Shallow foundation. Shallow strip foundation: construction device and technology
Looking at the project of a new house, the owner estimates how many cubes of concrete and tons of reinforcement will have to be laid in the foundation. Without this, there is no guarantee that the structure will not crack or squirm under the powerful pressure of the frozen soil.
Dig deeper won't let you down. It has been tested for centuries. However, there is a more economical way of building. It is called a shallow foundation and can be used for the construction of wooden log cabins and one-story frame-type buildings.
Obviously, shallow foundations save weight money and time. Unfortunately, it cannot be applied everywhere and not always. As we said, this a good option for low-rise buildings.
To lay small strip base it is possible only on condition that there is no heaving soil under it. Otherwise, the uneven rise of frozen soil or its subsidence in a waterlogged state will cause critical deformations of the concrete base of the house.
By its design, the shallow strip foundation is no different from the traditional one. The main difference is the depth of laying, which in this case does not exceed 50-60 cm. What is the main technological "highlight" of such a structure, which allows it to withstand seasonal ground fluctuations?
The answer is very simple. Performing the device of a shallow strip foundation, a thick layer of sand is laid under the concrete. It serves as a kind of buffer between natural soil and reinforced concrete. Small vibrations of the soil are absorbed by the sand cushion.
Once in the sand, water quickly goes into the underlying layers. Even if she freezes, then nothing terrible will happen. The sand cushion will evenly rise under the foundation, and after the ice melts, it will drop again. There will be no damage to the building from such movements.
As you know, an ordinary strip foundation is laid deeply (1.7-2.5 meters) not at all due to the insufficient strength of reinforced concrete. This depth is needed so that the sole is below the freezing mark. In this case, the forces of frost heaving do not put pressure on the sole from below and the house stands securely.
With this in mind, we do not advise anyone to joke with heaving soils. Build a shallow foundation only in areas where the groundwater is low and the soil does not have a lot of clay, silty sand, peat or loess loam in its structure.
The benefit from such a decision is obvious, since the estimate for the construction of a shallow foundation is 2-3 times less than the standard one. The complexity and construction time will decrease accordingly.
Construction technology
A shallow strip foundation is being built with your own hands in several stages. Let's consider each of them in detail.
1. Having marked the contour of the foundation on the site along the cord, you need to dig a trench... Its width should be 20-30 cm more than the width of the foundation. This is also necessary for laying a wide sand cushion that compensates for seasonal ground fluctuations.
2. Sand cushion 30-40 cm thick is poured... Pay special attention to the density of the sand. To do this, it must be moistened with water in layers and carefully tamped.
3. Prepare boards from OSB boards or edged boards, they are placed in a trench and reinforced with wedges and spacers made of wooden blocks. The spacing of the wedges and spacers must be at least 60 cm so that the raw concrete does not burst the formwork frame.
If you will make shields from a board, be sure to sheathe them from the inside with glassine, which will prevent the cement milk from leaving the concrete into the ground. As the formwork is being installed, you need to check its horizontal and vertical position. When knocking down the shields with nails, be sure to bend the exiting ends. This will facilitate further disassembly of the formwork.
4. Reinforcement of the foundation is carried out with reinforcement with a diameter of 12 to 18 mm... It is laid along the "tape" in two to four rows. Two reinforcing belts are put: lower and upper. They are tied into a single spatial structure with thick wire or welding.
Remember to leave a gap between the bottom chord of the reinforcement and the foot of the foundation. It should be between 40-50 mm. To do this, place plastic pipe cuttings under the rods. The upper belt should also not reach the level of concrete pouring by 3-5 cm so that the metal does not corrode.
Before the installation of the formwork, it is necessary to decide whether the shallow foundation will be insulated. If yes, then use it wisely and instead of sheathing the formwork with glassine, fix 4-5 cm thick foam sheets on it. They will insulate the structure and cut off the way for the solution to "escape" from the formwork.
5. Concreting also has its own subtleties.... Concrete should not be laid randomly, but in a continuous horizontal layer. If you fill it in short and high sections, vertical seams will form, which will weaken the structure. Thorough compaction of concrete by vibration or bayonet has also not been canceled (every 20 cm of the layer).
