Deep sand layer staged diversion type PLC composite pile comprehensive cofferdam construction method

Abstract

The invention discloses a deep sand layer staged flow guide type PLC composite pile comprehensive cofferdam construction method, and belongs to the field of hydraulic engineering, and the construction method comprises the following specific steps: I, constructing a temporary wharf according to field conditions, after the temporary wharf is constructed, assembling a steel floating box, and measuring and determining a PLC composite pile construction position; according to the method, the overall efficiency of field cofferdam engineering construction can be effectively improved, back pressure supporting, waterproof cloth air closing and double-machine inserting and piling construction are innovatively adopted, rapid construction of the oversized cofferdam can be achieved, and meanwhile safety and stability of the cofferdam are guaranteed. The on-site construction period is integrally shortened, repeated blocking, monitoring and early warning of the cofferdam are avoided, steel investment is reduced, higher safety and stability are achieved through mutual combination of the steel cofferdam and inner side earth and stone back pressure, the defects of a traditional steel cofferdam are overcome, and the requirements for environmental protection, energy conservation and emission reduction are met.

Description

Technical Field

[0001] The present invention relates to the field of water conservancy projects, and in particular to a method for constructing a comprehensive cofferdam of thick sand layers with staged diversion type PLC composite piles. Background Art

[0002] Water conservancy projects are constructed to control, regulate, allocate, protect, and develop and utilize natural surface water and groundwater to eliminate harm and promote benefits. They are key infrastructure facilities with outstanding social benefits. Cofferdams are a key temporary project during the construction of water conservancy projects. They are temporary retaining structures built to support the construction of permanent water conservancy facilities during the construction process. They prevent water and soil from entering the proposed building site and facilitate drainage, excavation, and building construction within the cofferdam.

[0003] Cofferdams can be classified into earth-rock cofferdams, concrete cofferdams, and steel sheet pile cofferdams. Considering that riverbeds with deep sand and soil offer excellent seepage conditions, the use of steel sheet pile cofferdams can extend the seepage channel and reduce the risk of piping. Due to the inherent properties of steel cofferdams, they are susceptible to deformation due to factors such as water level fluctuations, flow velocity, and winter ice formations. Furthermore, encountering rock formations during construction can make driving to the designed elevation difficult. Analysis and optimization efforts have resulted in a combination of interlocking steel sheet piles and steel pipe piles, which can increase the rigidity of the steel cofferdam to a certain extent. After the cofferdam is closed, airtight measures are implemented to address frequent leakage from the steel cofferdam. Furthermore, counter-pressure measures are implemented using the "entry-occupancy" method to address frequent deformation and abnormal monitoring data caused by external factors. This addresses the root causes of steel cofferdam leakage and monitoring issues, ensuring the safety and stability of the cofferdam throughout the construction process. At the same time, this achievement has the advantages of being safer and more stable than conventional steel sheet pile cofferdams; it has better construction efficiency than double-row steel sheet pile cofferdams; and it has less impact on the environment than earth-rock cofferdams; therefore, it is particularly important to invent a comprehensive cofferdam construction method for deep sand layers with phased diversion PLC combination piles. Summary of the Invention

[0004] The purpose of the present invention is to solve the defects in the prior art and to propose a comprehensive cofferdam construction method of a deep sand layer phased diversion type PLC composite pile.

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

[0006] The construction method of the comprehensive cofferdam with phased diversion type PLC composite piles in thick sand layer has the following specific steps:

[0007] Ⅰ. Build a temporary wharf according to the site conditions. After the temporary wharf is built, assemble the steel pontoon, and measure and determine the construction position of the PLC composite piles;

[0008] II. According to the construction location and construction sequence, the PLC composite piles are driven to the designed depth. After the PLC composite piles are driven, the cofferdam is closed and sealed;

[0009] III. After the cofferdam is closed, the internal drainage of the cofferdam is carried out, and after the drainage is completed, the construction of various hydraulic structures of the water conservancy project inside the cofferdam is carried out;

[0010] IV. During the construction period, the cofferdam shall be monitored and maintained in real time. After the construction is completed, the cofferdam shall be dismantled and the water environment shall be restored.

[0011] As a further solution of the present invention, the specific steps of constructing a temporary wharf according to the on-site conditions in step I are as follows:

[0012] P1.1: Set up a temporary wharf area according to site conditions, and drive a row of 12m steel sheet piles on the waterside of the temporary wharf, ensuring that the tops of the piles are flush with the temporary access road on site to form a flat construction surface.

[0013] P1.2: After draining the water inside the steel sheet piles, fill in earth and rock and compact it to reinforce the wharf structure. Then, dock the steel pontoon and clear the riverbed outside the steel sheet piles to 2.5m below the ground. If it is not possible to clear the riverbed to 2.5m below the ground on site, expand the temporary wharf area and extend it to the deep water area in the center of the river.

