Newly developed SKK method for the improvement of soft ground
Tetsuji Ozaki , Shigeyoshi Takahashi
Asahitechno Corporation, Tokyo, Japan, asahi_tokyo@space.ocn.ne.jp
The development of alluvial plain with constructing many infrastructures has been accelerated in many countries. One of
the serious problems facing the construction of some big structures such as airports, highways and so forth on alluvial
plains is improvement of soft ground to increase bearing capacity.
For the improvement of soft ground, drainage by a consolidation method is often used. In this method, the ground iscompressed, and then density is raised and intensity is increased. Two techniques have been commonly used.
One is consolidation settlement by embankment surcharge and another is by vacuum consolidation. The compressiveforce in the former is the weight of the embankment, and in the latter it is negative pressure. In a typical case, theembankment surcharge method requires a longer time for consolidation. On the other hand, the cost of the vacuum
consolidation method is higher than the embankment surcharge method. Due to this, a new method has been expected
in order to accelerate the consolidation processes and to reduce the cost.
The SKK (Su‐Kara‐Kahn) method proposed here is one of the drainage by consolidation methods and the advantage of thisSKK method is to considerably accelerate the consolidation processes.
In this method, the pore water can be rapidly drained by the combined use of vacuum pump, submersible pump, blowerand compressor. At first the vacuum pump and submersible pump are used to lower the groundwater level, and then all of
the equipment’ are used to drain the pore water.
As a result, compressive forces obtained by this method are the new effective stresses due to lowering groundwater leveland negative pressure. In addition, the combined use of the embankment surcharge method with this SKK method ispossible to accelerate the consolidation processes and decrease the time and cost for the ground improvement.
In this study, the SKK method has been applied to soft ground in several countries such as Japan, Korea, Singapore, and etc.
In conclusion, the SKK method is very preferable for the improvement of soft ground.
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Newly developed SKK methodfor the improvement of soft ground

Tetsuji Ozaki , Shigeyoshi Takahashi
Asahitechno Corporation, Tokyo, Japan

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Contents
1. Consolidation Dewatering Method
2. Outline of SKK Method
3. Example of Consolidation Dewatering
using SKK Method
4. Highlighted Items

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１ Consolidation Dewatering
MethodDefinition of Consolidation
Increase in strength of soft soil (clayey soil) due
to artificial compression
Dewatering of pore‐water due to compression ofsoft soil (clayey soil) and replacement of voids
with soil particles
Increase in density of soil and consequently
increase in strength of soil

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① Embankment Surcharge Method The compression of the ground due to weight of theembankment
 Replacement of voids with particles due to compression⇒ Increase in density and strength Long drainage time from clayey soil due to low permeability
 Embankment speed to be limited due to excess pore‐waterpressure (Without lowering ground water level) Shortening drainage time using vertical drain installation
 Drainage of pore‐water to the surface ground throughvertical drain More than one year is needed for completion of thedrainage

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Embankment Surcharge Method6

② Vacuum Consolidation Method
 Installation of vertical drain and drainage of ground water using vacuum
pump and consequently compression of the ground due to negative
pressure Drainage of pore‐water pressure due to negative pressure andreplacement of void with soil particles⇒ Increase density of ground Necessity of closed surface using a sheet or impermeable soil in order toincrease the negative pressure in the ground⇒ Recently development ofmethod of increase in negative pressure
 No limitation of embankment speed because of no increase in negative
pressure (without lowering the ground water level) Drainage of pore‐water through vertical drain to surface ground (upwardmovement) More than six months is needed for completion of the drainage
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Vacuum Consolidation Method8２ Su‐Kara‐Kahn（SKK） Method
Super‐Well‐Point Ｍｅｔｈｏｄ （ＳＷＰ Method）
・・・・・ Groundwater level‐lowering method
・・・・・Uses Submerged pump and Vacuum pump
Strengthens collection power of the groundwater
Su‐kara‐kan Method（ＳＫＫ Ｍｅｔｈｏｄ）
・・・・・ Ground consolidation method
・・・・・ Uses Blower pump and Compressor in addition toSubmerged pump and Vacuum pump
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SKK Method
Compression =Lowering Ground water level + Negative pressure + Embankment
Vacuum Pump
Exhaust Gas
Compressor
Drainage
Embankment

