River-side water-rich stratum foundation pit totally-closed freezing mechanism and reverse construction method

Abstract

The invention discloses a river-side water-rich stratum foundation pit totally-closed freezing mechanism which comprises a foundation pit well, a shield tunnel, freezing pipes and a freezing system, a plurality of freezing holes arranged in a circle are formed in soil on the outer side of the foundation pit well, and the freezing pipes are installed in the freezing holes; the freezing system comprises a freezing station, a liquid supply pipe, a liquid return pipe, a liquid supply main pipe and a liquid return main pipe, the liquid supply main pipe and the liquid return main pipe are connected to the freezing station, and the liquid supply pipe and the liquid return pipe are installed in the freezing pipes and connected to the liquid supply main pipe and the liquid return main pipe respectively. The invention further discloses a reverse construction method of the river-side water-rich stratum foundation pit totally-closed freezing mechanism. The device is compact in structure, the freezing pipe and the soil body around the freezing pipe are frozen through the freezing system, the strength and stability of the soil body are improved, underground water is effectively isolated, accidents such as water inrush and sand gushing of a foundation pit are avoided, and construction safety is guaranteed; the process is simple and easy to operate, the construction speed is high, and the construction cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of building construction, in particular to a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river and a reverse construction method. Background Art

[0002] During shield tunnel construction, a starting working shaft and a receiving working shaft of the shield machine need to be set up at the starting and ending points of the shield tunneling respectively. After completion, the working shafts are mostly used as permanent structures such as subway stations, drainage, and freight.

[0003] Currently, when the engineering geological structure is stable and has strong bearing capacity, the shield working shaft is generally constructed using the open-cut method, and the foundation pit is supported by "retaining piles + internal supports." However, when encountering water-rich and weak soils such as sand, silt, and silt, the lateral water and soil pressure on the foundation pit of the sequential construction method increases linearly from the top to the bottom of the pit. The retaining structure below the foundation pit is extremely prone to sand and water leakage, and may even cause safety accidents such as overall deformation and collapse of the foundation pit. In this case, underground continuous walls are generally used for soil support. This is not only costly and time-consuming, but also seriously squeezes the ground space, limiting the working surface and making it impossible to implement. Summary of the Invention

[0004] The purpose of the present invention is to provide a fully enclosed freezing mechanism and a reverse construction method for a foundation pit in a water-rich stratum near a river, so as to solve the above defects.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river comprises a foundation pit, a shield tunnel, freezing pipes and a freezing system, wherein the shield tunnel is vertically opened on the inner wall of the foundation pit, and the soil outside the foundation pit is provided with a plurality of freezing holes arranged in a circle, and the freezing pipes are provided in a plurality of ways and are installed one by one in the freezing holes; the freezing system comprises a freezing station, a liquid supply pipe, a liquid return pipe, a liquid supply main pipe and a liquid return main pipe, the liquid supply main pipe and the liquid return main pipe are connected to the freezing station, the liquid supply pipe and the liquid return pipe are provided in a plurality of ways and are installed one by one in the freezing pipes, and the upper ends of the liquid supply pipe and the liquid return pipe are respectively connected to the liquid supply main pipe and the liquid return main pipe.

[0007] Preferably, the foundation pit is composed of a crown beam and a first section well wall at the top, multiple standard section well walls in the middle, and a last section well wall and a sealing bottom layer at the bottom. The first section well wall, the standard section well wall and the last section well wall are tubular structures cast with reinforced concrete, and the crown beam, the first section well wall, the last section well wall and the sealing bottom layer are all reinforced concrete structures cast as one piece.

[0008] Preferably, the freezing station includes: a brine tank, a clean water tank, a refrigeration unit, a brine pump, and a cooling tower. The brine tank is used to store brine, the clean water tank provides make-up water for the system, the refrigeration unit reduces the brine temperature through a refrigeration cycle, the brine pump is responsible for circulating the brine in the system, and the cooling tower is used to dissipate the heat generated by the refrigeration unit; the brine tank is connected to the brine pump, and the outlet of the brine pump is connected to the liquid supply main through a pipeline. The refrigeration unit is connected to the brine tank through a pipeline for cooling the brine; the cooling tower is connected to the refrigeration unit to take away heat; the clean water tank and the brine tank are connected by a pipeline and a valve, and clean water can be added to the brine tank as needed.

[0009] Preferably, a sealed end cover is installed on the top of the freezing pipe, the lower end of the liquid supply pipe is installed to the bottom of the freezing pipe, and its upper end passes through the end cover and is connected to the liquid supply main pipe through a connecting branch pipe; the lower end of the return liquid pipe is installed to the upper side of the freezing pipe, and its upper end passes through the end cover and is connected to the return liquid main pipe through a connecting branch pipe.

[0010] Preferably, control valves are installed on the connecting branches connected to the liquid supply main pipe and the liquid return main pipe.

[0011] Preferably, a portal steel ring and a sealed safety door are provided at one end of the shield tunnel located on one side of the foundation pit, and the sealed safety door is installed on the inner side of the portal steel ring.

[0012] Preferably, the distance between two adjacent freezing holes is no more than 900 mm, and the distance between the freezing hole and the wall of the foundation pit is no more than 300 mm; assuming that the depth of the foundation pit is h and its diameter is D, the depth H of the freezing hole is no less than h+D.

[0013] Preferably, a number of temperature measuring holes arranged in a circle are provided in the soil outside the freezing hole, the distance between the temperature measuring holes and the freezing hole is not greater than 800 mm, the depth of the temperature measuring hole is the same as the depth of the freezing hole, and a temperature sensor is installed in the temperature measuring hole and connected to the control system of the freezing station.

[0014] Preferably, the bottom of the freezing pipe is sealed and welded with a bottom cone, and the length L of the freezing pipe lowered into the freezing hole satisfies: H≤L≤L+0.5, in meters.

[0015] Preferably, a reverse construction method of a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river comprises the following steps:

[0016] S1. Vertical freezing hole construction:

[0017] Plan the positions of several freezing holes and temperature measuring holes in a circle at a certain position outside the preset position of the foundation pit, then use a geological drill to drill holes, and finally install freezing pipes in the freezing holes;

[0018] S2. Freeze system installation:

[0019] According to the actual situation on site, the various equipment of the freezing station will be installed at the selected location and the pipelines and lines will be laid. The liquid supply pipe and liquid return pipe will be installed in the freezing pipe of the freezing hole. The liquid supply main pipe and liquid return main pipe will be laid above the freezing hole and connected to the freezing station. The liquid supply pipe and liquid return pipe will then be connected to the liquid supply main pipe and liquid return main pipe respectively. Finally, the temperature sensor will be installed in the temperature measuring hole and connected to the control system of the freezing station.

[0020] S3. Surrounding soil freezing:

[0021] Carry out equipment commissioning and trial operation of the installed freezing system, and adjust the operating parameters of the freezing system to achieve active freezing and maintenance freezing of the soil around the foundation pit;

[0022] S4. Freezing effect detection:

[0023] The soil temperature data is obtained through the temperature sensors installed in the temperature measuring holes. The flow rate and temperature data of the brine in the freezing system are obtained through the flow meters and temperature sensors installed in the various equipment and pipelines in the freezing station to evaluate the soil freezing effect. The freezing system operating parameters are adjusted in a timely manner to adjust the soil freezing effect.

