RU2781771C1 - Method for freezing soil - Google Patents

Abstract

FIELD: building.

SUBSTANCE: invention relates to soil strengthening, in particular, to the drainage of pits by freezing, and can be used in equipping the repair pit on a pipeline in waterlogged and swamped soils. Method for freezing soil includes installing zonal freezing apparatuses in the selected zone and creating of an ice and soil barrier by nitrogen freezing along the perimeter of the repair pit; wherefor the perimeter of the repair pit is measured, the thermophysical characteristics of the soil are determined on the four sides of the repair pit in order to determine the coefficient accounting for the above parameters of the soil, the length of the freezing apparatuses is measured, and the flow rate of liquid nitrogen is determined; wherein the required number of freezing apparatuses is determined from the following dependence:

,

wherein N is the number of freezing apparatuses; k is the coefficient accounting for the hydrophysical parameters of the soil; P is the perimeter of the pit, m; N is the length of the freezing apparatuses, m; q is the standard specific flow rate of the cooling agent, l/m³; Q is the flow rate of liquid nitrogen, l.

EFFECT: reduced labour intensity of work required for the installation of freezing tubes and optimised flow rate of the cryogenic agent ensuring the minimum required thickness of the ice and soil barrier along the entire perimeter of the repair pit.

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Description

SUBSTANCE: invention relates to soil strengthening, in particular, to draining pits by freezing, and can be used in arranging a repair pit on a pipeline in waterlogged and waterlogged soils. Artificial freezing of soils during the development of a pit makes it possible to make a solid fence of a rectangular or circular shape from frozen soil, which prevents the penetration of water-saturated unstable soils or groundwater into the mine. Such a fence perceives the pressure of the soil surrounding the pit or working, as well as the hydrostatic pressure of groundwater. As a rule, a refrigerant is used to freeze soils.

A known method of freezing soils during the construction of underground structures, including drilling wells with the installation of freezing columns in them, cooling the coolant through a refrigeration unit and circulating the coolant through the freezing columns [Shuplik M.N., Meskhidze Y.M., Korolev I.O. and other Construction of underground structures: Reference manual. M.: Nedra, 1990, p. 132-200].

The disadvantages of this method are significant energy costs for cooling the coolant and long periods of soil freezing.

The closest in technical essence and the achieved result to the proposed technical solution is the "Method of isolating the underground working zone from groundwater" according to patent RU 2211894, IPC E02D 19/14, published on 10.09.2003. A method for isolating a work area - an underground working zone from groundwater by creating an ice-ground enclosure using the nitrogen freezing method, including drilling wells with the installation of freezing columns with a liquid coolant in them, which are connected to a tank filled with a cryoagent to ensure circulation of the cooled liquid coolant through the freezing columns. The nitrogen freezing process includes the step of gradually cooling the area to be frozen with nitrogen gas. The disadvantage of this method is the overestimated labor intensity and overexpenditure of the cryoagent due to the fact that an indefinite number of freezing columns are first installed on it and then the minimum required thickness of the ice-ground barrier around the entire perimeter of the repair pit is provided, which, due to the heterogeneity of soils, leads to overexpenditure of the cryoagent.

The technical problem, the solution of which is provided by the use of the claimed invention, and which could not be solved by the implementation of analogs of the invention, is to reduce the labor intensity of the installation of freezing columns and optimize the consumption of the cryoagent to ensure the minimum required thickness of the ice-ground barrier around the entire perimeter of the repair pit.

To solve this problem, it is proposed in a known method of isolating the underground working zone from groundwater, including the installation of several zonal freezing devices in the indicated zone and the creation of an ice-ground barrier along the perimeter of the repair pit by nitrogen freezing, while the required number of freezing devices is determined based on the following

, где

, where

N is the number of freezing devices;

k - coefficient taking into account the thermal parameters of the soil;

P is the perimeter of the pit, m;

H - length of freezing devices, m;

q - standard specific refrigerant consumption, l/m ³ ;

Q - liquid nitrogen consumption, l.

The use of this method, which takes into account soil parameters when determining the optimal number of freezing devices to provide an ice-soil fence of the planned excavation, leads to a decrease in the labor intensity of preparatory work and optimization of the cryoagent consumption.

In FIG. 1, illustrating the prior art, shows a longitudinal section of a known device for zonal soil freezing, for which the claimed method is suitable. In FIG. 2 schematically shows the formation of an ice-ground barrier of length L with different distances between the freezing columns x _i .

The device for zonal nitrogen freezing of soils contains a body 1 of a freezing column with a head 2, a central supply pipe 3, a fairing 4. The central supply pipe 3 is connected to distribution pipes 8 at the level of the base of the fairing 4 by means of an adapter and a sealing device 10 in the form of a cryogenic evaporator. Distribution pipes 8 are made with perforation, cover the lower part of the fairing 4 and can be made both vertical and helical. In the upper zone, the fairing 4 is equipped with bumpers, which can be made in the form of inclined blades, and can also be provided with serrated perforations along the edges (not shown conventionally). The device is installed in the body 1 of the freezing well. Designations LN ₂ - liquid nitrogen, GN ₂ - gaseous nitrogen.

