What is a shallow strip foundation. How to make a shallow strip foundation correctly How to fill a shallow strip foundation correctly

When constructing foundations, one of the main problems is heaving soils, which are widespread in Russia. In order to prevent the negative impact of forces arising from frost heaving, shallow-depth strip foundations (MzLF) are successfully used in low-rise construction.

MzLF design

For the conditions of Russia, the definition of the most reliable design of foundations is of particular relevance in conditions where soils with the property of frost heaving lie at the base.

A special property of heaving soils is the ability to increase their volume during seasonal freezing - more details about the properties of heaving soils and the geography of their distribution in Russia can be found in the article “What are heaving soils, methods for determining them, choosing the type of foundation”.

When soil swells, its volume can increase significantly - by tens of centimeters, and the forces with which frost heaving forces will act on foundation structures can reach tens of tons. The deepening of the bottom of the foundation sole to a mark located below the depth of winter freezing does not save from the negative action of heaving forces, since the impact also occurs along the lateral surface.

The destruction of the walls of the house from the forces of frost heaving

To prevent the negative consequences of the impact of heaving forces, a special design was developed - a shallow strip foundation, or MzLF, which can be built with your own hands.

The features of the MzLF, in contrast to the conventional strip foundation, are as follows:

the depth of a shallow strip foundation is taken regardless of the freezing depth at a mark of no more than 30-40 cm from the soil surface after planning. This minimizes the effect of negative heaving forces on the side surfaces of the structure;
under the sole of the foundation, a do-it-yourself pillow is made of bulk materials - sand or ASG - a mixture of sand and gravel, the thickness of which is calculated depending on the complex conditions of the construction site. By replacing the soil under the sole, its heaving properties are eliminated, the bearing capacity of the compacted base is increased, and its deformations associated with thawing in the spring are reduced;
the foundation is necessarily reinforced with spatial frames that turn the strip foundation into a frame system of beams lying on an elastic foundation. The system of beams, rigidly fastened together, perceives and compensates for all uneven effects of heaving forces.

We present for demonstration a tape shallow foundation according to SNiPs in the context:

MzLF device

A shallow strip foundation on heaving soils can be built with your own hands, using the instructions of a typical technological map TTK "Device of a shallow strip reinforced concrete foundation".

The MzLF technology almost completely corresponds to the technology for constructing a strip monolithic foundation, which we described in the article “Strip foundation: from earthworks and cushions to pouring concrete and removing formwork”, and includes the following operations:

preparatory work - vertical layout of the site, marking and fixing the axes of the building, excavation of trenches for foundations;
pillow device under MzLF;
formwork installation;
reinforcement;
concreting;
caring for the concrete mixture laid in the formwork;
removal of formwork.

All these operations are described in detail in the above article, so here we will dwell in more detail on the points directly related to MzLF.

Pillow device

The cushion, due to which the heaving properties of the soil are eliminated and possible uneven deformations of the base are compensated, is the main element that distinguishes the MzLF from the usual strip foundation. The thickness of the cushion is determined by calculation (see section “Calculation of MzLF”).

The following bulk materials can serve as the material for the pillow device:

coarse sand and sand of medium size;
gravelly sand;
crushed stone;
blast furnace or boiler slag;
a mixture of coarse sand (not more than 40%) and gravel (not less than 60%).

Before installing the pillow, the bottom of the trench is cleaned, then the bulk material is laid in layers, with a layer thickness not exceeding 20 cm. Each layer must be carefully rammed with electric rammers, then the next layer is filled up and rammed again. The density of the cushion after compaction should be at least 1.6 t/m³.

If the groundwater is at a high level and there is a possibility of soaking with a perch, it is planned to lay a pillow on a layer of geotextile, which also covers the structure on both sides and on top. This prevents silting of the loose cushion material.

Foundation reinforcement

Reinforcement of the MzLF is carried out by spatial frames, in which the working reinforcement is located in the upper and lower parts of the foundation section.

Let's use an example to show how the foundation is reinforced.

A conditional foundation with a section of 400x400 mm is deepened by 400 mm from the soil surface, while being reinforced with a KP-1 spatial frame. The protective layer of concrete from the sole of the foundation is 65 mm, from the side surfaces of 30 mm, from the upper plane - 30 mm.

Cushion made of ASG - a mixture of gravel and sand (40% coarse sand, 60% gravel), the thickness of the cushion is taken by calculation, the width of the cushion is 200 mm larger than the width of the foundation, that is, it protrudes 100 mm from the side surfaces of the MzLF.

The spatial frame is assembled from six longitudinal rods of working reinforcement with a diameter of 12 class A3. In this case, the connection of the frames along the length with each other should be overlapped. The length of the overlap should not exceed 20 diameters of the rods that are connected, and be at least 250 mm. The rods should be connected in a run, that is, more than 50% of the connections should not fall into one cross section.

