Construction Technology of Inter-tidal Zone Vacuum Vacuum Preloading Combined with Surcharge Preloading Soft Foundation Treatment Construction Chen Shunyi (Xiaqiao Luqiao Construction Investment Corporation, Xiamen 361026, Fujian, China) Vacuum preloading under water and surcharge preloading soft foundation treatment.
1 Project Overview The comprehensive improvement project of Xiamen Island Third Pier to Tongyi Port section is located on the southwest side of Xiamen Island. The direction of the newly built coastline is north-south. The main contents of the project include: the total length of the newly built coastline is 1872.405m, and the new land area is about 15.1. Million m2, a new 500t public wharf, and flood protection tank culvert length and drainage pipe protection of 103.2m. The project implementation process is divided into two sections A and B, including underwater vacuum preloading and surcharge preloading test. In section B, the main work of B section is the new shoreline 405m) and the corresponding land backfill. The silt thickness of this section is between 14 and 30m. The original design of the soft ground treatment uses large diameter sand or Insert the plastic drainage plate + preloading plan for surcharge; the mileage number is K1 + segment. Due to the difficulties in the demolition of the two terminals of the township comprehensive terminal and the public pier, and the normal use of the terminal during soft base treatment must be ensured, the design changes. The soft ground treatment of the shoreline adopts underwater vacuum preloading combined with surcharge preloading scheme. The distance between the plastic drainage plates is 1.0mx1.0m and the square layout is an area of ​​9 428.5m2. The silt thickness in this section is 16~18m. Side No. K1+008. 556m, Gongjiu Wharf's approach base is K1+108.556m in the middle of the test section. The distance between the two wharfs is very short. Frequent ship movements are required. The public base pier requires special sealing measures for approaching the base of the bridge. In construction, it is necessary to focus on solving the problems of underwater vacuum pre-press sealing.
2 Process Principles and Processes Underwater vacuum preloading combined with surcharge preloading is to establish a vertical drainage channel by placing plastic drains in the silt reinforcement zone, and the overlying membrane is formed in a closed state to remove water and air from the soil. Vacuum is generated by negative pressure, negative negative hydrostatic pressure generated by vacuum is used, and water load is dominant and surcharge preloading is supplemented to reinforce soil, speeding up progress and strength of soil reinforcement, shortening construction period, saving raw materials, and saving investment costs .
The use of underwater vacuum preloading in combination with surcharge preloading in this test section is an additional insurance measure taken to prevent undersea vacuum from reaching the vacuum on land.
Underwater vacuum preloading combined with surcharge preloading process: construction preparations laying a sand cushion a sealing groove processing a plastic drainage board laying a drainage filter tube buried a deep monitoring tube a shop seal film installation of a vacuum pump device A vacuum pumping test is performed on a pile of preload for observation and monitoring of an unloading.
3 Construction Process 3.1 Preparation for Construction Remove debris, bamboo sticks and other debris in the construction area of ​​the section, remove and scrape the rafts on the bridges on the public side, and remove obstacles affecting the membranes, such as slopes for fruits and vegetables. .
3.2 Laying of sand cushions Laying sand mats in strict accordance with the design requirements, the mud content of medium coarse sand 3.3 sealing groove treatment before laying the film on the sea side mud surface (yellow zero, -3.61m), according to design requirements in the sand bag A layer of short-punch non-woven geotextile shall be laid on the slope surface of the cofferdam, and then a nylon sand bag shall be used to lay the protective surface; the sealing film shall be pressed into the mud surface 50cm above, and then the nylon sandbag shall be used to protect the surface; K1 The +175.318m location line intersects with the soft ground behind it. The base tank throwing sand will have an adverse effect on the vacuum degree of underwater vacuum preloading. Sealing treatment will be performed by applying cement mixing, and the other two sealing grooves will be excavated. Backfill sludge for treatment.
3.4 Set up plastic drainage board According to the design requirements, apply plastic drainage board. The drainage boards are arranged in a square, with a spacing of 1.0m, 4325 pieces on land, 456045m, 2768 pieces of water, and 30859m. After the formation of the secondary cofferdam in the vacuum preloading section, plastic drainage boards are applied behind the reclamation area.
