Construction method of large-diameter spiral blade spiral pile foundation in permafrost

Abstract

The invention discloses a construction method of a large-diameter spiral blade spiral pile foundation in permafrost, and belongs to the technical field of engineering foundation construction. The method comprises the steps that a frozen earth stratum is surveyed, and a pile position is determined; a plurality of holes are drilled around the pile position, the area defined by the holes is required to be larger than the diameter range of the spiral blades, and the hole depth is larger than the pile depth; electrodes are put into the holes, conducting media are injected into the holes, then the holes are electrified and heated, frozen soil between the electrodes is unfrozen to the degree that the spiral pile can be screwed into the holes for construction, and the electrodes are pulled out; then, the large-diameter spiral blade spiral pile is screwed to the designed depth through a machine according to a conventional method; a circulating refrigeration system is installed, and low-temperature saline water is circulated in a spiral pile hole for refrigeration; and after the unfrozen soil body around the screw pile is frozen back, the saline water in the screw pile hole is pumped and completely removed, and the hole is sealed by using a foam expanding agent. According to the method, the problem that the large-diameter spiral blade spiral pile in the permafrost is difficult to construct can be solved.

Description

Technical Field

[0001] The invention belongs to the technical field of engineering foundation construction, and in particular relates to a construction method of a large-diameter spiral blade spiral pile foundation in permafrost. Background Art

[0002] The foundation construction of transmission line towers is an important part of the construction of transmission line towers. However, in the Qinghai-Tibet Plateau, the ecological environment is fragile and permafrost is widely distributed. The excavation of traditional cast-in-place pile foundation construction is easy to cause environmental damage, the pile body maintenance time is long, and there is also the risk of hydration and thermal thawing of permafrost. It is difficult to meet the construction requirements of machinery, environment and quality for foundation construction in the permafrost area of ​​the Qinghai-Tibet Plateau. The large-diameter spiral blade spiral pile foundation has the characteristics of fast construction, high degree of mechanization and low environmental impact. It is an ideal type of transmission line tower foundation in the Qinghai-Tibet Plateau. However, due to the very hard permafrost, the spiral pile cannot be directly screwed into the frozen soil layer using traditional construction methods, which limits the application potential of spiral piles in related engineering fields in the Qinghai-Tibet Plateau. Summary of the invention

[0003] In view of the technical problem that in the above-mentioned prior art, spiral piles are difficult to screw into the inner strata in frozen soil areas, which limits the use of spiral piles in related engineering fields in frozen soil areas such as the Qinghai-Tibet Plateau, the present invention provides a construction method for spiral pile foundations with large diameter spiral blades in permafrost, which can not only realize the efficient and rapid construction of spiral piles in frozen soil areas, but also restore the frozen soil to its original state after the construction is completed, without causing damage to the frozen soil.

[0004] To achieve the above objectives, the technical means adopted by the present invention are as follows: A construction method for a large-diameter spiral blade screw pile foundation in permafrost is suitable for the construction of a large-diameter spiral pile foundation in permafrost areas, and the steps are as follows: S1: Accurately locate the pile hole position, conduct detailed survey on the frozen soil foundation within the pile foundation range, measure the soil layer distribution, electrical conductivity, resistivity, thermal conductivity, and heat absorption coefficient of the frozen soil foundation, level the site, and determine the entire construction plan; S2: Prepare construction tools, including drilling rigs, electrodes, power supplies, large-diameter spiral blade screw piles, cooling devices and brine; S3: Drill 4 to 6 holes at each pile hole precisely located in S1, such as around the pile hole, place electrodes in the holes, and fill all holes with conductive media (for the purpose of achieving electric heating and thawing); S4: Connect the electrodes in S3 with wires, check that the wires of each electrode are correctly connected, and then turn on the power switch to heat and melt the frozen soil within the pile diameter range; S5: After the frozen soil within the pile diameter is melted and softened by electric heating, the power supply is turned off to stop heating, the electrode is pulled out, and the large-diameter spiral blade screw pile is screwed into the soil to the designed depth; S6: Install a refrigeration cycle system, connect the spiral piles with water pipes and double-hole sealing pistons, the cooling device water outlet pipe passes through the double-hole sealing piston and enters the bottom of the first spiral pile, one end of another water pipe is connected to the sealing piston mouth, and the other end passes through the double-hole sealing piston of the adjacent spiral pile and enters the bottom of the adjacent spiral pile, and so on. After the entire cooling pipeline system is connected, start the cooling device, so that normal temperature salt water enters the cooling device for cooling, and the low-temperature salt water flowing out of the cooling device outlet reaches the bottom of the first spiral pile through the water inlet pipe, fills the first spiral pile, and flows out from its water outlet pipe and flows into the bottom of the next spiral pile, and performs cyclic refrigeration in sequence, so that the frozen soil that was previously heated and melted is cooled until it freezes again; S7: Use a pump to drain the residual salt water in the screw pile hole, then use a foam expansion agent to seal the top hole of the screw pile, and then carry out the construction of the superstructure.

