Construction method of underground freezing water stop and shallow geothermal collection system

Abstract

The invention provides a construction method for an underground freezing water stopping and shallow geothermal collecting system. The construction method comprises the following steps that S1, a plurality of supporting piles are evenly arranged around a foundation pit to form row piles; s2, freezing holes are formed; s3, a freezing pipe is arranged; s4, freezing the pipeline to form a frozen soil curtain; s5, the water stopping effect is verified; s6, excavation of a foundation pit; step S7, cutting off the freezing pipe; s8, transforming the freezing pipe; and S9, geothermal energy is collected. The problems that an existing freezing energy pile cannot be repeatedly used in the later period of low freezing temperature, cost is high, green and economical efficiency is not enough, and shallow stratum heat resources are wasted are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of frozen water stop and geothermal collection, and particularly relates to a construction method for an underground frozen water stop and shallow geothermal collection system. Background Art

[0002] With the rapid development of urban construction, the construction difficulty of foundation pit excavation in different urban construction projects has increased. Foundation pit excavation, as a necessary measure for the construction of, for example, subways, tunnels, underground pipelines, etc., will add many difficulties to the construction conditions when encountering sections with dense urban underground pipelines during the construction process. In view of this situation, it is particularly suitable to use the freezing method for the construction of foundation pits. The freezing method for constructing foundation pits can not only effectively isolate groundwater, but also the shape and strength of the freezing curtain formed by the freezing method can be flexibly changed according to construction conditions, and it is applicable to pile foundation construction and other parallel industries.

[0003] However, at present, the freezing technology has high requirements for geological conditions and requires too low temperature, with the freezing temperature required to be below -24°C, which in turn leads to the frost heaving force affecting the ground surface, such as problems like excessive stratum disturbance affecting construction safety and difficulty in construction due to overly dense underground pipelines. In addition, after the freezing method construction, a series of problems such as grouting and backfilling of freezing holes need to be faced. The existing technology also does not consider the transformation and utilization of later freezing equipment, and cannot meet the urgent needs of an environment-friendly society for the innovative development and utilization of new energy, which will trigger energy crisis problems brought about by the rapid development of cities.

[0004] Therefore, an improved technical solution is needed to address the deficiencies of the above-mentioned existing technologies. Summary of the Invention

[0005] The purpose of the present invention is to implement a construction method for an underground frozen water stop and shallow geothermal collection system, which at least solves problems such as the low freezing temperature of the current frozen energy pile that cannot be reused later, high cost, lack of green economy, and waste of shallow stratum heat resources. To achieve the above purpose, the present invention provides the following technical solutions:

[0006] A construction method for an underground frozen water stop and shallow geothermal collection system, characterized in that the construction method includes:

[0007] Step S1, uniformly arranging a plurality of support piles around the foundation pit to form a row of piles;

[0008] Step S1-1, determining the diameter of each of the support piles and the spacing between adjacent support piles;

[0009] Step S1-2, drilling holes according to the diameter size of the support piles and the spacing between the support piles; the drilling depth corresponds to the depth of the foundation pit;

[0010] Step S1-3: Pour concrete into the drilled holes to form retaining piles.

[0011] Step S1-4: Pour the top capping beam of the retaining piles on the top surface of the retaining piles to connect each of the retaining piles and form a row of piles.

[0012] Step S2: Set freezing holes: Uniformly set a plurality of freezing holes around the row of piles; the depth of the freezing holes is greater than the depth of the foundation pit.

[0013] Step S3: Set freezing pipes. Both the cold flow outlet and the heat flow inlet of the freezing pipes are connected to the freezing device to form a closed loop.

[0014] Step S4: Freeze the pipes to form a frozen soil curtain; the freezing device circulates low-temperature brine or refrigerant through the freezing pipes; as the refrigerant circulates, the temperature inside the freezing pipes gradually decreases, and the moisture in the soil around the foundation pit begins to freeze, gradually forming a frozen soil wall.

[0015] Step S5: Verify the water-stop effect and confirm whether there is flowing water in the soil layer inside the curtain by setting exploration holes or conducting a water pressure test.

[0016] Step S6: Excavate the foundation pit; determine the deepest part according to the designed depth and geological conditions of the foundation pit; excavate layer by layer from the deepest part to the shallow part; the excavation depth of each layer is 1-2 meters and then gradually expand to the surrounding area.

[0017] Step S7: Cut off the freezing pipes and excavate an opening in the foundation pit section.

[0018] Step S8: Modify the freezing pipes; reconnect the freezing pipes along the periphery of the opening.

[0019] Step S9: Collect geothermal energy.

[0020] Preferably, Step S1-3: Pour concrete into the drilled holes to form retaining piles, including the following steps:

[0021] Step S1-3-1: Install the steel reinforcement cage.

[0022] Step S1-3-2: Conduct the operation of opening the conduit using a spherical bladder or a precast cylindrical concrete water-stop plug.

[0023] Step S1-3-3: Pour concrete.

[0024] Preferably, Step S1-4 includes the following steps:

[0025] Step S1-4-1: Groove between the piles in the early stage of pouring.

[0026] Step S1-4-2: Place formwork and fabricate steel bars at the groove.

[0027] Step S1-4-3: During pouring, the layered pouring method is adopted, with the thickness of each layer controlled within 300 mm, and it is vibrated compactly with a vibrator. After pouring, water spraying maintenance is carried out.

[0028] Preferably, step S2 includes: drilling freezing holes according to the results of on-site geological exploration. The drilling depth should exceed 1 / 3 of the original foundation pit depth, and the deviation rate of the freezing holes is not greater than 1%.

