Geotechnical News June 2011
35
COMPUTING IN GEOTECHNICAL ENGINEERING
coded to aid in the identification (e.g.
clay is dark blue when
I
c
> 2.6 and sand
is light brown when
I
c
< 2.05, sand-silt
mixtures are light-dark green between
2.05 <
I
c
< 2.60
). The transition zones
that were automatically detected based
on the rate of change of
I
c
are identified
in red on the SBT
I
c
profile.
The auto-transition layer detection
approach used in the software
CPeT-IT
and
CLiq
is illustrated in Figure 3. The approach
allows the user to adjust the rate of
change of
I
c
(
Delta I
c
ratio
), the num-
ber of data points used to determine the
rate of change and the range of
I
c
over
which the detection should be carried
out.
The example shown in Figures 2
and 3 illustrate that for the CPT profile
presented there are potentially 14 tran-
sition zones in the 30m profile. Fig-
ure 4 shows the example CPTu profile
in terms of estimated undrained shear
strength ratio (s
u
/σ’
vo
) and OCR for the
clay layers both with and without the
transition zones. Figure 4 illustrates
that, although the automatic detection
of transition zones is not perfect, the
strength and stress history profiles are
improved when the transition zones are
mostly removed. For the same example
CPTu profile the estimated post earth-
quake settlements are significantly re-
duced when the transitions zones are
removed. If the transition zones are
not removed the post earthquake settle-
ments are conservatively large.
Interpretation of continuous CPT
results can be improved if CPT-based
software has the ability to both identify
and remove transition zones in inter-
layered deposits. However, the user
should have the option to control both
the identification and removal of any
transition zones so that interpretation
can be reviewed both with and with-
out any transition zones to evaluate
the impact on any interpretation and/
or design.
References:
Ahmadi, M.M., and Robertson, P.K.,
2005. Thin layer effects on the
CPT qc measurement. Canadian
Geotechnical Journal,
42
(9): 1302-
1317.
Lunne, T., Robertson, P.K., and Powell,
J.J.M., 1997. Cone penetration test-
ing in geotechnical practice. Black-
ie Academic, EF Spon/Routledge
Publ., New York, 1997, 312 pp.
Treadwell, D.D. (1976). “The influence
of gravity, prestress, compressibil-
ity, and layering on soil resistance
to static penetration.” PhD thesis,
University of California, Berkeley.
Jefferies, M.G., and Davies, M.P.,
1993. Use of CPTU to estimate
equivalent SPT N
60
. Geotechni-
cal Testing Journal, ASTM,
16
(4):
458-468.
Robertson, P.K. and Wride, C.E., 1998.
Evaluating cyclic liquefaction po-
tential using the cone penetration
test. Canadian Geotechnical Jour-
nal, Ottawa,
35
(3): 442-459.
Robertson, P.K., 1990. Soil classifica-
tion using the cone penetration test.
Canadian Geotechnical Journal,
27
(1): 151-158.
Robertson, P.K., 2010. Soil behaviour
type from the CPT: an update. 2
nd
International Symposium on Cone
Penetration Testing, CPT’10, Hun-
tington Beach, California. www.
cpt10.com.
Robertson. P.K., Campanella, R.G.,
Gillespie, D., and Greig, J., 1986.
Use of Piezometer Cone data. In-
Situ’86 Use of Ins-itu testing in
Geotechnical Engineering, GSP 6 ,
ASCE, Reston, VA, Specialty Pub-
lication, SM 92, pp 1263-1280.
Peter Robertson, Professor Emeritus,
University of Alberta, Technical Di-
rector, Gregg Drilling & Testing, Inc.,
Signal
Hill,
California
USA,
email:
