Centrifuge Modeling And Failure Mechanism Analysis Of Shenzhen 12·20 Catastrophic Landslide

The contradiction between population growth and unavailability of land has resulted in extensive development of urban underground space and a dramatic increase in construction waste in recent years.As dumping is the most common method for construction waste disposal,thousands of sites have been used for waste dumping and large amounts of land have been occupied.What's worse,many of these dumps are not carefully designed,constructed and monitored and might eventually result in the catastrophic slope failure.On December 20,2015,a catastrophic landslide occurred in a 110-m-high waste dump located in Guangming New District,Shenzhen,China(22°42?43.91?N,113°56?6.93?E).The landslide involved 2.51 × 106 m3 of construction waste,destroyed 33 buildings and killed 77 people.This failure event reflects a poor understanding of secure dumping and an irregular management in the dumping of construction waste in China.Understanding the failure mechanisms of construction waste slope and proposing methods of secure dumping are the key to prevent such disasters from happening again.This thesis studies the state of the dump before and after the failure,physical properties of the fill materials,process and mechanisms of the Shenzhen landslide from geotechnical investigation,laboratory test,centrifuge modelling,numerical and theoretical analyses.Factors including slope ratio,water content,filling rate and way of dumping have been analyzed.The main research works and conclusions are listed as follows:(1)Geotechnical investigation including field reconnaissance,witness interviews,remote-sensing images interpretation,borehole investigation,in situ penetration tests,etc.were carried out to investigate the failure process,dumping process,hydrotechnical and geotechnical conditions of the dump slope.Physical properties of fill materials were tested.As a result,the dump structure prior and post to the failure event were reconstructed.The dumpsite had been an abandoned quarry before receiving construction wastes beginning in February 2014.Within 22 months,5.83 × 106 m3 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.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(Dr)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 km2 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.(2)Unconsolidated undrained(UU)and consolidated undrained(CU)tests were performed on the completely decomposed granite(CDG)soil with different initial water content,which was the main component of the construction waste.The UU,CU shear strength and effective shear strength of CDG with different water content were obtained.It was found that the frictional angle of UU shear strength decreased with the increase of saturation as a result of increased confining pressure.When saturation was greater than 0.7,the frictional angle was significantly reduced to 3.4°-8.3°,which might explain the low-angle slip surface of the Shenzhen landslide.The frictional angel and cohesion of CU shear strength decreased with the increase of initial water content.(3)Two groups of centrifuge modeling test of the Shenzhen landslide were carried out.The build-up of excess pore-water pressure and the slope instability of a fill slope subject to rapid filling and high water level were studied in the first test.The dumping process of the Shenzhen dump was modeled,and the failure process of the Shenzhen landslide was reproduced successfully in the second test.Test results showed that high pore-water level in the model slope resulted in slope failure initiated at the toe of the slope,followed by an extensive sliding.Excess pore-water pressure was generated in the high water content part due to rapid filling.The ratio of excess pore-water pressure to overburden effective stress was about 0.54-0.85,and the ratio of pore-water pressure to overburden stress was about 0.78-0.91.The failure mode of the second test was that the high water content soil flowed forward under upper load and pushed the low water content soil at the front,resulting in the deep-seated landslide.This was the same with the failure of the prototype and indicated that the high water content part was the key factor in resulting in the landslide.(4)Numerical analyses of the centrifuge modelling results and the Shenzhen landslide were carried out.The shear strength of CDG was back-analysed according to centrifuge modelling results and veriyfied with the triaxial test results.The dump state was back-analysed and compared with the geotechnical investigation.Back-analysis of the centrifuge modeling showed that the effective fricitonal angle was 22°-27°,which was the same with the triaxial tests.Back-analysis of the prototype showed that high water level and excess pore-water pressure due to rapid filling rate collectively resulted in the catastrophic landslide.(5)Finally,factors influencing the height of slope and the capacity of dump were studied.The design,operation and moitoring method of secure dumping were put forward.Water content,slope ratio,way of filling and filling rate should be all considered in the design of dump to reach a maximum capacity.Initial water content of the fill,the drainage and filling rate should be controlled during operation.Water level,displacement and excess pore pressure should be monitored.