The construction of plastic drainage board meets the "Quality Inspection and Evaluation Standards for Harbor Engineering" (JTJ/T221-98), <Plastic Drainage Board Construction Regulations> (JTJ256-Plastic Drainage Board is applied with casing-type method to strictly control the verticality of the casing. Verticality deviation is not greater than *1.5%. The use of onshore equipment to apply plastic drainage board, hydraulic equipment used excavator refitted flapper machine, the application process should be catching up with the tide of water. Plastic drainage board measurement measurement strict Control, the allowable deviation of the plane position of the plastic drainage board is controlled to *100mm. Strictly controlling the sludge into the pipe during the playing lead to the return zone, the length of the setback zone must not exceed 500mm, and the number of the return zone should not exceed 5% of the total root number. When the plastic drain board is used to prevent the occurrence of kinks, breakage, tearing of the filter membrane, and the person in charge of the board should take care of it carefully, straighten out the board surface to prevent the board surface from kinking into the bushing; during windy weather construction, care must be taken to prevent the wind from damaging the board. The tearing of the membrane, to stop setting six wind above.
When cutting the plastic drainage board, the exposed length of the plastic drainage board above the sand cushion shall be greater than 200mm, and the exposed length deviation shall be *50mm. The bottom elevation of the plastic drainage board shall be strictly controlled and no shallow deviation shall occur. When geological changes are found to be large, When it is not possible to apply according to the design requirements, it is necessary to immediately contact the supervision engineer and obtain the consent of the design representative and supervision engineer before changing the design elevation.
After the plastic drain board is applied, it must be timely blocked with sand. pressure. Single root length is generally 4m; filter cloth outside the filter function of the filter cloth from the needle sticking hot-stiffened non-woven geotextile sewing, the unit area of ​​510-3cm / s, requiring only water vapor, sand.
All pipelines are laid out by the surveying staff in accordance with the design drawings. They are buried 30cm below the top surface of the sand cushion. Before being embedded in the sand cushion, the openings of the film outlet of the supervisor shall be temporarily covered with cloth. Prevent sand from entering the tube. When the length of the filter tube and the main pipe is more than 4m, it is connected by two-way or four-way connection; when it is connected to the filter pipe at the end of the main pipe, the three-way connection should be adopted. The filter tubes are arranged at a distance of 6m and the main pipe is arranged at a distance of 12m.
Plumbing holes are used for the water filtering pipe, and the connection point for the vacuum pipe line should be sealed strictly. To prevent the vacuum degree below the diaphragm from falling rapidly after the pump is stopped, check valves and door sections should be provided in the vacuum line.
After the filter tube is buried, the sand surface should be smoothed to remove debris such as stones and rubble.
3.6 The buried deep monitoring pipe The deep monitoring pipe shall be buried by the Tianjin Harbor Research Institute according to the observation needs. The observation data line shall pass through the sealing membrane to be laid for later collection and analysis of observation data.
3.7 Laying the sealing film Before laying the film, it is required to connect the film elbow with the filtering tube and fill the sand. The surface of the export pressure plate and the sand cushion should be flush with the surface, and the rubber gasket should be put down. Cut the film along the curved pipe mouth, place the rubber gasket and the upper pressure plate, grease evenly, and finally put the two nuts on the shelf. There is no sand material between the rubber gasket and the film. Sealing membrane processing in addition to considering the part buried in the sealing groove, due to sedimentation during the reinforcement caused by the settlement of the sand mat layer below the approach bridge, may tear the sealing film, need to reserve about 2m of film, folds attached to the base of the approach bridge. The sealing treatment of the approach bridge is rather special. After comparison of multiple schemes, a steel ring at the base of the approach bridge is used (the diameter of the steel ring is 700mm, which is 100mm larger than the base diameter of the approach bridge), and the reserved sealing film is inserted from above. The gap between the steel rims and the approach base was blocked with mud.
