The Ultimate Bearing Capacity of Shallow Foundation on Poorly Cemented Sandstone

• Main Authors: Jen-Chen Chang, 張振成
• Other Authors: Jyh-Jong Liao
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/wpjeyx

博士 === 國立交通大學 === 土木工程系所 === 96 === 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 .