軟岩河道遷急點之岩體穩定及沖刷機制探討

Abstract

河道遷急點附近動輒出現數十至數百公尺的劇烈倒退沖刷(算是岩體尺度之大規模破壞)，甚可能與岩體之構造破壞（導因於弱面發達）之遷急點岩盤邊坡破壞有極大關連，其破壞極可能受到水流導致之額外作用力的影響。本研究導出水流作用力與重力合併作用下遷急點岩盤邊坡之抵抗上舉、滑動、與翻覆之穩定安全係數。若水流作用力(拖曳力與上舉力)可知，則可計算遷急點岩盤邊坡之各種破壞機制對應之安全係數。本研究以計算流體力學(computational fluid dynamics ，CFD)數值模擬估算河道遷急點坡面上所受之水流作用力，配合穩定分析，探討遷急點岩盤邊坡之穩定性，進而瞭解遷急點驚人沖刷速率之原因與破壞機制。本研究以大安溪案址現地資料模擬河道遷急點沖刷，得知下游順向段之遷急點附近之急速沖刷的主控機制有二：(a)在坡頂處是以塊體抽離為主、(b)在坡趾處則是以平面滑動為主，中游水平段之遷急點沖刷則是以坡頂塊體抽離形成河階狀地形為主。上游逆向段之遷急點附近之急速沖刷的主控機制制為：(a)在坡頂處是以塊體抽離為主、(b)在坡趾處則是以傾倒翻覆為主。以上主控機制皆符合現地觀察之現象。透過塊體穩定分析之結果，可證實一旦考慮水流作用力，即使岩塊尺寸巨大至數公尺以上，亦有機會發生構造破壞而沖蝕流失。可解釋遷急點附近為何可有驚人之倒退沖刷速率。 影響塊體穩定之主要相關因素為塊體尺寸、流量及岩層位態，而含沙濃度影響並不大。

Intense knickpoint retreat up to tens or hundreds meters annually likely involves structural failure of jointed rock masses which has a lot to do with the slope instability adjacent to the knickpoint. In addition to the gravitational load, the slope instability is likely affected by the induced forces exerted by the strong current in the flow channel. In this thesis, the factors of safety for a rock block subjected to these loading against various failure modes, including uplift, sliding, and overturning, were formulated and calculated for a variety of conditions. The study made use of computational fluid dynamics (CFD) to simulate and to estimate the forces acting on a potential unstable rock block from the water current on a knickpoint. These forces from water flow, combined with the gravitational load, were then used to examine the factor of safety for different failure modes. The examination of the factor safety for the dominant failure mode can help to understand the true reason and failure mechanism for cases with an extremely high knickpoint retreat rate. This study examined the erosion rate of a reach in the Daan River related to knickpoint retreat. Major mechanisms of knickpoint retreat in different section along this reach were identified. For knickpoint in dip rock, the major failure mechanisms may include plucking of the rock blocks near the head of the knickpoint slope and plane sliding of the rock blocks at the toe of the knickpoint slope. For knickpoint in horizontal rock, the major failure mechanism is solely the plucking of rock blocks near the head of the knickpoint slope. For knickpoint in reverse rock, the major failure mechanisms may include plucking of the rock blocks near the head of the knickpoint slope and overturning of the rock blocks at the toe of the knickpoint slope. All the control mechanisms are in compliance with the actual phenomenon observable in field. Through the stability analysis, it appears even a rock block with its size a few meters large can become unstable when it is subjected to the forces from a strong current. This can explain the high erosion rate near a knickpoint in jointed rock masses. The major factors affecting the stability of the rock block may include block size, discharge, and the orientation of discontinuities.