What foundation is better to pour on loamy soil. Features of the foundation of a monolithic slab
Specialists of construction companies are often asked questions regarding the construction of country (cottage) houses on clay soils. The most common: “I have clay soil in my area. What foundation will ensure the reliability of building a house? In our review we will try to cover all the nuances of this topic.
Clay as a natural material and its features
Clay soils are different. If the percentage of pure clay in the soil fluctuates within from 5 to 10, then we have sandy loam. loam called soils in which 10-20% pure clay. And if it is more than 30%, then the soil is called “clay”.
The main feature of clay is the ability to quickly erode under the influence of water, while not letting it go deep. Clay layers can lie at a sufficient depth, and the water that has penetrated to them at low temperatures freezes and swells the soil. Therefore, clay soils are called heaving, and before starting construction, it is strongly recommended to conduct studies on the composition and uniformity of soils on the site. Otherwise, clay can behave unexpectedly, quickly turning a deep foundation into a ground one.
In the general case, river (alluvial) and glacial clays are distinguished. The first lies near water bodies, in lowlands and has high plasticity. Construction on such sites is contraindicated, and in exceptional cases, houses have a pile foundation. Concrete foundations can be confidently erected on glacial layers, but only if they are deep.
Choosing the right foundation
On clay soil, you can build your absolutely favorite type of foundation, it all depends on the specific site and the capabilities of the developer.
Regardless of the type of foundation chosen, it is recommended to backfill on clay soils: excavate the soil under the entire area of \u200b\u200bthe base and fill it with sand or gravel. The procedure is expensive, but increases the strength and reliability of the building, especially in areas with long periods of low temperatures.
So, the choice of foundation based on indicators of clay content and depth of soil freezing, as well as on the value of the groundwater horizon. If groundwater is located above the freezing level, then even before laying the foundation, it is necessary to create drainage trenches around it.
The foundation for a house on clay soil can be:
- tape, deep (especially in regions with low temperatures);
- slab shallow;
- piled.
Strip foundation on clay soil
For the construction of country houses on sandy loam and loam, with deep groundwater, a strip foundation can be laid. On clay soil it will monolithic reinforced concrete tape under all external and load-bearing walls, as well as under the heaviest sections of the future structure. This is the only possible option if the house is supposed to have a cellar, an underground garage or a basement.
After transferring the markup from the plan to the “nature”, a pit is dug along the stretched signal strings, the depth of which is greater than the level of freezing of clay soil. In regions with a cold climate, it can be 1.5 meters. The bottom is carefully leveled and covered with layers of crushed stone (10-15 cm) and sand (the deeper the ditch, the thicker the sand layer). The walls of the trench are laid with a waterproofing film or roofing material.
The width of the pit is calculated as the thickness of the walls with the finish, plus 10 cm for the blind area and 30% for the base of the foundation. At the bottom of the pit, a concrete mortar is poured with a layer of 3-5 cm (this will be the base), and after hardening (about 14 days), formwork and reinforcement tied into meshes are installed.
The development of building technologies has made it possible to use more expensive, but better fixed formwork. It additionally insulates and protects the foundation from displacement.
Concrete is poured continuously, in parts of 15-20 cm, with mandatory compaction of each of the layers. After 28 days, the concrete is gaining full strength and the formwork can be removed. Finished Foundation coated or sprayed with waterproofing materials, and if it remains to shrink for the winter - carefully cover.
Advantages:
- Sufficiently high bearing capacity.
- Durability (75-150 years).
- Possibility to build a basement or cellar.
Disadvantages:
- Great labor intensity.
- High material costs, and hence the total cost of the foundation.
- It is allowed to use only on loams and sandy loams, and in a deep-buried version.
slab foundation
Another name for this type of foundation is "floating" plate. This is an excellent option for clay soils, as the weight of the house is evenly distributed on the reinforced concrete base, and when the soil is washed out or moved, the entire slab will shift, and not individual parts of it.
By itself, the concrete slab has a slight depth (depending on the weight of the future house), but it does not lie on clay soil, but on a sand and gravel substrate.
