In Civil Engineering, Pile foundations are deep foundations used as type of foundation where the below soil is poor, i.e. either poor strength, no rock or proper bearing surface or when water is encountered at short depth from ground level.
Here we shall discuss about most commonly used types of Piles.
Major type of Piles for load transfer are
- End Bearing Piles – The load transfer from superstructure is transferred to the tip of the pile on hard strata.
- Friction Piles – The load transfer from superstructure is transferred by soil Friction.
Depending on the Soil Investigation Report and recommendations from Geo technical Engineer suitable type of Pile shall be adopted.
Since, for major loads single pile may not be sufficient below column/Pier/footing, Generally Group of piles are taken into consideration. Another important aspect is the total load carried by the group of piles is not same as sum of load carrying capacity of individual pile. In case of group of Pile, the load carrying capacity of individual pile may be less because of spacing of piles. The Pile foundations are installed in such a way that Piling should not affect already installed Pile.
The load carrying capacities are obtained from Pile Load tests. Initial Load test is carried out by Kentledge. This is also carried out by reaction Piles.
Test Piles are installed and loaded up to 2 to 3 times pile load or as per the load specified by the Engineer in charge/ up to Failure. This Pile casting and installation shall be similar to Working piles.
Generally, Piles have three types of Load carrying capacities
- Compressive capacity
- Tension Capacity
- Lateral capacity
Precautions while constructing Pile foundations
The Piling activity sometimes may affect the adjacent structures, necessary care and precautions and safety shall be ensured while driving pile. Preferably the spacing of Pile shall be 2.5 to 3 times dia of the Pile.
The cut off level of Piles shall be cast up to 1m above to remove any weak concrete and proper embedment of Pile Rebar into Pile cap.
For Deeper foundations the reinforcement cages shall be lowered with proper care, to avoid hitting edges,to maintain verticality. Cages may scrap the surface and soil may fall at the end bearing point of Pile. Necessary care shall be taken to avoid the same.
Settlement is another very important aspect in Pile foundations. As general practice, Settlement is related to Pile diameter and approx. settlement more than 10 % Pile dia should have Soil or supporting strata checked.
Geo Technical and some other precautions
In Geotechnical investigation necessary care shall be taken to check for Poor strata below hard rock just to be extra safe. Socketing into rock is preferred for end bearing Piles.
Pile integrity shall be checked for all the installed piles. This helps to know if any defects are in the pile.
Some international codes suggest Piles up to 600 dia are allowed settlement up to 15 mm. This should be very properly checked as per the respective Codal Provision or contract documents or recommendations of Local governing bodies or Piling specialists/suggested by Structural Engineer.
Rebar at the end of Pile should allow proper concrete flow and not obstruct. Rebar is required for lateral strength and the Rebar shall be accordingly provided for any Bending. Minimum Rebar shall be provided as per the Codal provisions.
In addition to Shear reinforcement, Inner reinforcement circular/rectangle of higher dia shall be provided to maintain the cage in shape for deep foundations.
Another important area is chipping of Pile. This is done after concrete has attained sufficient strength. Mostly this is done by Hammering/chiselling/Percussion jack Hammers.
Negative Skin friction is another very important aspect which needs thorough examination by Geotechnical Expert and also during testing of Pile and when arriving its capacity. Negative Skin friction causes problems on later date as well, so necessary analysis to be carried out.
Under Supervision, Loading testing on Pile shall be carried out.
Various codes for Pile foundations
EN 1997 – Eurocode
ACI 543 – US code
IS 2911 – Indian Code
AS 2159 – Australian code
National Building code of Canada – Canada