Discussion on construction scheme of cofferdam cutoff wall?

The following is the relevant contents of the cofferdam cutoff wall construction scheme brought by Zhong Da Consulting for your reference.

Banduo Hydropower Station on the Yellow River is the second cascade hydropower station planned by Longyangxia, the main stream of the Yellow River, with an altitude below 3,000 meters. The dam site is located at the exit of Banduo Canyon at the junction of Xinghai County and Tongde County, Hainan Prefecture, Qinghai Province, 6.5km away from the upstream Ciha Hydropower Station, 75km away from the downstream Yangqu Hydropower Station and Longyangqu Hydropower Station, and 176km away. The axis of the upstream cofferdam is located at 82m away from the upstream river, with the axis direction of SE 124, and the axis of the downstream cofferdam is located at 195m away from the downstream river of the dam axis, with the axis direction of NE79. Fracture hooks are developed in the rock mass, the integrity of the rock mass is poor, there is basically no strong weathering, and the water permeability of the rock mass is mainly weak.

The cutoff wall of this bid section includes the concrete continuous wall of upstream cofferdam and the concrete continuous wall of downstream cofferdam. The concrete diaphragm wall of the upstream cofferdam is a plain concrete diaphragm wall with a thickness of 0.8m, a top elevation of 2732.00m, a height of about 28m and an area of about 1370m2. The downstream concrete continuous cutoff wall is a plain concrete continuous cutoff wall with a thickness of 0.8m, a top elevation of 2726m, a maximum depth of 24m and an area of about 1, 3 10m 2.

According to the Technical Specification for Construction of Concrete Cut-off Wall in Water Conservancy and Hydropower Engineering, the orifice should be 2.0m higher than the groundwater level to prevent the hole from collapsing due to insufficient inner wall pressure. If the water level of the Yellow River rises to the elevation of the weir crest orifice less than 2.0m, it is necessary to do a good job in flood control and flood fighting, and put the equipment in a safe zone. After the flood recedes, the orifice is 2.0m higher than the water level of the Yellow River, so it should be resumed as soon as possible.

A, guide wall construction scheme

1. Construction deployment. Before the underground continuous wall is grooved, the guide wall should be built first. The quality of the guide wall directly affects the sideline and elevation of the underground continuous wall, and it is an important measure to guide the trenching equipment, store mud to stabilize the liquid level, maintain the stability of the upper soil and prevent the soil from collapsing. Therefore, the production of guide wall must be carefully constructed to ensure the construction quality. The cofferdam guide wall and dam foundation guide wall of this project are both L-shaped guide walls with a depth of 1.5m, a bottom width of 1.5m and a wall thickness of 30cm. The guide wall spacing is 900mm, and the concrete strength grade is C 15. The toe of the guide wall should be located on dense soil, and both sides of the guide wall should be backfilled with clay for compaction. Φ16 steel bar is adopted, and the steel cage is processed in the steel yard. After being transported to the site manually, the reinforcement shall be laid and bound manually.

2. Construction method. Survey and lofting: determine the excavation position of guide wall according to the axis of underground continuous wall. Excavation: After surveying and setting out, the guide wall shall be excavated by combining mechanical excavation with manual trimming. The excavation elevation is controlled by manual trimming. Cushion: according to the design width of the guide wall, pre-process the wooden pattern, fix it according to the axial position of the underground wall, check the size and then pour the concrete cushion. Erection of formwork and pouring of concrete: determine the position of guide wall on the concrete cushion, and then tie steel bars. The formwork is replaced by soil outside the guide wall, and the formwork is erected on both sides of the vertical edge. This project template adopts steel template, artificial shuttering. Concrete is transported by concrete pump, vibrated by vibrator, manually poured and plastered. Formwork removal and support: the formwork can be removed after 2 ~ 3 days of concrete pouring, and at the same time, 100mm square timber is supported on the inner wall in layers to prevent the guide wall from squeezing inward. The horizontal spacing of the square timber is 1.2m, and the upper and lower spacing is 0.8m ..

3. Division of slots. Immediately after the construction of the guide wall, draw the groove line on the top surface of the guide wall and mark the number of the unit groove with red paint; At the same time, the elevation of the top wall of each trough is measured and marked on the construction drawing for reference. Always observe the spacing, overall displacement and settlement of the guide wall, and make records, and do a good job of retest before forming the groove. When crossing the guide wall of construction road, steel plate must be used for overhead construction and filled with clay.