Under the entire ground floor soil is removed to a depth exceeding the thickness of the slab by 30-40 cm. With a high level of groundwater, a drainage system is installed from pipes with a slope from the foundation. Now, the base can be covered with sand and carefully compacted or poured with “lean” concrete. A formwork is mounted along the perimeter of the foundation and its inner part is covered with a waterproofing film or roofing felt. Reinforcement bars are tied into a mesh, mounted in a formwork and poured with concrete mortar filled with fine gravel. Filling is done in the shortest possible time, "at once". Moreover, it is better to pour one layer over the entire area of \u200b\u200bthe base on one day, and finish on the next, than to fill the entire height in parts.
- Resistance to subsidence and soil erosion, as well as seismic phenomena.
- Durability (up to 150 years).
- During the construction process, you can change the original layout of the house without additional changes to the foundation.
- Expensive materials and earthworks.
- The complexity of the process.
- The inability to build a basement without additional work.
Before you start building a foundation on clay, you need to know all the features of clay soil.
What you need to know about clay soil
Clay is a finely dispersed composition of particles that have the ability to absorb and retain moisture. Such soil has such a property as heaving. When freezing, the soil increases significantly in volume and can push the foundation of the house up. The clay base is subject to erosion by melt and rain water, which can have a devastating effect on the foundation on clay.
Clay soils are of several types:
- clay soil includes 30% or more pure clay;
- sandy loam - sandy sandy loam contains about 5% clay;
- loam contains up to 10% clay.
One type of clay soil - sandy loam
By the method of test drilling, the structure of the occurrence of soil layers is recognized. They drill a hole in the ground to the depth of the foundation of the house. To avoid errors in the analysis of the sample, it is better to contact the district department of architecture at the place where the construction master plan is linked and get a copy of the vertical survey.
From this document, it will become known about the qualitative composition of the soil, about the level of groundwater and about the depth of soil freezing. In most of the country, excluding the regions of the Far North, the depth of soil freezing reaches an average of 0.8 m. In the north, this figure reaches 2.4 m.
If the groundwater is above the freezing level, artificial drainage of clay soil will be required. To do this, construct drainage ditches and sewage outlets.
If the type of soil does not allow drainage of the base, then the foundation on clay will have to be built on piles.
Which foundation to choose
Pile foundation on clay
Before building a house, you need to carefully study the characteristics of the foundation and choose the right type of foundation on clay.
On clay soil, three types of building support base are applicable:
- strip foundation on clay;
- tape pile;
- monolithic slab;
- pilewort.
Strip foundation
The foundation is made only buried. A trench under the tape is dug to a depth below the freezing level of the soil. At its bottom, a pillow is made of two layers of sand and gravel. The trench should be wider than the foundation so that it is possible to install the formwork without interference. Then roofing felt or polymer film is laid. The waterproofing coating will block the path of moisture penetration to the foundation structure.
The height of the formwork must be higher than the top surface of the concrete. The edges of the waterproofing material lead to the same height.
The reinforcing cage is mounted nearby on the site, then its elements are lowered into the formwork. Sections of the frame are connected with knitting wire and form a single structure of reinforcement.
In cross section, the foundation of the house is a trapezoid. The bottom base will be wider by a third of the top surface. This type of construction gives high stability and increases the resistance to heaving of clay soil.
The concrete mixture is poured continuously. This will prevent the concrete from separating when it hardens. As liquid concrete enters the formwork space, it is compacted with a vibrator or an improvised tool (shovel, wooden manual rammer).
For the construction of the basement, they dig a pit. The foundation walls are made to the height of the basement.
Monolithic slab
A monolithic reinforced slab is the most expensive option for supporting a house. It is made of concrete grade not lower than 300. The depth of the foundation slab depends not only on the freezing of the soil, groundwater, but also on the total weight of the structure.
The reinforced concrete monolith creates the effect of a floating slab. This gives independence from any shifts in the clay base.
Works on the device of a sand and gravel cushion, waterproofing, formwork, reinforcement and concrete pouring are the same as in the construction of a strip foundation.
Strip pile foundation
A shallow foundation is erected for light wooden houses and for brickwork. A trench for a monolithic tape is dug 50 - 70 cm below the groundwater level.
To enhance the reliability and bearing capacity, supporting piles of the shufta type are installed every 2 meters below the level of soil freezing. After removing the formwork, all sinuses are filled with compacted sand. Watch a video on how to build a strip foundation on clay.
Pilewort
At the calculated points, piles are driven in until they reach dense layers of soil. In some cases, due to special soil conditions, they make pile bushes (piles from one point diverge downward in different directions) or install hanging piles (racks are driven in at an angle, providing stability due to lateral friction on the soil base).
