Application of Vacuum Preloading in Second Ring Road in Huzhou, Zhejiang

1 Project Summary The Second Ring Road in Huzhou City is designed as a Grade I urban road and is located on the west side of Huzhou City. The road section is a super-soft roadbed. To reduce the uneven settlement and excessive deformation of the road during the service period, the method of vacuum combined with surcharge preloading is adopted to reinforce the soft soil foundation to achieve the purpose of settlement after the concrete works, and also to make the roadbed Filling the earth in a relatively short period of time to ensure the safety of high-fill embankment construction, and there is no post-construction soil disposal problem, which greatly shortens the construction period. The scope of the project is a super soft-base high-filled road section between the section of K2+040~K4+233 of the Second Ring Road West, and the treatment area is 91000m2. The project began to be constructed on March 20, 2001, in October 2001. The successful completion of the 18th, the vacuum degree below the membrane all reached -90kPa above, the effect is obvious, the project quality is excellent.

Table 1 Vacuum Combined Surcharge Preloading Partition Table Area Stake Length (m) Width (m) Area (m2) Start Construction Time -80kPa Time Zone 1 May 5 May 18th Zone 2 May 1st May 18th 3rd district April 11th April 17th 4th district May 1st May 11th May 5th May 7th May 17th May 6th district April 1st April 11th 7th district March 20th March 28 Total 2 Engineering Geological Conditions The Second Ring Road West of Huzhou City is located in the northwest of the Hangjiahu Plain. The river network in the area is densely covered and the terrain is low-lying. It belongs to a typical soft soil area. The average ground elevation is in the Yellow Sea area of ​​24m. The ponds and paddy fields are dominated by local mulberry lands and dry lands. The topography is slightly ups and downs, and the relative elevation is within 2.0m.

The surface layer of the site along the site lacks a hard shell or a thin crust. The underlying soil of the embankment is a thick layer of soft silt or muddy soil, and the soil layer is mainly composed of layers of weakly permeable or impermeable cohesive soil. The coefficient of permeability coefficient of viscous soil is 1 (T7). The permeability of water in the vertical direction is generally less than the horizontal direction. When the roadbed is filled with large areas, especially when the local area is high-filled, it will easily cause uneven settlement of foundation soil. And excessive deformation, so that the roadbed suffered damage, resulting in roadbed landslides, instability and other adverse geological phenomena.Therefore, it is necessary to reinforce the soft soil foundation of high-filled road sections.

3 Construction of Vacuum Combined Surcharge Preloading Process The main construction steps of this prepressing method are: construction of line, leveling site, laying of vacuum filter pipe, excavation of membrane trench, laying of composite geotextile, and laying of three layers of polyvinyl chloride. Sealing film, vacuum pumping, cofferdam storage, preloading, etc.

Vacuum combined with surcharge preload profile as shown.

The following focuses on the construction process of this project.

Vacuum pipeline material selection and layout This project uses a U-PVC rigid plastic pipe with a diameter of 063. The pipe wall thickness is 3 and the texture is excellent. In the past, the thickness of the filter pipe was only 2.3 thick, so the membrane trench was excavated behind. During the process, although the excavator is running on the membrane, the filter tube will not be broken. The vacuum preload piping layout is as shown. The filter tubes are arranged in a sash shape at a spacing of 6m. Since the thickness of sand cushions in areas 1, 2, 4 and 5 is only 10 cm, the filter tube trench is dug to the gravel layer. To prevent the filter membrane from being punctured, the coarse sand is backfilled around the filter tube to protect it. The connection between the pipes is applied with a skeleton hose socket, the socket length is 200mm, and a certain shrinkage space is guaranteed. A 1.2m long, seamless galvanized steel pipe and a four-way reducer joint are used at the film outlet. The boundary of the reinforcement zone is approximately 30cm. For connection to a vacuum pump. The vacuum probe is embedded in strict accordance with the design drawings to visually reflect the degree of vacuum under the membrane.

The protection and laying of composite geotextiles Due to the long preloading time of the project and the small thickness of the sand cushion under the membrane, in order to prevent the membrane from being damaged in the vacuum process, it is tiled in the longitudinal direction along the roadbed in the pre-compression zone in the order of 4.2mX60m. Composite geotextile (composed of a layer of engineered geotextile and a layer of non-woven fabric), non-woven fabric and filter membrane are in direct contact, and the overlap between geotextile fabric and the block is lap joint, and the lap width is >30cm. It is due to the fact that the horizontal drainage cushion is grit, and the inner membrane of the membrane membrane is most vulnerable to damage. Therefore, geotextiles are laid to the bottom of the membrane membrane. During the first vacuum preloading process, because the geotextile inside the partial membrane trench was not laid to the bottom, the pressure under the membrane reached a certain level during the initial vacuum, and the membrane was adsorbed on the sand and caused damage. The decrease occurred and the repair was very difficult. In the construction of the back area, the experience was summed up. No such accident occurred and the vacuum degree under the membrane was stabilized.

After comparison and experience summary, the sealing film adopts three-layer blow-moulding polyvinyl chloride film. The biggest advantage of this plastic film is that after a special process, it replaces the previous film-pressing method, so it has no fine hole, uniform material, and strong flexibility. According to the actual size of each preloading zone, each side adds 6m length order sealing film, and the sealing film is heated and processed at one time in the factory. To ensure the safety of the sealing film during transportation, the sealing film is fully enclosed and wrapped with woven cloth. Sealing film performance indicators are as follows: After the sealing film is laid on the film carefully check the broken mouth, the general rupture port appears in the seam of the sealing film, the rupture should be timely repaired with PVC glue. The second and third layers of sealing film can be laid sequentially after inspection without defects, and the adhesive seam of the two layers of the film is staggered by lm. The sealing film at the film outlet of the testing instrument can be left to shrink, which is easy to seal due to the layered sedimentation tube and The water level pipe does not substantially settle in the pre-pressing process, and the sealing film is settling. Therefore, the sealing film is easily torn. Therefore, after the simple sealing and tying, a 0200 cement pipe is installed on the outer side of the water level pipe, and the interior is filled with silt and expansion. Soil, make sure to seal.

