Foundation of the house - solid foundation. It absorbs loads from the building structure and redistributes them to the ground. Acts as a kind of “sole” of any constructed object. The magnitude of the load is the main criterion for choosing the size of the base and its type. Strip foundations are the most popular in private housing construction. A seemingly simple basis, in fact, requires serious calculations using the standards laid down in SNiP and state standards.
Table of contents:
Tape type: advantages
The base in the form of a tape is a structure constructed from a continuous concrete belt, reinforced with reinforcing elements. It runs under all load-bearing walls, is necessarily fixed in the corners, and represents a closed contour along the entire perimeter of the base. Located in the ground, it repeats the exact plan of the building and the dimensions of the walls that will be mounted on the foundation.
Attention! A shallow strip foundation is erected in order to prevent first deformation and then destruction of the erected building as a result of changes in the standing of the soil, as well as its heterogeneous structure.
Among the main points in favor of the shallow type of foundation are:
- efficiency, minimal labor costs: the tape is laid only in those places where the maximum load is placed on the ground;
- service life 25 years (average): whether it is a lot or a little is a relative concept;
- ideal for light one-story construction (country houses, Finnish houses, frame buildings, etc.), the most structural wall materials are wood, foam concrete;
- minimum construction time.
There are also restrictions:
- It is necessary to carry out extremely accurate calculations in accordance with SNiP 2.02.01-83 and comply with the requirements for non-heaving soils. Take into account the resistance coefficient to the expected load. Design standards are also used for the construction of the building itself, taking into account wind, snow loads, and the weight of the foundation itself. So, with a belt weight of more than 12 tf/m.p. It is not advisable to build a strip, because Its width turns out to be large; it is better to make the base piled or in the form of a slab.
- Strip-type foundations are calculated using special reference books, which take into account tolerances and coefficients, which only a specialist can do. The task can be facilitated by calculation using a special program that can be found on the Internet.
- It is forbidden to erect buildings with a load per 1 linear meter of more than 5 tons.
- There are restrictions on the type of soil: it is prohibited to build a shallow foundation on clay and loamy soils. When the clay is heavily moistened, it becomes wet and its resistance to stress weakens. For the same reason, construction on deep black soil is problematic. The foundation is not being built on heaving soils from fine sand.
- In regions where there is a significant temperature difference (within 100 degrees), the risk of sole destruction increases. So, in the northern regions, where in summer the outside air temperature can reach +30 degrees or more. Partial defrosting of the soil occurs. The cold weather that follows freezes the thawed soil and increases the freezing layer. Cavities form in the soil into which the foundation can fall.
Laying depth: an important criterion for choosing the type of foundation
The depth of the trench is determined by calculation - you can’t make a mistake in this matter
The depth of the foundation in individual construction depends on a number of factors: the type of soil, the climate of the building area, the type and design features of the building itself (including weight, volume), as well as its purpose. The minimum depth level is 0.5 m (this is when the soil freezes to 2 m). But the most accurate indicators should be taken from SNiP. 2.02.01-83. The type of burial must be specified at the design stage of the house. It should be taken into account that the top layer (fertile) soil, usually about 1 meter thick, is not suitable for arranging the foundation for a house. The soil underneath is much more attractive. It is also important to know the heaving depth (freezing depth) of the soil in a particular region.
Reference. For the Moscow region and the capital itself, the average is in the range of 1.40 - 1.70 m. It must be taken into account when determining the laying depth. Arrangement of the base above this value is fraught with the risk of pushing it out of the soil when it becomes icy. In the case of non-heaving soils, when the soil freezes to 3 m, the foundation must be buried at least 0.75 m.
Algorithm for constructing a foundation strip for construction
After carrying out calculations that meet the requirements of SNiP, you can begin to build a shallow strip foundation with your own hands.
- At the construction site, the fertile soil layer is removed to the depth of the trench and 1.5 m on both sides of its boundaries.
- Trees, bushes and stumps are uprooted. Other vegetation is removed.
- Construction and other debris and wood remains are removed.
- The resulting potholes are filled up. The top layer of soil is leveled.
- Strictly observing the development plan, the dimensions of the trenches are marked.
- Using a level, pegs are placed at the extreme significant points, and the corners are necessarily covered. The twine is pulled.
- A trench is dug manually or mechanically. Be sure to check the distance between its parallel walls. Levels if necessary.
- The bottom of the resulting pit under the tape is covered with coarse sand. A “cushion” is formed, which should resist freezing of the trench filling.
- Waterproofing is being carried out. To do this, you can use a liquid concrete mixture. When hardened, it forms a hard coating that prevents the penetration of cement laitance from the base concrete composition into the soil.
- The formwork is installed using the chosen method (removable, non-removable).
- With a removable structure, the boards knocked into panels are strengthened from the inside with spacers, clamps, and from the outside - with supports. In this case, the nails are driven in from the inside without bending from the outside. This must be done so that the mixture being laid does not deform the walls under its weight and destroy the formwork.
- A frame is formed from reinforcement either in the trench itself or near it, and then in finished form inserted into the formwork.
- The concrete solution is gradually laid into the reinforced structure in layers. After each filling, it is pierced with a reinforcing rod (to remove trapped air and eliminate cavities).
- The mixture is compacted using a tamper or vibrator.
- The formed foundation is covered plastic film(roofing felt, roofing felt) to prevent the top layer of concrete from drying out and harden it evenly. At high air temperatures, the foundation under the shelter is periodically wetted.
- After 2-3 weeks, when the base has hardened, the formwork is dismantled. During this time, the foundation gains strength by only 70%.
- A layer of sand is first poured into the formed “pockets” between the trench wall and the base, moistened, and compacted.
- Then the soil is backfilled to the entire remaining depth. The pockets are completely removable.
- Walls and other façade elements are being erected. At the same time, under the weight of the structure, the reinforced concrete strip will strengthen for another half a month or even a month.
Accurate calculations, preparation of high-quality consumables and the help of friends will allow you to build a shallow shallow foundation on your own.