Do not rush to remove the formwork, even if you think the concrete is hard enough. It should take at least 5-6 days until it is disassembled.
6. After removing the formwork, coat the top and sides of the concrete or cover them with roofing material. The sinuses in the trench must be covered with sand, tamped down, then a layer of roofing material should be laid on the top and a concrete blind area made over it (width 80-100 cm, thickness 10-12 cm).
If all the steps are performed accurately and correctly, then you will only have to calculate the money saved and calmly proceed with the construction of the walls.
In low-rise construction of wooden or brick houses, as well as buildings made of aerated concrete and foam concrete, a shallow strip foundation (MZLF) is most often used. Such a base combines the advantages of buried and non-buried structures, but its cost is much lower.
To determine the main differences between the strip shallow foundation and the methods of its independent installation, we will consider its features, advantages and disadvantages.
Features of MZLF
For a standard buried foundation, it is necessary to dig a pit, the depth of which will be below the level of soil freezing. If you live in a region characterized by severe winters, then the depth of such a trench can reach 1.5 m, so you will have to use heavy construction equipment during construction.
If we consider the device of a shallow strip foundation, then it is a base laid along the perimeter of all load-bearing walls of the house. Moreover, its depth rarely exceeds 50 cm.
Among other advantages of structures of this type, it is worth highlighting:
- The possibility of erecting a strip foundation for a house from aerated concrete, foam concrete and other light materials, without using an excavator and mixer.
- Smaller volume building material and reduced labor costs.
- High structural strength.
- A wide range of materials and methods for its installation. For example, you can equip a belt monolithic foundation, pouring the base with concrete or making a more complex brick structure.
Also, thanks to the MZLF, you can easily insulate the basement rooms.
Of the disadvantages of such a light foundation, it is worth noting the fact that the shallow strip foundation on heaving soils will have insufficient strength. The fact is that during the season change, such soil rises and falls, which, in turn, can lead to deformation of the shallow tape. However, this problem is solved by using a drainage layer and sand filling, which will significantly reduce the loads from the soil.
Important! MZLF should not be laid on frozen ground or left unloaded in winter. Pouring the base should be carried out in a short time, before the onset of frost.
For more information about MZLF, see the video:
In order to make a shallow strip foundation with your own hands, you must first assess the type of soil prevailing in your area.
Assessment of soil composition and type
In order to determine the type of soil, it is not necessary to order an expensive service for the study of the geology of the site, it is enough to use the "old-fashioned" method. To do this, dig holes at several points, take some earth in your hand and try to roll it into a ball. If a:
- The ball has rolled up dense, and its consistency resembles plasticine, then this clay soil.
- After pressing the ball, cracks appeared on it - loam.
- The ball crumbled, it is sandy loam.
- You can't roll the ball at all, then there is sand in front of you.
For each individual breed, it is necessary to calculate the required resistance in kg / cm 2. This is necessary to determine the bearing capacity of the foundation, which, according to SP 22.13330, will be the following values \u200b\u200bfor:
- loam - from 1.8 to 2.8;
- sandy loam - 2-3;
- watered clay - 1-2;
- plastic clay - 2-3;
- medium-density clay - 3-5;
- dense clay - 4-6;
- gravel with sand - 5;
- sand of different fractions - 3-5;
- wet sand - 2-3.
After assessing the composition of the soil, it is necessary to calculate the structure and volume of the building material.
We calculate MZLF
To make a shallow strip foundation with your own hands, you need to decide on the type of foundation and its dimensions. Let's say we plan to build a base with an area of \u200b\u200b15 m 2, with a width of 5.5 m and a length of 6.5 m, with 4 horizontal rows of reinforcement and 2 vertical ones.
Based on this, we get the following rough drawing, the future fundamental foundation.
For such a building you will need:
- 402 m of reinforcement (120 m for horizontal rows, 192 m for vertical rows and 90 m for connecting rods);
- 2.02 "cubes" of timber for formwork;
- 153 bags of cement (50 kg each);
- 19,100 kg of sand;
- 27 550 kg of crushed stone.