Die groove is a key part related to the success of vacuum preloading. It must require that the film contact with the soft soil must have sufficient length. In addition, there is a certain pressure in the film groove backfill to ensure the seal between the seal film and the sludge due to the soft base. The back surface of the treatment area is backfilled with 50 cm of gravel and 3 m of slag. Therefore, it is difficult to construct the membrane ditch and the technical requirements are high. In addition, fishponds are located around the site and there is no suitable space for construction. Therefore, a composite geotextile is laid on the membrane surface and the backhoe runs on it. As the depth of excavation is deep, the pond embankment is liable to landslides and affects the villagers. Therefore, before the excavation, the sandbags, bamboo rafts, etc. shall be used for the support of the surrounding fish pond embankments, and then each excavation section shall be organized to organize composite geotextiles ( The protective edge) and the sealing film are stretched to the bottom of the die groove, and the sealing film is stepped into the mud forcefully, and then the sludge is backfilled in time. Backfill at least 1.5m of silt or clay, backhoe compaction, and then backfill the surrounding filler to 20cm below the membrane surface. The inner side of the mandrel groove is evenly repaired by artificial means. The slope of the silt is smooth and free of hard objects. The depth of the membrane ditch is determined according to the actual conditions on the site, and the cut off of the permeable layer is not less than 1.5m below the top surface of the impervious clay.

Because the original design of Zone 1 and Zone 2 is connected with a common membrane trench, but considering the backfilling of the slag to a depth of 5m, the possibility and safety of the opening cannot be guaranteed. Therefore, in order to ensure the quality of the project, it has been studied with the owner and the supervisor. We have merged District 1 and Excavation 2 into one area with an area of ​​43,000 m2. The two sealing films in the area have been heated together to form a whole site. Practice has proved that our solution is feasible. Currently, this reinforcement film is The vacuum degree has been reached - The IS vacuum pump system selected this time is a product developed by Guangzhou Sihang Engineering Technology Research Institute. It replaces the previous 3BA-9 vacuum pump with motor and pump combination. It has high efficiency and convenient disassembly and assembly. Small size, low damage and other advantages. There are a total of 119 vacuum pumps installed in the direct air preloading project of Erhuan West Road in Huzhou. The control area of ​​each pump is approximately -100 kPa on the IS type vacuum pump, which is far from the vacuum degree of traditional 3BA-9 vacuum helium pumps (-kPa). (Left and right), so the vacuum pressure delivered to the membrane can reach -90kPa or more, and the most helium can reach - %kPa. At the beginning of vacuum, in order to prevent vacuum preloading from causing instantaneous damage to the surrounding soil of the reinforcement zone and transient high load caused by the transformer Damage, strictly control the vacuum rate, generally half of the vacuum pump is turned on first, and then the number of vacuum pump stages is gradually increased. Carefully inspect the presence of leaks on the membrane surface and at the membrane groove and remediate them in time. Check the connection of the vacuum pump system one by one to ensure that the vacuum degree on the pump can reach -0.096 MPa or more when the gate valve is closed, so as to ensure that the vacuum pump system performs its best function. When the vacuum degree reaches 60kPa, after checking the no-leakage phenomenon, the membrane surface is filled with water and all the pumps are opened. The vacuum degree under the membrane is lifted to 80kPa. The vacuum degree under the membrane in all the reinforcement zones is above -90kPa, and the reinforcement is completed. The effect is obvious.

During the construction period, on-site personnel are strictly managed to observe changes in the degree of vacuum and to protect the on-site inspection equipment. At the same time, the stones and other hard objects in the padding are removed. The first level of stacking height is 70cm, and the filling is slag, which is designed for artificial laying. Due to the laying of a layer of composite geotextile, during the construction, our company proposed to begin laying an intermediate temporary road at one end of the reinforcement zone. In the construction, all the constructions were replaced by mechanical construction instead of artificial paving. Construction was performed on both sides in sequence and no vacuum preloading was performed. It has caused adverse effects, greatly increased the construction efficiency and shortened the construction period.

4 Analysis of Vacuum Preloading Effect According to the monitoring data of Nanjing Hydraulic Research Institute, there is basically no settlement in each section and the settlement after work is less than 30cm, which satisfies the acceptance criteria for the idle highway.

Table 2 Surface Settlement Set Statistical Table Area No. Calculated Settlement (cm) Actual Settlement (on) Residual Settlement (on) Consolidation Note: Since the vacuum preload settlement is mainly concentrated on the central axis of the consolidation zone, The data in the central axis of the reinforcement zone is dominated.

5 Concluding remarks Through the construction of this project, the IS-type vacuum system and wide-salt blown-seal film are used for the first time in the construction process. At the same time, the use of the filter tube arrangement and membrane groove treatment is more concise. A convenient and effective construction method is used to conduct in-depth research on the vacuum pre-press sealing technology. In addition, the vacuum pre-press treatment area is over 91,000 m2, which is the first time in Zhejiang Province; the single narrow strip shape area is as high as 43,000 m2, wide. The ratio of the web is as high as 15:1, which is the highest in China. The vacuum degree under the membrane reaches -90 kPa within 5 days and reaches up to -96 kPa. The vacuum pump negative pressure source pressure reaches full load -O.IMPa (the pressure of the traditional vacuum pump is around -96 kPa). , creating the most domestic.

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