Second, the mud preparation

1. Selection of pulping raw materials. According to the actual engineering situation and design requirements, this project plans to adopt high-quality Grade II calcium-based bentonite mud or high-quality clay mud to protect the wall. The dispersant is industrial sodium carbonate (NaCO3) produced by the recent chemical plant; The water loss reducing and viscosity increasing agent is CMC, and the mud prepared with fresh and clean fresh water will not adversely affect the mud performance. Before bentonite or high-quality clay enters the site, it is necessary to inspect the material source and manufacturer, and test the corresponding indicators. After each batch of bentonite comes into play, samples should be taken for slurry preparation test. If the standard of retaining wall cannot be reached, the quantity of materials used for slurry mixing shall be treated according to the field test results and the supervisor's instructions.

2. Material consumption plan. In the process of drilling rig construction, the main consumption materials are bentonite, alkali and clay. Bentonite consumes about 6 tons per day and clay consumes about 30 cubic meters per day.

3. Mud preparation and inspection. Excavate two slurry ponds in the upstream and downstream cofferdams, with the specifications of length× width× depth = =8m×6m×2m, and install LJ-400 high-speed refiner; The mixing time of each mud is 3 ~ 5min minutes, and the actual mixing time can be adjusted appropriately after being determined by experiments; Prepare mud according to the specified mixture ratio, and the error of adding various materials shall not be greater than 5%; Mud treatment agent is prepared into a certain concentration of aqueous solution before use to improve its effect. The concentration of soda water solution is 20%; Bentonite mud laboratory is set up next to the mud station. The newly made bentonite mud needs to be tested in the following table and meet the specified standards.

Third, make holes.

1. Drilling equipment. According to the stratum characteristics of this project, CZ-5 percussive drilling rig is mainly used for drilling the cutoff wall. According to the construction experience of similar projects, the drilling rig has good adaptability to the characteristics of large formation permeability, low content of boulders and boulders and low rock hardness, and is convenient for operation and maintenance.

2. Hole-making method. The trench construction method of cutoff wall adopts the method of "four drills and three splits", and the bucket pumps water to discharge slag. During the construction, the first-stage trough is built first, and then the second-stage trough is built. In the first stage, the main hole is drilled first, then the auxiliary hole is split, and finally the small wall is split to form the groove. Odd holes are main holes and even holes are auxiliary holes. The length of the main cofferdam is 0.80m, and the dam foundation1.2m. Each slot consists of four main holes and three auxiliary holes, with a single slot length of 6.8m and a slot spacing of 6m.

Fourth, hole cleaning.

After the final hole of the slotted hole is accepted, hole cleaning and slurry changing shall be carried out.

1. Acceptance criteria for borehole cleaning. After cleaning the slot and changing the slurry 1 hour, the following standards shall be met: the siltation thickness at the bottom of the hole shall not be greater than10 cm; ; Specific gravity of bentonite mud <1.10g/cm3; Viscosity of bentonite mud < < 30s；; The sand content of bentonite mud is less than 10%.

2. Hole cleaning method. The hole cleaning method adopts the method of pulling bucket to discharge slag. At the same time of hole cleaning, fresh mud is continuously added into the tank. When cleaning the hole, you can also run the drill bit to continuously stir the sediment at the bottom of the hole to completely remove the sediment. After cleaning a single hole, move the drilling rig to clean it hole by hole, effectively removing the mud and sand in the hole. If the depth of each hole in the unit groove section is different, the hole cleaning sequence is shallow first and then deep.

Five, wall concrete pouring

1. Concrete mix ratio and performance: the concrete of cofferdam cutoff wall is C20 concrete. At the same time, the concrete mixture ratio should be determined according to the concrete compressive strength, impermeability grade, elastic modulus and site materials, and can only be used after field test and confirmation by the supervisor.

2. Concrete material requirements: ordinary portland cement shall be adopted, with the grade of not less than P. O 32.5, and natural pebbles and gravels shall be preferred, with the maximum particle size less than 40mm and not more than 1/4 of the net spacing of steel bars, and the silt content not more than 1.0% and not more than 0.5%; Fine aggregate shall be medium-fine sand with fineness modulus of 2.4 ~ 3.0, and its silt content shall be no more than 3% and no more than 65438 0.0%. Quality and dosage of additives (water reducing agent, waterproof agent, air entraining agent, etc.). ) determined by testing and relevant regulations.

