Preservation of soil condition
Schematic diagram of the structure in section, taking into account the base
This option is appropriate for use if:
- there is a significant thickness of permafrost soil;
- buildings emit large amounts of heat and are small in size.
The calculation and justification of this option was made at the end of the last century. Now it is generally recognized and allows you to make the most of the high building qualities of permafrost soils.
The essence of this principle is to preserve the original state of the soil, both during the construction of the structure and during its subsequent use. This is acceptable given the economic feasibility of keeping the soil in its natural state.
It is easier than anything else to erect foundations on types of permafrost soils that are not plastic-frozen. In the presence of such soils, it is possible to reduce the temperature of the base to the required values. When calculating foundations laid on permafrost soils, the expected destruction and deformation under load are taken into account.
In this case, the main point is to prevent changes in the permafrost layer. That is, do not allow it to change its properties from the heat generated by the building in use. Therefore, the underground is arranged cold and well-ventilated (through the vents in the basement part or in the zabirka).
Features of permafrost soils
Erected foundations on permafrost soils (VG) have their own differences due to the special mechanical properties of the geological foundations. A sign of permafrost is observed during exploration work in ice-filled soil, thicker cover, zones of tectonic displacements.
The bearing capacity of VG depends on the mechanical properties of the so-called "ice cement", changes in temperature cycles and other phenomena. To calculate the foundation on permafrost, it is necessary to carry out a number of geological and permafrost exploration studies.
Permafrost soils are held together by piercing ice-cement bonds, which are elongated veins of ice that pass through the soil mass in both the vertical and horizontal planes. During the onset of the warm season, the ice-cement bonds can partially collapse (simply melt). As a result, the bearing capacity of the subgrade falls significantly. In areas with such conditions, the soil is unsuitable for construction.
Principles of designing the foundations of structures on permafrost soils
Today, designers use two main methods for designing foundations on perennial frozen soils (MM) for calculating foundation foundations on WG.
Method one
The method is based on maintaining the temperature of the water-gas, not allowing the permafrost to thaw. This design method is used for areas with thick layers of perennial frozen soils. The basic principles of the method were developed and implemented in the second decade of the twentieth century. Although many houses and buildings in cities such as Irkutsk, Chita and Khabarovsk were designed and built according to this principle at the end of the 19th century.
This method is based on the following provisions:
- the base of the foundation must be immersed in permafrost to a depth of at least 1 m;
- excavation is made under the foundation in such a way that then the formed sinuses are filled with non-porous soil;
- backfilling along the perimeter of the base of the structure in cross-section represents a trapezoid facing downward with the smaller top;
- construction sites must have an underground height of at least 0.7 - 1 m;
- along the perimeter of the underground, technological openings (air vents) are arranged in the walls for constant ventilation of the room.
Diagram of the structure of the foundation of the building according to the first principle
The purpose of the air vents is that, thanks to the through holes, the underground is constantly ventilated. Air currents carry warm air outside and carry air masses with a low temperature. It turns out a kind of refrigerator that does not allow heat from the house to penetrate into the frozen base. Frozen soil maintains a constant temperature and does not lose its bearing capacity.
As a result of observations over several decades, it was concluded that the permafrost boundary under the building was shifting upward. This was due to the lack of exposure to solar radiation, the vital activity of the active layer (DS). The figure shows how the M.M border changes:
Changing the border of the M.M roof under the building
The stability of a structure designed according to the 1st principle is determined by the formula:
KcQ + K1 (N + q)> K2TF
Q is the force against soil buckling;
N is the total load from the weight of the structure;
T is the degree of freezing of the lateral edge of the foundation with the ground;
q is the load from the building directed to the ground benches;
Kc - to-NT homogeneity;
K1 - to-nt overload (constant value equal to 0.9);
K2 - to-NT overload from heaving forces (constant value equal to 1.1);
F is the tangential heaving force.
Second Principle
This method of designing the foundations of buildings on VG allows some thawing of the soil directly under the building. For this, two methods are used:
- constructive;
- pre-building.
Constructive
The essence of the method consists in calculating the supporting structures of buildings and structures with a large margin of safety. The design allows for uneven settlement of the structure during long years of operation.
Possible deformations of the structure due to the formation of a thawing bowl
This phenomenon creates conditions of uneven settlement, and this, in turn, can threaten the integrity of the structures of the house. To prevent this from happening, design organizations, when calculating the foundations, lay a certain margin of safety.
Pre-building
The use of this design method is due to a number of reasons:
- Perennial frozen soil consists of heterogeneous rocks with different compression rates, both frozen and thawed.
- The base of the structure over the entire area is exposed to uneven heating (the presence of a boiler room, etc.).
There is only one way to resist the uneven settlement of buildings. It is necessary to design supporting structures with a sufficient margin of safety. For this, additional stiffening belts are installed from high-profile rolled metal products.
Permafrost region
Engineering and geological surveys
In calculations of the bearing capacity and structural features of the foundations of buildings and structures on the VG, data from the results of geocryological studies are used. Research is carried out by specialized design organizations in accordance with regulatory documents. Normative documents include SNiPs, Gosstandart and other recommendations.
The results of geocryological surveys include:
- characteristics of the geocryological data of the construction site - the area and depth of the WG, the average temperature, the height of the seasonal thawing of the soil, the level of groundwater, etc.
- data from laboratory studies and testing of soil samples in the field. Based on them, conclusions are drawn about the mechanical property of the soil both in frozen and thawed state, lithological form;
- the results of predicting changes in the permafrost and hydrogeological state of the soil depending on seasonal changes in temperatures, the thickness of snowfall, the height of the active layer.