Instead of class A3 fittings, you can use class A500C fittings, which cost 30% less and allow you to make connections by welding, which simplifies the fitting work. When connecting the working rods by welding, the length of the seam should not exceed 10 diameters, in this case - not less than 120 mm.

The working rods are connected into three-dimensional frames by means of clamps made of class A1 smooth reinforcement, installed with a step of 200 mm along the length.

In places where the walls intersect or adjoin each other and at the corners during the operation of the building, stress concentration occurs, so these places are reinforced by installing additional rods.

Strengthening the corner connection of frames

Strengthening is carried out by installing additional rods of the same working diameter of 12 mm as the working reinforcement, in the upper and lower levels of the frame. Additional rods, bent at a right angle, are attached to the intersecting working rods of the frames on the outside of the gusset using a tie wire. Additional trapezoidal rods are installed closer to the inside and welded to the connected rods according to GOST 14098-91-S23-Re for welding work.

Reinforcement of the T-connection

Strengthening of the T-shaped abutment is carried out by additional trapezoidal rods, which are welded to the main rods in two levels of the connected frames.

Reinforcement at the intersection of walls

Reinforcement at the intersection of the walls is carried out by welding additional trapezoidal rods in two levels of intersecting frames.

In this example, the width of the wall is equal to the width of the foundation. If the width of the foundation is calculated to be 600 mm greater than the width of the wall, then it is necessary to additionally reinforce the sole with flat meshes, the working reinforcement of which should be located across the sole. The diameters of the working reinforcement are accepted within 10-12 mm, class A3 or A500C, step 600 mm.

As structural reinforcement for grids, smooth reinforcement of class A1 (A240) with a diameter of 6 mm is used, or from high-strength wire of class Vr-1 with a diameter of 4-5 mm, which is laid with a step of 300 mm along the length. The connection of the working and structural mesh rods is carried out using a knitting wire at each intersection.

All reinforcement work should be carried out in accordance with the requirements of regulatory documents : SP 52-101-2003 "Concrete and reinforced concrete structures without prestressing reinforcement", SNiP 52-01-2003 "Concrete and reinforced concrete structures".

Rules for the production of works and special events

In addition to the main solution - the MzLF device with a compensating pillow, it is necessary to adhere to certain rules for the production of work and provide for additional measures that will help reduce the negative impact of heaving forces.

The rules for the production of works are as follows:

all work on the MzLF device should be carried out mainly in the summer. It is not allowed to build foundations on frozen foundation soils;
to prevent wetting of the foundation soils, it is necessary to perform a vertical layout of the site with a slope on each slope of at least 0.03 to drain surface water after precipitation from the building site and excavations for the foundation;
if the site is located in a low place, then it is necessary to protect it from the danger of flooding with surface water from neighboring, elevated areas by means of drainage ditches;
the process of building foundations - from preparatory work to the installation of a blind area - must be carried out in the shortest possible time, for which earthwork can only be started after all the preparatory work has been completed and all the materials necessary for construction have been delivered to the site;
on the site, it is necessary to preserve the vegetation cover of the soil as much as possible, which serves as a natural soil insulator;
after the installation of the MzLF, the sinuses of the trenches must be covered with non-rocky soil or the same material that was used to build the anti-rock cushion - sand, crushed stone or ASG - a mixture of sand and gravel with layer-by-layer compaction. This will prevent the effect of heaving forces on the vertical surfaces of the foundation;
it is impossible to leave the foundation after the device unloaded for the winter period, that is, it is necessary to immediately erect the walls of the building to the full design height and block it.

In order to minimize the possible negative impact of heaving forces, additional measures are provided:

- at a level of groundwater close to the foundations, wall drainage is arranged along the perimeter of the building with the laying of drainage pipes and drainage along the slope to a low place;
- effective additional insulation of the base under the sole of the foundation, which is arranged by laying insulation under the blind area. As a heater, it is best to use extruded polystyrene foam - EPS, specially designed for use in underground structures. We wrote about the properties of EPPS and its application in the article “Overview of technologies for warming houses with various types of foam (PSB, EPPS) with an analysis of the pros and cons, technical characteristics”;
- it is recommended to reinforce the blind area made of concrete with a mesh of high-strength wire of class Vr-1 with a diameter of 4 mm with a cell of 150x150 mm. Every 6 m along the length of the blind area and at the corners, it is necessary to arrange expansion joints by inserting a wooden board. In addition, for more efficient drainage of surface water along the edge of the blind area, it is necessary to make drainage grooves with a slope that provides for discharge into a low place;

the places around the building from which the fertile layer of soil was removed, after the completion of construction work, should be immediately covered with turf and it is advisable to plant shrubs. This will contribute to the warming of the soil and the retention of snow cover in winter, which also reduces the depth of soil freezing.

