The 2015 Shenzhen catastrophic landslide in a construction waste dump: Reconstitution of dump structure and failure mechanisms via geotechnical investigations

A catastrophic displacement of a waste dump occurred on December 20, 2015, in Guangming New District, Shenzhen, China. During this event, approximately 2.51 x 10(6) m(3) of construction waste slid out of the dump site affecting approximately 0.38 km(2), destroying 33 buildings and killing 77 people. This study tries to reconstruct the dump structure prior to the failure event, including evolution of dump geometry, geotechnical profile, hydrogeological conditions, and physical properties of fill materials. The dumpsite had been an abandoned quarry before receiving construction wastes beginning in February 2014. Within 22 months, 5.83 x 10(6) m(3) of construction wastes, some of which had high water content, filled the dumpsite forming a 110-m-high loose fill. The dominant component of the fill was completely decomposed granite (CDG) excavated from the adjacent underground construction. The dumping operation at the site was characterized by rapid filling, inadequate compaction and poor water drainage. Poor compaction on the fill materials resulted in low compactness. The measurements of soil density showed that over 80% of the collected CDG samples had a relative density (D-r) from 4% to 67%. The dynamic penetration tests conducted on the landslide bed indicated that the fill materials were loose to slightly dense at most of the penetration depth. No effective drainage facilities were provided at the dump site. A large quantity of water from a catchment area of 0.51 km(2) readily poured into the dump, resulting in a significant rise in the phreatic surface. The rapid filling rate of 5-7 m/month was very likely to have caused a build-up of excessive pore-water pressure in the lower poorly drained fill, which was verified by borehole exposure of the confined groundwater and the evaluation of consolidation degree based on the measurements of soil density. The fill materials within the landslide bed were found to have a consolidation degree of 33% on average. A deep-seated translational failure occurred on the lower poorly consolidated fill layer, forming a very fast-moving flow slide.