Pile Foundation Calculator — Pile Count, Spacing & Ground Beams

Number of piles = Total Building Load / (Pile Ultimate Capacity / Safety Factor), where Total Building Load is the UNFACTORED (characteristic/working/service) load — not the ULS-amplified figure. This is the BS 8004 working-stress method: the safety factor is applied to the resistance side to give the Safe Working Load, then compared against the working load. BS 8004 gives a single-pile safety factor of 2.0-3.0, with 2.5 typical. For a 500 kN working load with 250mm CFA piles (200 kN indicative ultimate capacity), safe working capacity = 80 kN, so you need approximately 7 piles. Pile capacity is wholly ground-dependent, so the structural engineer and a geotechnical site investigation determine the actual capacity, count and positions.

For friction piles, BS 8004 requires a centre-to-centre spacing of not less than 3 times the pile diameter (or the pile perimeter). For 250mm piles, minimum spacing is 750mm; for 450mm piles, it's 1.35m. End-bearing piles, which carry their load mostly at the base, can be spaced closer — not less than twice their least width. Closer spacing causes stress overlap in the soil, reducing the capacity of each pile. In practice, pile positions are determined by the building layout and load paths.

Ground beams are reinforced concrete beams that span between pile caps, supporting the building walls above. They transfer wall loads to the piles. Typical domestic ground beams are 450-600mm deep × 300-450mm wide with rebar reinforcement. They sit on a compressible material (e.g., Clayboard) so they can move independently of the soil.

Piled foundations are used when the soil near the surface cannot support the building loads — for example, soft clay, made ground, peat, or areas with large trees causing clay shrinkage. They transfer loads through weak soil to a stronger stratum below. Common in the UK for sites with significant tree influence on shrinkable clay. See our Foundation Calculator for conventional alternatives where ground conditions allow.

Mini-piles (150mm) cost approximately £800-1,500 each. CFA piles (250-300mm) cost £1,000-2,000 each. The rig mobilisation/demobilisation is typically £1,000-3,000 regardless of the number of piles. Ground beams add £80-150 per linear metre. A typical domestic piling scheme (15-25 piles + ground beams) costs £15,000-40,000.