Among all the options for foundation construction, shallow is especially popular.
It allows save money on excavation work and required materials, provide a reliable foundation for the future building and, what may be especially attractive, complete all the work on one's own.
What you need to know and how to deal with it task, we'll 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, the average depth of such a foundation is 50 cm what's really shallow, and quite doable on one's own dig a trench without involving outside labor and equipment.
It is a concrete reinforced tape on which they are subsequently built external And internal load-bearing walls. The best conditions for using a shallow foundation – non-heaving soils and low-rise a structure made of lightweight materials, which include:
- , frame-panel structures;
- Cellular concrete – expanded clay blocks, foam concrete etc.;
- Lightweight brickwork.
Considering that in most cases such a foundation is located higher level of soil freezing, it is necessary to take into account the soil and ensure styling matching pillow. You can read more about this in the technology section. devices such a pillow for the foundation.
Device
The shallow foundation is quite simple by design. On sandy(sand-gravel) cushion, concrete is poured into the formwork of the desired configuration and size. It's necessary strengthen fittings.
Cured coating waterproofed. The depth depends on the weight of the structure and the characteristics of the soil. Height of the foundation above the ground surface shouldn't be more height of the underground part of the foundation.
The possibility of using such a foundation can be limited the complexity of the terrain (construction on a slope), as well as the soil. Use on sapropelic and peat soils of shallow foundations are practically excluded.
How to calculate?
To calculate the parameters of a shallow foundation you need to know some soil parameters and future construction, including level groundwater flow, depth soil freezing, calculated weight buildings, snow, calculated resistance soil, etc. In general terms we will describe the calculation technology.
Initial data
We will take a one-story house by measurement as the initial data 10x8 m with walls made of aerated concrete with block sizes 600x300x200mm and one internal load-bearing wall. The density of this material is approximately 500 kg/m³. Estimated calculation, taking into account door and window openings, as well as one load-bearing structure interior wall, will show that the weight of the walls can be 15 t.
Similarly, you can calculate weight attic floor, gender, roofs, snowy loads(for example, for the Moscow region it is 160 kg/m2). You need to add here 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 budgies, etc. In order not to catch your Barsik for the purpose of weighing, the payload value is taken to be 180 kg/m2. This value is somewhat redundant, but allows you to take into account All possible indoor loads.
Calculation
Let's assume that the final weight of the house is 130 tons. Now let's determine the calculated pressure to the ground, for which we divide weight buildings to the area of the base of the foundation. Let us assume that the planned foundation will be width 35 cm. Then the area of its base will be approximately 15.4 m². Divide 130 by 15.4 and get that calculated specific pressure on the ground is 8.44 t/m².
Next, this value is needed compare with the calculated soil resistance. For example, for alumina this value is equal to 10 t/m². This means that the previously selected foundation size will provide that the soil will support the building.
Considering that width there must be a foundation more the width of the wall is approximately 10 cm, you may need to increase the width of the base of the house. This will lead to an increase masses foundation and will need to be done recalculation, but even a slight increase in the width of the foundation will provide stock according to the load on the ground.
Construction technology
Once everything is calculated, it’s time to start working on styling foundation. It can be divided into several stages.
Marking
According to plan marked territory for the future building, with the help pegs and a stretched rope determine the perimeter of the work.
Advice! It is advisable to remove the top fertile layer of soil over the entire area.
Trench
Is being dug up trench required depth. It is necessary not to forget to take into account not only the size of the foundation, but also the pillows under it. Width there must be a trench more the width of the base of the house so that nothing interferes with making the formwork.
After all the soil has been removed, it begins styling pillows.
At first recommended lay geotextiles, which will prevent mixing sand with soil. Sand, pre-moistened, is laid in layers along 15-20 cm and carefully compacted using vibrating plates.
After completing the formation of the pillow, you can proceed to installation formwork.
How to waterproof a strip foundation with your own hands:
Formwork
Most often produced from boards, but you can buy it at a hardware store, made made of metal or plastic. When installing formwork necessary watch closely verticality walls and holding permanent distances between them. To strengthen, pegs and bars are used, spacers are nailed on top, which will help later hold poured cement mortar.
The 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 provision underground ventilation.
They are installed perpendicular formwork and are covered with sand, which prevents concrete from getting into them and deformation when pouring the foundation.
Reinforcement
After completion installation of formwork before starting to pour concrete, necessary provide for it. In the case of the construction of light buildings, reinforcement can be used neglect, in other cases, perform it extremely recommended.
Reinforcement used with diameter 10-16 mm, which is located longitudinally along the entire length of the foundation. Usually, two reinforcement rods are laid in the lower and upper parts of the future foundation of the house, retreating from the edges several times, recommended by 5 centimeters. Transverse steel bars with a diameter of 6-8 mm. They are connected to each other with wire.
For reinforcement enough arrange 2 reinforcement belts, but for high altitude foundation and heavy loads, the number of belts can be increase.
Beyond necessity use welding equipment, with this connection method, due to heating, the structure of the metal and its mechanical characteristics change. Increased rigidity at the welding site may become cause of the breakup during concrete pouring, or may subsequently create internal tension in the foundation, preventing it from responding to temperature fluctuations.
Pouring concrete
Concrete grades are often used to construct the foundation M200. The finished mixture is poured in layers of approximately 15-20 cm.
Attention! Preferably every layer compact using a vibrator or tamper, or at least a piece of reinforcement pierce surface to remove possible air pockets.
The next layer needs to be filled before hardening previous one. After graduation concreting, foundation needed cover film and leave to harden from time to time wetting water.
Pre-hardening will occur in 3-4 days, then it will be possible dismantle formwork For further work, concrete will be suitable after 2-3 weeks.