[0014] As a further solution of the present invention, the temporary wharf described in P1.1 is usually set to 20 meters × 15 meters.

[0015] As a further solution of the present invention, the specific steps of assembling the steel pontoon in step I are as follows:

[0016] P2.1: Based on construction requirements, select three 3.6m × 22m × 2.2m pontoons and ten 2.8m × 3.2m × 0.92m pontoons. Adjust the length of the pontoons based on the length of the selected locking steel sheet piles or steel pipe piles.

[0017] P2.2: Use a 100t truck crane to lift the steel pontoons to a river surface with a water depth of at least 2.5m. Use steel pins to connect the pontoons to assemble a pile driver steel pontoon measuring 10.8m × 22m × 2.2m and a pile transport steel pontoon measuring 5.6m × 16m × 0.92m.

[0018] P2.3: After the steel pontoon is assembled, four 300×300H-shaped steel plugs are inserted into the preset positioning pile holes at the four corners of the pontoon to fix it. After the pontoon is fixed, the manipulator vibration pile driver is driven above the steel pontoon.

[0019] As a further solution of the present invention, the manipulator vibration pile driver described in P2.3 is equipped with three personnel, including one pile driver operator and two pile support personnel.

[0020] As a further solution of the present invention, the measurement and determination of the construction position of the PLC combination pile in step I

[0021] S1.1: Before the construction of the PLC composite piles, the surveyor will take a small boat to determine the axis. Using a total station and a level, the surveyor will accurately measure the axis of the cofferdam on the water surface according to the construction design drawings and reserve construction space within the cofferdam area.

[0022] S1.2: After completing the axis survey, determine the construction location of the PLC composite piles within the cofferdam area and draw a white control line between two adjacent PLC composite pile construction points. Based on the on-site construction conditions, the cofferdam construction area is extended to 10m above the river bank. At the same time, the cofferdam upstream and downstream and along the water flow direction are relaxed based on the construction boundary line.

[0023] S1.3: During the surveying and positioning process, conduct underwater exploration and geophysical exploration of the construction area simultaneously to check the riverbed conditions within the construction area and to detect any unknown factors such as obstacles, silt layers, and floating objects that may affect the cofferdam construction. If any obstacles are found, they should be removed in advance or appropriate treatment measures should be formulated.

[0024] As a further solution of the present invention, the specific steps of driving the PLC combination pile to the designed depth according to the construction position and construction sequence described in step II are as follows:

[0025] S2.1: Use locking steel sheet piles as guide piles. Weld 25a I-beam corbels to the steel sheet piles and drive them into the inner side of the foundation pit according to the measured positioning. Then, use HW400×400 steel as guide beams. Based on the single-layer and single-sided structure, fix the guide beams to the I-beam corbels of the guide piles. At the same time, cut off the part of the I-beam corbels that protrude from the guide beams.

[0026] S2.2: The manipulator vibratory pile driver uses a vibratory hammer to pick up the steel pipe pile / steel sheet pile from the transport ship and move it to the piling location. Two pile support personnel work with the pile driver to accurately place the steel pipe pile / steel sheet pile and ensure that the steel pipe pile / steel sheet pile is close to the guide beam. The pile driver then turns on the vibratory hammer and drives the steel pipe pile / steel sheet pile downward. At the same time, the pile support personnel constantly monitor the verticality of the steel pipe pile / steel sheet pile and instruct the pile driver to make adjustments.

[0027] S2.3: After the first steel pipe pile / steel sheet pile is driven to the design elevation, use the first steel pipe pile / steel sheet pile as a guide to lift and drive the second pile. Then, drive the steel pipe piles / steel sheet piles into the soil one by one until the cofferdam construction is completed.

[0028] As a further solution of the present invention, the guide piles described in S2.1 are about 0.8m away from the steel sheet piles inside the foundation pit, and the distance between each guide pile is 11.0m.

[0029] As a further solution of the present invention, special corner piles must be used at the corners of the foundation pit. Two steel sheet piles can be welded at 90 degrees. To ensure the sealing of the cofferdam, the gaps between the steel sheet piles are fully welded, and ribs are welded to prevent the steel sheet pile angles from deforming during the insertion process. Alternatively, separate steel sheet pile locks can be welded to the steel pipe piles / steel sheet piles at 90 degrees.

