Pile capacity from CPT: how the LCPC method reads a cone trace

Updated: 2026-08-13 · ISKRIT

A cone penetration test gives a resistance every two centimetres down the profile. A pile needs two numbers: what the shaft carries and what the toe carries. The whole method is the rule for getting from one to the other.

Toe resistance: the averaging window

The toe of a pile does not feel the soil at exactly its own level. It feels a zone above and below, and a single cone reading at the toe elevation is therefore the wrong input. The LCPC procedure averages the cone resistance over a window of about 1.5 pile diameters above and below the toe, after clipping values that depart too far from the window mean.

The clipping step is what makes the method robust. Without it, one spike from a gravel clast sets the capacity of the whole pile, and a designer reading the printout has no way to see that it happened.

Shaft friction: resistance divided by a factor

Unit shaft friction at each depth is the cone resistance at that depth divided by a friction factor that depends on the soil and on how the pile is installed, then capped at a limiting value for that combination. A driven closed-end pile in dense sand and a bored pile in the same sand get different factors, because the installation either densifies the soil around the shaft or relaxes it.

The thin dense layer trap

A half-metre of dense sand inside soft clay shows on the trace as a sharp peak. If the pile toe happens to land in it, the averaging window pulls in the soft material above and below and the capacity drops back toward the clay value, which is correct: a thin layer cannot carry a pile whose failure surface passes straight through it. A calculation that reads the peak directly will overstate the toe capacity several times over.

What the method does not give you

Settlement. Capacity and settlement are separate questions, and a pile that satisfies the capacity check can still settle more than the structure tolerates. The load-settlement curve is a second calculation on the same data, and for piles in service under working load it is usually the governing one.

The pile capacity calculator plots the cone trace with the averaging window drawn on it, so the toe value can be seen in the context that produced it rather than taken on trust.