3. After the slots are cleaned up, concrete pouring conduits are set, which are connected by threads with a diameter of φ 250 mm ..

4. The spacing and depth of concrete pouring conduits conform to the Technical Specification for Construction of Concrete Cut-off Walls of Water Resources and Hydropower (SL 174-96), the spacing between conduits is about 1.5m, and the spacing between two conduits is not more than 3.5m. ..

5. Concrete pouring method: Underwater concrete is poured by direct conduit method (250mm). 6m3 concrete is pumped to the notch storage tank, and the hopper sends the conduit into the slot hole for pouring construction. Before pouring, the transportation equipment should be routinely overhauled to ensure the smooth pouring process.

After the concrete is delivered to the notch, a special person shall be appointed to discharge the material to ensure that the concrete can enter the groove smoothly.

During pouring, control the hopper to discharge evenly, and pull out the conduit according to the rising speed of concrete. The depth of the conduit buried in concrete should be between1.0 ~ 6.0m ~ 6.0m. The supply strength of concrete should meet the requirement that the rising speed of concrete should not be less than 2.0m/h.

6. Underwater concrete pouring. (1) Overview. The final pouring elevation of concrete surface in this project is designed as upstream cofferdam of 2732.00m, C20 concrete design strength grade, downstream cofferdam of 2726.00m, C20 concrete design strength grade. (2) Preparation before pouring concrete. After checking the last procedure, fill the outside of the joint plate of the first stage tank section with sandbags or clay to prevent it from flowing around when pouring concrete. Lift the pouring frame, lower the conduit, and pour two conduits with an inner diameter of 250mm, which are connected by threads, and the threads are sealed with rubber sealing rings. The requirements under the conduit are as follows: two groups of conduits are used for the first and second pouring, and the spacing between the conduits is not more than 3.5m；; The distance between the primary tank conduit and the hole end or joint pipe is 1.0 ~ 1.5m, and that of the secondary tank conduit is 1.5m. When the tank bottom height difference is greater than 0.25m, the conduit should be placed at the lowest place within the control range. The distance between the bottom of the conduit and the bottom of the tank should be controlled within the range of 15 ~ 25 cm. Put a waterproof bladder or plug in the catheter. Hang the mud pump at the notch and connect the mud recovery pipeline.

7. Concrete pouring technology. After the preparatory work, it is required that the concrete supply capacity should be about 40m3/h, and the incoming materials should be uniform and continuous with good workability. After the concrete arrives at the site, the slump of concrete shall be tested as required. When pouring concrete, the first pouring concrete should meet the needs of concrete quantity in the pouring stage, ensure that the conduit is buried in concrete not less than 1 m, and make construction records. After concrete pouring, start the mud pump to recover mud. If the last remaining 5m mud has been seriously polluted, it will be pumped into the waste mud pool. After the bladder emerges from the mud surface, it is recycled for later use.

The concrete pouring under the mud is carried out by direct conduit method. When pouring, the depth of the conduit buried in the concrete shall not be less than 1.0m and not more than 6.0m. Keep the concrete surface in the tank rising evenly at a speed not less than 2m/h, and measure the depth of the concrete surface every 30min to ensure that the height difference of the concrete surface is controlled within 0.5m: When pouring concrete, the orifice should be provided with a cover plate to prevent sundries from falling into the groove.

When the concrete is blocked, the conduit can be lifted up and down by about 30cm. The mixing and conveying of concrete shall be continuous. If it is interrupted for any reason, the interruption time shall not exceed 40 minutes. Take out the catheter and put it in the designated position, rinse it clean and pile it up neatly.

After the completion of the whole cutoff wall construction, the working face should be cleaned in time, mainly to remove the construction waste slag and transport it to the waste slag yard designated by the supervisor by dump trucks.

After the completion of the whole cutoff wall construction, the construction platform, pulping station and other facilities should all be withdrawn, and the construction surface should be restored and cleaned up, and handed over to the corresponding bid section after acceptance by the supervisor.

For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd