Construction method of double-side steel pipe interlayer combined cofferdam body

Abstract

The invention discloses a construction method of a double-side steel pipe interlayer combined cofferdam body. The cofferdam body with the thickness of 3 m is constructed by adopting DN200 steel pipes, double-spliced 16 # A channel steel, phi 20 pull rods, waterproof geotechnical cloth, grids, waterproof soil bodies, sand bags or waterproof cloth. During construction, steel pipes are hit through a pile driving barge to form a closed loop, then double-spliced 16 # A channel steel back ribs and phi 20 pull rods with the interval of 50 cm are installed, the two ends of each pull rod are fixed through double nuts, then waterproof geotechnical cloth and grids are sequentially laid on the two sides in the weir body, sludge in a cofferdam enclosure area is grabbed through an overwater dredger to serve as waterproof soil, and the waterproof soil is filled into the weir body in a layered mode. And after the soil body stands and does not sink any more, sand bags or waterproof cloth is laid on the top face of the cofferdam body, water in the cofferdam is pumped and drained finally, and construction in water is converted into construction on land. The method has the advantages of being low in construction cost, good in environmental protection property, convenient to construct, stable in structure, low in daily maintenance cost and the like, and is suitable for water area construction within 3m in water depth.

Description

Technical Field

[0001] This invention belongs to the field of civil engineering construction technology, specifically relating to a construction method for a double-sided steel pipe sandwich composite cofferdam body suitable for water depths of less than 3m. Background Technology

[0002] In civil engineering construction, when foundation structure construction needs to be carried out in shallow water (water depth less than 3 meters), it is usually necessary to first construct a cofferdam to isolate the water body, thus converting the water-based construction into land-based construction. Commonly used cofferdam construction methods mainly include the following categories: 1. Island construction and cofferdam construction: Artificial islands are formed by filling a large amount of earth. Although the process is simple, it consumes a lot of earth resources and causes great damage to the ecological environment of the surrounding waters. After demolition, it is easy to cause soil erosion.

[0003] 2. Steel sheet pile cofferdam: It is a water-retaining structure formed by splicing steel sheet piles. It has high strength but expensive material cost, high requirements for pile driving equipment, and long construction period.

[0004] 3. Double-layer steel sheet pile cofferdam: The structure is formed by two layers of steel sheet piles and the middle backfill. Although it has good seepage prevention performance, the construction process is complicated, requiring professional equipment for interlocking connection, and the later maintenance cost is high.

[0005] 4. Earth-rock cofferdam: It is constructed using local earth and rock materials. It is relatively inexpensive but has weak erosion resistance and is prone to leakage or collapse when the water flow velocity is high.

[0006] The above methods have significant drawbacks in practical applications: island-building cofferdams are environmentally unfriendly, sheet pile cofferdams are costly, and earth-rock cofferdams lack stability. Therefore, there is an urgent need for a shallow water cofferdam construction method that balances cost, environmental friendliness, stability, and ease of construction. Summary of the Invention

[0007] To overcome the above-mentioned shortcomings, this invention provides a construction method for a double-sided steel pipe sandwich composite cofferdam body, which solves the problems of high cost, poor environmental protection, and complex construction in existing methods for water construction within a depth of 3m, and realizes the efficient transformation from water construction to land construction.

