How to Improve Soil Properties by Vacuum Preloading Method?

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The vacuum preloading method uses atmospheric pressure as a temporary surcharge to accelerate soil settlement and improve its characteristics. The process involves the application of vacuum pressure into a separated soil mass to decrease atmospheric pressure and pore water pressure, resulting in soil consolidation.

The vacuum preloading technique was proposed by Kjellman in 1952 to enhance the properties of fine-grained soil. It is suitable for various applications such as land reclamation and subgrade projects.

The method can be carried out at a low cost, without heavy machinery, and significantly reduces the time of construction. However, the major challenge in the process is maintaining adequate vacuum pressure.

How to Improve Soil Properties by Vacuum Preloading Method?

1. The arrangement for vacuum preloading involves a drainage system, sealing or isolation system, and vacuum pumps, Figure-1.
2. The drainage system consists of interconnected prefabricated vertical drain, horizontal filter pipes, and a sand layer. These components create a path for channelizing the vacuum pressure and water flow, Figure-2.
3. The thickness of the sand layer ranges from 0.3 m to 0.8 m, Figure-3.
4. The sealing system is used to prevent leakage of water and the air below.
5. The isolation system is made up of geomembrane (Figure-4), the soft clay itself, and the cutoff wall (slurry wall), if necessary.
6. Vacuum pumps generate a vacuum suction that spreads into the soil along the drainage system.
7. The area covered by one pump is based on its power; for instance, a pump with a power of 7.5 kW is used for an area larger than 1000m².
8. The vacuum suction decreases the atmospheric and pore water pressure, thereby creating a pressure difference between vertical drains and pore water in soils.
9. The pressure difference causes the pore water to flow towards the vertical drain, resulting in soil consolidation. Vacuum suction keeps taking out water and air and accelerates the process of soil consolidation.
10. A pressure of up to 80 kPa surcharge can be exerted on soft ground without stability problems.
11. Install vacuum gauges to monitor vacuum degree under the membrane, Figure-5
12. Terzaghi and Barron theories can be used as a base for the design of a preloading vacuum.
13. The factors like soil properties, site conditions, vacuum, drainage system, and the purpose of soil improvement control various design parameters such as drainage spacing and depth, period of surcharge and vacuum pumping, sealing measures, and individual block area.
14. Prefabricated vertical drain spacing may range from 0.7 m to 2.7 m. Keep a balance between cost and time while prefabricated vertical drain spacings are established.  
15. The depth of the vertical drains relies on soil profile. The usual treat depth is 20 m and exceeds up to 40 m in exceptional cases.
16. Terminate vertical drains around 1 m above the bottom of the soft soil layer.
17. Vacuum pressure creates an inward force into the subsoil. As a result, there is a slight chance of sliding failure. So, the influence of vacuum pressure on adjacent structures is low.
18. Assess soil settlement through the use of settlement plates. Place the settlement plates on the surface of a geotextile.
19. In a contaminated land, the system can collect water that can be treated later.
20. It has been used for land reclamation and subgrade projects.
21. If there is a shortage of sand in land reclamation projects, vacuum preloading can consolidate excavated and dredged soil to be used instead of sand.
22. The vacuum preloading is cost-effective and needs a short period to achieve a designated settlement. Nonetheless, it may cause uneven settlement.

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