Calculation of MzLF

The width of the shallow foundation and the thickness of the anti-rock cushion must be taken according to the calculation.

Let's look at an example of how the MzLF is calculated. Let's take the option for low-rise construction - a one-story residential building made of timber with two extreme and one middle bearing wall with dimensions in the axes of 8x8 m, the middle wall is located in the middle, that is, with a step of 4 m. For wooden light houses, the problem of soil heaving is especially relevant.

construction of external walls - a wall made of massive timber 150 mm thick;
the middle wall is a massive beam 150 mm thick;
floor height 3 m;
coating - with wooden load-bearing beams;
plinth 600 mm high made of monolithic concrete;
soils - loams are semi-solid, strongly heaving, since the site is located in a lowland.

First, we determine the load per 1 linear meter of the foundation for two design sections: 1 - the outer walls bearing the coating, 2 - along the middle wall, where the coating beams rest on both sides. For self-supporting walls, we will not do the calculation, we will take the width of the foundation constructively.

Pc - specific gravity of the base per 1 m2 = 1.5 t / m2 (according to table A);

hc - basement height, equal to 0.6 m;

Pbr - specific gravity of walls made of timber per 1 m² = 0.12 t / m² (according to table A);

he – floor height (3 m);

Pper = wood cover weight 0.223 t/m² (taking into account snow weight according to table A);

L - span of load-bearing walls (4 m).

We get: q1 = 0.6 x 1.5 + 0.12 x 3 + 0.223 x 4/2 = 1.72 t/m

For the middle wall:

Pbr - specific gravity of the middle wall of timber, taken according to table A = 0.12 t / m²;

Pc - weight of the plinth in the middle part = 1.5 t/m².

q2 = 1.5 x 0.6 + 0.12 x 3 + 2 x 0.223 x 4/2 = 0.9 + 0.36 + 0.892 = 2.15 t/r.m.

Determine the width of the foundation cushion by the formula:

b is the width of the foundation;

q is the load per 1 m of the strip foundation;

R is the design resistance of the soil base, which is taken according to table B, in our case for semi-solid loam R = 22.8 t / m².

We get for two sections:

As a result, we accept the width of the foundation from design considerations for all walls = 0.3 m.

It is also possible to calculate the width of the foundation according to the design resistance of the anti-heaving pillow, when the value of R is less than the value of Rp, where Rp is the design resistance of the soil of the anti-heam pad, which depends on the type of bulk material:

14 t / m² - for sand of medium size;
16 t/m² - for coarse sand;
21 t / m² - for sand and gravel mixture.

In our example, in any case, due to small loads, the width of the foundation is taken from structural considerations.

We determine the thickness of the pillow, for which two formulas are used:

From the conditions of resistance of the underlying soil:

where: R is the strength of the underlying soil (refractory loam R = 22.8 t/m²), determined from Table B given here.

t \u003d (A - C x D x q) / 1 - (0.4 x C x D x q / b)

A - coefficient, determined according to table B, value A for heated structures on heavily heaving soils = 0.5;

C - coefficient, which is equal to 0.1 - for heated buildings, 0.06 - for unheated buildings;

D - coefficient, which is determined from table D, the average value for heated buildings between a width of 0.2 and 0.4 m = 1.70 + 1.29 / 2 = 1.49

Note: the values above the line of the coefficient A are given for the most optimal depth of laying the base of the foundation of 0.3 m, under the oblique line - for foundations lying on the surface, that is, not buried.

We calculate the thickness of the pillow, based on the conditions of resistance of the underlying soil layer for load-bearing walls:

t \u003d 2.5 x 0.3 x [ 1 - (1.2 x 22.8 x 0.3) / 2.15)] \u003d 0.75 x (1 - 3.81) \u003d - 2.10 m

The result is a negative value, in this case the thickness of the pillow is taken equal to zero.

We calculate according to the second formula:

t \u003d (A - CxDxq) / [ 1 - (0.4 x C xD xq / b)] \u003d (0.5 - 0.1 x 1.49 x 2.15) / [ 1 - (0.5 x 0.1 x 1.49 x 2.15 / 0.3)] \u003d (0.5 - 0.32) / (1 - 0.53) \u003d 0.17 / 0.47 \u003d 0.36 m

The thickness of the pillow is taken according to the larger of the values \u200b\u200bobtained when calculating using two formulas.

As a result, we accept a pillow from design considerations with a thickness of 400 mm.

Note: same as for table B.

