| Summary: | 碩士 === 國立中央大學 === 土木工程研究所 === 88 === It makes the soil layer decision not very precise, because the cone resistance measured by traditional CPT maybe affected by the effect of soil interface, shear strength and compressibility. For this reason, this research installed a mini microphone into the cone tip to measure acoustic signal during cone penetration into soil so that to improve the accuracy of locating soil interface by using acoustic signal.
This research included three kinds of acoustic cone penetration tests： pure sand, pure clay and sand-clay layered specimens. Also, we measure cone resistance and acoustic signal to study the response of cone resistance, RMS sound pressure, and frequency spectrum in locating the soil interface.
In the test of pure sand specimens, the RMS sound pressure increase with the relative density of the sand. The RMS sound pressure of Fu-Long sand is between 50~100μV. In the figure of the relationship between RMS sound pressure and soil gain size offer by Tringale (1983), it matches the fine sand grade so that we can use the formula to predict the average gain size of sand. This research compares the response of acoustic signal of sand and clay, reveal that not only the amplitude of the acoustic signal of sand is greater than clay but also the consist of frequency is more complex than clay.
In the test of sand-clay layered specimens shows that it has transition coverage in cone resistance when cone is advanced through soil interface. The transition coverage increases with the increase of the relative density of sand layer. If we use cone resistance to determine soil interface, it could make a mistake when cone resistance between sand and clay layers are similar. But it can be improved by acoustic cone penetration test.
In the research of locating soil interface by the RMS sound pressure shows that as penetrating from sand layer to clay layer, the RMS sound pressure is limited or stable, the location of soil interface is the depth where the RMS sound pressure begin decay. Although using frequency analysis to determine the soil interface is not as clear as using the RMS sound pressure, but it still can be used for assistance.
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