軟砂岩之微觀組構及其對力學性質之影響

Abstract

軟弱岩石受環境條件之影響，可能逐漸弱化，而造成其強度下降或變形性的增加。本研究嘗試以人工方式模擬軟弱砂岩之弱化效應，以微觀方式量測分析其微組構的變化，並探討微觀特性與巨觀力學性質之間的關係。本研究嘗試以人為之乾溼循環來模擬軟弱岩石天然之弱化過程，除對每組弱化後的試體進行單軸壓縮試驗及波速量測，再對弱化後岩樣進行微觀特性之探討。 以往以製作薄切片觀察岩石材料顆粒組構的方法，對軟弱砂岩未必適用。因此特針對軟弱砂岩膠結鬆散之特性，嘗試建立一套適合微觀量測組構的試體準備方式，製作成岩樣厚切片。經過顯微鏡放大後拍攝得岩石微觀組構照片，再將照片加以影像處理方式進行分析，可以獲得關於岩石顆粒、孔隙及填充物的資訊。 將軟弱砂岩微觀組構變化與巨觀力學性質交叉比對，可知岩石單軸壓縮強度在第一次的乾溼循環後下降約 25% ，推測應為細顆粒填充物析出導致裂隙的增加造成的。隨著乾溼循環次數的增加，強度逐漸降低，而斷面孔隙的大小則趨向較為固定，由微觀量測推論大顆粒可能逐漸崩解為小顆粒，顆粒本身的稜角也變的較不明顯，顆粒間較易滑動所致。

Very weak sandstone may degrade when it is exposed to an unfavorable environmental condition. Its degradation usually results in a reduction in strength and an increase of deformability. This study attempts to investigate the microstructure change of very weak sandstone subjected to a controlled degradation process. The degradation process was simulated by a controlled number of wet-dry cycles. After controlled degradation, specimens of very weak sandstone were subjected to wave-velocity measurement and unconfined compression tests. Their microscopic features were also investigated using a microscope and subsequent image processing. Due to poor cementation, it is extremely difficult to obtain thin section of the tested material of the very weak sandstone. Instead, a section of rock specimen was cemented, trimmed and then polished (with a thickness of 10 mm approximately) to prepare for the microscopic observation in order to investigate the microstructure of the very weak rock material. Image processing enables the analysis and interpretation of the micro-contents including grain-particle, void, and infill. Comparing the microstructure and mechanical properties of the degraded very weak sandstone, it was found that the strength of the very weak sandstone decreases significantly after the first wet-dry cycle. The cause was likely due to the increase in the micro-fissures. Along with the increase in wet-dry cycles, the strength of the material gradually decreases without further noticeable change of the inter-particle voids. From microstructure observation, it was noted that large particles tend to break into smalle particles, and the particles' angularity tends to decrease. These changes may contribute to the decrease of material's strength.