Shallow strip foundation (MZLF): calculator, device, depth, reinforcement technology

The device of a shallow foundation (MZLF) and a method for calculating its depth and width

For country or individual construction, it is irrational to use a powerful deep strip foundation. The load from the building allows the use of a shallow type of strip foundations. But what design is considered a shallow support, and what are the requirements for it? This issue needs to be considered in detail.

For all types of strip foundations, including shallow foundations, the height depends on:

depth of soil freezing (climatic conditions of the area);
the presence of a basement in the house (the required mark of its floor, the height of the basement);
groundwater level (GWL).

In the first case, it is important to ensure that the sole is located below the layer of freezing soil. In the case of a shallow foundation, special measures are taken against frost heaving of the soil. This measure is necessary to prevent movement of the base of the building due to the forces of frost heaving. If the house provides for a basement and it is decided to use a shallow foundation, a sufficiently high base will be required, which will provide the necessary height.

Groundwater is often located at a sufficiently large depth, so the main problems with it arise when designing a deep belt. For a small foundation, it is important that they are located 50 cm below the sole.

Usage restrictions

The use of a shallow strip foundation is limited. It is not suitable for the following occasions:

on the building site, layers with different characteristics are joined (in the horizontal plane, that is, at different points of the site, different soils are located near the surface);
the groundwater level is closer than 1 m from the ground surface;
the presence of soil layers of organic origin (peat, silt, sapropel) within 1 meter from the surface of the earth;
strongly heaving soils (dusty sand, bulk soil);
buildings over two stories high.

It is important to note that massive brick and concrete houses are only allowed to be placed on shallow T-type foundations. Rectangular tape is used for buildings made of foam concrete, timber, logs and frame houses. The problem with strongly heaving soils is solved by replacing a layer with poor characteristics with medium or coarse sand. Groundwater at their high location can be diverted with the help of a well-arranged drainage system.

Determination of the laying depth

Factors that affect the value have already been cited previously. Each is considered separately below.

Soil freezing depth

The most important problem when supporting a shallow tape is the forces of frost heaving. They occur in the soil above the freezing mark, and for most of the country, this mark is in the range of 1.2-2.0 m. It is not possible to locate a shallow foundation and fully comply with the requirements for the depth of support, therefore, additional measures are required.

In the general case, the minimum depth of the tape can be taken from the table.

For most of the country, the value is 0.5 m. To prevent damage to foundation structures during alternate freezing and thawing of the soil, it is necessary to jointly carry out the following measures:

Warming. It is carried out in two stages. The first is the protection of the outer vertical surface of the tape. The insulation is attached along the entire height of the structure (underground and aboveground parts). The second stage of warming is the device of a warm blind area.
Sand cushion device. To prevent frost heaving under the support, a preparation with a thickness of 30-50 cm is arranged. Sand of medium or coarse fraction is chosen as the material, fine sand will shrink during operation.

It is most effective to place horizontal thermal insulation at the level of the base of the foundation. Vertical thermal insulation should cover the entire foundation.

As a heater, you can use polystyrene foam or extruded polystyrene foam (foam foam). They have very high thermal insulation characteristics and are resistant to biological attack, which is especially important in the existing operating conditions. When pouring a monolithic strip foundation, fixed polystyrene foam formwork is also quite often used. Its use greatly simplifies formwork and thermal insulation work, reduces financial and labor costs for the construction of a shallow foundation.

Important! The exact value of the soil freezing mark can be found by determining the type of soil and using, which presents a table of values for large cities of Russia.

Groundwater depth

To determine the presence of water in the soil, a study of the site will be required. With independent construction, it is carried out in two ways:

excerpt of pits manually;
development of wells using a hand drill.

After analyzing the vertical walls of the pit or the soil on the blades of the drill, they roughly determine the type of soil, its physical properties, as well as water saturation. The support of the strip foundation is possible 50 or more centimeters above the groundwater horizon. If the soil is heavily saturated with water, it is better to think about resting the house on screw piles.

You can also remove moisture from the strip base using a drainage device. To do this, drainage pipes are laid at the level of the base of the bearing with the necessary slope.

The influence of the laying depth and groundwater level can be combined in one table based on Table. 5.3.

Soils on which leaning occurs tape foundation	Depth soles
	if groundwater is located at a distance of less than 2 m from the freezing depth	if groundwater is located 2 or more meters below the freezing depth
Coarse-clastic and rocky rocks, gravelly sands, coarse and medium fraction	Does not depend on freezing Accepted by the minimum values in the first table	Does not depend on freezing, is taken according to the minimum values in the first table
Fine and silty sands*	Depends, no less than freezing depth is assumed
sandy loam
Clays, loams, coarse-grained rocks with silty filler		Depends, at least half of the freezing depth is accepted

Basement or first floor floor level location

If a decision has been made to construct a basement or technical underground, its floor level should be 20-30 cm above the level of the strip foundation support. But at the same time, it is important to remember that the buried part of the tape cannot be less than the above-ground one. This rule should be taken into account when choosing the height of the base. The same requirement applies to the floor of the ground floor if it is located close to the ground or below it.