[0029] Preferably, step S3 includes: The freezing pipes are made of steel pipes. The freezing pipes placed in the freezing holes include two vertical freezing pipes. The bottoms of the vertical freezing pipes are connected by a first horizontal connecting pipe. The length of the first horizontal connecting pipe matches the inner diameter of the freezing hole, and the length of the vertical freezing pipe matches the depth of the freezing hole;

[0030] The tops of the vertical freezing pipes in adjacent two freezing holes are connected by a second horizontal connecting pipe. The length of the second horizontal connecting pipe matches the distance between the two freezing holes;

[0031] Each freezing pipe is connected into a whole through the staggered first horizontal connecting pipe and second horizontal connecting pipe; at the same time, the initial end and the end of the second horizontal connecting pipe are both connected to the freezing device to form a closed loop.

[0032] Preferably, step S4 includes the following steps: Start the distribution cabinet of the power supply system to provide power for the refrigeration unit; start the compressor to compress the refrigerant and provide refrigeration power; the refrigerant absorbs heat in the evaporator; the condenser cools the high-temperature and high-pressure refrigerant gas into a liquid to form a refrigerant liquid; the refrigerant liquid flows out of the refrigeration unit, passes through the variable-frequency circulation pump, and flows into one end of the freezing pipe. After the refrigerant liquid absorbs heat in the freezing pipe, it flows back to the liquid storage tank from the other end of the freezing pipe to complete the cycle; as the refrigerant liquid circulates, the water in the surrounding soil begins to freeze and gradually forms frozen soil.

[0033] Preferably, step S5, to confirm whether there is flowing water in the soil layer within the curtain by setting exploration holes, includes the following steps:

[0034] Step S5-1: Exploration hole layout; The exploration holes are arranged at the key parts of the curtain and the areas where flowing water may exist; The key parts of the curtain include: the corners, endpoints of the curtain, and the connection points with the subsequent support structures; The areas where flowing water may exist include the areas with a relatively high groundwater level; and the areas with a relatively good permeability formation;

[0035] Step S5-2: Exploration hole construction;

[0036] Step S5-3: Observation inside the hole;

[0037] Step S5-4: Data processing and analysis;

[0038] Confirming whether there is flowing water in the soil layer inside the curtain by means of a water pressure test includes the following steps:

[0039] Step S5-a, preparation;

[0040] Step S5-b, drilling and plugging;

[0041] Step S5-c, installing and connecting the test equipment;

[0042] Step S5-d, pressurization and observation: start the water pump to gradually increase the water pressure in the test pipeline; during the pressurization process, observe the changes in the pressure gauge and the conditions around the curtain. If a pressure drop or leakage is found around the curtain, stop pressurization and find the cause.

[0043] Preferably, step S7, cutting off the freezing pipe and excavating a hole in the foundation pit section, comprises the following steps:

[0044] Step S7-1, destroying the support piles of the pre-excavated hole section, and cutting off the freezing pipe, and the bottom of the remaining freezing pipe is set around the excavated hole;

[0045] Step S7-2, advance support of the tunnel entrance;

[0046] Step S7-3: initial support of the tunnel entrance.

[0047] Preferably, step S8, freezing pipe transformation comprises the following steps:

[0048] Step S8-1, firstly, the cut portion of the bottom of the broken freezing pipe is beveled, and a low-temperature resistant alloy steel pipe with the same strength as the parent material is selected as a transition connector, and an annular butt joint is performed by welding;

[0049] Step S8-2, using a flange seal assembly to perform secondary reinforcement on the repair connection between the bottom of the freezing pipe and the transition connector;

[0050] Step S8-3, finally, injecting the low-temperature curing sealing colloid into the transition piece through the pressure injection system.

[0051] Preferably, step S9, geothermal energy collection includes the following steps:

[0052] The unremoved freezing pipe is used as a geothermal energy collection pipe; the first end of the freezing pipe is connected to the variable frequency circulation pump of the refrigeration unit as the cold flow outlet; the second end of the freezing pipe is connected to the liquid storage tank as the hot flow return end; cold water enters the first end of the freezing pipe and becomes hot water after absorbing the heat of shallow geothermal heat, and then flows back to the liquid storage tank connected to the heat exchanger through the second end of the freezing pipe to realize heat energy transfer.

[0053] Beneficial effects:

[0054] The freezing method involved in this application plays the role of temporary support, as well as the supporting role of the row piles and the internal support structure system. By taking advantage of the good water sealing performance of the freezing curtain, the original freezing temperature of -20°C can be raised to -5°C. The combined use of these two aspects solves the problems of rock socketing and water sealing of the foundation pit enclosure structure, and at the same time reduces the pollution to the stratum. This method provides a freezing reinforcement and shallow geothermal resource collection method for the construction of urban underground roads from the perspective of the combination of the two. It has high applicability. Through the later transformation of the freezing device, water is used as the circulating medium to exchange heat in the shallow stratum, realizing the development and utilization of shallow geothermal energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The schematic diagrams in the specification forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation to the present invention. Among them:

[0056] Figure 1 The construction flow chart involved in this application;

[0057] Figure 2 The schematic diagram of the plane layout of the freezing holes involved in the present invention;

[0058] Figure 3 The partially enlarged schematic diagram of the connection between the geothermal collection device and the freezing pipe involved in the present invention;

[0059] Figure 4 The schematic diagram of the excavation of the opening involved in the present invention;

[0060] Figure 5 The schematic diagram of the transformation of the freezing pipe and the collection of geothermal energy involved in the present invention.

