Thermal stabilization of foundation soils— a set of heat and reclamation measures aimed at ensuring a stable stable thermal state of soils in accordance with the chosen design principle of using soils as a foundation throughout the entire period of operation of the facility (STO Gazprom 2-2.1-390-2009).
When designing structures on permafrost soils (PMG), design organizations face the following problems:
1) Soils in a frozen state do not have the necessary bearing characteristics (high-temperature frozen soils), which leads to an increase in the number of foundation piles to absorb loads from the structure and an increase in the cost of the project.
2) The geological section at the construction site is represented by MMG of a non-merging type, which during the operation of the facility can lead to both their further thawing (foundation settlements) and freezing (heaving of foundations).
3) For technological reasons, there are restrictions on the installation of a ventilated underground under a heat-producing building or structure (or its height is not enough), which without additional measures can lead to thawing of the MMG.
4) In the area of distribution of MMG, the designed site falls on the area of distribution of thawed soils with low bearing characteristics.
5) Due to the remoteness of the construction area and the difficulties with the delivery of drilling and pile driving equipment, the Customer wants to reduce costs and is considering the option of arranging a shallow foundation instead of a pile foundation.
6) Heaving soils are widespread in the area, which has a negative impact on the foundations of structures and leads to their deformation (especially for lightly loaded foundations of masts, flyovers, small block boxes, etc.).
7) It is necessary to design a local soil dam, and soils with the required characteristics (low filtration coefficients) are not enough.
All these problems, to one degree or another, can be solved by applying soil thermal stabilization systems.
Our company performs both a complete set of design documentation for thermal stabilization of soils (sections: thermal engineering modeling of thermal stabilization systems with a forecast of the state of soils, geotechnical monitoring), and partial modeling of the interaction between a structure and the geological environment, variable calculations of thermal stabilization, etc. An example of a graphical application for the project can be viewed
An example of calculating the thermal stabilization of soils using BET
Instruments and devices used for thermal stabilization of the soil base: seasonal cooling devices ( SDA), year-round cooling devices ( KOU), open cooling devices ( OOU), heat-insulating screens, monitoring systems (loggers, thermocouples, benchmarks).
SOW ( in the literature, the name thermosiphons or single heat stabilizers can be found) - devices based on accelerated heat exchange between soil and air due to phase transformations and circulation of the coolant in a closed heat exchanger. The SDA consists of a condenser (which is located in the above-ground part) and an evaporator (underground part), sometimes a transit part is distinguished, which is important for an anchor-type SDA. The efficiency of the SOU largely depends on the ratio of the area of the evaporator to the total area of the condenser. At the moment, SDAs are widely used in all northern regions of Russia. SOU is installed both in a vertical position and horizontally. On some devices with a large length of the evaporative part, pumps are installed to speed up the heat exchange process.
SOU with a bifurcated system of radiators, in the upper part there is a crane for refueling (Republic of Komi, Vorkuta).
SOU with one radiator, in the upper part there is a crane for refueling (Republic of Komi, Vorkuta).
Sow with a bifurcated system of inclined V-shaped radiators. A similar form was conceived for more efficient operation with and without wind (Komi Republic, Vorkuta).
SOU with horizontal fins and the use of a sleeve that serves to control the freezing process, as well as to change the heat stabilizer.
The use of single SDA with horizontal fins for freezing a part of the site (Yamal-Nenets Autonomous Okrug, Yubileinoye field Gazprom dobycha Nadym).
Application of a vertical finned cooling system for freezing the core of the dam (Republic of Yakutia (Sakha), Yakutsk).
Model of interaction of horizontal thermal stabilization systems from single SDA with a building without a ventilated underground.
KOU - year-round thermal stabilizers are connected to refrigeration machines that are switched on in the warm season. Such systems are used as a rule in two cases. The first one is under difficult soil conditions (fluid soils, etc.), when it is necessary to freeze (lower the temperature) the soil (a) in a short time. The second is objects on a surface foundation with a high requirement for bearing capacity (large tanks), when it is not possible to apply a heat-insulating screen. The real application of KOU exists on the Kharasavey oil pipeline system. There is also a legend that there is a similar system under the building of Moscow State University to ensure the best bearing capacity of Jurassic clays.