This is an example of a standard MZLF construction with a height of 1,600 mm. The depth is calculated according to the following parameters:
- 0.4 m, if the base is protected from heaving soils;
- 0.45 m for sandy soil with low level groundwater;
- 0.5 m if clay soil predominates with a freezing level of 1 m;
- 0.75 if the freezing mark reaches 1.5 m;
- 1 m for soils with a freezing depth of 2.5 m.
Also, look for other types of foundations.
Types of shallow foundation structures
There are several types of MLF, presented in the table below.
Based on this classification, we will consider recommendations for choosing the type of a particular design:
- When building heated buildings with walls made of lightweight bricks or aerated concrete, it is recommended to orientate according to the following indicators:
- For heated frame houses, with wooden floors, it is recommended:
- For unheated log buildings:
After theoretical calculations, you can go directly to the construction of the foundation.
Construction of a strip shallow foundation
To build an unburied strip foundation with your own hands, you must perform the following stages of work:
Trench preparation
Before making a shallow strip foundation, remove the upper part of the soil from the territory allocated for the construction and mark the foundation. To do this, take wooden stakes and nylon thread and measure the corners. After that, check the distance between the walls with your project and dig a trench about 70 cm high and 30 cm wide.
Make sure that the walls of the trench remain vertical and the bottom level. The next step is to cover the trench with geotextile, which prevents mixing of the sand cushion and soil.
After that, coarse sand is poured onto the bottom of the trench (fine sand should not be used) or a mixture of sand and crushed stone, 10-15 cm high. Filling should be done in layers, with moistening and tamping of each subsequent layer. The final sand bed height depends on the type of soil. It is recommended to lay insulation (for example, roofing felt) on top of the prepared waterproofing layer, so you will insulate the shallow strip foundation.
Formwork
On top of the insulation layer, a formwork made of edged boards or OSB is installed. It is best to make a removable formwork, fastening its walls with ties in increments of 0.5 to 1 m.
On the back of the formwork, mark the concrete level.
Reinforcement
Place the reinforcement rods in the formwork, having previously cut the rods in accordance with the dimensions of the walls both in the cross-section and in the longitudinal section. In the corners and at the joints of the walls, perpendicular reinforcement rods must be tied up in order to increase the strength and durability of the reinforcement cage.
Helpful! If the foundation height is more than 30 cm, then you will need several layers of reinforcement.
For more information on reinforcement MZLF, see the video:
After pouring the first layer, it is necessary to punch holes in the concrete surface using a rod. This is necessary so that all the air leaves the solution.
Each next layer of cement-sand mixture must be poured before the previous one sets, so it is better to hire a team to work.
The final concrete layer must be leveled according to the markings applied to the formwork. After that, it is recommended to iron the concrete in order to increase the strength of the foundation and avoid surface cracking. To do this, sprinkle the concrete that has not yet set with dry cement (it is most convenient to use an ordinary sieve for this).
Final processing
After 3-5 days, when the base is set, and waterproof the foundation vertically. It can be done in several ways:
- Coat the surface with bitumen mastic in 2-3 layers.
- Cover the foundation with a roll of polymer-based or fiberglass-based material.
- Saturate the surface with the penetrating mixture.
Finally, the foundation will gain strength after 28 days, after which it will be possible to continue building the house.
Among all the options for the foundation, shallow is very popular.
It allows save on the earthworks and required materials, to provide a reliable basis for the future structure and, which can be especially attractive, to carry out all the work by yourself.
What you need to know and how to deal with this task, we will tell you.
What is a shallow foundation?
The name itself already quite accurately defines the features of this type of foundation: it buried into the ground, but finely, not deep. But seriously, on average, the depth of such a foundation is 50 cm, Oh really not deep, and it is quite possible by yourself dig a trench without involving outside labor and equipment.
It is a concrete reinforced the tape on which they are subsequently erected outdoor and internal load-bearing walls. The best conditions for the use of a shallow foundation - non-porous soils and low-rise a structure made of lightweight materials, which include:
- , frame-panel constructions;
- Cellular concrete - expanded clay blocks, foam concrete etc.;
- Lightweight brickwork.
Considering that in most cases such a foundation is located higher the level of soil freezing, it is necessary to take into account the soil and ensure styling appropriate pillow. You can read more about this in the dedicated technology devices such a pillow under the foundation.
