| Summary: | 碩士 === 中原大學 === 土木工程研究所 === 91 === ABSTRACT
The geologic structures such as faults, joints, bedding planes, inter-bedding, and foliation, are very common in Taiwan, and their in-situ orientations may be horizontal or inclined. The rock formation associated with these features, of course, yields a mechanical behavior totally different from that of a homogeneous, intact mass. Especially when subjected to surface loading, the bearing disturbance regions developed within it may not resemble the symmetrical zones I, I, III as ideally proposed by Terzaghi. The main objective of this thesis is to numerically study the bearing mechanism of a rigid strip foundation of width (B) sitting on an inclined two-layer ground, and to establish a preliminary prediction model of ultimate bearing capacity for such a case by use of the neural networks analysis.
The bearing behavior of such a mass was simulated by the FLAC code, in which finite-difference scheme is employed for both the spatial and time domains. The influence factors examined were the ratio of the distance between footing corner and bedding outcrop (D) to footing width (B), bedding inclination (��), the shear strengths (c1, c2, ��1, ��2) of two layers. The foundation loading mode used is strain-controlled, and the ultimate bearing capacity (qu) is determined from the loading versus settlement curve and the disturbed zone beneath the foundation is localized according to the displacement vector plots. For understanding the distinct bearing behavior of an inclined two-layer mass, two conditions were selected: one with a strong layer (S) overlying a weak layer (W), and another with W overlying S. Besides, the neural networks analysis with backward propagation algorithm (NNAB) was adopted to develop a general bearing capacity formulas for such a ground system, and about three thousand of cases were run by FLAC with formation properties: the friction angle (��) varying from 0 to 30�a and cohesion (c) from 0 to 1MPa. Three classes were considered: both layers cohesionless, one cohesionless and another cohesive, and both cohesive.
The simulation results show that: 1) for a S/W case, qu increases with both D/B and ��, with a trend approaching to the qu of a pure S mass; 2) for a W/S case, qu does not vary a lot with D/B and ��, and its value is very close to the qu of a pure W mass; 3) both (��,c) values of two formations affect qu to some certain extent (but not merely qu of each formation). The NNAB established a fair predictive model of qu for an inclined two-layer mass, with a prediction error less than 10%. In its linear model analysis, the most influential factor for each situation was also identified, and for instance, such a factor is bedding inclination (��) for a two-layered cohesionless system with a weaker top layer.
|
|---|
