ground improvement techniques

SHIVAM R. MISHRA081010240T.Y B.TECH CIVIL

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 formation

Major 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 re- proportioning 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 methods These methods are in general called as VIBRO-FLOATATION:

o Vibro Pier

o Vibro-Compaction

o Vibro-Replacement

o Vibro Concrete Columns

Impact methods1. The depth up to which soil can be compacted is 10-12 m2. All types of soil except clay-bearing and soil having high water table, can

be compacted

3. 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 compacted

under the pounding action of heavy hammer

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

Animation

Animation

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.

Loose soil before blasting

Densified soil after blasting

Underwater explosive compaction

explosive compaction with surface charges

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

Pre-loading…

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

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 sand

Sand 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

Grouting…

Grouting…

Jet Grouting…

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

Thermal Stabilisation by Cooling

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 are removed• A layer of thin metal is laid on ground• Soil is spread on this layer of metal strip and strip is bracketed around the

layer of soil• The same process of laying metal strips and soil alternatively is continued

till required height is achieved

Geo-products

• These include use of Geo-textiles, Geo-membranes, Geo-grids, Geo-spacers, geo-webs, geo-composites, etc

• These in general are porous of polypropylene polyester, nylon or PVC and their variations

• The main functions of these are :1. Separation2. Fluid transmission3. Reinforcement4. Filtration5. Containment6. Barrier

Pre-wetting• The technique is to flood the area prior to construction• As in natural expansive soils, extensive network of fissures and cracks

present initially, ponding process is easily facilitated• Because of pre-wetting, the water content will be closer to be attained

after construction, hence volume changes will be small subsequently• It is usual to treat the surface with a layer of lime to a depth of 0.3-0.5 m

after ponding• This treatment provides a working platform for construction and an

impermeable moisture barrier to retard subsequent desiccation of pre-wetted soil

THANKS…