[0030] When the steel pipe pile / steel sheet pile is of appropriate size, but the resistance of the locking ends on both sides is too great after insertion, and the pile cannot be driven to the designed elevation, a hand chain hoist should be used to assist or several adjacent steel pipe piles / steel sheet piles should be pulled up and re-inserted;

[0031] The cofferdam driving construction needs to extend to 10m to 15m above the river bank to prevent the river water from surging from the river bank to the inside of the cofferdam due to the rise and fall of water level. At the same time, it is convenient to close the precipitation wells in the cofferdam to reduce the seepage of river water from the river bank to the cofferdam. If hard rock layers are encountered or the riverbed sand is dense and piles cannot be sunk smoothly, thickened steel pipe piles with a height of 500mm from the top of the steel pipe piles are used, and two pile drivers are used for construction to make the pile top elevation reach the design elevation and ensure that the embedment depth meets the design requirements.

[0032] As a further solution of the present invention, the specific steps of closing and sealing the cofferdam in step II are as follows:

[0033] S3.1: After the PLC composite pile cofferdam is constructed, high-strength waterproof cloth is used as the cofferdam closure material. The selected high-strength waterproof cloth is spread along the water-facing surface of the cofferdam and fixed to the top of the PLC composite piles.

[0034] S3.2: Use sandbags to sink the waterproof cloth to the bottom of the water, with the bottom extending 5 to 8 meters outside the riverbed. The construction personnel will then check the fixing effect of the waterproof cloth and the airtightness of the cofferdam joints to ensure that the waterproof cloth, piles and sandbags form a complete waterproof closed system, and then fix the waterproof cloth and sandbags firmly.

[0035] As a further solution of the present invention, the specific steps of draining the interior of the cofferdam in step III are as follows:

[0036] S4.1: Set up a pumping efficiency of 1000m within the cofferdam 3 / h, 200 horsepower tractor water pump, and the number of water pumps is configured according to the volume of river water in the cofferdam at the construction site and the construction period requirements. The length of the water inlet pipe is 9m, and the length of the water outlet pipe is installed according to the actual situation. The diameter of the water inlet pipe and the water outlet pipe are both 0.6m;

[0037] S4.2: After the cofferdam is closed and sealed, tractor pumps are placed on the banks upstream and downstream where the water level is higher than the preset depth. Intermittent pumping is performed. First, the tractor pumps are used on the banks to pump the river water from the cofferdam. After pumping to the required depth, an excavator is used to fill the cofferdam with soil to create a construction platform for the tractor pumps. Repeat this pumping and filling process until the river water in the cofferdam is completely pumped out.

[0038] S4.4: During the pumping process, the water level changes are monitored in real time. As the water level drops, gravel is used for counter-pressure on the inside of the cofferdam. Sand and soil excavated from the river channel are used to fill the gaps inside the gravel counter-pressure according to the pumping situation. During the pumping and counter-pressure process, the construction personnel continuously check the stability of the cofferdam and the effect of soil and rock counter-pressure.

[0039] As a further solution of the present invention, during the construction period described in step IV, the cofferdam is monitored and maintained in real time. The specific steps are as follows:

[0040] S5.1: During the pumping process, check the locking areas of steel pipe piles or steel sheet piles for leaks in real time. If leaks are found, fill the locking areas of the steel sheet piles / steel pipe piles with fiber-rich cotton wool, leak-proof mortar, and sawdust. For wide pile gaps, use hemp root mixed with butter to seal the gaps, or adopt a comprehensive treatment method of spreading fly ash, sawdust, and expansive cement along the pile surface outside the cofferdam in the direction of water flow. For serious leaks, inject the required amount of cement slurry to seal the leaks. At the same time, use high-pressure water jets to press tape into the leaks for temporary sealing.

[0041] S5.2: During cofferdam construction, horizontal and vertical displacement monitoring points shall be arranged along the perimeter of the retaining structure at the top of the retaining structure, and monitoring points shall be arranged in the middle and corners of the perimeter, with the horizontal spacing between monitoring points being no greater than 15 m. The number of monitoring points on each side shall be no less than 3;

[0042] S5.3: Install the pile top settlement monitoring pile and the displacement observation pile simultaneously at the same point. After the displacement monitoring point is set up and the settlement at the observation point stabilizes, use a total station to measure the initial readings. After the horizontal displacement point of the pile body is set up after pumping is completed, monitor as required.

[0043] S5.4: Mark elevation data at 10 cm intervals on the outside of the cofferdam. Surveyors will use a steel ruler to measure the current water level elevation. This measurement will be recorded at a monitoring frequency of once a day. If the water level changes faster than a preset threshold, the monitoring frequency will be increased.