[0024] S5. Construction of crown beam and first section of shaft wall:

[0025] Excavate the foundation pit at the preset location, then carry out the following steps: pouring the cushion layer, welding and tying the steel bars, installing the formwork, pouring the concrete, removing the formwork, and curing the concrete, completing the construction of the crown beam and the first section of the well wall;

[0026] S6. Standard section shaft wall construction:

[0027] After the concrete strength of the crown beam and the first section of the shaft wall reaches %, continue to excavate the soil in the foundation pit, and then carry out the cushion layer pouring, steel bar welding and binding, formwork installation, concrete pouring, formwork removal and concrete curing in sequence to complete the construction of the second section of the shaft wall; similarly, carry out the construction of all standard sections of the shaft wall including the third section, the fourth section, and up to the Nth section of the shaft wall in sequence;

[0028] S7, construction of the last section of the shaft wall and bottom sealing layer:

[0029] Continue excavating the soil of the last section shaft wall and bottom sealing layer in the foundation pit; position and hoist the portal steel ring at the predetermined position of the shield tunnel on the inner wall of the foundation pit; weld and tie the steel bars of the bottom sealing layer and pour concrete; weld and tie the steel bars of the last section shaft wall and the shield portal outside the portal steel ring, and weld and tie the shield portal steel bars to the steel bars of the last section shaft wall; then proceed with formwork installation, concrete pouring, formwork removal, concrete curing, and installation of the sealed safety door, thus completing the construction of the shield portal, last section shaft wall, and bottom sealing layer;

[0030] S8. Dismantling of freezing system and melting and grouting:

[0031] The brine in the brine tank of the freezing system is heated and circulated, the freezing pipe is removed and the freezing system is disassembled, and finally the freezing hole is filled with grouting, thereby completing the entire process of the reverse construction method of the fully enclosed freezing mechanism of the foundation pit in the water-rich stratum near the river.

[0032] The beneficial effects of the present invention are:

[0033] (1) The present invention provides a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river. The mechanism has a compact structure. Several freezing holes are provided in the soil outside the foundation pit and freezing pipes are installed. The freezing pipes and the surrounding soil are frozen by the freezing system, thereby improving the strength and stability of the soil, effectively isolating groundwater, avoiding accidents such as water inrush and sand gushing from the foundation pit, and ensuring construction safety.

[0034] (2) The reverse construction method of the fully enclosed freezing mechanism of a foundation pit in a water-rich stratum near a river according to the present invention is that the main body of the foundation pit is constructed layer by layer from the ground to the underground, and the excavation is carried out alternately and cyclically. The main structure is formed layer by layer and constitutes a stable support system together with the frozen soil. The process is simple and easy to operate, which is beneficial to project quality control and has a fast construction speed. In addition, no internal support is required, which reduces construction costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a structural diagram of the fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river according to the present invention;

[0036] Figure 2 It is a structural schematic diagram of the freezing pipe and freezing system of the fully enclosed freezing mechanism of the foundation pit in the water-rich stratum near the river of the present invention;

[0037] Figure 3 The present invention is a process flow chart of the reverse construction method of the fully enclosed freezing mechanism of the foundation pit in the water-rich stratum near the river. DETAILED DESCRIPTION

[0038] The present invention is further described below with reference to the embodiments. It should be noted that these are merely examples and illustrations of the concept of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should be deemed to fall within the scope of protection of the present invention.

[0039] Example 1:

[0040] like Figure 1-2 As shown, a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river includes a foundation pit 1, a shield tunnel 2, freezing pipes 3 and a freezing system 4. The shield tunnel 2 is vertically opened on the inner wall of the foundation pit 1. A plurality of freezing holes arranged in a circle are provided in the soil 5 outside the foundation pit 1. A plurality of freezing pipes 3 are provided and are installed one by one in the plurality of freezing holes.

[0041] The freezing system 4 includes a freezing station, a liquid supply pipe 41, a liquid return pipe 42, a liquid supply main pipe 43 and a liquid return main pipe 44. The liquid supply main pipe 43 and the liquid return main pipe 44 are connected to the freezing station. There are several liquid supply pipes 41 and liquid return pipes 42 and they are installed one by one in several freezing pipes 3. The upper ends of the liquid supply pipe 41 and the liquid return pipe 42 are respectively connected to the liquid supply main pipe 43 and the liquid return main pipe 44.

[0042] The brine is frozen and cooled by the freezing station, and the low-temperature brine is input into the freezing pipe 3 through the liquid supply main pipe 43 and the liquid supply pipe 41, and then flows back to the freezing station through the return liquid pipe 42 and the return liquid main pipe 44 to continue freezing. The low-temperature brine in the freezing pipe 3 can freeze the soil around the freezing pipe 3, so that the water-rich soft strata containing sand, silt, silt, etc. are vertically frozen and reinforced, which improves the strength and stability of the soil, effectively isolates the groundwater, avoids the occurrence of accidents such as water inrush and sand gushing in the foundation pit, and ensures construction safety.

[0043] Example 2:

[0044] like Figure 1-2 As shown, a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river includes a foundation pit 1, a shield tunnel 2, freezing pipes 3 and a freezing system 4. The shield tunnel 2 is vertically opened on the inner wall of the foundation pit 1. A plurality of freezing holes arranged in a circle are provided in the soil 5 outside the foundation pit 1. A plurality of freezing pipes 3 are provided and are installed one by one in the plurality of freezing holes.

[0045] The freezing system 4 includes a freezing station, a liquid supply pipe 41, a liquid return pipe 42, a liquid supply main pipe 43 and a liquid return main pipe 44. The liquid supply main pipe 43 and the liquid return main pipe 44 are connected to the freezing station. There are several liquid supply pipes 41 and liquid return pipes 42 and they are installed one by one in several freezing pipes 3. The upper ends of the liquid supply pipe 41 and the liquid return pipe 42 are respectively connected to the liquid supply main pipe 43 and the liquid return main pipe 44.

[0046] The freezing station includes: a brine tank, a clean water tank, a refrigeration unit, a brine pump, and a cooling tower. The brine tank is used to store brine, the clean water tank provides make-up water for the system, the refrigeration unit reduces the brine temperature through a refrigeration cycle, the brine pump is responsible for circulating the brine in the system, and the cooling tower is used to dissipate the heat generated by the refrigeration unit; the brine tank is connected to the brine pump, and the outlet of the brine pump is connected to the liquid supply main 43 through a pipeline. The refrigeration unit and the brine tank are connected by a pipeline for cooling the brine; the cooling tower is connected to the refrigeration unit to take away heat; the clean water tank and the brine tank are connected by a pipeline and a valve, and clean water can be added to the brine tank as needed.

[0047] A sealed end cap 31 is installed at the top of the freezing pipe 3. The lower end of the liquid supply pipe 41 is installed to the bottom of the freezing pipe 3. Its upper end passes through the end cap 31 and is connected to the liquid supply main pipe 43 via a connecting branch pipe 45. The lower end of the liquid return pipe 42 is installed on the upper side of the freezing pipe 3. Its upper end passes through the end cap 31 and is connected to the liquid return main pipe 44 via a connecting branch pipe 45. Control valves 46 are installed on the connecting branches 45 connected to the liquid supply main pipe 43 and the liquid return main pipe 44.

[0048] The distance between two adjacent freezing holes shall not exceed 900 mm, and the distance between the freezing hole and the wall of foundation pit 1 shall not exceed 300 mm; assuming that the depth of foundation pit 1 is h and its diameter is D, the depth H of the freezing hole shall not be less than h+D.

[0049] There are several temperature measuring holes arranged in a circle in the soil 5 outside the freezing hole. The distance between the temperature measuring holes and the freezing holes is not more than 800mm. The depth of the temperature measuring holes is the same as the depth of the freezing holes. Temperature sensors are installed in the temperature measuring holes and connected to the control system of the freezing station.

[0050] The bottom of the freezing pipe 3 is sealed and welded with a bottom cone. The length L of the freezing pipe 3 lowered into the freezing hole satisfies: H≤L≤L+0.5, in meters.

