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SARTHAK
DAS TRAINING AT NALCO, ANGUL
At the beginning of
our vocational training in the second week of May, we are allocated our
first site where we are encountered with the installation and testing of bored
cast-in-situ pile for anode baking furnace of NALCO, Angul for the smelter
expansion project Phase-II. Before going
into the details and exact specification of piling work, we will be
discussing about the overall piling performed over here is done by Direct
Mud circulation method (DMC method). Initially, a hole is bored by using a
barrel (a hollow cylinder) by giving blows from a height of at least 2 mt.
After a hole is bored and mud is removed upto a depth of 1mt approximately,
the casing is inserted in the hole, which guides the chisel to give perfect
blow and also restrict the surface soil from collapsing. Now the chisel; is
fitted to the rig and it gives blows into the hole till we get a hard
strata. The bentonite slurry is continuously passed into the pile hole so
that it prevents the soil from collapsing within. After we get a hard /rocky
strata, the reinforcement cage with proper cover blocks is inserted. Then
the concrete of proper grade is filled through the trimic pipe, using a
funnel. Compaction is done by using a vibrator. The details shall be
specified further--
The work
involves construction of 500mm diameter straight shaft RCC bored
cast-in-situ piles. The approximate length of pile below the cut-off level
varies from 5.0m to 8.0m depending on occurrence of founding strata i.e.
weathered /disintegrated rock strata and approximate number of piles are as
follows -
Length of
the piles is indicative and may vary as per depth of founding strata to
terminate the piles.
Information on sub-soil profile
The soil layer is a silty sand with gravel and is about
3.0mt thickness. It is followed by very dense silty sand with gravel
/weathered and disintegrated sand stone.
Also the ground water tank was found to be varying from
1.2mt to 3.85mt depth below the Existing Ground Level.
Termination of piles
500mm diameter bored cast-in-situ RCC. piles is
terminated in strata consisting of very dense silty sand, weathered
/disintegrated rock deposits at a depth of 5.0mt to 9.0mt below the cut-off
level with a minimum of 1000mm embedment inside weathered /disintegrated
rock, strata. Incase hard /moderately weathered rock is encountered before
the pile is embeded 1000mm inside weathered rock, then
·
(i)
Chiselling shall be done for at least two hours with 1.5 MT minimum weight
and fall of height of 2.0mt.
·
(ii) Set
is established (less than 10mm) for 100 to 200 blows.
Safe load carrying capacity
The concrete
used is of M20 grade having minimum cement content of 400kg/m3 with maximum
water-cement ratio of 0.5. However the EIC may allow marginal adjustment in
the w/c ratio to obtain concrete of good workability.
Reinforcement
Reinforcement is high strength deformed bars of grade Fe
415 conforming to IS 1786(latest revision) and the same is weldable quality.To ensure dense and sound concrete up to cut-off level.
Approximate graded level was found to be EL.115.0mt. The segments of a
reinforcement project 50 times its diameter above cut-off level. For tremic
concreted piles, a sample of drilling fluid is taken from the base of the
bore hole in the first few piles and at suitable interval of piles
thereafter. Concreting shall not proceed if the density of the fluid exceeds
1250kg/m3. The sand content in the fluid shall not exceed 7%. The cement used shall be ordinary Portland cement
Portland
sand cement conforming to IS:455 or Portland Pozzolana cement conforming to
IS:1489. The coarse and fine aggregates for concrete should be
chemically inert, hard, strong, dense, durable, clean and free from veins,
adherent coatings and of limited porosity. Flaky and elongated pieces are
not used. Aggregates should not contain deleterious materials such
as iron pyrites, coal, mica, shale or similar laminated materials, clay,
alkali, soft fragments, sea shells, organic impurities etc. in such
quantities as to affect the strength or durability of concrete. For
reinforced concrete, any material which cause corrosion of the reinforcement
and aggregates which are chemically reactive with alkalis of cement should
not be used. Coarse aggregate is aggregate most of which is retained
on 4.75mm IS sieve. Fine aggregate is aggregate most of which passes 4.75mm
IS sieve but not more than 10% pass through 150 micron IS sieve. Admixtures such as CICO grade I or puddlo or equivalent
is used in concrete.