A shallow strip foundation (hereinafter referred to as MZLF) is one of the types of strip foundations, which is characterized by a shallow depth, significantly less than the depth of soil freezing, and a relatively small consumption of concrete mixture. This article discusses the main advantages and disadvantages of MZLF, the most common mistakes during their construction, a simplified calculation method suitable for private developers (not professionals), recommendations for constructing a foundation with your own hands.
The main advantages of MZLF are:
- economical - concrete consumption is significantly lower than during the construction of a conventional strip foundation. It is this factor that most often determines the choice of this technology in low-rise construction;
- reduced labor costs - less excavation work, less volume of prepared concrete (this is especially important when it is not possible to pour the finished mixture from a mixer);
- smaller tangential forces of frost heaving due to the reduced area of the lateral surface of the foundation.
However, during the construction of the MZLF, it is necessary to strictly adhere to the technology; a frivolous attitude to the process can lead to the appearance of cracks, and then all of the above advantages, as they say, will go down the drain.
The most common mistakes made when installing MZLF:
1) selection of the main working dimensions of the foundation without any (even the most simplified) calculation at all;
2) pouring the foundation directly into the ground without covering it with non-heaving material (sand). According to Fig. 1 (on the right) we can say that in winter time years, the soil will freeze to the concrete and, rising, drag the tape upward, i.e. the tangential forces of frost heaving will act on the foundation. This is especially dangerous if the MZLF is not insulated and a high-quality blind area is not equipped;
3) improper reinforcement of the foundation - choosing the diameter of the reinforcement and the number of rods at your discretion;
4) Leaving the MZLF unloaded for the winter - it is recommended to carry out the entire cycle of work (construction of the foundation, erection of walls, and arrangement of the blind area) one construction season before the onset of severe frosts.
Calculation of a shallow strip foundation.
The calculation of the MZLF, like any other foundation, is based, firstly, on the value of the load from the weight of the house itself and, secondly, on the calculated soil resistance. Those. the soil must withstand the weight of the house transmitted to it through the foundation. Please note that it is the soil that supports the weight of the house, and not the foundation, as some believe.
If an ordinary private developer can calculate the weight of a house if desired (for example, using our online calculator located), then it is not possible to determine the calculated soil resistance on your site on your own. This characteristic is calculated by specialized organizations in specialized laboratories after conducting geological and geodetic surveys. Everyone knows that this procedure is not free. Mostly, architects who design a house resort to it, and then, based on the data received, they calculate the foundation.
In this regard, it makes no sense to provide formulas for calculating the size of the MZLF within the framework of this article. We will consider the case when a developer carries out construction on his own, when he does not conduct geological and geodetic surveys and cannot accurately know the calculated soil resistance on his site. In such a situation, the dimensions and design of the MZLF can be selected according to the tables below.
The characteristics of the foundation are determined depending on the material of the walls and ceilings of the house and its number of storeys, as well as on the degree of heaving of the soil. How you can determine the latter is described
I. MZLF on medium and highly heaving soils.
Table 1: Heated buildings with walls made of lightweight brickwork or from aerated concrete (foam concrete) and with reinforced concrete floors.
Notes:
— the number in brackets indicates the pillow material: 1 — medium-sized sand, 2 — coarse sand, 3 — a mixture of sand (40%) with crushed stone (60%);
— this table can also be used for houses with wooden floors, the safety margin will be even greater;
— see below for foundation design options and reinforcement options.
Table 2: Heated buildings with insulated walls wood panels(frame houses), logs and beams with wooden floors.
Notes:
— the numbers in brackets mean the same as in Table 1;
- above the line value for walls made of insulated wooden panels, below the line - for log and timber walls.
Table 3: Non-buried foundations of unheated log and timber buildings with wooden floors.
Notes:
- above the value line for log walls, below the line - for walls made of timber.
Design options for MZLF on medium- and highly heaving soils, indicated in the tables by letters, are shown in the figures below:
1 — monolithic reinforced concrete foundation; 2 - sand filling of sinuses; 3 — sand (sand-crushed stone) pillow; 4 — reinforcement frame; 5 - blind area; 6 7 — waterproofing; 8 - base; 9 — ground surface; 10 - sand bedding; 11 - turf.
Option a. — upper plane The foundation coincides with the surface of the earth, the base is made of brick.
Option b.- the foundation protrudes above the surface by 20-30 cm, forming a low base or being part of the base.
Option c.- the foundation rises 50-70 cm above the ground, while it also serves as a base.
Option d.- non-buried foundation-basement; Table 3 shows that such foundations are used for unheated wooden buildings.
Option d.- used instead of options b. or V. when the width of the base of the foundation significantly exceeds the thickness of the wall (more than 15-20 cm).
Option e.— a shallow strip foundation on a sand backfill is used quite rarely on weak (peaty, silty) soils at a high level groundwater for wooden buildings. Depending on the size of the building, bedding is done either under each strip or under the entire foundation at once.
Reinforcement of a shallow strip foundation.
MZLF reinforcement is made with meshes of working reinforcement and auxiliary reinforcing wire. The working reinforcement is located in the lower and upper parts of the foundation, and it must be immersed in the thickness of the concrete by about 5 cm. The lower mesh works to deflect the foundation tape downward, and the upper mesh works to deflect the tape upward. There is no point in placing the working reinforcement in the middle of the tape (as can sometimes be seen on the Internet).
Table 4: Options for foundation reinforcement.
MZFL reinforcement schemes are shown in the following figure:
A.— a mesh with two working reinforcement rods; b.— a mesh with three working reinforcement rods; V.— T-shaped joint; G.— L-shaped corner joint; d.— additional MZLF reinforcement with a large sole width, when the sole is more than 60 cm wider than the base (the additional mesh is located only in the lower part.
1 — working fittings (A-III); 2 — auxiliary reinforcing wire ∅ 4-5 mm (Вр-I); 3 — vertical reinforcement rods ∅ 10 mm (A-III), connecting the upper and lower mesh; 4 — reinforcement for strengthening the corner ∅ 10 mm (A-III); 5 — connection with wire strands (twisting length is at least 30 diameters of the working reinforcement); 6 — additional working fittings ∅ 10 mm (A-III).
II. MZLF on non-heaving and slightly heaving soils.
Shallow strip foundations on non-heaving and slightly heaving soils do not have to be made only from monolithic concrete. Other local materials can be used, e.g. rubble stone, red ceramic brick. MZLF is laid at 0.3-0.4 meters without a sand cushion. Moreover, for wooden buildings and one-story brick (or aerated concrete) foundations, they don’t even need to be reinforced.
For 2- and 3-story houses with walls made of stone materials, MZLF is reinforced. Concrete foundations are reinforced according to the 1st reinforcement option (see Table 4 above). Foundations made of rubble or brick are reinforced with masonry mesh made from BP-I reinforcement ∅ 4-5 mm with a cell size of 100x100 mm. The nets are placed every 15-20 cm.
MZLF structures on non-heaving and slightly heaving soils are shown in the figure below:
1 - foundation; 2 - base; 3 - blind area; 4 — waterproofing; 5 — subfloor (shown conditionally); 6 - mesh made of wire reinforcement, 7 — reinforcement according to option 1 (see table 4)
Options a. and b.- for wooden and one-story brick (aerated concrete) buildings.
Options c. and Mr.— for two- and three-story brick (aerated concrete) buildings.
The width of the sole b is determined depending on the number of storeys of the building and the material of the walls and ceilings.
Table 5: Values of the width of the sole of the MZLF on non-heaving and slightly heaving soils.
Stages of construction of a shallow strip foundation and recommendations.
1) Before starting construction of the foundation, if necessary, it is necessary to ensure high-quality drainage of surface rainwater from neighboring areas from the building site. This is done by cutting out drainage ditches.
2) The foundation is marked and trenches are torn out. It is recommended to begin excavation work only after all necessary materials have been delivered to the construction site. It is advisable to organize the process of cutting out the trench, filling the tape, backfilling the sinuses and constructing the blind area as a continuous process. The less it is extended in time, the better.
3) The dug trenches are covered with geotextiles. This is done so that the sand cushion and sand filling of the sinuses do not become silted over time by the surrounding soil. At the same time, geotextiles allow water to pass through freely and do not allow plant roots to grow.
4) A sand (sand-crushed stone) cushion is poured layer by layer (in layers of 10-15 cm) with careful compaction. They use either manual rammers or area vibrators. Tamping should not be taken lightly. Shallow foundations are not as powerful as foundations poured to the full depth of freezing, and therefore freezing here is fraught with the appearance of cracks.
5) The formwork is laid out and the reinforcement frame is knitted. Do not forget to immediately provide water and sewerage to the house. If the foundation is also a plinth, remember about the vents (does not apply to buildings with floors on the ground).
6) Concrete is poured. Filling of the entire tape must be done continuously, as they say, in one go.
7) After the concrete has set (3-5 days in summer), the formwork is removed and made vertical.
8) Produced backfilling sinuses with coarse sand with layer-by-layer compaction.
9) A blind area is being constructed. It is advisable (especially with a small height of the foundation tape) to make the blind area insulated. This measure will further reduce the forces of frost heaving affecting the MZLF in winter. Insulation is made with extruded polystyrene foam.
As already mentioned at the beginning of the article, it is not allowed to leave the MZLF unloaded or underloaded (the building is not fully built) for the winter. If this happens, the foundation itself and the soil around it must be covered with any heat-saving material. You can use sawdust, slag, expanded clay, straw, etc. There is also no need to clear the snow on the construction site.
It is highly not recommended to build a shallow strip foundation in frozen soil in the winter.
In the comments to this article, you can discuss with readers your experience in the construction and operation of MZLF or ask questions that interest you.
The beginning of the construction of any building is laying the foundation. When choosing the type of foundation, various factors should be taken into account, ranging from soil characteristics to design features main building. Recently, shallow strip foundations have become popular, this is explained by its versatility, low costs construction works and the ability to erect a structure without the use of heavy construction equipment.
WHEN TO USE A SHOW FOUNDATION
Such a foundation can be safely erected for light suburban buildings of small number of floors. These can be houses made of wood and its varieties, from gas silicate and foam concrete blocks, frame-panel structures. It is possible to use a shallow foundation under brick houses provided that the walls have a slight thickness.
The height of buildings erected on shallow foundations should not exceed three floors.
Where is a shallow foundation used? In addition, a shallowly buried foundation is suitable for construction on soils prone to heaving, since a small depth protects the structure from the effects of heaving soil.
FEATURES OF CONSTRUCTION OF A SHALLOW FOUNDATION
When choosing a shallow foundation for a house, you should take into account some features of its construction:
The nuances of a shallow foundation Such a base requires mandatory protection from precipitation. Therefore, it is very important to provide for the organization of drainage and drainage system. This will move more water away from the foundation line.
One of the ways to protect a shallow foundation from precipitation and surface water is a blind area. Most professional builders insist on its arrangement.
At self-production For such a foundation, it is best to involve a large number of people to work on it to ensure quick preparation of concrete and its one-time pouring.
During the construction of a shallow foundation, only high-quality concrete should be used.
The main feature of this type of foundation is the need to complete the main construction before the onset of frost, since such a foundation cannot be left for the winter.
WHAT SHOULD BE TAKEN INTO ATTENTION WHEN CONSTRUCTION OF MZLF
A very important factor is the type of soil on the site. The fact is that on soils of organic origin and on clay, the construction of a shallow strip foundation is prohibited.
The groundwater level is no less important. Their very close location can cause rapid destruction of a shallow concrete strip.
The terrain determines the degree of difficulty in constructing a shallow foundation. If there is a strong difference in height, it is better to build a different type of foundation.
RULES FOR PERFORMING CALCULATIONS OF LOAD ON A SHALLOWLY BURNED BAND FOUNDATION
When calculating the load on a shallow foundation, it is important to take into account all parameters. The total load on the foundation of the house can be constant or variable. In the first case we are talking about the following:
Construction of the building and its features.
