Actes du colloque - Volume 1 - page 207

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Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
ILLICON would have significantly under predicted both the
excess pore pressure and the settlement measured at the Väsby
test fill. In addition, Mesri and Choi (1985a) also ignored an
important role of buoyancy in large deformations. In the time
period analyzed by Mesri and Choi (1985a), a 42% load
reduction was estimated due to buoyancy effect. This effect is
too large to be neglected. Taking into account this key aspect
would have even led ILLICON to further underestimate
measurements in a similar manner to the results of the elasto-
plastic model (SS) shown in Figure 4 and Figure 5.
In connection to another field aspect, Mesri and Feng (2009)
also present a summary data, detailed in Mesri et al. (1995),
claiming that field and laboratory vertical preconsolidation
stresses ( ′p) are the same. The authors of this paper found it
difficult to access these data in Mesri et al. (1995), in order to
analyze the quality of the test data (e.g. sample disturbance
effects) and to assess how and which procedures are used in the
determination of ′p’s, see further discussion in Degago (2011).
An exception to this was the data by Sällfors (1975) which
constitutes one third of the data gathered by Mesri et al. (1995).
Therefore the authors have thoroughly looked into the
laboratory and field ′p data as presented in Sällfors (1975).
A good starting point to evaluate the data by Sällfors is to
understand the background behind the data. Sällfors studied and
determined field ′p based on pore pressure response to an
applied stress increment. Then he proposed a method to directly
predict the field ′p based on laboratory CRS tests. Because of
the background and aim of the method, the ′p interpreted in
this way would naturally give a ′p lower than those determined
for laboratory cases (Olsson, 2010). In simple terms, the good
match between laboratory and field ′p of Sällfors (1975)
merely show that the Sällfors method serves its purpose.
However, Mesri et al (1995) took the final data and made a
wrong conclusion and this may relate to a lack of understanding
of the objective behind the data gathered by Sällfors (1975).
5 SUMMARY OF THE MAIN MISCONCEPTIONS
The most important misconceptions as observed in the works of
the advocators of hypothesis A arises from overlooking some
important aspects of clay compressibility. These are discussed
in detail in this paper and are briefly summarized as follows:
1. The importance of the load (effective stress) increment
that starts below and exceeds the initial p′c has not been
considered and understood properly by the advocators of
hypothesis A and its role has continually been underrated.
2. Without a unique and consistent EOP criterion, the
results from samples with different specimen heights can be
interpreted inconsistently resulting in misleading conclusions.
The discrepancy is not necessary significant within the duration
of laboratory tests, but become important when extrapolating to
field condition. EOP is of no interest when using a model based
on hypotheses B as it gives a smooth transition between primary
and secondary consolidation phases.
3. Effect of sample disturbance needs proper assessment.
Successful prediction of long-term field performance demands
use of high quality data with creep considerations. However, by
using results from tests on highly disturbed samples and
disregarding creep, one may obtain reasonably good estimate of
settlements.
6 FINAL REMARKS
Degago et al. (2009, 2010, 2011a and 2011b) and Degago
(2011) clearly showed that there exist definitive data to
demonstrate that the creep hypothesis B agrees very well with
the measured behaviour of cohesive soils. It is also illustrated
that that the isotache approach describes this soil response
properly. In closure, the main points of these studies with regard
to this paper are briefly stated as follows;
1. The experienced preconsolidation stress as well as EOP
strain are rate dependent even for EOP loading conditions and
this fact has been experimentally supported by several EOP
tests and field observations. All the experimental evidences that
were used to wrongly advocate hypothesis A actually imply
hypothesis B.
2. Hypothesis A would require that the soil element close
to the drainage boundary would wait for the global EOP state
before staring its secondary compression. However, various
tests conducted on sub-specimen compressibility clearly showed
that this does not hold true and the compressibility of a soil
element is controlled by prevailing conditions at that particular
element rather than what is happening elsewhere.
3. A model based on the isotache approach gives excellent
agreement between field measurement and numerical
simulations when soil data are derived from high quality
samples along with appropriate analyses assumptions.
4. Future developments related to the compressibility of
natural clays, such as modeling anisotropy and destructuration
effects, should be based on the isotache framework (hypothesis
B) along with use of soil data from high quality samples.
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