博士 / 國立臺灣大學 / 海洋研究所 / 87 / Eastern Taiwan is located on the suture of convergence and compression between the Eurasian plate and the Philippine Sea plate, which is characterized by its geological unstability with frequent earthquakes and abundant fault structures. The area offshore Huatung is to the east of the suture and above the Huatung Basin. Its rough topography is resulted from the collision between the Luzon Arc and the Eurasian plate. The area off Changyuan lies between the Chimei Canyon and Changpin Canyon and is part of the rough topography off Huatung. It is also a site chosen to be the location of the OTEC power plant. To investigate the stability of the sediment of the continental slope in the studied area, this research utilize the geophysical investigation and the analysis of the cores to study its sedimentary structure, physical, geotechnical and acoustical properties.
The bathymetric chart and the results of side scan and receiving array of air gun show that it is the slope failure, which leads to the rough topography and the winding canyon. Centered between the shelf break and the upper slope is the slope failure in the form of multiple retrogressive slides, rotational slide and debris deposits. The biggest failure is 5km in length and 700m in depth with a potential failure surface as its unconformity.
The analysis of the 12-box cores and the piston cores (0.5-3m in length) shows that the surface of the slope is covered with a layer of turbidity sediment of unequaled thickness (Bouma sequence D & E). The sediment mainly consists of silt and sandy silt. The sediment is composed of 49-90% silt, 7-29% clay and 0-25% sand. The clay minerals are mainly composed of illite and chlorite. The sediment is composed of terrigenous volcanogenic debris with carbonate less than 5%. The water content of the surface sediment is from 50% up to 120% and it drops to 25-40% with the increase of the depth of the core. The average bulk density of the sediment is 2.14 g/cm3 and the average special gravity is 2.75. The Atterberg limits show that the sediment is normal to inactive plastic-liquid. On the plasticity chart, all sediment fall within the CL and ML-OL sectors.
Different compressibility of the sediment corresponds to different shear strength and acoustical properties. When the sediment is underconsolidated, its shear strength is less than 10 g/cm2. The velocity is 1520 —1620m/s. When the sediment is normally consolidated, its shear strength is 20 - 60 g/cm2 and its velocity is 1630-1720 m/s. When overconsolidated, the shear strength increases to 30-140 g/cm2 with a velocity of 1640-1769m/s. The maximum thickness of its slump, calculated by the use of OCR, shows that the surface sediment in the area studied had undergone slumping activity. The thickness of the slump ranges from 100cm to 500cm.
The shear strength parameters CCu is 0.186 kg/cm2 and Cu is 25.5 degrees. The infinite slope stability analysis is used to examine the failure mechanism of the slope under static loading and earthquake loading. The results indicate that under static loading, when the angle of the slope is less than 25.5 degrees, there is stable deposit. When the angle is more than 25.5 degrees, the larger the angle of the slope, the thinner the layer of stable deposit becomes. At the Changpin Canyon and the Changyuan Plateau, the slope with an angle more than 45 degrees can support a maximum thickness of 3.1m. Under the earthquake loading, the smaller the angle of the slope, the larger the earthquake horizontal acceleration. In the studied area, with a 0.4-0.6 Cu/Po, a slope of 25 degrees has untrained slumping when the earthquake horizontal acceleration is 2% to 14%g. In a regression analysis in the earthquakes occurring in Taiwan in a 100-year period, it is estimated that an earthquake is capable of inducing a peak ground horizontal acceleration of 23%-27%g in Changyuan area.
When the sediment is consolidated, the velocity reveals the change of the elastic properties including the change of porosity and the rearrangement of the grain fabric. The attenuation coefficient indicates an energy loss after the interaction between water and grain or among grains when the acoustic source propagates through the sediment. By the use of vertical loading and void ratio of the silted turbidity, it is estimated that the average velocity is 2043m/s at the depth of 250m with a 0.4 dB/m/kHz attenuation coefficient.
| Identifer | oai:union.ndltd.org:TW/087NTU00279012 |
| Date | January 1999 |
| Creators | Sheng-Chung Lo, 羅聖宗 |
| Contributors | Min-Pen Chen, Jen-Chen Fan, 陳民本, 范正成 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 214 |
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