Construction technology of concrete cutoff wall for reservoir main dam.

Construction technology control of concrete cutoff wall of main dam of a reservoir

In this paper, the construction technology of concrete cutoff wall of reservoir main dam is discussed in detail, which ensures the engineering quality for reference.

In the reinforcement construction of a reservoir, the dam body and dam foundation of the main dam are treated with low elastic modulus concrete cutoff wall. The design thickness of the cutoff wall is 80 cm, which is placed on the weakly weathered rock and embedded in the weakly weathered bedrock not less than 0.5 m. The cutoff wall is arranged along the dam axis, with a total length of 266 m and divided into 33 grooves. 2 # ~ 32 # tanks are made of low modulus concrete, and 1# and 33# tanks are made of C 15 ordinary concrete. The average wall depth is 33.93m, the maximum hole depth is 42.70m, the minimum hole depth is 7.50m, the total footage12437.60m and the total seepage control area is 9054.90m2..

1 engineering geological conditions

The formation lithology is fine-grained porphyritic granite, which is distributed in the whole dam area. The rock is hard and has strong weathering resistance. The fully weathered zone at the left dam head is about 4m thick, and the strongly weathered zone is 0.30 ~ 1.00m thick. The thickness of the weakly weathered zone at the dam head is 0.60 ~ 2.00m.. According to the data, during the excavation of dam foundation at pile number 0+ 186, a diabase vein with a width of about 8m was encountered, and its strike was almost perpendicular to the dam axis. There is no fault passing through the dam area, and the joints at the dam heads on both banks are developed. Dam shell fill includes weathered sand, sandy loam and gravel. The core wall is filled with gravelly and sandy clay, partially with silty sand and silty clay, containing gravels and a small amount of plant roots, and it is in a state of being wet → saturated, plastic → hard plastic. Gravel content is about 5%, sand content is about 30%, reaching 40% locally. Gravel content is 7%, which is composed of siliceous shale, shale and granite. It is highly weathered and its particle size is generally 1 ~ 5 cm.

2 design requirements

2. 1 indicators of low elastic modulus concrete are as follows: 28d compressive strength is not less than 6.5MPa, and elastic modulus is not more than 2500MPa. The 28d tensile strength is not less than 0.4MPa, and the ultimate hydraulic gradient is not less than 150.

2.2 Slot connection of cutoff wall is treated by riding slot hole, with a hole diameter of 2 19mm, a hole depth of 8.0m and an elevation of 48.2m at the bottom of the hole, and C 15 fine stone concrete is backfilled.

3 concrete cutoff wall construction

3. 1 Layout of production facilities

3. 1. 1 concrete mixing system

According to the temporary construction site provided by the owner, the mixing station is arranged in the downstream delta of the dam, and the temporary construction bridge across the tailrace channel meets the needs of construction traffic supply. Choose two concrete mixers. The whole system is mainly composed of unloading platform, gravel yard, loader, concrete dosing machine, concrete mixer, concrete pump, unloading platform, cement tank and bentonite bin.

Concrete transportation: 1 towed concrete pump is selected to ensure the continuity of concrete transportation.

3. 1.2 Mud system

The pulping system is arranged at the site on the left side of the dam crest, and consists of pulping station, slurry pool, slurry supply pipeline and slurry recovery and purification facilities.

(1) Pulping Station: It is mainly composed of clay yard, feeding platform, pulper, pulp storage tank, feeding pipeline and water supply pipeline. Pulping configuration 1 4m3 horizontal double-shaft mixer.

(2) Mud pond: two slurry storage ponds and 1 sedimentation pond are set up, and 1 mud pond is built to ensure sufficient slurry supply during construction. There is a 1 6m3/min mobile air compressor for mixing mud.

(3) slurry supply pipeline: a slurry supply pump is set in the slurry pool for slurry supply. The slurry supply pipes from the slurry station to the construction tank are all laid with 150mm steel pipes, with a total length of about 500m m. ..

(4) Recovery and purification of mud: The mud generated by drilling and pouring concrete by drilling rig passes through the slurry discharge ditch and sedimentation tank, and is recovered and purified before use.

