RU2602538C1 - Method for reduction of action of forces of frost boil and increasing stability of pile foundations in permafrost zone - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to designing, construction and operation of structures in conditions of permafrost zone, particularly, to protection of buildings and structures of frost boil in areas seasonal frost penetrated soils. Method for reduction of action of forces of frost boil and increasing stability of pile foundations in permafrost zone involves immersing piles in permafrost. Method includes determining based on results of preliminary geothermal observations potential areas of occurrence of excess cryogenic heads near pile foundation and in perched water of supra-permafrost aquifer, in limited hydrodynamic zones, placing in said sections water reducing wells with perforated wall in lower part of its forming, immersed at a depth not above roof of permafrost rock. In said wells, installing hydrogeological discharge tubes, in each up to four pieces. Inner space of each of said hydrogeological water discharge tubes in beginning of period of freezing seasonally thawing layer is left free and tightly plugged from lower end of packer, and on top of sealed cover. Inner space of each hydrogeological discharge tube at beginning of period freezing seasonally thawing layer remains free and tightly plugged, and during formation of limited hydrodynamic zones in supra-permafrost aquifer, including perched water, and growth of cryogenic pressure in conditions of passage of depth of soil mass front freezing is performed step-by-step unloading cryogenic pressure, by varying hydrodynamic parameters of “soil foundation” by successive opening of unloading hydrogeological tubes at moments, determined based on analysis results of current geotemperature data, obtained during geotemperature control and monitoring of state of soil mass from beginning of transition period of average daily temperatures of ambient air below 0 °C.

EFFECT: technical result consists in reduction of action of forces of frost boil on pile foundations, higher stability and reduced risk of formation near them of frost mounts in permafrost zone.

1 cl, 4 dwg, 1 tbl

Description

The invention relates to the design, construction and operation of structures in the permafrost zone, and in particular to the protection of buildings and structures from frost heaving in seasonally freezing heaving soils. The decisive influence on the bearing capacity of the pile foundation in the permafrost zone with respect to the frost heaving forces arising from the crystallization of permafrost waters of the seasonal thawed layer is exerted by the magnitude of the forces and rates of freezing of piles with soil, the thickness of the freezing seasonal thawed layer and the degree of water saturation, the dynamics of the front passage freezing deep into the soil mass.

There is a method that increases the stability of the pile foundation, which consists in increasing the depth of the foundation during new construction. This allows you to ensure that the forces that keep the pile in the soil of the foundation exceed the tangential forces of frost heaving that arise in the seasonally thawed or seasonally frozen layer (Building norms and rules. Foundations and foundations on permafrost soils. SNiP 2.02.04-88, M. , Gosstroy of the USSR, 1990, p. 1).

The disadvantages of the method is that during the construction of the foundation piles are immersed to a great depth so that their increased bearing capacity can compensate for the frost heaving forces acting on them. For operating facilities, increasing the depths of the foundations is practically impossible and not economically profitable, therefore, with design errors or permafrost thawing, serious problems arise with ensuring the reliable operation of buildings and structures. In addition, arising hillocks of heaving can affect not only the foundation, but also directly on the structure erected on it.

There is a method of reducing the influence of tangential forces of frost heaving on a pile foundation, in which special constructions of the pipe-in-pipe type with an annular filler in the form of a lubricant and various anti-debris coatings of the surface of the upper end of the pile are used (Design of Industrial and Civil Facilities of the West Siberian Oil and Gas Complex ". RSN68-87. M. State Committee for the Construction of the RSFSR, 1987, p. 21-35).

The disadvantages of the method is that in the construction of the foundation of the pile used grease and various anti-polymeric materials applied to the side surface of the pile, located above the roof of permafrost. As a result, to ensure a given bearing capacity, it is necessary to increase the depth of the foundation. In addition, the risk of the effects of heaving (in this case, the formation of a heaving hill), which will directly affect the construction on the foundation, remains.

There is a method of increasing the stability of pile foundations in the permafrost zone, including placing a heat-insulating screen on the surface of the soil base and calculating its necessary parameters, while the heat-insulating screen is placed on the surface and inside the soil foundation, its dimensions, geometric configuration, and also the thermal properties of the material having spatial anisotropy , determined from the conditions of coincidence of the designed temperature field, ensuring the stability of the structure throughout about the operating period and the calculated temperature field obtained by solving the non-stationary two-dimensional inhomogeneous heat equation in rectangular coordinates by the finite difference method for an anisotropic medium with a moving phase boundary in it and representing an engineering-geological section of the soil of the base containing the designed heat-insulating screen, and the territory of construction adjacent to it (RU 2159308, IPC E02D 27/35 (2006.01), published on 04/20/2004).