[0044] S5.5: Select a river section with a gentle, straight flow and a length of not less than 10 meters, and set the flow rate monitoring frequency to once a day. Then, select waste on site to make a buoy, and select a canal section with a gentle flow, no bends and a length of 3 to 5 meters from the river section. Place the buoy on the water surface to drift, record the time required for the buoy to drift through the selected canal section, calculate the water flow speed, repeat the buoy drifting and water flow calculation 5 times, and take the average value as the water flow speed for that day. If the water flow speed change is higher than the preset threshold, increase the monitoring frequency.

[0045] As a further embodiment of the present invention, step IV is described.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] This phased diversion PLC composite pile cofferdam construction method for deep sand layers uses a PLC composite pile construction method. Before PLC composite pile construction, surveyors use a total station and level to measure the cofferdam axis on the water surface, reserving construction space and drawing a white control line. The cofferdam extends 10 meters toward the riverbank, with appropriate extension upstream and downstream based on construction requirements. Simultaneously, underwater exploration and geophysical surveys are conducted to remove obstacles. Locking steel sheet pile guides are then driven, I-beam brackets are welded, and HW400×400 guide beams are installed. Guided by the guide beams, a manipulator pile driver drives steel pipe piles / sheet piles one by one to the designed elevation. Upon completion of the cofferdam, high-strength waterproof sheeting is laid and sandbags are secured to seal the cofferdam, extending 5 to 8 meters from the bottom. A water pump is installed inside the cofferdam to drain water. At the same time, gravel and sand are filled for counter-pressure, and the stability of the cofferdam is monitored. During the pumping period, pile joints are checked for water leakage, and cotton wool, mortar, hemp silk, grouting and other methods are used to seal the leaks. During the construction period, pile top and pile body displacement monitoring points are set up to monitor water level and flow rate changes to ensure the safety and stability of the cofferdam structure. This can effectively improve the overall efficiency of on-site cofferdam construction. In addition, counter-pressure support, waterproof cloth airtightness, and double-machine insertion and construction are innovatively adopted to achieve rapid construction of super-large cofferdams while ensuring the safety and stability of the cofferdam itself. This saves on-site construction time as a whole, avoids repeated sealing and monitoring and early warning of the cofferdam, reduces steel investment, and the combination of steel cofferdam + inner soil and rock counter-pressure achieves higher safety and stability, avoids the disadvantages of traditional steel cofferdams, and meets the requirements of green environmental protection, energy conservation and emission reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0049] Figure 1 This is a flowchart of the comprehensive cofferdam construction method for the phased diversion type PLC composite pile in deep sand layers proposed by the present invention;

[0050] Figure 2This is a process flow chart of the thick sand layer phased diversion type PLC composite pile comprehensive cofferdam construction method proposed by the present invention. DETAILED DESCRIPTION

[0051] Reference Figure 1-2 The detailed steps of the comprehensive cofferdam construction method for phased diversion type PLC composite piles in thick sand layer are as follows:

[0052] A temporary wharf is constructed according to the site conditions. After the construction of the temporary wharf is completed, the steel pontoon is assembled and the construction position of the PLC composite pile is measured and determined.

[0053] Specifically, before the construction of the PLC composite piles, the surveyor took a small boat to determine the axis, and used the total station and level measuring equipment to accurately measure the cofferdam axis position on the water surface according to the construction design drawings, and reserved construction space in the cofferdam area. After completing the axis measurement and setting, the construction position of the PLC composite piles was determined in the cofferdam area, and a white control line was drawn between the two adjacent PLC composite pile construction points. Then, according to the on-site construction conditions, the cofferdam construction area was extended to 10m above the river bank. At the same time, the upstream and downstream of the cofferdam and the direction of the water flow were relaxed based on the construction boundary line. During the measurement and positioning process, underwater exploration and construction area geophysical exploration were carried out simultaneously to check the riverbed conditions in the construction area and to explore whether there were any unknown factors such as obstacles, silt layers and floating objects that affected the cofferdam construction. If obstacles were found, they would be removed in advance or corresponding treatment measures would be formulated.

[0054] It should be further explained that the temporary wharf area is set up according to the on-site conditions, and a row of 12m steel sheet piles are driven on the water side of the temporary wharf, and the pile tops are kept flush with the temporary access road on the site to form a flat construction surface. After the water inside the steel sheet piles is drained by pumping, earth and stone are filled in and compacted to strengthen the wharf structure. After that, the steel pontoon is docked and the riverbed outside the steel sheet piles is cleared to 2.5m below the ground. If the riverbed on the shore cannot be cleared to 2.5m below the ground on site, the temporary wharf area will be expanded and extended to the deep water area in the center of the river.

[0055] It should be noted that temporary docks are usually set up to 20 meters x 15 meters.