The reinforcement should be adequately secured and held
in position by metal chairs and spacers. Concrete spacers blocks of the same
strength as parent concrete shall be used to ensure correct cover of
reinforcement. All the reinforcement bars should be tied to form a rigid
cage to prevent displacement before or during concreting. The grade of concrete ranges between, or say matches with
M-20 with a minimum cement content of 400kg/m3. The slump of concrete ranges
between 100 to 180mm depending on the manner of concreting. The stirrups to the main bars should be tack welded. Care
should be taken to ensure bars do not come closer while the cage is lowered
down the hole. Proper cover and central placement of reinforcement should be
ensure by use of suitable concrete rollers or spacers cast specifically for
the purpose. The borehole bottom should be thoroughly cleaned to make
it free from sludge or any foreign matter before lowering the reinforcement
cage. This is called flushing. Concreting is done by tremic method. The concreting placing shall not proceed if the specific
gravity of fluid is near about the bottom of borehole exceeds 1.2. The
specific gravity of borehole mud is determined by using a hydrometer. The
top of the pile is brought above the cut-off level so as to permit removal
of all laitance and weak concreting before capping and to ensure good and
sound concrete at the cut-off level for proper embeded into the pile cap.
Pile testing
The head of the test pile should be brought to a proper
level and plane surface and with adequate space for proper seating of a jack
and dial gauges. The test pile surface should be preferred for testing
purposes on expiry of one week after casting the pile. Loading should be applied by the reaction method
consisting of an hydraulic jack, centrally against a loaded platform.
Supports of the of the platform shall be adequately designed .Special anchor
piles or any other special type of anchorage systems should be adequate to
ensure that the test shall be at least 25% greater than the maximum jaking
force required. Settlement of the pile is recorded by dial gauges
suspended from datum bars. Four dial gauses of 0.01mm sensitivity shall be
positioned on four corners around the pile. The datum bars should have rigid support of preferably of concrete
pillars of steel sections, embeded well into the ground .The supports shall
be located more than three times the pile diameter ,subject to a minimum of
1.5m clear way from the pile face and also sufficiently away from from the
supports of the loaded platform to avoid any disturbances on these accounts.
Movements near the supports near the datum should be avoided when the test
is in progress. Load corresponding to settlement specific requirements
which is based on the nature and the type of structure .50%of the final load
at which the total displacement equals to 10% of the pile diameter in case
of uniform diameter piles and 7.5% of bulb diameter in in acse of under
reamed piles.
The next part of our training is restricted to the road
work in the
township of
NALCO, Angul i.e. repairing of bitumen road from township gate no. 2 near
NH 42 to the existing oxidation pond of NALCO township. The process mainly involves five steps -
Preparation of surface for bitumen grouting, Supplying ,laying and grouting
of pot holes, 40 mm of premix carpeting, 25 mm of premix carpeting
,
6 mm of seat
coat. Before going into finest details ,it should be noted that
we are discussing about the flexible pavement (i.e. bitumen road) ,which is
found to be climate specific. It is almost impossible to construct a bitumen
road during the rainy season especially carpeting and barring water Bound Mechadam (WBM), grouting and earth work. Such a sort coming is encountered
due to a temperature specific work to be discussed further. Initially the damped road is repaired by removal of
damaged bitumen carpet, process called as grouting. it involves removal of
earth/murrom/stone (up to 150 mm ) from the damaged portion of the road and
stacking of the lead of 50 mm disposal and leveling with in a lead of 3 km
or as per the direction of the engineer in charge, namely Mr. S.K.Panda ,DM
(CIVIL), Project expansion. that it matches with the exising road profile. The existing road surface is thoroughly scarified and
cleaned if dust, loose materials and other foreign materials with the help
of wire, brush ,chisel picks etc. Before lying of bitumen grouting over a
tack coat as per direction. Thus a surface is prepared for bitumen grouting after
removal of dismantled WBM ,earth, damaged bitumen carpet, removal of earth
murrom, stone loose graits etc. stacking the same with a load of 50 m and
the breadth of road being 7.1m approximately. It is observed that the cleaning is carried out in such a manner
that stone material is exposed to a depth of 1mm to 2 mm without dislodging
the interlocking of the metal. All dust and other materials are thus removed
and carried away and dumped at designed places . For tack coat ,the bitumen used is of the grade 80/100 and heated
to a temperature of 177 degree Celsius is spread uniformly on the road
surface in the respective items.
