Pile Cap Design and Construction

Introduction of Pile Cap:

The pile cap is a type of thick concrete mat-like structure which rests on the pile (timber or concrete) which gives a stable foundation to the whole system. This concrete pile cap transfers the entire load of the superstructure to the into the piles, which gives you a stable bearing capacity.

The similar structure of the pile cap is a raft foundation, where there is also a concrete block-like structure is constructed.

Pile Cap Design: 

The pile cap is a mat-like structure which rests on the group of piles to transfer loads from the super structure. By IS 2911-2010, the pile cap could be arranged that the column’s load is spread at 45° out of the top of the cap.

1. Suppositions in the Pile Cap Design

1. The pile cap is must be rigidly set to the structure.
2. Though piles are slight elastic column, that’s why some distortions will be found, and we need to measure those in the plan.
3. The heads of the pile are pivoted to the pile cap to provide safety from the bending moment occur in a pile.

2. Limitations of Pile Cap Designing

1. Shape and form of the pile cap.
2. Quantity of steel is used in the pile cap.
3. Reinforcement placing in the pile cap.
4. Depth of pile.

3. Characteristics of Pile Cap Designing

1. The dimension of the pile is considered by the overhanging portion of the pile cap is not less than 100 mm, it would be highly preferable if you provide 150 mm.
2. The thickness of the pile cap is needed to be higher because the reinforcements of columns may come out and overlay to the reinforcement of the pile cap.
3. In pile cap construction, the clear cover must be more than 40 mm.
4. The pile cap’s span to thickness ratio must be not excess than 5. The pile cap must be relatively rigid to transfer the load to the pile section to downwards.
5. The thickness must not be below 500 mm, and it’s decreased to the 300 mm on the peripheries.
6. The pile must not less than 50 mm into the pile cap.
7. The upper coarse of pile cap is not less than 75 mm thick concrete.

Pile Cap Construction:

1. The pile cap is a mat-like structure which is made of cement, sand and small aggregates.
2. This mixture is placed in the required place, and it is supported by the framework to resist the whole structure from sagging and crack, this entire process is termed as shuttering and reinforcing.
3. In a pile, cap reinforcements are placed in the longitudinal section of the pile cap, and it creates a wire type of reinforcement.
4. At first, the reinforcement cage is placed in the pointed place; then the framework is done by timber to hold the wet concrete.
5. After the concrete has placed then the most vital job is to remove the air bubbles from the concrete; otherwise, the concrete gets porous, and it may be weaker.
6. After concrete placing, compaction and finishing happen then the concrete starts its internal reaction and it generates a lot of amount of heat that’s why we need to cure the concrete by the water curing process.
7. In the vast amount of concrete placing, there is a coolant that is used to reduce the heat, which is produced by hydration is called refrigerant coolant.
8. So, these are those actions which are very useful during the construction of the pile cap.

Piles Structure:

The pile is generally a strong cylinder structure which supports loads of superstructure and transfers it to the subsoil. The pile cap is constructed just above the pile.

Pile foundation is used in some following situations, those are-

1. Pile foundation is used when the subsoil is weaker, but the superstructure load is too heavy to carry that by shallow footing, then we use a pile foundation structure.
2. When the structure load is too heavy like a bridge, water tank, high rise building, etc., we need to use a pile foundation.

Piles are mainly two types, those are-

1. End-bearing Pile: In this type of pile, the end portion of this pile rests on the hard starts below the ground. In this type of pile is acting as a vertical column.
2. Friction Pile: when the structural load is too heavy, but the subsoil is not too hard, and we can not find hard strata in a reasonable depth, then we need to use friction pile. This type of pile dia is too large because to increase the surface area for the friction.

Pile Cap Foundations:

Foundation of the pile cap depends upon various points, those are-

1. Shape and Size of Pile cap

The shape and dimensions depend upon the numbers of the pile and the spacing between two piles. Like- Triangular pile cap for three piles, hexagonal pile cap for 6-7 piles, rectangular pile cap for other numbers of the pile, etc.

This type of pile cap is used to reduce the pile cap area because it’s compact the position of the pile. The pile cap should not be overhanging more than 150 mm. This minimum area provides sufficient length to:

1. Tension reinforcement has a large radius bend.
2. Make sure that enough reinforcement cover.
3. Must satisfy construction tolerance.