[0056] As a further solution of this embodiment, according to construction needs, three 3.6m×22m×2.2m pontoons and ten 2.8m×3.2m×0.92m pontoons are selected, and the length of the pontoons is adjusted according to the length of the selected locking steel sheet piles or steel pipe piles. A 100t truck crane is used to lift the steel pontoons to a river water area with a water depth of not less than 2.5m, and steel pins are used to connect the pontoons to assemble them into pile driver steel pontoons with dimensions of 10.8m×22m×2.2m and pile transport steel pontoons of 5.6m×16m×0.92m. After the steel pontoons are assembled, four 300×300H-shaped steel plugs are inserted into the preset positioning pile holes at the four corners of the pontoons for fixation. After the pontoons are fixed, the manipulator vibration pile driver is driven above the steel pontoons.

[0057] In addition, it should be noted that the manipulator vibration pile driver is equipped with 3 personnel, including 1 pile driver operator and 2 pile support personnel.

[0058] According to the construction position and construction sequence, the PLC composite piles are driven to the designed depth. After the PLC composite piles are driven, the cofferdam is closed and sealed.

[0059] Specifically, lock steel sheet piles are used as guide piles, 25a I-steel brackets are welded on the steel sheet piles, and inserted into the inner side of the foundation pit according to the measured positioning. Then, HW400×400 steel is used as the guide beam, and according to the single-layer single-sided form, the guide beam is fixed on the I-steel bracket of the guide pile. At the same time, the part of the I-steel bracket that grows out of the guide beam is cut off. The manipulator vibrates the pile driver with a vibrating hammer to clamp the steel pipe piles / steel sheet piles on the transport ship and move them to the piling site. Two pile support personnel cooperate with the pile driver to lift the steel pipe piles / steel sheet piles The sheet piles are accurately placed in the pile positions, and the steel pipe piles / steel sheet piles are ensured to be close to the guide beam. Then the pile driver starts the vibrating hammer and drives the steel pipe piles / steel sheet piles downwards. At the same time, the pile support personnel observe the verticality of the steel pipe piles / steel sheet piles at any time and instruct the pile driver to make adjustments. When the first steel pipe pile / steel sheet pile is driven to the designed elevation, the second pile is lifted and driven using the first driven steel pipe pile / steel sheet pile as a guide. After that, the steel pipe piles / steel sheet piles are driven into the soil one by one until the cofferdam construction is completed.

[0060] Specifically, after the construction of the PLC composite pile cofferdam is completed, high-strength waterproof cloth is used as the cofferdam closing material, and the selected high-strength waterproof cloth is spread along the water-facing surface of the cofferdam and fixed at the top of the PLC composite pile. Sandbags are used to sink the waterproof cloth to the bottom of the water, and the bottom extends 5 to 8 meters outside the riverbed. Afterwards, the construction personnel check the fixing effect of the waterproof cloth and the airtightness of the cofferdam joints to ensure that the waterproof cloth, pile body, and sandbags form a complete waterproof closing system, and then the waterproof cloth and sandbags are firmly fixed.

[0061] It should be noted that the guide piles are approximately 0.8m away from the steel sheet piles inside the foundation pit, and the distance between each guide pile is 11.0m;

[0062] Special corner piles must be used at the corners of the foundation pit. Two steel sheet piles can be welded at 90 degrees. To ensure the sealing of the cofferdam, the gaps between the steel sheet piles are fully welded, and ribs are welded to prevent the steel sheet pile angle from deformation during the insertion process. Alternatively, separate steel sheet pile locks can be used and welded at 90 degrees to the steel pipe piles / steel sheet piles.

[0063] When the steel pipe pile / steel sheet pile is of appropriate size, but the resistance of the locking ends on both sides is too great after insertion, and the pile cannot be driven to the designed elevation, a hand chain hoist should be used to assist or several adjacent steel pipe piles / steel sheet piles should be pulled up and re-inserted;

[0064] The cofferdam driving construction needs to extend to 10m to 15m above the river bank to prevent the river water from surging from the river bank to the inside of the cofferdam due to the rise and fall of water level. At the same time, it is convenient to close the precipitation wells in the cofferdam to reduce the seepage of river water from the river bank to the cofferdam. If hard rock layers are encountered or the riverbed sand is dense and piles cannot be sunk smoothly, thickened steel pipe piles with a height of 500mm from the top of the steel pipe piles are used, and two pile drivers are used for construction to make the pile top elevation reach the design elevation and ensure that the embedment depth meets the design requirements.

[0065] After the cofferdam is closed, drainage inside the cofferdam is carried out, and after the drainage is completed, the construction of various hydraulic structures of the water conservancy project inside the cofferdam is carried out.

