探討在具有長徑比之礫石層中隧道開挖引致之地表陷落

Abstract

台灣地質環境特殊，地形多山，在西部麓山帶及臺東縱谷平原分布著許多沖積扇的地形樣貌。其中花蓮舞鶴台地為第四紀的沖積扇，因中央山脈斷層作用將沖積扇抬升成為台地。因此河流沖積扇的特徵仍然會在地質組構中呈現，如顆粒大小的分布、排列方向等等。且礫石層顆粒大小差距大，使地層具有空間異質性；又因水流堆積而使礫石層具有覆瓦狀構造，有特定方向的顆粒排列使得地層具有空間異向性。如此的地層組構著實影響著地下工程的施工特性。 在2012年8月底，天秤颱風侵襲台灣，正在舞鶴台地中開挖的鐵路台東線NT隧道內發生了抽坍事故，在台地的表面發生了約10公尺深的陷落坑。查閱了此工程的鑽井資料以及地質調查報告，顯示抽坍處正位於舞鶴礫岩的地層中，如此，礫石層中的地質組構對於隧道抽坍並發生陷落坑的形成機制以及工程特性就成了我們需要深入探討的問題。 在此研究中利用物理模型試驗與數值模擬分析交叉比對，選定適當的參數來進行礫石層中陷落坑的發生過程與形貌進行研究。探討在不同厚度、不同深度的礫石層、以及不同的隧道深度的條件之下對於陷落坑行為的影響。 從數值模擬的結果中可以發現，當隧道深度越深的情形下，表面陷落的形貌寬度會增加，且陷落深度會減小。且在有特定排列方向之地層中，開挖隧道引致之地表陷落有偏離中央的情形，當層厚越厚時，其陷落坑偏離中央的距離越遠。

Taiwan is a mountainous country. Some gravel layers are located in western Taiwan mountain foot and Huatung Valley. Wuho Tableland is a Quaternary Period fluvial fan and risen by Central Range Fault. In this tableland, it still reserved the indication of fluvial fan that we can found, such as the spread of the gravel layers and gravel arrangement. And in the fluvial fan, the grain size and thickness of gravel layers are not equal, so that the stratum would be anisotropic and inhomogeneous. This stratum would affect the availability of geoengineering. Aug 28, 2012, Typhoon Tembin stroke Taiwan with heavy rain, especially for eastern Taiwan that causes sinkhole. The sinkhole is 10 meters in depth. From bore log and geologic investigation, we found the sinkhole is located in Wuho conglomerate. And we want to draw the connection between the configuration of the strata and the sinkhole. In this research, we analyzed into two aspects: experimental method and numerical simulation. Then, we set up sand-box experiment and verify with discrete element method (DEM). Finally, apply proper thickness, depth of gravel layer and depth of tunnel into this case. The results showed that with deeper tunnel comes shallower and wider sinkhole. And thicker gravel layers comes larger shift of sinkhole. This can be seen from both experiments and DEM simulation.