9a- Ground Improvement Techniques

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APPLICATION OF GEOTECHNICAL ENGINEERINGGEOTECHNICAL ENGINEERING

CE451A Dr. Rajesh Sathiyamoorthy, IIT Kanpur

GROUND IMPROVEMENT TECHNIQUES


Ground treatment is the controlled alteration of the state, nature or mass behaviour of ground materials in order to achieve an intended satisfactory response to existing or projected environmental and engineering actions.

Ground Treatment/Improvement

Ground/Soil Improvement: Enhancement of the inplace properties of the ground/soil by controlled application of technique suited to


of the ground/soil by controlled application of technique suited to the subsoil conditions.

For a particular situation/site more than one method of ground improvement technique may appear to be suitable. In such cases a relative study should be made for a proper selection. If necessary, a combination of more than one method may be more suitable.


• Mechanical properties are not adequate• Swelling and shrinkage• Collapsible soils• Soft soils• Organic soils and peaty soils

Need for Ground Improvement ‐ Concerns


• Sands and gravelly deposits, karst deposits with sinkhole formations

• Foundations on dumps and sanitary landfills• Handling dredged materials• Handling hazardous materials in contact with soils• Use of old mine pits

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Need for Ground Improvement ‐ Concerns

Swelling

Shrinkage


Collapsible soil

LiquefactionSoft clay

Sinkhole


Based on load intensity (exerted by the structure) and the subsoil investigation data for the proposed site, foundation design shall be carried out (including sizing and settlement analysis).

Need for Ground Improvement – Project specific

Ground improvement is needed:• If the net loading intensity of the foundation exceeds the allowable pressure

(IS 6403:1981)


(IS 6403:1981)

• If the resultant settlement (computed as per IS 8009) exceeds the acceptable limits for the structure both view point of distortions induced in the structure and from operation angle [even for relative low loading intensities] .

• If the dissipation of excess pore pressure due to enhanced loading (proposed structure) takes more time

• If stability of soil in slopes need to be enhanced substantially (by use of soil reinforcement)

• If subsoil is prone to liquefaction


When a project encounters difficult foundation conditions, possible alternative solutions are

• Avoid the particular site• Remove and replace unsuitable soils and redesign the

proposed structure accordingly

Need for Ground Improvement ‐ Strategies


p p g y• Enable cost effective foundation design• Attempt to modify existing ground• Reduce the effects of contaminated soils• Ensure sustainability in construction projects using ground

improvement techniques


• temporary: improvement is only during the application e.g. dewatering or ground freezing,

• short-term: improvement is aimed only for short-term application e.g. some forms of grouting, or use of diaphragm walls for ease of construction with longer term benefits

Nature of Improvement


for ease of construction with longer term benefits• long-term: improvement meant for permanent solution

e.g. soil nailing, vibro-replacement, curtain grouting of a dam, where the treatment is integral to the permanent works.

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Change of state‐the same ground but made stronger, stiffer, denser, more durable

Change of response ‐ the ground becomes a composite material with enhanced load‐

carrying or deformation characteristics through the incorporation of other

Effect on ground due to ground improvement



carrying or deformation characteristics through the incorporation of other materials

Change of nature‐the ground becomes a different material by inclusion of other

materials

Soil Reinforcement

•Geosynthetics• Fiber Reinforcement•Mechanically Stabilized Earth

• Stone Columns• Lime Columns

• Surface Compaction•Deep Dynamic Compaction

•Blasting•Drainage/Surcharge

Soil ImprovementChange in nature

Change in response

Various Ground Improvement Techniques


Lime Columns•Vibro‐Concrete Column•Ground Anchors• Soil Nails•Deep Soil Nailing•Micro piles (Mini‐piles)• Jet Grouting•Biotechnical

•Drainage/Surcharge•Electro‐osmosis•Compaction grouting • Soil Cement

• Lime Admixtures• Fly ash•Dewatering•Heating/Freezing

Soil Treatment

Change in state

General classification of ground modification techniques

Mechanical Modification

Hydraulic modification



Physical and chemical modification

Modification by inclusion and confinement

Combination of the above

• Type and degree of improvement required• Bearing capacity improvement, settlement reduction, permeability enhancement/decrease, long term/short term, liquefaction resistance.

• Type of soil, geological structure, seepage conditions• Type of clay/sand and foundation, role of pore pressure and

Factors affecting the selection of GIT


yp y/ , p pseepage, presence of difficult geological condition.

• Costs, equipment, specifications• Size of the project, availability of equipment, transportation costs, experienced contractors, specification of work, guidance documents.

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• Possible damage to adjacent structure or pollution of ground water resources

• Tolerable levels of loading and deformation, pore water contamination

• Durability of the materials involved• Short term and long term, corrosion, aggressive soil condition

Factors affecting the selection of GIT (cont.)


• Toxicity and corrosivity of any chemical additives• Government regulations may restrict the choice of additives

• Reversibility or irreversibility of the process• Lime added to expansive soil reacts in presence of sulphate

• Reusability of components such as steel, plastics, concrete etc

• Reliability of methods of testing, analysis and design• Good methods of testing, proven methods of design and analysis should be used and empirical approaches need to be avoided

• Feasibility of construction control and performance measurements• Documents of quality control and performance are required in

Factors affecting the selection of GIT (cont.)


major ground improvement projects

• Construction time• Construction time available, use of accelerated construction technique.