[0051] The brine is frozen and cooled by the freezing station, and the low-temperature brine is input into the freezing pipe 3 through the liquid supply main pipe 43 and the liquid supply pipe 41, and then flows back to the freezing station through the return liquid pipe 42 and the return liquid main pipe 44 to continue freezing. The low-temperature brine in the freezing pipe 3 can freeze the soil around the freezing pipe 3, so that the water-rich soft strata containing sand, silt, silt, etc. are vertically frozen and reinforced, which improves the strength and stability of the soil, effectively isolates the groundwater, avoids the occurrence of accidents such as water inrush and sand gushing in the foundation pit, and ensures construction safety.

[0052] Example 3:

[0053] like Figure 1-2As shown, a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river includes a foundation pit 1, a shield tunnel 2, freezing pipes 3 and a freezing system 4. The shield tunnel 2 is vertically opened on the inner wall of the foundation pit 1. A plurality of freezing holes arranged in a circle are provided in the soil 5 outside the foundation pit 1. A plurality of freezing pipes 3 are provided and are installed one by one in the plurality of freezing holes.

[0054] The freezing system 4 includes a freezing station, a liquid supply pipe 41, a liquid return pipe 42, a liquid supply main pipe 43 and a liquid return main pipe 44. The liquid supply main pipe 43 and the liquid return main pipe 44 are connected to the freezing station. There are several liquid supply pipes 41 and liquid return pipes 42 and they are installed one by one in several freezing pipes 3. The upper ends of the liquid supply pipe 41 and the liquid return pipe 42 are respectively connected to the liquid supply main pipe 43 and the liquid return main pipe 44.

[0055] The freezing station includes: a brine tank, a clean water tank, a refrigeration unit, a brine pump, and a cooling tower. The brine tank is used to store brine, the clean water tank provides make-up water for the system, the refrigeration unit reduces the brine temperature through a refrigeration cycle, the brine pump is responsible for circulating the brine in the system, and the cooling tower is used to dissipate the heat generated by the refrigeration unit; the brine tank is connected to the brine pump, and the outlet of the brine pump is connected to the liquid supply main 43 through a pipeline. The refrigeration unit and the brine tank are connected by a pipeline for cooling the brine; the cooling tower is connected to the refrigeration unit to take away heat; the clean water tank and the brine tank are connected by a pipeline and a valve, and clean water can be added to the brine tank as needed.

[0056] A sealed end cap 31 is installed at the top of the freezing pipe 3. The lower end of the liquid supply pipe 41 is installed to the bottom of the freezing pipe 3. Its upper end passes through the end cap 31 and is connected to the liquid supply main pipe 43 via a connecting branch pipe 45. The lower end of the liquid return pipe 42 is installed on the upper side of the freezing pipe 3. Its upper end passes through the end cap 31 and is connected to the liquid return main pipe 44 via a connecting branch pipe 45. Control valves 46 are installed on the connecting branches 45 connected to the liquid supply main pipe 43 and the liquid return main pipe 44.

[0057] The distance between two adjacent freezing holes shall not exceed 900 mm, and the distance between the freezing hole and the wall of foundation pit 1 shall not exceed 300 mm; assuming that the depth of foundation pit 1 is h and its diameter is D, the depth H of the freezing hole shall not be less than h+D.

[0058] There are several temperature measuring holes arranged in a circle in the soil 5 outside the freezing hole. The distance between the temperature measuring holes and the freezing holes is not more than 800mm. The depth of the temperature measuring holes is the same as the depth of the freezing holes. Temperature sensors are installed in the temperature measuring holes and connected to the control system of the freezing station.

[0059] The bottom of the freezing pipe 3 is sealed and welded with a bottom cone. The length L of the freezing pipe 3 lowered into the freezing hole satisfies: H≤L≤L+0.5, in meters.

[0060] The foundation pit 1 consists of a crown beam 10 and a first section of the shaft 11 at the top, multiple standard sections of the shaft 12 in the middle, a final section of the shaft 13 at the bottom, and a bottom seal 14. The first section, standard sections, and final sections are tubular structures cast in reinforced concrete. The crown beam 10, the first section, the final section, and the bottom seal 14 are all integrally cast reinforced concrete structures. A portal steel ring 21 and a sealed safety door 22 are installed at one end of the shield tunnel 2, located on one side of the foundation pit 1. The sealed safety door 22 is installed inside the portal steel ring 21.

[0061] like Figure 3 As shown, a reverse construction method of a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river comprises the following steps:

[0062] S1. Vertical freezing hole construction:

[0063] At a certain position outside the preset position of the foundation pit 1, several freezing holes and temperature measuring holes are planned to form a circle as a whole, and then a geological drilling rig is used to perform hole drilling operations, and finally the freezing pipe 3 is installed in the freezing hole.

[0064] S11. Arrangement of freezing holes and temperature measuring holes:

[0065] The structure of foundation pit 1 is circular, and freezing reinforcement adopts "cylindrical" freezing. A single row of freezing holes is arranged on the outside of the working well, with a hole spacing of 900mm. The freezing holes are 300mm away from the outer diameter of the well wall, and the depth of the freezing holes is twice the excavation diameter below the bottom of the well. That is, assuming that the depth of foundation pit 1 is h and its diameter is D, the depth H of the freezing hole is h+D.

[0066] Temperature measuring holes should be evenly arranged outside the freezing hole, with a minimum of five holes. The spacing between the temperature measuring holes and the freezing pipe should be 800mm, and the depth of the temperature measuring holes should be the same as the freezing holes. The freezing holes should be accurately positioned according to the construction control points and design drawings. The hole opening error should not exceed 50mm.

[0067] S12. Hole forming construction:

[0068] Freeze-hole construction uses a geological drill rig. Before construction, a slurry wall is constructed around the freeze-hole construction area to prevent slurry spillage, and a slurry circulation pool is set up to recycle the slurry. Once the drill rig is in place, a preliminary hole is drilled through the hardened pavement layer. To ensure drilling accuracy, the verticality of the drill rod is repeatedly checked during the first 5 meters of drilling, and the drill rig's position is adjusted as needed.

[0069] During the drilling of vertical freezing holes, the drill rig measures inclination every 15 meters, and any excessive deviation is promptly corrected. During this process, the inclinometer is first lowered, and the verticality of the freezing hole is measured using an inclinometer. All holes should undergo a final inclination measurement. After drilling is complete, the full length of the drill pipe is rechecked, and the hole is flushed with mud to ensure it is clean before lowering the freezing pipe.

[0070] The construction steps for the temperature measuring hole are the same as those for the freezing hole.

[0071] S13. Freezing pipe installation:

[0072] Before the freezing pipe 3 is lowered into the borehole, the pipe must be matched first to ensure that the concentric axes of the freezing pipe 3 coincide. The freezing pipe 3 is formed by welding multiple freezing single pipes.

[0073] Each individual freezing pipe is 12 meters long, and freezing pipe 3 must be lowered in sections. When lowering the first freezing pipe, first seal the bottom of the pipe and weld a bottom cone. Rebar is welded to the top of the freezing pipe to prevent it from falling into the hole. After the first pipe is lowered into place, use a crane to lift the second pipe, butt-join the ends, and weld them together to form a single piece. A 50x50mm steel plate is welded to the joint to secure it. This welding process continues until the required length is met. During welding, ensure a full weld to ensure sufficient strength for freezing pipe 3 to prevent breakage during removal. After freezing pipe 3 is installed, seal the end of the pipe with a wooden plug or other means to prevent foreign matter from falling into it. The length of freezing pipe 3 must not be less than the designed freezing depth and no more than 0.5 meters. The freezing depth refers to the effective freezing range required to ensure soil strength and stability and isolate groundwater during the freezing process. The freezing hole depth is an engineering design parameter designed to achieve this freezing depth. In actual construction, the length of the freezing pipe lowered into the freezing hole satisfies H≤L≤L+0.5 (in meters), which means that the length of the freezing pipe placed in the freezing hole must be adapted to the depth of the freezing hole to ensure effective freezing within the depth range of the freezing hole, thereby achieving the designed freezing depth requirements and ensuring the stability of the soil.

