Shallow strip foundation: step-by-step instructions for the device. Calculation and pouring of a shallow strip foundation Shallow strip foundation definition
With a depth of the sole of the underground supporting structure of 0.2 - 0.7 m, it is called shallow foundation(MZF) and has a minimum construction budget. There are shallow slabs, poles, tapes. Piles are always sunk below freezing, so they fall into the category of buried foundations by default.
Varieties of MZF
The main problem of the load-bearing structures of buildings operated underground is the frosty swelling of the soil, which occurs when the following conditions are present:
- high percentage of clay
- an abundance of moisture to saturate these grains
- freezing of the soil, leading to an uneven increase in volumes in different areas
Several solutions were used to solve the problem:
To reduce labor intensity, deadlines for the completion of facilities, and the construction budget, a set of measures was created to reduce or completely eliminate swelling in frost:
- insulation - blind areas and various foundation elements to preserve geothermal heat under all load-bearing structures and adjacent soils
Attention: Subject to these measures, it is possible to lay in the project a shallow foundation of a slab, strip or column type, which is indicated in the regulation SP 22.13330 for the foundations of dwellings.
It is customary to call prefabricated and monolithic structures shallow-depth, the sole of which lies at around 0.2 - 0.7 m. Therefore, on soils with a low clay content, drainage and insulation may not be required at all.
Tape MZLF
A strip foundation of small depth is economically justified for especially heavy wall materials. The restrictions for MZLF are:
- soils with low design resistance (silt, peat, dusty sand)
- high groundwater level (less than 1 m from the sole of the tape)
- height difference of more than 1.5 m (between opposite walls of the building)
There are prefabricated tapes from one row of FBS blocks and an armored belt on top of them, monolithic structures with maximum performance by default. The manufacturing technology of monolithic MZLF is as follows:
Attention: On the inner surface of the panels, polyethylene should be fixed to prevent leakage of cement laitance and facilitate stripping. It is better to insulate MZLF from the outside after waterproofing, since only in this case a continuous film layer is created.
Stove floating
In 90% of cottage projects, a shallow slab foundation is used. The technology is considered the most expensive, but provides the maximum bearing capacity of the underground structure.
Despite the high strength of a concrete slab with two layers of reinforcing mesh, serious loads from frost heaving occur on heaving soils. Therefore, modernized versions of the floating slab are popular with individual developers:
Attention: The minimum depth of laying slab foundations is regulated at 40 cm. This top layer is rich in humus, so it is forbidden to support load-bearing structures on it. Organics rot without access to oxygen for 12 - 36 months, after which, soil subsidence is inevitable.
Columnar MZF
A shallow columnar foundation is allowed to be equipped exclusively on dry soils with a groundwater level below 1.5 m from the sole. It is forbidden to erect on slopes, heaving soils. In addition, the poles have low resistance to tipping over, so they must be connected with a grillage:
Attention: For columnar foundations with a deepening above the freezing mark, a set of measures is required to prevent swelling - ring or wall drainage, underlying layer + backfilling with crushed stone / sand of the sinuses of the pits.
Construction technology
shallow foundation constructed much faster than a deep foundation. The construction budget is reduced, but conservation in the winter without load is not desirable.
Marking and excavation
Depending on the type shallow foundation marked in different ways:
- the floating slab is contoured with one cord along the outer perimeter of the formwork
- for MZLF poles and tapes, two strings are pulled on the cast-offs for the inner / outer formwork shield
The pit comes off only for the slab, the trench is more rational for the tape, pillars with a rare spacing. If the prefabricated loads from the weight of the building are small, separate pits are torn off for the pillars.
Attention: In any case, access to the surfaces of concrete structures for waterproofing is necessary. Therefore earthen formwork technology is wrong
Drainage and underlayment
A sand/gravel foundation pad replaces the layer of fertile soil excavated at the previous stage. The thickness of the underlying layer is 40 - 80 cm, depending on the geology of the site, the magnitude of the prefabricated loads from the dwelling.
Drainage is built into this layer along the outer perimeter of the blind area at the level of the base of the foundation. To do this, vertical wells are attached to the corners of the cottage, pipes perforated in the upper part (180 - 270 degrees) are laid between them.
Attention: Non-metallic material is laid in 10 - 15 cm layers, each of which is rammed with a vibrating plate. It is not recommended to spill sand with water, as this can lead to erosion of the lower layers of soil, the formation of perched water already in the process of construction.
