1 project overview
This project is the fourth bid section of Binjiang in the fourth phase of the standard pond in Hangzhou suburb of Qiantang River, focusing on flood control and ensuring the safety of towns such as Hangzhou and Puyan. Its pile number is 0+590-2+ 134, and its length is 1544m. The project is a first-class dike project designed according to the once-in-a-century flood (tidal water) standard. Through field investigation, the salient features of this project are as follows:
1) This bid section is an old embankment reinforcement project, focusing on open caisson sinking, concrete apron and slope protection construction. In the construction, it is necessary to formulate feasible technical and organizational plans to ensure that the project is completed on time and with high quality;
2) Because the construction site near the dike line of this project is narrow and the dike top is the main road for construction, temporary facilities for construction must be arranged reasonably;
3) Cofferdam construction of this project is the most critical project to ensure construction safety and progress. Effective technical and organizational measures must be taken to ensure the progress and quality of cofferdam construction and strengthen the maintenance of cofferdam.
2. Adoption of construction scheme
At the same time, according to the construction characteristics, natural conditions, construction period, etc. In principle, the method of combining parallel operation with flow operation is adopted. Considering the terrain of the site, the construction of this bid section is divided into three operation sections, which are bounded by the pile number 0+887 and the pile number 1+5 1 1. Considering that the second and third sections are relatively long and the first section is relatively short, two working faces are set in the second and third sections, extending from the middle to both ends, and the first section is set with one working face. Focusing on the throwing and filling of stone mixture, open caisson, apron protection, concrete slope protection and embankment back slope filling, the layout is carried out by the cross operation mode of first down and then up.
Considering the cofferdam construction is adopted in this project, and the working faces in the cofferdam are not affected by the tide of Qiantang River, so we can make full use of all available construction time, which is beneficial to the full development of the working faces and solves the contradiction of heavy workload and tight construction period, so we plan to adopt the cofferdam scheme for construction. Cofferdam is filled with soil bag, and earthwork is filled with sand suction pump. The construction is planned to be divided into three sections, the first section is 0+590~0+887, the second section is 0+887 ~1+511,and the third section is 1+5 16544. The concrete slope protection is constructed by slip-form technology, and the concrete is directly transported to the top of the dike by the mixing station with an engineering truck, and then discharged through the steel chute and vibrated by an internal vibrator.
3 survey lofting
Approach to do a good job of leveling, at the same time, the construction survey lofting work. Before construction, the survey control points shall be rechecked and set together with the owner and supervisor. According to the relevant provisions of the State Bureau of Surveying and Mapping on various investigation requirements, we should conscientiously do all the preparatory work before the investigation. Strictly implement the operation procedures during measurement to improve the measurement accuracy and ensure the quality.
According to the scale of the project, designate a special person to be responsible for the construction survey, and provide the survey data needed in the construction stage comprehensively and accurately. Plane setting in the construction stage: according to the coordinate points provided by the owner, locate the datum line and establish the coordinate control system, set the coordinate points and elevation control points at the corresponding parts of the embankment foot, and conduct the survey once. The accuracy of the coordinate control system is not lower than that of the first-class traverse survey, that is, the left-right error is less than 5, the relative median error is less than110000, the accuracy of the elevation control point is not lower than that of the third-class leveling survey, and the closing error should meet the specification requirements.
Construction surveyors shall uniformly number the survey marks, and compile them on the general construction plan, indicating relevant marks, mutual distance and elevation to avoid mistakes, and regularly check them during construction to avoid displacement. Coordinate points and elevation control points shall be set in places where the foundation is solid, unaffected by construction, not easy to be damaged and easy to be preserved, and concrete foundation shall be poured and protective piles shall be set. At the same time, in order to effectively ensure the measurement accuracy, before the construction of concrete, open caisson and sheet pile, the lofting measurement is carried out according to the control points, and the retest is carried out again to ensure the engineering accuracy.
The setting of plane control points mainly includes: standard dike center line sample (buried wooden stakes or concrete piles), open caisson center line sample (standing wooden poles), stone edge line sample of outer river side inhibition layer (standing wooden poles), apron edge line sample (standing wooden poles), slope protection edge line sample (standing wooden poles), inner slope earthwork edge line sample (standing wooden poles) and breakwater top edge line. According to the detailed observation and analysis of drawings and site, combined with the setting and measurement of plane control points, the method of traverse control measurement is adopted. The concrete measures are: using the relationship between the survey control points provided by the owner and the coordinate values of the piles in the standard pool, the angle and distance between the survey control points and the piles in the standard pool are calculated, and then the piles in the standard pool are determined by theodolite control. Using the relationship between the control points of each side line of the dike and the control points of the dike axis, the angle is controlled by theodolite, and the distance is measured by steel tape, and the control points of each side line are set. Each plane control point should be carefully protected, concrete should be poured in the center of the standard dike for protection, and other control points should be protected by rockfill; At the same time, the control points should be reviewed regularly to ensure accuracy.
