A slotted reinforced concrete foundation is called a monolithic strip reinforced concrete foundation of rectangular cross section, a feature of which is the laying of concrete directly into a dug trench - "into the thrust" of the soil. They are usually made in bound clay soils; they are not used in sandy soils, since the walls of the trench will crumble in them. The basement can be made as a single structure with a foundation or separately - from brick or block masonry (Fig. 1 a, b). In the first case, the formwork is set from the ground surface to the height of the base.

Slotted foundations are more economical than traditional ones, arranged in trenches using formwork (Fig. 1c). Therefore, they are more attractive in the construction of low-rise buildings. Until recently, only structures laid below the estimated freezing depth were used.
In traditional strip foundations, the load from the house to the foundation is transferred through the sole. Backfill soil resistance is not taken into account in the calculations. When constructing slotted foundations, due to the uneven sides of the trenches and dense (with vibration compaction or bayonet) concrete laying, a good adhesion of the side surface of the structure to the soil is obtained, which can take a significant part of the load from the house. Therefore, to obtain economical structures, the calculations take into account the resistance of the soil both along their sole and along the side surface. As will be shown below, this is not achievable in all soil conditions.
Slotted foundations laid below the freezing depth are calculated on the basis of sediment deformations and on resistance to the effects of tangential heaving forces.
When using shallow slotted foundations in heaving soils, in addition to the above calculations, it is necessary to perform a calculation according to the allowable heaving deformations. If the dimensions of the soles of slotted structures are determined by the permissible soil resistance calculated on the basis of physical and mechanical characteristics, then precipitation will be within acceptable limits and a separate calculation is not required.
Since the vast majority of construction sites are represented by heaving soils, for deep slotted foundations under low-rise buildings, the main thing is the calculation for stability, and for shallow ones - the calculation for stability and heaving deformations.
For buried structures, stability is ensured by the excess of the design load from the house over the maximum total tangential heaving forces (Fig. 2, curve 2). In this case, heaving deformations are equal to zero.

For shallow foundations, heaving deformations should be equal to zero when the soil freezes to the depth of their soles. Stability in this case is ensured at much lower total heaving forces than in buried foundations.

Patterns of interaction between slotted foundations and heaving soils

Soil freezing starts from the surface. As the freezing front advances into the soil in heaving soils along the side surface of the foundations, tangential heaving forces arise, the specific values ​​of which increase with decreasing air and soil temperatures (Fig. 2, curve 1).
The cementing component in the soil is ice, the amount of freezing of which with the concrete surface depends on the temperature of the soil. For example, in the Moscow region, negative mean monthly temperatures reach a maximum in January (Fig. 2, curve 3). In the same period, the specific tangential forces reach their maximum value. Further, with a decrease in the average monthly temperature in February, the specific shear forces decrease, but the total forces continue to increase for some time due to an increase in the freezing depth, and then also decrease (Fig. 2, curve 2).
If the calculated loads from the house are equal to or exceed the calculated total tangential heaving forces, then the foundation will be stable, and the heaving deformations are equal to zero. If the loads from the house are less than the total tangential heaving forces, then the foundation will move along with the soil. In this case, the sole comes off the base, and a cavity forms under it, which causes the accumulation of residual heaving deformations, since soil from the walls of the trench can get into it during the spring settlement of the house. The foundation in the spring may not return to its original position even if the load from the house is less than the friction forces of the soil. This phenomenon is often observed when using recessed slotted foundations for low-rise buildings built on heaving soils.. In all cases, the upward movement of the building indicates instability and, consequently, the unreliability of the foundation.
If the slot foundation is made in the form of a spatial rigid frame and the bending resistance of the cross section is sufficient to preserve the above-foundation structures, then damage to the masonry of walls in brick houses or in houses built from other masonry materials does not occur during heaving deformations. However, a roll of the whole house is formed, which can increase over the years.
When using shallow slotted foundations, the stability of the building is ensured by choosing the appropriate laying depth (Fig. 3b), and the permissible heaving deformations are ensured by arranging an anti-heaving pillow in the trench under the foundation. As a result, significant savings in concrete are obtained.
However, it should be borne in mind that as the foundations are deepened, it may be necessary to increase the width of their supporting part. In this case, the base can be left the same width (see Fig. 3 b).
If the groundwater during the production of work is located above the freezing depth, then it will not work to arrange a reliable foundation by tamping the anti-flood cushion. Therefore, the trench should be developed with a depth of 10...20 cm above the water level, and allowable heaving deformations should be ensured by widening the trench. That is, in this case, they proceed to the construction of conventional shallow foundations.

