Vacuum preloading (consolidation) method was initially proposed by Kjellman at 1952 and was worked out by the Royal Swedish Geotechnical Institute as a method for improving fine grain soil. This method utilises atmosphere pressure as a temporary surcharge for improving fine grain soft soil.

However, in the several following decades, this method was not widely being applied due to the difficulties for maintaining effective vacuum pressure. With the improvement of methodology and technology of producing better vertical drains and airtight sheets, this method was introduced and applied in many countries. Now, this method is widely applied in worldwide, especially in Euro, U.S.A, China, Japan and several other Asian countries.

Vacuum preloading method was introduced into China in 1960’s but was successfully applied until early 1980’s. Since then, this method has been employed in hundreds of projects in China for ground improvement. Furthermore, the development of construction machinery and technique allowed the method been applied in various soil conditions or site conditions.

The basic idea of vacuum preloading method is applying a vacuum suction into an isolated soil mass to reduce the atmosphere pressure and pore water pressure in the soil, resulting soil consolidation and effective stress enhance without change the total stress.

Basically, whole vacuum preloading system consists of drainage system, isolation system and vacuum pumps. Once generated in vacuum pumps, vacuum suction rapidly spreads into soils along drainage system, reducing atmosphere and pore water pressure and forming pressure difference between vertical drains and pore water in soils. This pressure difference cause the pore water flows toward vertical drain which means soil consolidation happens. Vacuum suction keeps taking out water and air and accelerating soil consolidation.

Drainage system is an interconnected network of PVD, horizontal filter pipes and sand layer, forming a complete path for spreading of vacuum suction and water flow.

Isolation system is used to cutoff leakage of water & air below it. It consists of geomembrane, the soft clay itself and cutoff wall if necessary.

In the lateral direction, vacuum suction exerts an inward force into the subsoil which is totally different with surcharge preloading where an outward force is exerted. As a result, in most cases, the sliding failure mechanism could be partly or totally eliminated when applying vacuum preloading method. This is a distinguished advantage when conducting ground improvement on very soft ground or beside existing buildings or infrastructures.

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Bored pile provides economical structural support for heavy load and is widely used in different ground conditions, the depth of processing can be up to 60 meters. There are mainly two types of machines used for site construction: drilled shaft and rotary drilling rig. Comparing that the precast pile is a soil displacement pile casting off-site, the bored pile is a replacement concrete pile that needs to be cast into drill holes in-suit.

Cement-Fly Ash-Gravel Pile or CFG Pile was developed based on stone column. When the stone column is applied in soft soil conditions, stone column is not reliable due to the confine resistance is relatively low. Cement and Flyash is mixed with gravel to work as binder therefore CFG column is a rigid column. Due to the CFG column is rigid column, this method could be applied in most of soil conditions. After many years development, now in most of CFG pile cases, lean concrete is used as more popular filling material. The lean concrete could be low grade (K50) to middle grade (K300). Some advantage of this improvement method is :

• CFG can be applied in most soil conditions, such as peat soil, soft clay, fat clay, sandy soil, complicated layered soil or hard soil conditions.
• CFG could use a lot of combinations of material, such as cement-fly ash-gravel, cement-lime-gravel, cement-sand (mortar).
• CFG could form a column from low stiffness column (similar to DCM column) until high stiffness column (similar to spun pile) which means the bearing capacity of CFG composite foundation could be designed in quite a big range to fit the project requirement.
• CFG pile construction fast and in some cases will be more cost-saving compare to other piles.

Deep Cement Mixing (DCM) is a soil improvement method where the in situ soil mixes with cement or other hardening agents such as (lime, slag, or other binders) at depths by augers. Deep mixing has been mostly used to improve soft cohesive soils, but sometimes it is used to reduce permeability and mitigate liquefaction of cohesionless soils.

The deep mixing method has the following advantages:

• Applicable for most soil types
• Installed at great depths
• Relatively fast installation
• Low noise and vibration level
• Formation of a DM wall for earth retaining and water barrier at the same location and time
• Less spoil soil, especially for the dry method

Dynamic compaction (DC) is a typical compaction method by impact energy. DC compact the soil by dropping a temper from a certain height to the ground. The dropped temper generates instantaneous impact energy to the ground, which shall significantly compact the granular soil in a very short time. The most effective part of such impact energy is the vertical energy which will be decreased along depth. Generally speaking, DC is most effective and economical when the target soil is not deep than 6 m.

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HVDM is a soft soil treatment method that is fast and combines vacuum drainage and deep dynamic compaction in designated cycles, so that soils at the project site can be improved through the effects of lowered water content and increased density. By using this method, soil strength and stiffness are improved, and the total and differential settlements after HVDM treatment are minimized.

The development of HVDM can be traced back to early 2000, when the inventor, Mr. Shi-Long Xu of Shanghai Geoharbour Group, began experimenting with the concept of high vacuum densification and applying it in a large scale to many well-known projects around Shanghai, such as Shanghai Pudong Airport Runway No. 2, Shanghai International Circuit, and the Shanghai port expansion.

Mr. Xu later filed patent applications and received Patent Cooperation Treaty approval for several separate but related soft ground improvement technologies. Among the three main patents are patent no. ZL 01127046.2, involving the use of multiple cycles of high vacuum process and varied dynamic compaction efforts (or mechanical compaction) to reduce water content in soft soils; patent no. ZL 200410014257.9, involving the combined use of surcharge preloading or vacuum consolidation, followed by HVDM; patent no. ZL 200510134966.5, involving the use of HVDM followed by construction of stone columns or other types of composite foundations. After initial successful applications in the Shanghai area, HVDM was expanded into other areas in China and other countries in Asia, such as Vietnam, Malaysia, and Indonesia. Currently, HVDM has become a major method used in land reclamation projects along coastal areas in China, with over 9 million meters square of land treated in the last 7 to 8 years.

HVDM can be described as a fast ground improvement technology utilising drainage, consolidation, and densification principles. HVDM is generally executed in a controlled manner based on feedback of on-site monitoring data collected for quality assurance and quality control (QA/QC) purposes.

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Stone Column (Vibro-replacement) is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increases the amount of densification, but provides a degree of reinforcement and a potentially effective means of drainage. Stone columns have been widely used worldwide for several decades. Most design methods for granular columns were developed based on stone columns. The columns installed by vibro-replacement (also called the wet method) are commonly referred to as stone columns or granular piles. During the tamping process, there is some degree of displacement toward the surrounding soil. There are 2 types of stone column installation method :

1. Wet Method
2. Dry Method

Wet method stone column use water pressure flow through the probe to vibro-replace the soil before the granular fill material is supplied through a feeding tube and form a “stone column”. Dry method stone column is similar to wet method but instead of water, air is used to help the penetration of the probe and maintain the stability of the hole.

Vibroflotation method is a typical compaction by vibration energy. This method sends high frequency vibrating probes into the ground which transfer continuous horizontal-spread vibration energy to granular soil to make soil densified. The improvement of vibroflotation can reach relative deep soil because the vibrating probes can be pressed and lifted by tubes. Vibroflotation is more effective and economical for soil depth beyond 6 m.

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