As is known, the design of an ice-ground barrier is based on the assumption that the ice-ground barrier consists of tangent regular cylinders with an axis - a column, with a length equal to the depth of the column. Therefore, the following formula for calculating the refrigerant consumption for the creation of an ice-ground barrier of a closed form was adopted

,

,

where

D - diameter of frozen cylinders,

H is the length of frozen cylinders,

P - the perimeter of the excavation being developed,

n - step of cooling piles,

q - coolant consumption for freezing 1 m ^{3 of} soil.

At the same time, based on the continuity of the barrier, the diameter of the frozen cylinders D is equal to the pitch n of the cooling piles: D=n, and the number of piles N, forming the perimeter of the pit P, is related to the pitch n by the relation

.

.

In addition, it is known that the value of the specific refrigerant consumption for freezing 1 m ^{3 of} soil depends on many factors: on its type, structure, humidity, temperature, and for swamps in the middle zone ranges from 600 to 1000 l / m ³ .

We take as a basis the value of 1000 l/m ³ and introduce the coefficient k, which takes into account the change in the specific characteristics of soils at the place of use of the proposed method.

Having made the appropriate substitutions in formula (1) and reducing the same parameters in the numerator and denominator, we obtain the following formula for determining the number of cooling piles of the ice-soil fence of the planned excavation

,

,

where

k - coefficient taking into account the thermal parameters of the soil;

P is the perimeter of the pit, m;

H - length of freezing devices, m;

q - standard specific refrigerant consumption, l/m ³ ;

Q - liquid nitrogen consumption, l.

Using the formula for determining the number of freezing devices, taking into account the actual thermal and physical characteristics of the soil on different sides of the planned excavation, as in the example shown below, it is possible to reduce the complexity of drilling and installation by 10-20% and reduce the coolant consumption by up to 10%.

An example of using the method. To create a repair pit of a square shape 5 × 5 m, adjacent to the edge of the swamp on one side, soil characteristics were determined on four sides of the perimeter, which showed, in comparison with known data, that one of the sides of the pit would require a specific flow rate of 600 l / m ³ , and three others - standard for 1 m ³ soil with a content of up to 30% water 1000 l of liquid nitrogen. Substituting the numerical values into the formula, we found that 5 freezing devices are needed for three sides of the pit, and 3 freezing devices are enough for one side. Thus, the number of freezing devices required to provide the ice-ground enclosure of the excavation was reduced from 20 to 18, that is, the complexity of drilling and installation was reduced by 10%. Since all the cooling piles are connected and fed at the same time, the savings in liquid nitrogen consumption was also 10%. Freezing work begins with drilling wells and installing freezing columns with supply pipes in them. In parallel, work is underway to build a freezing station, liquid nitrogen is used once (gas is released into the environment). Liquid nitrogen delivered to the construction site is discharged from tanks directly into freezing columns. Liquid nitrogen is delivered in tanks with a capacity of 1200, 3000 and 5000 liters and more, mounted on vehicles. Freezing columns for low-temperature freezing are connected in series in one system. Liquid nitrogen enters the feed (inner) pipe of the first freezing column. In the annular space of the column, liquid nitrogen evaporates, the resulting gas rises to the head of the column, from where it then flows through the pipeline into the supply pipe of the adjacent column, etc. It enters the atmosphere from the last column of the system with a temperature of about minus 40°C.

The formation of an ice-ground closed enclosure is judged by the increase in the water level in a drilled special hydrogeological control well. When the formation of a closed ice-ground barrier ends on one of the aquifers and its thickening begins, the water inside the frozen contour falls under the pressure of the thickening walls of the barrier, and its level rises in the control well.

After the repair work is completed, the frozen soils are thawed, which can be carried out either naturally or artificially, by pumping heated water or brine into the columns.

Claims (8)

A method for freezing soil, including installation of zonal freezing devices in the specified zone and creation of an ice-soil fence along the perimeter of the repair pit by nitrogen freezing, for which the perimeter of the repair pit is measured, the thermophysical characteristics of the soil are determined on four sides of the repair pit to establish a coefficient that takes into account the above soil parameters, measure the length of the freezing devices, determine the flow of liquid nitrogen, while the required number of freezing devices is determined from the following relationship

, where N is the number of freezing devices; k - coefficient taking into account the hydrophysical parameters of the soil; P is the perimeter of the pit, m; H - length of freezing devices, m; q - standard specific refrigerant consumption, l/m ³ ; Q - liquid nitrogen consumption, l.

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