軟弱岩質河道固床工之破壞機制及穩定

Abstract

台灣西部麓山帶為台灣人口稠密地區，其河道上建有許多如橋樑、取水工等設施，而麓山帶地層組成多為上新世與更新世的軟弱岩石，於台灣6~9月的洪峰期水流對河道侵蝕會特別嚴重，河道上常需設置固床工以保護河道。固床工下游常出現岩盤沖蝕造成下游河道下降，而影響下游河道的河相及設施的穩定性。隨著河床的下降，固床工上下游的高低差會隨著增加，甚至可達10公尺以上。若固床工發生破壞，則可能自固床工所在的河床產生遷急點上溯，危及上游之橋梁及取水工等設施。 台灣固床工破壞研究多以水流作用力觀點分析與歸納，但依現地調查之結果發現，固床工的破壞並非單靠水流的作用力所導致，而是與河道岩盤沖蝕有密切的相關。 本研究由地工的角度，從現地調查發現之破壞跡象探討固床工的破壞機制(過程)。使用羅楚鈞(2011)之方法計算沖刷坑深度與形狀，並配合數值模擬軟體COMSOL Multiphysics計算水流作用力後，以靜力學角度探討實際案例之固床工破壞原因及機制。從現有22座固床工調查分析結果顯示，共有18座出現下游岩盤沖刷的跡象，因此可判斷大部分的固床工的主要破壞原因為岩盤沖蝕導致沖刷坑發展後，固床工基礎受到淘刷，加上水流作用力而破壞。

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.