人造膠結不良砂岩之模型承載試驗設備建立與淺基礎承載試驗

Abstract

本研究旨在建立一可供膠結不良砂岩進行室內模型承載試驗之試驗設備，製作基礎模型試體及進行淺基礎承載試驗。藉由模型基礎承載試驗結果以觀察其承載行為，檢討現有基礎承載力分析理論及岩石承載破壞行為，並建立膠結不良砂岩淺基礎的破壞機制及承載力判斷依據等。 經研究結果顯示，人造膠結不良砂岩之比重約2.66、乾單位重約2.02 g/cm3、孔隙率約24%、單壓強度介於3~5MPa、凝聚力 = 0.06MPa及有效內摩擦角 =37.27o；其性質已接近模擬之天然岩材。 本研究承載試驗結果顯示，承載試驗之承載應力及沉陷曲線可略分為三個階段，即初始跡段、降伏階段及破壞階段，此曲線的尖峰值即為基礎之極限承載力。另外，由初始階段、降伏階段至破壞階段，發現人造膠結不良砂岩由於應力擴張現象，使得破碎與發裂的球形範圍向外伸展，除了放射狀的裂縫之外，並同時有某種程度剪動效應存在，而試體形成主動區、被動區、主要剪裂面與完整區等不同型態區域。上述現象顯示膠結不良砂岩不同於一般硬岩或土壤的承載行為，而是兼具脆性與塑性之間的特性。

This thesis aims to investigate the engineering performance of shallow foundation in poorly cemented sandstone. In this thesis, an artificial weak rock was developed as the foundation material. The mechanical behavior of the artificial rock is analogous to the natural sandstone. A set of laboratory loading equipments for foundation model was assembled. The loading capacity of the loading facility is up to 400 tons. Then, four sets of model loading tests were performed. Based on the results of the model loading tests, the loading stress v.s. settlement curves can be divided into three parts, i.e., the incipient stage, the yielding stage, and the fracture stage. The curve of the initial stage is approximately linear. It reflects the model material is still elastic. At the yielding stage, the micro-fractures are induced and model material behavior becomes inelastic. The deformation rate increases at the fracture stage. Then, the bearing stress decreases with the increase of settlement. Based on the observation of the failure modes of the foundation model, the failure foundation model can be divided into four particular zones, i.e., the active stress zone, the passive stress zone, the shear-crack zone, and undisturbed zone. The active stress zone locates beneath the footing and was induced by the stress concentration at the corner of the footing. The passive zone was formed by the lateral expansion of the active zone. The shear fractures increase with the increase of the loads. Then, a macro shear slip surface forms and extends to the surface. Since both the shear and tensile cracks were observed in the models, the loading behavior of model foundation of poorly cemented failure mode is little difference with those for hard rocks or soils. The existing solutions for estimating foundation bearing capacity for soils or hard rocks are not suitable for weak rocks.