Why pile diameter matters more than most people realise

Pile diameter is one of the most fundamental decisions in any CFA piling design. It directly affects how much load each pile can carry, how deep it needs to go to achieve that capacity, how much concrete is required, and ultimately what the foundation costs to build. It is not a number that gets picked from a standard list. It is calculated by a structural engineer based on the ground conditions at your specific site and the structural loads the pile needs to carry, and getting it right at the design stage avoids expensive changes once installation is underway.

The diameter range used in CFA piling and what drives the selection

CFA piles are typically installed in diameters ranging from 300mm at the smaller end, commonly used on residential extensions and lightly loaded structures, up to 900mm for heavily loaded commercial and multi storey schemes. The most frequently specified diameters across residential and light commercial work in the UK fall between 350mm and 600mm, with the specific size determined by the structural loads and the ground conditions rather than by preference or convention.

Larger diameter piles generate more shaft friction along their length and greater end bearing resistance at the tip, which means they can carry more load. However, a larger diameter also means more concrete per pile, a heavier reinforcement cage, and in some cases the need for a larger rig. The engineer will balance these factors to arrive at a diameter that is structurally efficient and practically achievable on site.

On sites where access is restricted or working space is tight, a smaller diameter pile at closer centres may be more practical than a larger diameter pile at wider spacing, even if both options achieve the same structural outcome. Capital Piling works closely with design teams at this stage to make sure the specified diameter is both right for the ground and workable on site.

How diameter is determined during the design process

The structural engineer begins with the load at each pile position, which comes from the building's structural frame or load bearing walls above. Using the soil parameters from the ground investigation, the engineer calculates how much shaft friction and end bearing a pile of a given diameter and depth can develop in the specific ground conditions on site. The diameter and depth are then adjusted until the calculated capacity exceeds the design load by the required factor of safety.

In cohesive soils such as London Clay, shaft friction along the pile length is the primary source of capacity, and increasing the diameter increases the surface area available to develop that friction. In stiffer or granular soils where end bearing contributes more significantly, diameter also increases the cross sectional area bearing onto the founding stratum at depth.

How diameter choices play out across different project types

On a typical two storey residential extension in London Clay, 300mm or 350mm diameter piles at depths of six to ten metres are a common outcome. For a terrace of new build houses, 400mm or 450mm piles are frequently specified. On a multi storey apartment block or commercial frame building with significant column loads, 600mm or larger piles at greater depths are more typical.

Where pile layouts include groups of piles beneath a single column or pile cap, smaller diameter piles at closer centres can achieve the same combined capacity as fewer, larger piles. The choice between these approaches depends on the specific load pattern, the ground conditions, and what is most cost effective for the project as a whole.