The upper ends of the piles are tied either with a metal profile, or with reinforced concrete beams or slabs. The most reliable and durable option for supporting a house is a grillage made of a monolithic slab on piles.
clay foundation
Clay is suitable for building a foundation for a greenhouse
The clay foundation is arranged for light buildings (greenhouses, sheds, utility rooms). In places where clay soils are available, a foundation is made of clay. Clay soil is mixed with water, adding sawdust, sand or any improvised filler to it. They dig a shallow trench and install the formwork. Then a continuous tape is formed from the resulting mixture in the formwork space around the entire perimeter of the future building.
The cross-sectional dimensions of the clay foundation are selected based on the type of structures resting on it. After the clay has dried, the formwork is removed. Around the resulting structure, firewood is laid out and kindled. In the process of burning wood, the clay foundation is fired. As a result, a strong support for light buildings is obtained.
The construction of foundations on clay soil is a very responsible business. When something happens to the upper building structures, it is managed by local repairs. And an incorrectly laid foundation can “destroy” the entire house.
If the developer is not confident in his knowledge and experience in the construction of structures of this type, then it is better to turn to specialists.
Clay soil in your area, despite its complexity, can serve as a good basis for building foundations and buildings.
Features of clay soil during the construction of the foundation
Clay soil consists of tiny flakes, between which moisture can accumulate well. This leads to the fact that such soil acquires the property of "heaving" - it solidly increases the volume when the moisture in it begins to freeze. This property must be taken into account when building a foundation on clay soil. Due to the fact that clay soil can displace the foundation in the cold season, the space around the foundation on your site will have to be covered with sand. Such backfilling will lead to some increase in the cost of construction.
Assessment of the state of the soil on the site
Before starting to draw up a foundation project, it is necessary to carry out survey work on your site.
The soil condition check consists of the following components:
- Evaluation of soil quality at various horizons,
- Checking the level of groundwater,
- Calculation of the level of soil freezing.
In order to carry out a comprehensive check of the state of the soil, it is necessary in the spring, during a high level of standing groundwater, to drill a pit about 2.5 meters deep at the site of the future structure. The soil raised from various horizons of the pit will give you a complete picture of the structure and moisture content of the soil. The depth of soil freezing is calculated based on the maximum winter temperatures for previous periods and the actual characteristics of the soil, like clay, for example, freezes to a level different from sandy soil.
Carrying out drainage works
In the event that a high level of groundwater is recorded on your site, and even more so if it exceeds the level of soil freezing, it is necessary to carry out drainage work before building the foundation. The construction of the drainage system includes the formation of trenches with pebble backfill around the future building, which will divert water away from the building. The drainage system can also be made in the form of pipes with holes, which are located underground and have a slope, which also helps to drain water.
Drainage system on siteTypes of foundations on clay soils
Depending on the characteristics of clay soils in your area, the degree of clay, sand and water content in them, professional builders recommend forming the following types of foundation structures for future buildings:
- Strip foundation
- pile foundation
- Combined strip-pile foundation
Each of them has some nuances in construction. Consider how to build such foundations with your own hands.
We build a strip foundation on clay soil
The strip foundation is a concrete monolith, which rests on the ground with a large area of \u200b\u200bits base. The concrete tape of such a foundation should be located under all load-bearing walls, regardless of whether they are external or internal.
The construction of this foundation is quite labor-intensive, but, nevertheless, such a strip foundation on clay soil can withstand the weight of a large, solid structure.
After drafting the strip foundation, it must be transferred to the area. Before that, it is better to remove a layer of fertile soil from the construction site, as it can be profitably used in other parts of your "estate".
Marking for the construction of a strip foundation (as well as any other) is made using pegs and stretched cords. When constructing a rectangle, not only its sides, but also its diagonals are measured with a construction tape measure. Thus, the construction of ideal right angles is achieved.
We build a columnar foundation on clay soil
On clay soil, it is also possible to build a columnar foundation. To do this, you can use both ready-made supports (reinforced concrete pillars or screw piles), and form bored piles.
Screw supports - made of durable metal pipes can be screwed into the ground with simple mechanical gates, but to place reinforced concrete piles, you will have to use the services of construction equipment.
