Resistência ao cisalhamento de um solo tropical não saturado considerando altas sucções
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2014-08-15
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Universidade Federal de Goiás
Resumo
Often overlooked, the mechanical behavior of unsaturated soils when subjected to high suctions
is relevant for several engineering problems. These range from common questions regarding
the behavior of near-surface desiccated clays to very specific challenges, such as the design of
nuclear waste disposal barriers. In this context, the objectives of this work are to experimentally
study the shear strength of a compacted unsaturated tropical soil considering high suctions, to
evaluate the performance of existing shear strength models, and to develop a model that
addresses the particular behavior observed under high suctions. The material studied, a residual
clayey soil from Goiania, GO, Brazil derived from mica schist, was semi-statically compacted
at optimum conditions corresponding to the Standard Proctor. An extensive experimental
program was undertaken, manly comprised of CDnsat triaxial tests for relatively low matric
suctions (i.e., lower than 300 kPa), CW triaxial tests for relatively high total suctions, and
Brazilian tensile tests also for high total suctions. The CW triaxial and tensile tests required the
measurements of suction before and after failure. These measurements were carried out using
both the filter paper technique for total and matric suction and a dew point potentiometer for
total suction. In addition, tests were performed to determine the soil-water characteristic curve
(SWCC) at unconfined condition. Data obtained from unsaturated triaxial tests also allowed the
measurement of the SWCC, specifically for the soil consolidated at different confining stresses
and for the soil subjected to deviator stresses. The experimental program was complemented
by consolidated-drained triaxial tests and isotropic compression tests, both under saturated
conditions. The monitoring of several stress, strain, and volume-mass state variables allowed
for a detailed evaluation of soil behavior. Based on the results of the experimental program, it
was determined that for total suctions ranging from zero up to approximately 10,000 kPa there
is significant nonlinear shear strength increase. Beyond that suction, shear strength decreases
as the soil desaturates. The shear strength loss depends on the net normal stress, with the soil
presenting smaller reductions in shear strength for higher confining stresses. A remarkable
relationship between the SWCC and the shear strength was observed, with the point of
maximum strength corresponding to the air-entry value of the micropores. The relationship
between shear strength and the water stored in micropores that are generally present within clay
aggregates indicates the importance of the mechanical stability of these soil aggregates. The
performance of different fitting and prediction models was evaluated considering both low
suctions and the wide range of suctions used in the experimental program. Existing prediction
models based on SWCC proved inaccurate for high suctions, requiring changes in their
formulations. A new family of models that consider the entire range of suctions, including the
loss of shear strength, was proposed. Finally, the prediction models using the SWCC of the soil
under deviator stress (i.e., peak conditions) was evaluated, showing that the use of the
unconfined SWCC presents superior prediction results to both low suctions and the wide range
of suctions.
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KUHN, V. O. Resistência ao cisalhamento de um solo tropical não saturado considerando altas sucções. 2014. 192 f. Dissertação (Mestrado em Geotecnia, Estruturas e Construção Civil) - Universidade Federal de Goiás, Goiânia, 2014.