OOU - various air blowing devices operating, as a rule, due to the natural movement of air. before the active use of the SDA, they were the main means for cooling the underground under the houses. The device consists of an air intake of various designs and an air duct (pipe). In the case of installation of OOU in the underground equipped with snow shields, when air passes from the street through a narrow opening, a throttling effect occurs, which lowers the temperature in the underground.
For the correct design of thermal stabilization systems, it is necessary to carry out thermal engineering calculations of the interaction of soils, structures and the thermal stabilization system for the entire period of operation. Carrying out modeling before reaching the design temperature is not enough, due to the possible overcooling of the soil and the activation of frost cracking. Our company has all the permits for design work on soil thermal stabilization, all calculations are made on our own certified software, created for the production of such work.
Thermal stabilization of soils
In recent decades, there has been an increase in the temperature of permafrost soils. This causes the risks of occurrence of beyond design stress-strain states of foundation soils, foundations, buildings and structures erected on such soils.
This serious problem every year affects an increasing number of objects operated on foundations composed of permafrost soils (uneven precipitation, foundation subsidence, destruction of structural elements, etc. occur).
The construction of buildings and structures on permafrost soils is carried out according to two principles:
The first principle is based on the preservation of the permafrost state of soils for the period of the entire operation of a building or structure;
The second principle implies the use of soils as bases in a thawed or thawed state (preliminary thawing is carried out to the estimated depth before the start of construction or thawing is allowed during operation;
The choice of principle depends on the engineering and geocryological situation. It is necessary to take into account and compare the appropriateness of the principles. The first principle implies that it is more profitable to keep soils in a frozen state than to reinforce thawed soils.
The second principle is more suitable when soil thawing leads to deformations of foundation soils that are in the range of acceptable values for a particular building or structure. This principle, for example, is suitable for rocky and hard-frozen soils, the deformations of which are small in the thawed state.
Thermal stabilization of soils
Thermal stabilization of frozen soils is designed to provide the possibility of erecting buildings and structures according to the second principle.
A number of measures are used to maintain soils in a frozen state. One of the effective and cost-effective methods is to lower the temperature of soils using heat stabilizers.
Soil thermal stabilizer (TSG) is a vapor-liquid siphon. This is a refrigerant-charged, seasonal cooling device for lowering ground temperatures.
TSG is immersed in drilled wells next to the foundation to lower the temperature of the soil mass, which is the base of the foundation. Part of the device is an evaporator that takes heat from the ground, and a condenser that gives off heat to the surrounding atmosphere.
In the heat stabilizer, natural convection circulation of the refrigerant occurs, which passes from one state of aggregation to another: from gas to liquid and vice versa.
The condensed refrigerant (liquefied ammonia or carbon dioxide) naturally, under the influence of the temperature difference, descends to the lower part of the TSG to the soil. After taking heat from them, it turns into steam and, evaporating, returns to the surface, where it again transfers heat to the surrounding air through the walls of the radiator-condenser, condenses. After the cycle repeats again.
The circulation of the refrigerant can be natural convection-gravity or forced. It depends on the design of the thermostat.
The type, design and number of heat stabilizers are selected on the basis of individual calculations for each object.
Thermal stabilizers have shown their effectiveness - with their help, it is possible to maintain soils in a permafrost state and ensure the strength and stability of the ice-soil slab under the structure.
The convection circulation of the refrigerant is based on the temperature gradient of the ground and the outside air.
During the summer period, as
only the temperature of the condenser - the upper, atmospheric part of the thermostat,
becomes higher than the coolant temperature,
circulation stops and the process is suspended with partial inertial thawing of the upper soil layer until the next cooling.
Schemes of installations according to the method of installation and design:
Single downhole thermal stabilizer (OST)
The simplest device that allows you to carry out installation work for both under construction and existing buildings and structures. OST can be installed both vertically and at an angle of inclination of 45 degrees to the surface;
Horizontal system of thermal stabilizers (HTS) is a system of evaporator pipes located in one horizontal plane in the soil mass, which is the base of the foundation. The refrigerant from the evaporator tubes is transferred to the surface condenser. The device of the GTS is advisable for new construction, when it is possible to construct a pit;
Vertical system of thermal stabilizers (VST) combines a horizontal system with evaporator pipes, which are connected to vertical evaporator pipes that go deep into the soil massif. This design makes it possible to freeze soils to a greater depth than according to the GTS scheme. The VST device is advisable for new construction, when a pit is possible;
thermostatic system, installed in the base of an existing building or structure using directional drilling.
