膠結不良砂岩的淺基礎承載力

Abstract

本研究旨在利用自行建立的室內模型承載試驗設備，經由淺基礎模型承載試驗探討基腳位於水平地表與邊坡穩定條件下之不同坡角及退縮距離之承載行為與破壞機制，並依據觀察結果嘗試提出一適宜之破壞模式，利用上限解定理提出膠結不良砂岩淺基礎承載力理論解。 本研究使用人造膠結不良砂岩試體進行一系列室內模型承載試驗，其試驗條件包含基腳位於水平地表、不同邊坡角度之坡頂(10°、20°及30°坡角)、及20度邊坡角度條件下之基腳置於距坡頂1倍與2.5倍基腳寬度之退縮距離。 人造試體經由各項物理及力學試驗證明，滿足模型相似律，適合作為模型承載試驗之用。模型承載試驗結果，顯示水平地表之膠結不良砂岩淺基礎承載力最大，邊坡坡頂承載力隨坡角增大而減少，10度與20度邊坡坡頂之極限承載力較水平地表條件約略成正比例的減少，而當坡角為30度時，極限承載力明顯較為減小；基礎承載力隨者基腳之退縮距離增加而漸增，當基腳之退縮距離為2.5倍基腳寬度時，其基礎極限承載力接近水平地表條件者。 膠結不良砂岩淺基礎承載曲線略分為四個階段，即應力調整階段、線性階段、非線性階段及極限破壞階段。而基礎破壞機制可分為基礎正下方之主動壓力區，最外側鄰近地表面之被動壓力區，及介於此兩區間傳遞應力之轉折區。基腳下方主動壓力區約略形成倒三角形狀，且隨坡角增大或退縮距離減少而更偏斜邊坡面，此區內材料類似顆粒被壓碎現象且呈現塑性變形，故主應力方向應為垂直向下；被動區為破壞滑動面上之區域，破壞滑動面為裂縫延伸至地表面所形成之，且當此破壞面形成時基礎隨即失去承載能力。被動區受到主動區的主動壓力推擠而形成，故此區主應力方向應為平行邊坡面。水平地表條件下是對稱形成的，其餘試驗條件下只有邊坡側形成被動區，且被動區域面積隨退縮距離增加而漸增；轉折區位於主動區與被動區之間，主應力方向於此區中旋轉。轉折區存在二至三條裂縫，將此區分為三至四個塊體。當邊坡基礎兩側對稱性愈高，則邊坡側與水平側破壞模式也愈趨於相似對稱，因此基礎承載能力也相對提高。膠結不良砂岩淺基礎破壞機制可歸類為多重塊體傳遞破壞機制(multi-block translation mechanism)的一種。根據試驗結果顯示，膠結不良砂岩兼具岩石裂縫與土壤塑性之特性，為土壤與岩石間過渡性之材料，因此目前適用於土壤及岩石的各承載力分析方法未能全盤適用。 本研究根據模型承載試驗結果提出一理想之假設破壞模式，其可分為基腳下方主動區、地表側之被動區以及介於主動區與被動區之轉折區，轉折區假設由三個楔形塊體所組成並且轉折區夾角隨坡角及基腳退縮距離變化。透過上限解定理提出膠結不良砂岩淺基礎承載力理論解，理論結果顯示接近承載試驗結果，因此本理論解可用於決定膠結不良砂岩材料基礎承載力，但於坡角較高條件下(即坡角大於 )需檢核其邊坡穩定的影響。

The present paper aims to investigate the bearing behavior and failure mechanism of a shallow foundation of level ground and on/behind the crest of a poorly cemented sandstone slope. As a marginal geo-material, the load-bearing behavior of soft rock may not be closely modeled by the common theories. A clear understanding of the actual failure mechanisms of poorly cemented sandstone is crucial for estimating its bearing capacity. It is attempted to develop a bearing capacity theory for the geomaterial according to the observed failure mechanism from model tests in this study Load-bearing model tests of strip footing on slope crest for slopes with various slope angles (0, 10, 20, and 30 degrees) and for footing at various setback distances (1 and 2.5 times of footing width) from 20° slope crest were conducted. The model rock slope was made of artificial rock that simulates natural poorly cemented sandstone. The comparison of various physical indices and mechanical properties supports that the mechanical properties of artificial soft rock are reasonablly close to the target natural soft rock. Based on a series of load-bearing model tests, it was found the ultimate bearing capacity decreases with the increase of the slope angle or the setback distance of footing. For slope angle greater than 30°, the influence of slope on the ultimate bearing capacity is more obvious. When the setback distance exceeds 2.5 times of the footing width or so, the ultimate bearing capacity is close to that of level ground. Referred to the load-deformation curves, the load-bearing process can be divided into four stages; namely, the initial stress-adjusting stage, the linear stage, the non-linear stage, and finally, the ultimate stage. The active zone, in a shape of an inverted triangle, exists under the footing base. It is noted, as the slope angle increases and setback distance decreases, the shape of the inverted triangle deformed more toward the sloping side. In the active zone, the foundation material deformed downward and laterally toward to the sloping side. Hence, the vertical stress is the major principle stress . When the shear fractures composed of the passive zone finally reached the slope surface, the footing foundation would lose its bearing capacity eventually. The passive zone was formed by crack extended onto the slope surface. The passive zone was pushed laterally and outward. In the passive zone, is parallel to ground. A transition zone, which may contain one or two radial cracks, is located between the active and the passive zones. In the transition zone, the shear cracks provided stress discontinuities between the active zone and the passive zone; it enables the major principle plane rotate progressively from the active zone to the passive zone. The fracture models in two sides of footing were symmetrical only in the level-ground case which has the largest bearing capacity. Based on the observed behavior from the model tests, the failure mechanism may be modeled as a multi-block translation mechanism. From the failure mechanism observed in the load-bearing model tests, it appears that common theories neither for soils nor for rigid rocks can fully model the bearing failure mechanism on poorly cemented sandstone. Unlike a plastic behavior in soil and brittle behavior in rigid rock, the failure mechanism for poorly cemented rock develops both plastic deformation and shear fractures in a progressive process. Based on the experimentally observed failure mechanism, a simplified plastic collapse mechanism is proposed and an upper-bound solution based on a muti-block translation mechanism is formulated. Failure zones were divided into the active zone, the transitional zone, and the passive zone. In this study, it assumed that the transition zone contained three rigid triangle wedges with two velocity discontinuities. The angle of the rotation angle of the major principle stress from the active zone to the passive zone is affected by slope angle and setback distance. The upper-bound solution agrees well with the experimental bearing capacity for slope angle less than .