Two members of the BGS landslide response team observed a landslide event at Wang Shan, China, on 13 September 2012.
During a conference field trip around Gansu Province, the team captured a video of the landslide in progress; including the downslope movement of rock debris and a tree fall.
The conference, the First International Symposium on New Techniques for Geohazard Research and Management, held in memory of the Zhouqu debris flow disaster of August 2010, combined a range of lectures focused on the instabilities of slopes and debris flows.
The Wang Shan landslide is located on the east-facing slopes opposite Dengqiao village (about 1500 m above sea level) along the G212 road, approximately 30 km south of Tanchang in Gansu Province, China.
The Wang Shan slope rises some 1100 m above the valley floor, to a height of 2600 m, part of a ridge leading up to the 3300 m high ‘And Shangduonao’. The Wang Shan slope comprises a landslide complex with a history of past movement, according to local information from villagers in Dengqiao.
Based on a comparison of our observations with interpretation of satellite images it seems that, recently, slope deformation has been accelerating. This most recent re-activation started on 12 September at approximately 4 p.m. The landslide was observed the following morning.
This summer has been wetter than average in this part of Gansu Province, and it is probable that this has resulted in a gradual loss of effective stress along pre-existing slip surfaces, causing the landslide to reactivate.
A surface model of the landslide has been generated using 123D Catch, free software that enables the user to generate three-dimensional models from photographs, and field observations.
The landslide is dominated by rotational movement of a number of large units, sliding along curved slip surfaces exiting the slope at approximately 1700 m above sea level.
Figure 1 shows the curved slip surfaces which are indicated by the thick dotted line running along the base of landslide units 1, 2, and 3).
This basal shear surface is very prominent and the BGS Landslide Team members were able to observe progression of the sliding bodies over the underlying, presumed intact, but highly fractured metamorphic bedrock. The upper limit of the complex landslide is defined by arcuate scars running at elevations of 1800 to 2300 m above sea level, the upper limits of units 5, 4 and possibly 6.
Unit 5 is a remnant of past instability and appeared not active at the current time. In between units 5 and 1 a small valley had formed and significant slope degradation was observed.
Landslide unit 1 was, at the time of observation, the most active part of the landslide. This unit appears to have formed in surface deposits that comprise loess (partly aeolian/partly reworked slope deposits). Along the front of this unit, loess deposits would collapse regularly and these would disintegrate on the steeply inclined run-out slopes below. The down-slope transport pathways, and their affected zones, of the disintegrating landslide materials are indicated by arrows and dotted lines.
The small landslide block indicated by 2 is in the centre of a small valley and is squeezed between 1 and 3. Unit 3, comprising significant proportions of loess, but also containing a much darker deposits, appeared to progress more slowly than unit 1 with fewer frontal collapse episodes during the period of observation.
Unit 4 is a large landslide complex that mainly comprises fractured bedrock. It is assumed that this feature was initiated by unloading of the toe of the slope as a consequence of progressive movement of landslide units 1, 2 and 3.