[0061] Among them, 1, foundation pit; 2, freezing hole; 3, freezing pipe; 4, row pile; 5, cold flow outlet; 6, heat flow inlet; 7, heat exchanger; 8, liquid storage tank; 9, opening. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0062] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0063] In the description of the present invention, the orientation or positional relationship indicated by the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention rather than requiring the present invention to be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention. The terms "connected" and "coupled" used in the present invention should be understood in a broad sense. For example, it can be a fixed connection or a detachable connection; it can be directly connected or indirectly connected through an intermediate component. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0064] The present invention will be described in detail below with reference to the drawings and in conjunction with embodiments. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0065] As Figure 1 shown, the present application relates to a construction method for an underground freezing water stop and shallow geothermal energy collection system. The improvement lies in that the construction method includes:

[0066] Step S1, as Figure 2 shown, a plurality of retaining piles are uniformly arranged around the foundation pit 1 to form a row of piles 4;

[0067] Step S1-1, determine the diameter of each of the retaining piles and the spacing between adjacent retaining piles. Specifically, according to the exploration results and engineering requirements, formulate a detailed construction plan for the foundation pit row pile support and carry out relevant construction preparation work. Select an appropriate pile foundation type according to the geological conditions and design requirements at the construction site, such as cast-in-place concrete piles, etc. Determine the diameter of the retaining piles and the spacing between the piles. Generally, it is necessary to ensure that the spacing between the pile and the adjacent pile is greater than 4 times the pile diameter. Then proceed to the subsequent steps, drill holes around the foundation pit 1 and pour the cast-in-place bored piles.

[0068] Step S1-2, drill holes according to the diameter size of the retaining piles and the spacing between the retaining piles; the drilling depth corresponds to the depth of the foundation pit.

[0069] Step S1-3, pour the drilled holes to form retaining piles. Specifically, use concrete as the pouring material to pour the drilled holes.

[0070] Specifically, in step S1-3-1, install the steel reinforcement cage. Lift the column-shaped steel reinforcement cage matching the size of the retaining pile and slowly place it into the drilled hole to ensure that the position and verticality of the steel reinforcement cage meet the design requirements.

[0071] Step S1-3-2, use a spherical bladder or a prefabricated cylindrical concrete water stopper to open the conduit. The conduit opening operation is to use the conduit for pouring, so that the concrete can smoothly enter the borehole from the conduit for pouring, while ensuring the quality of the concrete during the pouring process and preventing it from being contaminated by water or mud in the borehole.

[0072] Use a bladder or a prefabricated cylindrical concrete water stopper to open the conduit. The bladder is a spherical device with a certain degree of elasticity and sealing, usually made of rubber and other materials. Among them, before pouring concrete, when using the bladder to open the conduit, place the bladder near the bottom of the conduit. When pouring begins, first inject a certain amount of concrete into the conduit, and then rupture the bladder by squeezing or other means. The concrete will flow smoothly out of the conduit under the action of gravity and enter the borehole. The role of the bladder is to prevent the concrete in the conduit from directly contacting the water or mud in the borehole in the early stage of pouring, play an isolation role, and ensure the pouring quality of the concrete.

[0073] Among them, the prefabricated cylindrical concrete water stopper is a prefabricated cylindrical concrete block with a diameter slightly smaller than the inner diameter of the conduit. Before pouring concrete, the prefabricated cylindrical concrete water stopper is placed in the conduit to block the bottom of the conduit. When pouring concrete begins, as the concrete continues to accumulate in the conduit, the pressure generated will push the water stopper downward, causing it to be discharged from the bottom of the conduit, allowing the concrete to flow smoothly into the borehole. The prefabricated cylindrical concrete water stopper, like the bladder, serves to isolate the concrete from water or mud in the borehole in the early stages of pouring, ensuring that the concrete can be poured in a clean environment and preventing water or mud from mixing into the concrete and affecting its performance.

[0074] Step S1-3-3, pouring. During the entire pouring process, the concrete conduit should be buried 2 to 4 meters in the concrete (the minimum buried depth shall not be less than 1.5 meters) to avoid rolling the slurry near the rising surface of the concrete into the concrete. At the same time, the conduit should be lifted as the pouring progresses to avoid concrete voiding due to lifting too quickly or being unable to pull out the buried conduit due to lifting too late. During the pouring process, the slump, workability and setting time of the concrete should be strictly controlled. At the same time, the concrete should be poured continuously to avoid interruptions to ensure the uniformity and density of the concrete.

[0075] Step S1-4, casting the top crown beam of the supporting pile on the top surface of the supporting pile to connect each of the supporting piles to form a row of piles 4. Specifically, after the casting of the supporting piles is completed, excavation is carried out between the top piles, and the top crown beam is cast to form a row of piles 4 to ensure the structural stability and construction safety of the foundation pit 1. Specifically, the end face shape of the top crown beam is usually rectangular; the main function of the top crown beam is to connect multiple bored cast-in-place piles so that single isolated piles form an integral structure to improve the coordinated force and stability of the bored cast-in-place piles. The casting method of the top crown beam is as follows:

[0076] Step S1-4-1: In the early stage of pouring, groove cutting is carried out between piles, that is, on the ground or foundation between piles, a groove with a certain shape and size is excavated to provide space for the installation of the formwork and the arrangement of steel bars of the top capping beam.

[0077] Step S1-4-2: Place the formwork at the groove and fabricate the steel bars, and then carry out concrete pouring.

[0078] Step S1-4-3: During pouring, the method of layered pouring can be adopted, and the pouring thickness of each layer is controlled within a certain range (such as about 300 mm), and it is vibrated densely with a vibrator. After pouring is completed, sprinkle water for curing in time. The curing time should meet the design requirements.

[0079] Step S2: Set the freezing holes 2; a plurality of freezing holes 2 are evenly arranged around the row of piles; the depth of the freezing holes 2 is greater than the depth of the foundation pit 1. Specifically, as Figure 2 shown, around the row of piles and above the underground road, according to the results of on-site geological exploration, drill the freezing holes 2, and the drilling depth should exceed 1 / 3 of the original depth of the foundation pit 1. The deviation slope of the freezing holes 2 is not greater than 1%, ensuring the hole-forming quality. Specifically, in order to achieve that the deviation slope of the freezing holes is not greater than 1%, the drilling method is as follows: adopt the rotary drilling method to drill the freezing holes. Rotary drilling has the advantages of high drilling speed, stable hole wall, and simple construction. In order to ensure that the deviation slope of the drilled freezing holes is not greater than 1%, the electric logging method is used for inclinometry, such as the JJX-2 type or JX-4 type well inclinometer, etc.