[0008] The technical solution adopted by this invention to solve its technical problem is as follows: A construction method for a double-sided steel pipe sandwich composite cofferdam includes the following steps: 1) Pre-construction preparation: Survey the construction water area with a water depth ≤3m and a water flow velocity ≤1.5m / s, and determine the outline, axis and 1:1 slope ratio of the cofferdam; prepare materials and equipment, including DN200 steel pipes with a wall thickness ≥6mm, double-layer 16#A channel steel, φ20 tie rods, waterproof geotextile with a specification ≥400g / ㎡, biaxially oriented polypropylene grid, silt in the cofferdam with a moisture content of 30%-50% as impermeable soil, and sandbags or waterproof cloth; 2) Main construction: 2.1) Install DN200 steel pipes to form a closed loop, with a spacing of 50cm between pipes. The penetration depth should meet the requirements of ≥2m for silt layers and ≥1.5m for hard soil layers, with a verticality deviation ≤1%. 2.2) Weld double-segment 16#A channel steel back ribs, and simultaneously install φ20 tie rods at 50cm intervals, with both ends of the tie rods fixed with double nuts; 2.3) Waterproof geotextile and grid are laid sequentially on both sides of the dam body. The overlap of the waterproof geotextile is ≥20cm and the overlap of the grid is ≥30cm. Both are fixed. 2.4) Fill impermeable soil in layers to a thickness of 3m, let it stand until settlement is stable, and the settlement amount is ≤5mm / day for 3 consecutive days; 2.5) The top surface of the dam should be protected with sandbags or waterproof cloth; 3) Drainage and toe protection construction within the cofferdam: The water inside the cofferdam is pumped out in stages, and the excess silt is used to construct the toe protection on the water-facing side. The toe protection width is ≥1.5m and the slope ratio is 1:1.5. 4) Construction acceptance and maintenance.

[0009] Step 1) Material preparation also includes welding 10cm high and 8mm thick reinforcing rings at both ends of the DN200 steel pipe, and pre-connecting double-splitting 16#A channel steel with M12 bolts at 30cm intervals to form an integral back rib unit.

[0010] In step 1), the permeability coefficient of the waterproof geotextile should be ≤1×10⁻ 7 cm / s, in sandy loam sections, a two-layer fabric-one-film composite structure with a middle PE film thickness of 0.5mm is adopted.

[0011] In step 2.1), when installing the steel pipe, spiral blades with a spacing of 30cm and a width of 15cm are welded to the outside of the steel pipe in the sandy loam section, and the verticality is controlled by the H-shaped steel guide frame.

[0012] When installing the φ20 tie rod in step 2.2), both ends of the tie rod should extend ≥10cm beyond the outer side of the channel steel. Tighten it in three stages using a torque wrench: initial tightening 100N·m, secondary tightening 200N·m, and final tightening 300N·m. Fill the gap between the nut and the channel steel with a water-swellable sealing strip with a diameter of 2cm.

[0013] When laying the waterproof geotextile in step 2.3), the overlaps should be welded by hot melt welding at a temperature of 180-200℃ and a pressure of 0.3-0.5MPa or bonded with special adhesive, and fixed with nylon cable ties at 50cm intervals every 1m.

[0014] When laying the grid in step 2.3), use 20cm long U-shaped nails to drive into the soil at a 45° angle at the overlap, with a depth of 15cm and a fixing spacing of 50cm. The grid must be taut and without wrinkles.

[0015] Step 2.4) involves layered filling of impermeable soil, which includes: silt being filtered through a 5cm mesh screen and left to stand for 24 hours; the first layer being 30cm thick and compacted 3 times, and the second layer being 40-50cm thick and compacted 5 times, with a compaction degree ≥90%; and the surface being roughened manually after each layer is filled, with a roughening depth of 5cm.

[0016] In step 2.5), the top surface protection is specifically as follows: sandbags are stacked in a staggered manner with an overlap of ≥1 / 3 of the bag length, a thickness of ≥50cm, and the edges extend 50cm beyond both sides of the dam body. They are fixed with wooden stakes that are 30cm deep in the soil every 2m; or the waterproof cloth overlaps ≥50cm, and 3 sandbags are placed around the perimeter every meter.

[0017] Step 3) includes drainage and toe protection: during the staged drainage, first pump to 1m below the top surface of the weir, observe for 24 hours for no leakage, and then pump to the design elevation; the bottom layer of the toe protection is filled with 20cm thick stones with a particle size of 30-50cm, the surface is covered with grid and the edges are pressed with sandbags, and inclinometers buried at a depth of 5m are installed at the corners to monitor the slope, with an allowable deviation of ≤0.5%.

[0018] Due to the adoption of the technical solution described above, the present invention has the following advantages: 1. Low cost: Using silt inside the cofferdam as filling material reduces the cost of purchasing soil; using conventional steel pipes, channel steel and other materials reduces equipment and material investment.