What is this blind area?
protects foundation against soil erosion melt water, precipitation, retracting unnecessary moisture from the foundation of the house. In addition, it can also serve path around the house.
Her device not difficult. It consists of two parts:
- Reasons. To do this, lay a layer of sand, or several layers of gravel and sand.
- Decorative coating. In the simplest case, this is a layer of concrete, but you can use stone, pour asphalt, cover with gravel.
The slope of the blind area away from the house should be no less 1.5˚. Optimal, if the height difference between the edges is 8-10 mm.
Between the wall and the blind area you need to leave expansion joint width about 2 cm. Roofing felt, polystyrene foam is placed in it, sand or other waterproofing material.
The width of the blind area should be no less the width of the eaves of the house so that water rolling off the roof gets in at her, and not on the ground. In addition, another point can affect the dimensions - insulation of the foundation.
Why do you need foundation insulation?
After the foundation concrete has completely hardened, it needs to be covered several layers of bitumen mastic and cover waterproofing material, for example, roofing felt. The next step is . Reinforcement laid in concrete conducts cold well, and insulation measures will reduce this unpleasant effect.
In addition to actually improving the temperature indicators of the building, there is one more important moment.
Soil heaving
Considering the shallow depth of the foundation, higher frost boundaries, it is affected by vertical and tangential forces of frozen soil. This compensated cushion under the foundation, and lateral forces can be compensated by laying insulation under the blind area.
A good option would be to use extrusion polystyrene foam, which is buried to a shallow depth under decorative covering the blind area. Knowing depth soil freezing (remember, for the Moscow region this is approximately 1.4 m), if the width of the laid insulation, and, in fact, the blind area, will be no 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 ruined strip foundation:
Shallow strip foundation(MZLF) are used in the construction of low-rise buildings from lightweight pieces building materials. Relatively simple design, low consumption of materials and a high level of reliability make its use a technically optimal and cost-effective solution.
A structure of this type can be made by digging a pit around the entire perimeter of the house under construction or by digging trenches above the freezing point along the axes of load-bearing walls and partitions. The width and depth of the trench are determined by calculation (presented in gallery form towards the end of the article).
MZLF can be monolithic reinforced concrete or assembled from individual factory-made concrete blocks. It is possible to use rubble stone masonry or solid clay bricks, but the most durable and reliable structure is a reinforced concrete monolith.
MZLF scheme.
Conditions for using a shallow structure
Construction of a strip concrete foundation shallow(FMZ) when building a house will be appropriate in the case of:
- large depth of soil freezing;
- availability;
The lowest point of the foundation structure is always located above the maximum freezing level at a depth of no more than 1500 mm. Moreover, the greater the width of the base, the greater the weight load it can bear. The small depth of the MZLF allows it to rise with the house during winter heaving of the soil, while remaining intact.
Reduced load-bearing characteristics somewhat narrow the possible range of application of the structure and make it possible to use a shallow strip foundation only in the case of building houses from light building materials, which include:
- wood;
- cellular concrete blocks;
- frame-panel products;
- lightweight masonry made of hollow bricks.
The width of the sole depends on the building material. MZLF should not be used for the construction of large houses. The depth of soil freezing under the floor of such buildings will be less than in the adjacent area. As a result, winter swelling will be uneven, which can lead to structural failure.
The use of MZLF is prohibited on clay, peat and sapropel soils.
On weak and non-heaving soils.
On medium and highly heaving soils.
Conditions for calculating the foundation structure
In order to calculate the MZLF and determine the laying depth and width of the tape, it is necessary to take into account:
- type of soil to the depth of the calculated freezing point;
- groundwater level;
- freezing depth;
- distribution of weight load on the foundation;
- height differences on the construction site;
- calculated depth, which is equal to the distance from the bottom point of the foundation to the zero level of the building.
The depth of possible soil freezing can be determined according to SNiP 23-01-99* “Construction climatology”. If there are significant differences in height, it is structurally impossible to install an MZLF. In these cases, leveling the site or constructing a different type of foundation will be required.
The weight load acting on the foundation consists of a constant and variable component. Variables include snow and wind loads, the weight of engineering equipment, furniture, plumbing, etc. To determine the constant weight component, the height of the building, the materials used, the design of the roof and ceilings, and other design features are taken into account.
Design calculation
The reinforced concrete monolith of the foundation rises above the surface of the earth and rests on a cushion consisting of a layer of compacted crushed stone covered with a layer of sand.
Depending on climatic conditions, the depth of laying MZLF on heaving soils is determined by the requirements of SNiP II-B1-62 and is equal to:
- when freezing up to 1 meter - at least 500 mm;
- up to 1.5 meters – 750 mm;
- more than 1.5 meters – 1000 mm.
Therefore, builders usually make calculations for a shallow strip foundation, taking the laying depth as a value equal to the freezing depth, with a deduction of 20-25%. If this condition is met, the foundation for the house will be able to rise freely without destruction when the soil swells.
The height of the base above the ground surface can reach four times the width, but should not exceed the size of the underground part of the foundation.
The width of the structure under the load-bearing walls is equal to the mathematical ratio of the weight load (t/m) to the calculated soil resistance (t/m2), the value of which is given in the tables of SNiP 2.02.01-83.
The thickness of the crushed stone-sand cushion is determined according to the formulas and tables given in SNiP “Foundations and Foundations”. It depends on the degree of swelling of the soil, the thermal regime of the building and the material of the walls.
Expenses for the device MZLF
As practice shows, the cost of constructing a shallow foundation with your own hands depends on its size, design, elevation above ground level, number of lintels and some other factors. The average price is 5-6 thousand rubles. per linear meter.