2. Depth of Pile Cap

Depth of pile cap is another section in the pile cap foundation works. The following criteria are considered-

1. Shrinkage of clay
2. Shear capacity of the pile cap
3. Pile anchorage
4. Natural condition like frost attack
5. The water table of the ground surface

Naturally, the depth of the pile cap is much more than pad footing; that’s why it can carry more concentrated loads. Pile cap depth for different diameters of the pile-

Square Piling Caps

1. The square pile cap is one type of popular pile cap structure.
2. In this pile cap structure, all the piles are situated in a combined and compacted way.
3. The square pile cap is provided when the number of piles is even; it may be four, six, eight etc.
4. The square pile cap is highly used in bridge structure, high rise building, etc.

Pile Cap Reinforcement

1. Pile cap reinforcement and it’s calculation is one of the most essential things in the pile cap section.
2. Before starting the main calculation, we need to find out the main volume of the pile cap that is (length x breadth x height)
3. Bar bending schedule table for reinforcement:

1. In pile cap designing, we use reinforcement cage; it increases the strength of vertical concrete column when the soil condition is not good.
2. In the case of the prefabricated pile, the piles are directly placed in the site, and then the pile cap reinforcement works are done.

FAQ: Pile Cap Design and Construction

What is a pile cap?

A pile cap is a thick concrete mat-like structure that rests on piles (either timber or concrete) to provide a stable foundation for the superstructure. It transfers the entire load from the superstructure into the piles, ensuring stable bearing capacity.

How does a pile cap differ from a raft foundation?

While both pile caps and raft foundations are concrete structures providing support, a pile cap rests on a group of piles, whereas a raft foundation spreads the load over a large concrete slab resting directly on the ground.

What are the key design suppositions for a pile cap?

1. The pile cap must be rigidly connected to the structure.
2. Piles are slightly elastic and distortions must be measured in the design.
3. Pile heads are pivoted to the pile cap to protect against bending moments.

What are the main limitations in designing a pile cap?

1. The shape and form of the pile cap.
2. Quantity of steel used.
3. Placement of reinforcement.
4. Depth of piles.

What characteristics are essential in pile cap design?

1. Minimum overhang of 100 mm, preferably 150 mm.
2. Higher thickness to accommodate reinforcement overlaps.
3. Clear cover of more than 40 mm.
4. Span-to-thickness ratio not exceeding 5.
5. Minimum thickness of 500 mm, tapering to 300 mm at the edges.
6. Piles must extend at least 50 mm into the pile cap.
7. The upper coarse layer should be at least 75 mm thick.

What materials and processes are involved in constructing a pile cap?

1. Cement, sand, and small aggregates are used to create the concrete mix.
2. Shuttering and reinforcing framework support the structure.
3. Reinforcement cages are placed and held with timber frameworks.
4. Air bubbles are removed from wet concrete to prevent porosity.
5. Concrete curing involves water or coolant to manage heat from hydration.

What are the types of piles used in pile foundations?

1. End-bearing Pile: Acts as a vertical column with its end resting on a hard stratum.
2. Friction Pile: Used when hard strata are deep; relies on surface friction for load transfer.

What factors determine the shape and size of a pile cap?

The number and spacing of piles determine the shape and dimensions of the pile cap. Common shapes include triangular, hexagonal, and rectangular, based on the number of piles.

How is the depth of a pile cap decided?

Depth is influenced by factors like clay shrinkage, shear capacity, pile anchorage, natural conditions (e.g., frost attack), and the water table. It generally exceeds that of pad footing to handle concentrated loads.

What are the standard depth requirements for pile caps based on pile diameter?

1. 300 mm diameter pile: 700 mm depth
2. 350 mm diameter pile: 800 mm depth
3. 400 mm diameter pile: 900 mm depth
4. 450 mm diameter pile: 1000 mm depth
5. 500 mm diameter pile: 1100 mm depth
6. 550 mm diameter pile: 1200 mm depth
7. 600 mm diameter pile: 1400 mm depth
8. 750 mm diameter pile: 1800 mm depth

What is a square pile cap and where is it used?

A square pile cap is used when piles are arranged in a compact, square configuration, often for even numbers of piles like four, six, or eight. It is commonly used in bridge structures and high-rise buildings.

How is pile cap reinforcement calculated?

The volume of the pile cap is calculated first (length x breadth x height), followed by a bar bending schedule for reinforcement. This includes specifying the diameter, number, and length of rods used.

Why is reinforcement important in pile caps?

Reinforcement cages enhance the strength of the vertical concrete columns, especially in poor soil conditions. They provide additional stability and load-bearing capacity.

How does prefabricated pile placement differ from on-site reinforcement?

Prefabricated piles are directly placed at the site, followed by the installation of pile cap reinforcement. This method is often quicker and ensures better quality control compared to on-site casting.