[0080] Step S3: Set the freezing pipes 3. The cold flow outlet 5 and the heat flow inlet 6 of the freezing pipes 3 are both connected to the freezing device to form a closed loop. Specifically, as Figure 3 and Figure 4 shown, according to the design requirements, the freezing pipes are arranged in the soil layer, usually arranged in a circular or linear shape. The freezing pipes are made of steel pipes. The freezing pipes placed in the freezing holes include two vertical freezing pipes. The bottoms of the vertical freezing pipes are connected by a first horizontal connecting pipe. The length of the first horizontal connecting pipe matches the inner diameter of the freezing hole. The length of the vertical freezing pipe matches the depth of the freezing hole. The tops of the vertical freezing pipes in adjacent two freezing holes are connected by a second horizontal connecting pipe. The length of the second horizontal connecting pipe matches the distance between the two freezing holes. Each freezing pipe is connected into a whole through the alternately arranged first horizontal connecting pipe and second horizontal connecting pipe. At the same time, the initial end and the end of the second horizontal connecting pipe are both connected to the freezing device, which can be regarded as the cold flow outlet 5 and the heat flow inlet 6, forming a closed loop.

[0081] Among them, the bottom and top of the vertical freezing pipes are connected by horizontal through pipes parallel to the ground. Specifically, when the bottom and top of the vertical freezing pipes are connected by horizontal through pipes, connecting components such as flanges and quick connectors are installed at the bottom and top of the vertical freezing pipes, and then the horizontal through pipes are connected to these connecting components, thereby realizing the connection between the bottom and top of the vertical freezing pipes.

[0082] Taking the common flange connection as an example, flange plates are welded or installed at the bottom and top of the freezing pipes, and corresponding flange plates are also provided at both ends of the horizontal through pipe. They are tightly connected together by bolts to form a sealed connection channel, enabling the medium in the freezing pipes to flow through each other at the bottom. This connection method can not only ensure the connectivity between the bottom and top of the freezing pipes, but also ensure the sealing and stability of the connection, while avoiding problems such as damaging the structure of the freezing pipes, affecting the freezing effect, and causing groundwater leakage.

[0083] Step S4: Freeze the pipeline to form a frozen soil curtain.

[0084] The freezing device circulates low-temperature brine or other refrigerants through the freezing pipes; as the refrigerant circulates, the temperature inside the freezing pipes gradually decreases, and the moisture in the soil around the foundation pit 1 starts to freeze, gradually forming a frozen soil wall. Specifically, the freezing device includes a compressor, a condenser, an evaporator, and a refrigerant circulation pump. Compressor: Used to compress the refrigerant and provide refrigeration power. Condenser: Cools the high-temperature and high-pressure refrigerant gas into a liquid. Evaporator: The refrigerant absorbs heat in the evaporator to achieve a cooling effect. Refrigerant: Usually, brine or other low-temperature liquids are used as the coolant and are injected into the freezing pipes. Refrigerant circulation pump: Drives the refrigerant to circulate between the freezing pipes and the freezing device.

[0085] The process of freezing the pipeline is as follows: After the drilling of the freezing holes and the installation of the freezing pipes 3 are completed, the freezing system is started, and low-temperature brine or other refrigerants are circulated through the freezing pipes by the freezing device. Specifically, the power distribution cabinet of the power supply system is started to provide power for the refrigeration unit and other equipment. The compressor starts, compresses the refrigerant, and provides refrigeration power. The refrigerant absorbs heat in the evaporator to achieve a cooling effect. The condenser cools the high-temperature and high-pressure refrigerant gas into a liquid to form the refrigerant. The refrigerant flows out of the refrigeration unit, passes through the variable-frequency circulation pump, and flows into the freezing pipes (the initial end of the second horizontal through pipe). After the refrigerant absorbs heat in the freezing pipes, the refrigerant flows back to the liquid storage tank 8 from the freezing pipes (the end of the second horizontal through pipe) to complete the cycle. As the refrigerant circulates, the moisture in the surrounding soil starts to freeze and gradually forms frozen soil. After a period of freezing operation, the soil around the construction area will be frozen into a continuous, closed and frozen soil curtain with a certain thickness. The frozen soil curtain has high strength and stability and can effectively isolate the connection between groundwater and underground engineering.

[0086] Preferably, the freezing step also involves a monitoring system, including a temperature sensor: installed in the freezing pipes and the soil layer to monitor the temperature changes in real time. A data acquisition system: to collect and record the temperature data for easy analysis and control. A control system: to automatically adjust the operating state of the refrigeration unit according to the monitoring data.

[0087] Step S5, verify the water-stop effect; confirm whether there is flowing water in the soil layer within the curtain by setting exploration holes or conducting a water pressure test. Among them, confirming whether there is flowing water in the soil layer within the curtain by setting exploration holes includes the following steps:

[0088] Step S5-1, exploration hole layout: Determine the positions and quantities of the exploration holes according to the design requirements and geological conditions of the curtain project. The exploration holes should be arranged at the key parts of the curtain and the areas where flowing water may exist.

[0089] Among them, the key parts of the curtain include: the corners, endpoints, and the connection points with the subsequent support structures of the curtain. The stress distribution at the corners is relatively complex, and stress concentration is likely to occur, resulting in the curtain bearing greater pressure and deformation at these parts. It is a weak link in the curtain structure and requires key detection and reinforcement to ensure the integrity and stability of the curtain. The endpoints of the curtain are the connection points with the surrounding rock and soil masses or other structures. The connection quality here directly affects the water-stop effect of the curtain. If the endpoints are not properly treated, leakage channels are likely to be formed, so it is one of the key parts. When connected to the construction support structure, due to possible differences in the material properties, deformation laws, etc. of different structures, gaps or weak surfaces are likely to be generated at the connection points, which are areas with a high risk of leakage. Exploration holes are needed to accurately grasp the geological conditions at these parts so as to take corresponding treatment measures.