[0074] S2. Freeze system installation:

[0075] According to the actual situation on site, the locations for installing the various equipment of the freezing station will be selected and the pipelines and lines will be laid; the liquid supply pipe 41 and the liquid return pipe 42 will be installed in the freezing pipe 3 of the freezing hole, and the liquid supply main pipe 43 and the liquid return main pipe 44 will be laid above the freezing hole and connected to the freezing station, and then the liquid supply pipe 41 and the liquid return pipe 42 will be connected to the liquid supply main pipe 43 and the liquid return main pipe 44 respectively; finally, the temperature sensor will be installed in the temperature measuring hole and connected to the control system of the freezing station.

[0076] S21. Freezing station installation:

[0077] Calculate the freezing required cooling capacity based on the number of freezing pipes 3 and the freezing depth, and select appropriate freezing equipment and quantity based on the freezing required cooling capacity.

[0078] The freezing station is located on the ground near the entrance to foundation pit 1 (i.e., the working pit). The specific location will be determined based on the actual site conditions. The freezing equipment is selected based on the model, typically using two refrigeration units installed in parallel. Each refrigeration unit primarily consists of a brine tank, a fresh water tank, a refrigeration unit, a brine pump, and a cooling tower.

[0079] Freezing station installation process: equipment foundation layout → construction of equipment foundation (or anchoring anchor bolts) → equipment placement, leveling and fixing → pipe connection and cable laying between equipment → installation of electronic control system → freezer leak test → refrigeration unit fluorine filling and oil filling → water addition to brine tanks → salting → refrigeration system test operation of refrigeration unit → insulation of brine tanks and freezer low-temperature containers and pipelines.

[0080] S22, installation of liquid supply and return pipelines:

[0081] Install the sealed end cover 31, liquid supply pipe 41 and liquid return pipe 42 in the top pipe opening of the freezing pipe 3, and place the lower end of the liquid supply pipe 41 near the bottom of the freezing pipe 3, and the lower end of the liquid return pipe 42 near the upper side of the freezing pipe 3 near the end cover 31.

[0082] Inject clean water into the liquid supply pipe 41 for a pressure test. The initial pressure should be no less than 0.8 MPa. If the pressure drops by no more than 0.05 MPa over 30 minutes and remains stable for the next 15 minutes, the test is considered passed. If the pressure is unstable, the test fails and the freezing pipe 3 needs to be removed and the pipe re-welded or casing installed.

[0083] Install the annular liquid supply main pipe 43 and liquid return main pipe 44 and connect them to the freezing station. Then connect the upper ends of the liquid supply pipe 41 and liquid return pipe 42 to the liquid supply main pipe 43 and liquid return main pipe 44 respectively through the connecting branch pipe 45.

[0084] After the installed liquid supply and return pipelines have been leak tested and cleaned, they are insulated with 30mm thick insulation boards and the outside of the insulation layer is wrapped with plastic film.

[0085] Install the temperature sensor in the temperature measuring hole and connect it to the control system of the freezing station.

[0086] S3. Surrounding soil freezing:

[0087] The installed freezing system is debugged and tested, and the operating parameters of the freezing system are adjusted to achieve active freezing and maintenance freezing of the soil 5 around the foundation pit 1.

[0088] S31, Freeze system debugging:

[0089] (1) Fill the refrigeration unit with oil and fluorine according to the instructions in the equipment manual. First, check the refrigeration system of the refrigeration unit for leaks and flush it with nitrogen. After ensuring that there are no leaks in the system, fill the oil and fluorine.

[0090] (2) First, fill the brine tank with clean water, start the pump to circulate and flush the pipes, and empty the circulating water in the pipes; then fill the brine tank with about 1 / 4 of clean water, start the pump to circulate, and gradually add solid calcium chloride. The brine tank should not be filled too full with clean water to prevent the brine from overflowing when a certain amount of calcium chloride is added.

[0091] (3) When the brine density reaches about 1.15, start the refrigeration unit and add calcium chloride as the brine temperature drops until the designed brine density is reached.

[0092] (4) When melting calcium chloride, use a sieve to remove impurities and do not drop the packaging bag into the brine tank.

[0093] (5) Measure the brine flow rate of each connecting branch pipe 45 and adjust the control valve 46 to ensure that the brine flow rate in each connecting branch pipe 45 meets the design requirements.

[0094] (6) If it is found that the brine flow rate of individual connecting branch pipes 45 gradually decreases over time, indicating that there is air accumulation in the pipeline, a vent valve should be added in time.

[0095] S32, Active Freeze:

[0096] (1) During the equipment test run, the pressure, temperature and other state parameters should be adjusted at any time to ensure that the unit operates within the technical parameters required by the design. During the test run, the brine temperature, brine flow rate and temperature drop of the temperature measuring hole should be regularly checked, and the freezing system operating parameters should be adjusted in a timely manner. After the freezing system operates normally, enter the active freezing stage.

[0097] (2) During the active freezing period, a dedicated person shall be assigned to monitor the operation of the freezing station and measure the temperature 24 hours a day, including the brine temperature of the total brine outlet and the total circuit, the temperature of each group of circuits, and the temperature of each temperature measuring hole, and generate a temperature report to ensure that the temperature changes during the freezing process are fully under control.

[0098] (3) During the active freezing period, the surrounding patrols should be strengthened. It is strictly forbidden to take any precipitation measures in the confined water layer within 200m of the freezing area. At the same time, it is ensured that there is no concentrated water flow in the soil layer within the freezing area.

[0099] (4) Regularly calculate and accurately determine whether the permafrost curtain has formed a complete intersection circle and reached the thickness required by the design based on the actual measured temperature data.

[0100] (5) The designed active freezing cycle is set to 30-35 days. During this period, it is necessary to ensure that the single-hole flow rate of each freezing hole is not less than 5m3 per hour. Within 7 days after the start of active freezing, the brine temperature should be reduced to -18℃ or lower; by the 15th day, the brine temperature needs to be further reduced to below -24℃, and at the same time, the temperature difference between the outgoing and return brine should be controlled within 2℃. By the end of the freezing period, the brine temperature should be stabilized between -28℃ and -30℃, and the temperature difference between the outgoing and return brine should be controlled within 1.5℃. If the brine temperature or flow rate fails to meet the above design requirements, the active freezing time needs to be extended accordingly to ensure the expected freezing effect.

[0101] S33, Maintenance Freeze:

[0102] (1) When the thickness and average temperature of the freezing curtain meet the design requirements, maintenance freezing should be carried out during the excavation of foundation pit 1.

[0103] (2) During the maintenance and freezing period, a dedicated person shall be arranged to monitor the operation of the freezing station and measure the temperature 24 hours a day, including the brine temperature of the total brine outlet and the total circuit, the temperature of each group of circuits, and the temperature of each temperature measuring hole, and generate a temperature report to ensure that the temperature changes during the freezing process are fully under control.

[0104] (3) To ensure the stability of the frozen soil curtain, the brine temperature must be controlled to be no higher than -25°C. During the maintenance freezing phase, it is necessary to continuously monitor the operating status of the freezing system and regularly check and adjust equipment parameters to ensure the stability and durability of the freezing effect. Specific measures include regularly checking the operating status of the freezer, monitoring the brine temperature and pressure, and checking the sealing and stability of the freezing pipes.

[0105] S4. Freezing effect detection:

[0106] The soil temperature data is obtained through the temperature sensors installed in the temperature measuring holes, and the flow and temperature data of the brine in the freezing system are obtained through the flow meters and temperature sensors installed in the various equipment and pipelines in the freezing station to evaluate the soil freezing effect; the freezing system operating parameters are adjusted in time to regulate the soil freezing effect.