As a generalization, the following minimum values can be given for various soils (without taking into account water saturation):

clay - half the distance to the freezing mark;
sandy loam, coarse rocks, sands (except fine and dusty) - 0.5 m for most of the country, 0.75 for regions with a very harsh climate.

The correct choice of the mark of the sole of the strip foundation, as well as the adoption of measures against frost heaving (insulation, cushion, drainage) guarantees its stability and design bearing capacity.

A classic for low-rise buildings is with a low construction budget. The monolithic lattice structure makes it possible to make floors on the ground, has high spatial rigidity, and is suitable for any wall materials.

Tape shallow foundation step by step

The operation of any foundation is complicated by uneven heaving forces, insufficient calculated soil resistance. The technology became possible thanks to a set of measures to eliminate swelling:

the arable layer is replaced by sand, crushed stone, depending on the level of GWL
the blind area is insulated to a width of 0.6 - 1.2 m
the perimeter of the building is equipped with ring drains
backfilling of the sinuses of the trenches is carried out with an inert material

There is no heaving in non-metallic materials, excess moisture is removed by drainage sewage, a heat insulator retains the geothermal heat of the bowels. All work is best done at the pit stage in order to reduce the construction budget.

Calculation of parameters

Shallow strip foundation not regulated by depth only when providing the above measures to compensate for heaving forces. The design settlement must be within acceptable values, otherwise it is necessary to supplement the structure with bored piles. The standard parameters of the MZLF provide a 2 - 3-fold margin of bearing capacity for 2-storey brick cottages with an attic:

For MZLF tape, two reinforcement belts are usually sufficient, anchoring in L-shaped, T-shaped mates. Longitudinal rods 8 - 16 mm, reinforcement A400 ("corrugated"), clamps, anchors from rods 6 - 8 mm of smooth reinforcement A240.

Offset

Shallow strip foundation labeled in the standard way. Instead of pegs, it is better to use cast-offs, consisting of two pointed bars with a crossbar between them. If you set all the cast-offs according to the level, mark the axis of the wall, the side edges of the tape with dashes, the cords can be removed during earthworks, stretched later to install the formwork. They are carried out beyond the perimeter of the MZLF (1 - 1.5 m) so that the soil under them does not crumble. When splitting, consider:

MZLF is not recommended with a height difference of one and a half meters between adjacent walls. The lateral movements of the soil are too significant, it will be necessary to reinforce the tape with piles, retaining walls, which significantly increase the construction budget.

trench development

Even with the maximum depth of the MZLF tape, all work can be done independently. Special equipment is needed for pits, here you can get by with trenches. When excavating soil, common mistakes of an individual developer are:

Therefore, it is recommended to remove black soil from the entire perimeter on a bayonet. The dimensions of the pit are 1.2 m larger than the size of the foundation. This distance is necessary to insulate the blind area. Inside the trenches for the MZLF tape, additional trenches for drainage with a section of 30 x 30 cm are required.

drainage system

Clay soils swell only with abundant wetting by groundwater. Drainage reduces heaving forces by 40% without additional measures. To create an underground sewer circuit, you must perform the following work:

To increase the service life of the system, the lower layer of the natural filter is laid on geotextiles, with which the entire structure is covered from above after backfilling.

Underlayment

Adding non-metallic material to the bottom of the trench is necessary to solve the following problems:

base leveling
drainage to prevent concrete from getting wet
elimination of heaving forces

In different regulatory documents, the thickness of the sand, crushed stone cushion is not the same 20 - 80 cm or four times the width of the MZLF tape. In practice, a thickness of 40 cm is more often chosen, making it in layers (10 cm) with a mandatory seal (vibrating plate, rammer, wetting with water). You can alternate layers in any sequence, taking into account the distance from the base of the MZLF to groundwater.

Footing

A monolithic strip foundation is poured into the formwork, which drains the concrete with a lower underlying layer with high drainage qualities. Together with the liquid, part of the cement will leave, the structure will receive a decrease in strength. Therefore, the following technology is used:

The footing allows you to reduce the protective layer (lower) of the concrete of the MZLF tape to 1.5 - 3 cm, lay a waterproofing carpet on top of it (2 - 3 layers of rolled material). The supporting surface of the foundation increases, the strength of the structure, in which the cement laitance is completely preserved. It is convenient to mount formwork panels on the screed, you can additionally mark the axes. Plastic stands for fittings do not tip over when installing frames.