Installation of monolithic reinforced concrete MZLF
After the design calculations have been made and the project has been completed, you can begin to make a shallow strip foundation with your own hands. The work on constructing the MZLF consists of several stages, which will be described in the form of step-by-step instructions:
- preparatory;
- marking;
- earthworks;
- pouring concrete;
- base devices;
- hydraulic and thermal insulation devices;
- adding sand and building a blind area.
To complete the work you will need the following materials:
- sand and crushed stone;
- cement M400 or M500;
- steel reinforcement with a diameter of 8 and 12 mm;
- knitting wire;
- boards for formwork or ready-made panel elements;
- bitumen and bitumen primer;
- brick for leveling the base part.
When installing thermal insulation for FMZ, you can use polystyrene foam, which is secured with glue and plastic nails.
Preparatory stage
TO preparatory work include clearing and possible leveling of the construction site. In this case, uprooting of stumps and bushes should be carried out together with the roots in order to exclude the possibility of their germination in the future.
Everything must be delivered to the work site necessary materials and ensure their storage in such a way as to prevent damage from adverse external influences.
Marking
To perform correct and accurate markings, it is best to use a construction level and a measuring tape at least 10 meters long. Using a level, you can easily determine the required directions of rotation angles of the foundation tape. This is especially important with complex configurations.
To tension the marking cord, it is necessary to make beacons and pegs. The beacon is a U-shaped wooden structure of two stakes and a crossbar, the width of which should be greater than the width of the foundation.
Initially, the tensioned cords must run along the edge of the outer wall of the MZLF, and after completion of the excavation work, before installing the formwork, they must also be tensioned along the internal contours.
Checking the markings is carried out by measuring the diagonals inside all rectangles. They must be equal for both external load-bearing walls and partitions.
Digging a trench
Marking and excavation work. The required trench depth is equal to the sum of the depth of the reinforced concrete monolith and the height of the support pad. It is determined by the calculation described above.
For middle zone In Russia, the most commonly used design is with a cushion thickness of 200 mm and a recessed part of the foundation of 300 mm. In this case, the trench depth is 500 mm.
In order to prevent soil shedding and to have access to the surface when carrying out waterproofing work, it is necessary to make small slopes of 150-250 mm. Take the width of the bottom to be 200-300 mm wider than needed for the foundation itself.
Support pad for foundation
Sand pillow. As a cushion, you can use ready-made crushed stone-sand mixtures (SCS), which are sold on the construction market for road work.
Another option is to layer-by-layer backfill with crushed stone and sand, carefully compacting each layer. In this case, crushed stone should be at least ¾ of the total thickness.
After installing the cushion, its surface must be covered with geotextiles.
This non-woven material is capable of transmitting moisture in only one direction and will protect the foundation from its effects.
In addition, geotextiles will stop possible plant germination towards the concrete part of the shallow foundation of the house.
A layer of crushed stone. Formwork installation
Formwork design for shallow strip monolithic foundation installed vertically, to the full height of pouring the concrete mixture. In this case, part of it will be above ground level. It should be noted that concrete is not poured to the entire height of the foundation, since space should be left for a leveling layer of solid red brick.
To assemble the formwork, you can use boards or ready-made factory-made small-panel structures, which you can rent and not spend money on purchasing lumber.
The upper part of the formwork walls must be connected to each other by crossbars, and the walls themselves must be supported by inclined slopes every half meter. This will make the structure more durable and prevent it from falling apart under the pressure of the concrete mixture.
Do not forget to install sleeves made of metal or asbestos cement pipes. They will be required to enter utilities into the building. Since the MZLF is not a stable structure, the diameter of the sleeves must be at least 200 mm.
Formwork. Structural reinforcement
Metal reinforcement increases the strength of the foundation several times and ensures its safety even in the most difficult operating conditions. From metal rods using knitting wire, it is necessary to tie blocks with longitudinal, vertical and horizontal strings and place them inside the formwork. In this case, the lower longitudinal strings should not touch the bottom of the trench.
It is possible to connect rods by welding, but this technology is more complicated and, when heated, reduces the level of hardening of the metal. The recommended diameter of the longitudinal rods is 12 mm, vertical and transverse 8 mm. After installing the reinforcing blocks inside the formwork, you can begin pouring the concrete mixture.
Reinforcement schemes. 
Reinforcement.
Pouring concrete into the MZLF structure
There are two options for preparing concrete mixture. You can order it and buy it ready-made or make it yourself. The volumetric composition of a mixture of sand and crushed stone cement is determined as 1: 2.5: 4 for M400 cement and 1: 3: 5 for M500 cement.
Compaction of concrete is best done with a submersible construction vibrator. If it is not available, you can use a manual tamper, but in this case the concrete should be poured in layers of 30-50 mm and each individual layer compacted.
This way you can expel all air bubbles and ensure the necessary strength of the material.
If poured concrete hardens in hot and dry weather, it is recommended to cover the foundation with plastic film and periodically water it. The hardening time before masonry begins is at least 4 days.
Foundation protection
To protect a concrete monolithic structure from moisture, which can lead to gradual destruction, its surfaces are covered with hot bitumen and covered with waterproofing. Initially, the concrete is treated with a bitumen primer, which is used as a primer, and then waterproofing layers are applied.
Reduces heat loss of the building during the cold season and prevents the possibility of moisture freezing inside a monolithic reinforced concrete structure. Installation of insulation is a recommended, but not mandatory, condition for completing the work. This concludes our guide to building a strip foundation of shallow depth with your own hands.
Schematic cross-section: the insulation runs along the entire vertical wall of the tape combined with the base. Covered with cladding. The insulation should also be located under the blind area. 
