Cone Resistance (qc): Why This Parameter Governs Your Pile Design
A CPT chart often shows several lines: cone resistance, sleeve friction, friction ratio and sometimes pore pressure. For the design of a pile foundation, however, almost everything revolves around a single line: the cone resistance qc. Anyone digitizing an old CPT sounding can therefore usually restrict themselves to that one line.
What exactly does the cone resistance measure?
During a CPT, a cone with a 60° apex angle and a cross-section of usually 10 or 15 cm² is pushed into the ground at a constant rate (2 cm/s). The force on the tip divided by the cross-sectional area is the cone resistance, expressed in MPa. It is thus a direct, continuous measurement of the strength of the soil over depth — not an interpretation, but a measured value.
As rough rules of thumb for Dutch soils:
- < 0.5 MPa: peat and soft clay
- 0.5 – 2 MPa: clay, silty layers
- 2 – 10 MPa: loose to medium dense sand
- > 10 MPa: dense sand — the bearing stratum that piles are founded on
From qc to pile bearing capacity
In NEN 9997-1 (the Dutch national annex to Eurocode 7), the base capacity of a pile is calculated using the 4D/8D method (Koppejan): the average of the cone resistance over an interval extending 4 pile diameters below and 8 pile diameters above the pile tip level, with rules for cutting off peaks. The shaft friction likewise follows from qc, multiplied by a pile-type-dependent factor αs.
In concrete terms: the qc line determines the pile tip level, the capacity per pile and hence the number of piles. A difference of half a meter in the depth of the dense sand layer feeds directly into your design.
How accurately do you need to know qc?
More accurately than you might think — but in the places that matter. The bearing capacity calculation averages qc over intervals of several decimeters to meters, so small reading errors average out. What does matter:
- The depth of layer boundaries. The level at which the dense sand layer starts must be correct to within ± 0.1 m. So calibrate the depth axis carefully.
- Thin soft layers. A 30 cm peat layer within the sand pushes the average qc down considerably and can cause negative skin friction. Such a dip must not be lost when digitizing.
- Peaks don't need to be exact. Whether a peak is 18 or 20 MPa makes little difference after cut-off and averaging.
A carefully digitized scan is therefore perfectly usable for pile calculations; how to go about it is explained in our step-by-step guide to digitizing to GEF.
When do you need more than qc?
For some assessments, cone resistance alone is not enough. The friction ratio helps with classifying soil types, and pore pressure measurements (CPTU) are needed for, say, stability and settlement analyses in soft clay. For the majority of pile foundations in the Netherlands, however, a reliable qc line plus the ground level will get you a long way.