There seems to be something wrong with the section of your cofferdam. Is the backwater slope a little steep? As for the construction scheme, the following contents are for reference! 3. Construction technology 3. 1 construction process flow chart (see attached figure). 3.2 Cofferdam design: the width of the island top is B+9.0m(B is the design width of the line), the outermost layer adopts geotextile bag cofferdam, the top width is 3.0m, the outer slope is 1: 0.5, and the inner side is filled with sand (or clay). According to the engineering practice, navigation and water passage is reserved near the center of the river, generally at least 25m wide, so as to reduce the influence of backwater on the construction safety and quality. The height of cofferdam is calculated as H+2.0m (when H is a constant water level, the water depth+backwater height+1.0m) to ensure that the top of cofferdam is more than 2.0m higher than the water level. See the attached figure "Schematic Diagram of Plane and Section of Cofferdam in Water". Schematic diagram of plan and section of underwater pier cofferdam 3.3 Sand bag mud flushing pipe bag cofferdam positioning is mainly to pump the riverbed sediment sand into the earthwork bag with a mud pump, so that the mud flows out from the outlet above the earthwork bag. The main advantages of geotechnical mud pipe flushing bag are its large volume and heavy weight, and it is not easy to be washed away by the current when thrown into the water, and it is easy to form a cofferdam. However, in the construction, each bag of sand needs to be thrown to the designated location, especially before the cofferdam comes out. If the position is not accurate, 3.3. 1 Seal the bottom sand bag. First, draw river sand from other places at the beginning of the cofferdam, use a high-pressure mud pump to flush the sand to the riverbed surface, and spread a layer of sand (about 30cm thick) on the riverbed surface to prevent the sand bag from sliding due to silt at the river bottom. Then, seal the bottom of the sand bag, and throw a sand bag at the riverbed surface to seal the bottom. As shown in the figure, in order to realize that each bag of sandbags can be thrown to the designated position, a big ship is used to locate the sandbags upstream, and then the sandbags are thrown to the positioning ship from the downstream, and the big ship directs the sand throwing. When a positioning point is completed, the positioning ship is moved to the next point by using the traction wire rope on the positioning ship for sand throwing. As shown in the following figure (schematic diagram of sand throwing bag positioning), the sand throwing boat throws sand forward in the order of 1.2.3 ... The width of the back cover sandbags is two 8m sandbags, *** 16m, and all the back cover sandbags are filled along the river. 3.3.2 The bottom width of the cofferdam is 13.0m, and the top width is 3.0m. The bottom layer is13.0m, followed by 10.0m, 8.0m, 5.0m, and the top layer is 3.0m. (The specifications of geotechnical mud flushing pipe bags are mainly 8.0m, 5.0m, 3.0m. During cofferdam construction, swim around the shore A first, and then near the water surface in the middle of the cofferdam. When the height of upstream and midstream water surface cofferdams exceeds the water surface 1.0m, start. In deep water, geotextile bags are filled with 3/4 sand blasting method, and then directly thrown to the designated position. When the geotextile bag is close to the water surface, the sand throwing boatman can no longer enter the cofferdam area. At this time, the geotextile bag should be directly laid on the lower layer and filled with sand until the bag is filled to about 3/4 of the bag volume, and then the next bag is sealed and poured. In the process of sand filling, remember that geotextile bags should not be full or less than 1/2 bags. Full geotextile bags will break when thrown or squeezed, and too little sand washing will lead to insufficient weight of geotextile bags and waste of geotextile bags, which is not conducive to cost control. When the height of the cofferdam surface reaches the level of the downstream cofferdam (or is close to the level), a gap of about 5m shall be reserved, and no filling is required. The geotechnical mud-flushing bag on the periphery of the cofferdam has formed a cofferdam dam, which can be used for the next cofferdam sand flushing. 3.3.3 Cofferdam Sand Washing When the peripheral cofferdam is completed, start cofferdam sand washing. Sand washing is carried out from upstream to downstream, and from river bank to river channel. Finally, all the water in the cofferdam is squeezed out of the downstream gap with sand, and then the downstream gap is sealed. At this point, the cofferdam is completed. In the process of sand washing, geotextile bags are no longer used, but the sand in the sand boat is pumped out with high-pressure water and poured into the cofferdam. Sand is naturally deposited in the cofferdam and eventually forms the cofferdam. After the cofferdam is formed, the top of the cofferdam should be more than 1.0 meters above the water surface to ensure the construction of bored piles in the cofferdam. 3.4 Operating Points 3.4. 1 Construction Method of Mud Pump Pipe bags shall be grouted according to the grouting sequence, and the direction of the nozzle shall be adjusted in time during the grouting process, so as to make the pipe bags bear uniform stress and avoid deformation. Before the geotextile bag is formally constructed, the filling test should be carried out to explore the technical parameters of the pipe bag and soil during filling, such as grouting pressure, grouting time, consolidation speed and settlement rate, for the construction to master. Fill should be sandy soil, without harmful substances such as bark, grass roots, reeds, shells and stone slag. , and the qualified rear can construction use. In order to prevent the pipe bag from rolling, sliding and shifting during filling, before the pipe bag is positioned, the sand cushion is blown in the water in the cofferdam, and the total station is used to set out and insert bamboo. The positioning of bamboo piles should be accurate and the buried depth should meet the requirements. Bamboo piles and horizontal bamboo poles shall be firmly bound with 8# lead wire. In order to ensure the accurate position of the mud-filled bag, the geotextile bag should be forcibly fixed with bamboo piles. Geotextile bags must be connected to each other, and there can be no overlap. When there is an overlap, the straw bag or woven bag should be filled with soil in time to prevent the soil from losing from the overlap due to the impact of river water. When the pipe bag is 70% full, make a small hole on the inside of the top surface to make the water in the bag overflow, and beat the bag body with your hand to prevent the hole from blocking, ensure smooth dripping, reduce the pressure in the bag and prevent the pipe bag from breaking. Cutter suction dredgers are arranged in the sand fetching area, and the sand is directly dredged and filled on the sand carrier, transported to the cofferdam area, and dredged and filled into geotextile bags and dredging areas through mud discharge pipes. Through the reasonable arrangement of sludge discharge pipes, the smooth flow of dredger fill area is ensured. 