Browsing by Author "Cheng, Meng-Hsiung"

Now showing 1 - 2 of 2

Lessons from a failure case of an excavated floodway supported by precast cantilever pile walls
(2016-07-15)
Cheng, Meng-Hsiung
;
Huang, Ming-Wan
;
Pan, Yii-Wen
;
Liao, Jyh-Jong
;
土木工程學系
The Gangweigou floodway, across the Quiren and Rende districts in Tainan, Taiwan, was designed to bypass partial discharge in the Gangweigou River during heavy rainfall events for the purpose of flood control. This flood way was an excavated open channel supported by single or double rows of precast RC cantilever pile walls on each side of its banks; these piles were installed by the jetting-assisted method. Twenty days after the completion of the floodway, it was put to work for the first time during a heavy rainfall. Unexpectedly, the cantilever pile walls in several sections failed and caused the collapse of the slope behind the walls. This paper explores the failure mechanism and the real causes of failure in this case. Based on the conditions of hydrology and hydraulics, failure processes and the results of stability analyses, it was concluded that ignoring a possible bed incision or bank erosion was likely the primary problem with the improper design. The most critical condition was when the water level in the floodway quickly dropped, which resulted in rapid drawdown. The failure of the case could have been avoided if both the rapid drawdown condition and the potential of the channel bed incision were considered. (C) 2016 Elsevier B.V. All rights reserved.
Rapid channelization and incision into soft bedrock induced by human activity - Implications from the Bachang River in Taiwan
(2014-07-22)
Huang, Ming-Wan
;
Liao, Jyh-Jong
;
Pan, Yii-Wen
;
Cheng, Meng-Hsiung
;
土木工程學系
Many Taiwanese river channels have been affected by human activity, such as engineering works and gravel mining. Environmental conditions such as high seasonal precipitation and young sedimentary bedrock exaggerate the narrowing and incising of river channels. The Bachang River is located in southwestem Taiwan, and the 11-km-long channel at its midstream reach has been incised to a maximum depth of approximately 30 m within several decades. Therefore, the morphologic evolution of the Bachang River in southwestern Taiwan provides an illustration of channel changes in response to human activity. The historical data used for this analysis of morphology change include one set of topographic maps and six sets of aerial photographs from the past 100 years. The channel changes are analyzed qualitatively and quantitatively and include the channel planform, longitudinal profiles, channel incision, channel width, and channel cross sections. The results indicate that remarkable channel changes have occurred since the 1980s as a result of human influence. The 11-km-long study reach transformed from an alluvial-type channel to a gorge-type channel within several decades. The maximum accumulated depth of the channel incision is approximately 30 m, with a meter-scale annual average incision rate; the maximum width has been decreased to approximately one-sixth of the original width (448 m). The process of channel evolution is divided into four stages, and we have concluded that the causes of channel change include six factors from two categories: natural factors that include geological conditions, hydrological conditions, and the process of bedrock erosion; and human factors that include gravel mining, lateral structures, and levees. The initiation of channel evolution is triggered by human factors. Finally, we discuss the potential future channel evolution and lessons learned from the case study. (C) 2014 Elsevier B.V. All rights reserved.