岩質河床塊體抽離沖刷之微觀數值模擬與簡化分析

Abstract

於節理豐富之岩質河床，岩塊抽離機制最易成為主控之沖蝕機制。本研究以顆粒流數值模擬為方法，PFC3D軟體為工具，模擬節理發達岩體中岩塊抽離沖刷之行為與機制，檢視其微觀機制、探討影響岩塊抽離沖刷機制中塊體之運動條件及其受力行為之重要因子，進而提出簡化分析方法。為合理模擬節理面力學行為之要素，本研究先進行弱面直剪之模擬，結果顯示岩橋凝聚力及摩擦係數分別與最大摩擦阻抗及峰後摩擦阻抗呈正相關，而隨岩橋所占面積比越大，岩橋作用越為明顯。繼而模擬岩質河床表面岩塊之抽離行為，先模擬單岩塊虛擬試體之抽離行為，以探討影響塊體抽離之因子，整理虛擬試驗結果可知岩塊上舉速率隨岩橋面積比、岩橋鍵結強度、凝聚力以及節理勁度參數增加而下降；並隨著壓力波動平均差增高而上升。繼而模擬多岩塊岩床之同時受上舉力影響之抽離行為，探討塊體排列及岩橋分佈條件對岩塊抽離之影響，由模擬結果發現關鍵岩塊於塊體抽離中扮演重要角色。最後本研究基於虛擬沖刷試驗所整理之成果、以單岩塊抽離之模擬結果為基礎，提出一套簡化分析方法；關鍵岩塊之抽離行為則可視為單岩塊之抽離行為，考量其控制節理發達河床之沖刷速率。本文最後並以一案例展示此簡化分析法之應用。

Plucking is often the dominant mechanism of bedrock erosion when the rock riverbed is composed of heavily jointed rock masses. This thesis made use of particle flow simulation to model the bedrock incision by plucking. Through numerical simulation, the thesis looked into the microscopic features of the plucking mechanism and explored the important factors that may affect the kinematics and the kinetics of rock-block plucking in block scale. This study first simulated the direct shear on modeled weak plane to properly model the mechanical behavior on a discontinuity. Through simulation, it was validated that the peak strength increases with the increase in rock-bridge cohesion, whereas the post-peak frictional resistance increases with the rise in the frictional coefficient of the weak plane. The role of rock bridges tends to be more important as the area ratio of rock bridge increases. Subsequently, the study simulated the plucking behavior of jointed rock masses with virtual specimens composed of arranged blocks. Plucking simulation of single block and multiple blocks, respectively, were conducted. From the simulation of single block, it appears the uplift speed is negatively correlated with the following weak-plane characteristics: the area ratio and bond strength of rock bridges as well as the cohesion and the shear stiffness of weak plane. For the influence of the uplift force, the uplift speed increases as the mean value of pressure fluctuation increases. The plucking simulation of multiple blocks allows concurrent movements of adjacent blocks. The effects of the distribution of rock bridges and the orientation of joints were examined through simulation. It is shown that, for multiple blocks, there is a key block on the top layer to control the plucking of the jointed rock masses. Once the key block is removed, the uplift speed of other blocks will suddenly increase. On the basis of the simulated results, this study further proposed a simplified approach for the estimation of scouring depth by plucking during a specific flood event. For conservative concern, plucking speed was estimated using the results of single block plucking simulation. An example for applying the proposed simplified approach was also presented.