What is the application of high-blasting anti-seepage wall construction technology in reservoir cofferdam engineering?

The construction technology of high-spray cutoff wall is relatively simple, fast, has good anti-seepage reinforcement and lower cost than concrete cutoff wall. However, it is highly sensitive to the stratum and is generally used in loamy and medium-density conditions. The quality of the wall formed in the sand layer is better, but for the sand and pebble layer with larger particle size, or when it contains boulders, the penetration of the spray material is blocked, the slurry diffusion radius is limited, and the continuity of the wall is not easy to ensure. The project combines the hydraulic layout, topographic and geological conditions, water levels during the construction period and the actual situation of uninterrupted water supply from the water tunnel during the construction period. A temporary cofferdam is set up to enclose the water inlet tower and the spill gate together. The form of the cofferdam is: Earth-rock cofferdam, high-pressure jet spray anti-seepage wall.

The Laoshan Reservoir risk relief and reinforcement project started in October 2009. The project combines the hydraulic layout, topographic and geological conditions, water levels during the construction period and the actual situation of uninterrupted water supply from the water tunnel during the construction period. A temporary cofferdam is set up to enclose the water inlet tower and the spill gate together. The form of the cofferdam is: Earth-rock cofferdam, high-pressure jet spray anti-seepage wall.

1 Project Overview

The construction of Laoshan Reservoir started in 1958 and was basically completed in 1959. It has been in operation for 50 years. The original designed total storage capacity is 60.44 million cubic meters. It is a reservoir based on urban life and It is a medium-sized reservoir mainly used for industrial water supply, and also for comprehensive utilization such as flood control and breeding. In February 2004, the Shandong Provincial Water Conservancy Science Research Institute and the Shandong Provincial Water Conservancy Engineering Quality Inspection Center Station conducted a dam safety appraisal of the Laoshan Reservoir and determined that the reservoir was a Category III project. The main buildings include the dam, flood discharge tunnel, and auxiliary dam. Level 3 buildings. In order to make the reservoir flood control standard meet the 2000 calibrated flood standard, eliminate hidden dangers and ensure project safety, this Laoshan Reservoir risk elimination and reinforcement project was specially implemented.

High-blasting anti-seepage walls are mainly used in temporary cofferdam anti-seepage projects. Foundation pit drainage and safe flood control are the difficulties and key points in the construction organization of this project. The goal is to provide open excavation for diversion canals and water inlet towers. and create conditions for the construction of spill gates.

The main design engineering quantities are: the total length of the cofferdam is 330m, the top elevation is designed to be 46.2m, the maximum weir height is 14.2m, the top width of the weir is 6m, and the slope ratio on both sides is 1:2.0. The minimum thickness of the jet spray wall shall not be less than 0.40m. The cofferdam will be filled with 86,200 cubic meters of gravel and 5196m of sprinkler irrigation will be used. The hole spacing will be 1.0m. The holes will be constructed in two sequences, going 0.5m deep into the bedrock, from the top to 0.5m below the top of the cofferdam. The construction of the rotary spray wall uses a 76-type rotary vibration drill for drilling and grouting with a grouting pump.

2 Determination of main technical parameters

In order to ensure the smooth progress of high-pressure jet grouting construction and ensure the quality of construction, at the same time, to check the rationality of the design of jet grouting process parameters and the reliability of the wall In the early stage of on-site high-pressure jet grouting operations, high-pressure jet grouting process tests are carried out in sections with representative geological conditions according to the mix ratio selected in indoor tests. Through experiments, process parameters such as hole layout, hole depth, injection flow rate, pressure, rotation speed and lifting speed are determined. Publish papers.

3 Construction technical parameters and process control

3.1 Construction technical requirements and parameters

(1) The drilling hole spacing is 1.0m, and the bottom of the hole goes deep into the rock formation (weak , impermeable layer) 0.5m, the hole position deviation is less than 5cm, and the drilling deflection rate is less than 1.

(2) The slurry uses ordinary Portland cement, the grade is not less than 32.5, and the water-cement ratio is 0.5 :1--0.8:1 (weight ratio).

(3) High-pressure jet grouting adopts the triple pipe method. The nozzle adopts double nozzles with a diameter of 1.8mm. The high-pressure water pressure is 38~40MPa, the flow rate is 70~80L/min; the compressed air pressure is 0.7~0.8 MPa, flow rate 7~8m3/min; slurry pressure 26~32 MPa, flow rate 70~100L/min, density greater than 1.65g/m3; lifting speed 5~8cm/min.

(4) The permeability coefficient meets the order of n×10-4 ~ n×10-5 cm/s, the effective average thickness of the sprinkler-irrigated wall is 50cm, and the re-spraying at the contact point between the cofferdam base and the river bed shall not be less than 1 min. .

(5) Main construction machinery and equipment: high spray trolley (CYP hydraulic type), clean water pump (3DZ-2), grouting pump (HB80), grouting pump (BW-150), mixer , air compressor (WF3/6), 76-type rotary vibration drilling rig, etc.

3.2 Construction process methods

(1) Hole distribution and hole making. According to the construction plan, use levels and steel rulers to measure, set out and position the holes, arrange the hole positions, fix the position of the drilling rig, adjust the verticality of the drill pipe, and correct the deviation between the drill bit and the hole position. Use an ordinary 76-type rotary vibration drill to make holes. When encountering large boulders or boulders, the mud wall will form holes. In order to ensure the quality of the hole and the stability of the hole wall, the mud slurry should be adjusted appropriately, and the concentration and viscosity should be controlled to prevent shrinkage. diameter and collapse holes. During the hole-making process, make good hole-making records, especially the description of the lithology in the hole, to ensure that the drilling depth into the rock meets the requirements. According to the records, if the borehole reaches the designed depth but has not yet entered the relative water-proof layer, the borehole needs to be deepened and 1.0m deep into the relative water-proof layer. After the hole is formed and accepted, the casing should be immediately lowered to the bottom of the hole and the pipe mouth should be exposed for work. The surface is 20cm to prevent hole collapse, and the casing is made of φ110mm PVC pipe.

(2) Pulp making. The mixer is a WJG80-1 vertical mixer, the grouting pump is an HB80 type, and a BW-150 mud pump is used to recover the slurry. The cement is ordinary Portland cement, and two-stage mixing is used for slurrying. After the first stage is mixed for 12 minutes, it is filtered and put into the second stage mixer, and grouting is performed while stirring. Publish papers. During pulping, the water-cement ratio (0.5~0.8):1 is strictly controlled, and the specific gravity of the slurry is 1.6~1.7g/cm3. A hydrometer is often used to detect the slurry concentration to ensure that the slurry is concentrated, particle-free, uniform and stable, and has good fluidity.

(3) High spray grouting.

a Before high-spray grouting, conduct a test spray test on the ground for about 1 minute, adjust the spray direction, check whether the water and air nozzles are smooth, and whether the pressure can meet the design requirements. Then, slowly lower the spray boom to the bottom of the hole.

b After the grouting preparation is completed, proceed with normal injection. First, deliver slurry, water, and air according to the specified parameters for static spraying. After static spraying for 3 minutes, wait for the slurry to return to the orifice. After the situation is stable, perform high-spray grouting. The intermittent lifting method is used when spraying. Every time it is lifted 30cm, stop for 5 minutes. Under normal conditions of pulp supply, when the concentration of slurry returned to the orifice becomes smaller and cannot meet the design requirements, the concentration or amount of slurry should be increased.

c If there is a sudden drop or increase in pressure during the high-injection grouting process, or an abnormal concentration or amount of return grout at the orifice, find out the cause and deal with it in time.

d When resuming construction after high-jet grouting is interrupted for some reason, the nozzle should be lowered 30cm, and only after overlapping spraying treatment can it continue to be lifted upward and spray grouting, and the depth and time of the interruption should be recorded. . If the machine is shut down for more than 3 hours, the slurry conveying pipeline of the pump body should be cleaned before construction can continue.

During the grouting construction, various parameters of high-jet grouting, the amount of grout material used, abnormal phenomena and handling conditions were truthfully recorded.

(4) Backfill, seal holes and clean pipelines. After the high-jet grouting reaches the designed top elevation, the nozzle lifts out the orifice and continues to inject cement slurry into the hole to ensure the height of the slurry surface in the hole, maintain the grouting pressure in the hole, and replenish as it sinks until the slurry at the hole no longer sinks. . Use a trench with a depth of 100 to 150cm along both sides of the grouting hole opening, and use the back grout during grouting to fill the surrounding spray hole openings to ensure timely backfilling of the grouting hole.

3.3 Construction records

(1) Hole making record content: hole number, drilling depth, geological stratigraphic conditions, rock penetration depth, construction time and special problems that occurred during the drilling process Conditions, etc., especially the location and depth of changes in rock geology should be clearly recorded so that corresponding processing can be carried out during grouting.

(2) High-jet grouting record content: high-jet grouting time, rotation speed, lifting speed, grouting situation at the high-jet nozzle, cement dosage per hole, etc. At the same time, various equipment parameters during construction should be carefully recorded Record and make specific and appropriate adjustments according to the on-site conditions.

4 High spray effect and quality inspection

Immediately select a section of the cofferdam to excavate to a depth of 3 to 5m, and check the wall effect. The wall continuity is good, and most of the wall thickness is When it reaches 50-80cm, the cement slurry and sand and pebbles are well bonded, and the cross-sectional appearance is consistent with the concrete structure. It needs to be removed by drilling and blasting. Publish papers. Around the nozzle hole, the smallest thickness reaches 50cm. When encountering larger pebbles or stones, the wall-forming effect is slightly worse.

The axis of the temporary cofferdam anti-seepage wall project of Laoshan Reservoir is long, and there is no detailed geological data in the early stage. After design detection, the depth of underwater silt in the reservoir area is about three meters. The project uses high-pressure jet spray technology to complete the cofferdam. The construction of the anti-seepage wall ensured the smooth excavation of the diversion channel and the construction of the water inlet tower at the front of the drainage tunnel, and created conditions for the safe construction of the gate and flood discharge channel.

It is more difficult to construct on the pebble layer or the pebble layer containing large pieces of boulder, and the wall-forming effect is slightly poor. When encountering fine sand or small-grained gravel, it is easy to cause hole collapse during the drilling phase. Clay wall protection or casing follow-up technology can be used. During the implementation of the project, the appropriate parameters are determined through experiments, the hole spacing and construction technology are well controlled, and strict requirements are imposed on no slurrying or lifting, and the lifting speed is also adjusted to an appropriate value. This can basically ensure the construction quality and achieve the anti-seepage effect.

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