Collapse and Landslide Hazard Characteristic Along Railway Line in Chengdu-Lanzhou of China

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

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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.

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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. In this way, the crevice structure surfaces of the towering bed rock were weakened, and the structure surfaces were extended and connected. The rock mass, therefore, toppled and collapsed. The early collapse had a small scale and happened intermittently, however, the scale became larger to some degree because of the Vol. 19 [2014], Bund. Z 9308

“5·12” Wenchuan earthquake. The free surface of the dangerous rock mass extended, and the collapse and failing body on the landslide accumulated, too. (Shown in Figure 5 and Figure 6).

Figure 5: The dangerous rock mass collapses along the structure surface

Figure 6: The collapse rock blocks accumulates along the landslide

In a comprehensive analysis, the main deformation characteristics of the collapsed dangerous rock mass are as followed: the dangerous rock mass on the hill top is highly weathered, the crevices are developed, and the rock mass are broken a lot. The main failure mode of the collapse is collapsing, chip off-falling and falling along the steep and free structure surface, the extraversion structure surface and crevice surface; although the collapsed tier on the back hill is stable for the moment, there is no barricade at the landslide toe to support with the accumulation of the collapsed rocks. Therefore, engineering governing is needed. The front of the collapse body is only 20m far from the south teaching building of the school (shown in Figure 7), and several rock blocks ever rolled to the outside of the wall corner during the “5·12” Wenchuan earthquake (shown in Figure 8). Seriously threatening the life and property security of the 160 students and teachers in school, it need to be governed immediately. According to the standard in terms of the scales of the collapse and landslide, the standard of the disaster and risk of the geologic hazard, “The National Emergency Preplans for Geological Disasters” and the standard of risk, this collapse body is medium, and the grade of hazard object Vol. 19 [2014], Bund. Z 9309 is No.1. The geologic condition of exploration is complicated, and the grade of governing engineering is I.

Figure 7: The front part of the collapse body behind the school

Figure 8: Rock blocks rolling to the school wall

DEFORMATION CHARACTERISTICS AND HAZARD OF UNSTABLE LANDSLIDE (SCARP) BEHIND THE TEACHING BUILDING The scarp has been simply supported at the steep part (shown in Figure 9). The scarp is made of the Quaternary sandy pebble soil. The top part is farm land and classified as the second terrace, the soil is dry or a bit wet, compact, and mainly made of pebbles, rock blocks, cobbles and some boulder, filled with silt and fine sand, which are connected with each other well. The pebbles are mostly granite, slate and quartz sandstone. And the pebbles are secondary arris shape and one dimensional round shape. The sorting is rather poor, the grain composition is as follows: the grains bigger than 100cm account for 6-15%, the grains of 20-100cm account for 15-40%, the grains 5-20cm account for 20-30%. There are some slates size 1.3m-2.3m simply supporting at Vol. 19 [2014], Bund. Z 9310 the landslide toe, but the support has low strength. And the landslide surface faces outside. There is a pylon northwest at top of the landslide, which was greatly influenced by “5.12” earthquake. This area is obviously deformed and chip off-falling and classified as unstable soil landslide.

Figure 9: Simple support on the landslide surface

ANALYSIS OF DEFORMATION MECHANISM OF LANDSLIDE The collapse behind the school was induced by these factors: The landslide is high and steep. The dangerous rock mass on the hill top is highly weathered, with developed crevice. The structure surfaces are mostly gird shape and well-distributed. The rock mass is quite broken and the rock layer towards outside of the landslide and generally agrees with the direction of the landslide surface. Under the influence outside, the rock mass easily collapses, falling, chip off- falling or slides along the layer.

CHARACTERISTICS OF STRATUM AND STRUCTURE OF ROCK MASS According to the characteristics of stratum and the structure of rock mass of the landslide, the collapse dangerous rock mass is made of metasandstone, filled with dark grey and black charcoal and sandy phyllite and dark grey slate and sandy calc slate. The unstable landslide (scarp) behind the teaching building is made of pebbles. The bed rock is entirely thick and massive, structural stable. The rock properties of semi-compact or compact pebbles and cobbles are fine, but because the landslide is high and steep, and the rock mass exposures outside directly, under the negative factors from outside such as gravity and rainfall, the front part of the rock mass started to collapse along the crevices. Specially, during the “5.12” Wenchuan earthquake, since the addition of earthquake force, the landslide behind the school started deforming and chip off-falling. Vol. 19 [2014], Bund. Z 9311

THE EFFECT OF UNDERWATER ON THE LANDSLIDE During the exploration, the landslide behind the school is quite dry, there was no sign of underwater. During the rainy season or the continuous heavy rainfall, there might be perched water on the landslide. Among the rock mass on the landslide, the water permeability of pebbles is better, and it is easy for the atmospheric precipitation to transport from the landslide surface to the pores among the pebbles and spill from the landslide surface and landslide toe in a short time. The water not only increases the weight, but also decreases the stability of the landslide. In addition, while moving in the pebble soil, the rain decreases the strength of the fine grains, especially the clay, which fills in the pebbles, so that accelerates the disintegration of the pebble soil. Therefore, the underwater because of the permeation of precipitation is one of the most important factors of the deformation failure of the landslide.

HUMAN ENGINEERING ACTIVITIES The landslide behind the school is high and steep. A main reason that causes this problem is that before the construction of the school, due to the limitation of the area, the teaching building was designed very close to the landslide in order to enlarge the activity space of students. The designed distance is only 0.8 to 1.3m. And during the construction, the original landslide was excavated. Since the landslide became steeper, the school made a simple support with some slate at the landslide toe in order to prevent the collapse of the landslide. But the strength of the support is low and due to the lack of money at that time, there is no comprehensive and efficient construction to support the exposure landslide. As a result, the landslide deformation broke under the local weathering and denudation, which brought about the risk of the further destroy of the landslide. Another reason is that the school was constructed next to an unstable cliff and sandwiched into a small area between a cliff and a river without a geological hazards assessment to the construction land, which brought unsafety risk to the school.

CONCLUSION (1) 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. (2) The exposed strata of explored area are mainly Triassic middle Zagunao formation bed rock (T2Z), Pleistocene alluvium (Qpal+pl) and Quaternary Holocene alluvium (Q4al+pl). 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. (3) The main deformation characteristics of the collapsed dangerous rock mass are as followed: the dangerous rock mass on the hill top is highly weathered, the crevices are developed, and the rock mass are broken a lot. The main failure mode of the collapse is collapsing, chip off- falling and falling along the steep and free structure surface, the extraversion structure surface and crevice surface; although the collapsed tier on the back hill is stable for the moment, there is no barricade at the landslide toe to support with the accumulation of the collapsed rocks. Therefore, engineering governing is needed. This collapse body is medium, and the grade of Vol. 19 [2014], Bund. Z 9312 hazard object is No.1. The geologic condition of exploration is complicated, and the grade of governing engineering is I.

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