Steel Sheet Pile Cofferdam Construction Method

Steel sheet pile cofferdams are suitable for basic projects such as sandy soil, gravel soil, semi-dry clay, weathered rock formations, etc. with water depths above 4m and thick riverbed covering layers.

Steel sheet pile cofferdams are available in rectangular, polygonal, circular, etc. Steel sheet piles are available in straight, Z-shaped, trough-shaped, I-shaped, etc., and can be made into single-layer or double-layer cofferdams. In general bridge engineering foundation pit construction, rectangular and wooden guide frames are mostly used for shallow foundations, while circular and shaped steel are mostly used for deeper foundation pits.

Because of its good waterproof performance, single-layer cofferdams are often used. If a double-layer cofferdam is used, the middle of the interlayer of the double-layer cofferdam is usually filled with clay. Under special circumstances, underwater concrete is poured into the lower part of the interlayer to improve the anti-seepage capability. In the construction of steel sheet pile cofferdams, channel-shaped steel sheet piles are often used.

When constructing the steel sheet pile cofferdam, the top surface of the cofferdam is more than 0.5m higher than the highest water level that may occur during the construction period. The size of the working surface on the inside of the cofferdam must ensure that a distance of no less than 1.0m is maintained between the top edges of the foundation pit.

When the foundation is deep, the soil quality of the pit wall is poor, the amount of water seepage is large, and the side slope (pit wall) is easy to collapse, the distance from the inner slope foot of the cofferdam to the top edge of the foundation pit should be appropriately increased to ensure safety. At the same time, the depth of penetration of steel sheet piles into the soil and whether to use supports must be determined through inspection.

1. Construction Method:

1.1 Construction preparation:

After the new and old steel sheet piles are transported to the construction site, they should be inspected, measured, classified, and numbered in detail. At the same time, a short pile of the same model with a length of 2 to 3 meters should be used for the pass test on both sides of the lock. It is appropriate for 2 to 3 people to pull it through. , or use a winch to pull, the maximum traction force is ≤ KN, if possible, use an inspection trolley (as shown in Figure 1)

If the lock cannot pass or the pile body has defects such as bending, twisting, dead bends, etc., use cold bending, hot hammering (temperature does not exceed 800~1000℃), welding repair, riveting repair, cutting off, lengthening, and other methods to repair it.

At the same time, the strength of the joint is equal to that of other sections. When welding a long extension, use a strong clamp to clamp it flat to avoid deformation. When welding, first butt weld, then weld the reinforcement plate. For new piles or extended piles, make hanging pile holes at the end of the pile.

When the piles are piled and driven, plywood is provided every 4 to 5m. The plywood is clamped before the sheet piles are lifted. During driving, they are dismantled one by one for turnover.

Apply a butter mixture ointment (weight ratio: butter: asphalt: dry sawdust: dry clay = 2:2:2:1) inside the locks of the group piles and single piles to reduce friction during driving, and Enhance anti-seepage performance.

1.2 Guide frame installation and insertion method

When installing the guide frame, first perform positioning measurements. If the guide frame in the water is far from the shore or has formed a pier (or construction bridge), use the forward intersection method to locate it. The installation of the guide frame usually involves driving positioning piles or making a temporary construction platform.

The guide frame is manufactured in sections at the factory or on-site, assembled on the platform, and fixed on the positioning piles. When position piles are not set, they are hung directly on the floating platform. After a small amount of steel sheet piles are inserted and driven, the guide frame is gradually fixed to the steel sheet piles.

1.3 Lifting, driving, driving, and closing of steel sheet piles

After the steel sheet piles pass the inspection, they will be transported to the dock by two sets of flat cars. A maximum of four layers are allowed to be stacked in the order of pile insertion. The height difference between each layer should not be greater than 10mm with pads. The center lines of the upper and lower pads should be on the same vertical line. The allowable error shall not be greater than 20mm.

To place steel sheet piles, use an elevated cableway to transport the steel sheet piles horizontally and vertically, transport the steel sheet piles to the designated location, and then use two hooks to lift and lower the steel sheet piles to make the steel sheet piles vertical, take off the small hooks, and move to the placement position. , insert into the steel sheet pile lock that is already in place.

Before lifting, the lock mouth is filled with butter asphalt mixture. The arc clamps used to tighten the steel plates are released one by one when inserted into the lock.

