THESIS
2014
[xxv], 315 pages : illustrations ; 30 cm
Abstract
The deformation of subgrade soils under cyclic traffic loads is very crucial to pavement
performance and any differential deformation may induce cracking and rutting in the asphalt
and concrete layers of a pavement. In the field, subgrade soils are often unsaturated and
subjected to daily variations of water content (suction) and temperature. Many theoretical and
experimental studies have demonstrated that unsaturated soil behaviour is strongly affected
by suction and temperature. Most of these studies, however, focused on soil behaviour under
monotonic loading at strains larger than 1%. Cyclic behaviour of unsaturated soil at various
temperatures, particularly at small strains (less than 1%), are not fully understood.
The principal objectives of this research are to investigate...[
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The deformation of subgrade soils under cyclic traffic loads is very crucial to pavement
performance and any differential deformation may induce cracking and rutting in the asphalt
and concrete layers of a pavement. In the field, subgrade soils are often unsaturated and
subjected to daily variations of water content (suction) and temperature. Many theoretical and
experimental studies have demonstrated that unsaturated soil behaviour is strongly affected
by suction and temperature. Most of these studies, however, focused on soil behaviour under
monotonic loading at strains larger than 1%. Cyclic behaviour of unsaturated soil at various
temperatures, particularly at small strains (less than 1%), are not fully understood.
The principal objectives of this research are to investigate and to model cyclic behaviour at
small strains of unsaturated soil at various temperatures. A suction and temperature controlled
triaxial apparatus was newly developed in this study. Using this new apparatus, five series of
cyclic triaxial tests were carried out to investigate cyclic behaviour of unsaturated soil at
suctions ranging from 0 to 250 kPa and temperatures ranging from 20 to 60℃. To better
interpret cyclic triaxial tests, one series of triaxial compression tests was conducted to study
suction and temperature effects on the degradation of soil stiffness with strain. Furthermore, a
new constitutive model was developed using the bounding surface plasticity theory. This
model was proposed to improve the existing elastoplastic models for simulating cyclic
behaviour at small strains of unsaturated soil at various suctions and temperatures.
Experimental results from triaxial compression tests show that soil stiffness at small strains is
consistently larger at higher suction and lower temperature. The results of cyclic triaxial tests
illustrate that plastic strain of soil specimen accumulates with increasing number of cycles at
a decreasing rate. Each unsaturated specimen tested reaches a stable reversible response
within 100 cycles. The plastic strain accumulated during 100 cycles is larger at lower suction
and higher temperature. These observations are mainly due to suction hardening and thermal
softening. Also, the resilient modulus is strongly dependent on suction. At a given
temperature, when suction increases from 0 to 250 kPa, the resilient modulus increases by up
to one order of magnitude. Furthermore, the resilient modulus of unsaturated soil is slightly
affected by temperature. At the same suction, resilient modulus is smaller at 60℃ than at 20℃
by about 20%. Since current pavement design methods do not normally consider thermal
effects on soil behaviour, soil deformation induced by cyclic traffic loads may be
underestimated when in-situ temperature is significantly higher than room temperature.
Model predictions are in good agreement with experimental results under both monotonic and
cyclic loads. For triaxial compression tests, the model shows good predictions of the
degradation of soil stiffness with strain over a wide strain range from 0.001% to 1%. The new
model is able to predict effects of suction, temperature, drying and wetting and net stress on
small strain behaviour of unsaturated soil. For cyclic triaxial tests at various suctions and
temperatures, the non-linearity and hysteresis loop of stress-strain relation and accumulation
of plastic strain are all well captured by the new model.
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