Photo of insulation. Video on the topic
When constructing buildings that are light in weight (wooden garden houses, bathhouses, garages, sheds and even houses made of foam concrete and other lightweight materials), shallow strip foundations are often used to reduce the cost of construction.
It differs from a conventional strip foundation only in depth. During frost heaving, it, being essentially a rigid frame, lowers and rises along with the building. Since the displacement is equal, no destruction of the foundation occurs.
Anti-heaving
When water freezes, increasing in volume, it lifts the soil and “swells” it. Moreover, a building in the warm Crimea will behave completely differently than a building built in Siberia or the Urals. Clay soils that do not allow moisture to pass through are the most susceptible to heaving. In porous, dry soils, this phenomenon is noticeably less, because the rising soil fills the air voids.Soil heaving can damage even the strongest foundation
During construction shallow foundation located almost on the surface, the pressure on its walls is significantly lower. The action of the soil, which expands from below, is compensated by the weight of the building itself. In this case, a small depth must be compensated by the strength characteristics of concrete and a reliable reinforcement frame. However, such a small mass is being built - too much weight can damage the foundation.
Why do you need a blind area
In Finland and Sweden, where the air temperature in winter is quite low, houses built on a shallow foundation have long been not uncommon. Since the cause of soil heaving is freezing, builders once decided that they should fight not the effect (heaving), but the cause itself. On heaving soils, they stopped deepening the foundation and began to insulate it. Moreover, the further north the area, the greater the thickness of the insulation.They call the blind area a strip of waterproof material running along the perimeter of the building and tightly adjacent to its base. To drain water, it is made with a slight slope. Today, modern materials are used to insulate blind areas: expanded clay, polystyrene foam or penoplex.
Important! The width of the blind area should be equal to the freezing depth.
Insulation of the blind area
The thickness of the insulation layer depends on the type of soil and the structure of the building. Its width depends not only on the depth of freezing, but also on the extension of the roof: it must extend 20 cm from its edge. When erecting a small residential building on a shallow foundation, the basement itself is insulated simultaneously with the blind area to insulate the basement.
Important! The side parts of the foundation must also be waterproofed and insulated. The size of the trench for a shallow foundation should be increased by the width of the thermal insulation and bedding. When insulating the sole using polystyrene foam boards, the base area also increases.
Foundation insulation
If all the above conditions are met, shallow depth can be laid to a depth of 0.4-0.5 m. With highly heaving clay soils, this figure increases to 0.75 or even 1 m. It can be even greater when building on too mobile soil: on sand or in heavily swampy areas.
Foundation width
Minimum width strip foundation of shallow depth for light garden buildings and utility buildings is 25-30 cm. During construction small residential buildings this figure increases to 30-50 cm. Naturally, its width cannot be less than the width of the walls of the building, otherwise they will simply collapse.R – soil resistance: depends on the type of soil and is determined from the table;
P – support load; the specific gravity for each part of the structure made of different materials (roof, walls, etc.) is taken from standard tables SNiP II-3-79.
Knowing the total support area and length, you can calculate the width of the base of a shallow foundation, while the load per 1 cm2 should not exceed 2 kg. Moreover, the specific load from the building cannot be higher than 70% of the bearing capacity of the soil itself. If the calculated width is insufficient and does not satisfy this requirement, its size must be increased.
Important! The ratio of the depth of a shallow foundation to its width should not be more than 4:1.
Stages of foundation construction
1. Before starting work, the site and the area adjacent to it are carefully leveled. Next is done foundation marking:beacons (flags) are hammered into the corners of the future foundation, between which wire or a strong rope is stretched;
to check the correctness of the markings, a diagonal is drawn between them;
a blind area is marked at a distance of about a meter from the corners;
A rope is also fastened along its edges.
Important! The foundation should not be laid on a layer of fertile soil (chernozem); its loose structure cannot be considered a reliable foundation.
2. Trench digging under the foundation of a small building is done by hand, while the edges and bottom are leveled with a shovel. Its depth should be equal to the sum of the heights of the foundation and the pillow. The width of the trench should be increased by the width of the formwork walls by approximately 2.5 cm.
3. To distribute the load evenly over the entire area, it is laid before pouring pillow made of sand and gravel 20 cm high. Its thickness depends on the type of soil in a particular area. To avoid silting, its walls should be protected with a layer of waterproofing, for example, roofing felt. After backfilling, the pillow is carefully compacted and additionally spilled with water for better settling.
4. After the pillow layer is laid waterproofing made of polyethylene film or roofing felt. To secure it, it is poured with a liquid cement solution.
5. Device formwork. For its construction, you can use ordinary boards that are suitable in size. It is advisable to trim uneven boards from the inside. To prevent the wood from absorbing the solution, they must first be soaked in water. You can also use plywood or metal panels for formwork.
To prevent the mortar from protruding from the formwork walls, vertical supports are installed along its perimeter. When calculating the height of its walls, the size of the plinth (the above-ground part of the foundation) must also be taken into account. It is necessary to provide small openings in the formwork walls for ventilation, and temporary pipes to remove communications, which are removed after the solution has dried.
Formwork
6. The fittings are made of metal rods 10-12 mm. The number of its layers depends on the depth of the foundation: ideally it should be laid every 10-20 cm.
Foundation reinforcement rules:
the reinforcement should be placed as close as possible to the top and bottom of the foundation by 3-4 cm, in places that will bend the most; otherwise there will simply be no sense in such reinforcement;
the reinforcement must be located at least three rods horizontally without sagging;
it is allowed to connect reinforcement rods by welding or tying wire with overlapping each other; with a diameter of 10 mm, it must be tied with an overlap of 15-20 cm.
