3. Selection of characteristic values for CPT…
important and difficult task!
An unified procedure of selection does not exist.
Bias, subjectivity and different interpretations.
Introduction
measured value
derived value
characteristic value
design value
7. Filtering data
Remove picks related to
measurement anomalies, errors, and
very thin layers (lenses)
______________________________
!!!
Do not neglect important
information!
Do not remove data which represent
significant layers!
______________________________
The moving window method (Vessia
and Cherubini, 2007)
9. Characteristic qc values
Eurocode1990 “Basis of design” defines the characteristic value as a
5% fractile value, when a low value of the material is unfavorable.
works well for man-made
materials (low variability)
fails when applied to geotechnical
parameters (high variability)
EN 1997-1“Geotechnical design”, “the characteristic value of a
geotechnical parameter shall be selected as a cautious estimate of
the value affecting the occurrence of the limit state”
? How much ground is involved
? The required level of confidence
? Existing background information of site
? Number of samples and extension
10. Characteristic qc values
qcmean – mean value of qc for a specific depth
K – statistical coefficient (i.e. confidence level)
s – standard deviation of qc data
5 approaches
for K
11. Characteristic qc values
3 approaches for
mean values line
Best-fit line
(all the profile)
Best-fit line
(each layer)
Simple mean
(each layer)
17. Results
Difficult to get a single line (value) for characteristic values of qc:
- different available approaches,
- subjectivity during the cautious estimate process
- large variability in qc values.
