Technical Points for Vacuum Preloading in Large Land Reclamation Projects

The characteristics of vacuum preloading make it very suitable for large-scale dredging and dredging soil-building projects. To make full play of its advantages, it is necessary to closely combine its technical key points with the landfilling and dredging land reclamation project.

1.1 Vacuum preloading block

For large-area vacuum preloading, first of all, scientific and reasonable plane segmentation should be carried out. Whether the block is reasonable or not directly affects the construction period and cost, the larger the single block processing area, the shorter the construction period and the lower the cost, but it is subject to the current vacuum preloading. 

With the limitations of sealing technology, it is impossible to limit the processing area of ​​a single block. At present, the area of ​​a single block can reach 5*10000 square meters, but according to past experience, it is generally suitable to divide the block by 3*100000 square meters per block. A plurality of single blocks can be combined into one zone, and each can be constructed synchronously. Flowing operations are performed between the zones to reuse the vacuuming equipment and reduce the engineering input. The division of the blocks should be combined with the design load and reinforcement requirements of the site.

1.2 sand cushion

Sand pad material selection, coarse sand, mud content should be less than 5%, the thickness of the sand cushion layer depends on the construction process, if the backfilling method by artificial road, the thickness should not be less than 50cm, if it is filling, sand cushion Should be thicker.

The physical and mechanical properties of the flushed silt produced by landfilling and landing are extremely poor, and the surface of the dredged silt must be treated to ensure the smooth follow-up construction. Before the construction of the sand cushion layer, a layer of geogrid and geotextile can be laid in the area with large sludge content. If the treatment of the geotextile fabric is still carried out, the mulled mud phenomenon may occur, and the 801cm-1m thick medium may be laid first. Fine sand cushion, re-laying, coarse sand cushion, which can guarantee the quality of medium and coarse sand cushion, so that it has good horizontal drainage effect.

For the dredging process, in order to ensure the flatness of the sand cushion layer, an energy-eliminating head should be provided at the outlet of the blow-fill pipeline to reduce the flushing of the pipe head to avoid a large punching hole at the position of the pipe head.

1.3 Insert Plastic Drainage Vertical Drains（PVD Wick Drains）

As a vertical drainage channel for vacuum preloading, the plastic drainage board not only plays the role of vertical drainage, but also accelerates the consolidation of the soil, and plays a role in transmitting the vacuum. It must be fully functional, except that the material itself needs to have a good In addition to the water permeability, the spacing and depth of the installation are also very important. The drainage plate spacing is determined by the Taishaji and Balun consolidation theory according to the consolidation index of the foundation and the treatment period. The depth of the setting is determined according to the depth of the soft soil. In principle, it is required to penetrate the soft soil layer.

If the drainage board needs to penetrate the sandy soil that communicates with the outside world, the clay and the content of the clay should be analyzed. If most of the soil samples contain more than 15% of the powder and clay content, and there is cementation, it will not affect the vacuum preloading. Otherwise, the drainage board should be prevented from penetrating the soil layer or other special areas. Reinforcement plan.

1.4 Setting Sealing Wall

To ensure the reinforcement effect of vacuum preloading, effective sealing of the reinforced soil is a key part. The sealing form around the reinforcement zone is sealed groove and sealing wall. Due to the excessive water content of the surface dredger fill or the mud, or the thickness of the sand layer is large, it is not suitable to adopt the form of the seal groove. Therefore, the seal wall is generally used to seal the periphery of the reinforcement zone, due to the mud resources of the reclamation land. It is very rich, so it can be used as a sealing wall with mud mixing piles.

Since the construction of each block is carried out, a common sealing wall will appear between the adjacent two reinforcing blocks. During the vacuuming process, the mud particles in the wall will be affected by the soil shrinkage in the reinforcement area. The vacuum pressure is taken away, and the long time will cause the mud mixing pile to lose the sealing effect. For this reason, for the adjacent area where the vacuuming time interval is more than 15d, the common sealing wall should adopt a double-row mud mixing pile with a combination of length and shortness. This problem is solved by increasing the upper width of the wall. The length of the short pile is generally 3-4 meters, and the length of the long pile is determined according to the geological conditions.

1.5 Vacuum Preloading Soft Soil Processing

The vacuum system includes a filter tube, a filter tube outlet device, and a vacuum pump.
The filter tube is made of pvc plastic tube, the spacing of the filter tube should not be more than 6.5 meters, the depth of the filter tube is 0.3cm, and the vacuum gauge under the membrane is arranged between the two filter tubes.

The sealing film is made of 2 layers of polyethylene film, the size of which is predetermined according to the plane size of the reinforcement zone. For the adjacent two reinforcement zones, a narrow strip of vacuum film can be pre-buried in the common sealing wall in the later construction area. The remaining membrane joints are then bonded during subsequent construction. The second sealing membrane of the former zone is covered with the upper part of the common sealing wall and bonded to the sealing film of the latter zone to ensure that the sealing wall is sealed during vacuuming. There will be no air leaks.

Vacuum pump-7.5kw is arranged in a set of about 800 square meters, according to the order of motor, water pump, water tank, gate valve, check valve, and film outlet. The joints are sealed with rubber mat, glass glue, etc. The mouth and tank inlets are kept at the same level.