[0107] S41. Freezing effect assessment based on soil temperature detection:

[0108] The soil temperature data is obtained through the temperature sensor installed in the temperature measuring hole. The temperature sensor avoids direct contact with the temperature measuring tube to reduce measurement errors.

[0109] The data acquisition system records the soil temperature data transmitted by the temperature sensor in real time. The data acquisition system should be highly accurate, stable, and easy to operate to ensure the accuracy and integrity of the data.

[0110] Based on the recorded temperature data, use drawing software to draw a soil temperature distribution map or curve chart. The chart can intuitively understand the changing trend and distribution of soil temperature.

[0111] Based on the temperature distribution diagram, evaluate whether the freezing effect meets the design requirements. This mainly involves the following three points: ① Average freezing curtain temperature: Based on the temperature measurement data from the temperature measuring holes, use the freezing construction ice formation formula in the "Well Construction Engineering Manual" to calculate the average temperature of the freezing curtain. Confirm whether all freezing holes have been fully connected, and whether the average temperature of the frozen soil curtain and the thickness of the frozen soil in weak areas meet the design values. The average temperature is determined to be no higher than -10°C. ② Freezing curtain thickness: Calculate the thickness of the freezing curtain based on the temperature measurement data, and the thickness must be no less than 1.5m. ③ Temperature uniformity: Assess the uniformity of the soil temperature distribution to avoid localized overcooling or overheating.

[0112] S42. Freezing effect evaluation based on brine flow and temperature detection:

[0113] (1) Inspection of brine circulation system

[0114] Equipment Inspection: Check the working condition of brine pumps, pipes, valves and other equipment to ensure they are functioning properly and leak-free. Pay special attention to the tightness and insulation of brine pipes to prevent heat loss and ice blockage.

[0115] Circulation flow monitoring: Use flow meters and other equipment to monitor the brine circulation flow rate to ensure that it fluctuates within the design range. Too small a flow rate may result in poor freezing effect, while too large a flow rate may waste energy and increase system burden.

[0116] (2) Brine temperature monitoring

[0117] Monitoring point setting: Temperature monitoring points are set at the inlet and outlet of the brine tank and in the middle of the pipeline to accurately reflect the temperature changes of the brine.

[0118] Real-time monitoring: Use a high-precision thermometer or temperature sensor to monitor the temperature of the brine in real time and record the data.

[0119] (3) Temperature control and adjustment of the refrigeration system of the refrigeration unit: According to the monitored brine temperature data, the operating parameters of the refrigeration system are adjusted in time to ensure that the brine temperature remains within the set range. The brine temperature control standards are shown in Table 1 below:

[0120] Table 1. Brine temperature control standards

[0121]

[0122] S5. Construction of crown beam and first section of shaft wall:

[0123] The foundation pit soil is excavated at the preset position of the foundation pit shaft 1, and then the cushion layer is poured, the steel bars are welded and tied, the formwork is installed, the concrete is poured, the formwork is removed and the concrete is cured to complete the construction of the crown beam 10 and the first section of the shaft wall 11.

[0124] S51. Excavation of foundation pit soil:

[0125] (1) The excavation of the foundation pit is carried out by a long-arm excavator outside the well and an excavator inside the well. The excavator inside the well loosens the soil and then the long-arm excavator transfers it out of the well. The soil is excavated in layers, with each layer being 1m-1.5m. Parts that cannot be excavated by the excavator are excavated manually.

[0126] (2) The excavation process is monitored by surveyors throughout the entire process to control the excavation elevation. The foundation pit excavation is carried out in layers and sections evenly and symmetrically, and the construction principle of "vertical layering and full-section excavation from top to bottom" is followed during the excavation process.

[0127] S52, cushion layer pouring:

[0128] After the soil is excavated to the design elevation, the exposed surface of the base is immediately inspected to see if it meets the design and relevant specifications and regulations. The foundation pit is then leveled, the accumulated water in the pit is drained, and a 20cm thick cushion layer of C20 concrete is promptly poured to seal the foundation pit. The cushion layer is fully enclosed.

[0129] S53, Steel bar welding and binding:

[0130] (1) Before laying the reinforcement of the crown beam 10, clean the base surface first, lay the positioning reinforcement first, then lay the distribution reinforcement, and then insert the stirrups. The transverse reinforcement joints of the crown beam 10 are in the form of overlap, and the overlap length is not less than 35 steel bar diameters.

[0131] (2) After the reinforcement of the crown beam 10 is tied, the construction of the shaft wall reinforcement at the first section 11 of the shaft wall begins. The shaft wall reinforcement is laid first, followed by the horizontal reinforcement, and then the hook. The shaft wall horizontal reinforcement joints are overlapped, with the overlap length not less than 35 steel bar diameters, and the joint area percentage should not exceed 25%. The vertical reinforcement is connected with a straight thread sleeve, and the thread leakage at the sleeve end does not exceed 2.5. The sleeve position is sealed with a special plug and sealed with tape. The tie bars are arranged in a plum blossom shape.

[0132] S54, template installation:

[0133] (1) The template uses a curved surface template. Four templates form a full circle. First, the arc-shaped steel bars are processed according to the inner diameter of the structure. Then, horizontal and vertical bars are used to fix them. Finally, the support frame is welded to form a support frame. The height of the support frame is consistent with the excavation depth of each step. After the support frame is processed, 200*200*1000mm square wood is fixed on the surface of the arc-shaped steel bars with wire. The template is laid on the outside of the square wood. The template uses 15mm thick bamboo plywood. The area of ​​a single bamboo plywood is 1220*2440mm. Each template is assembled tightly, and the gaps are stopped by grouting strips. After the template is processed, it is assembled on the ground to ensure installation accuracy.

[0134] (2) The formwork support is erected in the order of positioning and laying out, base, vertical pole, horizontal pole, scissors brace, and diagonal pole. Top and bottom supports are set up on the support, and the adjustable height is controlled between 5 and 35 cm. After the first layer of horizontal poles is erected, they are adjusted by ground supports. Make the horizontal poles on the same level, and then proceed to the next layer. The diagonal poles are erected horizontally and vertically to form a truss unit. Horizontal scissors braces are set up at intervals of 4-6 standard steps along the height of the support frame, and scissors braces are set up on all four longitudinal surfaces of the support frame. The assembly height meets the requirements of a single well wall construction to ensure that the assembly is firm. After one-time formation, it can be hoisted and dismantled as a whole using a crane.

[0135] (3) The formwork support and the formwork are firmly fixed with main ribs, secondary ribs, and top supports to prevent collapse and slippage. Before installing the formwork, the residual concrete on the formwork surface should be cleaned and the isolation agent should be applied. When installing the formwork, stoppers should be used at the joints to prevent leakage. The joints should be tight and the support should be firm. The height difference between the joints of two adjacent formworks should not be greater than 2mm.

[0136] (4) During the template installation process, the surveying personnel shall promptly check the template position to prevent deviation.

[0137] S55, concrete pouring

[0138] A pump truck is used to pour concrete, and the "layered symmetrical continuous pouring construction method" is adopted to control the pouring height at one time, so that the material can be discharged in layers, compacted in layers, and the whole layer is promoted in a cycle. The thickness of each layer is 40cm. When pouring, the material discharge height is controlled and vibrated into place.

[0139] Concrete should be vibrated using an inserted vibrator. The horizontal insertion points should be 35-45 cm apart and 20-25 cm deep, maintaining a distance of 5-10 cm from the side formwork. The vibrator should be inserted quickly and removed slowly, with each point vibrating for 25-30 seconds, starting when the concrete stops sinking and a floating slurry appears on the surface. This is to prevent over-vibration and missed vibrations.

