Failure Mechanism and Stability Analysis of Control Grade Structures on Weak Rock River Beds
|關鍵字:||軟岩;固床工;破壞機制;穩定性分析;Weak rocks;Control grade structure;Failure mechanism;Stability analysis|
The Western Foothill is the crowded area in Taiwan. Therefore, many artificial structures, such as bridges and water intakes, are built on the river. In the western foothills of Taiwan, the geological province is composed of the Late Cenozoic sedimentary rocks. The floods during typhoon seasons usually induced serious bedrock incision; hence control grade structures were built to protect river bedrock from incision. The bedrock erosion at downstream of control grade structures resulted in elevation lowering. The occurrence might induce river and artificial structures instability. With the decline in the riverbed, difference of elevation between upstream and downstream of the structures would increase, even up to 10 meters or more. If the control grade structure suffered damage, knickpoints retreated from the control grade structure might endanger bridges and other facilities of upstream. According to the results of field investigation, it was found that the destruction of the control grade structures were not just caused by water force, but also related to river bedrock erosion. This study aims to propose the failure mechanism of control grade structures according to the field investigation. Using the approach proposed by Lo (2011) for estimating the shape and depth of the scour hole downstream weirs built on weak rocks, and the software COMSOLfor computing hydraulic forces, this study may investigate the failure reasons and the failure mechanism of the control grade structures. From the field investigation of 22 control grade structures, 18 control grade structures were found the phenomina of river bedrock erosion sownstream of the structures. Therefore, it appears that the most of control grade structures on weak rocks would fail because of scouring under the foundation downstream of the structures.