Collapse and Landslide Hazard Characteristic along Railway Line in Chengdu-Lanzhou of China Dongliang He School of Civil Engineering, Hunan University of City, Yiyang, Hunan 413000, China e-mail: [email protected] ABSTRACT The Chengdu-Lanzhou Railway locates in the Sichuan province and Gansu province. After the “5.12” Wenchuan earthquakes, a massive landslide, collapse, debris flow occurred, worsening the geological environment. The collapse of the mountain behind the Mao County valley gully elementary school is one of these many serious hazards, threatening the life and property security of 160 teachers and students in the school, as well as the residents around. In order to find out the form, structural morphology, scale, deformation behavior and the formation mechanism of the collapse hazard, studies on the structure forms, buried depth and the engineering construction, engineering geology emergency exploration to the collapse hazard has been carried out in this paper. Through varied reconnaissance means, such as geodesy, exploration and field testing, the causes, form, scale, scope, steady state and perniciousness of the collapse and the landslide next to the teaching building have been ascertained, and the deformation mechanism has been analyzed to provide a reference to the further assessment of prevention and control engineering, ensuring the successful construction of the Chengdu-Lanzhou Railway. KEYWORDS: landslide; collapse; Wenchuan earthquakes; assessment INTRODUCTION The Chengdu-Lanzhou Railway locates in the Sichuan province and Gansu province. It starts at Chengdu, going via Guanghan, Shifang, Mianzhu, Mao County, Songfan before arriving at Jiuzhaigou Valley, and extends northwards to the Hadapu station of the Lanzhou-Chongqing railway which is under construction. The main line is 457.592 km long. In this line, Mao County locates in the alp zones of the transition zones between the Sichuan Basin and the Western Sichuan Plateau. In this area, there are high mountains, deep valleys and extremely complicated geological conditions. The area is one of the most geologic hazard-prone regions in China (Chang et al., 2014; Ding et al., 2014; Xu et al., 2014; Yin, 2014). Particularly after the “5.12” Wenchuan earthquakes, a massive landslide, collapse, debris flow occurred, worsening the geological environment (Ding et al., 2014; Ge et al., 2014; Xu et al., 2014). The collapse of the mountain behind the Mao County valley gully elementary school is one of these many serious hazards, threatening the life and property security of 160 teachers and students in the school, as well as the residents around. In order to find out the form, structural morphology, scale, deformation behavior and the formation mechanism of the collapse hazard, which are beneficial to the - 9303 - Vol. 19 [2014], Bund. Z 9304 suggestions proposal about engineering measures, structure forms, buried depth and the engineering construction, engineering geology emergency exploration to the collapse hazard has been carried out in this paper. Through varied reconnaissance means, such as geodesy, exploration and field testing, the causes, form, scale, scope, steady state and perniciousness of the collapse and the unstable landslide (the scarp) next to the teaching building have been ascertained, and the deformation mechanism has been analyzed to provide a reference to the further assessment of prevention and control engineering, ensuring the successful construction of the Chengdu-Lanzhou Railway. COLLAPSE AND THE MORPHOLOGICAL CHARACTERISTICS OF THE LANDSLIDE The collapse of the valley gully elementary school consists of the dangerous rock mass and the falling accumulation of the hill behind the school and the unstable landslide (scarp) next to the teaching building. And the mode of collapse is toppling and scattering rock collapse. (1) Dangerous Rock Mass Located obviously the top of the back hill (shown in Figure 1), mainly induced by “5·12” Wenchuan earthquakes and its aftershock (Huang et al., 2014; Li et al., 2014; Yang et al., 2014; Yuan et al., 2014), as well as the heavy rainfall, the dangerous rock mass is similar to a “U” shape steep cliff body, generally upright, and more gentle at the top, with a landslide of its vertical plane between 70° to 90°. The dangerous rock mass consists of dark grey medium thick stratiform- massive metasandstone, quartz sandstone intercalated with dark black carbon, sandy phyllite and dark gray slate. Its free surface is facing to the northeast landslide. The free surface of the bed rock has a top elevation of 2623.45m and a bottom elevation of 2582.12m. The free surface of cliff is 41.33m high. Its maximum opening width is 134m, and 80m in the opening direction. There are some residual soil partly spreading at the top of the bed rock, and there are and a small amount of vegetation and bush. The rock is weathered strongly, and