ground improvement

Ground Improvement

ByNeeraj Singh

WHY GROUND IMPROVEMENT.• Development of infrastructures in cities

compelled the engineers to improve the properties of soil to bear the load transferred by the structures.

• As the land available may not be suitable for the structural load.

• The purpose of these techniques to increase bearing capacity of soil and reduce the settlement to a considerable extent.

Effects Of Ground Improvement• The process reduces the permeability

of soil mass.• The process reduce compressibility

and consolidation.• The process improves the bearing

capacity of soil mass.• The process is used to make an area

trafficable within short period of time for emergency & military purpose.

Methods for Soil ImprovementGround

Reinforcement

Mechanically Stabilized

Earth

Geosynthetics

Grouting

Vibrofloatation Column

Reinforced earth

GroundImprovement

Deep Dynamic

Compaction

Electro-osmosis

Pre-loading

GroundTreatment

Fly ash

Soil Cement

Heating/Freezing

Lime Admixtu

res

Dynamic compaction• This involves dropping heavy weights onto

the ground.• The weight causes the ground to compact.• The depth of influence D, in meters, of soil

undergoing compaction is conservatively given by

• D ½ (Wh)1/2

• W = mass of falling weight in metric tons.• h = drop height in meters

Dynamic compaction• Ground is consolidated by repeatedly

dropping dead weights and specially designed tampers

• Weights include: Flat bottomed and cone tampers

• Traditional weights are flat bottomed with cable

• Modern systems use cones with guide rails• Dynamic compaction is suitable for

granular soils, made-up and fill sites.

Typical weight (mass) 7-11 tonnes

Tamer drops and exerts known impact energy on strata

Pass 2 Pass 2 Zone compacted 2nd Pass

Zone compacted 1st Pass

Pass 1 Pass 1

Zone compacted 3rd Pass

Sound strata

Pass 1 and pass 2

Pass 3

50 – 150 kN/m2 Typical bearing capacity

Required treatment depth

Dynamic compaction

First drop Second drop

Plan of first two passes for compaction

9

Dynamic Compaction Example

Vibrofloatation• This technique involves densification

of granular soil using a vibratory probe inserted into ground.

• Relative density of up to 85% can be achieved.

• It is successful in loose sand soils up to 30 m and not applicable to clays(cohesive soil).

Vibrofloatation

Vibro floatation process.• The depth vibrator is positioned over the compacti

on point. Flushing, using water or air, is commenced and exits through jets in the nose cone of the vibrator.

• As a result of the induced vibrations and jetting, the soil is temporarily liquefied enabling the depth vibrator to penetrate the soil under its own weight.

• The vibrator has reached the specified depth. Flushing is stopped. The soil is densified by the vibrator induced vibrations.

• As the depth vibrator in slowly withdrawn, usually in stages of around 0.3m, a cylindrical compaction zone with a diameter of 2.0 to 4.0 m is formed around the vibrator.

Vibrofloatation

Freezing• On freezing, water expands in volume by about

9% .• Reducing temperature till freezing point at which

pore water freezes and soil is stabilized.• The strength achieved depends on freeze

temperature, moisture content and the nature of the soil.

• This method is costly and maintaining a low temperature is required.

• This is used while construction of tunnels through loose soils.

Heating• As soil is heated it water content is reduced

which causes settlement in soil.• Heat treatment of a clay soil to about

100°C the absorbed water is driven off further increasing strength.

• Heat treatment of a clay soil to about 400°C makes it non expansive and hard durable.

• It is expensive method due to cost of fuel usage and rarely used in practice.

Preloading• Preloading is a process to apply surcharge load on

to the ground prior to the placement of structure to consolidate the soil.

• Primary settlement will occur so as to increase the bearing capacity and reduce the compressibility of weak ground as water is flushed out.

• The temporary surcharge applied on the ground is generally more than the expected bearing capacity.

• It is more effective for soft cohesive ground and is removed after completion of process.

Preloading

Q1=actual structure Q2=preloading weight

Sand drains• Vertical drains are used to provide radial

drainage and accelerate the rate of consolidation by reducing the drainage paths.

• Accelerate the process of settlement in order to and gain in strength of soft cohesive soil.

• Vertical drains accelerate primary consolidation only.

• Secondary settlement is not speeded up by vertical drains.

Sand drains

GROUTING• Grouting is a process of ground improvement

attained by injecting fluid like material into subsurface soil.

• Grouting is the injection specially formulated cement of stable suspensions or liquid into pores, fissures or voids, or the jetting of cement mixtures at high flow rate and pressure into the soil to create soil- cement to increase the strength.

• Cement grouting.• Bituminous grouting.• Chemical grouting.• Polymer grouting.

