Structural Evolution And Collapsibility During The Deposition Of Loess-paleosols

Loess is widely distributed in China.Collapsibility is the most important characteristic of loess.Collapsibility of loess is closely related to its special structure and form,and its structure is related to its origin and evolutionary history.In order to explore the relationship between collapsibility and structural evolution of loess,this paper relies on the project of"Research on the mechanism of deep collapsible loess foundation of 4×1000 MW Project Q₂ of Huaneng Zhengning Power Plant Phase II",and takes dense samples of Q₄,Q₃,and Q₂ loess（S₀-L₄）strata at the research site for basic physical property tests and collapsibility tests.Representative samples of each loess and paleosol stratum were selected to test the pore size distribution curve（PSD curve）by mercury intrusion method,and the microstructure images of the sample were extracted by scanning electron microscopy.The structural changes before and after collapse of loess-paleosol profile samples were qualitatively and quantitatively analyzed.The relationship between the degree of collapsibility of loess layers at different times and their structural characteristics was discussed,providing a theoretical basis for improving the reliability of loess collapsibility evaluation.The main research work carried out is as follows:（1）Basic physical property tests were carried out on the loess-paleosol strata at the research site,and it was found that their physical properties changed regularly along the depth.The dry density increased overall with depth,while the porosity ratio decreased overall from shallow to deep.The water content and plastic limit of loess and paleosol were less than 25%,while the plastic limit was between 10%and 25%,and the liquid limit was between 25%and 40%.In addition,there were differences in physical properties between paleosol and loess.The indicators such as dry density,water content,saturation degree,clay content in paleosol were all higher than those in loess layer,while the saturation water content,silt content and sand content in paleosol were all lower than those in loess.（2）Indoor collapsibility tests were carried out on loess-paleosol layers at the research site.It was concluded that the initial collapsibility pressure of soil increased significantly with the increase of depth.The peak collapsible pressure fluctuates widely and increases with the increase of depth on the whole.The maximum collapsibility pressure did not exceed 2500 k Pa.The comparison was made among different collapsibility coefficients corresponding to 200 k Pa,300 k Pa test pressure,self-weight stress and peak pressure respectively.Among them,peak collapsibility coefficient had the largest value on profile,and under standard pressure of 300 k Pa,collapsibility coefficient of Q₃ layer was greater than self-weight collapsibility coefficient.Self-weight collapsibility coefficient of Q₂ layer was greater than standard collapsibility coefficient corresponding to 200k Pa and 300k Pa pressure.At the bottom of L₁ soil layer and S₁ soil layer,there was a remarkable difference between collapsibility coefficients corresponding to two standard collapsibility pressures of 200 k Pa and 300 k Pa respectively;while there was little difference in other soil layers range.Generally,the paleosols have weak collapsibility,but some paleosol layers also show medium to strong collapsibility under high pressure in this collapsibility test.（3）Analyzing the relationship between collapsibility and physical property indexes of strata at research site,it was found that collapsibility coefficient was negatively correlated with dry density,positively correlated with porosity ratio,negatively correlated with water content and saturation degree,negatively correlated with clay content,positively correlated with sand content,while the correlation between the indexes such as liquid limit,plastic limit,plastic index and clay content and loess collapsibility was not obvious.Due to many factors affecting collapsibility coefficient,there is no clear quantitative relationship between collapsibility coefficient and various single factors,but boundary values can be determined for each index to qualitatively determine whether it is the collapsible loess.（4）The pore distribution curves of loess and paleosol layers before and after collapsibility were obtained by mercury intrusion method.It was found that the pore size distribution curves of loess before and after collapsibility showed a bimodal distribution,with one high and one low representing the main dominant pores and sub-dominant pores respectively.The main dominant pores before wetting were in the mesoporous range,while the sub-dominant pores were in the microporous range.After collapsibility,there was little change in the position of sub-dominant pores,and most of the main dominant pores or mesopores were transformed into micropores.The distribution range of dominant pores in paleosols was wide.After collapsibility,the peak value of the curve shifted to varying degrees to the side of smaller particle size,and dominant pores transformed into micropores or small pores.（5）The scanning electron microscope extracted the plane images of the natural sections and rubber injection grinding discs of each layer of loess and paleosols.The particle contact mode and skeleton arrangement mode of the samples were observed before and after collapsibility.It was found that the support contact structure changed into a stable mosaic structure,and the particle connection mode of skeleton changed from point contact to face bonding,mostly flocculent-agglomerate bonding.Collapsibility leads to unstable aggregates to disintegrate,with a large amount of fine particles filling the pores and occupying pore space.Quantitative statistics of pore number and pore area ratio in grinding sheet plane images showed that after collapsibility,the ratio of micro-pore and small-pore area to total pore area increased,while that of mesopore and macropore area decreased.The number of large and mesopores decreased significantly,while that of micro-pores or small-pores increased.