[0140] S56, Formwork removal and concrete curing:

[0141] After the concrete is poured, it should be cured promptly. When the concrete reaches the required strength, the formwork is removed and the concrete is cured to prevent cracks or shrinkage.

[0142] S6. Standard section shaft wall construction:

[0143] (1) After the concrete strength of the crown beam and the first section of the shaft wall reaches 75%, the construction of the next section of the shaft wall can begin. The height of the second section of the shaft wall is about 3-5m.

[0144] (2) Continue to excavate the soil and pour the cushion layer in the foundation pit. The soil excavation and cushion layer pouring operations are the same as those in S51 and S52. In particular, a long-arm excavator or a telescopic arm excavator at the pit side is used in conjunction with an excavator inside the pit for excavation. However, it should be noted that the intersection of the two sections of the well wall must be excavated first with an excavator, and then manually chiseled clean with an electric pick before the steel bar binding construction can be carried out.

[0145] (3) Rebar welding and binding. The number and spacing of the vertical bars of the second section of the shaft wall are the same as those of the first section. The two layers of steel bars are connected by straight threaded sleeves. During construction, the vertical bars are connected first. The vertical bars are constructed on the soil side first and then on the soil side. After all the vertical bars are connected, the circumferential bars are laid and tied. The layout position, spacing, overlap form and length of the bars must meet the design requirements. The intersection of the horizontal bars and the vertical bars must be tied and tied firmly to ensure the stability of the structure.

[0146] (4) Formwork installation, concrete pouring, formwork removal and concrete curing are the same as steps S54 to S56.

[0147] By performing the above steps (1) to (4), the construction of the second section of the shaft wall is completed; similarly, after the concrete strength of the previous section of the shaft wall reaches the requirement, the construction of all standard sections of the shaft wall 12, including the third section of the shaft wall, the fourth section of the shaft wall, and up to the Nth section of the shaft wall, is carried out in sequence.

[0148] S7, construction of the last section of the shaft wall and bottom sealing layer:

[0149] S71. Soil excavation:

[0150] The soil excavation of the last section of the well wall 13 and the bottom sealing layer 14 is continued in the foundation pit, and the soil excavation operation is the same as step S51.

[0151] S72, tunnel door steel ring installation:

[0152] Pit 1 is the shield tunneling pit. A portal steel ring 21 is installed at the shield tunneling launch point. This ring measures 7100mm in diameter, 1000mm in width, and 5mm thick. It is divided into two semicircular sections, one above the other, and features 5cm-high wings on either side.

[0153] The height of the construction section is arranged reasonably. The position of the shield tunnel portal is completed in two steps. After the excavation is completed, the surveying personnel will determine the center position of the shield tunnel portal and use a crane to lift the portal steel ring 21 into place. The surveying personnel will check the position throughout the process to prevent deviations.

[0154] S73, bottom sealing construction:

[0155] The reinforcement of the bottom seal layer 14 and the portion of the steel bars in the final section of the shaft wall 13 connected to the bottom seal layer 14 are welded and tied together, and then the bottom seal layer 14 is poured with concrete and cured. The reinforcement of the bottom seal layer 14 is double-layered and bidirectional. Saddle bars are placed between the two layers to ensure the correct positioning of the upper layer of reinforcement. Positioning bars in the upper layer's vertical and horizontal directions are placed on the saddle bars. The steel mesh is tied in the same manner as the lower layer of reinforcement, ensuring that all intersections are securely tied.

[0156] S74, final section shaft wall and shield tunnel construction:

[0157] (1) Welding and binding of steel bars of the last section of the well wall.

[0158] The remaining steel bars of the last section of the shaft wall 13 are welded and tied. After the structural ring bars and vertical bars of the last section of the shaft wall 13 around the tunnel steel ring 21 are laid to the position of the tunnel steel ring 21, they are welded to the outside of the tunnel steel ring 21.

[0159] (2) Welding and binding of steel bars of shield tunnel portal.

[0160] Specifically, 10 reinforcement bars were added to the outside of the portal steel ring 21, with horizontal bars on the top and bottom, and vertical bars on the left and right. Ten reinforcement bars were added to the upper left, upper right, lower left, and lower right, parallel to the tangent line. Ten reinforcement bars were arranged circumferentially around the portal steel ring 21. The added reinforcement bars of the shield portal were tied and connected to the reinforcement of the final shaft wall 13. The portal steel ring 21 was constructed in two halves, with the vertical reinforcement bars connected to the shaft wall reinforcement using straight threaded sleeves.

[0161] (3) Concrete pouring construction.

[0162] After the welding and binding of the steel bars of the last section of the shaft wall and the shield tunnel portal are completed, the template installation, concrete pouring, formwork removal, concrete curing and installation of the sealed safety door 22 at the last section of the shaft wall and the shield tunnel portal are carried out, thereby completing the construction of the shield tunnel portal, the last section of the shaft wall 13 and the bottom sealing layer 14.

[0163] S8. Dismantling of freezing system and melting and grouting:

[0164] (1) Stop the freezing system from freezing the soil 5 and heat the brine in the brine tank of the freezing system. Circulate the hot brine in the freezing system pipes to melt a small part of the frozen soil around the freezing pipe 3.

[0165] (2) Use two 5t jacks to fix the upper end of the frozen pipe 3 and conduct a trial pull-out. When the pipe is pulled up about 0.5m, the circulation of hot brine can be stopped. Then use a 25t truck crane to connect to the upper end of the frozen pipe 3 and pull out the frozen pipe 3 by hooking. Since the frozen pipe 3 is long, it needs to be pulled out in sections. When cutting, ensure that the lower section of the frozen pipe 3 is firmly fixed to prevent it from sliding down. When pulling out the pipe, ensure that the frozen pipe and the hook are in line. If there is difficulty in pulling out the pipe, turn the frozen pipe in time and do not apply force blindly.

[0166] (3) The freezing system is disassembled and the freezing holes are filled with grouting, thereby completing the entire process of the reverse construction method of the fully enclosed freezing mechanism of the foundation pit in the water-rich stratum near the river.

[0167] The present invention provides a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river. The mechanism has a compact structure. A plurality of freezing holes are provided in the soil 5 outside the foundation pit well 1 and freezing pipes 3 are installed. The freezing pipes 3 and the surrounding soil are frozen by the freezing system 4, thereby improving the strength and stability of the soil, effectively isolating groundwater, avoiding the occurrence of accidents such as water inrush and sand gushing in the foundation pit, and ensuring construction safety.

[0168] The present invention provides a reverse construction method for a fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river. The main body of the foundation pit 1 is constructed layer by layer from the ground to the underground, and is carried out in an alternating cycle with earth excavation. The main structure is formed layer by layer, and together with the frozen soil, a stable support system is formed. The process is simple and easy to operate, which is beneficial to project quality control, has a fast construction speed, does not require internal support, and reduces construction costs.

[0169] Example 4:

[0170] like Figure 1-3 As shown, the fully enclosed freezing mechanism and reverse construction method of a foundation pit in a water-rich stratum near a river of the present invention are basically the same in structure and steps as those of Example 1, except that:

[0171] The fully enclosed freezing mechanism and reverse construction method of a foundation pit in a water-rich stratum near a river in this embodiment are applied to the power line crossing the Haihe Tunnel project of the Fuxingmen 110KV substation in the first phase of Tianjin Metro Line 10.

[0172] 1. Project Overview

[0173] The Haihe River Tunnel, a power cable tunnel for the Fuxingmen 110kV substation on the first phase of Tianjin Metro Line 10, undertaken by China Coal Third Construction Group, features a 3m inner diameter, 1,020m long, and two working shafts. The launch shaft is located northeast of the intersection of Yueyahe East Road and Huanyu Road, while the receiving shaft is located approximately 85m south of Zhafang Yong Road at the intersection of Taierzhuang Road and Zhafang Yong Road. From the launch shaft, the tunnel continues southward, crossing Huanyu Road and continuing along the Yueyahe East Road green belt and pedestrian walkway. The tunnel then crosses Haihe East Road, the Haihe River, and Taierzhuang Road to the receiving shaft on Zhafang Yong Road. Construction was carried out using a slurry shield machine with excellent sealing properties. The working shafts also serve as power shafts, ventilation shafts, and inspection shafts.