The width of the foundation cushion made of inert materials should be twice as large as the size of the tape, pillar. Under the floating slabs, the underlying layer is released to the outside by 40 - 60 cm.
Underlayment with waterproofing
A screed made of lean concrete with a thickness of 5 - 10 cm is necessary only for monolithic structures. If a shallow tape, a columnar foundation is constructed from prefabricated reinforced concrete elements, the footing is not used. The screed makes it possible to provide sealed joints of the waterproofing material, to exclude the absorption of cement laitance by the non-metallic material of the underlying layer.
Attention: The width of the footing is equal to the size of the underlying layer, that is, twice the tape, the section of the column.
As a waterproofing, a polyethylene film (minimum 15 microns) is usually used, glued to the footing, or rolled materials (TechnoNIKOL, hydrostekloizol), welded onto the concrete surface. The edges are released to be later launched onto the vertical surfaces of the concrete structures.
Formwork and reinforcement
Due to the variety of structures of shallow foundations, several formwork modifications are used:
- floating slab - boards along the outer perimeter or polystyrene L-shaped blocks
- tape MZLF - outer + inner deck
- pillars - a square of shields for the widening plate and vertical shields for the pillar
For each specified option, different reinforcement schemes are used:
Attention: Reinforcement is located inside concrete structures in compliance with the protective layer. The rods are recessed by 2 - 7 cm, the bottom row is laid on the footing with plastic spacers.
Concreting and waterproofing
Concrete should be laid in layers of 40 - 60 cm at intervals of 2 hours. Therefore, it is desirable to concrete any type of shallow foundations in one go. Waterproofing of accessible concrete surfaces is carried out with penetrating, coating, pasting materials after 70% curing.
Attention: individual elements within the same foundation (porch, tape on which the veranda will rest) are separated by a technological seam. Different operational loads will not lead to the destruction of walls, foundations, if the roof of the extension is independent of the roof of the main dwelling.
Thus, shallow foundations are not difficult for an individual developer. However, it is necessary to take into account the above recommendations to ensure the maximum possible resource of the building with a minimum construction budget.
For the conditions of Russia, the definition of the most reliable foundation design is of particular relevance in conditions where soils with the property of frost heaving lie at the base.
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.
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, described by us in the article "", 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 pillow is determined by calculation (see section "").
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 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.
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 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 must be done subject to 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 "";
- 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.
Initial data:
- 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.
- q1 = Pc x hc + Pbr x he + Pper x L/2
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:
- q2 = Pc x hc + Pbr x he + 2 x Pper x L/2
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.
We get:
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.
Table A
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.
Table B
We get for two sections:
- b1 = q1 / R = 1.72 / 22.8 = 0.07 m
- b2 = q2 / R = 2.15 / 22.8 = 0.09 m
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:
t \u003d 2.5 x bx [ (1 - 1.2 x Rx b) / q]
where: R is the strength of the underlying soil (refractory loam R = 22.8 t/m²), determined from Table B given here.
another formula:
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
Table B
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.
Table D
Note: same as for table B.
For a quick calculation, the MzLF calculator located here is intended.
Expert opinion
The manifestation of frost heaving forces is possible only if clay or sandy soils occur at the construction site, excluding rocky and coarse-grained soils, gravelly and coarse sands under the following conditions - a sufficient depth of winter freezing, which should be at least 0.5 m, close presence of groundwater from the sole of the foundation and other possibilities of moistening or soaking the soil.
When deciding on the use of MzLF, you should carefully analyze all the conditions. The design of the foundation - the width of the pillow, reinforcement, the thickness of the anti-rock pillow must be accurately calculated.
Use waterproof materials:
- Liquid polyurethane foam.
- Penoplex.
- Penofol.
- Foamed polyethylene, etc.
The choice of the most suitable material is determined by the situation and the capabilities of the owner. It is recommended to install the material both outside and inside the tape on all surfaces (except horizontal ones).
Final stages of work
The final stages are the backfilling of the sinuses and the filling of the blind area. Backfilling is done with absorbent material, ideally clean river sand. When moisture appears in the trench, it immediately passes to the bottom and is removed by the drainage system. Backfilling is carried out both from the outside and from the inside, since the appearance of groundwater is possible from everywhere.
The blind area is performed to protect the trench from rain or melt water flowing down the walls. Water flows along the concrete strip to the adjacent soil, where a drainage tray is equipped to drain wastewater into a drainage well.
