How to Design Pile Cap for Group of Piles in Foundation?

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Design of Pile Cap

A pile cap is the mat that rests on groups of piles and forms part of the foundation. The pile cap design, shape, and dimensions are discussed here.

Shape and Size of Pile Caps

The shape and plan dimensions of the pile cap depends on the number of piles in the group and the spacing between each pile. The most common shapes of pile caps are shown in figure below.

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

These pile cap shapes minimize the plan area for a symmetrical arrangement of piles about the load. The pile cap should overhang the outer piles by at least 150mm but should not be excessive, generally not more than the diameter of the pile diameter. This minimizes the plan area and thus the cost, while providing sufficient length to:

• Anchor the tension reinforcement with a large radius bend
• Ensure adequate cover to the reinforcement
• Satisfy construction tolerances.

Depth of Pile Cap:

The depth of pile cap is governed by the following criteria, where applicable:

• 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 criterias above, shear is the most critical criteria for selection of depth of pile cap. Generally, a pile cap is much deeper than a pad footing of similar dimensions, because it is subjected to much higher concentrated reactions, and consequently to much higher bending moments and shear forces. However, the increased depth gives the pile cap greater rigidity which is advantageous in spreading the load evenly to all piles. The Federation of Piling Specialists recommend the following most effective pile cap depths for a range of pile diameters as shown in table below:

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