7. Pouring concrete. There is no need to save when pouring a shallow foundation. Cement is selected grades M200-300. Filling occurs in stages in layers of no more than 20 cm. In this case, each layer is carefully compacted, and the formwork is tapped to squeeze out the resulting voids. It is removed after the last layer has completely dried for 8-10 days.
8. Waterproofing. A shallow strip foundation is covered with a dense layer of bitumen mortar, on top of which waterproofing is glued.
Important! To avoid cracks after pouring, the foundation should be covered with film.
Basic mistakes when laying a foundation
In order for the building to stand for as long as possible, the following mistakes should be avoided when laying the foundation:before starting work, it is necessary to completely remove the entire plant layer of soil (turf) by 15-20 cm; otherwise, the plants, and behind them the logs, will rot;
Sod removal
It is prohibited to pour a shallow foundation without preliminary filling sand and gravel mixture capable of passing water; otherwise, the wet soil, having frozen to the concrete in winter, will simply “pull” the tape up; This is especially dangerous in the absence of insulation;
a common mistake is incorrectly tying the reinforcement in the corners of the foundation with an overlap in the shape of a cross; for an ideal connection with the rods running lengthwise, its ends must be bend 90°;
Reinforcement errors
Tie porch or veranda foundation to the main one is prohibited, they must be separated expansion joint, otherwise a rupture may occur at the junction;
Expansion joint between the building and the porch or veranda
it is better to lay it with mineral wool or other heat-insulating material
The foundation cannot be left over the winter unloaded, the entire cycle of work on the construction of the building should be completed in one season; if the structure is left for a year, the soil will simply push it out or “tear” it.
One of the simplest foundations for buildings is a shallow strip foundation. Despite the ease of performing the work, there is always the possibility of doing something wrong, so before installing the MZFL you should familiarize yourself with the general technology. Today we will take a step-by-step look at the device of a shallow-depth tape.
Scope of application of MZLF
Shallow foundations are used for buildings with low loaded mass. As an example, we can cite houses on a steel or wooden frame, as well as buildings made of lightweight elements (foam concrete, PCB). Typically, the number of storeys in buildings at MZLF does not exceed two.
In the definition of MZLF, shallow means completely located in the frozen layer of soil; the underground part of the concrete strip rarely exceeds 500-700 mm. Frost heaving forces with this arrangement do not create tangential (bursting) 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 install shallow tape in areas with a general terrain slope of more than 2%. On steeper slopes shallow foundation can be built only after re-planning the soil to form a horizontal terrace.
The feasibility of using MZLF lies in the much lower consumption of materials and ease of design. If a basement is not planned for the building, a shallow tape will reduce the volume of concrete mixture and reinforcement by 2-3 times, while providing equivalent load-bearing capacity.
It is worth noting, however, that it will not be possible to establish a shallow foundation on loose peat, silty soils and mudflow sandy loam. Such soils have too low a density and high plasticity, and therefore require the construction of pile-grillage foundations resting on denser layers of soil. You should not install MZLF on soils with heaving indicators of more than 4% or if the groundwater level is located above the depth of occurrence, despite the fact that drainage of the site is not planned, so that you do not have to deal with the consequences afterwards.
Calculation of cross-section and configuration
Since the MZLF acts as a beam and not a stiffening rib, usually the cross-section of the tape is close in shape to a rectangle or trapezoid. Tape in the form of a T-bar or more sophisticated sections is almost never cast due to the fact that the material savings look too insignificant in comparison with the installation of 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 rigidity under the full design load from walls, roofs, snow, etc.
The width of the upper part of the foundation is determined by the maximum possible thickness of the wall, taking into account the layer of interior and facade finishing. When installing floors along joists, it may be necessary to form a protrusion or expand the foundation by about 50 mm.
The width of the tape in the plane of occurrence is determined entirely by the required load-bearing capacity. It is enough to divide the total mass of the building and calculate the average load per meter of the perimeter of the tape, and then calculate the sufficient cross-sectional area of the support in accordance with the characteristics of the soil. To create a sufficiently high safety factor, the thickness of the incompressible bedding is not taken into account.
Scheme of a shallow strip foundation: 1 - mother soil; 2 - backfilling with gravel or gravel-sand mixture; 3 - blind area; 4 - foundation reinforcement; 5 - shallow strip foundation with a wide base to evenly distribute the load; 6 - wall; 7 - gravel filling of the internal area of the house foundation
The height of the tape is determined as the component of its underground and above-ground parts. With the above-ground part, everything is simple - it must be at least 80 mm and no more than four widths 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 can 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 trench profile for the MZLF device must 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 installed on ground formwork, using board-and-panel formwork for reasons of containing cement laitance and the need to give the cross-section a trapezoidal shape. Let us immediately note that the distance between the pit walls and the formwork should be twice as large on the outside as on the inside.
Compensation for the forces of frost heaving is carried out through incompressible, non-heaving and hygroscopic bedding, as well as filling the lateral sinuses with similar material. A sand-gravel mixture with coarse sand and granite or basalt crushed stone of fraction 25-30 is used as backfill material. To stabilize the foundation, the prepared trench bottom is covered with a 30-50 mm preparatory layer of M 100 concrete without reinforcement.
The bedding at the bottom of the trench helps distribute the load on the supporting soil layer, increase the bearing area and bring into play forces with a horizontal vector of application. The recommendation regarding the thickness of the bedding, equal to two times the thickness of the tape, is rarely followed in practice; more often, on slightly heaving soils, preparation is limited to 25-30 cm.
However, you must remember that the more pronounced the heaving, the more responsibility falls on the bedding. Sometimes it is advisable to replace the soil down 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, the total content of steel reinforcement without prestressing is set to no less than 0.1%; a more realistic figure of 0.17-0.2% will ensure proper reinforcement without excessive strength, but with a significant margin of reliability.
