軟弱岩石水平磨蝕率及磨蝕行為

Abstract

台灣西部河川皆流經西部麓山帶地質區，而西部麓山帶地質區存在甚多地質年代極為年輕之地層，因此，部分河段於豪雨後產生軟岩河床劇烈沖蝕，長時間累積了數公尺至數十公尺的沖蝕量，形成峽谷地形。河道沖蝕可以分為多種型態，其中磨蝕沖蝕機制(abrasion)亦會與其他機制並存且交互作用，且水流夾帶河床載及懸浮載所作用之能量大於軟岩底床的抵抗沖蝕能力，容易造成局部劇烈沖蝕現象，因此磨蝕沖蝕於河道沖蝕行為扮演重要角色。 基於國內、外對於軟弱岩石的沖蝕行為之探討甚為缺乏，本研究利用室內多功能沖蝕儀，以管流為水平向磨蝕功能進行連續沖蝕試驗。針對沖蝕機制中的軟岩磨蝕行為加以探討，並估算軟岩受水流的沖蝕率及評估軟岩抗磨蝕能力。 本研究使用室內沖蝕儀之水平向磨蝕試驗段，為郭炳宏(2010)所建置。本研究於系列沖蝕實驗進行前進行重複性試驗，結果顯示在相同條件下具有試驗結果具有良好的重複性，且沖蝕量測值精度為0.02mm。沖蝕試驗試體來自大安溪、石岡壩、集集攔河堰鑽探岩心，分別以高流速 (9.88~15.07m/s)進行沖蝕試驗。泥岩試體規畫以不同的試驗方法，以低流速(2.27~5.09m/s)進行試驗避免高速水流造成試體局部沖刷，試驗時間持續30分鐘。含砂水流試驗則使用三種濃度(1000,5000,10000ppm)及兩種粒徑(過200號篩及渥太華砂)分別模擬河道中不同含砂量及粒徑大小的顆粒磨蝕行為。 清水沖蝕試驗結果顯示，沖蝕率排序為人造砂岩＞泥岩＞砂岩＞頁岩，與岩石消散、耐久性比較，沖蝕率為低耐久性>高耐久性，並且發現水平磨蝕與岩體膠結性有高度相關。含泥砂水流試驗結果顯示，以相同試體及水流條件下，沖蝕率為含泥砂水流>清水，相同濃度下沖蝕率為砂土>黏土，沖蝕率與粒徑為正相關。 清水試驗結果與郭炳宏(2010)沖蝕率結果共同繪製於岩石沖蝕率(mm/hr)-流速(m/s)關係圖中，與Briaud(2008)土壤取樣進行EFA試驗結果以及沖刷分區趨勢進行比較，發現軟岩和土壤的沖蝕特性有相當的差異，岩石沖蝕率顯示砂岩、頁岩均落於較低的沖蝕率分區，泥岩試體為(III)分區，人造砂岩試體則較接近土壤沖蝕率為高沖蝕率(II)分區中。本研究將岩石沖蝕率以流速、岩性、岩石性質、含泥砂水流、控制懸浮載粒徑為控制條件，將Briaud(2008)之岩石沖蝕率趨勢更加完整化，並且提高試驗流速以彌補先前沖蝕率試驗結果之不足。

In the Western part of Taiwan, most of major rivers flow through the geological region of Western foothill which is composed of young sedimentary rocks. Their engineering behaviors are tended to be like weak rocks, e.g. poor cementation, high deformability, and low strength. Severe erosion in river bedrock caused by heavy storm is often observed once the armor layer has vanished. The accumulated erosion depth from recently 10 to 20 years can be high unusually up to few tens meters, and hence of some canyon valleys in the rivers were formed. The abrasion is one of the important erosion mechanisms to bedrock. It is the phenomenon of the bedrock worn or cut by entrained bed load and suspension load. It is also commonly to couple with other mechanisms to accelerate the local erosion of weak rock. Due to the literatures related to the abrasion behavior for weak rocks are insufficient, this study aims to carry out series of laboratory abrasion test for weak rocks to investigate the behavior and determine the laboratory abrasion rate of the rocks . The multi-functional erosion test apparatus developed by Kuo (2010) is utilized for the abrasion test. Rock core specimen is installed under the horizontal flow section and then elevates the specimen till it protrudes 1 mm above the bottom of the horizontal flow. The flow velocity can be adjusted from 0.5 to 16 m/sec. The horizontal test section is made of transparent acrylic. The erosion depth of the specimen is measured with 10-2 mm precision, and the erosion rate is calculated. In this study, the rock specimens were cored from the Ta-an River (near XS44~45), the Tachia River (near the Shi-gang dam), and the Chou-shuei River (near the Chi-chi wier). The test flow veolocity is controlled at 9.88~15.07m/s. However, for mudstone specimens, flow velocity is lowered to 2.27~5.09m/s to prevent excess local scour occurred. In addition to the clean water, tests were performed in combinations of three sediment concentrations (1000, 5000, 10000 ppm) with two particle size (sieve through the # 200 soil and Ottawa sand) to simulate the nature river flow. The test results for clean water show, the erosion rate is man-made sandstone > mudstone > sandstone > shale. In classified with the rock slake and durability index, the erosion rate is low slake-durability > high slake-durability. It is found that the degree of rock cementation is highly correlated with abrasion erosion. The results also suggest that the flow with sediment could obviously increase the erosion rate. Furthermore, under the same sediment concentration, the particle size is positively correlated with erosion rate. The experimental data from Kuo(2010), Briaud (2008) and this study are drawing into the erosion rate – flow velocity chart (Briaud, 2008) for comparison and also increase its completeness.