| The Three Gorges Reservoir(TGR)provides important services such as power generation,flood control,shipping,and water resources regulation.It also has changed the pattern,hydrological rhythm and related surface processes of the Yangtze River.The TGR adopts the operation mode of "storage clear and drain muddy",with the maximum(175 m)and minimum(145 m)during the dry and rainy season,respectively.Consequently,an anti-seasonal fluctuation zone(WLFZ)with a verticle height of 30 m has been formed.In this zone,soil physical,chemical and mechanical properties undergo tremendous changes mainly attributed to the effects of water level rise and fall,coupled with repeated wet-dry alternation,resulting in soil instability and increased soil erosion.Thus,understanding the changes in soil characteristics and its dynamic mechanism in the WLFZ is of the most important for studying the mechanism of soil erosion and developing appropriate soil and water conservation technologies for both soil protection and bank protection.Study on soil structure changes and the response of soil mechanical properties could play an important role in understanding the mechanisms and patterns of the geomorphic process,geochemical process and ecological process in the WLFZ.The study can provide scientific basis for the technical research and development of environmental improvement,soil and water conservation and ecological restoration in the WLFZ.Therefore,the main objective of this study was to analyze soil structure and soil mechanical properties of the purple soils at different water-level elevations as research objects.Soil samples were collected at 150 m,155 m,160 m,165 m,170 m,175 m,and 180 m.The dry sieve method,wet sieve method and Le Bissonnais method were applied to analyze soil aggregate composition.Combined with the wet-dry alternating simulation experiments,the nondestructive detection technology of industrial micro-CT and synchrotron radiation-based micro-CT were used to detect the three-dimensional structure characteristics of soil body and soil aggregates in the WLFZ,and the changes in soil pore characteristics were quantitatively analyzed.The direct shear test was used to evaluate soil shear strength under different wet-dry alternation.Based on the partial least squares regression method,the response relationship of soil shear strength to soil structure was investigated under the wet-dry alternation in the WLFZ.The main innovative results were as follows:(1)Periodical water-level fluctuation in the WLFZ of the TGR had a significant effect on soil aggregate composition and stability.With the decrease of water-level elevations,the content of ?0.25 mm air-dried aggregates and water-stable aggregates decreased by 3.59% and 31.43%,respectively.Among them,the content of ?5 mm airdried aggregates and water-stable aggregates significantly declined by 19.71% and 85.20%,respectively.The content of <0.25 mm air-dried aggregates and water-stable aggregates significantly increased by 289.97% and 402.97%,respectively.Mean weight diameter and geometric mean diameter decreased significantly with the decrease of water-level elevations,and the maximum decreases were 66.43% and 81.35%,respectively.The percentage of aggregate destruction increased with the decrease of the water-level elevations,and showed the maximum increase of 463.06%.The findings suggest that the periodic flooding-exposed environment significantly reduced soil stability.Additionally,the longer the flooding period was,the more the soils become unstable.(2)Periodical water-level fluctuation in the WLFZ of the TGR and wet-dry alternation had a significant effect on pore characteristics of soil body and soil aggregate.Comparing the soils at the non-flooded elevation,soil porosity in the WLFZ decreased by 17.88 %?33.21%.The percentage of ?1000 ?m porosity significantly decreased by 20.66%?44.96%.With the decrease of the water level elevation,the percentage of elongated porosity decreased by 19.60%,while the percentage of regular and irregular porosity increased by 164.96% and 147.98%,respectively.Results of laboratory simulated wet-dry alternation experiments show that soil total porosity increased with the number of wet-dry alternations.After six times of wet-dry alternation,soil total porosity,porosity of ?1000 ?m and the percentage of elongated porosity significantly increased by 288.09%,420.67% and 10.62%,respectively.On contrary,the percentages of regular porosity and irregular porosity decreased by 59.62% and 17.13%,respectively.With the decrease of water level elevation,soil aggregate porosity gradually decreased and the maximum reduction was 56.16%.Comparing soil aggregates at 180 m,the porosity of ?100 ?m aeration pore in the WLFZ declined by 45.33%,while the porosity of <30?m storage pore and 30-100?m capillary pore increased by 57.97% and 114.84%,respectively.With the decrease of water level elevation,the percentage of elongated pores gradually decreased,with a maximum reduction of 21.18%,while the percentage of irregular pores and regular pores in the WLFZ increased,with a maximum increase of 75.49% and 66.43%,respectively.It indiceted that periodical water-level fluctuation and wet-dry alternation induced changes in pore characteristics of soil body and soil aggregate,which inceased with increasing of flooding time and depth.(3)Periodical water-level fluctuation in the WLFZ of the TGR and wet-dry alternation had a significant effect on soil shear strength.The results of this study revealed that the adhesion increased with the decrease of water level elevation,and the maximum increase was 141.61%;while the internal friction angle decreased with the decrease of water level elevation,and the maximum reduction was 48.32%.The results of laboratory simulated dry-wet alternation experiments showed that the adhesion force and internal friction angle significantly decreased with increasing number of wet-dry cycles.After 6 wet-dry cycles,the cohesive force and the internal friction angle decreased by 46.99% and 49.73%,respectively.It showed that the indexes of soil shear strength significantly changed with decreasing of water level elevation.(4)The partial least-squares regression method(PLSR)was applied to develop the regression models of soil shear strength indexes and soil structure indexes.In the cohesive model,soil structure indexes,such as 30-100 ?m porosity,organic carbon,regular porosity,total porosity and elongated porosity played an important role in soil cohesive.Among those indexes,the aeration porosity of 30-100 ?m and total porosity had a major influence on the cohesive force,thus,they were considered as the main structural characteristic factors controlling the cohesive force.In the internal friction model,soil structure indexes,such as organic carbon,total porosity,MWDFW,MWDwet,FDwet,aggregate total porosity,and ?100?m porosity play an important role in the internal friction angle;total porosity and organic carbon,in particular,had the greatest influence on the internal friction angle,and were considered as primary structural factors controlling the internal friction angle. |
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