These elements should be treated with the same attention as all the others, their creation cannot be neglected.
Useful video
In this video you will learn how the MZLF is installed:
Conclusion
A shallow strip foundation is a good option for building a private residential building with a small number of floors. It allows you to get a reliable and solid foundation without unnecessary labor and money.
The only condition for success is a thorough survey of the site and a preliminary calculation of the parameters of the tape in order to avoid errors and dangerous consequences.
In contact with
A shallow-depth strip foundation (hereinafter referred to as MZLF) is one of the types of strip foundations, which is characterized by a slight deepening, much less than the depth of soil freezing, and a relatively small consumption of concrete mix. This article discusses the main advantages and disadvantages of the MZLF, the most common mistakes in their construction, a simplified calculation method suitable for private developers (non-professionals), recommendations for building a foundation with your own hands.
The main advantages of MZLF are:
- efficiency - the consumption of concrete is much lower than in the construction of a conventional strip foundation. It is this factor that most often determines the choice of this technology in low-rise construction;
- reduced labor costs - less earthwork, less volume of prepared concrete (this is especially important when it is not possible to pour the finished mixture from the mixer);
- smaller tangential frost heaving forces due to the reduced area of the lateral surface of the foundation.
However, during the construction of the MZLF, it is necessary to strictly observe the technology, a frivolous attitude to the process can lead to cracks, and then all of the above advantages, as they say, will go down the drain.
The most common mistakes made with the MZLF device:
1) the choice of the main working dimensions of the foundation without any (even the most simplified) calculation at all;
2) pouring the foundation directly into the ground without sprinkling with non-porous material (sand). According to fig. 1 (on the right) it can be said that in the winter season the soil will freeze to concrete and, rising, drag the tape up, i.e. the tangential forces of frost heaving will act on the foundation. This is especially dangerous if the MZLF is not insulated and a high-quality blind area is not equipped;
3) incorrect reinforcement of the foundation - the choice of the diameter of the reinforcement and the number of rods at your discretion;
4) Leaving the MZLF unloaded for the winter - it is recommended that the entire cycle of work (construction of the foundation, erection of walls, and arrangement of the blind area) be performed one construction season before the onset of severe frosts.
Calculation of a shallow strip foundation.
The calculation of the MZLF, like any other foundation, is based, firstly, on the value of the load from the weight of the house itself and, secondly, on the calculated soil resistance. Those. The ground must support the weight of the house, which is transferred to it through the foundation. Please note that it is the ground that holds the mass of the house on itself, and not the foundation, as some believe.
If, if desired, an ordinary private developer can still calculate the weight of the house (for example, using our online calculator located), then it is not possible to determine the calculated soil resistance on your site on your own. This characteristic is calculated by specialized organizations in specialized laboratories after geological and geodetic surveys. Everyone knows that this procedure is not free. Basically, architects who make a project at home resort to it, and then they calculate the foundation based on the data received.
In this regard, it makes no sense to give formulas for calculating the size of the MZLF within the framework of this article. We will consider the case when the developer is building on his own, when he does not conduct geological and geodetic surveys and cannot accurately know the calculated soil resistance on his site. In such a situation, the dimensions and design of the MZLF can be selected from the tables below.
The characteristics of the foundation are determined depending on the material of the walls and ceilings of the house and its number of storeys, as well as on the degree of heaving of the soil. How can you determine the latter is described
I. MZLF on medium and strongly heaving soils.
Table 1: Heated buildings with walls made of lightweight brickwork or aerated concrete (foam concrete) and with reinforced concrete floors.
Notes:
- the number in brackets indicates the material of the pillow: 1 - sand of medium size, 2 - coarse sand, 3 - a mixture of sand (40%) with crushed stone (60%);
- this table can also be used for houses with wooden floors, the margin of safety will be even greater;
- options for foundation designs and reinforcement options, see below.
Table 2: Heated buildings with walls made of insulated wood panels (frame houses), logs and timber with hardwood floors.
Notes:
- the numbers in brackets indicate the same as in table 1;
- above the value line for walls made of insulated wooden panels, below the line - for log and timber walls.
Table 3: Unburied foundations of unheated log and timber structures with wooden floors.
Notes:
- above the value line for log walls, below the line - for walls made of timber.