The number of floors of the building and its total height.
The material that is planned to be used for the construction of walls and partitions of the building.
Load from roofing structure, including the weight of the roofing material.
In addition, seasonal loads from precipitation should be taken into account, especially in winter. It is in winter that the snow mass on the roof of the house exerts great pressure.
Variable loads include the number of residents, the weight of furniture and similar factors.
MZLF CONSTRUCTION TECHNOLOGY
The shallowly buried concrete strip of the base is quite simple to construct, so all work can be done independently. In addition, this process will require several assistants, but you can do without heavy construction equipment.
SET OF TOOLS FOR WORK
The construction of a shallow shallow foundation requires a set of the simplest tools:
Tool sets Shovel, you can prepare a shovel and a bayonet shovel.
Hammer and sledgehammer.
Electric drill, grinder with discs, screwdriver.
Roulette and building level.
Regular rope or construction cord.
TRENCH
When determining the width of the trench, the width of the wall and the thickness of the formwork structure are taken into account. The foundation monolith itself should be flush with the wall, but taking into account the formwork, the width of the trench should be slightly larger. If construction is carried out on loose or heaving soil, then this trench parameter should be slightly increased.
Before you start digging a trench, you need to mark the area using wooden pegs and construction cord or rope. Pegs are driven into the corners of the future foundation, and the cord is pulled between them around the perimeter. In this case, both the internal and external perimeter should be noted.
If construction does not require precise calculations of a shallowly buried concrete strip, then the depth of laying is determined by the location of the layer of hard clay. In this case, the bottom of the trench should be lowered 25-30 cm below the upper level of the clay layer.
Digging a trench For general information, it should be said that the soil most often has the following structure:
The top 10 cm is a layer of fertile soil.
The next 10-20 cm is occupied by a mixture of sand and clay.
Below this layer is hard clay, which is quite difficult to dig.
The bottom of the trench should be deepened by about 30 cm in order to build a foundation and a cushion for it. Thus, the total depth of the shallow foundation is about 0.7 meters. This parameter fully meets the requirements for the construction of frame, panel and other light-weight buildings.
CREATING A PILLOW
In order for seasonal ground movements and other changes in the soil to be evenly distributed over the entire area of the foundation base, it is necessary to create a kind of cushion of sand and crushed stone. The sand layer should be about 5 cm thick. The sand is poured into the trench, leveled, watered and thoroughly compacted. Crushed stone is poured in a layer of the same height, leveled well, compacted and filled with liquid cement mortar. As a result of the actions performed, the base of the foundation is obtained.
REINFORCEMENT
We reinforce MZLF The strength and reliability of any foundation is ensured by a frame made of reinforcing bars. It is not recommended to neglect the creation of a reinforcing belt, since it takes on the main load. For the same reason, you should not skimp on fittings or use low-quality elements.
To create a reinforcing belt, you need to take reinforcement bars with a cross-section of 10-12 mm and tie them together, creating two levels of two bars each. The width and height of the reinforcing frame are determined by the parameters of the foundation; the reinforcement must retreat from each edge of the foundation by 5 cm. The main rods of the frame must be connected to each other with vertical and transverse rods. For knitting it is best to use a special soft wire. If it is necessary to connect rods along the length, it is important to overlap the ends of the reinforcement by 10-15 cm.
FORMWORK CONSTRUCTION
The formwork can be erected before assembling the reinforcing belt, but for convenience this stage is postponed a little. The fact is that in a finished formwork structure it is much more difficult to knit the frame.
The main purpose of the formwork is to form smooth walls of a concrete strip, so for its manufacture it is necessary to use panels made of boards or plywood. The inside of the boards should be as smooth as possible, so the boards are well processed. In order for the structure to hold the heavy concrete mass inside, it is recommended to make supports and lintels. Supports are installed on the outside of the structure at a distance of 50-60 cm, lintels are secured along the upper edge of the panels so that the formwork does not move apart under the pressure of concrete.
POURING CONCRETE
A high-quality foundation can only be achieved by using high-quality concrete. Therefore, a factory-prepared solution is considered the ideal option. Most experienced craftsmen advise using this option whenever possible. However, with a small construction budget, savings are required at every stage, including the preparation of concrete. Great solution A mobile concrete mixer can become a problem; it can be rented, which will significantly save money.
The quality of concrete also depends on the components used and their quantity, therefore, maintaining proportions and quality ingredients will help to produce a durable and solid foundation for a house or other building.
It is recommended to pour concrete in one step, and the thickness of one layer should not exceed 20-25 cm. After pouring one portion, it is necessary to compact the concrete mass; for this purpose, use a vibrator or an ordinary metal rod. In the latter case, the concrete is carefully pierced in several places, removing air bubbles. Compaction of the concrete solution helps to increase the strength of the entire base.
After pouring the concrete mass, the surface of the foundation is well leveled and left for 28-30 days so that the concrete hardens and gains the necessary strength. During this time, the foundation is protected from debris and precipitation by covering it with plastic film. In addition, on hot days the surface is constantly sprayed with water to prevent the concrete from cracking. Maybe,
INSULATION AND WATERPROOFING
To avoid freezing of a shallow foundation, do not neglect the insulation and waterproofing of the base. In this case, the protection should cover both vertical and horizontal walls.
You can make a shallow strip foundation with your own hands. In this case, it is necessary to take into account the main factors that influence the quality of the foundation, take into account the features of constructing a foundation of this type and follow the technology of its construction.
Strip foundation is the most common type of foundation for building a house. Therefore, everything is clear with it, but what is the difference between a shallow foundation (MZLF) and a shallow foundation, how to calculate it and how to do it?
Characteristics - design principle
A shallow strip foundation or simply MZLF is similar in installation method to its counterpart, but has important differences:
- foundation laying depth up to 700 mm;
- located above the soil freezing zone;
- Designed for installation on swollen (heaving) soils.
The main feature of a shallow strip foundation is that it makes it possible to level out frost heaving of the soil. This is due to the fact that, despite the general rigidity of the structure, the MZLF, together with the weight of the entire structure, moves up and down depending on the time of year. Since the foundation is not deeply deepened, but moves evenly, it therefore does not collapse from such vibrations.
Schematic diagram of a shallow strip foundation
- Sand and gravel cushion
- Foundation tape
- Waterproofing layer
- Vertical (or coating) waterproofing
- Fittings (diameter 12)
- Fittings (diameter 8)
- Base
- Wall
Where can it be used - application
Shallow strip foundations are suitable for the construction of low-rise residential buildings and other structures made from materials that will not create significant pressure on the base of the foundation. Such structures and materials include:
- log cabins;
- cellular concrete - foam concrete, gas silicate blocks;
- lightweight bricklaying;
- frame-panel buildings.
When arranging a foundation of greater width, heavy houses made of timber or logs can be erected on it. But in this case, the soil will freeze to a shallower depth and there is a possibility of deformation of the foundation. Thus, if you plan to build a monumental building, it is better to equip a strip monolithic foundation.
At the same time, you should know that increasing the width of the shallow foundation strip makes it possible to build heavier houses with an attic. The large width of the foundation strip (and, accordingly, the base) helps to reduce the depth of soil freezing in the space under the floor.
What to consider when installing a shallow foundation
Shallow foundations are installed on heaving soils
It is prohibited to pour MZLF on biogenic organic soils, for example, peat, sapropel (freshwater deposits), as well as on clay. The photo shows that they are already appearance doesn't bode well.
- Level groundwater
The closer the water is to the surface of the earth, the more unstable the MZLF will be
If the terrain is characterized by a significant difference in height (a house on a slope), then installing a shallow strip foundation on it is quite problematic. In this case, a regular strip foundation is installed or a significant area under the MZLF is leveled. In terms of time and money, both options are equivalent.
- Laying depth
It represents the height from the lowest point of the foundation, the so-called sole, to the zero level (ground surface).
- Climate (depth of soil freezing)
It is quite common among builders to lay a shallow strip foundation at a height that is calculated using the formula - freezing depth minus 20%. This way you can be sure that the foundation will rise along with the building.
The minimum depth of a shallow strip foundation is regulated by SNiP II-B.1-62.
The depth of soil freezing for some Russian cities is given in the table.
How to calculate the load on a shallow shallow foundation
First of all, you should consider:
- design features of the structure;
- building height;
- planned number of floors;
- materials from which the walls will be built;
- coating weight;
Advice.
In general, the entire load can be divided into constant (calculated before the start of construction) and variable. The latter depends on the number of inhabitants, the weight of the furniture, etc.
- trench depth;
- pillow thickness;
- foundation strip parameters;
- quality of concrete.
You can additionally use
Calculation of a shallow strip foundation
1. The depth is determined by the proximity of groundwater and the depth of freezing.
2. Height above the ground surface = 4x width.
Good to know. The height above the ground is less than or equal to the depth.
3. The width is determined by the formula:
Where, D is the width of the foundation base;
q – design load on the foundation, t/m;
R – design soil resistance, t/m2. This indicator, for a laying depth of 300 mm, is given in the table.
4. The thickness of the cushion is determined from the conditions of the strength of the soil of the area.
For highly heaving soils, the following formula is used:
Where, tn is the thickness of the pillow;
A, C, W – coefficients;
A and C are determined from the tables below.
A W = 0.1 or 0.06 m2/t for heated and unheated structures.
Above the line - for MZLF with a laying depth of 300 mm, below the line - for non-buried foundations.
Advice.
Calculate the MZLF using both formulas and give preference to the larger value.
Cost of a shallow strip foundation
Varies between 4-6 thousand rubles. per linear meter. The price depends on the width, height, number of lintels and dimensions, for example, the cost of erecting the foundation of a 6x6 house will cost 70-80,000 rubles, and 10x10 = 120-150,000 rubles.
Types of shallow strip foundation (shallow foundation)
Depending on the type of MZLF, the technology of its design will differ. Therefore, you should briefly familiarize yourself with the main ones:
Strip monolithic shallow foundation
Poured directly onto construction site, resulting in a seamless tape
Strip block shallow foundation
The blocks are purchased ready-made or manufactured separately and only assembled at the construction site. Cement mortar is used as a fastening material.
Each of them has its own advantages and disadvantages. But, in general, the service life of the second is three times less than the first. Therefore, in this article we will dwell in detail on such a type as monolithic MZLF.
MZLF technology or how to make a shallow strip foundation with your own hands
The instructions are easier to understand if all the work is divided into clear steps. Let's not deviate from this scheme. So, the structure of a shallow shallow foundation is as follows:
Preparatory stage
1. The place where it is planned to pour the foundation is cleared of all unnecessary things.
Advice. Uproot trees that are nearby along with their roots.
2. On workplace deliver all required material and tool. Moreover, it is advisable to immediately determine the location of all this stuff, so as not to waste time searching for it later.
Marking for strip foundation
The importance of this stage is difficult to overestimate. Therefore, you need to not only “estimate” where you will dig a trench, but also make landmarks using a rope (if possible, a laser level would be better).
How to make markings for the foundation:
- take measurements around the perimeter;
- put beacons in the corners;
- check the diagonal between the corners;
- if necessary, move the beacons;
- make a blind area at a distance of at least a meter from the beacons;
- Attach a rope to the blind area boards, which will show the edges of the foundation.
The marking process is presented more clearly in the diagram.
How to dig a trench for a foundation
A foundation trench is a strip of excavated soil.
The depth of the trench is determined by the depth of the shallow strip foundation and cushion.
For example, the most common depth is 300 mm; the thickness of the cushion is about 200 mm, depending on the quality of the soil. Then the depth of the trench will be 500 mm.
Advice.
To prevent the edges of the trench from crumbling, make small slopes.
Considering the type of soil, it is better to start pouring the foundation immediately. Otherwise, it will crumble and some of the work will have to be repeated.
