乾/濕 砂岩力學行為差別初探

Abstract

台灣西部麓山帶屬第三紀沉積岩層，因其成岩時間不長而造成岩層本身膠結與壓密程度不佳，加上地體構造活動頻繁且複雜，造成許多岩體破碎等不利於地下工程之條件；且此特殊之地質背景，造成砂岩有受剪膨脹、遇水弱化及擠壓變形等特點，在此類岩層中進行地下開挖工程時，常易發生工程災害。 本研究以北投地區之木山層砂岩為研究對象進行三軸試驗，剪力施加時以純剪路徑進行而非以往之傳統三軸路徑，試驗類型分為即時剪壞及依時潛變試驗；試體狀態分為乾燥與含水試體兩類；藉由一系列三軸試驗之結果，希冀對木山層砂岩在遇水時力學行為之變化有進一步認知。 試驗結果顯示遇水後，單壓強度下降約45 ~ 55 %，純剪三軸試驗之剪應力強度下降約12 ~ 15 %；潛變試驗中，乾燥試體之剪應力達破壞強度之95 %時，則易在短時間內破壞；含水試體則在90 %左右有此現象。 就試體變形性而言，體積應力階段引致之體積應變，無論乾燥或含水試體，均隨體積應力增加而體積壓縮量的增量趨緩，以含水試體體積壓縮量明顯為大。 剪應力階段引致之體積應變，兩狀態之試體隨剪應力增加而由壓縮轉為膨脹，在接近破壞強度時，則呈現大量降伏至破壞，以含水試體之淨變形量明顯較為大；剪應力階段引致之剪應變，兩狀態之試體初始剪應變均呈線性增加，達塑性剪脹門檻後開始加速發展，在接近破壞強度時，呈現大量降伏現象至破壞，以含水試體之剪變形量明顯較為大。 就乾燥或含水狀態試體而言，觀察其塑性與黏塑性變形之發展行為，在體積變形部分，在低剪應力下為受剪壓縮（潛縮）、高剪應力時為受剪膨脹（潛脹）；且當剪應力比大於潛脹門檻後，在應力比相近時，其塑性體積應變與塑性剪應變之發展趨勢相似，即可將塑性變形視為瞬間之潛變變形，而可選擇適當之組成模式用以模擬試體之力學行為。

In the western Taiwan is almost sedimentary rock of tertiary period. However, these sandstones are often cemented and compacted not well, and sometimes full of unfavorable geological conditions, such as joints, faults and broken mass. Due to a special geological background, it may be caused the following characteristics of sandstone: shear dilation, wetting deterioration and squeezing. Several unsuccessful cases concerned with underground excavation through special geological zone , had been often occurred. In this research, it focuses on the mechanical behavior of dry and wet sandstones with Mushan sandstone which were collected from Beitou area. The experiments included immediately and time-dependent, and types of samples included dry and wet sandstones. Notably, we proceeded a series of pure shear stress tests in order to isolate the influence of volumetric stress and shear stress to the volumetric behavior. According to the experimental results, when samples were wet, which strengths decreased about 45-55% in uniaxial compressive tests, and decreased about 12-15% in triaxial compressive test. In creep tests, the dry samples were broken when shear strength reached to 95% of shear failure strength, and wet samples happened when shear strength reached to 90% of shear failure strength. When volumetric stress raised, both dry and wet samples, the amount of volumetric strain increment which brought about volumetric stress had been smaller and smaller. However, the deformation of wet samples was larger than dry samples. The volumetric strain which brought about shear stress, both dry and wet samples transformed compressibility into dilatability when shear stress had risen. Furthermore, The shear strain which brought about shear stress, both dry and wet samples were linear increment at first, and the shear strain developed fast when shear stress closed to shear distend threshold. The Samples would yield to failure when shear stress closed to failure strength. However, the deformation of wet samples was larger than dry samples. Both dry and wet samples, we had observed the deformation of plasticity and visco-plasticity, they had similar developments.