A layer of geotextile protective layer is laid on the top and bottom of the sealing film. At the elevation -0.3m, lay 1 layer of woven geotextile (100g/m2) or needled non-woven geotextile (0.00m, lay 1 layer of non-woven geotextile (150g/m2), soil leader, filter tube All use PVC pipe, diameter mm, wall is 335chf°mmie to 1 can withstand 40* ublishing (4) i laying the sealing film when the film should not pull I too 'tight, should be from above (*, preloaded When the pressure construction begins, it is necessary to lay the pore water pressure coefficient Ler 6 on the 1st. It is necessary to adjust the loading construction into the downwind of the wind, and the remaining amount around the extension and reinforcement area should be basically the same. Each side shall be 5m larger than the drawing size, and the elasticity of the membranes in all areas shall be approximately Uniform; laying from the beginning, the three-layer film at the same time.Construction workers should wear soft-soled shoes on the film, is strictly prohibited to wear shoes with nails on the film, each shop should be a layer of inspection, if there is a hole, timely glue repair. The inside of the ditch should be laid out with a film. When the film is too long, it can be folded at the bottom of the ditch. It must not be laid on the outside slope.
90m after the application of plastic drainage board, stacked sand bags to -1.50m to form a secondary cofferdam; secondary cofferdam to 0. 50m cofferdam top elevation, sealing film laying, requiring the sealing film to set up and down the geotextile Protective layer; The first layer of cofferdam sealing film shall be stacked and pressed into the mud surface 50cm, and then use sand bags to press the protective surface.
3.8 Installation of Vacuum Pump Installation Vacuum pump installation must be strictly connected in the order of motor, water pump, circulating water tank, gate valve, check valve, and film outlet. The connection is sealed with rubber mats, glass glue, etc. The water inlet and outlet of the water tank are kept On the same level.
Before the vacuum pump is installed, the vacuum pumping device (jet pump) shall be tested. Before the delivery, the pump shall be tested for acceptance. The test pump shall be operated with a vacuum pressure of 0.096 kPa.
3.9 Test vacuum According to the construction process, the jet vacuum pump and vacuum pipeline are connected according to the sealing requirements, and the vacuum is tested.
The operation of each jet pump, the tightness of the membrane and the degree of vacuum at each depth in the soil, the deep settlement of the soil layer, the total surface settlement of the soil, the lateral displacement of the depth of the soil layer, and the pore water pressure must be carefully checked during the vacuum test. Waiting for changes and finding problems should be dealt with promptly.
After 10 days, the value of the vacuum gauge under the membrane needs to reach 60~80kPa; if the value of the vacuum gauge below the membrane is lower than 60kPa, a comprehensive inspection should be performed to analyze the cause and deal with it in time.
When the value of the vacuum gauge under the membrane is maintained at approximately 60 kPa, it corresponds to a load of 80 kPa. When the value of the vacuum gauge under the membrane reaches more than 60kPa and is stable for a period of time, the water can not be started until the sealing system is determined to be in the inspection area. The height of the water-covering is generally when the test is started, ie, the vacuum gauge under the membrane should be performed. Various parameters such as values ​​and settlements are observed and collected.
3.10 Combined Surcharge Preloading Underwater vacuum pre-laminated vacuum gauges have reached the design requirements of 60 ~ 80kPa and stabilized for 20 days before the start of preloading. The geotextile protection material is used to protect the lower sealing film, so as to ensure that the preloading thrown sand does not affect the vacuum after the preloaded sand is thrown.
0.50~,+2.00m need to use sand bags as cofferdams, and the sand piles are laid during the high tide period. The sand throwers are used for throwing and filling, and leveling is done manually at low tide.
2.0m) Within 30 days after application, the sand should be frequently recharged and maintained at +2.0m elevation to ensure that the preload load is full for at least 1 month.
3.11 Construction observation and monitoring After the test vacuum is tested, the initial readings of various observation instruments are adjusted. After supervision and design acceptance, the underwater vacuum preloading can be officially started.
After the underwater vacuum preloading starts, various observation records or readings shall be made in time.
The value of the vacuum gauge under the membrane should be observed once every 2~4h. It is found that the vacuum pressure has decreased and the cause should be immediately found and handled in time.