[0005] Preferably, in the frozen soil area in step S1, the soil layers are distributed into a seasonally active layer and a frozen soil layer, and when measuring the thickness, the thickness of both the seasonally active layer and the frozen soil layer need to be measured.

[0006] Preferably, the electrode in step S2 is a φ16~φ25 steel bar with a sharp point at the lower end, and the length of the electrode is such that the top is exposed 10~15 cm above the ground after being placed in the drill hole; the pile diameter, wall thickness, pile length, and spiral blade thickness of the spiral pile all meet the requirements of the specifications, and can bear normally in the low temperature environment of the frozen soil area; the cooling device can cool the brine to minus ten degrees Celsius, thereby refreezing the thawed soil layer.

[0007] Preferably, the pile hole position is determined in step S3 strictly according to the design, and the range of frozen soil that can be melted by the electrodes in all the boreholes placed in each pile hole after being electrically heated is slightly larger than the pile body diameter of the screw pile, and the depth is consistent with the pile length of the screw pile; the injected conductive medium has good conductivity.

[0008] Preferably, the wires in step S4 are insulated sheathed aluminum wires, the wire load connecting the electrodes is large enough, and its size is determined according to the load of each line. The temporary line for electric heating must be laid on a pole, not less than 2.5m from the ground. Power can be supplied only after the wiring is confirmed to be correct, and the current of each branch line is measured at regular intervals. Once an abnormal situation is found, it must be handled in time. During the construction process, construction personnel must wear insulating gloves and shoes to work.

[0009] Preferably, the electric heating time in step S5 is determined based on field experiments to ensure that the frozen soil between the electrodes is thawed and softened to prevent the presence of frozen soil when drilling the screw pile and making it difficult to screw in.

[0010] Preferably, the diameter of the water pipe used in step S6 is slightly smaller than half the diameter of the reserved hole of the screw pile; the double-hole sealing piston can seal the hole of the screw pile so that the low-temperature brine entering the hole can only flow out through the other side of the sealing piston and reach the bottom of the next reserved hole through the water pipe, ensuring that all the thawed soil layers are re-frozen.

[0011] Preferably, in step S7, after the brine in the reserved hole of the spiral pile is drained, a small amount of remaining brine is placed in the absorption hole of the desiccant, and then the top of the pile is sealed using a foam expansion agent.

[0012] The beneficial effects of the present invention are: The present invention can solve the problem that large-diameter spiral blade spiral piles are difficult to construct in permafrost, so that spiral piles can be constructed efficiently and quickly in permafrost areas, and the permafrost can be restored to its original state after the construction is completed without causing permafrost damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a flowchart of the construction method of the present invention; Figure 2 It is the layout diagram of pile hole location and electrode location; Figure 3 It is a schematic diagram of the electrode plane layout and wire connection; Figure 4 Schematic diagram of heating the electrodes to melt the frozen soil between the electrodes; Figure 5 This is a schematic diagram of screw piles being driven into thawed frozen soil; Figure 6 This is a schematic diagram of the refrigeration cycle system returning to the frozen soil layer; Figure 7 It is a schematic diagram of the double-hole sealing piston structure; Figure 8 Schematic diagram of the refrigeration system to refreeze the soil layer; Fig. 9 This is a schematic diagram of the superstructure after the screw piles are installed; In the above figure, 1. seasonal active layer; 2. permafrost layer; 3. electrode; 4. screw pile; 4-1. pile hole; 5. heating to melt frozen soil; 6. frozen soil after refrozen; 7. melted frozen soil; 8. reserved holes; 9. conductive medium; 10. brine; 11. insulated aluminum wire; 12. power supply; 13. water pipe; 14. superstructure; 15. cooling device; 15-1. brine inlet; 15-2. low-temperature brine outlet; 16. double-hole sealing piston. DETAILED DESCRIPTION

[0014] The following will be combined Figures 1 to 9The present invention is described in detail, and the following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore only used as examples, and cannot be used to limit the protection scope of the present invention.