Device
The shallow foundation is quite simple by design. On the sandy (sand and gravel) cushion is poured into the formwork of the required configuration and dimensions. It is necessary reinforce fittings.
Cured coating waterproof... The depth depends on the weight of the structure, the characteristics of the soil. Foundation height above ground should not to be more the height of the underground part of the foundation.
The possibility of using such a foundation can be limited the complexity of the relief (construction on a slope), as well as soil. Use on sapropel and peat soils of a shallow foundation practically ruled out.
How to calculate?
To calculate the parameters of a shallow foundation, you need to know several parameters of soil and future construction, including level the occurrence of groundwater, depth freezing of soil, calculated weight buildings, snow, estimated resistance soil, etc. In general terms, we will describe the calculation technology.
Initial data
For the initial data we will take cottage metering 10x8 m with walls of aerated concrete with block sizes 600x300x200mm and one internal load-bearing wall. The density of this material is approximately 500 kg / m³... A rough calculation, taking into account door and window openings, as well as one load-bearing inner wall, will show that the weight of the walls can be 15 t.
Similarly, you can calculate weight attic floor, gender, roofs, snowy load (for example, for the Moscow region it is 160 kg / m2). Here you need to add weight insulation materials, facing bricks (if it will be used), metal for doorways, etc.
And don't forget about useful load, which consists of people, furniture, equipment, dogs, cats, cages with budgerigars, etc. In order not to catch your Barsik for weighing, the value of the payload is taken equal to 180 kg / m2. This value is somewhat redundant, but allows one to take into account all possible indoor loads.
Calculation
Suppose the final house weight is 130 tons... Now let's determine the calculated pressure to the ground, for which we divide weight buildings to the area of \u200b\u200bthe foot of the foundation. We will assume that the planned foundation will be width 35 cm.Then the area of \u200b\u200bits base will be approximately 15.4 m²... We divide 130 by 15.4 and get that estimated specific ground pressure is 8.44 t / m².
Further, this value must be compare with calculated soil resistance. For example, for alumina this value is 10 t / m²... This means that the previously selected foundation size will provide that the soil will support the building.
Considering that width the foundation should be more wall width by about 10 cm, you may need to increase the width of the base of the house. This will entail an increase masses foundation and need to be done recalculation, but even a small increase in the width of the foundation will provide stock by load on the ground.
Construction technology
After everything is calculated, it's time to start working on stacking foundation. It can be divided into several stages.
Markup
According to plan is marked the territory for the future building, using pegs and the stretched rope is determined by the perimeter of the work.
Advice! It is advisable to remove the top fertile soil layer over the entire area.
Trench
Dug out trench the desired depth. It is necessary not to forget to take into account not only the dimensions of the foundation, but also the pillows under it. Width trenches should be more the width of the base of the house so that nothing interferes with the formwork.
After all the soil has been removed, it begins stacking pillows.
First recommended to lay geotextiles that will prevent mixing sand with soil. Sand, previously moistened, is laid in layers on 15-20 cm and thoroughly rammed using vibrating plates.
After completing the formation of the pillow, you can proceed to installation formwork.
How to make a waterproofing strip foundation with your own hands:
Formwork
Most often made from boards, but you can buy it at a building store, made metal or plastic... When installing the formwork necessary follow closely verticality walls and holding permanent the distance between them. Pegs, bars are used to strengthen, spacers are nailed on top, which will help later keep poured cement mortar.
Formwork walls can be pull off wire. The inner walls of the boards must be smooth... Steel, asbestos-cement or other pipes should be installed in advance for securing ventilation of the underground.
They are installed perpendicular formwork and covered with sand, which prevents concrete from entering them and deformation when pouring the foundation.
Reinforcement
After completion setting up the formwork before starting to pour concrete, necessary provide for it. In the case of the construction of light buildings, reinforcement can be neglect, in other cases, perform it extremely recommended.
Reinforcement with a diameter of 10-16 mmwhich is located longitudinally along the entire length of the foundation. Usually, two rods of reinforcement are laid in the lower and upper parts of the future base of the house, retreating from the edges by several, recommended 5 centimeters. Transversely stacked steel rods with a diameter 6-8 mm... They are connected to each other by wire.
