Adg i-1-12-11[1]
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Transcript of Adg i-1-12-11[1]
CONSTRUCTION METHODS
Open Excavation – Excavation of entire area from surface – normally width equal or greater than depth
G L
d
b d
Trench – Excavation depth more than width
G L
d > b
b
Shoring – Temporary protection to walls of an excavation from collapse
Deep Foundation – Transferring the loads to soil / rock at considerable depths
Steel beams
Wooden planks
Open excavation may be in foundations for buildings, canals, sewer lines
When soil cannot retain itself vertically, excavation is sloped to safe angle of repose
Sides Vertical Sides Sloped
In very deep excavations with sloped sides, berms are provided to facilitate maintenance specially in canals
G.L.
BERMS
OVER EXCAVATION
Trench excavated beyond the required depth
Should not be filled back to the required depth by same soil
Specially in foundations, settlement takes place and structure develops settlement cracks or may collapse
METHODS OF EXCAVATION
Earthwork excavation may be i. Manually and ii. By machinery
Manually• Man and woman mazdoors are
employed
• Uses crow bars and spades and baskets
• Earth is excavated with crow bars, with spade it is loaded into basket. Soil in the basket is conveyed
• Manual excavation in deep and hard strata is very cumbersome, out turn will be less
In deep open excavations, slopes are provided to prevent earth from collapseIn such cases, and so much space is not available, basement walls are constructed
Trench portion
Beam
StrutInterlockingsheet piles
Conveyance of soil is also difficult as depth increases
Sheet piles are driven either by dropping hammer or by vibro sinker in two rows, the space in between being more than width of foundation
Earth is excavated, they will be strutted to prevent collapse
By Machinery
Very economical, quick and can be
excavated even in water without bailing
A number of earth excavation machinery
is available – out of which popular are
i. Shovel or excavator
ii. Clamshell
iii.Dragline
Shovel or excavator : It has track
wheel – bucket capacity – 1.8 m3 to 4.0
m3
Earth is excavated by bucket and
dumped into the tipper which
conveys to specified area
An excavator is estimated to
equivalent to 500 man mazdoors
Dumper
JCB ( Josh of Cirel Bomford)
It is a mini excavator cum loader with
loader bucket (1.1m3) at front and
excavation bucket (0.24m3) at back,
mounted on pneumatic types
ClamshellUsed to lift loose material
Bucket capacity varies from 0.25m3 to 2.0m3)
Useful for accurate spot dumping of material in a confined space
Digging and dumping in a vertical plane
DraglineIt drags bucket against the material to be dry and dump over long distance
In large size excavations like canals, the excavator cuts as rectangular trench. This is called box cutting.
The excavator drops the bucket and drags it to collect the earth – lifts – turns – drops into tipper
After box cutting, the slopes will be cut
Cut and CoverIn deep cuts, the depth of the bed is more
During rains, huge gullies formed and soil slopes eroded and settles at bottom
Thereby capacity of canal reduces – lining
the slopes with cement concrete or
revetment of slopes is costlier
In such cases the required section will be
constructed with RCC and it will be
covered with soil
This construction is called Cut and Cover
SLURRY WALL CONSTRUCTION
Slurry walls – diaphragm wall – Thin and deep concrete walls
HeartingClayey Sand Filter
Toe drainCasing
Casing
Rock Strata
Diaphragm (cut off wall)
Keyed to rock strata / impermeable strata
To arrest sub-soil flow
Mostly used in earthen dam construction
Thickness normally depends on depth but normally varies from 0.45 m to 1.5 m
Construction Technique involvesi. Excavation of narrow trenchii. Excavation by excavator – for deeper
depth of more than 25m – crane and clamp bucket / clamshell
iii. A segment of the trench is excavatediv. Then it is filled with “slurry”
v. Slurry exerts hydraulic pressure on the side of trench and prevents collapse
vi. Bentonite slurry is commonly used as excavation fluid
vii. Bentonite clay and water are combined in a colloidal mixer and resulting slurry is pumped into the trench
vii. If it is to be reinforced, mesh of reinforcements are inserted and concrete is placed from one end
viii. Concrete displaces the slurry and may flow into adjacent excavated portion. Otherwise, slurry will be pumped for reuse
ix. For low cost barriers – refilling of trench with a mixture of excavated soil, dry bentonite and bentonite slurry
x. Walls of this composition provides low cost impervious barrier
SLIP FORM SHUTTERS
Used for pier concreting, for towers
One set of form work only is used
After concreting the same will be lifted for second lift and concreting
Concreting can be every alternate day
Helps in rapid construction
In conventional method -
Form work is set for each lift.
After concreting – 24 hours time – shutters
Dismantled – lifted and arrange for next lift. Minimum it takes five days
In this method -
Forms placed as per shape of structure
Forms are of one to two metres high
Forms are held by yoke legs fixed on both sides work is set for each lift.
Yoke legs are connected by net of beams
Jacks fixed on the yoke and below head beam
Jacks fixed will lift the whole assembly
These jacks will be many in number operated hydraulically or pneumatically and inter connected
Jacks get support from foundation through jack rods – jack rods are 3 m length and connected by screwed studs
Head beams
Jacks
Yoke
Yoke Legs
Jack RodsForms
Finished surface
In conventional method -
Shuttering fixed for first lift and concretedAfter it sets, shutters removed and again assembled for second liftWhen centering ready it is concretedLikewise, every time shutters dismantled and reassembledOn an average, minimum 5 days required to concrete each liftIn slip form shuttering, every alternate day concreted
GUNITING / SHOTCRETING
Guniting applies cement mortar under high pressure on surface
Mixture of cement and sand is conveyed by compressed air at a velocity 100 metres per second
The mix is pre-hydrated to avoid segregation
Guniting also called shotcreting. In guniting, only cement and sand are conveyed
In shotcreting, in addition to cement and sand, aggregates also added and conveyed under pressure
Guniting used in i. Repair works of reinforced cement
concrete of columns and beams
ii. In strengthening of old workiii. Lining of water reservoirs, canals,
tunnels
iv. Arresting of leakages from concrete surfaces
v. Average thickness of shotcrete 50mm – with reinforcements it should not be less than 75mm
vi. Silica fume is added to improve adhesive and cohesive properties
vii. For more adhesive power – araldite compounds are added to surface before guniting
Apparatus consists ofi. Gun for material supplyii. Water tank for wateriii. Air compressoriv. A nozzle
Air Hose
Material Gun
Water from tanks
Procedure i. Make the surface clean and remove all
loose materialsii. Cement and sand mixed as per design
proportioniii. 5% of water added to the mix to avoid
segregationiv. Operate the nozzle