[0066] Specifically, a pumping efficiency of 1000m 3 / h, 200 horsepower tractor pump, and the number of pumps was configured according to the volume of river water in the cofferdam at the construction site and the construction period requirements. A 9-meter-long inlet pipe was selected, and the length of the outlet pipe was installed according to the actual situation. The diameter of the inlet and outlet pipes was 0.6m. After the cofferdam was closed and sealed, the tractor pumps were placed on the shore near the upstream and downstream water levels where the water level was higher than the preset value. An intermittent pumping method was adopted. The tractor pump was first used to pump out the river water in the cofferdam on the shore. After pumping to the specified depth, an excavator was used to fill the cofferdam with soil to fill out the construction platform of the tractor pump. The pumping and filling were repeated until the river water in the cofferdam was pumped out. During the pumping process, the water level changes were monitored in real time. At the same time, as the water level dropped, gravel was used on the inside of the cofferdam for counterpressure, and sand and soil excavated from the river channel were used to fill the gaps inside the gravel counterpressure according to the pumping situation. During the pumping and counterpressure process, the construction personnel continuously checked the stability of the cofferdam and the soil and rock counterpressure effect.

[0067] During the construction period, the cofferdam will be monitored and maintained in real time, and after the construction is completed, the cofferdam will be dismantled and the water environment will be restored.

[0068] Specifically, during the pumping process, the locking parts of the steel pipe piles or steel sheet piles are checked in real time for leakage. If leakage is found, the steel sheet pile / steel pipe pile locking parts are filled with fiber-rich cotton wool, leak-proof mortar and sawdust. For wider pile joints, hemp root mixed with butter is used to seal the joints to stop water, or a comprehensive treatment method is adopted in which fly ash, sawdust and expansive cement are spread along the pile surface along the water flow direction outside the cofferdam. For serious leakage, the required dosage of cement slurry is injected to seal the leak, and the high-pressure water jet is used to press the tape into the leak for temporary sealing. During the cofferdam construction, horizontal and vertical displacement monitoring points are arranged on the top of the support structure along the periphery of the support structure, and monitoring points are arranged in the middle and corners of the periphery. At the same time, the horizontal spacing between the monitoring points is kept no more than 15m, and the number of monitoring points on each side is no less than 3. The pile top settlement monitoring pile and the displacement observation pile are synchronized and set at the same point. The displacement monitoring point arrangement After completion, wait for the observation point to settle and stabilize, and complete the setting, use the total station to measure the initial reading. The horizontal displacement point of the pile body will be arranged after the pumping is completed, and monitor as required. Mark the elevation data at intervals of 10 cm on the outside of the cofferdam. The surveyor uses a steel ruler to measure the current water level elevation, and then conducts measurements and records at a monitoring frequency of once a day. If the water level velocity change is found to be higher than the preset threshold, the monitoring frequency will be increased. A river section with a gentle flow, a straight line, and a length of not less than 10 meters will be selected, and the flow rate monitoring frequency will be set to once a day. After that, waste will be selected on site to make a buoy, and a channel section with a gentle flow, no bends, and a length of 3 to 5 meters will be selected from the river section. The buoy will be placed on the water surface to drift, and the time required for the buoy to drift through the selected channel section will be recorded. The water velocity will be calculated, and the buoy drifting and water flow calculation will be repeated 5 times, and the average value will be taken as the water velocity for the day. If the water velocity change is higher than the preset threshold, the monitoring frequency will be increased.

[0069] In addition, it should be further explained that after the construction work inside the cofferdam is completed, the back pressure of soil and rock inside the cofferdam is gradually removed from the river channel to the river bank to prevent the back pressure of soil and rock inside the cofferdam from affecting the river channel. When the construction work inside the cofferdam is completed, the cofferdam backwater adopts the direct backwater method. Steel sheet piles are pulled out about 8 meters upstream and downstream of the cofferdam near the river bank as the water outlet. One steel sheet pile is pulled out upstream and two steel sheet piles are pulled out downstream to achieve the purpose of opening, so that water can flow into the cofferdam. At the same time, the speed of pulling out the steel sheet piles should be slow. The deformation of the surrounding steel sheet piles / steel pipe piles should be observed at any time to control the backwater speed. Before pulling out the piles and returning the water, a layer of geotextile is laid on the riverbed at the water outlet to prevent the riverbed from being eroded by the river water entering the cofferdam.

[0070] After the construction of the structure inside the cofferdam is completed and the water is returned, the temporary wharf is reset and the steel sheet piles / steel pipe piles are removed for reuse. Before pulling out the piles, carefully study the order of pile pulling methods. For closed cofferdams, the starting point of pile pulling should be more than 5 piles away from the corner piles. The starting point of pile pulling can be determined according to the situation when sinking the piles. If necessary, the jump pulling method can also be used. The order of pile pulling is best to be opposite to that of pile driving. When pulling out piles, first use a pile driver to clamp the pile head and vibrate it for 1min to 2min to loosen the soil around the pile, produce "liquefaction", and reduce the friction resistance of the soil to the pile. Then slowly vibrate upwards. Pay attention to the load of the pile driver when pulling out the pile. When you find that pulling up is difficult or cannot be pulled up, you should stop pulling out the pile. You can first inject a little downwards and then push it upwards. Repeat this process to pull out the pile.

