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