Slotted foundation construction technology for a private house
The slotted foundation is one of the clearest examples of the adaptation of a traditional strip foundation to clay soils. The absence of formwork over the entire height of the pour and the reduction in earthworks significantly reduces the cost of building the facility. Slotted foundations are popular for low-rise residential buildings, garages, utility buildings and other buildings.
Structurally, slotted foundations are comparable to monolithic strip foundations, only a trench is used instead of formwork. Outwardly, the trench is somewhat similar to a gap in the ground, hence the name "slit" foundation. The uneven sides of the earthen trench provide strong adhesion between the soil and the poured concrete mix.
The formation of the lower part of the slotted support occurs by means of soil, which acts as a formwork under the base of the foundation. Thus, the loads on the ground from the side of the building are transmitted by all surfaces of the foundation - the supporting plane and side walls, that is, the foundation transfers the full range of loads in the vertical and horizontal directions.
The laying of slotted foundations is carried out in clay soils. By pouring the concrete mixture into the expansion of the trench, a rigid spatial structure is created, which ensures the stability of the structure to weight loads and buoyancy forces of frost heaving. The manufacture of slotted foundations for houses built on sandy soils is not recommended. Sand does not retain the geometric shape of the walls, as a result, crumbling soil sharply degrades the quality of the poured concrete mixture and does not contribute to the creation of a workable foundation monolith.
The advantages of slotted foundations include:
- Significant reduction in the labor intensity of construction work. Statistics say that the transition to laying a slotted foundation reduces the volume of earthworks carried out by almost two times, the volume of work with formwork - up to 60-70%;
- Reducing the cost of concrete - up to 6% and reinforcement - up to 20%;
- The possibility of using trench technologies in cramped conditions when it is forbidden to carry out dynamic impacts on the ground, for example, near communications or near built buildings.
The main disadvantage of slot-type bases is the limitation of its applicability:
- It is allowed to pour only in clay soils to ensure the preservation of the shape of the trench when pouring the concrete mixture and compacting it;
- Use only on non-rocky soils, since high-intensity frost heaving can bulge and warp the erected house, due to the lateral adhesion of the foundation to the ground;
- Massive buildings are not erected on slotted supports.
Interaction of slotted bases with soil
When the air cools during the cold period of the winter season, the process of soil freezing begins. In heaving soils, the following process is characteristic: as the freezing front deepens from the earth's surface into the soil stratum, tangential heaving forces appear that are applied to the side surfaces of the foundations. With a decrease in soil temperature, the values of specific tangents and, accordingly, the total heaving forces Qf increase to almost 30 tf/m. The freezing of the soil into a single whole supports the ice, however, with spring warming, the ice loses its binding properties. As the temperature of the frozen soil decreases, the values of the total forces Qf reach their maximum and then begin to decrease. In the process of changing heaving tangential loads, two variants of events are possible:
- If the impact loads from the side of the built house exceed the values of the Qf indicators, the stability of the support will be observed, the heaving deformation will be zero;
- When the values of Qf exceed the loads from the side of the building, the foundation loses its stability and begins to move upward along with the frozen soil. In this case, the base of the foundation is separated from the soil base with the formation of a three-dimensional mini-cavity under it. In the process of spring subsidence of the building, associated with a decrease in heaving forces, soil from the walls of the trench gets into the resulting cavity. The foundation support can no longer return to its original position. The roll of the whole structure begins, increasing over the years.
Calculation methods
Depending on the depth of laying, slotted foundations are divided into two types:
- Deeply buried - laid below the depth of freezing of the soil;
- Shallow - used on non-rocky soils.
With regard to supports of the slotted tape type, it is necessary to use the instructions of the set of rules “Foundations of buildings and structures. Updated version of SNiP 2.02.01-83*, which regulates the calculations of foundations for two groups of limit states (clause 5.1.2):
- Calculations on the bearing capacity, attributable to the joint venture to the first group of limit states, which included the destruction of the structure, loss of position stability, etc.;
- Deformation calculations assigned by the SP to the second group of limit states, which included unacceptable displacements, etc.
Slotted structures of the foundations of the house, filled below the freezing depth, must be calculated for resistance to heaving tangential forces and sediment deformations. Shallow slotted bases, filled in heaving soils, are additionally calculated from heaving deformations. Reference values of the specific tangential heaving forces are given in Table. 6.10 "Design and installation of bases and foundations of buildings and structures." Based on them, the design load on the foundation is determined to make a decision on the applicability of a slotted strip base.
Construction stages
In the manufacture of slotted bases, the following stages of work are performed:
- Earthworks for digging a trench in accordance with the project;
- Installation of the formwork of the above-ground part to the required level - the future basement of the house;
- Reinforcement in accordance with the project;
- Pouring concrete mix;
Excavation
We dig a trench.
The laying of the trench begins with the removal of the top fertile layer and using it (if necessary) to level the site.
The trench is dug the same width as the width of the foundation. The depth of the trench is determined in the project. The side edges of the trench must be even and not collapse during all preparatory work. If it rained, then the formed puddles must be drained. And the “floated” soil is cut off to a dry layer.
It is allowed to expand the lower part of the trench for the supporting sole of the tape monolith. The device of a sand cushion is not mandatory for monolithic deep foundations, and sometimes it can be harmful. If a sand cushion is laid, vibrocompaction is necessary.
Arrangement of the formwork of the above-ground part
We expose the formwork and strengthen it with side supports.
To prepare the basement of the house, formwork is set up at the height of the basement from the level of the ground surface. It is allowed to manufacture the base as an independent structure of brickwork or block type.
Reinforcement
We lay the reinforcement cage in the trench.
Reinforcement is made with viscous reinforcement. Particular attention is paid to the corners. For more details, see the materials: how to choose.
We strengthen the formwork with additional transverse jumpers from above.
Pouring concrete mix
We pour concrete.
When preparing a concrete mix, it is customary to prepare it at least 10% more than the calculated need, completely filling all the irregularities in the ground with a solution.
The prepared concrete mixture is poured into the prepared trench. The best option is considered to be pouring immediately after preparing the trench, until the drying clay edges begin to crumble. To strengthen the concrete base, a compaction process is carried out, as a result, crushed stone / gravel lie as tightly as possible with the removal of excess water and air. The sealing options are bayonet or vibro seal.
We remove the formwork and remove the fertile layer of soil inside the foundation.
Conclusion
The practice of building light buildings has confirmed the cost-effectiveness of using slotted strip foundations. However, the specifics of the use of this type of foundation, depending on the category of soil, requires highly qualified designers in terms of performing calculations for the stability and deformation of house foundations. Often, builders do not conduct surveys to determine the properties of the soil in a new building, and the foundation design is accepted, reinsured, as for heavily heaving soils, which leads to an increase in the cost of construction. A well-founded solution of a slot foundation will reduce the complexity of construction and shorten the construction time of the house.
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