Design features of slot foundations

The load from the house is perceived by the soil along the side surface of the foundation and under its sole. If the base soils are non-rocky, then the allowable load on the foundations can be calculated as the sum of the calculated soil resistances. If the soils are slightly heaving, then the allowable load on the foundations should be taken only according to the calculated soil resistance under the sole. If the soils are medium or strongly heaving, then the allowable load should be taken according to the calculated resistance of the soil under the sole, taking into account the increase in the load on the foundations due to the negative friction of the soil that occurs in the spring on their lateral surface.
This is the first feature design of slot foundations, which requires explanation. In the spring, when the expanded soil thaws, the process of its consolidation (compaction) and subsidence begins. Due to the increased roughness of the side surface, part of the soil hangs on the foundations. The so-called negative (negative) friction appears, the general method for determining which is set out in SNiP 2.02003-85 "Pile Foundations", p.p. 4.11-4.13. The overall load on the foundations increases.
Such interaction of foundations with the soil lasts only a short time in the spring, but it occurs from year to year and can cause increased settlement of foundations.
Second feature, which should be taken into account when designing slotted foundations, is that due to the same roughness of the side surface, the tangential heaving forces increase, which should be taken into account when calculating foundations for stability.
The methodology for calculating strip foundations is described in detail in the article "Stability of the foundations of low-rise buildings in heaving soils" in the magazine "Councils of Professionals", No. 6, 2005, p. 21. Therefore, we note only the difference in the calculations for slotted foundations.

In the general case, the stability condition is determined from the expression:

γ 1 Q f = γ 2 Q d, (1)

where γ 1 , γ 2 - reliability factors equal to 1.1 and 0.9, respectively;
Q d - standard load from the house;
Q f - total tangential heaving forces acting on the side surface of the foundations are determined by the formula:

Q f = τ n k m ω S f, (2)

where τ n - specific tangential heaving forces, are determined according to table 6.10 of SP 50-101-2004 "Design and installation of bases and foundations of buildings and structures", 2005;
k - coefficient taking into account the ratio of the average monthly air temperature during soil freezing to the depth of shallow foundations or to the estimated freezing depth for buried foundations to the negative average monthly maximum temperature for the winter period, for buried foundations k = 1;
m - coefficient taking into account the width of the sinus and the type of soil used for backfilling; for slot foundations m = 1;
ω - coefficient taking into account the thermal regime of the house; for unheated houses ω = 2, for external foundations of heated houses ω = 1, for internal foundations of heated houses ω = 0;
S f - the area of ​​​​one side of the side surface of the foundation, located in the ground.

With an uneven side surface of reinforced concrete foundations with protrusions up to 20 mm, the value of the specific tangential heaving force (τ n) for slot foundations should be increased up to 1.5 times (SP, Table 6.10).
By solving expression (1) with respect to the value of Q d, it is possible to obtain the values ​​of the loads from the house, at which the stability of deep slotted foundations in heaving soils is ensured and, therefore, the possibility of their application. In table. the values ​​of such loads are given at a standard freezing depth of 1.4 m.

Table: Values ​​​​of loads from the house, at which the stability of deep slotted foundations in heaving soils is ensured

* Provided that during construction the heaving soil around the foundations will be protected from freezing.

The experience of long-term calculations of low-rise buildings shows that the range of characteristic loads for all houses is 2.0 ... 14.0 tf / m. In brick two-story houses, loads on individual foundations can reach 18.0 tf/m. As you can see, the area of ​​reliable application of deep slotted foundations in heaving soils under low-rise buildings is significantly limited.

Conditions for the reliable use of slotted foundations

1. The vertical walls of the trenches should not collapse until the end of the concrete placement.
2. The groundwater level during the execution of works must be below the bottom of the trenches. If, as a result of past rains, puddles have formed at the bottom of the trenches, they must be scooped out. If the soil in these places has come into a fluid or fluid-plastic state, it must be cut to the level of its original state.
3. Recessed slotted foundations are applicable in terms of stability under all houses, regardless of the thermal regime of the house in non-rocky soils, as well as under brick heated houses in 2 (and above) floors in slightly heaving soils. In all other cases, according to the condition of reliability, under low-rise buildings in heaving soils, deep slotted foundations are not applicable. Contact phone 353-55-75

L. Ginzburg, candidate of technical sciences, magazine "Dom" No. 10/2006