The sand cushion for the foundation is a mixture of sand and gravel. You can mix them, but it’s easier to pour them in layers. Moisten each layer with water and compact well. Since the structure of the pillow is quite porous, it is separated from the main foundation with a waterproofing film.
In principle, natural soil can serve as the basis for a shallow strip foundation, but its bearing capacity is much lower than the proposed sand and gravel cushion.
Install the entire formwork structure vertically. The supports are attached in increments of 500-600 mm. They are needed to ensure that the formwork does not fall apart or warp under the weight of the concrete.
Helpful advice.
Try to keep the boards adjacent to each other tightly and without sudden changes. Then you will avoid work on leveling the surface finished foundation under the cladding.
Pouring strip foundation
Obtaining concrete is an individual matter - you can buy (or rather, order) and have the concrete delivered with a mixer or make it yourself, using (and the second option, how to make it).
As for pouring concrete, the procedure is standard for such work - concrete is poured into the finished formwork.
Advice. If the formwork is watered, the concrete will lie more evenly around the edges.
When pouring concrete, try to beat it every 40-50 mm. height. This will “expel” air bubbles that could destroy the concrete in the future.
Reinforcement of a shallow strip foundation
If the foundation is not expected to bear a heavy load, this step can be skipped. But still, with MZLF reinforcement it will be much stronger.
Correct reinforcement of a shallow strip foundation:
- Fill the starting layer. This layer should be about 30% of the total height of the MZLF. The purpose of this action is to create a flat surface for laying the metal, as well as protect it from moisture.
- Knitting reinforcement for strip foundations. To do this, you need to tie it into blocks and lay it in a trench.
- Pour concrete to the desired height.
An example of a reinforcement cage for a strip foundation
An example of tying reinforcement with wire. The example shows that the connection is made with wire. This is a relatively new approach to joining, since resistance welding is more common.
But methods of connecting reinforcement by welding have several significant disadvantages, including:
- require a welding machine and a specialist who can perform welding work;
- due to the high temperature of the welding arc, the strength of the hardened reinforcement rod is lost;
- welding work is not applicable for fittings with a diameter of more than 20 mm;
- The most important thing is to increase the rigidity of the reinforced frame. Since a shallow strip foundation “plays” depending on the season (being a kind of flexible foundation), increased rigidity will lead to its destruction. And as a result, it will be required.
This is what the laid reinforcement will look like
Concrete protection
To avoid overdrying and the formation of cracks, the concrete poured into the formwork is covered with a film and periodically moistened with water.
Insulation of a shallow strip foundation
- Immediately after the foundation has gained the required strength, construction of the house should begin. Try to do all the work in one season. Because the soil under the foundation will freeze, and the MZLF, not pressed down by the weight of the house, will become deformed.
- If you cannot start construction, cover the MZLF and the area around it (200-300 mm) with straw or sawdust. To protect the soil from freezing.
- During the operation of the building, try to plant it near a shallow strip foundation perennials and shrubs. They will help reduce the freezing depth of the soil. They will also be able to retain a significant amount of snow, which will also reduce the freezing depth.
Shallow strip foundation video
Conclusion
In this article, we figured out how to make a shallow strip foundation with your own hands. By understanding each stage in detail, you will be able to prevent annoying mistakes and lay the foundation on which your house will last for decades. Most importantly, do not forget that before starting construction, order geological surveys, otherwise, there is a risk that the soil on your site is not suitable for installing an MZLF.
Shallow strip foundation (MSLF) is a type of strip foundation, which is located 0.3-0.7 m below the zero level. The construction of this type of foundation requires a minimum of financial and labor costs. MZLF is excellent for light buildings erected on various soils. The foundation has its advantages and disadvantages, as well as a special construction technology.
Shallow strip foundation: scope of application, pros and cons
The bearing capacity of MZLF, in comparison with foundations of other types, is assessed as average and largely depends on the type of soil on the site. A shallow foundation is suitable for the construction of country houses, timber buildings, frame houses, as well as outbuildings, bathhouses, barns, etc. For brick houses, large cottages made of foam concrete and aerated concrete blocks, a shallow foundation is not suitable. For such buildings, an excellent option would be a combined foundation on piles, for example, a shallow pile-strip type on bored supports.
It is advisable to build a foundation of this type on slightly heaving and non-heaving soils. The ideal option is sandy and sandy loam soils with low moisture content. The groundwater level must be at least 0.5 m below the laying depth. On clay, the construction of an MZLF is accompanied by difficulties, because Most clayey soils are classified as medium- and high-heaving soils. We will consider the features of constructing a foundation with a shallow laying depth on heaving soils below.
The advantages of this type of base include
- economical, concrete consumption is 30% lower than when constructing a conventional buried strip foundation or foundation with monolithic slab floors;
- ease of construction, you can build it yourself without the involvement of workers and special equipment;
- small amount of excavation work - a narrow trench with a depth of no more than 0.7 m is dug;
- small area of contact between the structure and the ground.
Among the disadvantages of MZLF they note
- pouring is carried out at stable temperatures above +10 C;
- limited use due to low load-bearing capacity
- construction is possible only on a flat surface with a slope of no more than 5 degrees;
- lack of a basement in the house.
Expert opinion
Dmitry Bogdanov
Ask a question to an expertIf you are planning to build an MZLF foundation and “freeze” the construction, then keep in mind that you cannot leave the foundation unloaded during the winter! When the soil freezes, it will push out the monolithic tape, which will lead to a violation of its integrity.
Features of the design of a shallow foundation
A shallow monolithic strip foundation is erected on a flat surface. When building a house on a slope, you will have to combine MZLF with pile foundation, using supports to level out the height difference. In terms of strength and resistance to deformation, the structure must comply with SNiP 2.03.01-84.
In cross-section, a shallow foundation looks like this:
Features of MZLF that must be taken into account during calculation and construction:
- The depth of the foundation base depends on the depth of soil freezing.
- Be sure to install a cushion made of dry bulk materials: a mixture of coarse sand and gravel.
- When the groundwater level is high, drainage is performed under and around the foundation.