[0071] It should be supplemented in the present invention that the cofferdam monitoring frequency, material selection and equipment selection of the deep sand layer staged diversion type PLC composite pile comprehensive cofferdam construction method are shown in the following table:

[0072] Table 1

[0073]

[0074] Table 2

[0075]

[0076] Table 3

[0077]

[0078]

[0079] Table 1 is the cofferdam monitoring frequency table, where the cofferdam monitoring items specifically include pile top horizontal displacement, pile top vertical displacement, pile body horizontal displacement, water level outside the weir, water level inside the weir, water flow velocity monitoring, and cofferdam leakage monitoring;

[0080] Table 2 is a material selection table. The materials selected in the present invention include IV type steel sheet piles, steel pipe piles, high-performance PVC double-sided plastic-coated canvas and sandbags;

[0081] Table 3 is an equipment selection table, in which the equipment selected by the present invention includes a manipulator vibration pile driver, a pile driver steel pontoon, a pile transport steel pontoon, a boat, a truck crane, an electric welder, a gas cutting tool and a diesel pump.

Claims

1. A comprehensive cofferdam construction method for phased diversion type PLC composite piles in thick sand layers, characterized by: The specific steps of this construction method are as follows: Ⅰ. Build a temporary wharf according to the site conditions. After the temporary wharf is built, assemble the steel pontoon, and measure and determine the construction position of the PLC composite piles; II. According to the construction location and construction sequence, the PLC composite piles are driven to the designed depth. After the PLC composite piles are driven, the cofferdam is closed and sealed; III. After the cofferdam is closed, the inside of the cofferdam shall be drained and after the drainage is completed, the construction of the hydraulic structures of the water conservancy project inside the cofferdam shall be carried out; IV. During the construction period, the cofferdam shall be monitored and maintained in real time. After the construction is completed, the cofferdam shall be dismantled and the water environment shall be restored.

2. The method for constructing a comprehensive cofferdam with a phased diversion type PLC composite pile in a thick sand layer according to claim 1 is characterized in that: Measure and determine the construction position of PLC combination pile as described in step Ⅰ S1.1: Before the construction of the PLC composite piles, the surveyor will take a small boat to determine the axis. Using a total station and a level, the surveyor will accurately measure the axis of the cofferdam on the water surface according to the construction design drawings and reserve construction space within the cofferdam area. S1.2: After completing the axis survey, determine the construction location of the PLC composite piles within the cofferdam area and draw a white control line between two adjacent PLC composite pile construction points. Based on the on-site construction conditions, the cofferdam construction area is extended to 10m above the river bank. At the same time, the cofferdam upstream and downstream and along the water flow direction are relaxed based on the construction boundary line. S1.3: During the surveying and positioning process, conduct underwater exploration and geophysical exploration of the construction area simultaneously to check the riverbed conditions within the construction area and to detect any unknown factors such as obstacles, silt layers, and floating objects that may affect the cofferdam construction. If any obstacles are found, they should be removed in advance or appropriate treatment measures should be formulated.

3. The method for constructing a comprehensive cofferdam with a phased diversion type PLC composite pile in a thick sand layer according to claim 2 is characterized in that: The specific steps for driving the PLC combination pile to the designed depth are as follows: S2.1: Use locking steel sheet piles as guide piles. Weld 25a I-beam corbels to the steel sheet piles and drive them into the inner side of the foundation pit according to the measured positioning. Then, use HW400×400 steel as guide beams. Based on the single-layer and single-sided structure, fix the guide beams to the I-beam corbels of the guide piles. At the same time, cut off the part of the I-beam corbels that protrude from the guide beams. S2.2: The manipulator vibratory pile driver uses a vibratory hammer to pick up the steel pipe pile / steel sheet pile from the transport ship and move it to the piling location. Two pile support personnel work with the pile driver to accurately place the steel pipe pile / steel sheet pile and ensure that the steel pipe pile / steel sheet pile is close to the guide beam. The pile driver then turns on the vibratory hammer and drives the steel pipe pile / steel sheet pile downward. At the same time, the pile support personnel constantly monitor the verticality of the steel pipe pile / steel sheet pile and instruct the pile driver to make adjustments. S2.3: After the first steel pipe pile / steel sheet pile is driven to the design elevation, use the first steel pipe pile / steel sheet pile as a guide to lift and drive the second pile. Then, drive the steel pipe piles / steel sheet piles into the soil one by one until the cofferdam construction is completed.