MATERIALS USED FOR GROUTING

• It is a technology in which high- pressure jets of cement grout are discharged sideways into the borehole wall to simultaneously excavate and then mix with the soil.

• The outstanding feature of jet grouting is the ability to treat a whole range of soils, from silty sands to cohesive deposits, by means of simple cement grouts.

• Jet grouting can be performed in soils with a wide range of soils and permeabilites.

JET GROUTING

Jet grouting

Reinforced earth• In this method soil is stabilized by introducing

thin strips in between such as thin metal strips or geosynthetics mesh.

• The mesh are parallel to each other and are connected to a barrier to keep soil contained.

• Barrier is extended into the backfill to prevent overturning and are made up steel, aluminum reinforced or plastic.

• Galvanized steel strips are used as reinforcement with 50-100 mm wide and several meters in length.

Reinforced earth• Friction develops between the

mesh and soil by which forces developed in the soil are transferred to the barrier.

• Back fill soil most likely used will granular soil.

• Barrier bottom end rest on small plain concrete footing.

Reinforced earth

Lime stabilization.• Lime stabilization is done by adding

lime to soil and is economical method of stabilization.

• Lime stabilization is used or clayey soil and is not effective for sandy soils.

• The reaction is very quick and stabilization of soil starts within few hours.

CHEMISTRY OF LIME TREATMENT• Drying: If quicklime is used, it immediately

hydrate and releases heat. Soils are dried, because water present in soil evaporate due to heat.

• Modification: After initial mixing, the calcium ions (Ca++) from hydrated lime migrate to the surface of the clay particles and displace water and other ions. The soil becomes friable and granular, making it easier to work and compact.

Advantages.• Swell reduction.• Improved stability.• Reduction in moisture-holding capacity

(drying).• Lime stabilization is not difficult to carry

out.• There is considerable savings on

aggregate and disposal charges i.e. same soil is used for stabilization.

Lime stabilization.

Fly–Ash & its use in stabilization.• Fly ash is a by-product of coal fired electric power

generation facilities; it has little cementitious properties compared to lime and cement.

• Fly ashes belong to secondary binders; these binders cannot produce the desired effect on their own.

• In the presence of a small amount of activator, it can react chemically to form cementitious compound that contributes to improved strength of soft soil.

• Fly ashes are readily available, cheaper and environmental friendly.

• Soil to be stabilized should have less moisture content for addition of ash.

Cement stabilization• Cement is the oldest binding agent since the

invention of soil stabilization technology in 1960’s .• primary stabilizing agent or hydraulic binder

because it can be used alone to bring about the stabilizing action .

• Cement reaction is not dependent on soil minerals, and the key role is its reaction with water that may be available in any soil .

• Hydration process starts when cement is mixed with water and other components for a desired application resulting into hardening phenomena of cement will enclose soil as glue.

Cement stabilization• It is ideal for granular soil mixed with fines will

require less amount of cement.• Clayey soils require high quantity of cement to

stabilize.• A well graded sandy soil require 5% cement

whereas poorly graded about 9%.for clayey soils 13% may be required.

• The quantity of water used for hydration must be sufficient for hydration and workable mix.

• Admixtures such as lime ,calcium chloride ,sodium carbonate can also be used.

Geotextiles• Geotextiles are any permeable,

synthetic, textile material used with foundation, soil, rock, earth, or any other geotechnical engineering-related material as an integral part of a man-made project, structure or system.

Types Of Geotextiles • Geotextile: Non Biodegradable Synthetics

Woven / Non Woven Fiber and perform discrete function i.e. separation, reinforcement, filtration, drainage and moisture barrier .

• Geomembranes : Geomembranes are also synthetic materials but they are impervious and made of thin sheets of rubber or plastic materials used primarily for lining and cover of liquid-or solid-storage facilities

Material• Natural materials:Natural fibres are

extracted from plants and are then converted into yarns by spinning. Natural fibres such as jute, coconut fabrics.

• Synthetic materials: Geotextile polymers are predominantly polypropylene (PP) (95% at present), polyester (PET) and polyethylene (PE) with some geotextiles (nonwovens) using combinations of polymers.

Woven Geotextiles

Nonwoven Geotextiles

Applications• In separation, layers of different sizes of solid

particles are separated from one another by the geotextile. Geotextiles are often placed underneath riprap to prevent underlying soil from eroding away.

• In drainage, the geotextile allows water to pass and in the special case of drainage "transmission," the geotextile itself acts as a drain to transmit water through soils of low permeability. Geotextiles can be wrapped around perforated drainpipes to filter out fines that can clog them.

• For reinforcement, the geotextile is used as a reinforcing element in the earth through either stress distribution or increase in soil modulus.

• In filtration, the fabric acts similar to a two-dimensional sand filter, allowing water tomove from the soil while retaining the soil.

Applications

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ground improvement

Engineering