With your own hands, you can form bored piles. To do this, large-diameter wells are drilled at selected locations (at all corners of the building, at the junction of internal load-bearing walls and at least every 2.5 meters in a straight line). A sand and gravel cushion is laid on their bottom and a 10-centimeter concrete support is poured. Pipes (made of polymers, asbestos cement or metal) are placed in the wells. A multi-tiered metal reinforcing structure is formed inside the pipes, which is then poured with concrete mortar.
We build a combined pile-strip foundation on clay soil
When building on clay soil, you can combine support pillars and a strip foundation. In this case, the pillars (or piles) will go below the freezing level of the soil, and the strip foundation will not be so deep.
When forming such a foundation, the metal frame of the supports and the metal frame of the strip foundation must be combined into a single structure.
Comments:
A monolithic foundation slab becomes the only way out when it becomes necessary to build a capital structure on soil with a weak bearing capacity. These types of soils include sandy and sandy loamy, clayey, loamy, peaty, as well as places where groundwater is close to the surface. The foundation of a monolithic slab distributes the weight of the walls and ceilings 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 soil under it. This is one of the most reliable types of foundation, and at the same time the most expensive.
Design features of a monolithic slab for 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 soil masses on which it stands. The foundation covers the entire area of the building that is built on it. When the soil freezes during winter frosts, it rises with it. At the moment of spring thaw, the ground sags, and the whole building sinks with it. At the same time, it is not subject to 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 structure of the building 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.
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How to make a monolithic foundation in the form of a solid slab
This type of foundation is usually laid under houses whose load-bearing walls are built from logs, wooden beams or various types of prefabricated wall blocks. The thickness of the concrete slab should be calculated based on the estimated weight of the walls and ceilings, to which one must not forget to add the weight of the residents and furniture. At the same time, its area should slightly exceed the living area of the first floor. Under the foundation, a hole of the appropriate size and shape is dug up to a depth of 50 cm, its bottom is leveled and poured with concrete mortar with a layer 10 cm thick. After it has completely solidified, a layer of coarse sand or fine gravel 20 cm thick is poured over it, which must be tightly compacted. 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 it is planned to live in the house all year round, then a layer of insulation. Styrofoam boards are best in this role. After that, the lower layer of reinforcement is placed, which is recruited in the form of a lattice of ribbed reinforcing bar with a thickness of at least 12 cm. The frame parts must be fastened together by welding, and in no case should be tied with wire. The width of the lattice cells should be approximately 25 by 25 cm. Then vertical racks are attached to this lattice, the length of which determines the future thickness of the slab.
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Pouring concrete to create a foundation slab
For an ordinary two-story frame house, the foundation slab should be 20 cm thick. 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. Next, you can start pouring the frame with concrete. Since there are increased requirements for the strength of this type of foundation, concrete must have a grade of at least M300. It is poured evenly in such a way 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 along the edges of the slab, called contour ribs.
An important criterion for choosing the type of foundation for a house is the type of soil occurring on the territory. Building on clay often requires additional costs. One of the options for building a house on clay is to cut out all the bad soil and then fill the pit with compacted draining soil.
This procedure requires a lot of excavation and entails an increase in construction costs. To improve the reliability of the base on clay, it is necessary to provide additional measures.
Types of clay soils
Before deciding which type of foundation to choose, you need to know the type of underlying soils and their characteristics. For this, engineering and geological surveys are carried out with laboratory tests of soils. The geological report will describe which soils prevail in the construction area.
Sandy loam - soil with a clay content of 10 percent
The type of soil, its moisture content and the percentage of sand particles affect the choice of foundation. According to the regulatory literature, clay soil is divided into:
- Clay. Here, its content in its pure form is more than 30%. This soil can be used under the base if it is homogeneous in composition and the freezing point of the earth is above the groundwater layer. It is characterized by high flowability and plasticity.
- Loam. It is sand with clay impurities. Its content in the soil is not more than 30%. Loam is also subdivided into light, medium and heavy.
- Sandy loam. Here the presence of clay does not exceed 10%. This type of soil is almost never used in construction, due to the high probability of its movement under the action of groundwater.
Depending on the origin and place of occurrence, glacial and alluvial clay is distinguished. Glacial soils are characterized by a high bearing capacity compared to other soils of this type. If the layer is large and deep, it can be used under the base. As the depth decreases, the plasticity of the clay increases.
The alluvial type of clay soil prevails in the lowlands, near water bodies. Due to its high ductility, its use in construction is very limited. Such soil can float, causing cracks in the building.