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 cross-section of the reinforcement can be divided into 4 bars for the upper and lower reinforcement lines.
If in MZLF the vertical distance between reinforcement lines exceeds 450 mm, add another row with rods whose thickness is at least 60% of the thickness of the main lines.
Structural reinforcement is performed with clamps or tying wire 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, MZLF reinforcement is accompanied by a number of anchorages. At turns and T-shaped junctions of the tape, each row of reinforcement in intersecting directions must be connected with bent embedded parts of the same section, the overlap of which with the main reinforcement is defined as 25 nominal diameters of the reinforcement. Anchoring with embedded studs may be required to connect to the base of the frame or masonry walls.
Concrete works
Before carrying out concrete work internal cavity It is recommended to cover the panel formwork with polyethylene film, which prevents the concrete mass from losing liquid until it sets. Afterwards, the reinforcement segments are installed, linked and distanced using plastic plugs.
Most often, bitumen mastics are used to protect against moisture, on top of which insulation based on fiberglass or cheaper roofing felt is rolled out. If the foundation does not require continuous waterproofing, the water barrier film remaining 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 this, all that remains is to make a blind area around the house, and the MZLF will be ready for further many years of operation.
Foundation work is not considered the most complex construction work, but in terms of responsibility it is one of the most critical building structures. Most mistakes during foundation construction lead to very serious consequences; correcting them is very difficult and expensive, and in some cases impossible. All foundation calculations must be carried out in accordance with the provisions of SNiP 2.02.01–83.
File for download. SNiP 2.02.01–83. Foundations of buildings and structures. SP 22.13330.2011
During calculations the following are taken into account:
- loads acting on the base of the foundation;
- soil indicators, presence and location of groundwater;
- maximum permissible deformations and load-bearing characteristics;
- characteristics of climatic zones and foundation depth.
This is very complex calculations, we will not dwell on them. For the simplest structures, builders take standard parameters for a given climate zone and soil characteristics as the foundation. They already have a large margin of safety, which ensures the reliability of the design.
Shallow strip foundation - parameters
Introductory table of the approximate width of the strip foundation
| Minimum tape width depending on the number of storeys | Tape width, cm The bearing capacity of the soil is 0.72 kgf/cm2. Example: silty, fine micaceous sands, silty sands, silty clay | Tape width, cm Soil bearing capacity 1 kgf/cm2 Example: clay, sandy clay, silty clay, silty inorganic silt | Tape width, cm The bearing capacity of the soil is 1.4 kgf/cm2. Example: sand, clayey coarse sand, silty crushed stone, clayey crushed stone | Tape width, cm Soil bearing capacity >1.92 kgf/cm2. Example: gravelly sand, gravel, crushed stone |
|---|---|---|---|---|
| Frame house 1st floor | 30 | 30 | 30 | 30 |
| Frame house 2 floors | 38 | 30 | 30 | 30 |
| Frame house 3 floors | 58 | 43 | 30 | 30 |
| Frame house, lined half-brick, 1st floor | 30 | 30 | 30 | 30 |
| Frame house, lined half-brick, 2 floors | 53 | 40 | 30 | 30 |
| Frame house, covered "half-brick" 3 floors | 81 | 60 | 40 | 30 |
| Brick house, 1 brick laying, 1st floor | 40 | 30 | 30 | 30 |
| Brick house, 1 brick, 2 floors | 73 | 53 | 35 | 30 |
| Brick house, 1 brick, 3 floors | 106 | 81 | 53 | 40 |
Tape width depending on wall material
A shallow strip foundation is one of the most frequently used options in private construction; in all respects it satisfies most developers. The construction of the foundation can be divided into three stages: marking, digging a trench and constructing formwork, pouring concrete and leveling.
Let's look at each stage in detail. We will talk about the stages of construction of a shallow reinforced foundation on a sand bed.
Work must be done carefully and slowly. It’s better to lose an extra hour or two during marking than to have problems later with the finished foundation tape.
Step 1. Drive wooden pegs around the perimeter of the foundation. In order to align the dimensions and angles of the tape, you need to prepare a basic device in the form of small benches. Drive two pegs into the ground at a short distance from the corners of the foundation and attach horizontal boards to them.
Step 2. Strengthen the nails in the boards and tie ropes to them. Try to first check the angle between the ropes with an ordinary large square. This will give you an approximation of the foundation lines.
Step 3. Align the corners of the tape; they should be exactly 90°. This must be done by checking the diagonals. The dimensions of the two diagonals cannot differ by more than two centimeters. Such scatter can be easily removed during the construction of the building box.
Step 4. Do not remove the ropes; use them to carefully mark the position of the tape with a shovel; it is advisable to dig a trench as deep as the bayonet of the shovel (about 20 centimeters).
Step 5. Now you can remove the ropes and continue digging the trench. Decide right away where you will put the land. It can be taken outside the perimeter of the building or leveled under it. In any case, the fertile layer must be completely removed.
Step 6. The width of the trench is equal to the width of the walls of the building. If the soil on the site is dense, then there is no need to do formwork in a trench; if the walls are crumbling, then you will have to do formwork along the entire height of the foundation.
The depth of the trench is within 60÷80 cm, taking into account a sand cushion up to twenty centimeters thick.
Video - Marking for the foundation
| Foundation marking schemes | Description |
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Manufacturing
You can use second grade edged boards or special waterproof plywood. We will take boards 20÷25 mm thick and 20 cm wide; to assemble the panels we will use slats and scraps of boards.
