ground improvement techniques
TRANSCRIPT
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…