Design options for MZLF on medium and highly heaving soils, indicated in the tables with letters, are shown in the figures below:
1 - monolithic reinforced concrete foundation; 2 - sand filling of the sinuses; 3 - sand (sand-gravel) pillow; 4 - reinforcing cage; 5 - blind area; 6 7 - waterproofing; 8 - plinth; 9 - ground surface; 10 - sand bedding; 11 - turf.
Option a.- the upper plane of the foundation coincides with the surface of the earth, the plinth is made of bricks.
Option b.- the foundation protrudes 20-30 cm above the surface, forming a low base or being part of the base.
Option c.- the foundation rises 50-70 cm above the ground, while it is also the base.
Option g.- non-buried foundation-basement; Table 3 shows that such foundations are used for unheated wooden buildings.
Option d.- used instead of options b. or V. when the width of the sole of the foundation significantly exceeds the thickness of the wall (by more than 15-20 cm).
Option e.- a shallow-depth strip foundation on sandy bedding is used quite rarely on weak (peaty, silty) soils with a high level of groundwater for wooden buildings. Depending on the size of the building, backfilling is done either under each tape, or under the entire foundation at once.
Reinforcement of a shallow strip foundation.
Reinforcement of MZLF is made with meshes of working reinforcement and auxiliary reinforcing wire. The working reinforcement is located in the lower and upper parts of the foundation, while it must be immersed in the concrete thickness by about 5 cm. It makes no sense to place working reinforcement in the middle of the tape (as you can sometimes see on the Internet).
Table 4: Foundation reinforcement options.
The MZFL reinforcement schemes are shown in the following figure:
A.- mesh with two bars of working reinforcement; b.- mesh with three bars of working reinforcement; V.- T-shaped joint; G.- L-shaped corner joint; d.- additional reinforcement of the MZLF with a large width of the sole, when the sole is wider than the base by more than 60 cm (additional mesh is located only in the lower part.
1 - working fittings (A-III); 2 - auxiliary reinforcing wire ∅ 4-5 mm (Вр-I); 3 - rods of vertical reinforcement ∅ 10 mm (A-III), connecting the upper and lower grids; 4 - reinforcement for reinforcing the corner ∅ 10 mm (A-III); 5 - connection with wire twists (the length of the twist is at least 30 diameters of the working reinforcement); 6 - additional working reinforcement ∅ 10 mm (A-III).
II. MZLF on non-rocky and weakly rocky soils.
Shallow strip foundations on non-rocky and slightly heaving soils do not have to be made only from monolithic concrete. You can use other local materials, such as rubble stone, red ceramic bricks. MZLF is laid at 0.3-0.4 meters without a sand cushion. Moreover, for wooden buildings and one-story brick (or aerated concrete) foundations, you can not even reinforce them.
For 2 and 3-storey houses with walls made of stone materials, MZLF is reinforced. Concrete foundations are reinforced according to the 1st reinforcement option (see table 4 above). Foundations made of rubble or brick are reinforced with masonry meshes made of Vp-I reinforcement ∅ 4-5 mm with a mesh size of 100x100 mm. Nets are laid every 15-20 cm.
The structures of the MZLF on non-rocky and slightly heaving soils are shown in the figure below:
1 - foundation; 2 - plinth; 3 - blind area; 4 - waterproofing; 5 - draft floor (shown conditionally); 6 - mesh of wire reinforcement, 7 - reinforcement according to the 1st option (see tab. 4)
Options a. and b.- for wooden and one-story brick (aerated concrete) buildings.
Options in. and Mr.- for two- and three-story brick (aerated concrete) buildings.
The width of the sole b is determined depending on the number of storeys of the building and the material of the walls and ceilings.
Table 5: The values of the width of the sole of the MZLF on non-rocky and low-rocky soils.
Stages of construction of a shallow strip foundation and recommendations.
1) Before proceeding with the construction of the foundation, if necessary, it is necessary to ensure high-quality drainage of surface rainwater from neighboring areas from the building spot. This is done by cutting drainage ditches.
2) The foundation is marked and trenches come off. It is recommended to start earthworks only after the delivery of all necessary materials to the construction site. The process of extracting the trench, pouring the tape, backfilling the sinuses and constructing the blind area is desirable to organize continuous. The less time it takes, the better.
3) Dug trenches are covered with geotextiles. This is done so that the sand cushion and the sandy backfill of the sinuses do not eventually become silted up with the soil surrounding them. At the same time, geotextiles freely pass water and do not allow plant roots to germinate.
