| Summary: | 博士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === The soil strata in southwestern Taiwan are dominant in low-plasticity silty sand with fine particles and low-plastic. In engineering design, low-plasticity silty sand was considered as general fine-grained soil, so the engineering properties of low-plasticity silty sand were ignored before. During the 1999 Chi-Chi earthquake, serious soil liquefaction damages were observed in central Taiwan. The post-earthquake study indicated that most soil liquefactions were occurred in silty sand deposits with high fines content. Therefore, the engineering properties of low-plasticity silty sand have been of great research interests in soil liquefaction. However, difficulties occurred in undisturbed sampling of sensitive low-plasticity silty sand material with high fines content and high water content. And the undisturbed engineering properties of low-plasticity silty sand have yet to see. In 2004 to 2005, several catastrophic subway construction failures occurred in south Taiwan. Results of forensic investigation indicated that piping failure of non-plastic silty sand is the dominated factor causing serious tunnel and excavation pit collapses. However, because the limitations of traditional test method, so the engineer can’t understand the internal erosion properties of low-plasticity silty sand.
In an effort to investigate the engineering properties of low-plasticity silty sand, this study had adopted a recently developed “Gel-Push” sampling technique to obtain undisturbed soil samples. Results of cyclic triaxail tests on undisturbed and remolded specimens indicate that, higher non-plastic fines content of silty sand would result in lower cyclic liquefaction resistance. Such low-plasticity silty sand deposits would have less liquefaction resistance when they are subjected to disturbance. And this phenomenon would become much more noticeable on the remolded soil specimens.
In order to investigate the internal erosion properties of low-plasticity silty sand, the Flexible Wall Pin Hole Test Device, FWPH, was developed by combining concepts of the conventional Pin Hole test and the triaxial test. Test results of FWPH showed that internal erosion potential of low-plasticity silty sand was affected by its fines content, density and confining pressure. Low-plasticity silty sand at loose state has much higher internal erosion potential than it at dense state. Moreover, when higher confining pressures were maintained, low-plasticity silty sand had less internal erosion potential. Most of all, higher fines content of the low-plasticity silty sand, higher internal erosion potential is clearly observed.
Finally, comparing the triaxial test results of undisturbed specimen with the analysis results of traditional soil liquefaction potential assessment method indicated that the traditional assessment method will underestimate the liquefaction potential in low-plasticity silty sand deposits. Therefore, the triaxial test analysis of undisturbed specimen was recommended to adopt for soil liquefaction potential assessment in low-plasticity silty sand deposits.
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