Choosing a foundation on clay
When choosing a foundation for a house, it is necessary to determine the soils that lie on the construction site, as well as several very important parameters:
- Soil water content.
- The percentage of clay particles.
- Ground water level.
- The depth at which the soil freezes.
To find out the water level in the soil on the site, you can look into the well or do engineering and geological surveys. In each region, the depth of freezing of the earth is different, therefore, to determine this parameter, it is necessary to use the regulatory literature and select it from the tables given in it.
On the same clay soil, you can build different types of foundation. The main criterion for choosing a foundation is the material of the walls, the number of storeys of the building, the presence of a basement. For buildings with a heavy load (wall material brick and concrete) on any type of soil (sandy loam, loam, clay) and the presence of a basement, a deep strip foundation is used.
Base types
When the water level in the soil is below the freezing height of the soil, a monolithic strip foundation with an enlarged sole is used. The bearing area becomes larger, which allows you to evenly distribute the pressure. It is necessary to divert water from the tape bases by laying a sand cushion and drainage pipes.
Another option is a shallow strip foundation. This type of foundation is best used for light wooden houses. The soil should contain a minimum amount of clay particles and the groundwater level should be below the base. This type of foundation is tied into a rigid structure to distribute the impact of the soil along the entire length of the tape. With this option, there are large bending forces, so two reinforcing belts are arranged. The shallow strip foundation is a general floating system that is immune to minor disturbances from the soil. Watch the video on how to build a strip base on clay.
With a lot of pressure from the future home, the presence of heaving soils and low groundwater, choose a slab foundation. In most cases, it is expensive, but also the most sustainable. This type of foundation is a floating slab on which a large house can be built. It is possible to make a floating slab in the form of a concrete strip.
A floating foundation is well suited for building on clay. The load from the house is evenly distributed, and when the earth is blurred or shifted, the entire slab will move completely. The floating slab is not deep, but always on a sand and gravel cushion.
If groundwater is located close to the surface of the earth, then it is better to use a pile foundation on clay. Piles are driven or screwed to the depth of hard rock. The support of piles on dense rocks ensures the stability of the foundation, and hence the building as a whole. It is worth remembering that if the choice fell on this type of foundation, then the owner of the house will have to build a house without a basement.
To fix the piles, they are tied with a bar or channel. The timber is used for wooden houses, the walls of which are made of logs or timber. If the walls are brick or foam concrete, then a channel is used. A special head is put on the piles, and the timber is tied with the help of holes in it.
Not everyone decides to choose a floating columnar foundation for a building on clay. It can only be used for wooden houses. Instead of a monolithic structure, FBS blocks or masonry from wall blocks are used. Due to the small supporting surface of the log, the lower crowns of the log house are made of timber, reduce the pitch of the pillars, preventing the wood from collapsing. For this configuration, both a floating slab and a concrete strip are used. Watch the video on how to build a concrete slab on clay.
The construction of a house from foam blocks has become widespread. It can be attributed to light materials, with minimal pressure on the soil compared to brick or concrete. For houses made of foam blocks, the foundation is used shallow tape and pile.
Causes of foundation deformations and measures to prevent them
When building a house on clay, you should be aware of their features. Depending on the type of clay, the content of sand particles reaches a minimum of 5%. Under the action of groundwater, sand can be washed out, which will entail subsidence, heaving and plasticity.
The most common causes of damage to building foundations due to clay are:
- House drawdown. When the sandy component is washed out by groundwater, cavities appear, which leads to subsidence of the house. This is typical with a high location of groundwater and high pressure on the base.
- Heaving. It appears during the construction of the foundation above the freezing level of the soil, which contains loams or water-saturated clays. When freezing, water expands in size and acts on the base from below.
- Effects on the foundation from the side. The more it is in depth, the higher is the load. Clay, due to its plasticity, acts on the base from the side, which entails the possibility of cracks or splits.
To take into account the impact of the earth from below, it is necessary to lay a reinforcing cage throughout the volume, which connects the top and bottom of the base.
For minimal lateral impact, the sole of the base is made wider than the upper. For waterproofing, it is advisable to cover the side with machine oil or tie the base with a film. With a decrease in the base area from the side, the action of forces during soil heaving decreases.
The depth of soil freezing can be changed down by insulating the soil with special materials around the building.