[0019] 2. Good environmental performance: It realizes the resource utilization of silt, reduces the transportation and mining of soil, and reduces the impact on the surrounding ecological environment.

[0020] 3. Convenient construction: The process is simple and the steps are clear. No special equipment is required, and construction can be organized quickly, shortening the construction period.

[0021] 4. Stable structure: The double-sided steel pipe frame and tie rods form an integral load-bearing system, which is reinforced by the seepage prevention layer and grid, resulting in strong impermeability and deformation resistance.

[0022] 5. Wide applicability: Suitable for waterways, lakes and other water bodies with a depth of up to 3 meters, and can cope with various geological conditions such as silt and sandy loam. Attached Figure Description

[0023] Figure 1 A schematic diagram of the cofferdam structure of this invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] A construction method for a double-sided steel pipe sandwich composite cofferdam body, comprising DN200 steel pipe 1, double-layered 16#A channel steel 2, φ20 tie rod 3, waterproof geotextile 4, geogrid 5, impermeable soil 6, and sandbags or waterproof fabric 7. The specific construction steps are as follows: 1) Pre-construction preparation Site survey: Survey the water depth of the construction area (≤3m, flow velocity ≤1.5m / s), the thickness of the silt layer in the riverbed, the type of the underlying soil, and the surrounding environment, and draw a site topographic map. Determine the outline of the cofferdam based on the survey results. The cofferdam outline should preferably be rectangular, with a cofferdam axis, a slope ratio of 1:1, and a toe protection range.

[0026] Materials preparation: DN200 steel pipe 1: wall thickness ≥6mm, material Q235B, length determined according to water depth and burial depth, total length of steel pipe = water depth + burial depth + 0.5m height above water surface, wherein the burial depth is ≥2m in silt layer and ≥1.5m in hard soil layer, and 10cm high and 8mm thick reinforcing rings are welded at both ends.

[0027] Double-section 16#A channel steel 2: The specifications of a single channel steel are 160×63×6.5mm. They are pre-connected with M12 bolts at 30cm intervals to form an integral back rib unit. The length is consistent with the height of the steel pipe above the water surface.

[0028] φ20 tie rod 3: made of threaded steel, with its length designed according to the 3m thickness of the weir.

[0029] Waterproof geotextile 4: Specifications ≥ 400g / ㎡, permeability coefficient ≤ 1×10⁻ 7 cm / s, cut according to the perimeter of the weir + 2m, heat-seal the edges, and the width is 5cm; in special geological sections such as sandy loam, a two-layer cloth and one-layer film composite structure with a middle PE film thickness of 0.5mm can be used.

[0030] Grating 5: Biaxially oriented polypropylene grating 5, with longitudinal and transverse tensile strength ≥20kN / m.

[0031] Impermeable soil body 6: Use silt from inside the cofferdam. The moisture content needs to be tested and should be between 30% and 50%. If there are many impurities, it needs to be filtered with a filter screen with a pore size of 5cm.

[0032] Sandbag 7: Woven bag, size 40×60cm, filled with soil to 70% of the bag capacity, filled with impermeable soil; or waterproof cloth (same as the waterproof geotextile 4 above).

[0033] Equipment preparation: Piling vessel, equipped with GPS positioning system, verticality monitoring instrument and H-beam guide frame; dredging vessel, grab bucket capacity ≥1m³, with filter screen installed; water pump, head ≥10m, flow rate calculated based on water volume in the cofferdam; total station, level, electric welding machine, torque wrench, excavator, etc., all equipment must be tested and qualified before arrival on site.

[0034] Safety and environmental protection measures: Delineate the construction warning zone, set up buoys and warning signs, and prohibit unauthorized vessels from entering; equip with life jackets, fire-fighting equipment and impermeable tarpaulins; prepare a temporary sludge storage site and lay an impermeable membrane; communicate with the local environmental protection department to clarify the requirements for sludge storage and wastewater discharge.

