Study On Microcosmic Characteristics And Physical And Mechanical Properties Of Deposited Loess In The Yellow River Delta

The deposited loess in the Yellow River Delta is formed by the sediments originated by soil erosion on the Loess Plateau,which is delivered to the estuary area through the Yellow River.In the Yellow River Delta region,the deposited field is now under different land uses.At present,there is a lack of understanding of the soil structure and physical properties of such deposited loess.In this paper,the deposited loess in the Yellow River Delta was collected,The quantitative and qualitative research on the microcosmic characteristics of deposited loess by using x-ray diffraction method,mercury intrusion method and scanning electron microscope.Soil mechanics tests are used to determine the physical and mechanical properties of the soil,Under different land-use types(tidal flat,grassland and farmland)and at different soil depths(0-20 cm,20-40 cm,and 40-60 cm),to analyze the differences and changes of microcosmic characteristics and physical and mechanical properties and the influence of micro characteristics on macro physical and mechanical properties.This will supports the land use planning and development of the Yellow River Delta,vegetation and crop management,and improvement of saline-salinized loess.The experimental results show that different land-use types change the microcosmic characteristics of the deposited loess at different soil layers.In the Yellow River Delta,the primary minerals of the loess are mainly quartz and feldspar,and the secondary minerals are mainly calcite,illite,and kaolinite.The volume,average pore diameter,and median pore diameter of the deposited loess under different land-use types decrease with soil depth.With the increase in the soil depth,the tidal flat sediment loess pores change from small pores to micro pores,those of the grassland deposited loess change from medium and small pores to micro pores,and those of the farmland deposited loess changed from medium pores to small and micro pores.The microstructure unit of loess is large round and block-shaped particles.The contact method is face-to-face mosaic.The particles are arranged very well dense.The pores are long gaps and arranged closely.The microstructure unit of grassland soil is large.The particles and detrital particles are mainly,with increasing depth,the inter-particle pores are filled with detrital particles,and the point-to-point contact and point-to-surface contact become surface-to-surface contact of soil particles,and the mosaic pores become inter-particle pores;agricultural land deposition Loess granular units are pile-shaped and rounded.The pores in the 0-20 cm soil layer are overhead pores that penetrate through the large pores.As the soil layer deepens,the denser the soil,the overhead pores change pore mosaic,point-point contact into surface-surface contact.Land-use type changes the physical and mechanical properties of deposited loess.The particles of deposited loess of three land-use types in the Yellow River Delta are mainly silt and fine sand.The particle size distribution is concentrated in a narrow band.The soil bulk density is large and increases with the soil depth.The porosity is small and varies with the soil layer.The overall porosity from large to small is in the order as: farmland,grassland,tidal flat land.Land-use type can change the physical characteristics of deposited loess.Deposited loess has extremely poor water conductivity and strong spatial variability.The vertical and horizontal saturated hydraulic conductivity of each soil layer of tidal flat deposited loess is in the range of 0.03~2.42 mm/h,the grassland is in the range of 0.15~10.90 mm/h,and the farmland is in the range of 1.02~15.88 mm/h,all of which decrease with the soil depth.The infiltration capacity is in the order as: farmland,grassland,tidal flat,and land-use type changes the hydraulic characteristics of deposited loess.Under each land-use type,the shear strength of the deposited loess increases with the soil depth,and the cohesive force changes in the same way,all tend to increase with soil depth,but the differences between the soil layers are not large;the internal friction angle is larger overall From the largest to the smallest,they are in the order as: tidal flat,grassland,and farmland.There is no significant change trend between the soil layers of the same land-use type.The land-use type changes the mechanical properties of the deposited loess.The microcosmic characteristics of deposited loess affect its hydraulic and mechanical properties.Among them,the vertical and horizontal saturated hydraulic conductivity are negatively and linear correlated with the micro pores content(p<0.01);the horizontal saturated hydraulic conductivity are positively and linearly correlated with the average pore diameter(p<0.01).The cohesion force is negatively and exponentially related with the average pore diameter(p<0.01),and is linearly and positively correlated with the clay particle content(p<0.05).The internal friction angle is linearly and negatively correlated with the clay content(p<0.01).The results from this study can supply scientific supports to farmland cultivation and soil water and salt management.