床面剪應力及顆粒彈跳引致岩石磨蝕之微觀數值模擬

Abstract

台灣豐、枯水期之水量變化非常大，造成河道的不穩定，也使得河床上之跨河結構物因沖刷而破壞，岩床沖蝕問題對於台灣來說是個不可忽視的問題。當岩床或岩岸屬於地質年代較為年輕之軟弱岩層時，岩石河床沖蝕尤其更為嚴重。數值模擬方法可任意控制變因、可大量進行分析，耗時較短之優點，又不受試體取樣與準備不易之困擾，本研究因而嘗試以數值模擬方法為工具，將數值模擬視為「虛擬沖蝕試驗」，代替實體試驗。本研究建立磨蝕沖蝕機制及顆粒彈跳撞擊兩種岩床沖蝕機制模型，進行虛擬岩床沖蝕試驗，以便透過模擬結果探討影響沖蝕機制與行為之重要因子，並探討沖蝕過程中材料中之消散能量變化與鍵結破壞狀況。 就岩床沖蝕模擬而言，當床面剪應力增加時（遭磨蝕）破壞顆粒數也會增加。岩床磨蝕試驗結果顯示對應新鮮岩盤材料的磨蝕啟動剪應力遠高於一般河道可能出現之床面剪應力尺度，顯示現地之岩床若未經乾溼循環或風化等因素強度大幅弱化，清水所造成之床面剪應力可能甚難以造成岩床之顯著磨蝕。 由顆粒彈跳撞擊之模擬結果，可發現（遭彈跳撞擊）破壞顆粒數會隨撞擊顆粒速度、角度及尺寸等撞擊因子增加而隨之增加。此外，就材料力學性質之影響而言，當楊氏係數增加時破壞顆粒數也增加，而試體張力強度增加時破壞顆粒數則降低。此外，比較破壞顆粒數與累積消散能量，可發現兩者間呈良好之相關性。

River discharge in Taiwan varies a lot during flooding season; this situation often results in unstable river channel. The intensive erosion of bedrock during flood may also endanger the stability of cross-river structures, especially for cases of river bed composed of soft rocks. This study makes use of numerical simulation as “virtual erosion test” to explore the mechanisms of rock erosion. In the simulation, rock material is modeled as a granular assemblage with inter-particle bonding; the erosion process is simulated as particles’ release due to de-bonding. Virtual rock specimen are subjected to boundary loads from either bed shear stress or saltating particle to simulates physical erosion experiments. The purpose of this thesis is to investigate the important factors that may control the erodibility of soft rock subjected to bed shear stress and particle saltation; also, the dissipated energy and bonding failure associated with the occurrence of rock erosion are studied. For erosion due to bed shear stress, simulated results show that the number of de-bonded particles raises as the bed shear stress increases. It is found the shear stress required for the initiation of a de-bonded particle in fresh soft rock material is significantly higher than the typical in-situ bed shear stress that may occur in a flood. It appears clear water current alone may hardly erode a fresh rock material. However, it is common to see the degradation or weathering of soft rock exposed to the periodical variation of water level (i.e., subjected to drying-wetting cycles). These weakening processes are likely to cause the rock material become more erodible. Saltating abrasion can be a consequence of impacts of gravels traveling along with water flow; the impact results in the local failure and causes abrasion of river-bed rock material. The major factors affecting the erodibility and erosion rate are examined through a series of virtual erosion tests. Simulated results show that more de-bonded particles may occur for a condition with higher impact speed, higher impact angle, larger gravel size, higher Young’s modulus, or lower rock strength. Besides, a good correlation relationship between the number of de-bonded particles and the accumulated dissipated energy is notable.