Course : S0892 - Ground Improvement
Method
Year
: 2010
SOFT SOIL
(PROBLEMS & STABILISATION METHOD)
Session 2 - 7
COURSE 2
Content:
• Soft Soil Problems
• Stabilization Methods:
–
–
–
–
–
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Preloading & Vertical Drain
Stone Column
Dynamic Compaction
Chemical Stabilization
Reinforcement
SOFT SOIL PROBLEMS
• LOW BEARING CAPACITY
• HIGH SETTLEMENT
• LIQUEFACTION
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SOFT SOIL PROBLEMS
LOW BEARING CAPACITY
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SOFT SOIL PROBLEMS
SETTLEMENT
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SOFT SOIL PROBLEMS
LIQUEFACTION
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STABILIZATION METHOD
PRELOADING & VERTICAL DRAIN
• Encountered problems on fine grained
soft soil are low strength, and high
compressibility
• Preloading by surcharge embankment
applied to reduce compressibility and
increases soil strength
• Fine grained soil possessed very low
permeability that consolidation process
takes very long time
• Vertical drains provide drainage paths
to reach the surface
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PRELOADING & VERTICAL DRAIN
BASIC PRINCIPLE
Surcharge
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PRELOADING & VERTICAL DRAIN
BASIC PRINCIPLE
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VERTICAL DRAIN DESIGN
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VERTICAL DRAIN DESIGN
D = 1.05 S
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D = 1.13 S
VERTICAL DRAIN INSTALLATION
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VERTICAL DRAIN + VACUUM
• Vacuum load can reach greater than 80kPa (equals to 4 – 5 m
height preloading fills with g = 17 – 18 kN/m3)
• Rapidly decreases excess pore water pressure
• Inward lateral movement
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VACUUM CONSOLIDATION
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CONVENTIONAL VD VS VACUUM
CONSOLIDATION
CONVENTIONAL VERTICAL DRAIN
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VACUUM CONSOLIDATION
STABILIZATION METHOD
VIBRO COMPACTION
•
•
•
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Compaction of loose granular soils
by penetration of a vibratory probe
or vibroflot.
Should combine with sand/stone
column if applied in fine grained
soil
Affected area by the compaction
energy : 1.5 - 4 m
STABILIZATION METHOD
STONE COLUMN
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STONE COLUMN DESIGN
BEARING CAPACITY SINGLE
STONE COLUMN
Formula 1 
 '