[0035] 2) Main construction Drive DN200 steel pipe 1 to form a closed loop of cofferdam: Positioning and setting out: Use a total station to set out the position of steel pipe piles along the design axis, mark a point every 50cm, and add control piles at the corners of the cofferdam to ensure that the axis deviation is ≤5cm.

[0036] Piling Operation: After the piling vessel is in place, it is aligned with the pile location using GPS, the vessel is leveled, and the steel pipe is placed into the guide frame. Vibratory pile driving is employed, initially using a low frequency of 10Hz vibration, accelerating to 15-20Hz in the silt layer; deceleration is applied near the hard soil layer. In sandy loam sections, spiral blades are welded to the outside of the steel pipe, with a blade spacing of 30cm and a width of 15cm. The final penetration depth meets the requirements, and the verticality deviation of the steel pipe is ≤1%, monitored in real time using a verticality monitoring instrument.

[0037] Closed-loop treatment: Drive the steel pipe piles in a clockwise direction, with the last steel pipe connected to the first one, and the gap controlled within 2cm. Weld a 20cm wide and 8mm thick connecting steel plate to form a closed loop.

[0038] Install double-sided 16#A channel steel back ribs and φ20 tie rods 3: Rib installation: Fully weld double-section 16#A channel steel back ribs along the inside of the steel pipe. The weld height should not be less than 6mm. After installation, check the verticality deviation to be ≤1%. If necessary, use a jack to correct it.

[0039] Tie rod installation: Drill 22mm diameter holes at 50cm intervals on the back rib, insert φ20 tie rod 3 through each hole, extending at least 10cm beyond the outer side of the channel steel at both ends, and fit double nuts. Tighten in three stages using a torque wrench: initial tightening 100N·m → secondary tightening 200N·m → final tightening 300N·m, with a 1-hour interval between each stage. Fill the gap between the nuts and the channel steel with water-swellable sealing strips.

[0040] Laying waterproof geotextile 4 and geogrid 5: Waterproof geotextile laying: Lay it sequentially from the bottom to the top of the cofferdam, starting with the inner side closest to the center of the cofferdam and then the outer side, extending 30cm beyond the top surface. The overlap width should not be less than 20cm. Use hot-melt welding at a temperature of 180-200℃ and a pressure of 0.3-0.5MPa or special adhesive for bonding. Fix it to the steel pipe or channel steel every 1m with nylon cable ties at 50cm intervals.

[0041] Geogrid installation: Lay it on the inside of the waterproof geotextile, with an overlap width of not less than 30cm. Use 20cm long U-shaped nails at a 45° angle to drive them into the soil to a depth of 15cm at the overlap. Fix the spacing at 50cm to ensure that the geogrid is taut and wrinkle-free.

[0042] Layered filling of impermeable soil 6: Silt treatment: The silt in the area enclosed by the cofferdam is dredged by a dredger, filtered through a filter screen, and then stored in a temporary silo for 24 hours to allow the moisture content to meet the requirements.

[0043] Layered filling and compaction: After the first layer is filled to 30cm, use an excavator with a plate compactor to compact it along the axis of the dam body, repeating 3 times. From the second layer onwards, each layer increases in thickness to 40-50cm, and the number of compaction passes increases to 5. Compaction degree is tested every 10m using the ring cutter method, ensuring ≥90%. After each layer is completed, the surface is roughened manually with a rake to a depth of 5cm to ensure tight bonding with the upper soil layer.

[0044] Settlement and Monitoring: After the impermeable soil in the dam body has been filled to the designed height of 3m, filling is stopped, and the dam body is left to stand for observation. Ten observation points are evenly set on the top surface of the dam body. The settlement is measured daily using a level instrument according to the "front-back-left-right-center" five-point method. When the settlement is ≤5mm / day for three consecutive days, the dam body is considered to be settled stably. If the top surface of the dam body is lower than the design elevation after settlement, silt needs to be added to the design height, and the dam body is left to stand for stability again.