4) In layers (layers of 10-15 cm), a sand (sand-gravel) pillow is poured with careful tamping. Use either manual rammers or platform vibrators. Do not take ramming lightly. Shallow foundations are not as strong as foundations filled to the full depth of freezing, and therefore a freebie here is fraught with the appearance of cracks.
5) The formwork is exposed and the reinforcing cage is knitted. Do not forget to immediately provide for the supply of water and sewer to the house. If the foundation is also a plinth, remember about the ventilation (does not apply to buildings with floors on the ground).
6) Concrete is being poured. The filling of the entire tape must be carried out continuously, as they say, in one go.
7) After the concrete has set (3-5 days in summer), the formwork is removed and vertical is made.
8) The sinuses are backfilled with coarse sand with layer-by-layer tamping.
9) A blind area is being built. It is advisable (especially with a low height of the foundation tape) to make the blind area insulated. This measure will further reduce the frost heaving forces affecting the MZLF in winter. Insulation is made with extruded polystyrene foam.
As mentioned at the beginning of the article, it is not allowed to leave the MZLF unloaded or underloaded (the building was not fully built) for the winter. If nevertheless this happened, the foundation itself and the soil around it must be covered with any heat-saving material. You can use sawdust, slag, expanded clay, straw, etc. It is also not necessary to clean the snow on the building spot.
It is highly discouraged to build a shallow strip foundation in the winter season in frozen ground.
In the comments to this article, you can discuss with readers your experience in the construction and operation of the MZLF or ask questions that interest you.
For buildings with walls made of small-format materials (block, brick), a shallow strip foundation is the best solution. In terms of the cost-effectiveness of the construction budget, it is in third place after pile and columnar foundations. Provides a multiple margin of safety, allows you to make an exploitable basement level.
Step-by-step instructions for arranging a shallow strip foundation (MZLF)
On heaving soils, it is recommended to insulate the sole of the MZLF tape, on sands, sandy loams, you can do with the insulation of the blind area. Therefore, the technology for the worst geological conditions will be considered below. In order to build a shallow strip foundation on their own, the developer needs to perform the following steps:
- calculate the width of the tape, the cross section of the reinforcement, draw up a reinforcement scheme;
- make trenches (cottage without basement) or foundation pit (ground floor);
- lay drains for drainage, insulate the sole;
- make a sub-base, mount the formwork, lay reinforcement;
- concrete the tape, not forgetting about the communication input nodes, ventilation ducts;
- provide care for concrete, waterproof all edges of the tape in a convenient way after stripping;
- for an operated basement, it is necessary to insulate the outer walls of the tape.
Then it remains to insulate the blind area, line it with waterproof material, integrating a storm drain into the outer perimeter. At each stage, there are nuances, without knowing which you can drastically reduce the resource of an underground structure.
Geology, geodesy and calculation
To build a shallow strip foundation with your own hands with a high operational resource, it is not necessary to order geological surveys. It is enough to dig holes in several places, to determine the composition of the soil at different depths visually:
- if it rolls into a tight ball that can withstand being squeezed by fingers without breaking, it is clay;
- loam in a similar situation will be covered with a network of cracks;
- the sandy loam will partially crumble;
- it is impossible to roll sand into a ball in your hands.
Each specified rock corresponds to its own design resistance (kg / cm2), which is necessary to calculate the bearing capacity of the foundation. It is taken from tables:
- loam - 1.8 - 2.8;
- pebbles with dusty clay - 4 - 4.4;
- sandy loam - 2 - 3;
- crushed stone with sand - 6;
- clay - 1 - 2 (watered), 2 - 3 (plastic), 3 - 5 (medium dense), 4 - 6 (dense);
- gravel with sand - 5;
- sand of different fractions - 3 - 5;
- dusty, wet sand - 2 - 3.
The minimum value of the design soil resistance is observed for silty sand (1 kg / cm2, uncompacted slag dumps, sand, ash, industrial waste. If the developer is not sure about the composition of the soil, he can play it safe by assuming the design resistance to unity on a normal site.
Or 0.8 units if it is suspected that the soil in the building spot is bulk, there is no self-consolidation due to the period of the embankment being less than 24 months. This is guaranteed to provide a solid safety margin for a high operational resource of the foundation.
