Hydro-mechanical behaviour of loess at elevated and sub-zero temperatures

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 &quot;problematic soil&quot;, 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.</p

A New Evolution Model for Weighted Directed Networks

The most of the recent models of directed weighted network evolution capture the growth process based on two conventional assumptions: constant average degree assumption and slowly growing diameter assumption. Such evolution models cannot fully support and reflect the dense power law and diameter shrinkage in the process of evolution of real networks. In this paper, a new evolution model, called BBVd, is proposed for directed weighted networks by extending BBV model with the idea of the Forest Fire model. In BBVd, new directed edges are established with probabilities computed based on in/our-strength of nodes, with dynamical evolution of weights for local directed edges. The experimental result shows that the generated networks using BBVddisplay power-law behavior for the node strength distributions, and moreover, it satisfies the densification power laws and has shrinking diameter.</jats:p

An unfrozen water retention curve for capturing soil density and specific surface effects

Unfrozen water retention curve (UWRC) defines the relationship between temperature and unfrozen water content in frozen soils. Although many models have been proposed for the UWRC, these existing models cannot predict UWRC well over a wide temperatures range. In this study, a new UWRC model is proposed with explicit considerations of both capillarity and adsorption. In this model, capillarity is considered dominating when the freezing of soil pore water at higher temperatures (above -2oC), whereas the effects of adsorption pronounce at temperatures below -2oC. Moreover, effects of void ratio on the freezing of capillary water are incorporated. The proposed model was applied to predict UWRCs of silt and clay at different initial void ratios over a wide temperature range (from -50 to 0oC). Predicted results by this new model are compared with predictions by three well-known existing models. The new model can capture the density effects on UWRC. Moreover, the new model can predict better UWRC over a wide temperature range since it explicitly considers both effects of capillarity and adsorption

Effects of Specimen Preparation Method on the Volume Change of Clay Under Cyclic Thermal Loads

Previous investigations of the volume change of soil with different fabric patterns have been mostly carried out at a constant temperature. To investigate the influence of the specimen preparation method on the volume change of saturated clay under cyclic thermal loads, reconstituted, intact and recompacted specimens were tested. Thermal axial strains of these specimens in a normally consolidated state were measured using a temperature-controlled invar oedometer apparatus. The soil fabric of each specimen was evaluated using a scanning electron microscope (SEM) and a mercury intrusion porosimeter (MIP). All specimens showed continuous contraction as the number of thermal cycles increased, albeit at a decreasing rate. After five heating and cooling cycles with temperatures ranging from 15 to 70°C, the accumulated plastic axial strain of the reconstituted specimen was 38% and 68% larger than those of the intact and recompacted specimens, respectively. The SEM visualisations and MIP measurements demonstrate that these observed differences can likely be attributed to different distributions of clay particles in the soil specimens (with a 28% clay content). In the intact and recompacted specimens, most of the clay particles formed silt-size aggregates. In the reconstituted specimen, the clay particles filled the spaces between silt particles and the soil fabric was homogeneous overall

An unfrozen water retention curve for capturing soil density and specific surface effects

Unfrozen water retention curve (UWRC) defines the relationship between temperature and unfrozen water content in frozen soils. Although many models have been proposed for the UWRC, these existing models cannot predict UWRC well over a wide temperatures range. In this study, a new UWRC model is proposed with explicit considerations of both capillarity and adsorption. In this model, capillarity is considered dominating when the freezing of soil pore water at higher temperatures (above -2oC), whereas the effects of adsorption pronounce at temperatures below -2oC. Moreover, effects of void ratio on the freezing of capillary water are incorporated. The proposed model was applied to predict UWRCs of silt and clay at different initial void ratios over a wide temperature range (from -50 to 0oC). Predicted results by this new model are compared with predictions by three well-known existing models. The new model can capture the density effects on UWRC. Moreover, the new model can predict better UWRC over a wide temperature range since it explicitly considers both effects of capillarity and adsorption.</jats:p