[0045] Lay sandbags or waterproof tarpaulins: Sandbag laying: Sandbags are laid along the full width of the top surface of the dam, with staggered overlaps, an overlap length of ≥1 / 3 of the bag length, a laying thickness of ≥50cm, and the edges extending 50cm beyond both sides of the dam. They are then fixed with wooden stakes, spaced every 2m, and buried 30cm into the soil.

[0046] Waterproof tarpaulin installation: Lay a waterproof tarpaulin to cover the top surface, with an overlap width of ≥50cm, and weigh it down with sandbags around the perimeter, 3 sandbags per meter.

[0047] 3) Drainage and toe protection construction within the cofferdam Pumping water out of the cofferdam: A staged pumping method is adopted. First, pump the water to a level 1m below the top surface of the cofferdam and observe for 24 hours to check the seepage. If the leakage is serious, repair it with quick-setting cement and geotextile. Then pump the water to the design bottom elevation, exposing the construction surface. During the pumping process, use water pumps to discharge the water to the water area outside the cofferdam. Assign a dedicated person to monitor the deformation of the cofferdam with a total station. The allowable deviation of the steel pipe displacement is ≤3cm. If the steel pipe is found to be tilted or the tie rod is loose, stop the pumping immediately, reinforce it, and then continue.

[0048] Toe Protection Construction: After drainage, clean the remaining silt inside the cofferdam and transport the excess silt to the upstream side of the cofferdam for toe protection. The toe protection is constructed in layers, each layer being 50cm thick, ≥1.5m wide, with a slope ratio of 1:1.5, and the top flush with the bottom of the cofferdam. A 20cm thick layer of rubble can be placed at the bottom of the upstream toe protection layer to form a filter layer, upon which silt is then filled. After filling, a layer of geogrid 5 is laid on the surface of the toe protection and secured with sandbags. Inclinometers are installed at the corners of the cofferdam, and data is recorded every 2 hours; a slope change ≤0.5% is considered stable.

[0049] 4) Construction acceptance and maintenance Acceptance criteria: Weir dimensions: thickness 3m±5cm, length and width meet design requirements, axis deviation ≤10cm.

[0050] Structural stability: verticality of steel pipe ≤1%, no loosening of tie rods, settlement ≤5mm / day; no obvious cracks in the dam body, seepage ≤0.5L / (m²·h).

[0051] Seepage prevention effect: After water is pumped out from inside the cofferdam, the water level rises by ≤5cm within 24 hours.

[0052] Routine maintenance: Regularly check the tightness of the tie rod nuts and tighten them once a month; observe the dam body for settlement or tilting, and monitor it once a quarter. If the waterproof geotextile is found to be damaged, repair it promptly with fabric of the same specification, with an overlap of ≥30cm; when the embankment is eroded, reinforce it with sandbags or silt. The top surface of the dam body should be trimmed to the design slope ratio of 1:1, and the surface should be covered with grass seeds for protection, such as bermudagrass or ryegrass. After the grass seeds germinate, water them regularly to prevent soil erosion.

[0053] Example 1: Construction of bridge piers in small river channels (water depth 2m, silt layer thickness 3m) 1. Pre-construction preparation: The riverbed was surveyed and measured to be 2m deep with a flow velocity of 0.8m / s. The riverbed consisted of a 3m thick layer of silt with a water content of 42%, and the bottom layer was silty clay with a bearing capacity of 120kPa. The design outline was laid out, and a 6m long DN200 steel pipe was prepared, with 0.5m protruding above the water surface and 3.5m embedded in the soil, along with other materials. The GPS of the piling vessel was calibrated to an accuracy of ±2cm.

[0054] 2. Main construction: Steel pipe driving: Use a vibratory hammer at a frequency of 15-20Hz to drive the pipe 3.5m into the soil, with a verticality deviation of 0.8%, and weld it to form a closed loop.

[0055] Support installation: Weld double-channel steel back ribs, tighten tie rods to three torque levels, and fill the gaps between nuts with water-stop strips.

