Piling is required for the following reasons:

  • When soil is soft and there is no solid sub-stratum for the foundation at reasonable depth.
  • When raft foundation is very expensive or practically impossible.
  • When the building carries heavy concentrated loads.
  • When the building is constructed along the sea shore or river bed.



Piling For Building Construction

·        End Bearing Piles

·        Friction Piles


END BEARING PILES

  • These piles are used to bear vertical loads.
  • They are driven into the ground until hard bed is reached.
  • The load of the superstructure is transferred to the hard strata through these piles.
  • These piles act like columns carrying structural load, below ground level.
  • The bearing piles may be cast-insitu or may be precast.
  • For pre-cast piles, the piles are driven in the ground either manually or mechanically.
  • For cast-insitu piles, the bores of the required diameter are excavated until the hard strata is reached.
  • The bore is cleaned, de-watered and pile cage is slowly lowered down to the hard strata.
  • As the depth is more, the concrete is poured through tremie or shoots to avoid segregation.



WORK PROCEDURE FOR BORE CAST IN-SITU PILES

  • Set the tripod in exact position on centre of pile.
  • Make initial boring to hold the temporary casing.
  • Drive the temporary casing upto hard strata.
  • Bore upto hard strata with bailer (sludge pump).
  • Remove muck and water from sludge pump.
  • Chisel the hard rock with appropriate heavy chisel to have socket of minimum one diameter of pile diameter in hard rock.
  • To avoid side collapse use Bentonite powder and water while boring in soft strata.
  • Measure the depth of pile bore at regular intervals of 30 minutes.
  • Approve the strata only when penetration is less than 5 cm for 30 minutes of chiseling.
  • Cut the reinforcement cage according to depth and lower it down into pile bore.
  • Lower the tremie pipe (200 mm diameter) in pile bore. Keep the tremie pipe bottom approximately 30 cm above approved strata.
  • Install hopper on the top of tremie.
  • Prepare concrete by mixer and transport to pile location by wheel barrow.
  • Pour the concrete in hopper to its full capacity.
  • Remove the steel plug so as to make proper flow of concrete inside tremie and thereby in pile bore.
  • Ensure that the tremie pipe is embedded in concrete at least 1.5 m, repeat the procedure till concreting is done upto existing G.L.
  • Remove Tremie and Hopper.
  • Pour extra concrete inside pile bore, to ascertain no cavities are left.
  • Lift the temporary casing gradually and shake the reinforcement cage to ensure for proper cover and compaction.




QUALITY CHECKS

  • Mark the reference point before driving the casing.
  • Check the distances of references from time to time.
  • Collect and check strata after every 30 minutes.
  • Ensure anchorage with minimum depth in Hard strata equal to diameter of respective pile.
  • Ensure that last 30 minutes penetration in Hard  strata is less than 5 cm.
  • Use M-20 grade concrete with slump of 150 mm.
  • Cast 6 concrete cubes for every batch of concreting.
  • Check the compressive strength of cube at 7 days and 28 days of curing.
  • Maintain the log book for every pile for the following details.

a.     Depth of pile.
b.    Socketing (Anchoring) Depth.
c.     Chiseling Hours.
d.     Theoretical and Actual consumption of cement.
e.     Date of Concreting.


FRICTION PILES

There are various types of friction piles ; however, most common and widely used in building construction is ‘under-reamed’ pile.


UNDER REAMED PILES INSTRUCMENTS USED FOR WORK

  •  Auger with special blades – to excavate the pile hole.
  • Reaming tool – to form the bulb.
  • Bucket – to collect and remove the excavated soil from the bulb.
  • Extension rod – to extend the length of reaming tool or auger.
  • Tripod and pulley block – to take out the reaming tool or auger from hole.



WORK PROCEDURE

  • Give the samples of the soil excavated from trial bore, at various depth to the Structural Consultant.
  • Collect the R.C.C. drawing from the Structural Consultant.
  • Mark the centre of the pile on the ground.
  • Excavate about 30 cm of the soil manually.
  • Erect the tripod stand exactly over the pile centre.
  • Auger having helical blades are inserted in the bore.
  • The diameter of the blades goes on increasing and it is equal to the bore diameter at the top.
  • Auger is rotated manually, applying downward pressure.
  • After sufficient depth (1.00 m) excavated auger is taken out with the help of chain and pulley block.
  • Mud and water should be removed from the bore and the procedure should be repeated till the desired depth of excavation is reached.
  • Approximately at every 2 m of excavation, reaming tool with bucket is inserted in the bore to excavate a bulb.
  • After the formation of bulb, the tool is pulled up and excavated soil collected in the bucket is removed.
  • Pile cage is lowered down as per the drawing and design.
  • Maintain the pitch of spiral stirrups as per the drawing.
  • Ensure the concrete covering to the main steel, plumb of the cage etc.
  • Pour concrete of the desired grade.
  • Mix the concrete with the mixer and using concrete platform.
  • Compaction may be done by rodding.
  • After casting the bore, concrete pile cap or plinth beam is cast over the pile as per the given design.
  • From this cap/beam the columns of the super-structure should be taken out.
  • Keep the pile bars including spiral stirrups above the G.L. for required lap length.