Calculating the width of the tape is not difficult:
W = combined load / design resistance / length of the perimeter of the tape
The prefabricated load is calculated by adding the weight of all structures (foundation, ceilings, roofing, walls, partitions, cladding, insulation), furniture, residents, snow, wind load. The last values are also taken from the SP tables for a particular region of operation. Practice shows that, taking into account the thickness of the walls, a width of 40 - 50 cm is enough for an MZLF tape with a double margin of safety on the most problematic soils.
The depth of the MZLF is selected as follows:
- 0.4 m - if protection against heaving forces is provided;
- 0.45 m - on sandy soils with a low level of GWL;
- 0.5 m - when clay freezes by 1 m;
- 0.75 m - at a freezing mark of 1.5 m;
- 1 m - in regions with a freezing mark of 2.5 m.
To provide protection against winter swelling, the following technologies are used:
- replacement of the soil under the sole of the tape with non-metallic material (0.4 m minimum);
- drainage system around the perimeter, blind area and storm water on the surface;
- insulation of the MZLF sole and blind area.
Drainage removes moisture, soils are not saturated with water, which reduces heaving forces. A layer of insulation allows you to save the geothermal heat of the bowels, stop freezing from the outside from the contact of the soil with cold air. Therefore, if you include in the estimate insulation, non-metallic material for the foundation cushion, drainage sewer pipes, the budget of the MZLF, buried by 40 cm, will still be less than tapes at the level of 1 m without a heat insulator and drainage systems.
Scheme of insulation MZLF. Instead of foam, it is better to use extruded polystyrene foam.
The height of the tape above ground level is selected depending on the project:
- for an operated basement floor, it is better to deepen it by 1 m, raise it above the surface by 1.7 m, ensuring the ceiling height in the underground is 2.2 - 2.5 m;
- a height of 0.4 m above the zero mark is convenient for the manufacture of a small technical underground, in which communications can be placed, the manufacture of ventilation products (25 cm from the blind area), five steps of the porch;
- concreting flush with the ground (or at a level of +0.2 m) allows you to reduce the construction budget for the manufacture of floors on the ground (no need for a slab, floor beams).
Any of the indicated dimensions allows you to normally lay two layers of reinforcing mesh inside the tape with a protective layer of 1.5 - 4 cm in accordance with the standards.
Marking and earthworks
The fertile soil layer is removed completely, trenches are dug under the pillow.
They begin the device of a shallow strip foundation with markings in accordance with the recommendations. Unlike columnar, pile foundations, not axes are brought into the building spot, but cords are pulled along the outer and inner perimeter of the tape:
- mark of the first corner of the main facade - 3 m from the border of the site, 5 m from the center line of the street (3 m from the axis of the passage);
- the wall of the main facade - the tension of two cords along the cast-offs (bar 60 - 80 cm between two pegs), indicating the outer, inner face of the MZLF;
- side walls - after finding the right angle for each wall using the triangle method (5 m hypotenuse, 4 m, 3 m legs), cords are pulled along the cast-offs for them similarly to the previous method;
- rear facade - two cords in exactly the same way;
- internal walls - similar to the indicated options.
Then it is necessary to mark the foundation pits for separate foundations of the power structures of the cottage, equipment weighing more than 400 kg. These are a porch, an internal staircase, a fireplace, a stove, pumping equipment, a backup generator, etc.
The horizontals of all cast-offs are aligned in a common plane with a level or a laser plane builder. Cords should be located 5 - 7 cm below the upper side of the formwork.
In the considered version of the MZLF with a warmed sole, additional earthworks will be required. The trenches for the tape will have to be expanded by 0.5 - 0.8 m inside the perimeter of the building, 0.8 - 1.2 m outside. This is necessary for laying a heat insulator, making a drainage system, providing access to the outer walls of the waterproofing tape, pasting with polystyrene foam.
For earthworks, it is not necessary to pull the cords, it is enough to draw the contours of the trenches on the soil with lime mortar. If there is a basement, a pit is made, corner cast-offs should not interfere with excavation by special equipment.
Sand or gravel substrate
In different regulatory documents, the thickness of the underlying layer under a monolithic foundation of non-metallic materials has a different value. For example, in the norms it is three times the width of the MZLF, the construction allowance (Sazhin's edition) is regulated by 30 - 80 cm, depending on the soil. In practice, the most commonly used method is:
- 20 cm of sand - spilled with water or compacted with a vibrating plate;
- 20 cm of crushed stone - ramming with a vibrating plate or manual bayonet in layers.
