the house is one-story, 15 cm may be enough.But if the building area is large enough, the thickness should be increased to 25-30 cm. Then you can start pouring the frame with concrete. Since increased requirements are imposed on the strength of a foundation of this type, concrete must have a grade of at least M300. It is poured evenly so that the reinforcement completely goes into its thickness to a depth of at least 3-5 cm. The surface of the slab is leveled using a building level. If it is necessary to ensure the frost resistance of concrete, special additives are added to it, and thickenings are made at the edges of the slab, called contour ribs.

The service life of the structure will depend on the choice and correctness of the construction of the base. But even before these processes, it is worth conducting a detailed analysis of the terrain on which the construction will be carried out. It is especially important to have a complete understanding of the characteristics of the soil. It is with this data in mind that the grounds can be calculated. Further, we will talk about what kind of foundation can be on clay soil, which one is better for building a house, and also the features of clay soils will be given.

To carry out the classification of soil rocks, scientists analyze a combination of several factors, among which there is a percentage of particles of a certain composition in the earth.

Certain soil rocks are composed of several impurities. For example, sand and clay. It does not happen that the land in a certain place consisted exclusively of one component (in natural conditions).

It is because of the different concentration of impurities that clay soils are divided into:

Clay. In such a rock of soils, more than 30% of pure clay mass is present. Exact calculations will be carried out by experts, but for an independent determination, it is enough to take a little soil and roll up the "sausage" too. We can safely say about the large presence of clay in the soil, if this "sausage", even after a long time, will succumb to deformation - it will retain its plasticity. Before making the foundation in this case (and in the others), you need to find out the depth of soil freezing, which should not reach the groundwater level.
Loam. To understand that you will have to build a foundation on loam is quite simple, using the previous method with clay "sausage". Only in this case the plasticity will disappear quickly and the soil will dry out quickly, which will be accompanied by the destruction of the “hand made product”. The percentage of clay in loamy soil is in the range of 10-30%.
Sandy loam. The content of clay grains in this case is about 5-10%.

If sandy loam soil dominates at the construction site, then it is worth considering that it is significantly influenced by groundwater. That is why you need to use moisture-resistant materials to create the base of the house, take care of the vapor barrier of the walls and protection against subsidence of the structure.

It is worth considering not only the presence of pure clay in the soil, but also its features. For example, a foundation on glacial clay will have the required margin of safety only if the clay layer is wide enough (it will not be washed out by groundwater). Construction is also dangerous in the presence of soils containing alluvial clay - its plasticity does not allow reaching the required foundation strength.

How to choose a base?

What is the best foundation for construction on clay soil can be accurately determined by geological surveys. Experts carry out an accurate analysis of soils, the depth of water occurrence, and the level of freezing. Only after receiving a complete picture of all factors can you choose the ideal base for a particular house, capable of supporting the weight of the structure and guaranteeing a durable service.

The main types of bases that are used when building a house on clay:

Floating. A monolithic slab is erected under the building, additionally reinforced with reinforced rods. The reinforced concrete structure can be delivered from the manufacturer or made by yourself. Its feature is stability in the presence of a shift in soil layers. When choosing such a foundation, it is necessary to correctly calculate the level of its deepening, and also take into account a rather large indicator of construction costs.
Tape. Such a foundation on clay soil should be chosen with extreme caution. It is necessary to analyze the climatic conditions of the area, choose the right material for the structure, taking into account the characteristics of the soils. In any case, the base should be deepened more than the level to which the soil freezes through.
Pile. This option is optimal (if not even the only reasonable) in cases of groundwater concentration in the immediate vicinity of the earth's surface. The use of driven or screwed-in supports makes it possible to reach those layers of soil, the density index of which can ensure reliable fixation of the structure. But it is worth considering the moment that the use of such a base is not suitable for the construction of a house in which a basement is planned.

When arranging the pile foundation, it is especially important to make the correct calculation of the number and method of positioning the supports, and also to install them with perfect evenness.

What foundation would be better for the construction of a building on clay, even an experienced specialist will not immediately answer - it will be wrong. As already mentioned, it is necessary to carry out a general analysis of the soils, the materials used, the climate of a particular region, the structural features of the building, its dimensions and much more.

What should be considered when building in an area with clay soil?

Saving on material for the foundation can end in the most unfavorable way for the owner of the structure and its inhabitants.
It is imperative that a cushion of sand, gravel and crushed stone or lean concrete is built under the foundation - it will improve the shock-absorbing qualities of the base, and will also allow more evenly transfer the load to the soil.
For maximum stability of the base, it is constructed so that the lower part is approximately 30% larger than the upper one.
After pouring the concrete solution, it is imperative to compact it. For this purpose, special vibration equipment is used. Exposure to vibration helps to remove air from the base structure, which has a positive effect on strength.
A structure on a clay base does not exclude the need for drainage and insulation works.

If you have problems with determining the depth of soil freezing, then you can contact the architectural department of any regional center - they should have such information.

A foundation on clay for any structure, and especially for a house, must exactly correspond to the conditions of a particular construction and be fully constructed in full accordance with the project documentation.

What is the best clay foundation for a home? updated: February 26, 2018 by: zoomfund