the structural surfaces, such as the joints, cleavages and fractures, are developed, well-distributed and generally grid shape. The main occurrence of the structure surface are 335°-350°∠56°-75°, 235°∠50°, 50°∠86°. According to the exploration, there is no tension fissure at the top of the hill and no falling accumulation at the bottom of the dangerous rock mass. Vol. 19 [2014], Bund. Z 9305 Figure 1: The collapse of the valley gully elementary school (2) Collapsed Body of the Back Hill It was formed because of the scattering of the collapse bed rock on the top hill. The collapse body was shaped in a fan (shown in Figure 1), the front arc is 258m long and 2240.80m in elevation, and the fanhead is 2,495.0 m in elevation. The relative height difference is 254.20m. The main axis of the collapse body is toward northeast, agreeing with the landslide, and the landslide angle is 40° to 50°, averaging 45°. The tier in the front is 3m to 6m high. The collapse body is totally 50,000m3, being made of rock blocks, like sandstone and slate. The biggest block is 1.5×2×2.5m3. The dangerous area is about 15,500m2. (3) Unstable Landslide (Scarp) behind Teaching Building The scarp is closely next to the back wall of school (shown in Figure 2 and Figure 3). Their distance is only 0.8 to 1.0m. The scarp is 130m long, 2,236.24m in elevation at the bottom, 2248.63m in elevation at the top. The relative height difference is 12.39m. The landslide is 75° to 80°. Although simply constructed with some rubble, now there are some crevices, deformation and convex since the influence of “5·12” Wenchuan earthquakes, aftershock and heavy rainfalls. The scarp is made of the Quaternary sandy pebble soil. These factors indicate that the scarp is extremely unstable and dangerous and it needs to be treated and governed. Vol. 19 [2014], Bund. Z 9306 Figure 2: The north side of scarp Figure 3: The south side of scarp COLLAPSE AND LANDSLIDE GEOLOGICAL STRUCTURE CHARACTERISTICS According to the ground survey and well exploration, the exposed strata of explored area are al+pl mainly Triassic middle Zagunao formation bed rock (T2Z), Pleistocene alluvium (Qp ) and al+pl) Quaternary Holocene alluvium (Q4 . The composition of the landslide is mainly sandy pebble soil. The dangerous rock mass is mainly metasandstone, sandy phyllite and calc slate and so on. According to the exploration, the dangerous rock mass consists of dark grey medium thick stratiform-massive metasandstone, quartz sandstone intercalated with dark black carbon, sandy phyllite and dark gray slate. The rock mass is strongly weathered, the joints are developed and distributing in grid shape. The main occurrence of the structure surface are 335°-350°∠56°-75°, 235°∠50°, 50°∠86°. The rock is quite broken. Due to the influence of structure, earthquakes, heavy rainfall, the structure surface was destroyed and collapse was made by the gravity (shown in Figure 4). Vol. 19 [2014], Bund. Z 9307 Figure 4: The free surface of collapsed rock mass (2) Characteristics of Collapse Rock Mass According to the exploration, since the major collapse happened because of the earthquake, the broken rocks scattered and covered the lower part of the landslide, and distributed in fan because of the shape of landslide. The rock blocks are different in size, the blocks of middle part are little smaller than those of two sides, and the blocks at the bottom are bigger than the upper. The fan in the front is 3m to 6m high. The free surface of the collapse body is loose in structure, and locates in the bed rock, with some residential soil distributing on it. (3) Characteristics of Unstable Landslide (Scarp) behind Teaching Building The scarp is upright. And its posterior is used as farm land. The area is the second Zagunao terrace. And the top of the scarp is stable now, being made of grey-taupe sandy pebble soil. It is a bit wet and high compactness, being filled with silty sand. And the pebble is mainly made of metasandstone, slate, granite and so on. The scarp now had been simply supported with some rock blocks, which partly deformation failure and broken during the “5·12” earthquakes. DEFORMATION CHARACTERISTICS AND HAZARDS OF THE COLLAPSE ON THE BACK HILL According to the field investigations, the deformation characteristics of collapse and landslide can be evaluated in terms of two parts, the collapse on the back hill and the unstable landslide (scarp) behind the teaching building, respectively. The collapse is on the top of the back hill. And the dangerous rock mass is made of dark grey medium thick stratiform-massive metasandstone, quartz sandstone intercalated with dark black carbon, sandy phyllite and dark gray slate. This is a semi-hard rock collapse and had been developing for a long time, more than a decade. The early collapse started with various geological factors both inside and outside, especially the rain weathering.