[0090] Among them, the areas where flowing water may exist include areas with a high groundwater level: A high groundwater level means that the pores in the rock and soil masses are filled with a large amount of water and have a certain water head pressure, and the water has strong fluidity. In such areas, once there are defects in the curtain or the construction quality is poor, leakage is likely to occur. Therefore, exploration holes need to be arranged to monitor the groundwater flow situation in order to detect potential leakage channels in a timely manner.

[0091] The areas where flowing water may exist also include areas of strata with good permeability: Such as strata with strong permeability such as sand layers and gravel layers. The groundwater has little resistance to flow in these strata and is easy to form flowing water. Through exploration holes, the permeability characteristics of these strata and the movement laws of groundwater can be accurately grasped, and the freezing temperature of the freezing pipes can be reasonably adjusted to change the permeability coefficient and thickness of the curtain to ensure the water-stop effect of the curtain.

[0092] Step S5-2, exploration hole construction: Use appropriate drilling equipment to drill the exploration holes according to the predetermined positions and angles. The drilling depth should reach the bottom of the curtain or the depth required by the design.

[0093] Step S5-3, In-hole Observation: After the exploration hole is drilled, use observation equipment (such as water level gauges, flow meters, etc.) to observe the water level, water flow velocity, etc. in the hole.

[0094] Specifically, determine whether there is flowing water in the soil layer within the curtain by measuring the water level: Place the water level gauge into the exploration hole and measure the change of the water level in the hole. If the water level is stable and consistent with the surrounding groundwater level, it is preliminarily judged that there may be no obvious flowing water at this position. However, if the water level shows an obvious rising or falling trend, or there is a large difference from the surrounding groundwater level, there may be flowing water.

[0095] It is also possible to determine whether there is flowing water in the soil layer within the curtain by detecting the water flow velocity: Place the flow meter probe at an appropriate position in the exploration hole and record the water flow velocity data. When there is flowing water, there will be a certain numerical display of the water flow velocity, and the water flow velocities at different positions may be different.

[0096] Step S5-4, Data Processing and Analysis: Process and analyze the observed data to determine whether there is flowing water in the soil layer within the curtain. If flowing water is found, further analyze the source and scope of the flowing water.

[0097] Among them, confirming whether there is flowing water in the soil layer within the curtain by means of a hydraulic test includes the following steps:

[0098] Step S5-a, Preparation: Determine the test pressure and test time of the hydraulic test, and prepare the equipment and materials required for the test, such as water pumps, pressure gauges, water pipes, etc. Check the equipment to ensure that equipment such as pressure pumps, pressure gauges, and pipelines is in good condition.

[0099] Step S5-b, Drilling and Sealing: Select an appropriate position within the curtain for drilling. The drilling depth should reach the bottom of the curtain or the depth required by the design; Remove the debris in the hole to ensure the hole is unobstructed; Use sealing materials to seal the drilling hole to ensure that the seal is tight and there is no leakage.

[0100] Step S5-c, Installing and Connecting Test Equipment: Install equipment such as water pumps and pressure gauges on the test pipeline, and connect the water supply pipeline and monitoring instruments to ensure that all connections are tight and there is no leakage.

[0101] Step S5-d, Boost Pressure and Observe: Start the water pump and gradually increase the water pressure in the test pipeline. During the pressure boost process, closely observe the changes in the pressure gauge and the conditions around the curtain. If a pressure drop is detected or leakage occurs around the curtain, immediately stop boosting the pressure and find the cause. Specifically: Slowly inject water into the borehole, record the initial water level and pressure; keep the water level stable and observe the pressure changes; slowly increase the water pressure and record the pressure and water level after each pressure increase. Observe whether the water level and pressure are stable, record any abnormal fluctuations, maintain the set pressure for a period of time, observe the changes in pressure and water level, and continuously record the pressure and water level data to ensure there are no significant changes. Gradually reduce the pressure and record the pressure and water level changes during the pressure relief process, paying attention to whether the water level recovers as the pressure drops. Through the above tests, analyze and judge whether there is flowing water in the soil layer within the curtain. Generally, stable pressure and water level indicate no flowing water.

[0102] Step S5-c, Data Processing and Analysis: Record the pressure change data and observation results during the test. Analyze and process the data to determine whether there is flowing water in the soil layer within the curtain. If flowing water is found, further analyze the source and scope of the flowing water and take corresponding treatment measures. It should be noted that during the water pressure test, ensure that the test pressure does not exceed the bearing capacity of the curtain and the surrounding soil layer to avoid damage to the curtain and the soil layer. At the same time, closely monitor the conditions around the curtain during the test. If any abnormal situation is found, immediately stop the test and take corresponding treatment measures.

[0103] Step S6, Excavate Foundation Pit 1. Specifically, select appropriate machinery to excavate in the predetermined excavation area, determine the excavation sequence and corresponding slopes, and generally follow the principle of "excavate deeper parts first and in layers". Among them, "excavate deeper parts first" means that during the excavation of the foundation pit, give priority to excavating the areas with greater depth, and then excavate the relatively shallower areas. This can prevent the shallow soil layer from being subjected to excessive pressure during excavation, resulting in collapse or deformation. It can start from a corner or one side of Foundation Pit 1 and gradually advance towards the center or the other side. Adopting excavation from the periphery to the center can help reduce the soil pressure at the edge of Foundation Pit 1 and prevent soil collapse. At the same time, it can also facilitate the construction of the subsequent support structure.

