| 摘要: | 岩坡因不連續面與坡面之傾向關係，可能形成順向坡、斜交坡及逆向坡，其工程行為截然不同，因工程配置方位與不連續面位態之關係亦將顯著影響岩體的工程特性的差異，稱為岩體工程行為之異向性（anisotropy of engineering behaviors）。
斜交角之分析及模擬係屬三維問題，本文採用數值軟體PFC3D（Particle Flow Code in three Dimension）建立一套生成BPM（Bonded Particle Model）配合DFN（Discrete Fracture Network）建構具有一組不連續面之合成岩體邊坡模型，能合理模擬岩坡之力學行為。本文透過參數研究，包括：斜交角（不連續面傾向\alpha_j與坡面傾向alpha_s之夾角, \left|\alpha_j-\alpha_s\right|）、坡角（
eta_s）、坡高（H）、不連續面傾角（
eta_j）及摩擦角emptyset_j，探討在不同斜交角下，平面破壞（planar failure）及傾覆破壞（toppling failure）之崩塌行為。根據數值模擬結果，求取崩塌體之波及範圍、位移方向及崩塌能量，並以崩塌能量為量化指標，決定平面破壞之臨界斜交角及傾覆破壞之臨界逆斜交角。
研究結果顯示：（1）當岩坡符合不連續面摩擦角≧30˚、坡角≦75˚、坡高≦60m之條件時，其臨界斜交角均≦20˚。此一結果與一般工程實務及技術規範揭示當斜交角小於20˚時，易發生順向坡平面破壞的經驗一致。然而，當岩坡具不利因子時，即高坡角、大坡高、中等不連續面傾角及不連續面低摩擦角，其臨界斜交角有可能超過20˚，尤其不連續面摩擦角10˚時，臨界斜交角接近40˚。（2）根據傾覆破壞模擬結果，其臨界逆斜交角均<\mathrm{30°。此一結果與一般工程實務及技術規範揭示當逆斜交角小於30˚時，易發生逆向坡傾覆破壞的經驗相符。然而，當岩坡具不利因子時，即高坡角、高不連續面傾角及低不連續面摩擦角，其臨界逆斜交角有可能超過30˚，尤其不連續面傾角mathrm{75˚，其臨界逆斜交角接近50˚。（3）當岩坡滿足平面破壞三條件，即平行度條件（left|\alpha_j-\alpha_s\right|<\left|\alpha_j-\alpha_s\right|_{cri}）、見光條件（βs>βj）及滑動條件（βj>∅j）時，隨不連續面傾角增加，位態評分調整扣分及崩塌能量先增後減，兩者分析方法趨勢相似，其結果與潛在滑動體積相關，且能反映災害程度。然而，以極限平衡分析狹義順向坡(\left|\alpha_j-\alpha_s\right|=0°)分析岩坡穩定性，不連續面傾角越大，安全係數越小。此一現象未能反應潛在滑動體積。（4）傾覆破壞分析結果顯示，不連續面傾角增加，崩塌能量越大，反之亦然。根據極限平衡法對狹義逆向坡（left|\alpha_j-\alpha_s\right|=\mathrm{180}°）穩定性分析，隨不連續面傾角增加，安全係數越小。三種分析方法比較結果，趨勢相符皆為一致。（5）斜交角及逆斜交角=0˚，模擬分析得到的崩塌體位移方向與坡面傾向一致。但當斜交角或逆斜交角>0˚，崩塌體之位移方向會介於坡面傾向與不連續面傾向之間。;The dip slope, oblique slope, and anti-dip slope may present different engineering behaviors, which are dominated by the oblique angle defined by the angle between discontinuities and slope orientations on a rock slope. Additionally, the angle between the direction of an engineering configuration and the dip direction of discontinuity also influences the engineering characteristics of a rock slope. These phenomena display the anisotropy of engineering behaviors.
Due to the slope stability analysis on the oblique angle needing a three-dimensional numerical simulation, the Particle Flow Code in Three Dimension (PFC3D) is adopted to simulate a three-dimensional synthetic rock slope model (SRSM) herein, including the parts of intact rocks and discontinuities, which are generated by the bonded particle model (BPM) and the discrete fracture network (DFN), respectively. This simulation technique can successfully simulate the planar and toppling failures that are consistent with the field observations. This paper conducts a series of parametric studies with the oblique angle (the absolute value of the angle between the dip direction of discontinuity, \alpha_j, and the dip direction of a slope, \alpha_s, \left|\alpha_j-\alpha_s\right|), the slope angle (\beta_s), slope height (H), the dip of discontinuity (\beta_j), and the friction angle on SRSM simulations to investigate the influences on engineering behaviors of planar failure and toppling failure by these parameters. In this paper, rock blocks sliding from planar failure or falling from toppling failure are referred to as collapse blocks. The impact area of landslide, the displacement vectors of collapse blocks, and the energy release of landslide are further analyzed during the simulation process. One of the analyzed results, the energy release of landslide, is employed to determine the “critical oblique angle” for planar failure and the “critical inverse oblique angle” for toppling failure.
According to the numerical simulation results, the following conclusions can be drawn: (1) For the planar failure simulated by SRSM under the friction angle of discontinuity ≥ 30˚, the dip of slope ≤ 75˚, and the slope height ≤ 60m conditions, the critical oblique angle simulated by this paper is consistent with the critical oblique angles revealed in engineering practical experiences and the national codes, which appear 20˚. However, the critical oblique angle may exceed 20˚ even near 40˚ when multiple unfavorable factors, i.e., a moderate dip of discontinuity, a steep slope, a tall slope height, and a low friction angle of discontinuity, appear. (2) For the toppling failure simulated by SRSM, the critical inverse oblique angle simulated by this paper is similar to that in engineering practical experiences and the national codes, the former appears 30˚ and the latter appears 30˚. Similarly, the critical inverse oblique angle may exceed 30˚ even near 50˚ when multiple unfavorable factors appear, such as a high dip of discontinuity, a steep slope, and a low friction angle of discontinuity. (3) When rock slope satisfies three conditions of planar failure, i.e., parallelism condition (\left|\alpha_j-\alpha_s\right|<\left|\alpha_j-\alpha_s\right|_{cri}), daylight condition(\beta_s>\beta_j), and sliding condition (\beta_j>\emptyset_j), the energy release of landslide increases with the decreasing dip of discontinuity and vice versa. (4) For toppling failure simulated by SRSM, the energy release of landslide increases with the increasing dip of discontinuity and vice versa. However, for toppling failure calculated by limited equilibrium analysis, the safety factor of a slope decreases with the increasing dip of discontinuity. (5) When oblique angle and inverse oblique angle=0°, the mean of displacement vectors of collapse blocks is consistent with the dip direction of a slope, and it appears near the middle of the dip direction of a slope and the dip direction of discontinuities under the other conditions. |