Step 1. Production of formwork panels. There is no need to make them very large, otherwise it will be difficult to install and dismantle. The length of the shields depends on their height, but in any case, try to total weight the structure did not exceed 50÷60 kilograms. This weight can be lifted without much effort by two people. Try to fit the boards without large gaps, nail the vertical posts at a distance of 50–60 centimeters. More accurate indicators depend on the parameters of the foundation tape.
Shields – photo
Use nails for nailing; self-tapping screws will not only increase the cost of the formwork, but will also create problems during its disassembly. Almost all lumber after dismantling the formwork can be used for further construction of the building, it is advisable not to damage it again. You are unlikely to be able to accurately calculate the length of the shields; leave a margin of approximately 1.5÷2 meters in length; you will make this shield after accurately installing all the previous ones. In this position it will be possible to take exact measurements up to the corner and make the shield of the required length.
Step 2. Start laying out the formwork for the foundation. This kind of work cannot be done alone; you need to call an assistant. Lower all the panels one by one into the trench, fix the position with pegs and corner stops. Spacers should be inserted between the panels so that they do not change their position while fixing the formwork. Spacers do not work while concrete is being poured, they are only important during the assembly of the formwork. We advise you to immediately prepare spacers for the template; the quantity is determined taking into account the length and height of the foundation strip and the parameters of the panels.
Step 3. Fix the panels securely; it is better to place corner supports more often than to level the foundation later. Place your shields along the rope and constantly monitor their position. The joints of the boards are sealed with a board; fastening must be done in this place. Drive the pegs as deep as possible; the corner supports to the shields must be fixed with special small stops made from scraps of boards. You must exclude even the slightest possibility of violating the integrity of the formwork while pouring concrete. Such an “accident” always comes at a cost.
Step 4. During the installation of formwork, it is always necessary to provide for its subsequent dismantling. This means that the nails should be driven into places where they can be removed. Place all dismountable units only on the outer accessible side of the formwork.
Step 5. Check that the formwork is installed correctly. Pay attention not only to the linearity of the planes, but also to the reliability of fixation. Apply multidirectional forces to the formwork - if even slight vibrations of the structure are detected, immediately install additional stops. Remember that correcting formwork installation errors while pouring concrete is an extremely thankless task.
Assembled formwork - photo
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Video - Making formwork
Step 6. Place plastic tubes in the formwork at the locations of ventilation ducts and utility lines. Secure the top boards of the formwork panels with boards or tie them with wire to prevent expansion.
Our foundation must be reinforced - its load-bearing capacity increases significantly. For reinforcement, construction reinforcement of periodic profile Ø 10 mm is used. The reinforcement can be tied together, which takes a very long time, or you can only lay horizontal bars while pouring concrete. We, of course, choose the second option. In terms of strength, a strip foundation loses almost nothing, and much less work and money will be required.
If everything is fine, you can start pouring concrete.
Pouring concrete
We will make the concrete ourselves from grade 400 cement, sand and gravel or crushed stone.
Nowadays, such heavy physical work is no longer done manually; you need to purchase or borrow a small concrete mixer with a drum volume of approximately 0.2 m3.
Before starting work, you need to calculate the required amount of building materials; calculations are made taking into account the total volume of the foundation tape. It’s easy to calculate the volume; multiply the length by the width and height. For one cubic meter of concrete you will need 325 kg of cement, 760 kg of sand and 1100 kg of crushed stone. These are approximate figures, they are needed only to guide the amount of materials. When purchasing materials, increase their quantity by 10%. You need to buy even more sand for a sand cushion.
No one weighs individual ingredients to the nearest gram when making concrete. When preparing concrete, take two shovels of sand and three shovels of crushed stone or gravel for one shovel of cement. Water is used as needed, the specific amount is determined experimentally, and the concrete must have optimal viscosity.
Step 1. Pour a layer of sand ≈15÷20 centimeters thick into the trench. The sand needs to be compacted well; use any available means for this or make a simple device.
Step 2. Pour the concrete mixture around the perimeter of the building, do not allow large differences in height. If you have an electric vibrator, great, it can compact concrete across the entire height of the foundation at once. If tamping is done manually, then the concrete layer should not exceed 25÷30 centimeters in height.
Step 3. Simultaneously with pouring, pay attention to the “behavior” of the formwork; if you detect any bulges or curvatures, immediately take measures to eliminate them.
Step 4. We decided that we would perform the foundation reinforcement in a simplified way. The rods are placed directly on the concrete. In total, we will have four rows of reinforcement in the tape, two in each layer. Cover the sand cushion with a layer of concrete approximately 30-40 centimeters thick. Level it horizontally and lay two rows of reinforcement. There is no need to try too hard, the rods will then be filled with mass, and a small angle of inclination (if any remains) will not have any negative effect.
Step 5. Continue pouring the foundation around the perimeter, when twenty to thirty centimeters remain to the top of the tape, lay the second row of reinforcement. Make sure that the ends of the reinforcing bars overlap by approximately 20÷30 centimeters.
Step 6. Level the zero mark of the strip foundation; the height variation in the corners cannot exceed two centimeters. Use a stretched rope; with its help you will perform the leveling faster and with better quality.
It is highly advisable to fill the entire foundation within one working day.
In most cases, this is possible for baths, but it all depends on the specific size, ability to organize construction work, professionalism and the number of workers. If you have to leave part of the work for the next day, then the liquid concrete in the formwork must be leveled horizontally as much as possible. The presence of “steps” with large differences in height significantly reduces the load-bearing characteristics of the foundation. The formwork should be removed two weeks after pouring the concrete. If the weather outside is hot and dry, it is recommended to moisten the concrete generously with water several times a day. A quickly drying foundation will not have the calculated strength indicators.
