Quantification of Sample Disturbance in Soft Clay

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Quantification of Sample Disturbance in Soft Clay
Steven E. Poirier, University of Massachusetts
Researchers have struggled with the evaluation of
the effects of sample disturbance of soft clays
on results of geotechnical laboratory testing for
many years. As the monetary and time costs for
major engineering works escalate, the importance
of obtaining accurate stress-strain-strength data
from soil samples becomes increasingly critical.
Inaccurate evaluation of in situ soil properties
in the laboratory due to the effects of sample
disturbance may lead to hazardous under-design or
costly over-design of engineering projects.
Laboratory testing of soil samples is important
to determine strength and settlement properties,
which are the two most important geotechnical
soil parameters for design. However, during
sampling, disturbances may occur, affecting these
measured properties. The two types of
disturbance are mechanical (physical) and stress
relief (due to replacing the natural stress state
with zero total stress). There are several
recommendations for measuring the degree of
disturbance of a sample, but most of these
methods provide relative disturbance there is
not an accepted procedure for quantifying sample
sample disturbance.
Example of Disturbance Effects on Strength Axial
Strain vs log Effective Vertical Stress for CRS
at 12.3 m Depth in Drammen Clay (from Lunne et
al. 1997)
Example of Disturbance Effects on Strain Shear
Stress vs Axial Strain for CAUC Tests at 6.1 m
Depth in Drammen Clay (from Lunne et al. 1997)
The research will involve obtaining conventional
and high quality soil samples (block samples)
from a site deposited with soft clay (Newbury,
MA), extensive laboratory testing and soil
property assessment of these samples, and
advancing suction measurement technology and its
application to the evaluation of sample
disturbance.
Conventional soil samples (for camparison) will
be obtained from the Newbury test site using
Shelby tubes, while high quality block samples
will be obtained using the Sherbrooke Block
Sampler. The Sherbrooke sampler is generally
accepted to provide the highest quality (least
mechanical disturbance) of all soil samplers.
High quality samples are samples with minimal
disturbance. Since stress relief is unavoidable,
high quality samples imply minimal mechanical
disturbance.
Sherbrooke Sampler
A device for measuring soil suction (suction
probe) for measurements will be developed, and
the use of soil suction for evaluating sample
disturbance will be investigated. A controlled
laboratory testing program will be undertaken to
determine the validity of the suction probe as a
tool for measuring suction values that develop in
samples. Field and laboratory tests will be
conducted at the Newbury site (soft clay
deposit). Laboratory tests will include
one-dimensional consolidation, triaxial, and
direct simple shear testing while reconsolidating
to the in situ stresses. Evaluation of sample
disturbance using the suction probe will be
compared to other state of the art techniques of
sample disturbance evaluation.
Suction Probe
Application of Sherbrooke Sampler from Lefebvre
and Poulin (1979)
References Lefebvre, G. and Poulin, C. A New
Method of Sampling in Sensitive Clay. Canadian
Geotechnical Journal 161, 226-233. 1979.
Lunne, T., Berre, T., and Strandvik, S. (1997).
Sample Disturbance Effects in Soft Low Plastic
Norwegian Clay. In Almeida (Ed.), Recent
Developments in Soil and Pavement Mechanics (pp.
81-102). Rotterdam Balkema.