[0015] like Figure 1 As shown, the present invention provides a construction method for a large-diameter spiral blade spiral pile foundation in permafrost, comprising the following steps: S1. Conduct detailed survey of the frozen soil area to be constructed and accurately locate the position of the pile hole (such as Figure 2 4-1), measure the soil layer distribution, electrical conductivity, resistivity, thermal conductivity and thermal absorption coefficient of the frozen soil foundation, level the site, and determine the entire construction plan; S2. Prepare necessary construction tools such as drilling rig, electrode 3, power supply 12, screw pile 4, cooling device 15 and brine 10. Electrode 3 uses φ16~φ25 steel bar with a sharp tip at the bottom. The length of electrode 3 is sufficient to expose 10~15cm of its top above the ground after being put into the drill hole. A hole 8 is left in the middle of the screw pile 4. Its pile diameter, wall thickness, pile length, spiral blade thickness and other parameters meet the requirements of the specification, and it can bear normal load in the low temperature environment of frozen soil area. S3, such as Figure 2 As shown, at each pile hole position determined according to the design (such as pile hole 4-1), multiple holes of designed depth are drilled around each drilling position, and the area surrounded by the holes is required to be larger than the diameter range of the spiral blades, and the hole depth is greater than the pile depth. The electrode 3 is placed in the borehole, and the brine mixture is filled in all the boreholes as the conductive medium 9; S4, such as Figure 3 As shown, all electrodes 3 that have been drilled to the designed depth are connected to the insulated aluminum wires 11. After checking that the wires of each electrode are properly connected, the power supply 12 is turned on to heat and melt the frozen soil 5 between the electrodes; S5, such as Figure 4 , Figure 5 As shown, after the frozen soil 5 between the electrodes is heated for a period of time and melts and softens, the heating time is determined according to the actual geological conditions on site, the voltage level, the freezing depth and field experiments, etc., to ensure that the frozen soil area 7 through which the screw pile 4 passes when it is screwed into the soil is thawed and softened after the heating is completed, the power supply is turned off to stop heating, the electrode 3 is pulled out, and the screw pile 4 is screwed into the melted frozen soil 7 by using a drilling rig to a designed depth; S6, such as Figure 6 , Figure 7 , Figure 8As shown, a refrigeration cycle system is installed, and the reserved holes 8 of the spiral pile 4 are connected by a water pipe 13, wherein the water pipe at one end of the reserved hole of the spiral pile enters the bottom through a double-hole sealing piston 16, and the water pipe is connected to the other outlet of the sealing piston, and then the cooling device 15 is started, and the normal temperature brine 10 is placed in the container, enters the cooling device 15 through the water inlet 15-1 for cooling, and the low-temperature brine flowing out from the water outlet 15-2 of the cooling device 15 reaches each spiral pile 4 through the water pipe 13, and finally flows back into the container to form a circulating cooling; the diameter of the water pipe 13 used is slightly smaller than half of the diameter of the reserved hole 8 of the spiral pile; the sealing piston 16 can seal the spiral pile hole 8, so that the low-temperature brine 10 entering the hole can only flow out through the other side of the sealing piston, and reach the bottom of the next reserved hole through the water pipe, thereby performing reciprocating cooling; the cooling time needs to be determined according to the actual situation on site and experiments to ensure that the thawed soil layer 7 is completely frozen again; S7, such as Fig. 9 As shown, a pump is used to drain the residual brine 10 in the screw pile hole 8, and a certain amount of desiccant is put into the hole to absorb the small amount of brine remaining in the hole. Then, a foam expansion agent is used to seal the top hole of the screw pile, and the upper structure 14 is constructed on the screw pile 4.

