THESIS
2017
xxii, 228, that is xxv, 228 pages : illustrations ; 30 cm
Abstract
Thermo-Hydro-mechanical behaviour of loess is important for the serviceability and
ultimate limit states of high speed railway embankment. In semi-arid areas, loess
subgrade is subjected to daily heating-cooling and seasonally freezing-thawing. Many
theoretical and experimental studies have demonstrated that soil behaviour is
significantly affected by temperature. Most of these studies, however, focused on
compacted soils. Loess, which is well known as a "problematic soil", has a metastable
and high porosity structure under unsaturated conditions. Effects of soil structure (soil
particle arrangement) on hydro-mechanical behaviour of loess at elevated and
sub-zero temperatures has not been well studied.
The principal objectives of this research are to investigate the hydro-mecha...[
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Thermo-Hydro-mechanical behaviour of loess is important for the serviceability and
ultimate limit states of high speed railway embankment. In semi-arid areas, loess
subgrade is subjected to daily heating-cooling and seasonally freezing-thawing. Many
theoretical and experimental studies have demonstrated that soil behaviour is
significantly affected by temperature. Most of these studies, however, focused on
compacted soils. Loess, which is well known as a "problematic soil", has a metastable
and high porosity structure under unsaturated conditions. Effects of soil structure (soil
particle arrangement) on hydro-mechanical behaviour of loess at elevated and
sub-zero temperatures has not been well studied.
The principal objectives of this research are to investigate the hydro-mechanical
behaviour of loess at elevated and sub-zero temperatures. The oedometer, shear box
and triaxial apparatus were modified by adding temperature control systems. With
reference to the design of serviceability state, volume change behaviour of loess under
cyclic heating-cooling and freezing-thawing was investigated through the newly
developed oedometer and triaxial apparatus, respectively. For oedomter tests, loess
with different specimen preparation methods (intact, compacted and reconstituted)
was tested. With reference to the design of ultimate limit state, shear behaviour of
intact and compacted loess at elevated temperatures were studied through the shear
box apparatus. Furthermore, a new model for unfrozen water retention curve (UWRC)
was proposed by considering stress effects. The performance of the new model and
three popular existing models were compared through simulating measured UWRCs.
Experimental results showed that the plastic strains of intact, compacted and
reconstituted loess accumulated but at a reducing rate under cyclic heating-cooling.
The accumulated plastic strain of the reconstituted loess was about 68% and 38%
larger than those of the intact and compacted loess, respectively. Shear stiffness and
dilatancy of compacted loess increase by up to 47% and 63% respectively with an
increase in soil temperature from 20 to 60
oC. On the contrary, shear stiffness and
dilatancy of intact loess decrease by 35% and 68%, respectively. The difference is
because heating-induced plastic strain (0.4%), in addition to plastic strain-hardening
effects, also destroys the resistant structure of intact loess. These differences are likely
attributed to the different soil structures among the three types of specimens. Triaxial
test results showed that phase change between liquid water and ice occurs during
freezing-thawing. During phase change, the unfrozen water content of loess increased
with an increase of confining stress. This is because higher confining stress results in
smaller void ratios. More liquid water could be maintained due to the larger capillary
forces (ice-water interface).
Comparisons between measured and calculated results illustrate that the new model
improves the calculation of UWRC over existing UWRC models. Furthermore, the
new model can well capture stress effects on UWRC, which could not be captured by
existing models. The better performance obtained for the new model is mainly due to
the separation of the freezing of capillary and adsorbed water.
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