- The base on which the monolithic tape is installed is compacted as much as possible.
- It is necessary to arrange a blind area to drain rainwater and snow.
Taking these features into account, we can conclude that the backfill and blind area are an integral part of the shallow foundation. Requirements for the blind area are specified in SNiP 2.02.01–83.
Calculation of a shallow strip foundation
Calculating the MZLF, which is being built on low- and non-heaving soils, is not difficult. During the calculation, three main parameters are determined:
Laying depth
Determined on the basis of SN “Foundations and Foundations”. The document states the following minimum values depth of foundation base:
- when the soil freezes less than 2 m - 50 cm;
- when the soil freezes to a depth of 3 m - 75 cm;
- when the soil freezes more than 3 m - 100 cm.
For most regions middle zone the laying depth of the MZLF will be 50 cm. For light buildings, for example a frame barn or a small country house this parameter can be reduced to 30 cm.
Width of monolithic tape
In order not to make complex calculations, we recommend taking the width of the sole based on the table:
| Wall and ceiling materials | Number of floors | MZLF sole width, m |
| Walls made of lightweight brickwork or aerated concrete with reinforced concrete floors | 1 | 0,6 |
| 2 | 0,8 | |
| 3 | 1,2 | |
| Timber frame walls with timber ceilings | 1 | 0,4 |
| 2 | 0,4 | |
| 3 | 0,6 | |
| Log walls with wooden ceilings | 1 | 0,3 |
| 2 | 0,4 | |
| 3 | 0,6 | |
| Walls made of timber with wooden floors | 1 | 0,2 |
| 2 | 0,3 | |
| 3 | 0,4 |
Height above ground level
The higher the monolithic strip rises above ground level, the better the floors of the house will be protected from dampness and cold. However, the height of the foundation to maintain stability and load-bearing capacity must correlate with its width. The best option: the height of the tape above the zero mark is equal to its width.
Expert opinion
Dmitry Bogdanov
Professional builder. 18 years of experience
Ask a question to an expertExample: The laying depth is 50 cm. The width of the monolithic tape according to the table is 30 cm. This means that the height above ground level will be 30 cm, and the height of the entire monolithic tape will be 80 cm.
Expert opinion
Dmitry Bogdanov
Professional builder. 18 years of experience
Ask a question to an expertThe height of the ground part of the MZLF should not be lower than the snow level. The snow depth value depends on the region (you can find it on the Internet). For regions of the middle zone, this value does not exceed 8-10 cm.
Calculation of MZLF on heaving soils
When building a house on heaving soils, more complex calculations are performed, the purpose of which is to determine the heaving deformation. It is quite difficult to make such a calculation yourself, so you need to entrust it to professional designers or use a ready-made table:
| Name and degree of soil heaving | Number of floors of the building | Width of foundation base b, m | Thickness of the pillow t, m | Foundation design option | Reinforcement option |
| clays, loams and sandy loams, fine and silty sands, moist – medium sandy | 1 | 0,3 / 0,2 | 0,6/0,7 | G. | 3 |
| 2 | 0,3 / 0,2 | 0,5 / 0,6 | G. | 3 | |
| 3 | 0,3 / 0,2 | 0,4 / 0,5 | G. | 3 | |
| clays, loams and sandy loams, fine sands and dusty moist – highly heaving | 1 | 0,3 / 0,2 | 0,7 / 0,8 | G. | 4 |
| 2 | 0,3 / 0,2 | 0,6 / 0,7 | G. | 4 | |
| 3 | 0,3 / 0,2 | 0,5 / 0,6 | G. | 4 |
In column 2 “Sole width” and column 3 “Cushion thickness”, the values for heated and unheated rooms are indicated through the / sign. The column “Reinforcement option” indicates the minimum number of reinforcing bars that must be used to reinforce the monolithic strip.
Construction of a shallow foundation: construction technology
The technology for constructing MZLF is not complicated; filling can be carried out according to SNiPs 3.03.01-87, 2.02.01-83 or according to our instructions. Reason for frame house 10 x 10 m can be poured in 1-2 days. Before you start pouring, you need to decide where you will take the concrete solution. There are two options:
- 1order concrete class B22.5...B17.5 at the nearest RBU. In this case, the mixture will be delivered to you at the specified time by a concrete mixer truck. If the ABS cannot reach the pouring site, then a special hose is used through which the concrete solution will be supplied to the formwork. Ordering a sleeve will slightly increase the cost of ABS services. In addition, you will have to pay for every hour of downtime of the special vehicle.
- 2 prepare concrete yourself. At the same time, you will not depend on RBU and will spend significantly less money, however, the quality of the concrete mixture will be slightly lower. When mixing concrete, the recipe must be strictly followed. To prevent concrete from starting to set ahead of time, you can use special additives. You can begin preparing concrete immediately after installing the formwork.
Installation of MZLF: step-by-step instructions from A to Z
Preparatory work and marking
The installation of a shallow foundation begins with site preparation, which consists of removing debris and uprooting stumps. The top layer of soil with vegetation is cut off. If necessary, leveling and filling of soil is carried out, followed by compaction.
The marking is carried out as follows: the perimeter of the future foundation is established, beacon pegs are driven into the corners. Ropes are stretched along the future monolithic tape.
Expert opinion
Dmitry Bogdanov
Professional builder. 18 years of experience
Ask a question to an expertThe diagonal between the corners must be strictly 45 degrees. You can check the evenness of the markings using a tape measure and a building level.
Construction of trenches and cushions
The depth of the trench depends on the depth of the foundation and the thickness of the cushion. The width of the trench is made 10 cm wider than the calculated thickness of the monolithic tape. This is necessary for installing the formwork. When constructing MZLF on loose soils, the trench slopes can be strengthened with boards. Pillowing is necessary to reduce the impact of frost heaving forces on the foundation. The thickness of the cushion, as a rule, is 20-30 cm for slightly heaving and non-heaving soils. For heaving ones - determined according to the table given above.