[0104] Specifically, this application adopts the method of excavating Foundation Pit 1 from the center to the periphery. The specific excavation method is as follows:

[0105] According to the designed depth and geological conditions of Foundation Pit 1, determine the deepest part, which is generally the central area of Foundation Pit 1 or the part with deeper foundation requirements. Excavate layer by layer from the deepest part to the shallow part.

[0106] Starting from the ground, first conduct layered excavation of these deepest parts, that is, the excavation process is divided into several layers and carried out layer by layer. The excavation depth of each layer is determined according to the soil quality and construction conditions. Usually, the excavation depth of each layer is 1-2 meters and then gradually expands towards the surrounding areas. This can ensure that the soil layer with a greater depth in the central area is excavated first, reducing the disturbance to the surrounding soil mass. After the excavation in the deep central area reaches a certain depth, start gradually excavating towards the relatively shallow areas around. At this time, the excavation in the shallow layer area can be coordinated with the subsequent excavation and construction processes in the deep layer area to ensure the uniform excavation depth of the entire foundation pit 1. During the excavation process, corresponding support, dewatering and other work will be carried out simultaneously to ensure the safety and stability of the excavation. To maintain the stability of foundation pit 1, usually a certain slope is set at the edge of foundation pit 1, and the slope direction inclines from the edge of foundation pit 1 towards the center. This can prevent the slope from collapsing and ensure the construction safety.

[0107] Step S7, excavate the opening 9 in the foundation pit section;

[0108] Step S7-1, damage the support piles of the pre-excavation opening section and cut off the freezing pipes. The bottom of the remaining freezing pipes is arranged around the excavation opening.

[0109] Step S7-2, advance support for the opening. Specifically, when excavating the opening 9 in the foundation pit section, according to the geological conditions, advance support measures such as grouting with small advance pipes and construction of large pipe roofs are carried out to ensure the stability during the excavation of the opening, and the cut-and-cover method is selected for the excavation of the soil under the road.

[0110] Specifically, the grouting with small advance pipes includes the following steps:

[0111] Construction preparation: Prepare small advance pipes, grouting equipment, grout materials, etc., and measure and set out the position of the opening 9 and the hole positions of the small advance pipes.

[0112] Drilling: Drill holes around the opening 9 with a drill according to the designed angle and depth.

[0113] Insert the small advance pipes: Insert the processed small advance pipes into the drilled holes. The front end of the small advance pipes should have a taper, and the tail end should be welded to the steel arch or connected to the steel mesh to enhance the support effect.

[0114] Seal the hole orifice: Seal the hole orifice with materials such as cotton yarn and cement slurry to prevent the grout from leaking out during grouting.

[0115] Grouting: Inject the prepared grout (such as cement slurry, cement mortar, etc.) into the small advance pipes through a grouting pump, so that the grout penetrates into the surrounding soil mass under the action of pressure, playing a role in strengthening the soil mass and stopping water. During the grouting process, control the grouting pressure and grouting volume well.

[0116] Effect inspection: After grouting is completed, the grouting effect is inspected by methods such as core drilling and water pressure test. If the design requirements are not met, supplementary grouting should be carried out.

[0117] Specifically, the construction of the large pipe shed includes the following steps:

[0118] Construction preparation: It includes site leveling, equipment selection, material procurement, etc. At the same time, a concrete guide wall is constructed at the portal 9 as the guiding and supporting structure of the pipe shed.

[0119] Surveying and setting out: Accurately measure parameters such as the position, angle, and length of the pipe shed to determine the specific positions of the drill holes.

[0120] Drilling: Use a large drilling rig to drill according to the design parameters. During the drilling process, pay attention to controlling the direction and perpendicularity of the drill hole, and monitoring equipment such as an inclinometer can be used for monitoring.

[0121] Installing the pipe shed steel pipes: Place the processed steel pipes into the drill holes one by one, and connect the steel pipes through screw threads or welding. The pipe shed steel pipes generally use seamless steel pipes with a relatively large diameter, and grouting holes should be drilled on the pipe wall according to the design requirements.

[0122] Grouting: After the pipe shed is installed, grouting operations are carried out. First, seal the hole opening, and then inject the grout into the pipe shed through the grouting pipe, so that the grout fills the gap between the steel pipe and the hole wall and penetrates into the surrounding soil to achieve the purpose of soil reinforcement. The grouting material is usually cement slurry or cement-sodium silicate double-fluid slurry.

[0123] Quality inspection: Inspect the installation quality and grouting effect of the pipe shed, such as checking whether the spacing, angle, and length of the steel pipes meet the design requirements, and detecting the soil reinforcement effect after grouting by means of geological radar, etc.

[0124] Step S7-3, initial support at the portal; Specifically, after the excavation of the portal 9, install the steel arch frame in a timely manner. The steel arch frame is laid along the portal wall according to the shape of the portal. Spray concrete on the steel arch frame, and after the concrete stands still and reaches the final setting state, an initial support structure is formed to control soil deformation and maintain the stability of the excavation face.

[0125] Step S8, freezing pipe modification; Reconnect the freezing pipes around the portal. Specifically, as Figure 5 shown, since the excavation of the underground road inward will cause the original freezing pipe 2 to be cut off, after the portal 9 is built as the underground road portal, select a suitable iron pipe to weld the bottom of the original cut-off freezing pipe 3, and reconnect and overlap the freezing pipes to make them a complete circuit.

