Some useful construction tips for effective pile cap design

A pile cap refers to the mat that is supported with groups of piles and builds part of the foundation. This construction document briefly focuses on the pile cap design, shape and dimensions.

The shape and plan dimensions of the pile cap are based on the number of piles in the group and the distance amid each pile. Illustrated below, the most recognized shapes of pile caps.

These pile cap shapes reduce the plan area for a proportioned arrangement of piles concerning the load.

The pile cap overhands the outer piles with minimum 150mm but must not be extreme, usually not more than the diameter of the pile diameter.

This reduces the plan area as well the cost, whereas offering enough length to:

• Anchor the tension reinforcement with a large radius bend
• Guarantee sufficient cover to the reinforcement
• Settle construction tolerances.

Depth of Pile Cap: The depth of pile cap is influenced with the following factors, where relevant:

• Shear capacity of pile cap (beam and punching shear)
• Shrinkage and swelling of clay
• Frost attacks
• Pile anchorage
• Water table and soluble sulphates
• Holding down bolt assemblies for steel columns
• From the factors above, shear is considered as the most vital factor for choosing the depth of pile cap.

Usually, a pile cap is much thicker as compared to a pad footing having similar dimensions, as it is dependent on greater concentrated reactions, and subsequently to much greater bending moments and shear forces.

However, with the expanded depth, the pile cap gets the superior hardness that facilitates to disperse the load uniformly to all piles.

The Federation of Piling Specialists suggest the following most useful pile cap depths for a slew of pile diameters as demonstrated in following table:

Ref.: theconstructor.org

Pile Diameters (mm)		Depth of Pile Cap (mm)
300 350 400 450 500 550 600 750		700 800 900 1000 1100 1200 1400 1800