Significant disadvantages of the method is the fact that in conditions of limited information on the geocryological structure, including the results of geotechnical monitoring (geological and technical monitoring) and variability over time, especially under the anthropogenic impact of the operation of engineering structures, it is practically impossible to make a long-term heat engineering calculation and forecast the permafrost behavior with established heat-insulating screens. As a result, the risk of unacceptable deformations of the foundation and destruction of the structure is seriously increased.

A known design to prevent frost heaving of the soil, including a system of wells, including damping wells drilled near or inclined under the foundation in the underlying soil layer to the depth of freezing and filled with a working fluid, each damping well contains a garland of interconnected containers, and the lower part of the well is predominantly perforated containers filled with a dry salt-sand mixture in a ratio of 1: 2, and one or more are placed in the upper part of the well wells filled with saline with a composition of at least 0.5 kg of salt per 10 liters of water, while the tanks are equipped with diametrically located outlets communicating with the interior of the wells closed with removable heads (RU 2513078, IPC7 E02D 27/35, published on 20.06.2004) .

Significant disadvantages of the method is that garlands of interconnected containers filled with a dry salt-sand mixture are lowered into damping drills near or inclined under the foundation to the depth of freezing of the well. The method is not applicable in the zone of continuous distribution of permafrost, labor-consuming, economically expensive. But most importantly, when ice is thawed in the active layer, water-soluble salts from damping wells, dissolved in water, will fall into the near-pile space, which will cause degradation of permafrost and lead to a decrease in the bearing capacity of the foundation. As a result, unacceptable risks of operating buildings and structures.

Closest to the claimed technical solution is a method of protecting the pile foundation from frost heaving, characterized in that it provides for freezing the foundation in a soil base, which is carried out by artificially freezing the array of the surrounding soil below the depth of seasonal freezing, and the artificial freezing of the soil mass begins when a negative is established average daily ambient temperature and freeze on each side of the foundation an array of g Unt equal to double the width of the foundation, the depth of freezing zone is determined based on a relationship

2.2h _sp ≥h _np ≥1.7h _sp ,

where h _np is the depth of the zone of artificial freezing of the soil, m;

h _SP - the depth of the natural seasonal freezing of the soil from the surface, m,

and to increase the freezing strength of the soil with the foundation, the temperature of the frozen soil is set in the range from -6 to -8 ° C.

Cryostructuring water-soluble polymer components, for example, polyvinyl alcohol, are introduced into the frozen ground, and the surrounding soil foundation is artificially frozen using seasonally acting devices for cooling and freezing the soil, including soil and air heat exchangers and a thermally insulated connecting heat pipe, and the devices are installed in the soil through the outer contour of the foundation with the placement of a soil heat exchanger in the middle of the depth of the freezed n, including immersion piles in permafrost (RU №2227195 published: 20.04.2004).

Significant disadvantages of the method according to these inventions is that when it is used, seasonal cooling devices (SOUs) are installed that are installed on each pile support. In some cases, this leads to the activation of dangerous geocryological processes in the case of permafrost soils with a high moisture content. One of them is the creation in the permafrost aquifer of areas of backwater (locally limited lenses of supercooled permafrost waters). It is they that lead, in particular, to the development of heaving mounds in the inter-pile space and the impact on engineering structures, as well as the forces of frost heaving of piles. And this just significantly increases the risk of emergencies in operating buildings and structures, which is observed in practice.

The tasks to be solved by the claimed technical solution are: reducing the effect of frost heaving forces on pile foundations, increasing their stability and reducing the risks of formation of heaving bumps near them in the permafrost zone in areas of continuous permafrost with high moisture content, typical for the Far North Western Siberia.

When implementing the claimed technical solution, the problem is solved by achieving a technical result, which consists in increasing the stability of pile foundations, as well as in eliminating the formation of permafrost tufts, frost cracks and their impact on pile foundations of objects