Flow of ground water to surface

BlowerSuction Air pipe

Initial Ground
Water Level
Ground Water Level

Fixed
QinTAKO
Suction of Air

MovableQin TAKO
Supplyof Air↓

Soft Ground(Low Permeability)

Submerged Pump

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Principle of SWP Method
Groundwater

The air in the well is suckedout by vacuum pumpAir（Well tube）（Separated screen）

Decompressed air（‐0.050.07Mpa）▽Groundwater flows toward theholes at the lower part of well
Air pressure and water level iscontrolled to prevent air coming
into the wellA large amount of groundwater canbe pumped up
Rapid groundwater drawdown

PaGroundwater is pumped up bysubmerged pump

Pw

Saturated water withnegative pressure▽P（Holes）“ In the SWP method, groundwater flows toward the bottomof the well due to negative pressure and gravity, and is thenpumped up by a submerged pump”
11Comparison of Methods
MethodCompression
Force

Drainage

ProblemConstruction
PeriodCost

Embankment Surcharge
MethodEmbankment

Vacuum Consolidation
MethodNegative Pressure

SKK Method

Embankment, Negative
Pressure, Reduction of Uplift(Increase in Effective Stress)‐ Improved Permeability
using particle drain‐ Drainage upward‐ Improved Permeability
using particle drain‐ Drainage upward‐ Improved Permeability due
to formation of drainage root
by Qin‐TAKO‐ Drainage downward‐ Increase in pore‐waterpressure‐ Possibility of sliding‐ Limitation ofembankment speed‐ No increase in pore‐waterpressure‐ No possibility of sliding‐ No Limitation ofembankment speed‐ No increase in pore‐waterpressure‐ No possibility of sliding‐ No Limitation ofembankment speed

6 Months ~ 1 Year

3 Months ~ 6 Months

More than 1 Year

Ａ＞Ａ＜Ａ12

1 Comparison of Effective Stress (Compression Force)
Original

VacuumConsolidation

Embankment Surcharge
40KN/m2

water Level

40KN/m2 + α

80KN/m2▽▽ Ground

SKK▽
Embankment
Surcharge
Negative
Surcharge

RemovalUpliftSoil
WeightEffective
StressUplift(Neutral Stress)

Effective Stress

Effective Stress▽

Effective Stress (Large)

Total Stress
Vapor
Drain

2 Comparisonof Drainage▽WaterPressure
Increase▽

Suction Due
To Negative Pressure

GravityForce
Suction DuePTo Negative Pressure
Low Efficiency
of Drainage

Low Efficiency
of Drainage

High Efficiency
of Drainage

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３ Example of Consolidation
Dewatering using SKK Method• Place: Kanto region,independent houses planning area
• Year: 2012
• Objective: Ground improvement
・ Geology： Alluvial cohesive soil layers
Depth around 30m• Improved area: Around 1050m2• Construction Method SKK Method14

Outline of Method

Position of SWP / Settlement Plate,
Surcharge Embankment Area

Arrangement ofSuction Pipe / Supply Pipe

Settlement CurveComparison of Moisture Content (Wn)
Before and After Improvement

Comparison of Initial Void Ratio (eo)Before and After Improvement

Comparison of Consolidation Yield Stress (pc)
Before and After Improvement

Fixed Qin TAKO⬇･･Embankment（H=2m）↑Location of SWP⬆
Movable Qin TAKO

Head of ＳＷＰ
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lifting pipe

Notch tank

Compressor↓
Blower pump
Vacuum pump

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Fixed Qin TAKO

Installation ofwell tube
Movable Qin TAKO

Settlement plate24Gage pressure ‐0.08MPa（Inside of the well）

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４ Highlighted Items
1 Finding the mechanisms of consolidation
・ Comparison of the effect of compression forces
・ Mechanisms of drainage
2 Construction methods
・Using the alternative compression force instead ofembankment（ex. Vibration roller）
・Rationalization of Qin‐TAKO（position, moving, lifting）
・Using the vertical drain for drainage
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