4. The method for constructing a comprehensive cofferdam with a phased diversion type PLC composite pile in a thick sand layer according to claim 3 is characterized in that: The specific steps for closing and sealing the cofferdam in step II are as follows: S3.1: After the PLC composite pile cofferdam is constructed, high-strength waterproof cloth is used as the cofferdam closure material. The selected high-strength waterproof cloth is spread along the water-facing surface of the cofferdam and fixed to the top of the PLC composite piles. S3.2: Use sandbags to sink the waterproof cloth to the bottom of the water, with the bottom extending 5 to 8 meters outside the riverbed. The construction personnel will then check the fixing effect of the waterproof cloth and the airtightness of the cofferdam joints to ensure that the waterproof cloth, piles and sandbags form a complete waterproof closed system, and then fix the waterproof cloth and sandbags firmly.

5. The method for constructing a comprehensive cofferdam with a phased diversion type PLC composite pile in a thick sand layer according to claim 4 is characterized in that: The specific steps for draining the inside of the cofferdam described in step III are as follows: S4.1: Set up a pumping efficiency of 1000m within the cofferdam 3 / h, 200 horsepower tractor water pump, and the number of water pumps is configured according to the volume of river water in the cofferdam at the construction site and the construction period requirements. The length of the water inlet pipe is 9m, and the length of the water outlet pipe is installed according to the actual situation. The diameter of the water inlet pipe and the water outlet pipe are both 0.6m; S4.2: After the cofferdam is closed and sealed, tractor pumps are placed on the banks upstream and downstream where the water level is higher than the preset depth. Intermittent pumping is performed. First, the tractor pumps are used on the banks to pump the river water from the cofferdam. After pumping to the required depth, an excavator is used to fill the cofferdam with soil to create a construction platform for the tractor pumps. Repeat this pumping and filling process until the river water in the cofferdam is completely pumped out. S4.4: During the pumping process, the water level changes are monitored in real time. As the water level drops, gravel is used for counter-pressure on the inside of the cofferdam. Sand and soil excavated from the river channel are used to fill the gaps inside the gravel counter-pressure according to the pumping situation. During the pumping and counter-pressure process, the construction personnel continuously check the stability of the cofferdam and the effect of soil and rock counter-pressure.

6. The method for constructing a comprehensive cofferdam with a thick sand layer by phased diversion type PLC composite pile according to claim 5 is characterized in that: During the construction period described in step IV, the specific steps for real-time monitoring and maintenance of the cofferdam are as follows: S5.1: During the pumping process, check the locking areas of steel pipe piles or steel sheet piles for leaks in real time. If leaks are found, fill the locking areas of the steel sheet piles / steel pipe piles with fiber-rich cotton wool, leak-proof mortar, and sawdust. For wide pile gaps, use hemp root mixed with butter to seal the gaps, or adopt a comprehensive treatment method of spreading fly ash, sawdust, and expansive cement along the pile surface outside the cofferdam in the direction of water flow. For serious leaks, inject the required amount of cement slurry to seal the leaks. At the same time, use high-pressure water jets to press tape into the leaks for temporary sealing. S5.2: During cofferdam construction, horizontal and vertical displacement monitoring points shall be arranged along the perimeter of the retaining structure at the top of the retaining structure, and monitoring points shall be arranged in the middle and corners of the perimeter, with the horizontal spacing between monitoring points being no greater than 15 m. The number of monitoring points on each side shall be no less than 3; S5.3: Install the pile top settlement monitoring pile and the displacement observation pile simultaneously at the same point. After the displacement monitoring point is set up and the settlement at the observation point stabilizes, use a total station to measure the initial readings. After the horizontal displacement point of the pile body is set up after pumping is completed, monitor as required. S5.4: Mark elevation data at 10 cm intervals on the outside of the cofferdam. Surveyors will use a steel ruler to measure the current water level elevation. This measurement will be recorded at a monitoring frequency of once a day. If the water level changes faster than a preset threshold, the monitoring frequency will be increased. S5.5: Select a river section with a gentle, straight flow and a length of not less than 10 meters, and set the flow rate monitoring frequency to once a day. Then, select waste on site to make a buoy, and select a canal section with a gentle flow, no bends and a length of 3 to 5 meters from the river section. Place the buoy on the water surface to drift, record the time required for the buoy to drift through the selected canal section, calculate the water flow speed, repeat the buoy drifting and water flow calculation 5 times, and take the average value as the water flow speed for that day. If the water flow speed change is higher than the preset threshold, increase the monitoring frequency.

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