The specified technical result is achieved by the fact that the method of reducing the effects of frost heaving and increasing the stability of pile foundations in the permafrost zone includes immersing piles in permafrost soils, while near the foundation piles, in the tops of the permafrost aquifer, water-reducing wells with a perforated wall are placed in limited hydrodynamic zones the lower part of its generatrix, submerged to a depth not higher than the roof of permafrost rocks, into which they are set hydrogeologically unloading tubes, up to four pieces in the well, the inner space of which at the beginning of the freezing season of the seasonally thawed layer remains free and hermetically sealed, the sealing of the unloading tubes is achieved due to the fact that a packer is installed at the lower end of each tube, and the upper end is closed with a sealed cover, with the onset of winter, during the formation of limited hydrodynamic zones in the permafrost aquifer, including in the upper reaches, and the growth of cryogenic pressures under conditions of deep passage l the soil massif of the freezing front stage-by-stage unloading of cryogenic pressures is performed, unloading of cryogenic pressures changes the hydrodynamic parameters of the soil-foundation system, eliminating the conditions of formation of heaving tubercles, unloading of cryogenic pressures is carried out by successively opening discharge hydrogeological tubes, opening the tube by removing it with a sealed cover top and removal of the packer, for example, inside the well with a manual drilling tool, the opening of the pipes is carried out at moments determined emye an assay geotemperature current data obtained in the process of controlling and monitoring geotemperature precoat array state from the beginning of daily average outside temperatures of the transition period is below 0 ° C.

An analysis of the geothermal field allows one to judge the nature of the advancement of the freezing front from the surface deeper into the soil massif and the possible changes in the thermobaric parameters of the permafrost aquifer, to record the dynamics of reduction in the thickness of the seasonal thawed layer during freezing. The phased opening of the permafrost aquifer at certain points in time leads to a decrease in its water saturation, an increase in freezing rates, a decrease in the thickness of the seasonal thawed layer, the formation of limited hydrodynamic zones (locally bounded taliks), a decrease in the effect of frost heaving forces on the pile, and a multiple reduction in the probability of formation of heaving tubercles stabilization of the hydrodynamic system as a whole. The likelihood of developing negative geocryological processes and phenomena such as heaving mounds, frost cracks, and their effect on the pile foundations of the objects is sharply reduced. The problem is achieved due to the fact that the inventive method violates the necessary condition for the occurrence of soil heaving W> W _fh (here W is the soil moisture before freezing, W _fh is the humidity of the heaving limit).

In FIG. Figure 1 shows an example of interpretation of regime geothermal observations in the form of a temporary hydrodynamic model of a permafrost aquifer. The model visualizes the position of the roof of permafrost rocks and potential areas of excessive cryogenic pressures in which water-reducing wells should be installed and phased unloading should be carried out.

In FIG. 2 shows a hydrogeological discharge pipe installed in a dewatering well. The depth of the water-reducing well depends on the specific conditions at the installation point and should be equal to or greater than the depth of immersion of the roof of permafrost (determined by the interpretation of geo-temperature observations).

In FIG. 3 shows a schematic diagram of the opening of a hydrogeological discharge tube.

In FIG. 4 shows a schematic diagram of unloading operations in order to reduce frost heaving forces and increase the stability of pile foundations.

In the inventive method, figures 2, 3, 4 show: hydrogeological discharge pipe 1, which consists of a metal or polypropylene pipe with a length of 1.5 to 2.5 m and Dy 30-50 mm Tube 1 is equipped with a sealed packer 2 from the lower end, and a sealed cover 3 with the upper end. Sealing is achieved by the presence of a layer 4 of rubber or sealant, a water-reducing well 5, perforation holes of the water-reducing well 6, the base of the seasonally frozen layer 7, the roof of permafrost 8, CMC - seasonally frozen layer, STS - seasonally thawed layer, IMF - permafrost. The figure 4 shows: the engineering structure 9, the pile foundation of the engineering structure 10, and also shows the change in the lower border (sole) of I, II, III CMC after unloading in time.

The method is as follows.

In places near the pile foundation 10, where potential areas of the occurrence of excessive cryogenic pressures were determined by preliminary geo-temperature observations, water-reducing wells 5 are installed with perforations 6 located at the bottom of the well body (in the lower part of the well wall). The depth of the wells 5 is not higher than the roof 8 of permafrost. Up to four hydrogeological discharge tubes 1 are installed in each well 5. Each tube 1 is closed at the bottom by a packer 2, and on top by a cover 3. Sealing of the internal space of the tubes 1 from the side of the packer 2 and the cover 3 is provided by a sealing layer 4 made of rubber or sealant. The well 5 through the perforation holes 6 is filled with water to a level corresponding to the groundwater level of the active layer.

