Studies On Static And Seismic Stability Of Landfills With High Water Level By Centrifugal Model Tests

In order to save land resources,landfills in China will be developed highr,larger,and steeper.Once a slippery disaster occurs,leakage of waste not only brings direct loss of life and property to residents,but also causes serious pollution of the environment of the region.The high leachate level is one of the main causes of the major landsilde hazards of landfill.China is located between the Circum-Pacific seismic belt and the Eurasian seismic belt.The earthquake environment is complex,and the intensity and frequency of earthquakes are very high.The earthquake can cause redistribution of the stress field and seepage field of the landfill,leading to the gradual evolution of the disaster,triggering the instability of the landfill.Hypergravity centrifugal model test has irreplaceable advancement in research of instability mechanism of the landfill and has been successfully applied to simulate the instability problem of loose soil slopes.Considering the characteristics of complex components,high organic content,and high water content of waste in China,attention should be paid to the study of the stability of landfills with high water level and under earthquake.Combines laboratory experiments,model tests,theoretical analysis,numerical simulation and other research methods to reveal the failure mechanism of landfills under different triggering factors,and establish progressive failure analysis methods of landfills.The main work and research conclusions are as follows:(1)Combined with the study of the basic phase and reinforcement of MSW,and the laboratory tests on different particle sizes of the basic phase,the MSW strain hardening mechanism was explained and the method of determining the MSW shear strength parameters was discussed.There are a large number of free or particulate-wrapped organic fiber in the basic phase particles of different sizes,and the smaller the particle size is,the more free fiber are in the particle group.The shearing process may be accompanied by breakage and slippage of the fiber.CU test stress path phase transition point corresponds to the pore water pressure stability point and the turning point of the stress-strain curve.Before the phase transition point,there was no obvious reinforcement process.After the phase transformation point,the fiber reinforced effect was obvious.It is recommended to use a slightly conservative "phase transition point method" to determine the shear strength parameters of the CU test of the MSW.The shear strength parameter obtained with the stress ratio peak is similar to the shear strength parameter obtained by the "transition point method".(2)Improvements have been made to the existing methods for the preparation of synthetic solid waste.The peat plays a leading role in the strain hardening characteristics of the prepared synthetic waste,but degradation of the organic fiber in the peat will lead to the weakening of reinforcement effect.The organic fiber content of the used peat was measured,and the effect of organic fiber content on the stress-strain characteristics of peat and synthetic waste was analyzed.The higher the organic fiber content incorporated in waste,the higher deviated stress peak and the damage strain.When the content of organic fibers is fixed,the peak value of the deviator stress decreases and the failure strain decreases with the increase of clay content and decrease of the sand.The organic matter content was used to describe the degradation degree of peat,and synthetic wastes similar to partially degraded wastes in Qizishan Landfill were reconstituted.At the same time,the applicability and dynamic characteristics of the prepared synthetic waste in the hypergravity environment were studied.(3)Based on previous studies on instability of hydraulically induced landfills,several typical failure modes of landfills are summarized,and the critical water level and influencing factors are discussed.For landfills without a waste dam,continuous slipping is generally caused by the global instability along the liner interface,and depending on the type of slope,penetration cracks or partial slips will occur.For the landfills with waste dams,there is no obvious slide along the liner interface,but penetration crack failures occur;for landfills with steep slopes and high levels of degradation of solid waste,the transition from local slip to whole slip induced by erosion of the foot slope may occur.Based on the analysis results,the limit equilibrium analysis method of double/tri wedge was improved.The active wedge and passive wedge interface were changed to the actual slip surface in the process of slip.The safety factor obtained is closer to 1.0,which is close to the unstable state.(4)Focus on the dynamic response of the high water level landfill and the stability under the earthquake,a series of centrifuge shaker tests were conducted to reveal the failure mechanism of the earthquake-induced landfill.The dynamic response of the landfill under high water level conditions was explored.The acceleration response in the landfill shows a significant elevation effect,and the acceleration peak increases significantly with the increase of the elevation.At higher water levels,the acceleration decays locally after the earthquake.Under different water level,the response of pore pressure is quite different.The absolute value of the difference in pore pressure is linearly increasing with the peak acceleration at the corresponding height.The seismic settlement of the landfill is mainly represented by instantaneous settlement,occurs mainly during the seismogenic period and quickly stabilizes after the earthquake.The settlement shows a non-linear increase with the increase of magnitude.(5)The results of centrifugal model tests for seismic response of landfill with different slope types and water level heights were analyzed.The effects of water level rise and earthquake and slope type were analyzed,and the seismic instability failure mechanism of the high water level landfill was studied.The acceleration amplification factor firstly increases with the rise of the water level,and when the water level reaches a certain height,the acceleration response of the earthquake weakens,and decreases as the water level rises.With the increase of the water level,the pore pressure change value under the earthquake has gradually changed from positive growth to negative growth.Settlement of the landfill firstly decreases and then increases as the water level rises.After the earthquake at high water level,the tension cracks were first observed at the shoulder position,and then they were extended downward to form penetration cracks under the effect of the high water level seepage.Earthquake will significantly reduce the water level when the landfill slide.The cracking after the earthquake plays an important role in the instability of the landfill.The higher the frequency of occurrence of the earthquake,the lower the water level when the landfill significantly slip.Steeper slopes will reduce seismic stability.(6)Combining the Hangzhou Tianziling Landfill Project and the Chengdu Municipal Solid Waste Sanitation Disposal Compatibilization Project,the three-wedge limit equilibrium analysis method based on the actual slip plane and the earthquake stacks obtained will be proposed.The response law and failure mode is applied to stability analysis and water level control.Within the allowable range of deformation of the landfill,the stability of the landfill can be improved to a certain degree by utilizing the stiffening properties of the solid waste deformation.Stability analysis based on the deformation of the landfill can more accurately grasp the stability.In this paper,based on the centrifugal model test,the sliding mechanism of the high water level landfill under water level rise and earthquake is revealed.The corresponding development rules of pore pressure and deformation are obtained,and the stability based on the actual instability slip surface is proposed.This study will help to improve the understanding of the seismic response of high-water level landfills,provide scientific basis for the design and stability control of landfills in China,and have theoretical and practical value for the study of the stability of landfills.