[0126] Specifically, the structure is optimized for the condition of the freezing pipe fracture caused by the excavation of the underground road. When the underground road construction advances to the area where the freezing pipes are laid, the excavation operation will cause structural fractures in the originally laid freezing pipes 2. To solve this technical problem, after the main structure construction of the portal 9 is completed, a step-by-step repair process is adopted: Step S8-1, first, bevel the cut at the bottom of the fractured freezing pipe, that is, process a bevel of a certain shape at the cut to meet the requirements of welding, connection or other processes. Select a low-temperature-resistant alloy steel pipe with the same strength as the base material as the transition connector, and implement a circumferential butt joint through a hot-melt welding process, and weld the transition connector at the cut of the freezing pipe 2. Step S8-2, use a flange seal assembly to secondary reinforce the repair connection at the bottom of the freezing pipe and the transition connector to form a multi-layer sealing structure. The flange can be the same flange used in Step S3 as described above. Step S8-3, finally, inject a low-temperature curing sealing colloid into the transition connector through a pressure injection system to construct a composite anti-seepage system.

[0127] This innovative method can restore the fractured freezing pipes to form a series-connected closed circulation loop, effectively ensuring the continuity of the freezing curtain. In particular, the transition connector adopts a corrugated compensation structure design. Specifically, a component with a corrugated structure is used between the bottom of the freezing pipe and the transition connector to achieve the design method of the compensation function, which is mainly composed of a corrugated pipe and the connecting components connected thereto. Utilize the stretchability and flexibility of the corrugated pipe to adapt to the dimensional changes and displacements of the freezing pipe caused by factors such as thermal expansion and contraction, pipeline deformation, and equipment vibration, thereby protecting the safe and stable operation of the pipeline system. When the freezing pipe undergoes expansion or displacement, the corrugated pipe will stretch or compress accordingly to absorb the deformation stress brought by the formation displacement, and avoid damage to the freezing pipe and the connector due to excessive stress. Among them, the welding process parameters are controlled within the current range of 380-420A, and the interlayer temperature is kept not lower than 150°C to ensure the ductility of the weld metal under low-temperature conditions. After testing, the pressure-bearing capacity of the repaired freezing system reaches 105% of the original design standard, and the leakage rate is lower than 0.3L / (m·d), which is significantly better than the conventional repair process.

[0128] Step S9, geothermal energy collection; use the originally undemolished freezing pipe as the water delivery pipe and serve as the geothermal energy collection pipe; the first end of the freezing pipe is used as the cold flow outlet end and is connected to the variable-frequency circulation pump of the refrigeration unit (power 3kW, flow rate 15m 3 / h), the second end of the freezing pipe serves as the heat flow return end and is connected to the liquid storage tank 8; cold water enters the first end of the freezing pipe, absorbs the heat of the shallow geothermal energy and then becomes hot water, and then returns to the liquid storage tank 8 connected to the heat exchanger 7 through the second end of the freezing pipe, realizing heat energy transfer. Specifically, the originally undemolished freezing pipe is used as the water delivery pipe. The first end of the freezing pipe, that is, the initial end of the second horizontal through pipe, is connected to the variable-frequency circulation pump of the refrigeration unit to realize intelligent adjustment of the flow rate, ensuring that the residence time of the water flow in the freezing pipe ≥ 120 seconds. The countercurrent heat exchange mode is adopted, and the cold water forms a reverse flow with the geothermal gradient in the water delivery pipe. Usually, geothermal resources transfer heat from the hotter area inside the earth to the colder area near the surface, with a gradually changing temperature gradient, that is, the geothermal gradient. The cold water flows from the area with a relatively lower temperature along the water delivery pipe towards the area near the surface with a relatively higher temperature, that is, in the opposite direction of the geothermal gradient. This situation is called "forming a reverse flow with the geothermal gradient". This reverse flow is beneficial for the cold water to fully absorb the geothermal heat. Because during the reverse flow process, the cold water continuously exchanges heat with the hotter geothermal rocks or geothermal water around it as it flows along the pipe, and as the geothermal temperature it encounters gets higher and higher, it can continuously absorb heat, thus improving the geothermal collection efficiency. Through computational fluid dynamics (CFD) optimization, the optimal flow rate is determined to be 0.35 m / s. At this time, the Nusselt number can reach 125, and the heat exchange efficiency is increased by 40% compared with the conventional system. Among them, the Nusselt number reflects the strength of the convective heat transfer process. The larger the Nusselt number, the stronger the convective heat transfer, which means that the fluid can quickly transfer heat from one place to another, and the heat exchange efficiency of the equipment is higher. While the smaller the Nusselt number, the relatively weaker the effect of convective heat transfer, and the heat transfer depends more on the thermal conductivity of the fluid itself, and the overall heat exchange efficiency is relatively low.

[0129] The hot water returns to the liquid storage tank from the second end (the end of the second horizontal through pipe) of the water delivery pipe. The liquid storage tank 8 adopts a phase change material heat storage tank, and paraffin / graphite composite phase change material (latent heat value 220 kJ / kg) is filled in the phase change material heat storage tank. The step-by-step storage of sensible heat and latent heat is realized through a plate heat exchanger, and the system thermal inertia is increased by 60%. In this way, the collection of geothermal energy is realized. The collected geothermal energy can ultimately achieve road constant temperature to facilitate deicing roads in rainy and snowy weather.

[0130] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are within the scope of the claims of the present invention awaiting approval.

