軟岩沖蝕之微觀數值模擬

Abstract

台灣西部麓山帶常出露極年輕地層，岩層因成岩時間短故強度有限。此區域河川中下游段之軟弱岩床受過度沖蝕之作用，往往產生劇烈岩床沖蝕，不但造成河道不穩定，更威脅河道上人工構造物之結構安全。軟弱岩層河床的劇烈沖蝕行為已成為台灣西部麓山帶河川穩定的重要課題。 本研究首先經由現地勘查，探討軟弱岩層地區的沖蝕潛勢與影響因素。本研究首先以大安溪卓蘭附近的大峽谷段至蘭勢大橋段及嘉義八掌溪沖蝕劇烈河段的沖蝕現象為例，進行現地軟岩河床之沖蝕機制勘查、成因探討，並歸納不同岩性岩層中所產生相異型態之沖蝕機制。 以數值模擬為手段，搭配本研究繼而運用個別元素法程式為工，針對軟弱岩層「虛擬試體」進行模擬分析以進行沖蝕數值模擬，虛擬試體由含鍵結顆粒集合體所構成。基本上此種模擬可視為利用虛擬試體」來進行「虛擬沖蝕試驗」，用以探討軟弱岩床在不同的沖蝕機制下的沖蝕速率、破壞模式及影響範圍，並探討影響沖蝕速率之主要因子。虛擬沖蝕試驗所得之相關結果如下： 1.就床面剪應力之沖蝕作用，當模擬水流所造成之床面剪應力越大 時，受沖蝕脫離之顆粒會較多。若由小而大逐漸增高床面剪應力，可找出啟動沖蝕之臨界剪應力。床面剪應力所造成沖蝕破壞之作用皆係出現剪應力超過其剪力強度所產生之剪向鍵結破壞所致。 2.砂、礫石顆粒於水中漂流、彈跳、撞擊岩床，便可造成岩石河床中之磨蝕作用。在撞擊速度大、高撞擊角度及大撞擊顆粒的作用條件下，受沖蝕脫離之顆粒數目最多，形成破壞之區域也會越大。高角度撞擊時主要造成「變形磨損」(deformation wear)。低角度撞擊時則主要造成較表層之破壞，或可稱「切削磨損」(cutting wear)。撞擊所生之鍵結破壞大多為剪向破壞。 3.於相同之沖蝕作用力下進行模擬沖蝕試驗，於岩石材料具相對較高強度條件下，受沖蝕脫離之顆粒較少，反之亦然。因此可說明材料強度乃軟岩沖蝕速率的重要因子。於固定強度下當勁度較高時，顆粒間之鍵結於受顆粒撞擊作用產生較大程度之破壞範圍，可能係因撞擊後局部範圍之應力較高，因而導致較多之鍵結破壞。

The outcrops in the northern and central regions of Western Taiwan often contain young and weakly cemented sedimentary rocks. Recently, intensive erosion cases have been found in front of several checkdams or bridges in Taiwan; the extent of river incision and widening had resulted in serious threat to the stability of river channel and cross-river structure. This study started with a series of site investigation to examine the erosive potential of soft rock riverbed and the important factors affecting the erosion of soft rock. The investigated sites included a number of river sections (along the Da-An River and Ba-Chang River as exampled cases) of which intensive erosion of soft rock riverbed had occurred. The site investigation aims to identify the erosion phenomena and their major causes, and to classify different erosive mechanisms in various conditions of soft rock. Subsequently, this study made use of particle flow simulation to model the erosion of soft rock in microscopic scale. The numerical simulation can be regarded as a mean of “virtual erosion test” with a virtual rock specimen that contains a number of particles with inter-particle bonding. The virtual erosion tests can be carried out to investigate the erosion rate of soft rock due to various mechanisms, the failure mode, and affected zone of erosion. The major factors that affected the erosion rate can also be examined through the virtual erosion tests. The following were found from a series of virtual erosion tests. Shear stress acting on the top surface of virtual specimen may result in the de-bonding of grain particles; the evidence from virtual erosion tests reveals the mechanism of erosion by shear. The de-bonding of particles is a result of shear failure in the shear bonding between a pair of particles. Beyond a threshold level of shear stress (i.e., the threshold shear stress), a higher shear stress tends to increase the number of de-bonding grain particles; consequently, the erosion rate increases. Saltating erosion is a result of sand or gravel traveling along with water flow and striking on the riverbed. The striking impact results the local failure (which is a zone of de-bonding particles adjacent to the impact location) on riverbed rock and causes abrasion or wear. Larger area of local failure zone may occur for a condition with higher impact speed, higher impact angle, or larger gravel size. Gravel impact for a higher impact angle causes deformation wear. On the other hand, impact for a lower impact angle causes cutting (shallow) wear. Most bonding failure in both cases appears to be shear failure. Under a same acting condition (either bed shear or impact) for virtual erosion tests, the number of de-bonding particles decreases with an increase in rock strength; and vice versa. Hence, material strength plays an important role affecting the erosion rate of soft rock. For a fixed rock strength, the size of failure zone (or the number of de-bonding particles) caused by a gravel impact is larger for a higher rock stiffness probably due to a higher local stress level in this condition.