[0056] The impermeable layer is laid using 400g / ㎡ geotextile, which is hot-melt welded and fixed with nylon cable ties; the geogrid overlaps by 30cm and is fixed with U-shaped nails.

[0057] 3. Filling and protection: After being filtered through a screen, the silt was left to stand for 24 hours. It was then filled in layers, with the first 30cm layer compacted three times and the subsequent 40cm layer compacted five times, achieving a compaction degree of 92%.

[0058] After standing for 7 days, the settlement stabilized with a maximum settlement of 3mm / day. A 50cm thick sandbag was laid on the top surface for protection.

[0059] 4. Drainage and foot protection: Drainage is pumped down to the design elevation in stages, with no obvious leakage; the water-facing side is protected with silt for a width of 1.5m, with the bottom layer filled with rubble and the surface covered with 5 layers of grating for fixation.

[0060] 5. Acceptance results: All indicators meet the standards, the weir is stable, and it meets the construction requirements of bridge piers and abutments.

[0061] Example 2: Foundation construction of a lake sluice gate (water depth 2.8m, including sandy loam interlayer) 1. Pre-construction preparation: The lake has a water depth of 2.8m and a water flow velocity of 1.2m / s. The geological conditions consist of 1.5m thick silt + 1m sandy loam + silty clay. Spiral blades welded to the outside of the steel pipe should be used to address the sandy loam soil. A composite geotextile consisting of two layers of fabric and one layer of membrane should be prepared.

[0062] 2. Main construction: Steel pipe installation: In the sandy loam section, the pull-out resistance is enhanced by spiral blades, and the penetration depth is 4m to penetrate the sandy loam soil with a verticality deviation of 0.6%.

[0063] Support and seepage prevention layer: The tightening torque of the tie rods meets the standard, and the composite geotextile is bonded and overlapped to ensure the seepage prevention effect.

[0064] 3. Filling and protection: After the silt is treated, it is filled in layers with a compaction degree of 91%. After standing for 10 days, the settlement is stable, and the top surface is protected with waterproof cloth.

[0065] 4. Drainage and foot protection: The drainage was successfully pumped to the design elevation, the foot protection construction met the requirements, and the inclinometer monitoring showed a slope change of 0.3%.

[0066] 5. Acceptance results: The weir structure is stable, has good seepage prevention performance, and meets the construction conditions for the sluice gate foundation.

[0067] The parts not detailed above are existing technologies and therefore have not been described in detail.

Claims

1. A method for constructing a double-sided steel pipe sandwich combined cofferdam body, characterized in that, Comprising the following steps: 1) Preparation before construction: survey the construction water area with water depth ≤3 m and water flow velocity ≤1.5 m / s, determine the contour, axis and 1:1 slope ratio of the cofferdam; prepare materials and equipment, the materials including DN200 steel pipe with wall thickness ≥6 mm, double-spliced 16#A channel steel, φ20 pull rod, waterproof geotextile with specification ≥400 g / ㎡, two-way stretch polypropylene grid, cofferdam silt with water content 30%-50% as impermeable soil and sandbags or waterproof cloth; 2) Main construction: 2.1) Form a closed loop by driving DN200 steel pipes, the spacing between steel pipes is 50 cm, the depth of soil entering meets the requirements of silt layer ≥2 m and hard soil layer ≥1.5 m, and the verticality deviation is ≤1%; 2.2) Weld double-spliced 16#A channel steel back ribs, and simultaneously install φ20 pull rods at a spacing of 50 cm, and the two ends of the pull rod are fixed with double nuts; 2.3) Lay waterproof geotextile and grid on both sides of the cofferdam body in turn, the overlap of the waterproof geotextile is ≥20 cm, and the overlap of the grid is ≥30 cm, both are fixed; 2.4) Fill impermeable soil in layers to 3 m thick, and stand still until the settlement is stable, the settlement amount is ≤5 mm / day for 3 consecutive days; 2.5) Lay sandbags or waterproof cloth on the top surface of the cofferdam body for protection; 3) Construction of cofferdam internal drainage and toe protection: drain the water in the cofferdam in stages, use the excess silt to make the toe protection on the water-facing side, and the width of the toe protection is ≥1.5 m and the slope ratio is 1:1.5; 4) Construction acceptance and maintenance.