3. 1.3 wind, water and electricity system

(1) gas supply system

On-site construction platform layout 1 set 10m3/min mobile air compressor, which is used for hole cleaning and slag suction in tank section; A 6m3/min mobile air compressor/kloc-0 is arranged beside the mud system at the right downstream of the dam for overturning and stirring the mud storage tank.

(2) Water supply system

There are 1 fixed or floating water pump houses in the reservoir area on the upstream side of the left bank of the dam. Two 5-inch multistage centrifugal fresh water pumps are installed to pump the water to the reservoir built on the top of the hillside on the left bank of the dam, and then send it to all construction water points through pipelines to meet the construction water demand. The drilling rig adopts direct water supply to solve the problem.

(3) Power supply system

Set up 8-meter pole low-voltage lines at the transformer to the office living area, living area and construction area. When the line is erected, it conforms to the safety code for electricity consumption, and the electricity consumption for construction and living lighting is measured separately. The installed transformer has a capacity of 900kVA, which basically meets the power demand, and has 1 generator. In order to ensure the safety of electricity use, all main power switches are air switches and equipped with electric shock protection devices. The low-voltage line is connected to the motor with a rubber cable.

3.2 Engineering construction

3.2. 1 Punching

Division of (1) unit slots

According to the geological conditions, wall depth, construction methods and many other factors and the characteristics of this project, in principle, the slot length is divided into 8m and * * * is divided into 33 slots. Considering the actual situation, adjust 1# slot and 33# slot to 10m and 8m.

(2) Manufacture of concrete guide groove and construction platform

The top elevation of the cutoff wall is 56.2m, and the axis of the cutoff wall is located at the downstream side of the original dam crest. In order to meet the requirements of the width of the construction platform of the cutoff wall, the lateral core wall and gravel downstream of the dam body are widened to an elevation of 56.7m, and then the diversion trench is made. The trench with the depth of 1.2m and the width of 1.6m is excavated by the hydraulic backhoe excavator. Through manual trimming, the core soil is used as the bottom formwork and side formwork of the diversion trough, and the diversion trough is made of cast-in-place reinforced concrete continuous plate girder with a height of1.5m. ..

After the guide groove is completed, a construction platform will be built. The total width of the construction platform is16.0m, and the drilling rig walking platform is located at the upstream side of the cutoff wall with a width of 6.0m. The drilling rig trajectory is parallel to the center line of the cutoff wall. In order to prevent uneven settlement of foundation during construction, ballast and gravel are filled between sleepers. The construction platform on the downstream side is used for heavy machinery and equipment, cranes, bit repair, slag tapping, etc. , 8.0m wide, cast-in-place concrete, with stone cushion under the concrete. A slurry discharge ditch is set at the front of the construction platform, and waste slurry will enter the slurry recovery pool through the slurry discharge ditch. The slurry discharge ditch is built with M7.5 slurry block stone and plastered with M 10 mortar.

(3) Mud preparation and transportation

Mud has the functions of stabilizing the hole wall, suspending cuttings, washing the hole bottom and cooling the bit.

According to the actual situation of the main dam stratum, the mud is mainly prepared from the original hole, and the insufficient part is obtained by 1 4m3 mud mixing mechanism. Mud flows into the slurry storage tank through the filter screen, and the mud in the slurry storage tank is often turned by high-pressure air injection to maintain the uniformity of the mud. When transporting mud, blow it with high-pressure air for 30 minutes, and then discharge the mud. Mud pump transports mud from slurry storage tank to each tank section through 150mm steel pipe. During normal production, samples should be tested for specific gravity, viscosity and sand content. In order to adjust the slurry performance in time. Mud should be purified and recycled, and waste mud should be removed in time.

(4) drilling with a drilling machine

① Selection of pore-making machinery

Twenty-six impact drilling rigs were put into the project to form holes, and the drilling splitting method was the main hole-forming process. Because the geological condition of this project is clay layer, when drilling with impact drill, a long hollow drill is equipped, which can improve the drilling efficiency in soft strata such as clay layer, sand layer and loam layer. The bit has large cutting force and stable center of gravity. The construction practice in similar projects shows that the construction technology can accelerate the drilling speed, ensure the verticality of drilling, and is not easy to collapse, and its efficiency is not inferior to that of hydraulic grab. At the same time, we also actively explore various hole-making technologies to speed up the construction progress.