[0174] The receiving well is a circular well with an outer diameter of 12.6m, a wall thickness of 0.8m, a bottom plate thickness of 1.2m, and a depth of 29.5m. The concrete grade is C40. The working space is extremely limited due to the Fuxing River 10m to the east, a 24-story residential complex 13m to the west, a high-voltage corridor to the south, and the Haihe River 80m to the north. The area is primarily composed of soft strata consisting of silt, silt sand, and mud. The groundwater is abundant and well-developed, with high head and rapid recharge, and the geology is complex and variable.

[0175] 2. Application

[0176] Construction of the receiving pit (i.e., Excavation Pit 1) commenced in December 2022, was completed in August 2023, and passed final acceptance in July 2024. The receiving pit's retaining structure utilizes vertical freezing, with a single-row arrangement of 55 holes, spaced 891 mm apart. Each hole reaches a depth of 49.360 m (penetrating the confined water layer and penetrating 2 m into the 11-1 silty clay). Five temperature measurement holes are evenly spaced outside the holes, 757 mm apart from the freezing pipes and 50 m deep. Freezing began on January 1, 2023. After January 10, the brine temperature dropped to -20°C, with the temperature difference between the inlet and outlet brine remaining within 2°C. By January 18, the brine temperature had dropped below -28°C, with the temperature difference between the outlet and outlet brine remaining within 1.5°C. After 30 days of freezing, temperature measurements confirmed that the average temperature of the frozen wall had dropped below -10°C, and the wall had completed its perimeter, achieving the designed thickness of 1.5 m.

[0177] After 35 days of freezing, and after analysis of the freezing effect confirmed that construction conditions were met, foundation pit construction began. Maintenance freezing was continued throughout the excavation process, and the excavated surface soil showed good self-sustaining properties, with visible frost and no water seepage. Shaft wall construction employed a reverse construction method, alternating between excavation and shaft wall construction. Each construction session ranged in depth from 3 to 5 meters, and each took approximately 15 days. Horizontal displacement of the surrounding soil and ground subsidence remained within the design range, with no warning signs observed.

[0178] The fully enclosed freezing mechanism and reverse construction method of the foundation pit in the water-rich stratum near the river of the present invention have been successfully applied in this project. The technical staff gave full play to the advantages of the company's internal system resources, and aimed at the technical difficulties of deep foundation pit construction in a narrow site under strong water-rich soft soil geological conditions, and effectively solved the problems of foundation pit deformation and water leakage through the enclosure freezing + reverse construction method. And through continuous practice and summary, a complete set of construction processes has been formed. Compared with the traditional enclosure pile + forward construction method, this technology has high construction safety, low site requirements, small impact on the surrounding environment, low construction cost, and no pollutants such as mud are generated, which has no adverse effects on the surrounding environment, effectively ensuring construction safety. This technology combines the two major advantages of the group company in freezing and civil construction, and provides a new technical option for the field of deep foundation pit construction in a narrow site under strong water-rich soft soil geological conditions.

[0179] Example 5:

[0180] like Figure 1-3 As shown, the fully enclosed freezing mechanism and reverse construction method of a foundation pit in a water-rich stratum near a river of the present invention are basically the same in structure and steps as those of Example 1, except that:

[0181] The fully enclosed freezing mechanism and reverse construction method of a foundation pit in a riverside water-rich stratum of this embodiment are applied to the expansion of the Shenchang Interchange Pump Station and the renovation project of the Zhongshan Interchange Pump Station in Xuzhou City.

[0182] 1. Project Overview

[0183] China Coal Third Construction Group is undertaking the Xuzhou Shenchang Interchange Pump Station Expansion and Zhongshan Interchange Pump Station Renovation Project. The Shenchang Interchange Pump Station is located in Xuzhou's Gulou District, southwest of the Shenchang Interchange Bridge, at the intersection of Second Ring Road North, Longhai Railway, Tongpei Road, and Second Ring Road West. The main construction work involved excavating the collapsed pump station inlet pipe and reburying a DN1500 steel pipe connected to the pump station. The steel pipe was connected to the existing box culvert via a fan-shaped connection well. The inlet pipe foundation was reinforced with compacted grouting, and the steel pipe was encased in reinforced concrete.

[0184] The foundation pit for the Shenchang Interchange Pump Station is 10.2 meters long in the east and west directions and 7-15 meters long in the south and north directions, with a maximum excavation depth of 18.95 meters. The groundwater on the site is of the infiltration-evaporation type. The primary aquifers are the first and second layers of silt soil and the fifth layer of silt sand, which have a loose structure, medium permeability, and good water-yielding properties. During the survey, the groundwater depth in the borehole was measured to be 4.3 meters, with an initial groundwater level of -4.6 meters and a stable water level of -4.6 meters.

[0185] 2. Application

[0186] The excavation and support for the foundation pit (i.e., Pit 1) of this project was constructed using the freezing method. The freezing pipes were arranged at a 65° incline on the north and south sides, with a freezing thickness of 2m. The east and west sides were vertically frozen, with a freezing thickness of 2.5m. A total of 120 freezing holes were constructed, including 54 inclined freezing holes, 66 vertical freezing holes, and 8 temperature measurement holes, each with 4-6 temperature measurement points. The freezing holes were constructed using Φ108×8mm 20# low-carbon seamless steel. The hole spacing was 900×900mm, and the excavation cross-section was an inverted trapezoid. The pit bottom elevation ranged from 32.45 to 33.18m. The freezing wall had a compressive strength of 3.6MPa, a flexural strength of 2.0MPa, and a shear strength of 1.5MPa.

[0187] The refrigeration system officially began operation on April 12, 2023. The temperature dropped rapidly after startup. On April 18 (7 days of freezing), the brine temperature and return temperature had reached below -25°C. By April 26 (15 days of freezing), the brine return temperature had reached below -28°C, both meeting the freezing design requirements. The return temperature difference met the design requirements (within 2°C). The freezing design reached below -28°C before excavation. This project reached below -28°C on April 26 (15 days of freezing). The brine temperature is basically around -30°C, and the return temperature difference is 1°C.

[0188] After 35 days of active freezing, the reverse construction of the foundation pit began after the freezing effect analysis met the construction conditions. Maintenance freezing was continued during the excavation process. The soil on the excavation surface had good self-supporting properties, the average temperature of the frozen wall was ≤-10℃, and no water seepage occurred.

[0189] The fully enclosed freezing mechanism and reverse construction method of the foundation pit in the water-rich stratum near the river of the present invention have been successfully applied in this project. The artificial freezing method is used to freeze the soft water-rich stratum into a frozen soil layer with stable structure and strong bearing capacity, forming a freezing reinforcement reverse construction technology for the working shaft of the shield tunnel in the strong water-rich stratum, which solves the leakage risk at the source, enables the foundation pit soil to be supported in time and effectively, reduces the risk of deformation and collapse, can significantly reduce construction costs, shorten project costs, and achieve good construction results.