For reinforcement enough arrange 2 belts of reinforcement, but for a high foundation height and heavy loads, the number of belts can be to enlarge.
Beyond the need use welding equipment, with this method of connection, due to heating, the structure of the metal and its mechanical characteristics change. Increased stiffness at the weld can become cause of the break during pouring with concrete, or may subsequently create internal stress in the foundation, preventing it from reacting to temperature fluctuations.
Pouring with concrete
Concrete brands are often used for the foundation. M200... The finished mixture is poured in layers approximately 15-20 cm.
Attention! Desirable each layer tamp using a vibrator or rammer, or at least a piece of reinforcement to stab surface to remove possible air pockets.
![](https://i0.wp.com/6sotok-dom.com/wp-content/uploads/2015/09/melkozaglublenniy_lentochniy4.jpg)
The next layer must be filled before solidification the previous one. After endings concreting, the foundation is needed shelter film and leave to freeze from time to time wetting water.
Pre-hardening will take place 3-4 days, then it will already be possible dismantle formwork. For further work, the concrete will be suitable through 2-3 weeks.
What is this blind area?
protects foundation from soil erosion by melt water, atmospheric precipitation, diverting unnecessary moisture from the base of the house. In addition, it can also serve track around the house.
Her device not difficult... It consists of two parts:
- Foundations... For this, a layer of sand is laid, or several layers of gravel and sand.
- Decorative coating... In the simplest case, this is a layer of concrete, but you can use a rock, pour asphalt, cover with gravel.
The slope of the blind area to the side of the house should be at least 1.5˚. Optimally, if the height difference between the edges is 8-10 mm.
Leave between the wall and the blind area expansion joint order width 2 cm... Roofing material, expanded polystyrene is laid in it, sand or other waterproofing material.
The width of the blind area should be not less the width of the eaves of the house, so that the water rolling down from the roof gets on her, not on the ground. In addition, one more point can affect the dimensions - insulation of the foundation.
Why do you need foundation insulation?
After the concrete of the foundation is completely frozen, it must be covered several layers of bituminous mastic and shelter waterproofing material, for example, roofing felt. Next step - . Reinforcement laid in concrete conducts cold well, and measures for insulation reduce this unpleasant effect.
In addition to actually improving the temperature indicators of the structure, there is one more important moment.
Heaving of soil
Given the shallow depth of the foundation, higher freezing limits, vertical and tangential forces of the frozen soil act on it. it compensated cushion under the foundation, and lateral forces can be compensated for by laying insulation under the blind area.
A good option would be to use extrusion expanded polystyrene, which is buried at a shallow depth under decorative covering the blind area. Knowing depth freezing of the soil (recall, for the Moscow region it is about 1.4 m), if the width of the installed insulation, and, in fact, the blind area, will be not less this value, then the soil at the point of contact with the foundation will not freeze.
IN video below is shown in detail device finely cut strip foundation:
When the depth of the base of the underground supporting structure is 0.2 - 0.7 m, it is called shallow foundation (MLF) and has a minimum construction budget. There are shallow slabs, pillars, tapes. Piles are always submerged below freezing so they fall into the recessed foundations category by default.
Varieties of MLF
The main problem of the supporting structures of buildings operating underground is frosty swelling of soils, which occurs when the following conditions are present:
- high percentage of clay
- an abundance of moisture to saturate these grains
- freezing of the soil, leading to an uneven increase in volumes in different areas
Several solutions were used to solve the problem:
![](https://i2.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/2-3-300x135.png)
![](https://i2.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/3-3-300x185.png)
To reduce labor intensity, delivery time of objects, construction budget, a set of measures was created to reduce or completely eliminate swelling in frost:
![](https://i1.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/4-3-300x225.png)
- insulation - blind areas and various foundation elements to preserve geothermal heat under all supporting structures and adjacent soils
Attention: If these measures are observed, it is possible to lay in the project a shallow foundation of a slab, strip or columnar type, which is indicated in the regulations of SP 22.13330 for the foundations of dwellings.
