軟砂岩弱化之力學行為及連體損壞力學之初步探討

Abstract

台灣中北部之麓山帶地區之軟砂岩，在地質年代上屬於甚年輕，具有固結差，膠結不 良，強度不高，且孔隙率大、透水性良好，遇水易弱化的特性，而會對工程之安全性產生 一定程度之影響。因此，了解軟砂岩的真正弱化機制及其影響，並期能進一步掌握且預測 其行為是極重要的課題。 本研究以台中縣和平附近之軟砂岩為材料，人工地模擬軟砂岩在大自然環境下可能發 生弱化的一些條件，來對軟砂岩的弱化機制及其對材料之力學行為之影響性作初步之定性 探討，同時試圖基於以連體損壞力學理論所建立的組合律模式來詮釋並預測材料受弱化後 之強度等力學性質之改變。軟砂岩試體經過各種弱化模擬條件予以弱化後，使用MTS 815 岩石力學試驗機進行軟砂岩的單軸多階段加壓/解壓/再壓試驗，來觀察材料的強度、彈性 模數以及變形性受弱化模擬條件的影響性。 由本研究之實驗結果發現，經過弱化之後之軟砂岩試體，在強度以及初始彈性勁度上 均表現出較為弱化試體為低之現象，而破壞應變則增大，顯示材料受弱化造成材料損壞而 使力學性質改變，造成不利之影響。而本研究提出力學模式來描述軟砂岩之應力應變行為 ，此模式整合一裂縫閉合雙曲線模式與一連體損壞力學模式。由本文所提出之模式對實驗 結果之模擬與比較可看出此模式確實可合理描述軟砂岩之應力應變行為。 The weak sandstones in the northen region of Taiwan generally are in gener- al poorly cemented and degradable when exposed to water. They usually hold low shear strength and high void ratio. Due to the tendency of degradation, engineering in this type of rock formation may require special concern, and the degradation mechanism of the weak sandstones deserves more attention. As a preliminary and qualitative investigation of the degradation of weak sandstones, this thesis aims to artificially model some possible in-situ degradation mechanisms in laboratory and attempts to make use of the theory of continuum damage mechanics to decribe and interprete the mechanical behavior of the degraded weak sandstone. Following different artificial degradation processes, the specimens are subjected to uniaxial loading- unloading-reloading compression test. The stress-strain curves are obtained and then used to study the influence of degradation on the strength,elastic modulus and deformability of the weak sandstones. From the experimental results, it is noted that degradation results in the reduction of strength and elastic modulus and the increases of the failure strain. On the basis of the experimental results, a constitutive model is proposed to describe the mechanical behavior of weak sandstone. The constitu -tive model contains a hyperbolic pore/crack closure model and a continum damage mechanics model. By comparing the simulated and experimental results,i t is demonstrated that the proposed model can closely simulate the stress- strain relation of weak sandstone with just five material parameters.