ground improvement

07-Feb-19
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INTRODUCTION
What is ground improvement and Why is it required?
Soil as an engineering material has different uses:
• To support and anchor structures
• As a construction material in earth structures such
as embankments, dams, etc.
Soil has many different types of properties. Some of
these properties may fall short of requirement for a
particular type of use of soil.
The measures to improve the soil in situ upon the
deficient properties to suit for the required uses are
called ground improvement methods.
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Common Problems Soils
• Soils with inadequate mechanical properties
• Swelling soil (expansive soil)
• Collapsible soils
• Soft soils
• Organic soils and peaty soils
• Sands and gravelly deposits
• Karst land with sinkhole formations
• Foundations on dumps and sanitary landfills
• Dredged materials
• Soils containing hazardous materials and mine spoils
Introduction
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What can be done when problem soil is
encountered
• Avoid the particular site
• Remove and replace unsuitable soils
• Modify the design of the structure according to
soil condition
• Use effective foundation design for the particular
soil
• Use ground improvement techniques
Introduction
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Hydraulic Methods
Dewatering
• Pumping out
• Electro-osmosis
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Ground Improvement using Electro-osmosis
(Electro-kinetic Stabilisation)
• Electro-kinetic stabilisation is a dewatering technique where an
electrical potential difference is applied through a soil mass.
• The potential difference (voltage) forces the ions dissociated in
groundwater to migrate to the electrodes.
• The anions (-ve ions) move to anodes (+ve electrodes) and the
cations (+ve ions) move to the cathodes (-ve electrodes).
• While the ions move through the pore-water to the electrodes,
they drag water molecules with them. It happens because the
water molecules are dipoles, and there are electrostatic force
between the ions and water molecules.
• The technique is suitable for soils with low permeability (k=10-4
– 10-6 cm/s), where other methods of dewatering do not work,
and where permeation or chemical grouting is not feasible.
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Electro-kinetic Stabilisation
• In clay soil, pore sizes are small, so the permeability is less.
• In addition to that, there are –ve charge concentrations on the
flat sides of clay plates, which attract and attach cations.
• Moreover, there are +ve charge concentrations at the edges of
clay particles, which attract and attach anions.
• These cations and anions attract water dipoles and the water
dipoles become attached with the ions.
• This makes movement of pore water less free.
• When the ions are pulled by an electric potential towards the
electrodes, the ions also pull the attached water molecules. 8

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• Clays having higher cation exchange capacity (CEC) have more
cations which can be replaced or displaced. Such soils are
suitable for electro-osmosis.
• If groundwater has dissolved ionic salts, such soils are also
suitable for electro-osmosis.
• The electrodes, particularly the cathodes are made of hollow
pipes, which are used as wells.
• Water travelling to electrodes are pumped out from there.
• During the electro-osmosis process, many water molecules
dissociate to become H+ and O2- ions. The H+ ions move to
cathodes and O2- ions move to anodes where they become H2
and O2 gases. This also reduces the amount of water in soil.
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 Electro-osmotic discharge, = , where
• ke = electro-osmotic permeability (cm/s per V/cm),
• ie= electro-potential gradient (volt/cm) = voltage applied across the
electrodes divided by the distance between the electrodes, and
• A=area of flow between the electrodes.
 For most soils, ke is in the range of (0.4-0.6) x10-4 cm/s. Unlike
hydraulic conductivity, it does not depend upon grain size.
 The applied voltage should not be more than 0.5 V/cm to
prevent higher loss of energy because of heating.
 The anode tubes are usually 25mm or 50mm in diameter. They
should not be spaced closer than 0.7m, otherwise two or more
anodes may behave as a single one.
 The electrodes may corrode very fast. They should be replaced
when current drops below 30% of the initial.
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Preloading
• Simple preloading
• Preloading with vertical sand drains
• Wick drains or sand wicks or PVDs
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Surface Compaction
• To increase dry density of soil
• Control parameters:
o Thickness of layer
o Moisture content
o Number of passes
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Deep Dynamic Compaction
or Heavy Tamping
• Loose sands or silty soil is densified by impacts of heavy loads
dropped from large heights.
• A weight of 6-20 tonnes of steel or concrete is used
• Drop height is about 10-20m, sometimes even upto 30 m. A crane
is used to lift and release the load.
• The pounding of the soil is done in a grid pattern covering the
whole area to be improved.
• One coverage of the grid is called a tamping pass. More than one
tamping passes may be required at site.
• The heavy impact densifies loose sands and silty soils up to 4.5 to 9
m deep. The depth of improvement depends upon the total
amount of energy as well as the type of soil. A tamper of 90kN
falling through a height of 21-24m may compact sand deposits up
to 7.5-9m depth.
• It provides an economical way of improving soil for mitigation of
liquefaction hazards.
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Deep Dynamic Compaction or Heavy Tamping
Clip1: Dynamic Compaction of Mine Spoils
Clip2: Technique
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• Effective zone of influence – the depth up to which
significant improvement occurs due to heavy tamping.
