Long-Term Settlement Characterization of High-Filling Foundation in the Mountain Excavation and City Construction Area of the Yan’an New District,China

Mountain Excavation and City Construction(MECC)represents a prominent anthropogenic endeavor aimed at facilitating urban expansion in the Loess Plateau of China.It is important to comprehend the long-term settlement behavior at MECC engineering sites to effectively assess the project’s success in reshaping landscapes and expanding urban areas.In this study,a typical MECC project,specifically the upstream area of the Liujiagou Valley within the new district of Yan’an City,is selected as a case study to investigate long-term settlement characteristics.The research involved conducting creep tests on soil samples with varying dry densities and moisture content and continuous in-situ monitoring of ground surface settlement at 17 specific points.Furthermore,a numerical model was developed and calibrated using the in-situ monitoring data to predict the long-term settlement.The findings reveal that an increase in soil dry density and a decrease in soil water content contribute to reduced deformation.Notably,settlement primarily manifests within the filled areas,with greater soil thickness exacerbating settlement effects.Over time,cumulative settlement exhibits a progressively diminishing rate of deformation until it attains a stable state.These results provide insights for assessing the long-term stability of MECC projects,facilitating decision-making in future endeavors within this region.

Pore evolution and shear characteristics of a soil-rock mixture upon freeze-thaw cycling

The changes in pore structure within soil-rock mixtures under freeze-thaw cycles in cold regions result in strength deterioration,leading to instability and slope failure.However,the existing studies mainly provided qualitative analysis of the changes in pore or strength of soil-rock mixture under freeze-thaw cycles.In contrast,few studies focused on the quantitative evaluation of pore change and the relationship between the freeze-thaw strength deterioration and pore change of soil-rock mixture.This study aims to explore the correlation between the micro-pore evolution characteristics and macro-mechanics of a soil-rock mixture after frequent freeze-thaw cycles during the construction and subsequent operation in a permafrost region.The pore characteristics of remolded soil samples with different rock contents(i.e.,25%,35%,45%,and 55%)subjected to various freeze-thaw cycles(i.e.,0,1,3,6,and 10)were quantitatively analyzed using nuclear magnetic resonance(NMR).Shear tests of soil-rock samples under different normal pressures were carried out simultaneously to explore the correlation between the soil strength changes and pore characteristics.The results indicate that with an increase in the number of freeze-thaw cycles,the cohesion of the soil-rock mixture generally decreases first,then increases,and finally decreases;however,the internal friction angle shows no apparent change.With the increase in rock content,the peak shear strength of the soil-rock mixture rises first and then decreases and peaks when the rock content is at 45%.When the rock content remains constant,as the number of freeze-thaw cycles rises,the shear strength of the sample reaches its peak after three freeze-thaw cycles.Studies have shown that with an increase in freeze-thaw cycles,the medium and large pores develop rapidly,especially for pores with a size of 0.2–20μm.Freeze-thaw cycling affects the internal pores of the soil-rock mixture by altering its skeleton and,therefore,impacts its macro-mechanical characteristics.