以滾筒實驗模擬土石流侵蝕軟弱岩盤峽谷之現象

Abstract

這個實驗的目標是研究土石流造成軟弱岩盤峽谷的沖刷。實驗的方法是簡化模型再利用轉輪模擬重複不斷的土石流事件。而岩盤模型的材料是選用石膏粉、石英砂及水的混和。製模是結合河床及河岸的形狀並將環狀模型分成小塊製作。而模型的強度決定於水與石膏粉的比例且配合圓柱式體抗壓強度測試。選擇的顆粒為常被用以模擬土石流的細磨石而部分實驗是含水顆粒流，部分是乾顆粒流。而記錄隨固定間隔時間，逐步磨蝕的模型在滾輪直徑方向的長度所採用的量測方法為雷射掃描搭配影像分析技術。模型自原始的矩形河槽經由定時的顆粒流侵蝕漸漸演化成類似倒著看典型鑰匙孔的形狀，正好可比擬現地的峽谷。模型的河床出現了波浪狀正如現地岩層與岩層交接處。而實驗的成果說明飽和顆粒流可造成模型強度降低，另外模型配比強度及轉輪的轉速也是造成沖刷量明顯增加的原因。

The aim of this thesis is to investigate soft bedrock canyon erosion by debris flows. The approach adopted is to set-up simplified experiments in a rotating drum. Soft bedrock is modeled by mixtures of sand and gypsum, formed into blocks with erodible wall and bottom. Different strengths are obtained by varying the water to gypsum ratio, and characterized by cylinder compression tests. Small mill stones are used to model the debris flow grains, with and without water. The gradual erosion of a preformed channel along the perimeter of the drum is then monitored by laser scanning at regular time intervals. Starting from a rectangular shape, debris flow abrasion leads to the gradual development of an inverted key hole shape, similar to the shape of bedrock canyons observed in the field. Wavy canyon surfaces are observed, which appear linked to the locations of vertical joints between gypsum blocks. It is found that saturation with water reduces the strength of the canyon material, and leads to significantly increased rates of erosion. The erosion rate also increases for mixes of reduced strength, and for faster drum rotation rates.