- Scope of MZLF
- Section and configuration calculation
- Excavation and preparation
- Reinforcement and anchoring
- Concrete works
- Waterproofing and foundation maintenance
One of the simplest bases for buildings is considered to be a shallow strip foundation. Despite the ease in the production of work, there is always the possibility of doing something wrong, therefore, before setting up the MLF, you should familiarize yourself with the general technology. Today we will take a step-by-step look at the device of a shallow tape.
Scope of MZLF
Shallow foundations are used for buildings with a low loaded mass. As an example, we can cite houses on a steel or wooden frame, as well as buildings made of lightweight elements (foam concrete, PCB). Usually, the number of storeys of buildings on the MZLF does not exceed two.
In the definition of MZLF, shallow – means completely located in the frozen soil layer, the underground part of the concrete tape rarely exceeds 500-700 mm. The forces of frost heaving with this arrangement do not create tangential (breaking) loads, but the entire building, along with the foundation, dynamically moves following the expansion of the soil. For this and other reasons, it is not recommended to arrange a shallow tape in areas with a general relief slope of more than 2%. On steeper slopes, a shallow foundation can only be built after replanning the soil to form a horizontal terrace.
The expediency of using MZLF lies in a much lower consumption of materials and ease of design. If a basement floor is not planned for the building, the shallow tape will reduce the volume of concrete mix and reinforcement by 2–3 times, while providing an equivalent bearing capacity.
However, it is worth noting that it will not be possible to establish a shallow foundation on loose peat, silty soils and mud sandy loam. Such soils have too low density and high plasticity, therefore, they require the construction of pile-grillage foundations based on denser soil layers. You should not arrange a MZLF on soils with heaving indicators of more than 4% or if the GWL is above the burial depth despite the fact that drainage of the site is not planned so that afterwards you do not have to deal with the consequences.
Section and configuration calculation
Since the MZLF acts like a beam and not a stiffener, usually the section of the tape is close to a rectangle or trapezoid in shape. Tape in the form of a tee or more sophisticated sections is almost never cast due to the fact that the material savings seem too small compared to installing more complex formwork.
The calculation of the MZLF is carried out in two directions: sufficient bearing capacity of the soil in the plane of occurrence and its own structural strength, which will allow the tape to maintain its rigidity at full design load from walls, roofs, snow, etc..
The width of the upper part of the foundation is determined by the maximum possible wall thickness, taking into account the layer of interior and facade finishing. When constructing floors on logs, it may be necessary to form a ledge or expand the foundation of the order of 50 mm.
The width of the tape in the plane of occurrence is determined entirely by the required bearing capacity. It is enough to divide the total mass of the building and calculate the average load for each meter of the belt perimeter, and then calculate the sufficient cross-sectional area of the support in accordance with the characteristics of the soil. To create a sufficiently high safety factor, the thickness of the incompressible bed is not taken into account.
The scheme of a shallow strip foundation: 1 – parent soil; 2 – backfilling with gravel or gravel-sand mixture; 3 – blind area; 4 – foundation reinforcement; 5 – shallow strip foundation with a wide base for even distribution of the load; 6 – wall; 7 – gravel bedding of the inner area of the foundation of the house
The height of the tape is defined as a component of its underground and aboveground parts. Everything is simple with the above-ground part – it should be at least 80 mm and no more than four times the width of the upper edge of the tape. In turn, the height of the underground part can be determined taking into account several factors:
- the foundation should not lie on the border of dissimilar soils;
- the minimum depth of the foundation is 35–40 cm, but, depending on the intensity of heaving and the depth of freezing, the height of the underground part may increase by an additional 60–80%;
- to maintain the required strength characteristics of reinforced concrete products, the ratio of width to height should be no less than 3: 5.
