| Summary: | 博士 === 國立臺灣大學 === 土木工程學研究所 === 90 === Tunnel construction in western Taiwan sedimentation strata may encounter cases of excess deformation, namely, squeezing. This difficulty increases the cost and the construction time of the project. Based on the previous study, squeezing may be caused by the following characteristics of sandstone: wetting deterioration, shear dilation of foothill sandstone as well.
To evaluate the mechanical characteristics of sandstone, fifteen kinds of sandstone in the western Taiwan were selected as tested material in this research. The types of mechanical experiments included uniaxial and triaxial tests. Notably, to isolate the influence of hydrostatic stress and shear stress to the volumetric behavior, a series of pure shear stress tests have been performed. The measured deformations were then decomposed into elastic and plastic components further exploring the stress-strain behavior. The analysis results indicate that: (1) significant plastic volumetric strain comes up at low levels of volumetric stress, and locks at higher levels of confining stress; (2) apparent coupling behavior exists between shear stress and elastic volumetric strain; (3) a set of elliptical plastic potential surface is proposed according to the distribution of plastic flow. An elasto-plastic model was then proposed to characterize the behaviors of the sandstones. Through the simulations of different stress paths for various sandstones, it is found that good agreements arise between the predictions and the experimental results.
Besides, the correlation of mechanical behavior and microstructure (and composition) of sandstone was accessed. Sandstone samples were subjected to petrographic analysis and nine parameters were obtained. Based on the stepwise multiple regression results, mathematical functions were developed for estimating the uniaxial compression strength (UCS) and the ratio (R) of saturated strength to dry strength from the petrographic and petrophysical index. The research revealed that the major influence factor of UCS is porosity and the minor factor is grain area ratio (GAR). Otherwise, the major influence factor of R is GAR and the minor factor is porosity. According to the results, a mechanical classification of foothill sandstone was proposed, and sandstones were classified into eight kinds based on two index, porosity and GAR.
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