Steel sheet piles are driven block by block (group) to the bottom of the entire cofferdam (a rectangular cofferdam can be on one side). They are first inserted and closed, and then driven in block by block (group). Rectangular cofferdams are generally inserted on the upstream side first and closed on the downstream side. There are three insertion sequences for circular steel sheet pile cofferdams, as shown in the figure below:

It can be seen from the figure that a and b have one less closing point than c, and the cumulative error of point b is greater than c. Both ab and ab may be affected by the backflow before closing, causing the pile feet to move outward, and causing difficulty in closing. c is less affected by the backflow. Where the flow rate is high, use C-type insertion.

In the A-type insertion method, insertions are added in sequence from both sides symmetrically to the downstream, and when the downstream is closed, the number of additions on both sides is roughly equal, and a maximum difference of 8 groups is allowed.

When the verticality of the steel sheet piles is good, drive the piles to the required depth in one go; when the verticality is poor, drive them in two steps, that is, drive all the piles to about half the depth, and then drive them to the required depth the second time. depth.

1.4 Pumping water and plugging leaks

After the steel sheet piles are inserted and driven, water can be pumped and excavated. Design a supported cofferdam. Support it first and then pump water. Check whether each node is tight and whether the wooden wedges between the sheet piles and the guide frame are tight to prevent accidents due to pumping.

The pumping speed cannot be too fast, and changes in the cofferdam must be observed at any time. When the lock mouth is not tight and leaks, use cotton wool to plug it on the inside, and at the same time, sprinkle a large amount of sawdust or rice bran at the leak, so that it will be carried by the water to the leak and block it on its own. For the leak at the pile foot, use the following three methods, or adopt measures such as concrete bottom sealing.

1.5 Pull-out piles

Before pulling out the steel sheet piles, the supports in the cofferdam should be removed from bottom to top, and water should be poured until it is 1 to 1.5m higher than the water level outside the cofferdam to balance the internal and external water pressure, so that the extrusion force of the sheet piles disappears, and Separated from part of the concrete (referring to the part with underwater concrete seal).

Then select a group or a piece of steel sheet piles that are easier to pull out downstream. First, slightly hammer and vibrate each pile to raise the height by 1~2m. Then raise all the steel sheet piles by 1~2m one by one. After they are loosened, divide them into two starting from the downstream. The piles are pulled out one by one from the side to the upstream. For piles with curled pile tips and deformed locks, the capacity of the pile-pulling equipment can be increased to pull adjacent piles together and perform underwater cutting if necessary.

2. Mechanical Equipment Configuration

On the shore or in shallow water, use simple scaffolding or directly use mechanical pilings such as pile drivers, cranes, and poles. When piling in deeper water, arrangements must be made according to the requirements of the machinery used on the construction site and the equipment required for water operations.

When driving steel sheet piles, a lighter pile frame should be used. Generally, the hammer weight should be greater than the pile weight and the hammering energy should be appropriate.

3. QC

3.1 When splicing steel sheet piles, the steel sheet piles at both ends should be aligned and tightly clamped in a strong clamp for welding. It is required that the gap between the ends of the two steel sheet piles should not be greater than 3mm, and the misalignment on the cross-section should not be greater than 2mm. Use new steel sheet piles. When testing, it is necessary to have factory certification documents for its mechanical properties and chemical composition and to measure the dimensions in detail to check whether it meets the requirements.

3.2 The two ends of the assembled steel sheet piles should be flush with each other, with an error of no more than 3mm. The steel sheet pile group should be consistent up and down, with an error of no more than 30mm. All locks should be coated with waterproof mixed materials to ensure that the locks are tightly caulked.

3.3 To ensure the smooth closing of the inserted piles, the pile body is required to be vertical, and the number of steel plates around the cofferdam must be evenly distributed. To ensure that the pile body is vertical, a guide wood fixed on the cofferdam support is installed on the first group of steel sheet piles, and the guide wood is Inserted to make the first group of steel sheet pile piles vertical. Since the upper and lower widths of the steel sheet pile groups are not completely consistent, and the lock gap is not completely consistent, the pile body may still be tilted. Measurements will be strengthened during the construction to find out the tilt and make timely adjustments. , so that the inclination of each group of steel sheet piles along the perimeter of the cofferdam and in the vertical direction is not greater than 5‰;

At the same time, to make the periphery of the cofferdam evenly divided by the number of steel sheet piles, the positions of each group of steel sheet piles are drawn on the cofferdam guide beam according to the actual width of the steel sheet pile group to disperse the width error, and when inserting the piles, according to the Adjust the plane position of the steel sheet piles so that the error is no more than ±15mm. When there are still difficulties, adjust several groups of steel sheet piles on both sides of Helongkou without inserting them into the river bed and adjust them in a suspended state.

