As per the definition, the pile cut off level is the elevation up to which the concrete of the pile head is chipped or cut off while exposing rebar. It is an important design parameter in case of pile foundation that allows the rebar of pile to embed in pile cap and uniformly distribute the load of the pier thereby acting as one monolithic structure.
All the piles under the pile cap are chipped or cut to equal depth that is mentioned in the pile construction drawings or specification.
Pile Cut Off Level Meaning
As per IS 2911-1-2, the definition of pile cut off level is as follows;
It is the level where a pile is cut-off to support the pile caps or beams or any other structural components at that level.
Difference between Pile Cut Off Level & Founding Level
The pile cut off elevation must not be confused with foundation level or pile toe level. The pile toe level is actually the top level of the bedrock up to which the pile has to be bored or driven.
The Reason for Pile Chipping or cutting off pile heads?
So, why pile chipping is required before the pile cap works?
Please note that the concept of pile cut off level only belongs to the cast in situ concrete piles. In such piles, the borehole is made in the ground by percussion or rotary method of drilling. As a result we have mud or underground water at the founding level.
To concrete in such a mud we use tremie method of concreting also termed as wet process. In this method a steel shaft connected with a hopper at the top is used to pour concrete in the hole. But this tremie pipe is lowered up to the founding level and the concrete slurry is lowered below the water level. The concrete, by pressure, displaces the slurry and overflow from the bore hole. It is very important that the concrete is continued to be poured until good quality concrete is formed above the design pile cut off level. This will avoid contamination of slurry within the design pile length. This top few meters of pile consists of weak concrete as well as contaminated debris.
So, there are two main reasons of cutting or chipping off pile heads. The first one is to develop the pile cage rebar into the pile cap. The second is to remove the weak or poor quality concrete.
Pile Cut off Level Specification
Want to know how to calculate pile cut off level? In every pile construction project, the design details give the information necessary for setting out and layout of piles, cut-off levels, finished cap level, layout and orientation of pile cap in the foundation plan and the safety, capacity of each pile.
The specification of pile cut off level is as follows:
Pile cut off level calculation can be done as:
Where cut-off level is less than 2.5 m below the ground level, concrete shall be cast to a minimum of 600 mm above cut-off level. For each additional 0.3 m increase in cut-off level below the working level, additional coverage of minimum 50 mm shall be allowed. Higher allowance may be necessary depending on the length of the pile.
When concrete is placed by tremie method, concrete shall be cast up to the ground level to permit overflow of concrete for visual inspection or to a minimum of one metre above cut-off level. In the circumstances where cut-off level is below ground water level, the need to maintain a pressure on the unset concrete equal to or greater than water pressure should be observed and accordingly length of extra concrete above cut-off level shall be determined.
Pile Cutting Procedure?
The cut off level of pile can be above or below the ground. When it is above ground, Concrete should overflow from the pile head on completion. Due to tremie method the upper part of pile consists of weak concrete as well as some contaminated debris of boring.
- In case the pile cut-off below the ground, the concrete level should be raised to allow for around 1 m for trimming off the weak concrete.
- The procedure / method statement for pile cutting is as follows:
- After completion of pile casting group for a pile cap, the soil around the pile is removed with the help of excavator and a depth designed for the pile cap is achieved.
- Pile head chipping is commenced after minimum of 7 days when the concrete has achieved 70% of its compressive strength.
- The concrete of the pile head is chipped or cut by using a jackhammer or hand-arm vibration syndrome (HAVS) one by one. But make sure not the damage the pile dowels. In case of damage to the rebar, dowels will have to be drilled upto the required development length.
- If the concrete is unsound even below the pile cut off elevation, the Engineer may ask to repair the concrete before commencing the chipping procedure. In every condition, the pile must be free from any cracks or poor quality concrete.
- After removing the chipped concrete, pile cap rebar is fixed followed by concrete pouring.