| 摘要: | 中國四川省大光包山崩為2008年汶川地震誘發之規模最大的巨型山崩，亦是百年來全球體積最大山崩之一，土方量約高達10.51億立方公尺。根據過去的認知，此種規模的楔型破壞發生的可能性極低，因此，了解大光包巨型山崩的發生機制與幾何特性具有指標性的意義。本研究利用遙測影像分析、野外調查、室內實驗與邊坡穩定分析等方法進行此一山崩之案例研究。經現地調查發現，此山崩為楔型岩體滑動，其南側主要受控於白雲岩(dolomite)之層面，而北側斷裂面受控於一組高角度走向節理，上述兩組弱面之交線約以傾角14°向N70°E傾沒。本研究利用災前災後地形資料、推測的地質構造(南側層面偏轉以及北側鋸齒狀節理)和災前災後磷礦工棚位置限制滑動範圍，以建立此山崩之幾何模型。結果顯示塊體滑動距離約1.9公里且過程中產生逆時針偏轉，與典型楔型滑動略有不同。根據現地觀察得知，主要滑動帶以角礫(breccia)為主，內部夾有數層延伸性良好、約數公厘厚的黃色及紅色泥層。本研究採集大光包山崩滑動面之白雲岩以及泥層，進行XRD (X-ray diffraction)分析、SEM (scanning electron microscope)微觀觀察與旋剪試驗，並由試驗結果引入擬靜態邊坡穩定分析(pseudo-static slope stability analysis)搭配強地動資料，以了解此山崩之發生機制與運動特性。XRD結果顯示，白雲岩和泥層的組成極為相似，主要皆為白雲石所構成。根據旋剪試驗在正向應力11.47MPa(白雲岩)和1~3MPa(泥層)以及剪動速度0.0009~1.3m/s的條件下之結果，白雲岩和泥層之尖峰摩擦係數落在0.52~0.96和0.75~0.92之間，而穩態摩擦係數則介於0.1~0.57和0.13~0.74之間，若將所得摩擦係數引入擬靜態邊坡穩定分析，塊體在未受地震力之前顯得非常穩定(F.S. = 2.45)，並在水平地震力達到0.32 以上時產生破壞 (F.S.< 1)。由於白雲岩和泥層之旋剪結果顯示，穩態摩擦係數在速度1.3m/s的條件下經過長距離剪動後皆可低於0.25(~tan(14°)；楔形體弱面交線傾角)，換言之，一旦塊體被啟動，此山崩塊體將受慣性力產生高速且長距離滑移。;The Daguangbao landslide which triggered by the 2008 Wenchuan earthquake is one of the largest earthquake-triggered landslides in the world over the past century. Therefore, it is important to well document this landslide, such as the geometry of the sliding mass, the dominating structures, and the failure mechanisms. In this study, remote sensing images analysis, field investigation, laboratory experiment and slope stability analysis were adopted to characterize this Daguangbao landslide. Based on the remote sensing images and pre- and post- DTMs, as well the field works, the dominating structures of this landslide are folded bedding plane and a steeped-out joint system, which outcropped at the south and north of the landslide site respectively. Accordingly, this landslide is a gigantic, atypical wedge failure. With the inferred slip planes, the intersection line is curved and counterclockwise rotated, which fit the trajectory of mining tent well. Meanwhile, the intersection line will be daylighted. That is, rock mass shear-off on the toe of this landslide, as assumed by most of the researchers, is no more required with adopting the inferred sliding surfaces. According to the proposed wedge shape, pre- and post- DTMs, the volume of moving mass is about 10.51 × 108 m3 with a travel distance of 1.9 km. The characteristics of the sliding surface on the south part of the landslide site were carefully investigated. The identified slip zone was composed of breccia and gouge layers of several centimeters thick. The intact dolomite rocks adjacent to the slip zone and the thin gouges were sampled to conduct X-ray diffraction analysis. The results show that the mineral of the gouges is nearly identical to the dolomite country rocks. The friction coefficients of the dolomite discontinuity and gouges near the sliding surface were measured utilizing a rotary shear apparatus under a normal stresses of 11.47 MPa (dolomite discontinuity) and 1~3 MPa (gouges) with slip rates of 0.0009~1.3 m/s. A threshold velocity of 0.001 m/s was identified that the slip strengthening behavior under slowly shearing turned into slip weakening. The peak friction coefficient of the tested dry dolomite discontinuities and wet gouges are 0.52~0.96 and 0.75~0.92 and the steady-state friction coefficient of the two samples are 0.1~0.57 and 0.13~0.74, respectively. The wedge analysis shows that the slope is quite stable (F.S. = 2.45) without the seismic force. However, the gigantic wedge can be triggered by the Wenchuan earthquake based on the pseudo-static slope stability analysis. Moreover, the friction coefficient of the gouges under large shear displacement will drop below 0.25 (~tan(14°); the intersection line plunged 14°) when the shear velocity exceeds 1.3 m/s. That is, the gigantic wedge can be speeded up by the inertial force generated by the earthquake and keep moving rapidly with long run-out. |