 v  hs . tan 2  45o  v 
2 

Formula 2 
v  25.cu
Formula 3 
v  4.PL
Where:
Allowable Bearing Capacity

qc(all )  v
FS
FS = Factor of Safety = 3
v = vertical stress in stone column
hs = passive resistance of surrounding soil (the effect of loading shall be considered)
v’ = effective friction angle of stone column (35o – 40o)
cu = undrained cohesion of surrounding soil
PL = Pressuremeter limit presssure
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STONE COLUMN DESIGN
Available design methods with respect to settlement:
(1) Equilibrium Method
(2) Priebe’s Method
Derived from unit cell idealisation, stone column is
(3) Granular Wall Method
modelled to be a concentric body in a composite soil
mass.
(4) Greenwood Method
(5) Incremental Method and
(6) Finite Element Method
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STONE COLUMN DESIGN
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STONE COLUMN NOTES
• Typical design values per column in range of 200 to 300
kN/column
• Young modulus of stone column in range of 40 to 70
Mpa
• The settlement of single stone column under design load
usually in range of 5 to 10 mm
• The maximum settlement of sotne column group is
100mm
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STONE COLUMN INSTALLATION
vibrator makes a hole in
the weak ground
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hole backfilled
..and compacted
Densely compacted stone
column
STONE COLUMN INSTALLATION
WET METHOD
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STONE COLUMN INSTALLATION
DRY METHOD (TOP-FEED METHOD)
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STONE COLUMN INSTALLATION
DRY METHOD (BOTTOM-FEED METHOD)
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STONE COLUMN INSTALLATION
BOREHOLE METHOD (RAMMED COLUMN)
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VIBROFLOTATION
Vibro Compaction Method by using a self vibrating probe or a vibroflot
Vibroflot (vibrating unit)
Length = 2 – 3 m
Diameter = 0.3 – 0.5 m
Mass = 2 tonnes
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VIBROFLOTATION
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VIBROFLOTATION INSTALLATION
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STABILIZATION METHOD
DYNAMIC COMPACTION
• A method of improving the ground using that
involves very high energy waves by hammering
the earth
• Pounders weighing 15 to 40 tons are released
in free fall from a height of 10 to 40 m.
• Most suitable for granular soil
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DYNAMIC COMPACTION
Cost effective method for:
- sand densification
- soil compaction
- ground compaction
- land fill treatment
- unconsolidated fill or soil treatment
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DYNAMIC COMPACTION
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DYNAMIC COMPACTION
For design, suitability assessment, and determination of
optimum field operation parameters rely mainly on :
- Empirical Equations
- Field Pilot Tests
- Past experiences
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DYNAMIC COMPACTION
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DYNAMIC COMPACTION DESIGN
• Effective Depth Improved
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DYNAMIC COMPACTION DESIGN
• Applied Energy
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DYNAMIC COMPACTION DESIGN
• Design Chart
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DYNAMIC COMPACTION DESIGN
• Design Procedure
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DYNAMIC COMPACTION MECHANISM
IN COHESIONLESS SOIL
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DYNAMIC COMPACTION NOTES
Important notes for deep compaction
(1) Effective depth will not exceed 15 m in practice (Lukas, 1986).
(2) Depth of compaction (D) is proportional to the square root of the impact energy (Metric
Ton*Meter) /drop. Different relations have been proposed.
D = (W*h)1/2 for cohesive material (Menard and Broise, 1975)
D = 0.5 (W*h)1/2 for cohesionless material (Leonards et al., 1980)
Actually, D = 0.3~0.7 (W*h)1/2 from field observation. Use a higher value for loose soils.
(3) Maximum improvement occurs within a zone between 1/3 to 1/2 the depth of compaction.
(4) If a line is attached from the crane to the weight, the efficiency of the energy reduces by 20%
(Lukas, 1986).
(5) The ground water has to be at least 2 m below existing ground or 0.6 m below the bottom of
craters.
(6) An area of 5,000 to 10,000 m2 is required to be economical.
(7) Vibration and noise (115 to 120 dB at source) may be a concern to the nearby developments.
Figure 1.3 shows the estimated vibration velocity due to the compaction (Lukas, 1986).
(8) Minimum 34 ~ 50 m clearance from any structure.
(9) The number of repeated drop on the same spot should be limited to drops (Lukas, 1986)
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STABILIZATION METHOD
CHEMICAL STABILIZATION
• INJECTION GROUTING
• DEEP SOIL MIXING
• JET GROUTING
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CHEMICAL STABILIZATION
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INJECTION GROUTING
1. install grout pipes
using drilling or
driving techniques.
2. The mortar-like grout,
injected under certain
pressure through the
pipes.
The grout pipe is then lifted
some distance (0.3 to 1.5
m), and the injection
process is repeated.
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3. Grouting can stiffen and
strengthen the soil layer
by in creasing its density,
and acting as a
reinforcement.
Grouting may also be used to
re-level a structure that
has been damaged by
differential settlements.
DEEP SOIL MIXING
• Soil Mixing is the mechanical
blending of the in situ soil with
cementitious materials.
• Strengthen soft and wet
cohesive soils in a very short
time period
• Treatment is possible up to
depths of 30 m
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DEEP SOIL MIXING
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JET GROUTING
• Uses high kinetic energy (20-60 MPa) liquid spurt (jet) for chopping
up the ground around and mixing it with binding agent.
• Can be used for treating most soil types
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JET GROUTING TYPES
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JET GROUTING
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CHEMICAL STABILIZATION – RESULT
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STABILIZATION METHOD
REINFORCEMENT
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REINFORCEMENT
BASIC PRINCIPLE
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REINFORCEMENT MATERIAL
GEOSYNTHETIC
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