[0190] The above is an exemplary description of the invention. Obviously, the specific implementation of the present invention is not limited to the above-mentioned method. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river, characterized in that: The invention comprises a foundation pit (1), a shield tunnel (2), a freezing pipe (3) and a freezing system (4), wherein the shield tunnel (2) is vertically opened on the inner wall of the foundation pit (1), a soil body (5) outside the foundation pit (1) is provided with a plurality of freezing holes arranged in a circle, and the freezing pipes (3) are provided with a plurality of freezing holes and are installed one by one in the plurality of freezing holes; the freezing system (4) comprises a freezing station, a liquid supply pipe (41), a liquid return pipe (42), a liquid supply main pipe (43) and a liquid return main pipe (44), wherein the liquid supply main pipe (43) and the liquid return main pipe (44) are connected to the freezing station, wherein the liquid supply pipe (41) and the liquid return pipe (42) are provided with a plurality of freezing pipes (3), and the upper ends of the liquid supply pipe (41) and the liquid return pipe (42) are connected to the liquid supply main pipe (43) and the liquid return main pipe (44) respectively.

2. A fully enclosed freezing mechanism for foundation pits in water-rich riverbeds according to claim 1, characterized in that: The foundation pit (1) is composed of a crown beam (10) and a first section well wall (11) located at the top, a plurality of standard section well walls (12) located in the middle, a last section well wall (13) and a bottom sealing layer (14) located at the bottom, wherein the first section well wall (11), the standard section well wall (12) and the last section well wall (13) are tubular structures cast with reinforced concrete, and the crown beam (10), the first section well wall (11), the last section well wall (13) and the bottom sealing layer (14) are all reinforced concrete structures cast in one piece.

3. The fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river according to claim 1, characterized in that: The freezing station includes: a brine tank, a clean water tank, a refrigeration unit, a brine pump, and a cooling tower. The brine tank is used to store brine, the clean water tank provides supplementary water for the system, the refrigeration unit reduces the brine temperature through a refrigeration cycle, the brine pump is responsible for circulating the brine in the system, and the cooling tower is used to dissipate the heat generated by the refrigeration unit; the brine tank is connected to the brine pump, and the outlet of the brine pump is connected to the liquid supply main through a pipeline. The refrigeration unit is connected to the brine tank through a pipeline for cooling the brine; the cooling tower is connected to the refrigeration unit to take away heat; the clean water tank and the brine tank are connected by a pipeline and a valve, and clean water can be added to the brine tank as needed.

4. The fully enclosed freezing mechanism for foundation pits in riverside water-rich strata according to claim 1 is characterized in that: A sealed end cover (31) is installed on the top of the freezing pipe (3); the lower end of the liquid supply pipe (41) is installed to the bottom of the freezing pipe (3), and its upper end passes through the end cover (31) and is connected to the liquid supply main pipe (43) through a connecting branch pipe (45); the lower end of the liquid return pipe (42) is installed to the upper side of the freezing pipe (3), and its upper end passes through the end cover (31) and is connected to the liquid return main pipe (44) through a connecting branch pipe (45).

5. A fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river according to claim 4, characterized in that: Control valves (46) are installed on the connecting branch pipes (45) connected to the liquid supply main pipe (43) and the liquid return main pipe (44).

6. The fully enclosed freezing mechanism for foundation pits in riverside water-rich strata according to claim 1 is characterized in that: A gate steel ring (21) and a sealed safety door (22) are provided at one end of a shield tunnel (2) located on one side of a foundation pit (1), wherein the sealed safety door (22) is installed inside the gate steel ring (21).

7. The fully enclosed freezing mechanism for foundation pits in water-rich riverbed strata according to claim 1 is characterized in that: The distance between two adjacent freezing holes is no more than 900 mm, and the distance between the freezing hole and the wall of the foundation pit (1) is no more than 300 mm; assuming that the depth of the foundation pit (1) is h and its diameter is D, the depth H of the freezing hole is no less than h+D.

8. A fully enclosed freezing mechanism for a foundation pit in a water-rich stratum near a river according to claim 7, characterized in that: A plurality of circularly arranged temperature measuring holes are provided in the soil (5) outside the freezing hole, the spacing between the temperature measuring holes and the freezing hole is no more than 800 mm, the depth of the temperature measuring holes is the same as the depth of the freezing hole, and temperature sensors are installed in the temperature measuring holes and connected to the control system of the freezing station.

9. The fully enclosed freezing mechanism for foundation pits in water-rich riverbeds according to claim 7, characterized in that: The bottom of the freezing pipe (3) is sealed and welded with a bottom cone, and the length L of the freezing pipe (3) lowered into the freezing hole satisfies: H≤L≤L+0.5, in meters.

10. A reverse construction method of a fully enclosed freezing mechanism for a foundation pit in a riverside water-rich stratum based on claims 1-9, characterized in that: The steps include: S1. Vertical freezing hole construction: A plurality of freezing holes and temperature measuring holes are planned to form a circle at a certain position outside the preset position of the foundation pit (1), and then a geological drilling machine is used to perform hole drilling operations, and finally a freezing pipe (3) is installed in the freezing hole; S2. Freeze system installation: According to the actual situation on site, the various equipment of the freezing station are installed at the selected location and the pipelines and lines are laid; the liquid supply pipe (41) and the liquid return pipe (42) are installed in the freezing pipe (3) of the freezing hole, the liquid supply main pipe (43) and the liquid return main pipe (44) are laid above the freezing hole and connected to the freezing station, and then the liquid supply pipe (41) and the liquid return pipe (42) are connected to the liquid supply main pipe (43) and the liquid return main pipe (44) respectively; finally, the temperature sensor is installed in the temperature measuring hole and connected to the control system of the freezing station; S3. Surrounding soil freezing: Conducting equipment debugging and trial operation on the installed freezing system, and adjusting the operating parameters of the freezing system, thereby achieving active freezing and maintenance freezing of the soil (5) around the foundation pit (1); S4. Freezing effect detection: The soil temperature data is obtained through the temperature sensors installed in the temperature measuring holes. The flow rate and temperature data of the brine in the freezing system are obtained through the flow meters and temperature sensors installed in the various equipment and pipelines in the freezing station to evaluate the soil freezing effect. The freezing system operating parameters are adjusted in a timely manner to adjust the soil freezing effect. S5. Construction of crown beam and first section of shaft wall: Excavation of the foundation pit soil is carried out at a preset position of the foundation pit (1), followed by pouring of the cushion layer, welding and binding of steel bars, installation of formwork, pouring of concrete, removal of formwork and concrete curing, to complete the construction of the crown beam (10) and the first section of the well wall (11); S6. Standard section shaft wall construction: After the concrete strength of the crown beam and the first section of the shaft wall reaches 75%, the soil excavation is continued in the foundation pit, and then the cushion layer is poured, the steel bars are welded and tied, the formwork is installed, the concrete is poured, the formwork is removed and the concrete is cured to complete the construction of the second section of the shaft wall. Similarly, the construction of all standard sections of the shaft wall (12) including the third section of the shaft wall, the fourth section of the shaft wall, and up to the Nth section of the shaft wall is carried out in sequence. S7, construction of the last section of the shaft wall and bottom sealing layer: Continue to excavate the soil of the last section shaft wall (13) and the bottom sealing layer (14) in the foundation pit; locate and hoist the portal steel ring (21) at the predetermined position of the shield tunnel (2) on the inner wall of the foundation pit (1); weld and tie the steel bars of the bottom sealing layer (14) and pour concrete; weld and tie the steel bars of the last section shaft wall (13) and the shield portal outside the portal steel ring (21), and weld and tie the shield portal steel bars to the steel bars of the last section shaft wall (13); then install the template, pour concrete, remove the template, maintain the concrete, and install the sealed safety door (22), thereby completing the construction of the shield portal, the last section shaft wall (13) and the bottom sealing layer (14); S8. Dismantling of freezing system and melting and grouting: The brine in the brine tank of the freezing system is heated and circulated, the freezing pipe is removed and the freezing system is disassembled, and finally the freezing hole is filled with grouting, thereby completing the entire process of the reverse construction method of the fully enclosed freezing mechanism of the foundation pit in the water-rich stratum near the river.

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