Why? Concrete is a unique material. Under ideal conditions, the strength of concrete structures constantly increases. During the first 14–15 days, concrete gains 70–80% of maximum strength, then the rate of increase in strength slows down and after 30–40 years its strength increases by only a fraction of a percent. But it is increasing! Of course, this only happens in cases where the structure is not affected by precipitation, temperature changes, external forces, etc. That is why it is not recommended to remove the formwork in less than two weeks. Do not rush to dismantle it; during this time it is better to prepare for the next stage of construction.
Prices for concrete mix M400
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Video – Seminar on pouring the foundation
Now that you already have a general understanding of foundation construction technology, you can give some practical advice. With their help, it will be possible not only to simplify and facilitate a number of construction works, but also to improve their quality and reduce cost.
How to make concrete
A lot depends on the proper organization of work; we give advice from experts who have solid practical experience.
The concrete mixer must be in a stationary place, and ready-made concrete is transported to the foundation by wheelbarrows.
There are some “craftsmen” who advise dragging the concrete mixer along the foundation as it is filled. They transport water, sand, cement and gravel separately in buckets to each new location. Looking at this “technology” is both funny and painful. This organization of work increases the labor intensity at least twice and increases the time by the same amount.
What do we recommend? The concrete mixer must be stationary; install it taking into account the possibility of access. Place a barrel of water with a volume of approximately 200 liters near the concrete mixer. Bring sand and gravel to separate piles, and let the cement lie on pallets in bags. All materials must be available for filling into a concrete mixer without transfer.
Throw a bag of cement onto the pile of sand and use a shovel to cut it approximately in half. Half a bag of cement is just the norm for one batch of concrete. Turn on the concrete mixer, pour one and a half to two buckets of water into it, take half a bag of cement with your hands and pour it into the water. You can immediately throw gravel or crushed stone; the pebbles will break up small piles of cement well. Next, throw sand and gravel into the mixer one at a time. The calculation is simple - for one shovel of cement you need two shovels of sand and four shovels of gravel.
The specific amount depends on the size of your shovels; for the first batch, you can measure the amount of cement in half the bag, this will give you a rough idea. In the future, you will gain experience and you will already be able to determine the quality of concrete by eye by the viscosity of the mass. If the solution is too thick, add water in small portions. If it turns out liquid, add sand; gravel has almost no effect on the viscosity of concrete; it absorbs little water. I had to add a lot of sand - throw another shovel of cement into the concrete mixer. The cement that spilled onto the sand is picked up and used.
With water, you also can’t guess right away; the amount of water depends on the moisture content of the sand and gravel. For the first batch, it is better to take slightly less water and then add it as needed. Too much water can cause trouble - there won't be enough volume in the concrete mixer to add large amounts of sand and gravel.
Ready concrete must be transported to the foundation by wheelbarrows. When planning the organization of work, it is necessary to provide for such a number of people that the work proceeds in an assembly line manner, without anyone expecting anyone. The time for pouring the foundation with concrete should be equal to the time it took to make it. These indicators are influenced by many factors: the distance to the foundation strip, the volume of the concrete mixer, the professionalism and hard work of the workers.
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Important
Perhaps this is all we can advise beginners; in the future you will gain your own experience and you will be able to change and improve the algorithm for making concrete.
How to assemble formwork
Absolutely all individual nodes work in bending or compression. Why are we talking about this? And then, when assembling the formwork, there is no need to use huge nails and then bend them five centimeters from the back side. Not a single nail works to pull out. In corner stops, the nails are subject to bending forces and very insignificant pulling forces. Using normal nails will not reduce the strength of the structure, but it will make your work much easier during its dismantling.
Most of the boards after formwork are used for further construction - after removing the formwork, immediately disassemble it and clean the boards. There are cases when high-quality boards are used for formwork - we recommend covering them with plastic film. Install the film on the inside of the formwork, fix it with an ordinary stapler. The cost of the film is pennies compared to the cost of lumber. No master will process boards contaminated with cement on machines, and polyethylene will keep the materials in their original condition.
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How to dig a trench
It would seem that there is nothing simpler - dig deeper and throw further. But those who have never dug themselves think so. Each job has its secrets, here are just a few of them.
- First you need to trim the grass roots along the entire length of the trench. Using a shovel, cut the turf along the line on both sides of the foundation strip to a depth of approximately ten centimeters. This will not only make it easier to remove the sod, but will also make the edges smooth.
- The first excavated soil should be thrown away as far as possible from the edges of the tape. You will still have to throw it further than you might think. It's easier to do this before you get too deep. Over time, the trench will become deeper, and more effort will have to be spent to throw out the earth. And if you have already made a mound near the edges, then you will have to transfer it too.
- Increase the depth by the spade bayonet along the entire length of the trench, immediately leveling the edges. Then clean up the loose soil with a shovel, starting work from the place where you finished digging with a bayonet shovel. This way you won’t be compacting the soil yourself. When you’ve finished clearing the ground, repeat it all over again, work with a bayonet shovel along the entire length, then clean the bottom of the trench.
Video - How to dig a trench for a foundation
We guarantee that in this way you will be able to dig the foundation much faster and without “heroic feats of labor.” It is impossible to talk about all the intricacies of construction work in one article. Do not perform monotonous work for a long time, this accelerates physical and mental fatigue; if possible, change it after an hour or two.
We can give one universal piece of advice: look for logic in every work. All actions should be aimed at one thing - obtaining maximum results with minimal losses. It doesn't matter what the loss is: time, money or effort. If you listen to our advice, then work will always be a joy, not a punishment. Especially if it is done with your own hands and for yourself.
Video - Do-it-yourself shallow strip foundation