4 temporary cofferdam construction technology
In view of the fact that the seawall of this project is reinforced by the old dike, according to the hydrological tide level, some sub-projects such as open caisson and apron at the toe of the dike are in the water level fluctuation area during the construction period. In order to start the construction of this part of the project after entering the site, avoid the influence of high tide and low tide on the construction progress and quality, and facilitate the development of the construction surface, which is conducive to speeding up the construction progress, it is planned to build a temporary cofferdam to form a closed construction area and eliminate stagnant water. Because the tidal water in Qiantang River is influenced by many factors, such as riverbed topography, accompanied by strong tidal bore, turbulent tidal bore, high head, fast velocity and strong impact, the cofferdam must resist the impact of large tidal bore and the scouring of beach cofferdam sediment. In order to ensure the safety of the cofferdam and the smooth implementation of the project, the proposed scheme is as follows:
For the temporary cofferdam structure, it is considered to adopt the dredger-fill pipe bag cofferdam. The designed main cofferdam is about 1.570 m long and arranged along the outer side of the apron. Each woven bag is about 50m ~ 100m long, and the earthwork is filled with sand suction pump. It is planned to start construction in three sections. The center line of the cofferdam is about 35m~40m away from the dike axis, and a certain distance is left from the excavation of the toe of the slope, which is convenient for foundation excavation and other related processes and ensures safety. The design cofferdam has a top elevation of 9.2m, a top width of 4m and a bottom width of11m.. The dredged fill of woven bags on both sides is fixed with steel pipes, φ 48 steel pipes are set at the middle and top of the bag as Rachel steel pipes, and the spacing is1.5m. A layer of woven cloth with impermeable coating is laid at the bottom of the cofferdam, and dredged fill is filled in the middle.
1) material selection. The cofferdam is filled with mud from the beach, sampled after filling, and implemented after being approved by the supervisor. Mud-filled pipe bag is made of geotextile woven bag, with a length of 50m~ 100m and a diameter of 2m~3m. Polypropylene woven fabric with tensile strength greater than 25kN/M and latitude and longitude of 14× 14/ inch was sewn with 35 three-ply nylon threads, and the bag and interface were sewn three times. When sewing each package, sew 4 ~6 pieces. Steel pipe adopts φ 48 steel pipe for shaping construction scaffolding;
2) Construction technology. Before filling, clean up roots, turf, rocks, corrosion, etc. At the bottom of the pipe bag. First, the steel pipe is driven by water flushing method. After the steel pipe is driven, it is supported and fixed with steel pipe, and the diagonal brace and the vertical steel pipe are fixed with butterfly buttons to make them well connected. Then put a geotextile bag and fill it with a sand suction pump. Before large-scale construction, try to fill it on the beach. After obtaining certain test data, such as slurry concentration, bag size, slurry screening pressure, slurry screening time, consolidation speed, deposition rate, etc., comprehensive construction is carried out. The suction pump is placed on the ship. When blowing mud, control the position of the pipe bag according to the lofting, and then tie the two ends and the middle part of the pipe bag firmly with iron wire and tie it firmly on the fixed pile to avoid displacement during blowing mud. When filling mud, loosen the iron wire until the bag body is stable. When filling, pay attention to adjust the direction of the delivery nozzle in time to avoid deformation and displacement of the bag body due to uneven stress, and manually disturb the mud filling body to accelerate the consolidation of the soil body. At the same time, pay attention to control the screen pulp pressure to prevent bag breakage. The dredger fill shall be filled in layers, and the upper bag shall be filled after the lower bag is consolidated. When blowing mud, the slope toe on both sides of the cofferdam should be carried out at the same time, and a layer of bags should be blown to fill the mud core between the bags. The dredged bag should be protected in time and the slope protection should be done as soon as possible.