• Zone of major improvement – ½ to 2/3 of effective
zone.
• Drop energy = energy per blow = weight of the
tamper X the height of fall = WH
• Total energy = energy of all the drops of the tamper =
∑WH
• Effective zone of influence = α√(WH), usually α = 0.3-
0.5, when W and H are in tonne and metre
respectively.
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• A crater is formed where the tamper falls on the ground, it is called
an imprint.
• Local liquefaction occurs beneath the imprints if soil is saturated or
nearly saturated.
• When the excess porewater pressure from the dynamic loading
dissipates, it is called the recovery. Densification occurs upon
recovery.
• The imprints are to be filled with granular fill and may require
shallow compaction.
• In granular soils above water table, the blows form a hard plug of
densified material in the form of a bulb under the imprints.
• In medium to stiff clay, the tamping will cause soil to develop
fractures, leading to expulsion of water and some densification.
Clay soils however, require more number of tamping passes
compared to granular soils.
• This method is not useful in soft saturated clay, as the soil bulges
out.
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• In the first tamping pass, the grid is laid wide, with
increased drop heights. This compacts the deep layers.
• In the second tamping pass, the drop weight or drop
height is reduced and the grid is made at closer
spacing. This pass compacts the layer at medium dept.
• At the end, the grid spacing is further reduced, and at
the same time the drop height or drop weight is also
reduced. This will compact the layer nearer to the
surface.
• However, if foundation level is at more depth, the final
pass may not be required.
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Induced settlement:
Soil type settlement in % of depth
Natural clays 1-3%
Clay fills 3-5%
Natural sands 3-10%
Granular fills 5-15%
Peaty soil 7-20%
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Benefits:
• Economic to improve large open areas
• Quick results
• Good for granular and high organic soils
Drawbacks:
• Not suitable for soft saturated clays or silty clays
• Cannot be conducted in confined built-up areas
• Structural damage may occur to nearby buildings within 20m
(peak particle velocity>40mm/s)
• Minor architectural damage may occur upto a distance of 40m
(peak particle velocity>10mm/s).
• No damage may occur beyond 60m, but it can still cause
annoyance to people because of vibration (peak particle
velocity>2.5mm/s) as well as because of noise.
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Verification of Improvement:
Improvement in soil properties upon heavy
tamping has to be verified and reported.
In-situ tests before and after heavy tamping are
conducted for verification of improvement :
o SPT
o Dynamic Cone Penetration Test (DCPT)
o Static Cone Penetration Test (SCPT)
o Menard’s pressure meter test, etc.
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• Vibro-compaction is a method of creating compacted columns of
gravel or sand-gravel using the mechanism of vibro-floatation.
• Vibroflot is a large and heavy tubular probe, with nozzles at the
bottom as well as at the top ends.
• It is lowered into the ground using a crane, simultaneously forcing
water jets at high pressure to come out through the bottom
nozzles.
• The force of the water jets dislodges soil at the bottom and digs
downward. The weight of the vibroflot sinks it further down in the
liquefied soil.
• The probe is also fitted with an eccentric weight, which can be
rotated about the vertical axis at high speed.
• Rotating it produces a centrifugal force, creating a cyclic vibration in
all horizontal directions, because of which the probe continuously
hammers at the surrounding soil. The vibratory eccentric force
induces densification of surrounding soil.
Vibroflot and Vibro-Compaction
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• It may be difficult to pull out the vibroflot from deep
ground because of friction and soil deposited on top of
it.
• At the time of extraction, water jets are forced out
through the top nozzles, which liquefy the soil
deposited above the vibroflot, and the probe can be
pulled out. Granular fill is poured in to the hole created
by the vibroflot.
• Probes are inserted in grid pattern at a spacing of 1.5
to 3 m.
• Vibratory probe compaction is effective if silt content is
less than 12-15% and clay is less than 3%.
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• Length of vibrating unit –
about 2m
• Diameter of vibrating unit –
about 0.4m
• Weight of vibrating unit –
about 1800 kg
• Speed of rotation – about
1800 rpm
• Centrifugal force – 90-160 kN
(depending upon the mass of
the eccentric weight)
• Water pressure – about 700-
1000 kN/m2
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Spacing
1.8-4.2m
Clip: Vibro-compaction Technique
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Ground Type Relative Effectiveness
Sands Excellent
Silty Sands Marginal to Good
Silts Poor
Clays Not applicable
Mine Spoils Good (if granular)
Dumped Fill Depends upon nature of fill
Garbage Not Applicable
• The sand and gravel particles rearrange into a denser state, with RD = 70-
85%.
• The ratio of horizontal to vertical effective stress is increased significantly.
• The permeability of the soil is reduced 2 to 10 fold, depending on many
factors.
• The friction angle typically increases by up to 8 degrees.