The latter method does not require the development of pits, trenches, reinforcement, and allows you to preserve the natural structure of the soil. It is permissible to install a soil thermal stabilization system in parallel with the construction of the building or structure itself, which speeds up the construction process.
Technical and economic indicators in the application of soil thermal stabilization
Thermal stabilization of soils using various TSG systems can reduce the cost of construction by up to 50% and reduce the construction time of facilities by almost 2 times.
"Thermostabilization of soils" (download in PDF format)
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Seasonally operating cooling units (SOU) are designed to maintain the soil in a frozen state, which ensures the stability of buildings, structures on piles, and also preserves the frozen soil around power transmission line supports and pipelines, along railway embankments and highways. The technology of seasonally operating cooling devices is based on a heat transfer device (thermosiphon), which extracts heat from the soil in winter and transfers it to the environment. An important feature of this technology is that it is natural-acting, i.e. does not need external energy sources.
The principle of operation of all types of seasonally operating cooling devices is the same. Each of them consists of a sealed pipe, which contains a coolant - a refrigerant: carbon dioxide, ammonia, etc. The pipe consists of two sections. One section is placed in the ground and is called the evaporator. The second, radiator section of the pipe, is located on the surface. When the ambient temperature drops below the temperature of the ground where the evaporator lies, refrigerant vapors begin to condense in the radiator section. As a result, the pressure decreases and the refrigerant in the evaporative part begins to boil and evaporate. This process is accompanied by heat transfer from the evaporator to the radiator.
Heat transfer using a thermosyphon
Currently, there are several types of designs of seasonally operating cooling devices:
1) Heat stabilizer. They are a vertical tube of a thermosiphon, around which the soil is frozen.
2). It is a vertical pile with an integrated thermosyphon. A thermal pile can carry some load, such as an oil pipeline support.
3) Deep Seasonal Cooling Unit. It is a long (up to 100 meters) thermosyphon pipe with an increased diameter. Such cooling devices are used for thermal stabilization of soils at great depths, for example, for thermal stabilization of dams and dams.
four) . This type of cooling device differs from the heat stabilizer in that the installation of the evaporator pipe is carried out at a slope of about 5%. In this case, it is possible to install an inclined evaporator tube directly under buildings erected on concrete slabs.
5) Horizontal cooler. A feature of the horizontal seasonally operating cooling device is that it is installed completely horizontally at the level of the prepared bulk base. In this case, the building is erected directly on non-sagging soil, located on the insulation layer and evaporator pipes. The advantage of horizontal cooling units is that they can be used in two configurations: on slab and pile foundations.
6) Vertical Cooling System. This type of seasonally operating cooling devices is similar to a horizontal cooling device, but unlike it, in addition to horizontal evaporator tubes, it can contain up to several tens of vertical evaporator tubes. The advantage of this system is more efficient maintenance of the soil in a frozen state. The disadvantage of vertical systems of cooling devices is the difficulty of their repair and maintenance.
A separate subdivision of the city of Vladimir LLC NPO Sever is a plant equipped with equipment for the production of technical means for thermal stabilization of soils and engineering and geocryological monitoring. This plant is a full-fledged manufacturer of thermal stabilizers. Monthly production of heat stabilizers is 2000 - 2500 pcs. (depending on sizes), plus related products. The manufacturer of heat stabilizers has technical equipment that allows the entire production cycle to be carried out without the involvement of contractors. Currently, work is underway to install an automatic line, which will simplify the production of heat stabilizers and increase the productivity of products. Stocks of raw materials, materials, components and semi-finished products allow us to quickly respond to the needs of customers and deliver products in the shortest possible time.
Soil thermal stabilizers are manufactured in accordance with TU 3642-001-17556598-2014, certified according to the voluntary certification system (ROSS RU.AV28.N16655) and in the field of industrial safety (S-EPB.001.TU.00121).
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