Reinforcement

With a height of MZLF from 70 cm (together with the basement), it is inconvenient to lay reinforcing cages inside the formwork. Therefore, at first the tape is reinforced, then shields are installed around the frames. Work is carried out in stages:

The ideal option is to bend the bar 90 degrees, run to the adjacent wall, overlap with the next bar. The adjacent rod in the same row, on the contrary, is launched from another wall so that the joints are located at a minimum distance of 60 - 80 cm. The lower, side protective layers are created by polymer parts put on bars at intervals of 0.7 - 1 m. Bars of one wall laid in the corners on the rods of another without bending are considered to be an interruption of the reinforcing belt even when connected by welding.

Longitudinal rods are made of A400 reinforcement with a periodic section of 8 - 16 mm. Anchors, transverse, vertical rods, clamps made of smooth 6 - 8 mm A240 reinforcement.

formwork

Therefore, at small depths of MZLF, it is recommended to mount formwork panels to the entire height of the tape with a margin of 7 cm. When installing the upper edge of the shield above the design mark, the concrete is guaranteed not to splash out during vibrocompaction, leveling.

Depending on the height of the foundation, formwork panels are made of edged boards or plywood. This will allow the use of lumber after stripping at the stage of manufacturing partitions, roofing.

In the absence of an underground (floors on the ground), ventilation ducts in the MZLF tape are not needed. If beam ceilings are used, pipes must be installed in the formwork panels. The total area of the vents should be about 1/400 of the size of the basement.

concreting

The mixture is laid inside the formwork in layers, in one direction. The thickness of the layer depends on the size of the internal vibrator head used for compaction. The normal quality of the rammer is evidenced by the absence of large gravel, air bubbles on the surface, and the presence of cement laitance.

It is forbidden to drop concrete from a height of 1 - 1.5 m, the gaps inside the formwork are more than 2 mm. For MZLF, small amounts of work are characteristic, which allows you to fill in the tape for the reception. If staged concreting is planned, vertical partitions are mounted in the middle thirds of the straight sections. After pouring, the first three days require a wet compress (sawdust constantly wetted with water) or watering from a watering can on the surface.

Protection of MZLF from moisture, soil heaving

After stripping, concrete structures are protected from moisture, swelling in several ways:

The most effective are volumetric or complex waterproofing. In the first case, concrete is modified with special additives during mixing or impregnated with Penetron to impart water-repellent characteristics. In the second option, concrete structures are treated with a primer, coated with mastics, pasted over with films, rolls, membrane materials.

Today, when the popularity of building individual residential buildings has increased incredibly, the device of a shallow strip foundation has become the most relevant. In fact, when erecting low-rise buildings made of light material: wood, timber, frame-panel panels, it is not advisable to use a monolithic foundation, which must be located below the freezing level of the soil.

Device Technology

Of course, every home owner wants to have not only a house that will stand for many years, but also wants to save money when building such a building. For light, one-story buildings, it is quite acceptable to arrange a base that is located above the level of soil freezing. With sufficiently high-quality insulation, such a foundation will fully provide heat and the required strength.

A strip base of this type, despite the fact that it may be located in the thickness of the cut soil, must be at least 50 cm above the groundwater level.

The advantages of this type of bases include the following:

But still, it should not be used on soils that have increased swelling. In any case, before proceeding with direct actions to build a shallow foundation, you need to know some of the nuances.

during the construction of the base, it is necessary to provide for drains that will help prevent excessive accumulation of water from it.
if you plan to mix the concrete mortar yourself, then you should definitely make sure that all layers of the foundation are poured immediately. It is not necessary to fill the foundation in layers for several days, because of this, a butt joint is formed between the layers, leading to loss of strength.
to prevent loss of moisture from the solution, care should be taken that the top of the base is covered with a film, and a pillow of sand or gravel must be arranged under the foundation.

Construction works

The technology of a shallow strip foundation, which is used in construction, begins with a preliminary marking of the trench. Usually used for shallow foundations, the depth of the trench is 70 cm, and the width is 80 cm. When it is dug, a layer of sand is poured to the bottom. Here it is desirable to use material of large fractions. The thickness of the sand layer is about 20 cm. After backfilling, the material must be moistened with water and carefully compacted.

The so-called "cushion" layer is arranged under a shallow monolithic foundation in order to partially replace heaving soil with it. Sand is practically not subject to heaving, and even if it is oversaturated with moisture, it will not cause critical deformations of the foundation itself during freezing.

After the sand is poured and rammed into the prepared trench, the formwork is made. For this, ordinary, not thick boards are suitable. Small shields are made of them, which are lowered into the trench. But there is a small nuance here: it is necessary to install formwork panels strictly according to the level, both in the vertical and in the horizontal plane.