Claims

1. A construction method for an underground freezing water stop and shallow geothermal collection system, characterized in that The construction method comprises the following steps: Step S1, evenly arranging a plurality of support piles around the foundation pit to form a row of piles; Step S1-1, determining the diameter of each of the support piles, and determining the spacing between adjacent support piles; Step S1-2, drilling according to the diameter of the support piles and the spacing of the support piles; the drilling depth corresponds to the depth of the foundation pit; Step S1-3, pouring the drilled holes to form support piles; Step S1-4, casting a top crown beam of the supporting pile on the top surface of the supporting pile to connect each of the supporting piles to form a row of piles; Step S2, setting freezing holes: a plurality of freezing holes are evenly set around the periphery of the pile row; the depth of the freezing holes is greater than the depth of the foundation pit; Step S3, setting a freezing pipe, wherein the cold flow outlet and the hot flow inlet of the freezing pipe are both connected to the freezing device to form a closed loop; Step S4, freezing the pipes to form a frozen soil curtain; the freezing device circulates low-temperature salt water or freezing liquid through the freezing pipes; as the freezing liquid circulates, the temperature in the freezing pipes gradually decreases, and the moisture in the soil around the foundation pit begins to freeze, gradually forming a frozen soil wall; Step S5, verifying the water-stopping effect, confirming whether there is flowing water in the soil layer inside the curtain by setting a test hole or a water pressure test; Step S6, excavation of foundation pit; according to the design depth of the foundation pit and geological conditions, determine the deepest part; excavate layer by layer from the deepest part to the shallow part; the excavation depth of each layer is 1-2 meters and then gradually expands to the surrounding areas; Step S7, cutting off the freezing pipe and excavating a hole in the foundation pit section; Step S8, freezing pipe modification: reconnecting the freezing pipe around the hole; Step S9: geothermal energy collection.

2. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S1-3, pouring the drilled holes to form support piles, includes the following steps: Step S1-3-1, installing the steel cage; Step S1-3-2, using a spherical bladder or a prefabricated cylindrical concrete water stopper to open the conduit; Step S1-3-3, pouring.

3. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S1-4 includes the following steps: Step S1-4-1, in the early stage of pouring, slotting is performed between the piles; Step S1-4-2, placing a template in the groove and preparing steel bars; Step S1-4-3, during pouring, adopt the layered pouring method, control the pouring thickness of each layer within 300mm, and use a vibrating rod to vibrate and compact it. After pouring is completed, watering and curing are carried out.

4. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S2 includes: drilling freezing holes according to the results of on-site geological exploration, the drilling depth must exceed 1 / 3 of the original foundation pit depth, and the deflection rate of the freezing holes is not greater than 1%.

5. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that Step S3, comprising: the freezing pipe is a steel pipe, the freezing pipe placed in the freezing hole includes two vertical freezing pipes, the bottoms of the vertical freezing pipes are connected by a first transverse through pipe, the length of the first transverse through pipe matches the inner diameter of the freezing hole, and the length of the vertical freezing pipe matches the depth of the freezing hole; The tops of the vertical freezing pipes in two adjacent freezing holes are connected by a second transverse through pipe, and the length of the second transverse through pipe matches the spacing between the two freezing holes; Each freezing pipe is connected into a whole by means of the staggered first transverse through pipe and the second transverse through pipe; at the same time, the initial end and the tail end of the second transverse through pipe are both connected to the freezing device to form a closed loop.

6. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S4 includes the following steps: starting the power distribution cabinet of the power supply system to provide power to the refrigeration unit; starting the compressor to compress the refrigerant to provide refrigeration power; the refrigerant absorbs heat in the evaporator; the condenser cools the high-temperature and high-pressure refrigerant gas into liquid to form a freezing liquid; the freezing liquid flows out of the refrigeration unit, passes through the variable frequency circulation pump, and flows into one end of the freezing pipe. After the freezing liquid absorbs heat in the freezing pipe, it flows back to the liquid storage tank from the other end of the freezing pipe to complete the cycle; as the freezing liquid circulates, the moisture in the surrounding soil begins to freeze, and frozen soil is gradually formed.

7. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that Step S5, confirming whether there is flowing water in the soil layer in the curtain by setting a test hole, includes the following steps: Step S5-1, arranging the test holes; the test holes are arranged at key parts of the curtain and areas where flowing water may exist; the key parts of the curtain include: the corners and end points of the curtain, and the connection with the subsequent support structure; the areas where flowing water may exist include areas with high groundwater levels; and areas with good permeability. Step S5-2, construction of a test hole; Step S5-3, observation inside the hole; Step S5-4, data processing and analysis; Confirming whether there is flowing water in the soil layer inside the curtain by means of a water pressure test includes the following steps: Step S5-a, preparation; Step S5-b, drilling and plugging; Step S5-c, installing and connecting the test equipment; Step S5-d, pressurization and observation: start the water pump to gradually increase the water pressure in the test pipeline; during the pressurization process, observe the changes in the pressure gauge and the conditions around the curtain. If a pressure drop or leakage is found around the curtain, stop pressurization and find the cause.

8. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S7, cutting off the freezing pipe and excavating a hole in the foundation pit section, includes the following steps: Step S7-1, destroying the support piles of the pre-excavated hole section, and cutting off the freezing pipe, and the bottom of the remaining freezing pipe is set around the excavated hole; Step S7-2, advance support of the tunnel entrance; Step S7-3: initial support of the tunnel entrance.

9. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S8, freezing pipe transformation includes the following steps: Step S8-1, firstly, the cut portion of the bottom of the broken freezing pipe is beveled, and a low-temperature resistant alloy steel pipe with the same strength as the parent material is selected as a transition connector, and an annular butt joint is performed by welding; Step S8-2, using a flange seal assembly to perform secondary reinforcement on the repair connection between the bottom of the freezing pipe and the transition connector; Step S8-3, finally, injecting the low-temperature curing sealing colloid into the transition piece through the pressure injection system.

10. The construction method of the underground freezing water stop and shallow geothermal collection system according to claim 1, characterized in that, Step S9, geothermal energy collection includes the following steps: The unremoved freezing pipe is used as a geothermal energy collection pipe; the first end of the freezing pipe is connected to the variable frequency circulation pump of the refrigeration unit as the cold flow outlet; the second end of the freezing pipe is connected to the liquid storage tank as the hot flow return end; cold water enters the first end of the freezing pipe and becomes hot water after absorbing the heat of shallow geothermal heat, and then flows back to the liquid storage tank connected to the heat exchanger through the second end of the freezing pipe to realize heat energy transfer.

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