5 concrete aproll and slope protection construction technology
5. 1 Concrete apron construction
1) The newly-built concrete apron dike is located from the outside of the slope protection foot to the open caisson position, with a length of 8.5m, a thickness of 40cm and a top elevation of ▽ 6.0m; Aproll is a reinforced concrete structure with drainage holes. There are 3m thick M7.5 cement mortar cushion and 17cm thick gravel cushion under the apron, and a layer of 400g/m2 nonwoven geotextile is laid. Non-woven geotextile under apron is laid manually, and gravel cushion should be laid immediately after laying to avoid the geotextile from aging due to sun exposure or outdoor exposure for too long.
2) Laying gravel cushion and M7.5 cement mortar cushion. Gravel cushion shall be laid manually by excavator, and it shall be leveled manually. Gravel shall be transported directly to the bottom of embankment by dump trucks and piled up near the apron. Gravel is laid on the geotextile on the inner slope of open caisson by excavator and leveled manually. Gravel cushion should be laid evenly and continuously with sufficient thickness, and the elevation after laying in the whole range should meet the design requirements. Thickness control: before construction, the foundation surface shall be laid out, marked with wooden stakes, and the laying thickness between wooden stakes shall be controlled. The gravel cushion shall be made of high-quality mud-free graded gravel. M7.5 Cement mortar cushion is produced by mixer, transported to the required position by engineering vehicle, and paved and leveled manually. The thickness of mortar cushion should be sufficient, and it should be laid continuously, spread and wiped to achieve dense effect;
3) The apron reinforcement is made in the on-site processing plant. Before fabrication, steel bars should be straightened and derusted. Steel bars shall be straight without local distortion, and coiled steel bars and bent steel bars shall be straightened. After the steel bar is straightened by straightening machine, the cross section of the steel bar should not be reduced by more than 5% on the scratched surface of the cylinder mould. Steel bar intersection is bound with iron wire, the end of stirrup is bent inward, the corner of stirrup is fastened with the intersection of steel bar, and it is bound with artificial vertical bar;
4) The steel formwork of this project is formed and assembled, and the steel formwork is customized in the special local shape. The formwork erection shall be firm, stable and good in integrity, and the flat-fell seam of the formwork shall be flat and smooth, and there shall be no slurry leakage between joints. Template should besmear brushs release agent, make the concrete surface smooth after demoulding;
5) When mixing, due to the reinforcement in the apron, the concrete should be divided into two grades. After the steel binding is completed, pouring without warehouse surface is carried out. With 15T crawler crane and 1m3 horizontal tank, concrete without apron surface is poured, compacted by internal vibrator, leveled by wood trowel, and survived by calendering iron plate for three times.
5.2 concrete slope protection facing construction
The slope protection veneer is located at the water's edge, from the outside of the breakwater wall at the top of the dike to the square foot of the dike. Should choose secondary concrete, the largest particle size is not more than 40 mm, the side form adopts steel template, template coated with colorless isolation agent, template joints sealed with rubber strip, to avoid slurry leakage. The upper die adopts self-made steel wire drawing die, which is pulled by chain block and pushed by sliding mode.
The concrete is mixed by the on-site mixing station, transported to the top of the dike by the engineering vehicle, and directly fed into the formwork through the chute. Manual liquidation, layer by layer. The slipform will be raised to the height of 20cm~30cm at a time, which can be increased or decreased according to the temperature change. φ50 plug-in vibrator vibrates. In order to facilitate the transportation of chute concrete, the slump of concrete should not be too small, and segregation or bleeding should not occur during transportation. Concrete pouring shall be carried out after demolition and excavation (part of the foundation surface shall be treated according to the requirements of phase II concrete, and the foundation shall be washed with water first). When vibrating the upper concrete, the lower concrete should be inserted 5cm to facilitate the combination of the upper and lower concrete. Insertion points should be evenly arranged, with a vibration point every 40cm, vibrating obliquely on the concrete surface until the concrete surface is submerged, no longer sinking, and basically no bubbles escape. The sliding time of sliding mode should not be too early, which can be determined according to the actual situation. After sliding, the concrete will not sink due to the vibration of the upper concrete, and the surface of the concrete will be compacted and brushed in time. Construction joints shall be caulked with asphalt slabs, which shall be straight, beautiful and full.
6 conclusion
Combined with the first-class dike project, this paper mainly introduces the construction of open caisson sinking, concrete apron and slope protection in the old dike reinforcement project. Effective technical and organizational measures must be taken in the construction to ensure the progress and quality of cofferdam construction and strengthen the maintenance of cofferdam.
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