Excavation and preparation
The profile of the trench for the MZLF device should have a width 2.5 times greater than the estimated width of the tape and a depth greater than the height of the underground part by two widths. This is due to the fact that MZLF is rarely arranged on soil formwork, using a board-board formwork for reasons of containing cement milk and the need to give the section a trapezoidal shape. Immediately, we note that the indentation of the pit walls from the formwork should be twice as large from the outside as from the inside..
Compensation of frost heaving forces is carried out due to an incompressible, non-heaving and hygroscopic bedding, as well as filling the lateral sinuses with similar material. A sand-gravel mixture with coarse sand and granite or basalt crushed stone of fraction 25-30 is used as a filling material. To stabilize the foundation, the prepared trench bottom is covered with a 30-50 mm preparatory layer of concrete M 100 without reinforcement.
Backfill at the bottom of the trench helps to distribute the load on the support layer of the soil, increase the bearing area and attract forces with a horizontal vector of application. The recommendation regarding the thickness of the backfill, equal to two values of the thickness of the tape, is rarely observed in practice, more often on slightly heaving soils, they are limited to preparation of 25-30 cm.
However, you must remember that the more pronounced the heaving, the more responsibility is placed on the bedding. Sometimes it is advisable to replace the soil up to the depth of freezing and expand the external sinuses to the shape of an inverted wedge, the base of which corresponds to the width of the blind area.
Reinforcement and anchoring
For MZLF, the total content of steel reinforcement without prestressing is set at least 0.1%, a more realistic indicator of 0.17-0.2% will provide proper reinforcement without excessive strength, but with a significant margin of safety.
The minimum value of the protective layer for the underground part of the foundation is 60 mm, the maximum is no more than half the width of the tape. Working reinforcement is made by bars with a periodic profile of such a diameter that the total section of the reinforcement can be divided into 4 bars for the upper and lower reinforcement lines.
If in MZLF the vertical distance between reinforcement lines exceeds 450 mm, add another row with rods, the thickness of which is at least 60% of the thickness of the main lines.
Structural reinforcement is made with clamps or wire for tying in increments of 2-2.5 times the average width of the foundation. The diameter of the rods used for the manufacture of structural reinforcement should not be less than 50% of the diameter of the working reinforcement.
In addition, the MZLF reinforcement is accompanied by a number of anchors. At bends and T-shaped joints of the tape, each row of reinforcement in intersecting directions must be connected with bent embeds of the same section, the overlap of which with the main reinforcement is defined as 25 nominal reinforcement diameters. Anchoring with embedded studs may be required to bond to the base of the frame or wall masonry.
Before carrying out concrete work, it is recommended to cover the inner cavity of the panel formwork with a polyethylene film, which prevents the loss of liquid from the concrete mass until it sets. After that, the reinforcement segments are installed, their alignment and distance using plastic plugs.
MZLF is poured with concrete of grade 350-450 in accordance with the calculation of concrete products for structural strength. The filler should be used heavy, fraction – no more than a tenth of the smallest linear size of the tape.
When pouring concrete, it is pinned in parallel, and then vibration shrinkage. Fortunately, the dimensions and low density of the tape reinforcement do not create obstacles to the spreading of the concrete mixture.
It is allowed to remove the formwork in 10-12 days after pouring, concrete gains full strength in 4 weeks. However, for MZLF, shrinkage is recommended for a year before starting the erection of walls with masonry, while frame structures can begin to be mounted already after 3 weeks.
Waterproofing and foundation maintenance
After setting, the concrete tape needs periodic moistening for a more even flow of cement hydration. Following the breakdown of the formwork, the concrete is dried, then the coating or pasting waterproofing is applied and the foundation is insulated.
Most often, bituminous mastics are used to protect against moisture, on top of which insulation based on fiberglass or a cheaper roofing material is rolled. If the foundation does not require continuous waterproofing, the remaining film of the hydro-barrier is sufficient..
The sinuses around the foundation are filled with ASG immediately after the waterproofing has dried. Backfilling is carried out in layers of 30–40 cm with careful compaction. After that, it remains only to make a blind area around the house, and the MZLF will be ready for further long-term operation..