室內軟弱岩石於含砂水流之磨蝕率

Abstract

我國西部麓山帶所出露的岩層主要為上新世及更新世的地層，此種地層之地質年代較為年輕，普遍具有岩石顆粒組成膠結不良、單壓強度較低、遇水容易軟化並且極易受到水流侵蝕。台灣西部主要河川中下游的河道大多流經此類地層之上，當大雨、颱風等洪水事件發生時，河道中岩盤出露處於高流速以及高泥砂濃度的流況下，容易造成大幅度的下切、沖蝕等現象產生，此種現象不但會造成河道不穩定，也會導致許多跨河構造物因基礎結構的不穩定而造成傾倒、破壞的現象，因此含砂水流對於軟岩沖蝕行為之研究對於台灣河流穩定是一門非常重要的課題。 由於國外對於岩石沖刷之文獻多以硬岩為主，其結果並不適用於軟岩，而國內對於軟岩沖刷之研究並不多見，且又以清水磨蝕為主，因此建立含砂水流對於軟岩沖刷之儀器及試驗方法是必要的。 本研究以郭炳宏(2010)及吳佳諺(2011)建構之多功能沖蝕試驗儀中的磨蝕沖蝕試驗段進行改良，並建構一套自動量測系統取代以往之影像判釋法來量測試驗數據。本試驗規劃以1000、5000ppm兩種試驗水流濃度及流速為2m/s、5m/s、10m/s，來模擬常時水流、豪雨大雨以及颱風來襲時之沖刷流況，試驗試體則選用大安溪卓蘭層砂岩、及集集攔河堰頭嵙山層砂岩，單壓強度分別約為3.8~9.2MPa(大安溪)，平均單壓強度為6.5MPA、及23.1~33.7MPa(集集攔河堰)，平均單壓強度為28.4MPA，而八掌溪六重溪層泥質粉砂岩擾動岩心知單壓強度小於0.5MPa。 由本研究之試驗結果顯示，於相同的試驗水流流速及相同岩性下，磨蝕沖蝕率與試驗水流的泥沙濃度呈現良好之正相關性，大安溪砂岩在三種流速下(2、5、10m/s)，泥沙濃度增加5倍狀況下(1000-5000ppm)，沖蝕率最大可增加將近55%(流速2m/s)，平均增加約50%；集集堰砂岩在三種流速下(2、5、10m/s)，泥沙濃度增加5倍狀況下(1000-5000ppm)，沖蝕率最大可增加將近195%(流速5m/s)，平均增加約160%；八掌溪泥質粉砂岩在兩種流速下(1、2m/s)，泥沙濃度增加5倍狀況下(1000-5000ppm)，沖蝕率最大可增加將近60%(流速2m/s)，平均增加約50%。於相同的試驗水流濃度下，試驗水流的流速與試體之磨蝕沖蝕率呈現良好之正相關性，大安溪與集集堰的砂岩，在流速為2m/s增加至5m/s時，沖蝕率最大可增加約20%(濃度1000ppm)，平均增加約15%；在流速為5m/s增加至10m/s時，沖蝕率最大可增加約105%(濃度1000ppm)，平均增加約80%；八掌溪泥質粉砂岩在流速為1m/s增加至2m/s時，沖蝕率最大可增加約50%(濃度1000ppm)，平均增加約40%。不同岩石性質所的沖蝕率亦有些許差異，三種試體以集集堰砂岩之岩性最佳(Id2(岩石消散耐久指數)約93%，平均單壓強度為28.4MPa)、大安溪砂岩次之(Id2約65%，平均單壓強度為6.5MPa)、八掌溪泥岩最弱(Id2約9%，平均單壓強度為0.4MPa)；在相同試驗條件下，集集堰砂岩與八掌溪泥岩最大之沖蝕率差異可達到約190倍(2m/s, 1000ppm)；其次是大安溪砂岩與八掌溪泥岩之最大沖蝕率差異約70倍(2m/s, 1000ppm)；集集堰砂岩與大安溪砂岩之最大沖蝕率差異則僅約2倍(5m/s, 1000ppm)。

The geological province of the Western Foothill in Taiwan is composed of the Late Cenozoic sedimentary rock. The uniaxial compressive strength of the young sedimentary rock is typically less than 25 MPa and occasionally less than 1 MPa for mudstones. This type of rock is usually weak and poorly cemented, and easily eroded once exposed on the riverbed. Major rivers in the western Taiwan are generally along the east-west direction and flow cross the Western Foothill. With the drastic difference of river discharge between dry and wet seasons, the floods during typhoon season are often induced significant bedrock incision, and threatened the stability of the cross-river infrastructures. The sediment concentration is apparently increased during floods; hence it is important to understand the abrasive processes of weak rock under turbidity flow. 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 under turbidity flow. The Multi-functional Erosion Test Apparatus (META) developed by Kuo (2010) is improved and utilized for the abrasion test. The improvements of META include an automatic measurement system for specimen erosion and a modification on the rectangular cross section pipe of the test section. The abrasive tests are conducted in three flow velocities (2, 5, 10 m/s) and two sediment concentrations (1000 and 5000 ppm with the sediments pass through #200 sieve) with three different sites of core specimens, which are the sandstone specimen from the Ta-an River, the sandstone specimen from the Chi-chi weir, and the mudstone specimen from the Ba-chang River. Each test is repeated twice under the same test conditions for correctness; totally 36 tests are executed. According to the test results, we found the following correlations and trends: 1. Under the same flow velocity, the abrasive rate is highly positive-correlated with the sediment concentration. 2. Under the same sediment concentration, the abrasive rate is highly positive-correlated with the flow velocity. 3. Under the same flow velocity and sediment concentration, the abrasive rate for the relatively durable and high strength specimen (e.g., the sandstone specimen from the Chi-chi weir with Id2=93% and UCS=28 MPa) is significantly lower than the weak and low strength specimen (e.g., the mudstone specimen from the Ba-chang River with Id2=9% and UCS=0.4 MPa); the maximum difference of the abrasive rate could be up to 190 times (2m/s, 1000ppm).