Effect of Slope Inclination on the Loading Behavior of Shallow Foundation In a Poorly Cemented Sandstone
Dr. Jyh-Jong Liao
|關鍵字:||人造膠結不良砂岩;邊坡傾角;淺基礎模型承載試驗;極限承載力;破壞機制;artificial poorly cemented sandstone;slope angle;shallow foundation;ultimate bearing capacity;load-bearing behavior|
This thesis aims to investigate the loading behavior of shallow foundation in poorly cemented sandstone on slope crest where slope angles are 10 degree,20 degree and 30 degree . In this thesis, an artificial soft 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 samples were prepared and loaded in a steel box with the dimension of 60 cm x 30 cm x 65 cm. To observe the fracturing of the specimen during test, a observed window was made on the upper and central part of one of the wall of the box using high strength glasses . Then, twelve sets of model loading tests were performed in the laboratory. Base on these tests, not only the bearing capacity but also the mechanisms of progressive failure are investigated by image processing analysis. We explore the behavior of the shallow foundation of poorly cemented sandstone by the progressive failure and the analyzed results. To prepare the model material the sandstone was first gently crushed and ground in order to appropriately represent actual grain size distribution of the natural sandstone. After grinding, the rock grains passing #30 sieve and retaining on the #200 sieve were adopted as the basic model material. The solution passing #200 sieve was used as the cementing material for producing the artificial soft rock model. The particles and the solution with fines were mixed thoroughly with the ratio of 1:1 by volume. After that, their mixture was poured into a model container. The load on the specimen was slowly increased up to 250 tons step by step. After the consolidation stage completed, the specimen was extruded from the container and was exposed to electric fans for three weeks to produce an air-dried specimen; Then the specimen was ready for a load bearing tests. The results of model bearing test on slope crest indicate that a load-settlement curve is analogous to that on horizontal specimens. It also can be roughly divided into the incipient stage, the linear stage, the non-linear stage, and the final stage. The peak point of the load-settlement curve is the ultimate stress and decreases varies with slope angles. Generally speaking, the bearing capacity of 10 degree slope is approximately about 91 percent of horizontal specimens; the bearing capacity of 20 degree slope is approximately about 79 percent of horizontal specimens; the bearing capacity of 30 degree slope is approximately about 63 percent of horizontal specimens. At the incipient stage, the specimens have no obvious cracks. And at the linear stage , cracks just beneath both edges of footing could be observed, these cracks were likely due to the stress concentration near both footing edges and extended outward with the increasing bearing load. At the non-linear stage, as the cracks grows completely, it reaches the peak point of the load-settlement curve and an active zone right under the foundation base was clearly formed. At the failure stage, the active zone slide toward the slope faces and the bearing stress decreases. After tests, the failure zones and mechanism were also identified. Failure zones were divided into the active zone, the transitional zone, and the passive zone. Not only the horizontal specimens but the inclination specimens, it was found that the bearing behavior on poorly cemented sandstone is distinct from the cases on hard rock or soil, with both plasticity and brittle characteristics. The bearing capacity formulas commonly used for soil or hard rock is not necessarily suitable for the case of poorly cemented soft sandstone.
|Appears in Collections:||Thesis|