To correctly backfill means making 2 layers: the first is coarse sand, which is poured, and then moistened and compacted, the second is fine crushed stone or gravel. Blast furnace or boiler slag can also be used as a second layer. If the groundwater level is high, it is recommended to put a layer of waterproofing under the cushion: roofing felt or geotextile, and fill it on top.
Installation of formwork and reinforcement
The formwork is made from planed boards, the thickness of which is no more than 3 cm. The formwork is mounted in a trench, and its height is 5-10 cm greater than the height of the monolithic tape. A reinforced frame is installed into the finished formwork. Reinforcement of the monolithic tape is carried out in accordance with SNiP 52-01-2003 and SP 52-101-2003. The reinforced frame can contain from 3 to 6 longitudinal reinforcement bars connected to each other by vertical jumpers.
The reinforcement scheme is determined based on the width of the foundation strip. The reinforced frame is assembled from corrugated reinforcement with a cross section of 12 mm, class A3. For a small structure, you can use A500C class reinforcement, the cost of which is much cheaper. The frames are interconnected with an overlap, forming a reinforcement unit. The length of the overlap should not exceed 20 diameters of the rods (20 x 12 = 24 cm). The connections are secured by welding.
For corners where the walls of the future house intersect, it is necessary to strengthen the reinforcement frame by installing additional vertical rods, which are welded to the longitudinal rods. This strengthening allows strip foundation withstand critical loads, operating at the intersection of walls.
You can lay the reinforced frame on a prepared cushion, but it is better to do it on the starting layer of concrete. The thickness of the starting fill should not exceed 20% of the entire height of the tape. Concrete pouring helps create a more even surface on which the reinforced frame is laid. If you decide not to fill the starting layer, then support fungi should be used to raise the reinforced frame above the surface of the pillow by 5-7 cm.
Expert opinion
Dmitry Bogdanov
Professional builder. 18 years of experience
Ask a question to an expertThe reinforced frame should be located in space, nowhere in contact with the bedding and formwork.
Pouring concrete
Filling should be done at a temperature of +10 C and above. Before pouring, the formwork must be moistened, then the concrete will lie more evenly. The mixture must be poured in layers, the layer thickness should not exceed 40 cm, optimally 20-30 cm. Each layer is subjected to 5-10 minutes of vibration compaction. This technology does not allow voids to form inside the concrete. To supply the concrete mixture to the formwork, you must use an elastic sleeve or chute.
Layer-by-layer foundation pouring
After pouring is completed, the formwork is covered with a vapor-proof film. Hardening of the concrete lasts 25-30 days, after which the formwork is dismantled and the gaps between the foundation and the trenches are filled with soil.
Insulation of a shallow foundation
Professional builders recommend be sure to insulate the MZLF. It is recommended to do this immediately at the time of construction of the structure. An insulated foundation will protect the floors of the house from dampness and cold, which is especially important when arranging the floor “on the ground”. In this case, in the absence of thermal insulation, all the heat from the floor will go into the ground.
There are external and internal thermal insulation MZLF. External - when the insulation is attached to the outside of the monolithic tape, internal - from the inside. External insulation is considered mandatory, but internal insulation is usually done if the house has a basement. What insulation should I use? There are a lot of options. The most popular are:
- Penoplex. Dense thermal insulation, excellent heat conservation. Its service life is significantly longer than that of conventional foam. Penoplex is resistant to rodents, mold and practically does not absorb moisture. In terms of price/quality ratio it is best material for thermal insulation of shallow foundations.
- Expanded polystyrene. The extruded material is slightly inferior to penoplex in terms of physical and mechanical properties. However, the price of polystyrene foam boards is 20-30% cheaper. It can be used in dry and dusty soils with minimal moisture.
- Polyurethane foam. Liquid spray thermal insulation is an expensive option, but it has many advantages: no joints between slabs, a service life of at least 50 years, minimal water absorption and resistance to aggressive environments.
Features of construction of MZLF on heaving soils
Heaving soils do not allow moisture to pass through, preventing it from going deeper, so precipitation accumulates near the surface of the earth. When erecting a building made of blocks or wood on heaving soils, drainage must be done, and the backfill must be at least 30 cm. When building houses, a set of protective measures is taken for the foundation:
- The backfill is made from non-heaving soil. Waterproofing must be installed under the sand and gravel cushion. It is better to use geotextiles, which prevent siltation well.
- Drainage is installed at the level of the sole of the monolithic tape. Drainage pipes are laid at a distance of at least 1 m around the foundation. The slope of the pipes depends on their diameter: the smaller the pipes, the greater the slope should be.
- Installation of insulated blind area. The blind area helps drain water from the MZLF. A layer of insulation under the scaffold prevents freezing of heaving soils around the house.
- Storm sewer installation. The main purpose of a storm drain is to effectively remove precipitation from the site. For a shallow foundation, a well-executed storm drain will avoid flooding and subsequent freezing of heaving soils.
If these conditions are met, a shallow foundation built on heaving soils can last for many years without problems. To avoid mistakes during construction, it is recommended to familiarize yourself with SP 45.13330.2012, which describes all measures to protect MZLF on difficult soils.
Prefabricated MZLF on sandy soils
Most sandy soils are classified as slightly or non-heaving. In areas with such soils, shallow prefabricated foundations can be erected from ready-made reinforced concrete blocks. Installing ready-made blocks is much easier, and their cost differs minimally from the cost of concrete mixture and formwork. For prefabricated MZLF, the following types of blocks are used:
- FBS – rectangular solid structures;
- FL – trapezoidal pillow blocks with increased load-bearing capacity.
FL blocks are excellent for frame houses and outbuildings made of aerated concrete, and FBS are considered universal and have a wide range of applications. The sizes of finished reinforced concrete products are used small so that you can install them yourself without the involvement of a construction crane. The technology for installing a prefabricated shallow foundation is described in detail in SNiP 3.03.01-87. Just like a monolithic tape, the blocks are mounted on a prepared cushion. It is necessary to deepen the FBS in accordance with the calculations performed. To strengthen the base, the blocks are connected to each other with reinforcing mesh at the intersection of the walls.