A sand cushion of 20 cm is laid, it must be rammed with a vibrating plate.
There are projects that use only sand (40 cm) or only crushed stone of a similar layer thickness. In other MZLF with solid operating experience, a mixture of PGS was used. In any case, an individual developer needs to know the features of inert materials:
- when wet, sand almost completely loses its bearing capacity;
- crushed stone retains strength, has draining properties (lack of capillary moisture);
- it is easy to lay a layer of rolled material on the sand for waterproofing the MZLF sole (relevant for high groundwater levels);
- the sharp edges of the rubble will definitely damage this waterproofing, so you have to fill in the screed-footing to protect the hydroglass from damage.
On top of the sand, crushed stone (thickness 20 cm) is laid and compacted.
At the same stage, drains are laid along the perimeter of the tape at the level of its sole. They are looped into a common circuit with a single slope to an underground reservoir, where effluents will be collected by gravity.
With a 40 cm deepening of the MZLF, it is possible to completely get rid of the heaving forces by lining the outer plane of the tape with a 5 cm layer of expanded polystyrene, continuing the heat insulator layer horizontally at the level of the sole of the tape. The layer width is 60 cm, the thickness of the insulation is 5 cm. If the foundation lies at a depth of 0.7 - 1 m, the insulation of the blind area at a depth of 30 - 40 cm becomes a more economical solution. In the latter case, heaving forces do not disappear completely, but decrease to an acceptable level.
formwork
This stage of construction of the MZLF is regulated, which provides terminology, instructions for the manufacture of formwork. Most often, shields are made of materials:
- edged board - a minimum thickness of 5 cm, the pros are the reuse of lumber (for example, partitions, roofing), cons - a long assembly of panels;
- multilayer plywood - bakelite, moisture-resistant modifications are too expensive, private developers use multilayer birch sheets;
- OSB - can also be used after demoulding, has moisture resistance, retains geometry.
The only serious requirement for formwork panels is the absence of gaps in the joints of more than 2 mm. The footing is often poured without formwork, using the walls of the trench as it. Formwork installation technology for low-depth tape has several options:
- removable - the upper side of the shields is 5 - 7 cm higher than the design mark, to the inner wall of the decks it can
- attached extruded polystyrene foam;
- non-removable - assembled from polystyrene blocks, jumpers give structural rigidity.
The first option is preferable to the others, since after pouring the surface must be waterproofed. It is better to mount a layer of insulation on top of a waterproofing carpet.
We expose and strengthen the formwork.
Shields are fixed vertically with couplers (step 0.5 - 1 m), jibs. Inside them, at different heights, it is necessary to make holes for the through passage of the sleeves. In the underground part, engineering systems will be introduced through these pipes. Above the ground, these holes will become ventilation vents through which moisture will be removed from the underground.
Reinforcement and pouring
Foundations on heaving soils experience both compressive and tensile loads. Therefore, the tape is reinforced in two planes - at the upper edge and near the sole. However, the rods must be protected with concrete to prevent corrosion. The minimum protective layer is 1.5 - 4 cm. The monolithic foundation should be reinforced with bars of 8 - 16 mm with a periodic section in the longitudinal direction. To impart spatial rigidity to the reinforcing cage, rectangular clamps made of smooth 6–8 mm reinforcement are used.
One of the possible schemes for reinforcing corners and mates.
Contrary to the misconception that thicker rods are needed when prefabricated foundation loads increase, this is not true. The calculation is made according to the cross section of the tape - it should contain 0.1% of the reinforcement of the total area. The calculation of reinforcement can be done on your own in the following way:
- determination of the total section - you need to multiply the height of the tape by the width (in millimeters), divide by 100;
- search for a section of a longitudinal bar - is performed according to a table with reinforcement schemes (for example, when choosing a scheme 2 + 2, a thick bar is required, 3 + 3 is thin, but in larger quantities);
- adjustment - is made taking into account the protective layer (4 cm), the requirements (40 cm maximum between the rods in one row);
- clamps should be a maximum of ¼ thinner than the main rods.
We reinforce the corners according to the scheme presented above.
Armoframe is connected by wire manually or mechanically. Welded meshes are used by professional developers who operate with large volumes of work. Each weld is a potential source of corrosion.
We reinforce the conjugations.
The main requirements for reinforcing MZLF are:
- overlap spacing in adjacent rows of one belt at least 60 cm;
- overlap length 50 rod diameters.