It is customary to call prefabricated and monolithic structures shallow, the sole of which lies at the level of 0.2 - 0.7 m. Therefore, on soils with a low clay content, drainage and insulation may not be required at all.
MZLF tape
A shallow strip foundation is economically justified for particularly heavy wall materials. The restrictions for MZLF are:
- soils with low design resistance (silt, peat, silty sand)
- high ground level (less than 1 m from the sole of the tape)
- height difference of more than 1.5 m (between opposite walls of the building)
There are precast tapes from one row of FBS blocks and an armored belt on top of them, monolithic structures with maximum performance by default. The manufacturing technology of monolithic MZLF is as follows:
![](https://i1.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/5-3-300x155.png)
Attention: On the inner surface of the boards, polyethylene should be fixed to prevent leakage of cement laitance, which facilitates stripping. It is better to insulate MZLF from the outside after waterproofing, since only in this case a continuous film layer is created.
Floating plate
In 90% of cottage projects, a shallow slab foundation is used. The technology is considered the most expensive, but it provides the maximum load-bearing capacity of the underground structure.
Despite the high strength of a concrete slab with two layers of reinforcing mesh, heavy loads from frost swelling arise on heaving soils. Therefore, modernized floating slab options are popular with individual developers:
![](https://i1.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/6-300x201.png)
Attention: The minimum depth of laying slab foundations is regulated by 40 cm. This top layer is rich in humus, therefore it is forbidden to support supporting structures on it. Organic matter decomposes without oxygen for 12 - 36 months, after which soil subsidence is inevitable.
Columnar MZF
Shallow columnar foundation it is allowed to equip exclusively on dry soils with a groundwater level below 1.5 m from the sole. Erection on slopes is prohibited, heaving soils... In addition, the pillars have low resistance to overturning, so they must be tied with a grillage:
![](https://i2.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/7-1-300x218.png)
Attention: For columnar foundations with a deepening above the freezing mark, a set of measures is required to exclude swelling - annular or wall drainage, underlying layer + backfilling with crushed stone / sand of the pits sinuses.
Construction technology
Shallow foundation is built much faster than deep. The construction budget is being reduced, but it is not desirable to preserve it in the winter without load.
Marking and excavation
Depending on the type shallow foundation is marked in different ways:
- the floating slab is contoured with one cord along the outer perimeter of the formwork
- for poles and strips of MZLF, two strings are pulled on the rags for the inner / outer formwork shield
The pit comes off only for the slab, the trench is more rational for the tape, pillars with a rare location pitch. If the prefabricated loads from the weight of the building are small, separate pits are torn off for the posts.
Attention: In any case, access to the surfaces of concrete structures is required for waterproofing. Therefore, earthen formwork technology is incorrect.
Drainage and underlying layer
A sand / gravel foundation cushion replaces the layer of fertile soil removed at the previous stage. The thickness of the underlying layer is 40 - 80 cm, depending on the geology of the site, the magnitude of the prefabricated loads from the dwelling.
Drainage is built into this layer along the outer perimeter of the blind area at the level of the basement sole. To do this, vertical wells are attached at the corners of the cottage, between them are perforated in the upper part of the pipe (180 - 270 degrees).
Attention: Non-metallic material is laid in 10 - 15 cm layers, each of which is rammed with a vibrating plate. It is not recommended to spill sand with water, as this can lead to the erosion of the lower layers of the soil, the formation of a top water already during the construction process.
The width of the foundation pad made of inert materials should be twice as large as the size of the tape, the post. Under floating slabs, the underlying layer is released outward by 40 - 60 cm.
Concrete with waterproofing
Lean concrete screed 5 - 10 cm thick is required only for monolithic structures. If a shallow tape, a columnar foundation is constructed from precast reinforced concrete elements, no concrete is used. The screed allows to ensure tight joints of the waterproofing material, to exclude the absorption of cement milk by the non-metallic material of the underlying layer.
Attention: The width of the foundation is equal to the size of the underlying layer, that is, twice the tape, the cross-section of the column.
As a waterproofing, a polyethylene film (minimum 15 microns) is usually used, glued to a footing, or roll materials (TechnoNIKOL, hydroglass), fused to a concrete surface. Edges are released to be later fired onto vertical surfaces of concrete structures.
