In geo-energy and geo-environmental engineering, soils are often subjected to heating and cooling cycles in many earth structures, such as high-level nuclear waste disposals and energy piles. The thermally induced soil plastic strains can have dramatic consequences for the serviceability of these geo-structures. To study the response of these geo-structures during thermal cycles, the thermal strains of the soils surrounding the geo-structures should be firstly understood. Up to date, there are already numerous experimental and numerical studies on the thermal strains of fine-grained soils. However, current studies mainly focus on the thermal strains of fined-grained soils under isotropic stress conditions, while soils are always under anisotropic stress condition in practical en...[ Read more ]

In geo-energy and geo-environmental engineering, soils are often subjected to heating and cooling cycles in many earth structures, such as high-level nuclear waste disposals and energy piles. The thermally induced soil plastic strains can have dramatic consequences for the serviceability of these geo-structures. To study the response of these geo-structures during thermal cycles, the thermal strains of the soils surrounding the geo-structures should be firstly understood. Up to date, there are already numerous experimental and numerical studies on the thermal strains of fine-grained soils. However, current studies mainly focus on the thermal strains of fined-grained soils under isotropic stress conditions, while soils are always under anisotropic stress condition in practical engineering. Consequently, the responses of the geo-structures cannot be accurately predicted and determined. On the other hand, some phenomenon such as the flow rules of the thermal plastic strains, the thermal dilatancy of the soil have not been well understood. Furthermore, beyond the fine-grained soils, little researches have been reported so far on the thermal strain behaviors of coarse-grained soils (i.e., sand) under thermal loads, although sand is always encountered in the practical geotechnical engineering.

The first part of this research aims at studying the thermal strain behaviors of saturated sand. Saturated Toyoura sand specimens with different relative densities (D_r = 21% - 90%) and confining stresses (p’ = 50kPa and 200kPa) were firstly isotopically compressed. Then, volume changes during two cycles of heating and cooling (temperatures from 23 to 50°C) were investigated. Meanwhile, the Discrete Element Method (DEM) was also adopted for a more insightful and comprehensive understanding on the thermal strain behaviors of the sand. It is found that thermally induced plastic volumetric contractions would only dominate for the loose sand initially on the ‘wet’ side of the critical state (i.e., low relative density or high confining stress). The magnitude of thermal plastic volumetric contraction increases significantly with the Been’s state parameters. The development of plastic contraction is attributed to particle rearrangements, which are induced by the thermal expansion/contraction and sliding of individual particles. On the contrary, the volume changes of dense packing initially on the ‘dry’ side of the critical state during thermal cycles are almost elastic, mainly because thermally induced particle rearrangements are relatively negligible due to its highly interlocked microstructures. Under anisotropic stress condition, not only plastic volumetric strain but also plastic deviatoric strains are mobilized for loose sand during heating. Furthermore, the dilatancy of the granular packing during heating tends to gradually decrease, especially under relatively high stress ratios.

In the second part of this research, experimental studies on the volumetric and deviatoric strains of Kaolin clays were carried out. Reconstituted Kaolin clay specimens were firstly prepared, saturated and then consolidated to different OCRs and stress ratios (η = 0, 0.25M and 0.45M, where M is the critical state stress ratio) under drained conditions. Afterwards, they were subjected thermal cycles between 23˚C - 50˚C by steps. For Kaolin clay specimens under stress ratios of 0, 0.25M and 0.45M, plastic volumetric strains of 0.51%, 0.42%, 0.25% and deviatoric strains of 0, 0.21% and 0.27% are mobilized after one heating-cooling cycle (23˚C→50˚C→50˚C). At anisotropic stress condition, clay specimens show increasing dilatancy during heating. When clay specimen is subjected to multiple thermal cycles under a higher stress ratio, the accumulation of thermal deviatoric strains is more pronounced than that of thermal volumetric strains with the number of thermal cycles.