In order for the dry boards of the formwork panels not to draw water from the concrete mortar, they must be lined from the inside with roofing material or thoroughly moistened with water.

Reinforcing cage

In order for the foundation for the building to withstand a large load, it was a solid and monolithic structure. Be sure to lay inside the base a metal skeleton welded from reinforcement bars.

For this, a rod having a diameter of 16 mm is suitable. They are welded together in the form of a lattice or tied with knitting wire. If you choose which option is better, then welding is preferable, since everything must be fixed rigidly.

The metal frame is assembled near the trench, and then lowered into it, checking the evenness using the building level.

Pouring concrete mortar

The device of a shallow strip foundation obliges to pour a concrete solution in such a way that it forms as many air voids as possible.

That is why the solution is ideally better to fill in several layers. The thickness of each layer is approximately 20 mm. After pouring, the concrete is carefully bayoneted, removing excess air. Experts recommend protecting the foundation from the outside, by installing special blind areas.

Shallow foundation on heaving soils

In order for a shallow foundation on very heaving soils (clay, loamy soils), you should take care of the drainage system of the base. To do this, drill wells in the amount of two pieces. The distance between them is maintained at least 2, but not more than 3 meters. The depth of the well in the maximum size is not limited, but the minimum must correspond to the value of the value by which the soil freezes through.

Crushed stone is poured into the drilled well, then a pipe is placed, which is also covered with crushed stone from the inside. A formwork is placed around the pipe, and a layer of sand is poured into the openings, carefully rammed, then a concrete solution is poured. Reinforcement is necessarily lowered into the concrete mixture.

Drainage and waterproofing

The use of a shallow strip foundation must be compatible with a high level drainage system, due to the shallow location of the foundation.

A trench is dug on the outside of the prepared base. Its depth should reach a layer of gravel or sand of the base itself. At the bottom of the new trench, crushed stone is also poured and carefully rammed. The next step is the installation of blind areas, the width of which varies from 50 to 100 cm. A pipe is often used, but special material that is not subject to decay, such as acrylic or fiberglass fabric, can also be used.

The task of the drainage system of any foundation is to divert excess water from it, which is formed during the melting of snow and during rains. Dry crushed stone will ensure the dryness of the soil itself, which is under it, which will help to avoid heaving during frost.

Not only good drainage can increase the durability of the base, but also waterproofing. It not only increases the stability of the base from precipitation. Often, a shallow foundation in severe frosts freezes through. Waterproofing and a layer of expanded polystyrene will help to significantly reduce heat loss from the living space.

Coating waterproofing is the least expensive and quite effective method. True, there is one drawback: the surface of the base must be completely dry. Otherwise, the waterproofing layer, which is used as bituminous mastic, will not last long.

In addition, the protection layer must be covered with a dense film or geotextile. This is due to the fact that with a temperature difference, the heaving soil can increase in size, raising the base. Therefore, in order to avoid mechanical damage to such insulation, protection is needed.

Mistakes to avoid

The construction of a shallow strip foundation today has become very popular among private developers. Often, the owners of future houses, in order to save money, try to lay the foundation for the construction on their own. In the process of work, the most common mistakes should be avoided.

The pillow under the foundation should ideally be made of river sand. It should be poured in layers of no more than 20 cm, carefully tamping each.
It is impossible to leave the shallow foundation not immersed in the ground for the winter period. If you feel that you will not have time to complete the construction work on time, then it is imperative to build a layer of protective thermal insulation around the foundation of a temporary nature. For this, sawdust, slag, expanded clay are suitable.
It is impossible to start construction work on the construction of the foundation when the soil has not yet completely thawed. All work should begin only when the groundwater has receded and the soil has completely thawed.
Experts recommend using additional reinforcement to give greater strength to the base for the internal walls of the structure.

Knowing the technology of the foundation device, it is not difficult to build it on your own. The main thing is not to rush and avoid common mistakes.

In private low-rise construction, a shallow strip foundation is used as the most economical and, moreover, reliable construction solution. This type of foundation combines a number of advantages inherent in buried and non-buried foundations, so its use is quite widespread.

What is the difference between a shallow foundation?

To carry out the foundation of a buried structure, a pit is required with a depth below the level of soil freezing. In areas with severe winters, this depth exceeds one and a half meters, so you can’t do without heavy equipment on the site. A shallow strip foundation is a base made along the perimeter of all load-bearing walls, while the depth of the structure usually does not exceed half a meter.

The advantages of a shallow foundation include:

Ease of implementation and low cost of work;
The possibility of building a foundation without the use of construction equipment;
Strength sufficient for the construction of small houses, baths or frame buildings;
The possibility of warming the basement;
A wide range of materials and technologies for its construction: it can be monolithic, made by pouring concrete, or made of concrete blocks or bricks.