Formwork and reinforcement
Due to the variety of shallow foundations structures, several formwork modifications are used:
- floating plate - boards around the outer perimeter or polystyrene L-shaped blocks
- mZLF tape - outer + inner deck
- pillars - a square of shields for the widening plate and vertical shields for the pillar
For each specified option, different reinforcement schemes are used:
![](https://i1.wp.com/fundamentdomov.ru/wp-content/uploads/2016/06/11-300x200.png)
Attention: The reinforcement is located inside the concrete structures, observing the protective layer. The rods are recessed by 2 - 7 cm, the bottom row is laid on the footing using plastic spacers.
Concreting and waterproofing
The concrete should be laid in layers of 40 - 60 cm with an interval of 2 hours. Therefore, it is advisable to concrete any type of shallow foundations in one go Waterproofing of accessible concrete surfaces is carried out by penetrating, coating, pasting materials after 70% of strength gain.
Attention: individual elements inside one foundation (porch, tape on which the veranda will rest) are separated by a technological seam. Different operational loads will not lead to the destruction of walls, foundations, if the roof of the extension is independent of the roof of the main dwelling.
Thus, shallow foundations are not difficult for an individual developer. However, it is necessary to take into account the recommendations given to ensure the maximum possible resource of the building with a minimum construction budget.
One of the simplest foundations for buildings is considered to be a shallow strip foundation. Despite the ease in the production of work, there is always the possibility of doing something wrong, therefore, before setting up the MLF, you should familiarize yourself with the general technology. Today we will take a step-by-step look at the device of a shallow tape.
Scope of MZLF
Shallow foundations are used for buildings with a low loaded mass. As an example, you can cite houses on a steel or wooden frame, as well as buildings made of lightweight elements (foam concrete, PCB). Usually, the number of storeys of buildings on the MZLF does not exceed two.
In the definition of MZLF, shallow - means completely located in the frozen soil layer, the underground part of the concrete tape rarely exceeds 500-700 mm. The forces of frost heaving with this arrangement do not create tangential (breaking) loads, but the entire building, together with the foundation, dynamically moves following the expansion of the soil. For this and other reasons, it is not recommended to arrange a shallow tape in areas with a general relief slope of more than 2%. On steeper slopes, a shallow foundation can be built only after replanning the soil to form a horizontal terrace.
The expediency of using MZLF lies in a much lower consumption of materials and ease of design. If the building is not planned ground floor, shallow tape will reduce the volume concrete mix and reinforcement by 2-3 times, while providing an equivalent bearing capacity.
It is worth noting, however, that it will not be possible to establish a shallow foundation on loose peat, silty soils and mud sandy loam. Such soils have too low density and high plasticity, therefore, they require the construction of pile-grillage bases, based on denser soil layers. You should not arrange the MZLF on soils with heaving indicators of more than 4% or if the GWL is above the burial depth, despite the fact that drainage of the site is not planned so that later you do not have to deal with the consequences.
Section and configuration calculation
Since the MZLF acts like a beam, not a stiffener, usually the section of the tape is close to a rectangle or trapezoid in shape. Tape in the form of a tee or more sophisticated sections is almost never cast due to the fact that the material savings seem too small compared to installing more complex formwork.
The calculation of MZLF is carried out in two directions: sufficient bearing capacity of the soil in the plane of occurrence and its own structural strength, which will allow the tape to maintain its rigidity at full design load from walls, roofs, snow, etc.
The width of the upper part of the foundation is determined by the maximum possible wall thickness, taking into account the layer of interior and facade finishing. When constructing floors on logs, it may be necessary to form a ledge or expand the foundation of the order of 50 mm.
The width of the tape in the plane of bedding is determined entirely by the required bearing capacity. It is enough to divide the total mass of the building and calculate the average load for each meter of the belt perimeter, and then calculate the sufficient cross-sectional area of \u200b\u200bthe support in accordance with the characteristics of the soil. To create a sufficiently high safety factor, the thickness of the incompressible bed is not taken into account.