Claims

1. A method for constructing large-diameter spiral blade screw piles in permafrost, characterized in that: The construction method is suitable for use in the construction of large-diameter spiral blade screw piles in frozen soil, and comprises the following steps: S1: Accurately locate the pile hole position, conduct detailed survey on the frozen soil foundation within the pile foundation range, measure the soil layer distribution, electrical conductivity, resistivity, thermal conductivity, and heat absorption coefficient of the frozen soil foundation, level the site, and determine the entire construction plan; S2: Prepare construction tools, including drilling rigs, electrodes, power supplies, large-diameter spiral blade screw piles, cooling devices and brine; S3: Drill several holes of designed depth at each pile hole position accurately located in S1, place electrodes in the holes, and fill all the holes with conductive medium; S4: Connect the electrodes in S3 with wires, check that the wires of each electrode are properly connected, turn on the power switch, and heat and melt the frozen soil between the electrodes; S5: After the frozen soil between the electrodes is melted and softened by electric heating, the power supply is turned off to stop heating, the electrodes are pulled out, and the large-diameter spiral blade screw pile is screwed into the soil to the designed depth; S6: Install a refrigeration cycle system, connect the spiral piles with water pipes and double-hole sealing pistons, the cooling device water outlet pipe passes through the double-hole sealing piston and enters the bottom of the first spiral pile, one end of another water pipe is connected to the sealing piston mouth, and the other end passes through the double-hole sealing piston of the adjacent spiral pile and enters the bottom of the adjacent spiral pile, and so on. After the entire cooling pipeline system is connected, start the cooling device, so that normal temperature salt water enters the cooling device for cooling, and the low-temperature salt water flowing out of the cooling device outlet reaches the bottom of the first spiral pile through the water inlet pipe, fills the first spiral pile, and flows out from its water outlet pipe and flows into the bottom of the next spiral pile, and performs cyclic refrigeration in sequence, so that the frozen soil that was previously heated and melted is cooled until it freezes again; S7: Use a pump to drain the residual salt water in the screw pile hole, then use a foam expansion agent to seal the top hole of the screw pile, and construct the superstructure on the screw pile.

2. The construction method according to claim 1, characterized in that: In step S2, the electrode is made of φ16~φ25 steel bars with pointed lower ends, and the length of the electrode is such that the top of the electrode is exposed 10~15cm above the ground after being placed in the drilled hole; a hole is reserved in the middle of the spiral pile, and the pile diameter, wall thickness, pile length, and spiral blade thickness all meet the requirements of the specifications, and can bear normally in the low temperature environment of frozen soil areas; the cooling device can cool the brine to minus ten degrees Celsius, thereby refreezing the thawed soil layer.

3. The construction method according to claim 1, characterized in that: In step S3, the distance and depth between the boreholes are designed according to the size of the screw pile and the actual engineering conditions. The electrodes in all the boreholes placed in each pile hole can melt the frozen soil in a lateral range greater than the diameter of the screw pile body after being electrically heated, and the longitudinal depth is consistent with the length of the screw pile.

4. The construction method according to claim 1, characterized in that: In step S3, the conductive medium is a salt water mixture.

5. The construction method according to claim 1, characterized in that: In step S4, the wire is an insulated sheathed aluminum wire, and its size is determined according to the load of each line. The line is laid on a pole, not less than 2.5m from the ground. Power can be supplied only after the wiring is confirmed to be correct, and the current of each branch line is measured at regular intervals. Once an abnormal situation is found, it must be handled in time. During the construction process, construction workers must wear insulating shoes to work.

6. The construction method according to claim 1, characterized in that: In step S5, the heating time is determined according to the actual geological conditions on site, voltage level, freezing depth and field experiments, so that after the heating is completed, the frozen soil area through which the screw pile passes when it is screwed in is thawed and softened.

7. The construction method according to claim 1, characterized in that: In step S6, normal temperature brine is placed in a container, and the low temperature brine cooled by the cooling device reaches each screw pile through a water pipe, and finally flows back into the container to form a circulating cooling; the diameter of the water pipe used must match the aperture of the double-hole sealing piston of the sealed screw pile to ensure that no leakage occurs during the circulating refrigeration process; the low temperature brine flows from the water pipe passing through the sealing piston hole into the bottom of the screw pile, flows out from the water pipe passing through another sealing piston hole, and flows into the bottom of the next screw pile, and so on, to ensure that all the thawed soil layers are re-frozen.

8. The construction method according to claim 1, characterized in that: In step S7, after the salt water in the reserved hole of the screw pile is drained, the remaining salt water in the hole is absorbed by a desiccant, and then the top of the pile is sealed using a foam expansion agent.

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