Experimental Study On Temperature Effect On Engineering Properties Of Clayey Soils

This thesis focuses on the temperature effect on engineering properties of clayey soils. Taking three types of clayey soils in Nanjing as test samples,5-50℃as the test temperature range that is determined by the urban seasonal temperature difference and urban heat island effect in Nanjing. a series of experimental studies have been done in the temperature effects on the engineering properties of three types of clayey soils, including their adsorbed water content, Atterberg limits, permeability, swelling properties and strength etc. and the related mechanisms of the temperature effect on engineering properties are analyzed and the corresponding findings are presented as follows:1. Adsorbed water content of three types of drying clayey soils were tested under various temperatures by unit-weight method. It showed that as the environmental temperature rose, adsorbed water content in the three types of clayey soils decreased linearly and the change rate was proportional to the hydrophilic mineral content. The heating process speeded up the thermal motions of water molecules in bond water. The water molecules in the diffusion layer break away from the adsorptive force and turn into free water, leading to a decrease in adsorbed water content.2. As the temperature rose, the plastic limit of three types of clayey soils hardly changed, but the liquid limit changed remarkably. In the clayey soils with no organic matter, some bond water transform into free water, causing the liquid limit and the plastic indexes'reduction, and the reducing rates depends on the hydrophilic mineral content. For the clayey soils containing a small amount of organic matter, the heating process make the organic matter activation energy increase and the adsorption ability enhance, which leads to the liquid limit increase. The relationship between the Atterberg limits and temperature is established through laboratory study, and the conclusion of decreasing of electric double layer thickness is proved.3. The permeability coefficients of three soils samples in the same dry density increased linearly with the rise in temperature and, the amplitude of variation showing Xiashu soil(S2)> Silt (Sl)> Bentonite(S3). The change rates of the permeability coefficients under the temperature rise differed among the densities of soil samples. The change rates of the infiltration coefficients of lower-density soil samples were higher than those of higher-density soil samples. The higher density of clayey soils, the lower infiltration coefficients was. The two primary causes of the increase in permeability are the attenuation of the electric double layer thickness and the decrease in water viscous resistance. Combined with the experiment on adsorbed water content, it is proved that the forms and thermal variations of water in soils directly affected the permeability of clayey soils.4. As the environment temperature rose, the non-load expansion rates of each sample linearly increased, but the growth rates were distinct. The samples with higher inflation rates had lower thermal expansion rates. Several factors cause the thermal expansion:on one hand, soil particles and water expanded during heating; on the other hand, the changes of electric double layer thickness was a major factor of the thermal inflation rates difference.5. The temperature changes influenced the strength of clayey soils. Temperature had great effects on the cohesive force and unobvious influence on internal friction angle. For clayey soils with lower content of hydrophilic minerals, when the moisture content was lower(w<17%), the shear strength showed the thermo-hardening phenomenon. However, the thermal-softening phenomenon was shown when the moisture content was higher(w>22%), and the temperature effect on shear strength was more obvious when the dry density of sample is higher. The cohesive force of clayey soils with higher content of hydrophilic mineral decreased with the temperature rose, showing thermal-softening on strength, and its strength was more sensitive to temperature change. Moreover, the mechanism of thermal hardening and softening of strength of unsaturated clayey soils relate to the change of the electric double layer thickness and viscous resistance of water, but also affected by soil mineral composition, saturation, density, and other aspects.6. The influence on the engineering properties of clayey soil caused by the environment geothermal field change in urban heat island environment is complex, slow and long-term. According to the test results and the measured data of Nanjing urban heat island effect intensity, when the annual average urban heat island effect intensity is 2.14℃, the adsorbed water content of S3 decrease 0.66%, the strength reduce 0.85kPa; when extreme urban heat island effect intensity is 3.98'C, the permeability coefficient of S2 even increase 1.45×10-1cm/s. The influence on engineering properties of clayey soils caused by UHI effect is remarkable, and it should be payed higher attention to.Due to limits in time and laboratory conditions, this thesis is merely preliminary for the temperature effect research on the engineering properties of clayey soils, and further work on this research topic is analyzed and predicted in the final part of the paper.