Hot mix plants
Mechanical mixes are used for preparation of premix bitumen .The
hot mix plant of capacity 6 to 10 TPH is installed at a site in the NALCO
township .Stone chips of specified size is thoroughly mixed dry in the
mixture of the plant.
The binder is heated at a suitable temperature
for the grade of bitumen and added to the mixture drum at a specified rate
per cubic meter of the stone aggregate and thoroughly mixed till stone chips
are completely coated with bitumen.
Premix stone chips shall be carried in
hydraulic operated tractor of capacity
3 to 4 cubic meter to work size and unload these materials in the
hopper of the mechanical paver.
A mechanical paver is a machine which provides
or feed the premix to the surface of the road with a particular thickness
uniformly according to to the height of the plate above the surface .The
hopper send the premix to the rear of the paver machine by a revolving
arrangement where the plate provide certain thickness to premix.
It should be noted that premix has to be
immediately transferred from hot mix palbt to the premix of 40 mm thick
,with stone aggregate over tack cote is spread manually mechanical paver to
match the required slope and camber of different area and size. After laying
of premix rolling shall take place with a road roller of 8 to 10 tonne
capacity proceed towards the center longitudinally .
Wheels of the roller are continuously moistened
to prevent the premix adhering and being picked up.
After preliminary rolling and honeycombing ,high
spots and depression shall be rectified by adding or removing the premix as
per requirement and the surface shall be rolled again for compaction .camber
is checked at every stage .
Premix shall coat ,mixed in a mechanical mixture
should be applied after lying and rolling of the carpeting. Proportion of
stone aggregate ,bitumen sand etc is specified later.
Traffic may be allowed on the road surface
preferably 24 hrs after providing the seal coat. It is the duty of the
contractor to provide indicator necessary to prevent traffic movement on the
freshly compacted premix.
Bitumen Premix Carpet
Coarse Aggregate:
The aggregate consist of crushed stone ,hard ,tough,
durable rock of uniform quality and should clean free from excess of dust,
flat or elongated pieces ,soft or disintegrated stone clay or deletericious
matter.
Sand
The sand shall consist ofg clean ,hard ,durable uncoated
coarse dry particles and shall be free from injurious amount of dust, Soft
or flaky particles ,organic matter etc.
Bisnder
The binder shall be bitumen of penetration 80/100
confirming of IS specification .Bitumen is procured in a sealed drum.
To sum up ,the following processes are followed with a certain
specification:-
·
Firstly
,grouting is performed for 150 mm. Again bitumen carpeting is done for
(63-40 mm) total average thickness being 75 mm.
·
Supplying
lying and grouting of potholes with 40 mm stone aggregate up to avg
thickness of tack coat of 80/100 bitumen of approved quality of material at
0.75 kg ig spread per square meter of road surface including rolling with a
(8 to 10 T) road roller.
·
Further we
are concerned with laying the 40 mm premix carpeting with the premix being
prepared at the hot mix plant installed at the site and manual spreading by
using stone aggregate at 5.4 m3 per 100 sq matre (with 60%20 mm down and 40%
12.5mm down aggregate) with bitumen of grade 80/100 of approved quality at
56 kg/m2 for 20 mm down stone aggregate over a tack coat at 0.750kg/m2 of
the road surface.
·
Next process
involves the lying of 25 mm thick premier carpeting with the premier being
obtained from the hot mix plant of capacity (6 to0 10 )T PHand manual
spreading by using stone aggregate 12.5 mm down at 3.37 m3 per 1000 m2 with
a bitumen grade of 80/100 of approved quality at 64 kg of hot bitumen per
m3 of stone aggregate over a tack coat of 0.75 kg/m2 of road surface .
·
Finally there
is the lying of 6 mm thick seal coat by using stone aggregate obtained by
100% passing 6.3 mm sieves at 0.9 m3 per 100 sqm with bitumen of grade
80/100 of approved quality at 98 kg per m3 of stone aggregate over a coat of
tack coatat 0.5 kg per m2 of the road surface.
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