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Field investigations of landslides triggered by Lushan, Sichuan Ms 7.0 earthquake – the landslide hazards between Lingguan Town and Shuangshi Town

2014/5/27 13:42:36

The field investigations of landslides on April 22 cover the landslide hazards on the two sides of Lushan County – Lingguan Town, Lingguan Town – Shuangshi Town, Shuangshi Town – Weita – Taiping Town and Shuangshi Town – Lushan Countyhighways.

From Lushan County to Lingguan Town, landslide hazards become dense from sparse and the scale also shows a trend from small to large (Fig. 1 shows two small landslides of different types on a slope far from Lingguan Town). The highway from Lushan County to Lingguan Town is seriously affected by the landslides on the two sides. After rush repair, this road section has resumed traffic. In the section about 1km from Lingguan Town, two deep rock landslides on a large scale are developed. The estimated volume is above 10,000m3. After rush repair, traffic is resumed now. In this area, the mountains are high and the slopes are steep (almost vertical). Secondary slides are highly likely under aftershocks, so no vehicles are allowed to stay here.

Fig. 1Small rock collapse and superficial destruction-type soil landslide near Lingguan Town

The cross-mountain highway from Lingguan Town to Shuangshi Town was in a rush repair stage this noon. Due to a large landslide mass, it is estimated that this section is still not open to traffic. Starting from Lingguan Town, our emergency team consisting of 9 people followed road rush repair machines and planned to go to Shuangshi Town and carry out study of seismogenic structure. Fig. 2 shows a bulldozer clearing a loose accumulation body generated by a bedding rock landslide. Approximately, the volume of this accumulation body is hundreds of cubic meters. The superficial destruction-type debris flows and small collapses developed on the slopes on the two sides of the highway in this section are densely distributed. As shown in Fig. 3, 2~3 such small landslide hazards can be found every 100m on average. Usually, these types of landslides are small on scale and may partially or completely block the highway. As the scale is small, the clearing is fast. In this section of the highway, a great many of rock collapses are also presented. Consequently, large rock blocks become rolling stones. Some of them roll down from a higher slope position. Along the route, obvious vegetation destruction traces can be seen. The rolling stones caused by some typical rock collapses are shown in Fig. 4. The reason for traffic failure of this cross-mountain highway section till present is three large ~ huge deep rock landslides (Fig. 5). The scientific research team walked through these large landslides (Fig. 6) and entered Shuangshi Town smoothly. Besides, wide fissures are developed at many points on the slopes above this section of highway and it is extremely likely that the rock masses lose stability under the disturbance of external force (Fig. 7).

Fig. 2  A bedding rock landslide

Fig. 3  Superficial debris flow and small collapse-type landslide

Fig. 4  Photos of some rolling stones resulting from collapse

Fig. 5  Large – huge deep rock landslide posing difficulty to rush repair of the highway

Fig. 6  Field scientific research team walked to Shuangshi Town

Fig. 7  Huge rock almost losing stability

Some landslides are also developed on the two sides of Shuangshi Town –Weita –Taiping Town Highway, but they are inferior to the landslides developed betweenLingguan Townand Shuangshi Town in terms of quantity and scale.

Fig. Some landslides developed on the two sides of Shuangshi Town Weita Taiping Town Highway

On the two sides of the highway from Shuangshi Town to Lushan County and in the area near Shuangshi Town, many landslides (right of Fig. 9) are developed on slopes whose direction is consistent with seismic wave propagation direction as the mountains are high and the slopes are steep (left of Fig. 9) and they are near the epicenter. They are mostly superficial debris flows. The landslides developed on the slopes cover almost a half of the natural slope areaand have destroyed the vegetation on the slopes.

Fig. 9  Topography and landslides on the two sides of the highway from Shuangshi Town to Lushan County

Among the areas investigated today, the area between Lingguan Town and Shuangshi Town, including the areas near Lingguan Town and Shuangshi Town, is an area with the most serious landslides, mainly because of the following reasons: (1) The landform is steep and the gradient is above 50° in many areas and the natural slopes in some areas are almost vertical. (2) It is close to the epicenter and seismic oscillation is violent. (3) Due to the influence of long-term tectonic activity in the area, joints are developed in rock masses and the strength is low. (4) In this area, rainfall is large, spring water often gushes out from rock fissures along the route, these spring-water fissures may form cut-through planes in a long time and weaken slope stability. As a result, it is extremely likely to generate many large-scale coseismic landslides in the case of a strong earthquake.