The reinforcement project of Laoshan Reservoir started in 2009 10. Combined with the hydraulic layout, topographic and geological conditions, water level during construction and the actual situation of uninterrupted water supply during the construction of water conveyance tunnel, a temporary cofferdam is set up to enclose the intake tower and spillway gate. Cofferdam forms are earth-rock cofferdam and high-pressure jet grouting cutoff wall.
1 project overview
Laoshan Reservoir 1958 was started, and 1959 was basically completed. It has been in operation for 50 years, and the original designed total storage capacity is 60.44 million cubic meters. It is a medium-sized reservoir with comprehensive utilization of urban domestic and industrial water supply, flood control and aquaculture. In February, 2004, Shandong Research Institute of Water Conservancy and Shandong Water Conservancy Engineering Quality Inspection Center conducted dam safety appraisal of Laoshan Reservoir, and determined that the reservoir was a Class III project, and the main buildings such as dam, spillway tunnel and auxiliary dam were all Class III buildings. In order to make the flood control standard of the reservoir reach the once-in-a-year check flood standard in 2000, eliminate hidden dangers of diseases and ensure the safety of the project, the Laoshan Reservoir reinforcement project is specially implemented.
High-pressure jet grouting cutoff wall is mainly used for temporary cofferdam seepage control project. Foundation pit drainage and safe flood control are the difficulties and key points in the construction organization of this project, and the goal is to create conditions for the open excavation of diversion canal, intake tower and spillway gate construction.
The main design quantities are: the total length of cofferdam is 330m, the top elevation is 46.2m, the maximum weir height is 14.2m, the weir top width is 6m, and the slope ratio on both sides is 1: 2.0. The minimum thickness of the jet grouting wall is not less than 0.40m m, the cofferdam is filled with 86,200 pieces of ballast, the sprinkler irrigation is 5 196m, and the hole spacing is1.0m. The construction is divided into two continuous holes, with the hole depth of 0.5m and the hole top below 0.5m.. Model 76 rotary vibration drilling rig is used for drilling and grouting pump grouting in the construction of jet grouting wall.
2 Determination of main technical parameters
In order to ensure the smooth construction of high-pressure jet grouting, ensure the construction quality, and test the rationality of the design of jet grouting process parameters and the reliability of wall formation, at the initial stage of on-site high-pressure jet grouting operation, a high-pressure jet grouting process test was carried out in a section with representative geological conditions according to the mixture ratio selected by indoor tests. The hole arrangement mode, hole depth, injection flow, pressure, rotation speed and lifting speed are determined through experiments. Publish a paper.
3 Construction technical parameters and process control
3. 1 Construction technical requirements and parameters
(1) hole spacing 1.0m, hole bottom is 0.5m deep into rock stratum (weak impermeable layer), hole position deviation is less than 5cm, and well slope is less than 1%.
(2) The cement slurry is 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 triple pipe method, with double nozzles, nozzle diameter 1.8mm, high-pressure water pressure of 38 ~ 40 MPa and flow rate of 70 ~ 80 L/min; The pressure of compressed gas is 0.7 ~ 0.8 MPa, and the flow rate is 7 ~ 8 m3/min；; Mud pressure is 26 ~ 32 MPa, flow rate is 70 ~ 100 L/min, and density is greater than1.65g/m3; The lifting speed is 5 ~ 8 cm/min.
(4) The permeability coefficient should be in the order of n×10-4 ~ n×10-5 cm/s, the effective average thickness of the wall formed by sprinkler irrigation is 50cm, and the contact part between cofferdam base and riverbed should be re-sprayed at least 1min.
(5) Main construction machinery and equipment: high-pressure jet trolley (CYP hydraulic type), fresh water pump (3DZ-2), grouting pump (HB80), grouting pump (BW- 150), mixer, air compressor (WF3/6), 76 rotary drilling rig, etc.
3.2 Construction technology and method
(1) hole arrangement and hole making. According to the construction plan, use level and steel rule to measure, set out and locate, arrange hole position, fix the position of drilling rig, adjust the verticality of drill pipe, and correct the deviation between drill bit and hole position. When ordinary 76 rotary vibration drilling rig is used to drill holes, mud will protect the wall when encountering boulder or boulder phenomenon. In order to ensure the quality of hole formation and the stability of hole wall, the slurry should be properly adjusted, and the concentration and viscosity should be controlled to prevent hole shrinkage and hole collapse. In the process of hole-forming, make hole-forming records, especially the description of lithology in the hole, to ensure that the depth of drilled rock meets the requirements. According to the records, if the drilling depth reaches the design depth and has not yet entered the relative water-resisting layer, it is necessary to deepen the drilling, and after the acceptance of hole formation, it will go deep into the relative water-resisting layer 1.0m, and immediately run the casing to the bottom of the hole, and the nozzle will be exposed to the working face for 20cm to prevent hole collapse. The casing adopts φ 1 10mm PVC pipe.
(2) pulping. The mixer adopts WJG80- 1 vertical mixer, grouting pump adopts HB80 and BW- 150 mud pump to recover slurry. Ordinary portland cement is used as cement, and two-stage mixing is used for pulping. After the first-stage mixing 12 minutes, it is filtered and put into the second-stage mixer, and grouting is carried out while stirring. Publish a paper. When pulping, strictly control the water cement ratio (0.5 ~ 0.8): 1, and the specific gravity of the slurry is 1.6 ~ 1.7g/cm3. Mud concentration is often detected by hydrometer to ensure mud concentration, particle-free, uniform and stable, and good fluidity.
(3) High pressure jet grouting.
A, before high-pressure jet grouting, spray for about 1 min at the site interview, adjust the spray direction, check whether the water nozzle is unblocked and whether the pressure can meet the design requirements, and then slowly lower the spray bar to the bottom of the hole.
B. After the grouting preparation is completed, normal injection shall be carried out. Firstly, the slurry is transported according to the specified parameters, and electrostatic spraying is carried out with water and gas for 3 minutes. After the slurry returns to the orifice and the situation is stable, high-pressure jet grouting is carried out. Intermittent lifting method should be adopted when spraying, and it should be stopped for 5 minutes every 30 cm. Under normal slurry supply conditions, when the return slurry concentration at the orifice becomes smaller and fails to meet the design requirements, the slurry inlet concentration or slurry inlet quantity should be increased.
C, in the process of high-pressure jet grouting, if the pressure suddenly drops or rises, and the orifice slurry concentration or quantity is abnormal, find out the reason and handle it in time.
D, high-pressure jet grouting is interrupted for some reason, and when the construction is resumed, the nozzle should be lowered by 30cm. After overlapping jet treatment, jet grouting can be continuously lifted upwards, and the interruption depth and time should be recorded. When the downtime exceeds 3h, the slurry conveying pipeline of the pump body should be cleaned before the construction can continue.
E, truthfully record the parameters, slurry consumption, abnormal phenomena and treatment of high-pressure jet grouting during grouting construction.
(4) Backfilling, hole sealing and pipeline cleaning. After the high-pressure jet grouting reaches the designed top elevation, the nozzle raises the orifice and continues to inject cement slurry into the orifice to ensure the height of the slurry surface in the orifice, maintain the grouting pressure in the orifice and make up with the sinking until the slurry at the orifice no longer sinks. Trenches with a depth of 100 ~ 150 cm shall be built along both sides of the grouting orifice, and the surrounding spouted orifice shall be backfilled with slurry during grouting to ensure timely backfilling of the grouting orifice.
3.3 Construction Records
(1) Hole-forming records: hole number, drilling depth, geological stratum, rock penetration depth, construction time, special circumstances during drilling, etc. In particular, the changing position and depth of rock geological conditions should be clearly recorded so as to make corresponding treatment during grouting.
(2) Record the contents of high-pressure jet grouting: high-pressure jet time, rotating speed, lifting speed, slurry return from high-pressure jet port, cement dosage per hole, etc. At the same time, we should carefully record all kinds of equipment parameters in the construction process and make appropriate adjustments according to the site conditions.
4 High-pressure spray effect and quality inspection
Then select a section of cofferdam to excavate to a depth of 3 ~ 5m, and check the wall-forming effect. The continuity of the wall is very good. The thickness of most walls is 50 ~ 80 cm, and the cement slurry is well cemented with sand pebbles. The appearance of the section is consistent with the concrete structure, which needs to be demolished by drilling and blasting. Publish a paper. Around the nozzle hole, the part with the smallest thickness also reaches 50cm. When encountering coarse gravel or block stones, the wall-forming effect is slightly worse.
The cutoff wall of temporary cofferdam in Laoshan Reservoir has a long axis, and there is no detailed geological data in the early stage. After design and testing, the depth of underwater silt in the reservoir area is about three meters. High-pressure jet grouting technology is used to complete the construction of cofferdam cutoff wall, which ensures the smooth excavation of approach canal and the construction of intake tower at the front end of spillway tunnel, and creates conditions for the safe construction of gate and spillway.
It is difficult to construct in pebble layer or pebble layer containing large boulder sand, and the wall-forming effect is slightly poor. However, when encountering fine sand or gravel with small particle size, it is easy to cause hole collapse during drilling, so clay wall protection or casing follow-up technology can be adopted. When the project is implemented, the appropriate parameters are determined through experiments, the hole spacing and construction technology are well controlled, and it is strictly required not to return slurry or lift, and the lifting speed is also adjusted to a suitable value, which can basically ensure the construction quality and achieve the anti-seepage effect.
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