② Hole position control

On the axis of the cutoff wall, semi-permanent axis control points are set on the left and right bank slopes, single hole center points are set on the top surfaces of the upper and lower sides of the trough plate beam, and the hole number and position of the single hole are marked. Temporary benchmark piles for the axis of the cutoff wall are set on the upstream side of the construction trough to check the quality of each hole.

③ Hole-forming technology of impact drilling rig

Select 1.3 ~ 2.5t cross impact drill to make holes. In order to ensure that the hole position of the final hole is not less than 80cm, repair welding should be carried out in time during construction. The drilling of main hole and auxiliary hole mainly depends on the impact cutting effect of drill bit. For the split of the small partition wall between the main hole and the auxiliary hole, it is necessary to move the drilling rig to the center of the small partition wall. When dividing, it is necessary to properly control the paddling so that the paddling is smooth and stable. When drilling, put a slag receiving cage in the main hole, and hoist the chopped sand out of the slot hole with a hoist. When drilling holes in bedrock, different drilling methods are adopted according to the weathering degree of rocks. In case of weathered and broken rocks, the method of "tapping and taking lightly" is adopted, and in case of hard rocks, the method of "tapping and impacting" is adopted.

Identification of bedrock

The depth of groove embedded in bedrock must meet the design requirements. In the process of pore formation, the bedrock surface is determined as follows.

(1) According to the geological profile of the center line of the cutoff wall, when the hole depth is close to the predicted bedrock surface, start sampling, and then determine the bedrock surface according to the properties of the rock sample;

(2) Determine the bedrock surface according to the elevation and drilling conditions of adjacent holes;

(3) When it is difficult to determine the bedrock by the above method, or when there is doubt about the bedrock surface, use the core to drill rock samples for determination and verification.

All rock samples are appraised on site by geological engineers, supervision engineers and engineers from the construction unit of design institute, and the rock sample appraisal form is filled in, and the owner unit participates in the acceptance.

3.2.3 Hole cleaning and slurry changing

After the completion of drilling, conduct a comprehensive inspection on the quality of drilling. After passing the inspection, the hole can be cleaned and the slurry can be changed. In this project, the inner air duct suction fan is used to clean the hole and change the slurry, and a duct filled with underwater concrete is used as the suction pipe, and the high-pressure air duct is located in the duct. Advantages: After hole cleaning, it is not necessary to lift and dismantle the conduit, but only dismantle the bent pipe part, so that underwater concrete can be poured, which wins time.

3.2.4 Quality Control of Low Elastic Modulus Concrete

Quality of concrete raw materials: the aggregate is first-class crushed stone, that is, 5 ~ 25mm. The specification requires that the over-diameter content is not more than 5% and the over-diameter content is not more than 10%. The batching deviation of various materials: cement, admixture and water are 65438 0%, and sand and gravel are 2%. In order to strengthen the quality management of concrete pouring, this project implements the concrete pouring certificate system, all raw materials are sampled and tested by the owner or supervisor, and the construction gradation adjustment sheet is made, which can only be started after being checked and signed by the supervisor.

3.2.5 Concrete pouring under mud

Mix low elastic modulus concrete in strict accordance with the provided construction mixture ratio. Check the quality of bagged bentonite during concrete mixing. Don't get wet and agglomerate, and don't mix with water. Mix the cement, bentonite and sand evenly, and then add water to prevent bentonite from caking and sticking to the mixer, so as to make the mixing quality uniform.

Concrete pouring adopts "direct conduit method", the inner diameter of conduit is 230mm, and the wall thickness is 4 mm, and the arrangement spacing of conduit should be carried out according to the actual situation and specification requirements. After the catheter is assembled, a pressure closing test shall be conducted.

After the concrete is discharged from the mixing station, it is directly transported to the storage hopper of the construction platform by the concrete towed pump, and flows into the conduit through the discharge chute of the storage hopper. Samples should be taken from the notch and machine mouth of each channel section for compression resistance, impermeability, elastic modulus and tensile strength tests. When pouring concrete, designate a special person to draw indicator diagram and pouring curve diagram, fill in the record of catheter removal in time, measure the concrete surface, and check the depth of catheter buried in concrete to absolutely prevent the catheter from being "empty".

The above are collected and sorted by Zhong Da Consulting Company.

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