Among the shortcomings, it should be noted its insufficient strength for buildings built on heaving soils. This problem can be solved by sand filling and drainage, which will reduce the load from the soil.

A shallow foundation cannot be made on frozen ground, and also left unloaded for the winter, so the time frame for pouring the foundation and building walls is rather short and is 4-5 months in the middle lane.

Materials for making a shallow foundation

The easiest and fastest way to make a shallow foundation is to pour a monolithic concrete base. However, options are also possible: the foundation can be made of concrete blocks or.

For a monolithic foundation, concrete of a grade of at least 200 is used, as well as reinforcement from a metal bar D12-D16. Filling is carried out either immediately or in horizontal layers, there should not be vertical joints in the foundation - this will lead to its destruction.

If the concrete is prepared independently, and it is impossible to prepare large volumes immediately, you can make a shallow foundation from do-it-yourself concrete blocks.

Technology for the implementation of a monolithic shallow foundation

The fertile layer of soil is removed from the site and markings are made using pegs and nylon twine. Carefully measure the angles, compare the distances between the walls with the project. By marking, they dig a trench up to 70 cm deep. In this case, it is necessary to start from the lowest corner of the site, and align the horizontal plane of the trench along it. The walls of the trench must be strictly vertical. If the soil is loose, you can install props.
Geotextiles are laid at the bottom of the trench - a construction fabric that allows water to pass through, but prevents soil particles from entering the sand. The geotextile is laid in such a way that its edges are higher than the planned sand bedding.
The bottom of the trench with the laid geotextile is covered with coarse sand. Backfilling is carried out in layers, with each layer being shed with water and compacted. The height of the sand bed depends on the type of soil. On sandy soils, 20 cm of sand is enough; on saturated clay soils, its layer should be half a meter.
A formwork made of planks is installed over the trench, continuing the walls of the trench. The boards are fastened with nails or self-tapping screws, while the ends of the nails sticking into the formwork should be avoided, otherwise the formwork will then be difficult to disassemble. On the formwork from the inside, mark the level of pouring concrete. When marking, use the building level.
Reinforcing bars are placed in the prepared formwork. The metal bar is cut to the size of the walls in the longitudinal and transverse sections, and with the help of wire knitting they are fixed to the reinforcing mesh. All corners and joints of the walls must be tied with perpendicular reinforcement rods - this will preserve their tensile strength during soil movements. If the height of the foundation is more than 30 cm, several layers of reinforcement are required. Welding cannot be used when attaching a bar for several reasons: a corrosion-unstable area appears at the welding site, in addition, a hardened bar loses its tensile strength during welding and may burst during operation. As a result - a crack in the foundation, gradually transmitted to the wall of the building.
Start pouring concrete formwork. The first layer is poured and the concrete is evenly distributed over the formwork, punching it in several places with a bar to remove air voids. Each subsequent portion of concrete must be poured before the previous one has set, so it is better to pour it in a large team. The very last layer is leveled according to the markup, smoothed with a rule and sprinkled with dry cement with a sieve - this measure leads to a stronger setting of the upper part of the foundation and prevents cracks when it dries.
Concrete maturation lasts 28 days and depends on weather conditions. At the same time, the construction of light buildings can be started in a couple of weeks, for the construction of a brick house, you will have to wait for the entire ripening period.

Technology for the implementation of a shallow foundation of concrete blocks

This technology allows you to perform a shallow foundation in stages, which is important when building a foundation with your own hands. At the same time, concrete blocks can be made independently.

The form for blocks is made of plywood in the form of a rectangular frame with walls, but without a bottom, while it is more expedient to make it into 4-6 blocks, dividing the structure with removable partitions. All parts are impregnated with machine oil - this facilitates the removal of finished blocks. The bottom is a flat surface covered with polyethylene.
Concrete is mixed: take 1 part of cement, 4 parts of sand mixed with crushed stone, expanded clay or other filler. Fiber can be added if desired. The resulting dry mixture is poured with water and mixed, the resulting concrete should be thick enough, but freely fit into the mold.
Fill the form halfway and lay the chopped reinforcing mesh. Fill the form completely, leveling the surface of the concrete. You can also make blocks hollow by inserting glass bottles into the mold.

Products are dried for 2-3 days at positive temperature, after which they are taken out of the mold and dried for another 2 weeks.

Laying the foundation as a whole does not differ from a monolithic one, the only difference is that the formwork above the soil surface is not performed, a monolithic base is poured from concrete in the ground, reinforcing it with a bar, and the ground layer is laid out from concrete blocks in several rows, using ordinary cement for masonry solution.

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