ground improvement techniques

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Contents Introduction Alterations of ground after formation Need for Ground Improvement Ground Improvement potential Improvement techniques Modern Techniques

Introduction Soil improvement is the alteration of any property of a soil to improve its engineering performance which may either be a temporary process or a permanent This can be done by reducing the pore water pressure, by reducing the volume of voids in the soil, or by adding stronger materials. The result of an application of a technique may be increased strength, reduced compressibility, reduced permeability, or improved ground water condition

Alterations of Ground after formationMajor causes of alterations are Seasonal moisture variation Water seepage and surface erosion Vegetation Temperature variation Vibration Mining subsidence and pumping Construction operation

Need for ground improvement Rapid urbanisation and industrial growth driving demand for land . In order to meet this demand, land reclamation & utilization of unsuitable or environmentally affected lands have been taken up Where poor ground conditions make traditional forms of construction expensive, it may be economically viable to attempt to improve the engineering properties of the ground before building on it To improve load bearing capacity and shear strength

Ground Improvement potentialAll ground conditions may not be the same but different in varieties and generally of three types: Hazardous: A regional/local field condition is such that a regular design approach or an economical treatment technique may not be feasible and construction in such a location may result in ultimate disaster. As far as possible such location must be avoided Poor: A local condition including regional conditions which may require special design &/or special treatment for development Favorable: A local condition including regional conditions for which normal design and ground treatments are suitable

Improvement techniques

Mechanical compaction Soil stabilization methods Dynamic compaction methods Pre loading Drainage methods Diaphragm Wall Compaction piles

Grouting & Injection Electrical methods Thermal methods Rock Anchors/Bolt Reinforced Earth technology Geo-products Pre-wetting

Mechanical Compaction This method utilizes mechanical compactors and rollers which reduce voids This method has very low depth of influence and hence can be used only for shallow soils In the event of weak soil being present up to certain depth, the same is replaced with a layer of good soil Sheepsfoot roller and vibratory rollers are used for cohesive and cohesionless soils respectively

Soil stabilization techniques This method can be used only for shallow soils which includes: Mechanical: This method employs mixing and reproportioning of soils with sandy/gravely material Chemical: This method employs the use of lime, calcium & sodium chlorides, admixtures like lignin, water-proofers, aggregants, dispersants, natural and synthetic polymers Cementing: This method employs use of Cement. Lime and/or Flyash and/or bituminous (asphalt) material may be used

Dynamic Compaction This method has a higher depth of influence as compared to mechanical compaction methods and hence can be used for deeper soils This method includes: Vibration methods Impact methods Micro-Blasting

Vibration methodsThese methods are in general called as VIBRO-FLOATATION:o Vibro Pier o Vibro-Compaction o Vibro-Replacement o Vibro Concrete Columns







Vibro Concrete Columns


Impact methods1. 2. 3. The depth up to which soil can be compacted is 10-12 m All types of soil except clay-bearing and soil having high water table, can be compacted The Depth of influence/improvement is given by : D = n(W*H)0.5 where n is a constant which depends on various factors like:

Material to be compacted Applied force Contact area Presence of energy absorbing layers

These methods are: Rapid Impact compaction: In this method, the soil is compactedunder the pounding action of heavy hammerAnimation

Dropping of heavy weight: In this method, steel or concrete weightaround 500-600kN is dropped from a height of 40-50m with the help of a Animation crane

Rapid Impact Compaction


Dropping of heavy weight


Micro-blasting The micro-blasting technology is used for the improvement of subsoil under civil and hydro engineering structures. It employs high-energy explosions to modify the surrounding soil. The energy generated by the explosion of 1 kg of TNT = the energy of 5 tons of tamper falling free from a height of 100m. Applications: harbour areas, reclaimed islands, breakwaters, road embankments and airfield pavements, nuclear power plants, dams, etc.

Underwater explosive compaction

explosive compaction with surface charges

Loose soil before blasting

Densified soil after blasting

explosive compaction with hidden charges

Pre-loading It is used to densify sanitary landfills Pre-load is applied in the form of an imposed earth-fill left for a long period over an area to be compacted When soil is being laid on the garbage and kept for long period, the leach-ate present in it is squeezed out, as a result, there is reduction in volume of garbage and more space is available


Graph: Time vs. Settlement

Drainage Methods This is includes several methods like: Dewatering systems: these include methods like construction of sumps/ditches/trenches and well-point system Drains: these include open, closed, horizontal and vertical drains

Diaphragm wall It s the generalized term used for stone columns, vertical sand drains In which a vertical wall is constructed to prevent collapse of sides by resisting the lateral pressure and add to stability of structure of soil Thus Diaphragm wall include :a) Sand Drains b) Stone columns

Sand Drains Sand and sandy soils are known to be the one of the most cohesionless soils and therefore drainage through sand layers is very easy and convenient Horizontal or vertical layer of sand are placed to facilitate easy drainage and also to compact the cohesive soil around


Stone Columns

The method of installation is similar to that of compaction piles The size of stones used is 6-40 mm The spacing of stone columns is kept between 1-3m It is particularly applied to soft soils It is not suitable for highly organic soils

Compaction Piles Compaction piles are used to compact loose granular soils, thus increasing their bearing capacity The compaction piles themselves do not carry any load. They may of weaker material like sandSand compaction piles : It consists of driving a hollow steel pipe with closed bottom & fitted with collapsible doors Sand is filled in the hollow steel pile Similar to Tremie method , the pipe is slightly lifted Due to the load the bottom plate opens out and sand backfills the void created during driving of pipe The refilled sand prevents the surrounding soils from collapsing

Settlement piles

Grouting & Injection Grouting is normally done to fill the cracks present in soil or rock strata It proves effective in following situations

When foundation is to be below ground water table When site is located in an area where direct access is restricted When design of foundation is restricted by no. of boundaries & contact zones Excavation is not possible besides the proposed structure

The grout used may be a combination of cement slurry with different admixture Bentonite slurry may be commonly used grout, it is basically a highly plastic clay which has 2 advantages Due to its highly plastic nature it can enter into even its smallest present crevice It compacts quickly & forms a monolithic structure It is costly and hence used in special situations like to protect the sides of tunnels from collapsing



Jet Grouting


Grout Plan & Section

Electrical methods Electro-osmosis This method is employed for cohesive soils (clays) Metal strips are inserted and a well point system is also employed The current is passed through metal strips thus becoming the anode and well point system as cathode Water being charged with anions flows to well point system and is pumped out

Thermal Stabilisation by Heating


Formation of Thermal Gradient in soil

Flow of pore water

Dielectric Constant

Particle Electric Repulsion

Strength of soil

Thermal Stabilisation by Cooling


Pore water in soil freezes

Ice is formed

Soil is reinforced

Non-vibration sensitive barriers/walls are formed

Soil Strength

Rock Anchors Rock bolts are tensile units employed to keep rock mass in compression It is installed as nearly perpendicular to joints as practicable The ordinary types consist of rods installed in drill holes by driving and wedging, by driving and expanding, or by grouting with mortar or resins Bolt heads are then attached to rod and twisted against a metal plate to impose the compressive force on mass Fully grouted rock bolts, provide more permanent bolts than ordinary types Rock bolts are used in slope stabilization, open excavations, in tunnels, caverns, mines, concrete dam foundations to provide resistance to uplift and sliding

Reinforced Earth Technology

In this technology, the sides to be retained are fixed with sheet piles The soil on the backside of the sheet piles ar


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