2. The method of constructing a double-sided steel pipe sandwich combined cofferdam body according to claim 1, characterized in that, The material preparation in step 1) further includes welding a 10 cm high and 8 mm thick reinforcing ring at both ends of the DN200 steel pipe, and pre-connecting the double-spliced 16#A channel steel with M12 bolts at a spacing of 30 cm to form an integral back rib unit.

3. The method of constructing a double-sided steel pipe sandwich combined cofferdam body according to claim 1, characterized in that, The permeability coefficient of the waterproof geotextile in step 1) is ≤1×10⁻ 7 cm / s, and a two-cloth-one-film composite structure with a middle PE film thickness of 0.5 mm is used in the sandy loam section.

4. The method of constructing a double-sided steel pipe sandwich combined cofferdam body according to claim 1, characterized in that, In step 2.1), when driving the steel pipe, spiral blades with a spacing of 30 cm and a width of 15 cm are welded outside the steel pipe in the sandy loam section, and the verticality is controlled by an H-shaped steel guide frame.

5. The method of constructing a double-sided steel pipe sandwich combined cofferdam according to claim 1, characterized in that, In step 2.2), when installing the φ20 pull rod, the two ends of the pull rod protrude outside the channel steel by ≥10 cm, and are tightened in three stages using a torque wrench, in the order of initial tightening 100 N·m, re-tightening 200 N·m, and final tightening 300 N·m, and the gap between the nut and the channel steel is filled with water-swelling sealing strip with a diameter of 2 cm.

6. The method of constructing a double-sided steel pipe sandwich combined cofferdam according to claim 1, characterized in that, In step 2.3), when laying the waterproof geotextile, the overlap is welded or bonded with special glue at a temperature of 180-200℃ and a pressure of 0.3-0.5 MPa, and is fixed at a spacing of 50 cm per 1 m using nylon straps.

7. The method according to claim 1, wherein In step 2.3), when laying the grid, U-shaped nails with a length of 20 cm are used to punch into the soil at an angle of 45°, with a depth of 15 cm, a fixed spacing of 50 cm, and the grid needs to be tightly stretched without wrinkles.

8. The method of constructing a double-sided steel pipe sandwich combined cofferdam body according to claim 1, characterized in that, In step 2.4), the layer-by-layer filling of impermeable soil includes the following steps: the silt is filtered through a 5 cm diameter screen and left to stand for 24 hours; the layer thickness is 30 cm for the first layer and 40-50 cm for the second layer, and the compaction is 3 times for the first layer and 5 times for the second layer, with a compaction degree of ≥90%; the surface is manually roughened after each layer is filled, with a roughening depth of 5 cm.

9. The method of constructing a double-sided steel pipe sandwich combined cofferdam body according to claim 1, characterized in that, The specific steps of the top surface protection in step 2.5) are as follows: the sandbags are overlapped and stacked in staggered manner, and the overlap is greater than 1 / 3 of the length of the bag, and the thickness is greater than 50 cm, and the edge is 50 cm away from the two sides of the weir body, and every 2 m is fixed by a wooden stake with a depth of 30 cm into the soil; or the waterproof cloth is overlapped by greater than 50 cm, and 3 bags of sandbags are pressed every meter around the periphery.

10. The method of constructing a double-sided steel pipe sandwich combined cofferdam according to claim 1, characterized in that, The specific steps of the drainage and foot protection in step 3) are as follows: when the water is pumped out in stages, it is first pumped out to be lower than the top surface of the weir by 1 m, and then pumped out to the design elevation after 24 hours of observation without leakage; the bottom layer of the foot protection is filled with 20 cm thick block stones with a particle size of 30-50 cm, the surface is paved with a grid and the edges are pressed with sandbags, and the corners are provided with a slope measuring pipe with a buried depth of 5 m to monitor the slope, and the allowable deviation is less than or equal to 0.5%.

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