| Summary: | 碩士 === 國立高雄應用科技大學 === 土木工程與防災科技研究所 === 99 === A saturated cohesive soil with its water content attain to the liquid limit, viscosity of water slow-moving soil flowing, can be used to the maximum static friction angle of soil increases, but when the water content increases to a certain limit, the saturated cohesive soil began to collapse, the study to name the water content of the collapse is flow limit. Water content of flow limit is minimum water content of Slope occurred debris flow.
The soil used in this study was primarily taken from Mountain Hsiendo in Siao-Lin Village, Kaoshiung, Taiwan. Inclined board with the geometry of 45cm×30cm was used to investigate the relationship between water content and the maximum static friction angle for the soil of the grain size less than 0.420mm (sieve #40) to mix 0%-90% soil of coarse particle the grain size greater than 0.420mm (sieve #40). Because water of viscosity the results have shown that there exists an inverse relationship – the maximum static friction angle would decrease as the soil water content increases. However, the maximum static friction angle would be raised up 75~80° while soil water content has reached the flow limit. This is extremely close to the slope sliding angle of Mountain Hsiendo.
A homogeneous soil with its water content lower than the liquid limit would behave a circular failure mode. This is caused by the shear strength of the interior soil layer potential failure surface conquered by the external driving shear force, leading to a sequenced displacement transmission. The failure surfaces are of stepped slope and multiple curves. When the soil water content exceeds its specific flow limit, the failure mode becomes instantaneous flow collapse. The curved surface remains on the failure slope, while the collapsed soil would form a sedimentary plain at the slope toe. This is the case of Mountain Hsiendo in Siao-Lin Village with the failure depth of 84m, and was called “deep collapse” by press NHK, Japan.
If the rainfall amount is greater than the infiltration capacity of the soil, the water content of the soil colluviums would exceed its flow limit. For example, the vast amount of rainfall brought by Typhoon Morakot has caused a cyclic loading effect to the soil. In this study, soil with water content greater than its flow limit was settled in the chamber (30cm×30cm×60cm), tilted for 24 hours and restored to horizontal for another 24 hours. These steps were iteratively repeated until the 48-96th hour at which the vertical tensile crack occurred in the soil chamber. This flow collapse test is thus, contrast with Rainfall intensity duration curve of Siao-Lin Village during Typhoon Morakot, can be used to simulate the deep flow collapse (84m) at Mountain Hsiendo in Siao-Lin Village under a 48-96 hour continuous rainfall during Typhoon Morakot.
In the test chamber while the soil with the water content greater than liquid limit and tilted at 75~80°, the flow collapse occurs within 1 sec, which can be used to evaluate that Siao-Lin Village was destroyed within 8-9 sec.
Electro-Osmotic Solar Energy Method can be used to improve the shear strength of soil and to prevent the occurrence of deep flow collapse when tilted at 55~79°, as well as when the soil water content reaches 100% during rainfall. While it should be mentioned that the deep soil (84m) strength improvement in Siao-Lin Village require more sophisticated considerations.
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