Design of Pile Cap
The focus of this discussion is the pile cap, which is a crucial component of a foundation system. Specifically, a pile cap refers to the mat that sits on top of a group of piles and serves as a support structure for the entire foundation. The design, shape, and dimensions of the pile cap are essential considerations that must be carefully evaluated to ensure the stability and durability of the foundation. Proper design of the pile cap is crucial to ensure that the load distribution is uniform and that the piles are protected from excessive stress or deformation. Therefore, the shape and dimensions of the pile cap must be tailored to the specific site conditions and the loads that the foundation will be subjected to. Overall, the pile cap is an essential element of a foundation system, and its design must be carefully planned and executed to ensure the long-term safety and stability of the structure.
Shape and Size of Pile Caps
The design of pile caps is influenced by two factors: the number of piles in the group and the spacing between each pile. Based on these considerations, the shape and plan dimensions of the pile cap are determined. Various shapes of pile caps are commonly used in construction, and these are illustrated in a figure.
Fig.: Triangular Pile Cap for 3 piles
Fig.: Hexagonal pile cap for 6 to 7 piles
Fig.: Rectangular pile cap for all other number of piles
To minimize the cost, symmetrical arrangements of piles about the load can be used with pile cap shapes that have a small plan area. For such designs, it is important that the pile cap overhangs the outer piles by a minimum of 150mm, but not excessively, which is generally not more than the diameter of the pile. This approach reduces the plan area and ultimately lowers the cost of the construction.
Additionally, the design of pile caps should ensure that there is sufficient length to anchor the tension reinforcement with a large radius bend, provide adequate cover to the reinforcement, and satisfy construction tolerances. These factors are critical to ensure the structural integrity and stability of the construction.
Therefore, when designing pile caps, it is essential to consider the symmetrical arrangement of piles about the load, overhang distance, diameter of the pile, length of tension reinforcement, cover to reinforcement, and construction tolerances to achieve a cost-effective and stable construction.
Depth of Pile Cap:
The depth of a pile cap is determined by various criteria, such as shear capacity, shrinkage and swelling of clay, frost attacks, pile anchorage, water table and soluble sulphates, and holding down bolt assemblies for steel columns. However, the most critical criteria among them is shear capacity. In selecting the depth of a pile cap, the shear capacity needs to be given priority, as the pile cap is subjected to higher concentrated reactions, resulting in higher bending moments and shear forces.
Compared to a pad footing of similar dimensions, a pile cap requires a much deeper foundation. This is because the pile cap is subjected to higher concentrated reactions and greater bending moments and shear forces. A deeper pile cap provides greater rigidity, which is beneficial in distributing the load evenly to all piles.
The Federation of Piling Specialists recommends specific depths for pile caps, depending on the diameter of the piles. These recommended depths have been proven to be effective in ensuring the stability and strength of pile foundations. By following these guidelines, the appropriate depth for a pile cap can be determined, taking into consideration the critical criteria mentioned earlier.
Table: Pile cap depth for different pile diameters
| Pile Diameters (mm) | Depth of Pile Cap (mm) |
| 300 | 700 |
| 350 | 800 |
| 400 | 900 |
| 450 | 1000 |
| 500 | 1100 |
| 550 | 1200 |
| 600 | 1400 |
| 750 | 1800 |