• Increase of strength

• Reduce distortion under stress (Increases stress‐strain modulus)

• Reduce compressibility (volume decreases due to a reduction in air voids or water content under loads)

• Prevent detrimental physical or chemical changes due to

Objectives of Ground Improvement Techniques


environmental conditions (freezing / thawing, wetting / drying)

• Reduce susceptibility to liquefaction

• Reduce natural variability of borrow materials and foundation soils

Soil Densification

Preloading and drains

Ground Improvement Techniques ‐ Themes



Injection and Grouting

Soil Reinforcement

Replacement / Admixures/ Heating / Thawing

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Soil Densification


• A simple ground improvement technique, where the soil is densified through external compactive effort

• By application of shock and vibration to the subsoil and thereby causing arrangement of the soil structure from a loose to medium dense state

Significance

Soil Densification


medium dense state.

Methods• Shallow compaction• Dynamic compaction / consolidation• Vibrocompaction• Dynamic replacement• Blasting

Strategies for shallow compaction process are• In the case of constructed fills, specify placement conditions (water content, density, depth of layers, etc.)

• Select appropriate equipment (roller compactor, tamping) and method of operation (number of passes, patterns of tamping,etc.).

Shallow compaction

Soil Densification


• Set up adequate control procedures (type and number of tests, statistical evaluation, etc.).

Field Compaction Methods• Smooth wheel roller• Vibratory roller / plates• Pneumatic rubber tired roller• Sheepsfoot roller• Impact roller

Advantages of Compaction• Increases shear strength• Reduces compressibility• Reduces permeability• Reduces liquefaction potential• Controls swelling and shrinking• Prolongs durability

Shallow compaction

Soil DensificationSheepsfoot roller –

very effective for claysVibrating plate –primarily for granular soils and for

very small areas


Smooth wheel roller ‐ effective only to 200‐300 mm depth;

Pneumatic rubber tired roller‐ eight‐wheeled machine designed for rolling hot‐mix‐asphalt surface treatments and soil‐stabilization

Impact roller – provides deeper (2‐3m) compaction; used in runways

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Deep compaction techniques are required when in–situ soil extending to large depths does not meet the requirements of performance criteria specified for the

Deep compaction

Soil Densification


criteria specified for the expected loading and environmental conditions.

Methods• Dynamic compaction/

consolidation• Vibro‐compaction• Dynamic replacement• Blast densification

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Deep compaction ‐ Classification

Soil Densification

Dynamic compaction• It is the compaction of unsaturated or highly permeable saturated granular materials by heavy tamping. The response to tamping is immediate.

Dynamic consolidation• The improvement by heavy tamping of saturated cohesive materials in which the response to tamping is largely time dependent.


• Excess pore water pressures are generated as a result of tamping and dissipate over several hours or days after tamping.

Dynamic replacement• The formation by heavy tamping of large pillars of imported granular soil within the body of soft saturated soil to be improved

• The original soil is highly compressed and consolidated between the pillars and the excess pore pressure generated requires several hours to dissipate

• The pillars are used both for soil reinforcement and drainage

Deep compaction – Dynamic compaction / consolidation

Soil Densification

• Technique involves repeatedly dropping a large weight from a crane

• Weight may range from 6 to 172 tons• Drop height typically varies from 10 m to 40 m• Degree of densification achieved is a function of the energy input (weight and drop height) as well as the


saturation level, fines content and permeability of the material.

• 6 – 30 ton weight can densify the loose sands to a depth of 3 m to 12 m

• Done systematically in a rectangular or triangular pattern in phases

• Each phase can have number of passes; primary, secondary, tertiary, etc.

Soil Densification



Typically this technique should not be carried out within 15m of existing services and 30m from existing buildings.

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Soil Densification



Soil Densification

Deep compaction – Vibrocompaction

• A loose soil or non‐homogeneous granular fill can be compacted to depth by the penetration of vibrating probes or vibroflots (in a horizontal direction).

• The main purpose of Vibrocompaction is to increase the density of the insitu soils by


vibration.• Effective treatment at depths 2m to 20m.• Optimised and localised treatment offering flexible solution for differing soils.

Soil Densification

Deep compaction – Vibrocompaction

• Using the rigs pull down force, water jetting and sustained vibration allows the probe to penetrate to the design depth.

• Water jetting is then decreased, and the resulting soil cone facilitates the re‐arrangement of the soil particles. Granular fill can then be incorporated at the surface (top feed) into the created void. The water flow down the vibrating probe assists in the transportation of the backfill down to the compaction zone


probe assists in the transportation of the backfill down to the compaction zone at the vibrating probes base.

• The vibrating probe is gradually lifted to produce a cylinder of compacted ground of 2‐4m diameter, depending on the soil and vibroflot used.

• This densification creates a conical crater, that is then filled with the granular backfill, as the probe is removed

Soil Densification

Deep compaction – Dynamic replacement

• Dynamic replacement is an extension of dynamic /vibrocompaction to highly compressible, organic and weak soils.

• In this application, the tamping energy drives granular fill material down into the compressible soils to form a large diameter soil reinforcement column, with a


diameter of 2‐3.5m. • Popularly known as stone columns.

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Soil Densification

Blast densification

• Controlled blasting / Explosive compaction is a densification technology used to densify loose, saturated, medium to coarse materials.

• Explosive compaction requires installing plastic casings ranging in size from 80 mm


to 130 mm in diameter to any depth. • The holes are charged with multiple, pre‐packaged explosive cartridges separated by gravel stemming.

• The method is faster than preloading and cheaper than deep cement mixing.

• It can densify coarse grained soil up to 40 m. It is effective when the soft soil layer to be replaced is less than 10 m.

Soil Densification

Comparison

• Soils in zones A and B can be compacted by the deep vibratory compaction method vibrocompaction(also called vibroflotation),


• Soils in zone C are often found on sites where liquefaction due to earthquakes is of concern. These soils can be compacted during the installation of Stone Columns.

• Soils in zone D are not compactable by vibration, but can be substantially reinforced, stiffened and drained by installing Stone Columns

while soils of zones C and D cannot be compacted by vibration alone.