1.6 Water Cover on The Sealing Membrane

After the vacuum is started and the vacuum under the film reaches 60 kPa, the film can be covered with water. The effect of water covering is: a, to avoid the aging of the sealing film under the long-term exposure of sunlight, so that cracking is easy, b, the tiny pores in the reinforcing zone are filled by the infiltration of water, and the sealing property of the reinforcing zone can be further improved. Strengthen, c, the weight of the water increases the preloading load, which can speed up the construction progress.

Under normal circumstances, the depth of overburden does not exceed 0.5m. It is pointed out in particular that seawater is very cheap and convenient for reclamation land. If the depth of overburden is properly deepened, the construction period can be exchanged at a very low cost. Overburden preloading is gradually being promoted.

Vacuum Preloading Soft Soil Improvement Technique Application Prospects In Land Reclamation

With the rapid development of our social economy, the land resources available for economic construction in the coastal areas are becoming more and more tense. Therefore, the pace of reclamation and land reclamation is accelerating with the needs of economic development, and its engineering technology is also constantly developing and maturing.
In recent years, there are mainly two ways to reclaim land for land reclamation. One way is to open a mountain for reclamation, and the other is to make land by dredging the dredged soil. The former method is more and more damaging to the environment due to high cost, and its use is increasingly restricted by government departments. The latter method is not only low in cost, relatively environmentally friendly, but also has a short construction period. More importantly, with the increase of port construction, the landfilling of dredged soil can minimize the external throwing of dredged soil in port engineering. While greatly reducing the cost of dredging projects, the use of dredged soil as a reclamation resource, so that the dredged soil that should be discarded has a value, can be described as a multi-pronged.

Because landfilling by dredging and filling land has great advantages in both technical and economic benefits, this land reclamation method is more and more accepted by people and has been widely used in reclamation projects in coastal areas. . This land reclamation method also brings greater application prospects to vacuum pre-loading foundation treatment technology.

The application prospect of vacuum pre-loading in land reclamation

The purpose of reclamation is to provide land areas that can be used for economic construction. If the site is to meet the requirements of use, ground treatment must be carried out. Vacuum preloading is widely used in large reclamation projects because the technology has the following characteristics:

1) Save pre-compressed materials and reduce project cost. Vacuum preloading uses atmospheric pressure as the motive force for preloading. On the one hand, it does not need to use a large amount of piled materials, such as sand and stone, as preloading load, which solves the trouble of supplying preloaded materials, and on the other hand, the atmosphere Pressure is a free resource, which reduces engineering costs, especially for large-area soft-base treatments, which can greatly save engineering investment.

2) Fast consolidation and short construction period. Since the foundation does not undergo shear deformation under the action of vacuum pressure, no matter the degree of vacuum during the loading process, the foundation will not be unstable when applied once. With the maturity of vacuum preloading reinforcement technology in China, the vacuum degree under the film can reach about 85 kPa. Although the speed of consolidation does not depend on the magnitude of the preloading load, the overload is greater when the load is equal. The speed of residual settling must be faster. Since the load of the vacuum preload is large and the loading process is fast, the reinforcement time is short.

3), The construction is simple, the vacuum preloading is achieved by pumping air, then the unloading can be stopped as long as the pumping is stopped, that is, there is no problem that a large amount of unloading sand needs to be treated, which is in areas with poor external traffic conditions, It looks very good. In addition, vacuum preloading does not require the use of a large number of construction machines during loading, construction disturbance is small, construction accidents are not easy to occur, and after the required vacuum is stably maintained, the remaining work is only daily inspection and maintenance.

4), The reinforcement effect is good, the degree of consolidation of the vacuum preload can reach more than 90%, which can eliminate most of the settlement of the foundation. Moreover, since the atmospheric pressure is evenly distributed throughout the reinforced zone, the vertical deformation of the soil is also uniform throughout the reinforced zone (constrained by the surrounding soil, and the settlement at the boundary of the reinforced zone is slightly smaller), so The vacuum preloading strengthens the soft foundation, and the uneven settlement of the site will be smaller.

For sites with large ground design loads, such as container yards, if the preloading scheme is used, when the treatment area is large, the amount of earthwork used for the loading will be very amazing. Due to the impact of the stacking process, the construction period will be It has become very amazing. Due to the impact of the stacking process, the construction period will become very long. For example, in a project in Nansha, Guangzhou, the disposable treatment area is 1.50*10~5 square meters. If the preloading scheme is used, the purchased heap is required. Load sand 4*40~5 cubic meters, then divide the treatment area into three large blocks and use the piled sand in stages. The required construction period is not less than 2 years, and the construction cost reaches 400 million yuan. In the case of increasingly tight supply of sand and gravel resources, It is very difficult to obtain such a large amount of sand in a short period of time. The handling of a large amount of unloading sand is also extremely difficult. What is more important is that even if the engineering cost and construction difficulty are aside, such a long period of time cannot be used for investors or Accepted by the owner.

Because the vacuum preloading soft soil improvement has the advantages of low cost, short construction period, saving earthwork and ensuring the reinforcing effect, the treatment of large-area deep soft soil, especially the port engineering with relatively high ground design load, has the advantage that the preloading can not be compared. As mentioned above, the Guangzhou Nansha foundation project uses the vacuum preloading scheme to not only greatly reduce the project cost, but also the total processing time is only one year. Obviously, the application prospect of vacuum preloading in the landfilling project of dredging and filling soil is Very bright.