When it cannot be closed smoothly, special-shaped steel sheet piles are made according to the actual size of the closing opening as shown in the figure. However, it is necessary to control the difference in the upper and lower widths of the special-shaped steel sheet piles to not exceed 2% of the pile length.

3.4 When using spliced long steel sheet piles, the spliced joints of the steel sheet piles cannot be on the same section of the cofferdam, and the joints of adjacent piles are staggered up and down by at least 2m. Therefore, piles must be prepared in advance when assembling the steel sheet piles. During transportation and storage, stack the piles smoothly according to the pile insertion. When inserting the piles, hang them in the specified order.

3.5 When driving steel sheet piles, when the verticality of the steel sheet piles is good, the piles should be driven to the required depth in one go. When the verticality is poor, the driving should be done in two steps, that is, all the piles should be driven in first. After reaching about half the depth, hit the required depth a second time.

3.6 When driving piles, pile caps must be installed on the tops of the piles to avoid damage to the tops of the piles (the picture shows the pile caps of three groups of combined piles). Do not hammer too hard to avoid bending of the pile tips, making it difficult to pull out the piles.

3.7 During the use of the steel sheet pile cofferdam, prevent the water level inside the cofferdam from being higher than the water level outside the cofferdam. Set up a connecting pipe at a low water level, and then seal it when pumping water into the cofferdam. When pumping water into the cofferdam, water leaks from the lock of the steel sheet pile. A large amount of fine cinders, wood chips, rice bran and other fine materials are sprinkled outside the cofferdam. The suction of leaking water is attached to the lock mouth to block the water, or you can use laths, cotton wool, etc. to wedge into the lock mouth to caulk the seams in the cofferdam. When spreading cinders and other materials to block the leaks, you must consider the direction of the water flow and try to get as close as possible to the leaks. The leaks are relatively small. When it is deep, put it in bags and place it near the leak and slowly spread it. At the same time, when the water is pumped from the cofferdam to the support guide beams on each layer, use wooden wedges to wedge the gaps between the guide beams and the steel sheet piles layer by layer to make the guide beams tight. The beam is uniformly stressed.

3.8 After the cofferdam is used when removing the steel sheet piles, first cut off the welding between the steel sheet piles and the guide beam wooden boards, and then fill the cofferdam with water until it is 1~1.5m higher than the water level outside the cofferdam to balance the water pressure inside and outside so that it is consistent with the back seal. After the concrete is separated, select a group or piece of steel sheet piles downstream that are easier to remove. First, hammer and vibrate slightly, and then raise the height by 1~2m. Then raise all the steel plates by 1~2m one by one until they are loose, and then start from the downstream. Remove them one by one from both sides upstream.

4. Security Measures

To ensure safety during construction, when constructing steel sheet pile cofferdams, safety work must be given top priority and prevention should be given priority.

4.1 Provide safety ideological education to operators, improve safety awareness, and implement a certificate-based employment system. Those without training or certificates are not allowed to perform operations.

4.2 When using elevated cableways for horizontal and vertical transportation, the ground ridges at both ends must be firmly buried, and the lifting capacity of the cableway must be measured to ensure that the cableway has sufficient bearing capacity during transportation.

4.3 Designate a dedicated person to be responsible for daily inspection and maintenance work, and assign a dedicated person to give signals when using the ropeway to avoid random commands when there are many people and safety accidents.

4.4 The main tower of the pile driver is fixed with a winding rope to prevent the pile frame from shaking and causing serious accidents when the wind is strong.

4.5 The wheels of the pile driver on the track should be checked regularly to ensure that there are no hanging wheels. If necessary, the wheels or pads should be adjusted to keep the wheels pressed against the track.

5.6 When people climb the pile driver tower and walk on the guide ring near the water, they should wear a safety helmet and fasten their seat belts.

4.7 When pulling out piles, vibrate for 1 to 2 minutes first, and then slowly start the winch to pull out piles. After it becomes loose, pull it out while vibrating to prevent recklessness.

4.8 Check all pulleys and wire ropes every day. Pay special attention to the wear of the pulley shafts and wire ropes. If safety is endangered, repair or replace them in time.

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