It is forbidden to join the bars at the corners and in the junctions of the walls with a crosshair. In these areas, one of the rods should bend, go to the adjacent side by 40 - 60 cm, and then overlap with the next piece of reinforcement. To provide a protective layer at the bottom of the tape, the lower bars are laid on concrete pads or polymer racks.
- 3.7 / 1.9 / 1 - for grade B 22.5 (corresponds to M 300);
- 3.9 / 2.1 / 1 - for grade B 20 (corresponds to M 250);
- 4.8 / 2.8 / 1 - for grade B 15 (corresponds to M 200);
- 5.7 / 3.5 / 1 - for grade B 12.5 (corresponds to M 150);
- 7/4.6/1 - for grade B 7.5 (corresponds to M 100).
Manual mixing is not recommended, grade M 7.5 is used exclusively for footings. The mixing time of the components inside the concrete mixer is 1.5 minutes. The most demanded modifiers for concrete mixtures that improve their properties.
Do not forget to lay sleeves in the formwork for communications and air, if necessary.
Before pouring, only exfoliating rust, fresh corrosive deposits are removed from the reinforcement, on the contrary, it improves the adhesion of reinforced concrete components. Therefore, professionals often soak the rods in water for 2 - 3 to finish knitting frames.
When concreting, the following requirements must be observed:
- laying the mixture in one direction in layers of 40 - 60 cm for compaction with the nozzle of a deep vibrator;
- filling the formwork in one go or installing vertical partitions, continuing concreting after 75% concrete strength, removing the film from its surface at the junction;
- technological breaks of more than 2 hours between individual concreting operations are not allowed, since after this period hydration begins;
- it is forbidden to dump the mixture from heights greater than 2 m (the level of 0.5 m is considered optimal).
A monolithic foundation should not have technological seams in the corners. If pouring in one step is unrealistic for these volumes, the partition is mounted in the middle third of the span.
The nozzle of the deep vibrator is lowered into the mixture for a few seconds until the appearance of milk, the cessation of air bubbles. When rearranging the working body, the vibration zone should overlap the previous one by a third. The disadvantages of manual baying with a reinforcing bar are the increase in work time.
Concrete care and stripping
The technology of laying the mixture without fail includes the care of concrete in the formwork up to a curing of 50 - 70%. These measures ensure the protection of the mixture:
- from waterlogging - shelter with a film in the rain;
- from freezing - a heating cable, a layer of straw, warm formwork in winter;
- from intensive evaporation - a film or a layer of sawdust on the surface with irrigation with a scattered jet (watering can) in the first three days.
This avoids shrinkage caverns, cracking, internal stresses. Instead of sawdust, tarpaulin, burlap or sand is used. Humidification in the heat begins after 8 hours, maintaining the compress in this state regularly. For example, at a constant temperature of + 30 degrees, stripping is possible on the 4th day, at + 20 degrees on the 8th day, at + 10 degrees in two weeks. In the off-season (average temperature + 5 degrees), stripping is carried out on the 29th day.
The waterproofing technology makes it possible to increase the resource of a partially buried reinforced concrete structure up to 70-150 years. A monolithic foundation is protected from moisture in several ways:
- coating with bituminous mastics in 2 - 3 layers, resource 15 - 30 years;
- pasting with rolled materials on a polymeric, fiberglass basis (2 - 3 layers), a resource of 20 - 40 years;
- impregnation with penetrating mixtures or adding them to concrete during manufacture, resource 50 - 100 years.
After stripping, we apply waterproofing to the side faces of the foundation.
In the first two versions, the surface of the MZLF tape is pre-primed with primers in one layer. Comprehensive protection includes priming, coating with mastics, pasting with hydrostekloizol. This allows you to increase the service life up to 70 - 100 years. Penetron products change the molecular structure of concrete to almost the entire depth. Protection is not afraid of mechanical damage and has the same resource as concrete.
We insulate the outer walls of the foundation to the full height. The insulation is glued to bituminous mastic, it is forbidden to use mechanical fasteners, because. this will lead to the destruction of waterproofing and concrete. Further, it is desirable to insulate the blind area around the foundation according to the scheme given above.
If necessary (high groundwater level) do.
The considered technology is suitable for individual construction. Following the above recommendations, even a non-professional can calculate the cross section of the tape, reinforcement. Step-by-step instructions will allow you to make MZLF without errors, providing a high foundation resource.