Ground subsidence observation point, timing, by special observation subsidence target location. After throwing sand to 0.00m, three surface subsidence markers (observed by observing the amount of sand matting caused by the pre-vacuum preparation period) should be placed and moved to the membrane surface during film deposition. After completion of film laying, one film surface subsidence mark is arranged for every 1500m2, and a total of six are arranged, and the settling observation error requirement is not more than 2mm. From the start of vacuuming to the waiting period of the second-stage surcharge (, + 2.00m), and In the early stage of secondary loading, it is observed once a day. After the second-stage surcharge, the sedimentation rate is less than 2.72h and observed once, if the settlement is slowed down to observation one time.
Before the construction of the plastic drainage plate, the surface elevation map within 7m of the extension area and the surrounding extension shall be measured and mapped into a topographic map; after the vacuum preloading is completed, the ground elevation in the above range shall be measured again and mapped. At the same time, the cross-sections of the settlement in both directions are measured.
Observed data data should be firstly collated and accurate, seeking truth from facts, and the relationship curve between the amount of subsidence and time should be made in time. The abnormal value should be re-observed once. The difference in closing should meet the requirements of the relevant regulations.
Underwater vacuum preloading combined with surcharge preload monitoring.
Benchmark observations: observations were made every 2 days prior to laying, vacuum heads were observed 10 days after the film was applied, and observations were made once every 2 days.
Under-film and vacuum pump vacuum observation: vacuum preloading full load (60~70kPa) for 20d, the vacuum table value of the vacuum level under the film and the vacuum pump is observed once a day.
Two-stage surcharge observation: The secondary pile load + 2.0m is used for vertical deformation observation of the foundation every day. The surface settlement at the center point of the surcharge does not exceed 20 degrees, and sand is often added within one month after the full load is loaded. During low-vacuum combined with surcharge preloading full load, inspection and maintenance of vacuum equipment should be strengthened. The vacuum combined with surcharge preload for 70 days, the measured ground settlement amount is greater than 2 mm/d for 10 consecutive days and nights, and the ground settlement calculated from the measured settlement curve is greater than 80%. The pre-compression can be terminated by experience collection.
Vacuum combined with surcharge preload settlement and consolidation calculation. In the calculation, the load is used to backfill the top surface of the bank behind the revetment, 4.50m, and the ground load is 10kN/m2. The settlement of the foundation is calculated using the layered summation. The mud is considered as the underconsolidated soil and the sludge is considered as the normal consolidation soil. The final settlement calculated using the load is 140cm, the settlement after the vacuum stability is 120d is 112cm, and the main consolidation residual sink 3.12 unloading underwater vacuum preloading with the surcharge preload 70d, the vacuum gauge reading under the membrane meets the design requirements 80kPa technical standard, the actual measured ground subsidence is not more than 2mm/d for 10 consecutive days and nights. The ground settlement calculated by the measured settlement curve is greater than 80%. After inspection and acceptance by the supervision, design and monitoring units, vacuum preloading can be terminated.
After the vacuum preloading is terminated, the reinforced ground elevation is immediately measured and the monitoring unit is responsible for core detection.
The unloading should be carried out according to the elevation required for the construction of the superstructure of the bank protection project.
4 Effect testing Underwater vacuum preloading combined with surcharge preloading Soft foundation treatment During the implementation of the test section, the Tianjin Harbor Research Institute conducted vacuum vacuum observations, shallow and deep settlement displacement observations for the entire process, based on on-site observations and Monitoring data, combined with the final coring test data analysis, the time of the soft ground treatment reinforcement process in this test section and the consolidation of the consolidated sludge, strength and total settlement and other indicators meet the design requirements.
The cumulative total settlement of the experimental observations in this test section reached 165cm, which is larger than the calculated value of 140cm. This project has been completed for nearly a year now. After one monthly post-construction settlement observation, the post-construction settlement observations in this test section are smaller than those of other soft ground treatments.
5 Conclusion Practice has proved that soft ground treatment of underwater vacuum preloading combined with surcharge preloading in the intertidal zone is successful in the implementation of this project, and the construction process is feasible. Through the practice of this test section, to provide similar experience for similar engineering design and construction in the future, I believe that after constant experience of colleagues, the soft ground treatment method for intertidal zone vacuum preloading with surcharge preloading is bound to be similar. Gradually promoted in the project.
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