Pile caps are thick reinforced concrete mats that rest on top of concrete or wooden piles (driven, bored, or cast-in-situ) to offer stability to the foundation.
The main purpose of the construction of the pile cap is to distribute the load of the columns, which is usually too large for large construction, for underlying piles.
A similar type of foundation for pile cap is the rough foundation.
In the case of the raft, which is a thick and broad RCC slab resting directly at the base, the load from the columns is carried directly through the raft to the mud strait.
The raft can only be used for a stable hard soil layer capable of carrying huge loads without failure, otherwise, pile and pile cap are commonly used.
Function of pile cap:
1. To evenly distribute a single load coming from the column to the piles.
2. Stabilizing the pile, thus increasing the overall efficiency of the pile group.
3. To provide the necessary joint resistance stresses established by the superstructure and ground movement.
Main features of pile caps:
- A minimum grade of concrete that can be used to manufacture piles and pile caps is M25.
- In the case of small piles like 2-3 piles, an entire pile cap can be inserted unbroken.
- For larger pile caps casting is done in layers.
- The reinforcement bars of the columns are embedded in pile caps up to their growth length.
- After casting, the pile caps are fixed for 28 days and the shuttering is removed, the adjacent soil is compacted and leveled using a compactor.
- When the number of piles in a group is very high, piles are constructed in the form of raft instead of piles.
Design of Pile Cap:
The pile cap is a mat which is rested on the numerous piles and it is a part of the foundation.
Shape and Size of Pile Caps:
The size and plan dimension of the pile cap will depend upon the variety of piles within the group and the spacing between each pile.
Pile caps are thick reinforced concrete mats that rest on concrete or woodpiles (driven or bored and cast-in-situ) to offer stability to the foundation.
These pile cap sizes reduce the planning space for the symmetric arrangement of the pile concerning the load.
The pile cap should enable the outer pile to exceed a minimum of 150 mm however should not be extreme, usually not higher than the diameter of the pile.
This reduces the plan area thus the cost while providing sufficient length to pile cap as follows:
- Tension reinforcement anchor with a large radius bend.
- Ensure adequate cover for reinforcement.
- Satisfactory construction tolerance.
Depth of Pile Cap:
The depth of the pile cap is governed by the following criteria as follows:
- Shear Capability of Pile Cap.
- Soil shrinkage and swelling.
- Frost attacks.
- Pile anchorage.
- Water table and soluble sulfates.
- Holding bolt assemblies for steel columns.
From this criterion, shear is the most important criterion for the selection of pile depth.
Generally pile cap is deeper than the pad footing because it is subjected to concentrated loads which results in much larger bending moments and shear forces.
However, the increased depth gives the pile cap greater stiffness which is beneficial in spreading the load evenly for all piles.
Piling specialists suggest the following simplest pile cap depths for federation pile numbers, as shown within the table:
Pile cap depth for different pile diameters:
|Pile Diameters (mm)||Depth of Pile Cap (mm)|
Also read: Sheet Piles, Pitched Roof & Micropiles