Study On Properties And Genesis Of Sensitive Silt In Middle Reaches Of Fenhe River In Shanxi Province

Sensitive soil is a kind of soil with strong structure,which will change its strength and deformation characteristics obviously after being disturbed.In this paper,the distribution range of sensitive silt was delimited according to the geological survey of the first terrace of Fenhe River and the physical and mechanical properties of the sensitive silt were studied systematically.At the same time,the causes of sensitivity of silt were analyzed by various characterization methods and numerical simulation,and the corresponding methods for improving properties of sensitive silt were hence put forward.The main work and achievements of this paper are as follows:(1)According to the landform,geological structure and hydrogeological conditions of the first terrace of Fenhe River in Shanxi Province,the zoning range of sensitive silt of the first terrace of Fenhe River is determined,and the sensitivity of silt is tested by sampling within the zoning range.The results show that Jinyang Lake area and Qingxu water-rich area are the main distribution areas of sensitive silt in the middle reaches of Fenhe River because of their abundant water supply and good stratigraphic structure,and most of the silt in them is medium and high sensitive soil.(2)The physical properties of sensitive silt are studied from five aspects:basic physical properties,mineral composition,soluble salt,grain size distribution and microstructure.The results show that the plasticity index of air-dried silt is lower than that of natural water content,the interaction between soil particles and water is irreversible;the highest content of clay mineral in the silt is illite,and active amorphous allophane is also found in the silt;the combined pretreatment of boiling and sodium hexametaphosphate solution could achieve the best dispersion effect using the fractal dimension(D_f)based on the fractal model as a quantitative index to evaluate the dispersion effect of different pretreatment methods;the particle frequency distribution curve of silt presents bimodal Gaussian distribution;the pore size of the undisturbed silt sample is mainly small pore with pore size of 1.0-0.1?m,and the pore type is mainly intergranular pore;the arrangement orientation of silt aggregate structure is low,and the micro-structure morphology has the flocculation structure of edge-edge and edge-surface connection,and the macroporous honeycomb structure with arched chain shape.(3)The compressibility and thixotropy of sensitive silt are studied by one-dimensional consolidation test and unconfined compressive strength test,and the mechanical properties of sensitive silt with different disturbance time are analyzed.The results show that the structural yield stress is the key turning point of the compression characteristics of the sensitive silt.When the pressure reaches the structural yield stress,structural strength is destroyed,the settlement of undisturbed soil increases suddenly,and the consolidation coefficient increases suddenly.At the same time,the primary consolidation ratio decreases suddenly,and the contribution of secondary consolidation to settlement begins to increase,while the ratio of the secondary compression coefficient C_?and the compression index C_cunder the corresponding consolidation pressure is basically a constant of 3.1×10^-4.The extension of the compression curves of remolded and undisturbed silts coincides approximately at 0.58e₀.The faster the loading speed is,the larger the settlement is,and the compression curve tends to be more linear so that the characteristic point of silt compression disappears.The existence of structural strength makes the stress-strain curve of sensitive soil show strain softening characteristics similar to that of over-consolidated soil.After thixotropy,the recovery of each index of sensitive soil is time-dependent,the unconfined compressive strength is linear recovery,and the elastic modulus is step-wise recovery.The settlement of sensitive silt after disturbance is proportional to the disturbance time under the same pressure,and the additional settlement caused by disturbance is contributed by both primary consolidation and secondary consolidation.The cumulative strain of sensitive silt under axial cyclic loading can be expressed by empirical formula.(4)Jingle red clay and Dongshan loess were sampled according to the topography and hydrodynamic transport conditions.By means of metallographic microscope scanning,XRD,IR and XRF testing methods,the formation reasons of the sensitivity of silt in Middle reaches of Fenhe River were analyzed from the aspects of geomorphological characteristics,particle surface micromorphology,mineral composition and chemical weathering degree.The results show that the sensitive silt is deposited in the low salt static water environment of the Jinzhong faulted basin by the alluvial-proluvial transportation of the upstream red clay and Dongshan loess,and its sensitivity is lower than that of marine sedimentary soft clay because of the unstable flocculation and honeycomb structure formed by illite and allophane.(5)Based on the test results of sensitive silt,the corresponding PFC2D discrete element model was established for calibration.By changing the mesoscopic parameters of the model,such as average particle size,particle size ratio,intergranular bonding strength and friction coefficient,the relationship between the mesoscopic parameters and the macroscopic strength was established through multi-parameter variance analysis and multiple linear regression analysis of the simulation results.The results show that among the mesoscopic parameters,the intergranular cementation strength has the greatest influence on the macroscopic strength of silt,and strengthening the intergranular cementation strength by cementitious materials can increase the stability of microstructure and hence improve the soil properties.Cement has the best effect when the water content is low,but when the water content is high,adding a small amount of semi-hydrated gypsum or polyurethane or both to replace the same proportion of cement can reduce the cement dosage and improve the strength of cement-soil.