3.4.2 Pipeline installation Pipeline layout adopts the combination of floating pipe and submerged pipe. (1) Reduce mutual interference with other projects; (2) Meet the row spacing requirements of dredgers; (3) Reduce the times of installation and disassembly, and increase the working time of the dredger; (4) Ensure safe and civilized construction in the construction process; (5) Meet the flatness requirements of the reclamation area. 3.4.3 Elevation control of dredger fill area The elevation and flatness of dredger fill area is an important link that directly affects the surface leveling of cofferdam. In the process of hydraulic fill, the branch direction of hydraulic fill is designed in detail and the height of hydraulic fill is strictly controlled. (1) According to the situation of the reclamation area, the on-site management personnel specially responsible for reclamation will make disclosure with the pipeline team in time every day to determine the direction and extension distance of the reclamation pipeline, so as to make the layout of the reclamation pipeline conform to the construction plan. (2) The on-site management personnel and surveyors of the project department responsible for hydraulic fill must go to the construction site every day to observe and control the hydraulic fill elevation in the hydraulic fill area, set an elevation ruler around the mud outlet, observe and check regularly, and control the hydraulic fill elevation. (3) In order to ensure the quality of the dredger fill area and prevent silt accumulation near the sluice or in the corner area of the dredger fill area, an energy dissipation nozzle is installed at the nozzle of the dredger to fully diffuse the water-sand mixture discharged from the dredger pipeline, which is beneficial to meet the requirements of dredger fill smoothness. (4) The mud outlet is moved and adjusted according to the mud and sand measurement in the reclamation area, so that there is no long-term dead water area in this area. (5) In the process of construction, the elevation of the dredger fill area is often observed by using the elevation ruler together with the elevation measurement of the dredger fill area. (6) In order to reduce the earthwork loss in the process of hydraulic fill, when the pipeline in the hydraulic fill area is erected, hydraulic fill can be carried out across several mud outlets. In order to control the concentration of tail cement slurry, there is a special person to check at the sewage outlet. When the concentration of tail water rises, the drainage outlet should be raised or the hydraulic filling area should be changed in time to ensure that the tail water discharge meets the requirements of environmental protection. Cofferdam construction
3. 1 construction procedure
Construction preparation → whole slope cleaning → membrane bag laying → tensioning and positioning → anchoring and coping → soil returning and reinforcement → anti-seepage blanket construction → weir separation construction.
3.2 Construction Difficulties and Measures
(1) Positioning and laying of membrane bags: Before construction, the riverbed should be properly cleaned to ensure that the weir foundation is flat and free of sharp objects, so as to avoid damaging the membrane bags. Bag laying is started at the deepest part of the river bed, and construction is started at low tide and advection, which is convenient for operation. The film bag is positioned by bamboo piles. According to the size of the weir, bamboo is inserted in the four corners of the paving place, and the corner pull ring of the membrane bag is sleeved on the bamboo. When filling sand, the membrane bag sinks into the river bottom along the positioning pile.
(2) Cofferdam closure: Different from other types of cofferdams, other types of cofferdams are generally caused by closure, and the closure method is closure. Especially, when the cofferdam construction height reaches the water level change zone, no individual gap can be left in each flood tide and ebb tide, and the same membrane bag can be completed in each flood tide and ebb tide to avoid the scouring of the weir body by concentrated water flow.
(3) Seepage prevention of cofferdam: The leakage of cofferdam mainly includes three parts: the contact surface between the cofferdam body and the original riverbed; Contact surface between weir body and bank slope; Between the film bags. Especially the first two parts, are the most destructive and dangerous places. The main anti-seepage measures are:
① Make use of the characteristics of high mud content, good compactness and poor water permeability of cowhide sand to prevent seepage by itself.
② Leakage channels are easily formed between membrane bags. On the one hand, don't fill the cowhide sand too full, and let the film bag adjust its density by itself; On the other hand, it is also possible to fill cohesive soil between membrane bags and then cover them with woven soil bags.
(3) Two layers of geotextile are laid on the water-facing side of the weir body from the weir top to the weir foot, and the geotextile is 3 meters deep into the riverbed, and the woven cloth is fixed with a sand-wrapped plum-blossom pressing surface; In the lap joint of plastic woven cloth, sandbags should be used to fill it tightly.
(4) Local problem handling:
(1) At the junction of weir body and bank slope, clay should be tamped and embedded into bank slope to form embedded seepage-proof cut-off.
(2) The outer weir foot should be filled with soil and compacted as impervious body.
(3) During the construction, the weir shall be raised by layers and stages. Choose a reasonable time, fill the height of about 30cm every day, strictly control the filling speed and strengthen observation.
(4) During foundation pit pumping, the water level drop should be strictly controlled, about 50cm per day, and attention should be paid to the dewatering and seepage channels of the weir body to avoid centralized seepage and piping.