With the onset of cold weather, as the liquid freezes in the well 5 in a downward direction, the central part of each hydrogeological discharge tube 1 freezes into the ice cork of the well 5 to the depth of freezing of the current position of the sole 7 of the seasonally frozen layer. When the moment comes when it becomes necessary to unload the cryogenic pressure, the upper sealed cover 3 is removed from the first discharge pipe 1, and the packer 2 located in the lower part of the pipe 1 is torn off inside the well 5 (for example, with a manual drilling tool). As a result, the channel for the outlet of pressure water through the first tube 1 is open and through it the excess water is removed from the reservoir due to the pressure that arose when the freezing front moved down. The freezing rate of soils in this case changes, or rather increases. An almost complete cessation of frost heaving of soils was established with an increase in the rate of their freezing up to 6-8 cm / day. Unloading of a permafrost aquifer at first leads to a significant increase in the rate of freezing of soils, and eventually, with the redistribution of cryogenic pressures within limited hydrodynamic zones, to its reduction to zero values. Sequential opening of several tubes during the passage of a cold wave deep into the soil mass forms a cyclic unloading process, guaranteeing the complete exclusion of cryogenic heaving processes.

A schematic diagram of the unloading operations of a permafrost aquifer, a decrease in the influence of frost heaving forces and an increase in the stability of pile bases 10 in the permafrost zone is shown in FIG. 4, which schematically shows a real industrial experiment. Three hydrogeological discharge tubes 1 are installed in each water-reducing well 5, equipped with packers 2, hermetic covers 3 and sealing layers 4, as they are opened, the hydrodynamic regime of the permafrost seasonally thawed aquifer (STS) changes. Then there is an increase in the freezing rate with crystallization of permafrost waters without the formation of a massive cryogenic texture, the sole of the 7th seasonally frozen layer is immersed (I, II and III - a schematic change in the CMC boundaries after unloading in time) and, in most cases, its merging with roofing of 8 permafrost rocks. At the same time, the tangential forces of frost heaving do not develop or their influence is negligible to affect the lateral surface of the pile foundation 10 of the engineering structure 9. The probability of the formation of annual tubercles of heaving at the unloading site and the likelihood of their impact directly on the structure of the engineering structure are practically zero. located on a pile foundation 10.

As an example of the implementation of the proposed method can serve as an engineering structure 10 - site ABO gas and piping piping compressor shop DKS-1V GP-1V of the Yamburg gas condensate field.

In the period from 2009 to 2014, four cycles of unloading were carried out in the winter periods (December - April). The repeated formation of heaving tubercles in the area observed in previous years has stopped; no new deformations of the pile foundations associated with the influence of frost heaving forces have been recorded. In addition, a roof rise of permafrost rocks and a decrease in the thickness of the seasonally thawed layer were recorded in the industrial experiment research area, in other words, the permafrost zone aggravated, which can be seen from the table below

Between 2008 and 2013, the average soil temperature in the range 2.0–10.0 m decreased by 0.2–0.7 ° C, while the thickness of the seasonally thawed layer decreased from 2.3–4.0 m to 1 , 8-2.2 m.

Thus, it has been experimentally proven that the claimed method leads to the achievement of the goal, namely, to reduce the effect of frost heaving forces on pile foundations and to increase their stability, as well as to exclude the formation of permafrost tufts, frost cracks, and their effect on pile foundations of objects.

Claims (1)

A way to reduce the effects of frost heaving forces and increase the stability of pile foundations in the permafrost zone, including immersion of piles in permafrost, characterized by the fact that according to the results of preliminary geo-temperature observations, potential areas for the occurrence of excessive cryogenic pressures near the pile foundation and in the tops of the permafrost aquifer in limited hydrodynamic zones place in these areas water-reducing wells with a perforated wall at the bottom of its generatrix, submerged to a depth not higher than the roof of permafrost rocks, hydrogeological discharge pipes are installed in these wells, each in up to four pieces, while the internal space of each of these hydrogeological discharge pipes is left free and hermetically sealed at the beginning of the freezing period of the seasonal thawed layer from the lower end by the packer, and on top by a tight cover, with the interior of each hydrogeological discharge tube at the beginning of the freezing season the melted layer remains free and hermetically sealed, and during the formation of limited hydrodynamic zones in the permafrost aquifer, including in the upper reaches, and the growth of cryogenic pressures under conditions of penetration of the freezing front into the soil massif, the cryogenic pressures are unloaded in stages, changing the hydrodynamic parameters of the system "Soil-foundation" by successive opening of discharge hydrogeological pipes at moments determined by the analysis of current geothermal temperatures data obtained in the process of geothermal control and monitoring the state of the soil mass from the beginning of the period of transition of average daily atmospheric air temperatures below 0 ° C.

Images