The scheme of a shallow strip foundation: 1 - parent soil; 2 - backfilling with gravel or gravel-sand mixture; 3 - blind area; 4 - foundation reinforcement; 5 - shallow strip foundation with a wide base for even distribution of the load; 6 - wall; 7 - gravel bedding of the inner area of \u200b\u200bthe foundation of the house
The height of the tape is defined as a component of its underground and aboveground parts. Everything is simple with the above-ground part - it must be at least 80 mm and no more than four times the width of the upper edge of the tape. In turn, the height of the underground part can be determined taking into account several factors:
- the foundation should not lie on the border of dissimilar soils;
- the minimum depth of the foundation is 35-40 cm, but, depending on the intensity of heaving and the depth of freezing, the height of the underground part may increase by an additional 60-80%;
- to maintain the required strength characteristics of reinforced concrete products, the ratio of width to height must be no less than 3: 5.
Excavation and preparation
The profile of the trench for the MZLF device should have a width 2.5 times greater than the calculated width of the tape and a depth greater than the height of the underground part by two widths. This is due to the fact that MZLF is rarely arranged on soil formwork, using a board-board formwork for reasons of containing cement milk and the need to give the section a trapezoidal shape. Immediately, we note that the indent of the walls of the pit from the formwork should be twice as large from the outside as from the inside.
Compensation of frost heaving forces is carried out due to an incompressible, non-heaving and hygroscopic bedding, as well as filling the lateral sinuses with a similar material. A sand-gravel mixture with coarse sand and granite or basalt crushed stone of fraction 25-30 is used as a filling material. To stabilize the foundation, the prepared trench bottom is covered with a 30-50 mm preparatory layer of M 100 concrete without reinforcement.
Backfill at the bottom of the trench helps to distribute the load on the support soil layer, increase the bearing area and attract forces with a horizontal vector of application. The recommendation regarding the thickness of the backfill, equal to two values \u200b\u200bof the thickness of the tape, is rarely followed in practice, more often on slightly weeping soils they are limited to preparation of 25-30 cm.
However, you must remember that the more pronounced the heaving, the more responsibility is placed on the bedding. Sometimes it is advisable to replace the soil up to the freezing depth and expand the external sinuses to the shape of an inverted wedge, the base of which corresponds to the width of the blind area.
Reinforcement and anchoring
For MZLF general content steel reinforcement without prestressing is not less than 0.1%, a more real indicator of 0.17-0.2% will provide proper reinforcement without excessive strength, but with a significant safety margin.
The minimum value of the protective layer for the underground part of the foundation is 60 mm, the maximum is no more than half the width of the tape. Working reinforcement is made with bars with a periodic profile of such a diameter that the total section of the reinforcement can be divided into 4 bars for the upper and lower reinforcement lines.
If in MZLF the vertical distance between the lines of reinforcement exceeds 450 mm, add another row with rods, the thickness of which is at least 60% of the thickness of the main lines.
Structural reinforcement is made with clamps or wire for tying in increments of 2-2.5 times the average width of the foundation. The diameter of the rods used for the manufacture of structural reinforcement should not be less than 50% of the diameter of the working reinforcement.
In addition, the reinforcement of the MZLF is accompanied by a number of anchors. At turns and T-shaped joints of the tape, each row of reinforcement in intersecting directions must be connected with bent embeds of the same section, the overlap of which with the main reinforcement is determined as 25 nominal reinforcement diameters. Anchoring with embedded studs may be required to bond to the base of the frame or wall masonry.
Concrete works
Before carrying out concrete work, it is recommended to cover the inner cavity of the panel formwork with a polyethylene film, which prevents the loss of liquid by the concrete mass until it sets. After that, the reinforcement segments are installed, their alignment and distance are made using plastic plugs.
Most often, bituminous mastics are used to protect against moisture, over which insulation based on fiberglass or a cheaper roofing material is rolled. If the foundation does not require continuous waterproofing, the remaining hydro-barrier film after pouring is sufficient.
The sinuses around the foundation are filled with ASG immediately after the waterproofing has dried. Backfilling is carried out in layers of 30-40 cm with careful compaction. After that, it only remains to make a blind area around the house, and the MZLF will be ready for further long-term operation.
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