Experimental Study And Numerical Simulation On The Collapsibility Of High Embankment Loess In Yan'an New District

In this paper,the Yan'an New District construction project with the largest scale of “Cutting mountain,filling the trenches,reclaiming land and building cities” in the domestic loess area was used as a natural testing area,seizing the impact of “high pressure and water” on deep excavation of high embankment projects.Through consolidation tests and Collapsibility test,engineering mechanical properties and parameters of undisturbed soil and compacted loess under water and high pressure were obtained.Combining advanced testing methods such as electron microscope scanning and energy spectrum analyzer,the changes in the microstructure,particle size composition and soluble salt composition of the undisturbed and compacted soil before and after compaction and collapsing were analyzed to explore the collapsibility and deformation mechanism under different pressures and moisture contents.Using discrete element software PFC2 D,based on the size of loess particles,pore structure and particle size composition,the numerical method was used to construct the undisturbed soil and compacted soil models,and the load was applied for numerical simulation.The results achieved can be summarized as:(1)The compressive deformation and collapsibility characteristics of the samples with 22.2%,20.2%,18.2%,14.2%,12.2%,10.2%,natural and saturated moisture contents at various pressures from 50 k Pa to 1600 k Pa were tested by the single-line method.The samples include undisturbed loess and the compacted loess with 89%,92%,and 95% compactness.Collapsibility coefficient and compression amount of loess under normal stress at all levels was obtained.The influence of different water content and different pressure on collapsibility of loess was studied.The collapse coefficient was found to have a peak with the pressure change and decreased with the increase of the moisture content.At the same time,the compaction of loess can significantly reduce the coefficient of collapsibility and eventually convert to non-collapsed loess.(2)Based on SEM images,the microstructure of loess was qualitatively analyzed,and the differences between the structure of the compacted loess and the original loess in the structure of the matrix and the debris were compared,and the microstructures of the loess and the original loess under different pressure and moisture content were compared.From the microscopic mechanism,the preliminary analysis of the consolidation and collapse characteristics of loess was carried out.At the same time,using IPP image processing and statistical software for quantitative analysis,the distribution of macropores,mesopores,small pores,and micropores in each loess sample was obtained,revealing that the pressure and water content changes have an effect on the distribution of pores in undisturbed and compacted soils.In the process of consolidation and consolidation,the reduction of large and medium pores was found to be the main factor;and the dissolution of soluble salts was an important factor in the process of collapsing.The compaction of soil debris was obviously increased.(3)Through the analysis of energy spectrum,the chemical composition of the skeleton and adhesive of the undisturbed loess and the compacted loess were analyzed.The content of each element was counted,and then the main mineral components were obtained.The difference in elemental content between the undisturbed loess and the compacted loess debris particles at each compaction level was compared,and the conversion relationship between the skeleton and the detritus in the compaction process was obtained.In detrital minerals,feldspar content is higher than quartz,and there are also some carbonate minerals.Skeleton composition in loess is mainly hydromica and kaolinite.(4)Based on the laboratory consolidation collapse test data,a two-dimensional particle flow model was established.The meso-level parameters were calibrated by multiple numerical tests.The consolidation and compression processes of undisturbed loess and compacted loess were numerically simulated,and the contact force distributions of each model during continuous compression and progressive compression were obtained,as well as the compressive deformation,surface porosity and contact stress.The calculation result is reasonable,which lays the foundation for the numerical study of the follow-up embankment project.