• Enforced settlements of the compacted soil mass are in the range of 2 % to
15 %, typically 5 %
• The stiffness modulus can be increased 2 to 4 fold.
• The sand and gravel particles rearrange into a denser state, with RD = 70-
85%.
• The ratio of horizontal to vertical effective stress is increased significantly.
• The permeability of the soil is reduced 2 to 10 fold, depending on many
factors.
• The friction angle typically increases by up to 8 degrees.
• Enforced settlements of the compacted soil mass are in the range of 2 % to
15 %, typically 5 %
• The stiffness modulus can be increased 2 to 4 fold.
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• The grain size distribution of the backfill material is an
important factor for effective densification.
• Brown (1977) defined an index called Suitability Number
(SN) for rating backfill. The smaller its value, the better is
the suitability of the material as backfill.
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• Blast-densification is a ground improvement technique,
which uses energy from confined detonations of explosive
charges placed within the soil mass.
• It is usually applied for densifying loose, relatively clean,
cohesionless soils.
• It increases the density of loose granular deposits, above or
below the water table.
• The explosive waves temporarily liquefy the soil, causing
the soil particles to rearrange to a higher relative density as
excess pore pressure dissipate.
• It has been used to treat soils to depths of up to 40m.
• As depth increases, the amount of the charge (explosive)
necessary to destroy the soil structure and liquefy the soil
increases.
Blast Densification or
Explosive Compaction
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Blast Densification or Explosive Compaction
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Material and Equipment Required:
• Industrial-Grade Explosives:
Types:
• Nitroglycerin
• TNT, PETN
• Dynamite (nitroglycerine + a clay called kieselguhr)
• Emulsions
• Gels
• ANFO - ammonium nitrate and fuel oil (diesel)
• Detonators
• Fuses
• Electric Cords
Blast Densification or Explosive Compaction
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• Excess pore pressure and settlement due to
explosion are related to the ratio where
Nh= Hopkin’s number = W1/3/R
W= weight of explosives, in equivalent to
kilograms of TNT (1 Kg of TNT ≡ 4612 KJ of
energy)
R= radial distance from point of explosion, m.
• If Nh is less (0.09 to 0.15), liquefaction does not
occur and the equation can be used to estimate
safe distance from explosion.
Blast Densification
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• Following relationships were obtained for
some sandy soils in Netherlands based on
statistical analyses of field results:
ΔU/ σ’= 1.65+0.65lnNh
Δh/h = 2.73+0.9lnNh
• For optimum densification, the ratio of ΔU/ σ’
should be more than 0.8.
Blast Densification
Clip
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Deep Mixing Method
• The Deep Mixing Method
(DMM), a deep in-situ soil
stabilization technique using
cement or other stabilizing
agent, was developed in Japan
and in the Nordic (Scandinavian)
countries in the 1970s.
• Soil is stabilized from a deep
layer to the top layer by mixing
with a binder, usually cement.
• It is a reinforcement method of
soil, like stone columns.
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• The deep mixing equipment consists of
₋ one or more augur-like tool(s),
₋ a binder dispensing system, and
₋ a rig to support and operate the tools.
• The binder or stabilizer or reagent is a
reactive material which strengthens soil.
• Binders may be cement, lime, fly ash,
gypsum, blast furnace slag, or any other
material which reacts with soil and
hardens it. Cement is the most common
binder.
Deep Mixing Method
• Binder may be applied in slurry form, then the method is called
Wet Mixing Method. When it is applied in powder form, the
method is called Dry Mixing Method. 36

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Deep Mixing Method
Image courtesy JAFEC USA
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• After curing, columns of soil-cement are created inside soil.
• Multiple soil-cement columns can be created in one go.
• If adjacent soil-cement columns touch one another, they form
a soil-cement wall.
Deep Mixing Method
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Applications of DMM
• To transfer load from structure to deep soil, like a pile
group do.
• To reduce liquefaction potential of soil.
• Cut-off wall - to prevent seepage.
• Excavation support wall.
• Environmental remediation – to confine harmful waste.
Deep Mixing Method
Courtesy Keller
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Advantages and disadvantages:
• Applicable in all type of soils except soils
having large cobbles or boulders.
• Requires large open space.
• Less noise or vibration as compared to blast
densification or heavy tamping.
• Useful up to a depth of 40m.
• Economic if used in large scale.
• Quick result as compared to PVD.
Deep Mixing Method
Clip1 Clip2
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• This is a variation of deep mixing method.
• It is a mass stabilization method.
• The whole soil is treated to a shallow depth,
leaving no space between the treated columns.
• Dry mixing is used.
• It offers a cost-effective solution for ground
improvement or site remediation when dealing
with substantial volumes of very weak or
contaminated superficial soils with high water
content, such as deposits of dredged sediments,
wet organic soils or waste sludges.
Mass Shallow Mixing Method
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Mass Shallow Mixing Method
Courtesy Keller
Clip 42

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