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How to use CPT results?

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CPT results give you the cone resistance and the local or total friction. Somethimes also the internal friction angle or some other parameters are mentioned.


I was talking to some other engineers about how the CPT results are used and I got some different methods.

1) using the exact values of the CPT result

For the bearing capacity of a pile, we use a xls spreadsheet where we have to input the coneresistance. So we input all the CPT-values of the cone resistance to the end of the CPT test.

We also have a similar spreadsheet for the bearing capacity of a fondation slab, retaining wall,..


2) using a "mean" CPT result

A collegue doesn't use the exact values of the CPT results but he divides the results in "blocks with +-equal results". Then he uses these mean block-results in the xls spreadsheets.


3) using "soil-type" based on CPT results

A collegue (another company) uses the CPT results to determine the soil type (sand, clay,...) or layers of soil-type (soil behavior charts). Then he uses this soil-type in his GEO software with predetermined soil types. So he doesn't input CPT results directly but only the soil types.


4) using "soil-type" in 3D FEM software

Similar to (3) but using the soil-type as the "soil" in an 3D FEM analysis.


I was wondering what others are doing with CPT results.

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We either use CPT results directly with D.7. annex of EN 1997-2 to calculate the resistance of a pile and then check settlement with characteristic loads with elastic space method.


Another way is we draw the CPT result diagrams from all bores next to each other and then superimpose them on onto another to visually inspect and decide on the "mean" values for all "layers". These mean values then are used with the D.7. annex.

Best of luck!

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