Sediment Initiation And Transportation Of Cohesiveless Soil Under Seepage Flow

Seepage is an important factor leading to earth dam instability and riverbank erosion.Sediment movement under seepage flow has become an important research topic in water conservancy projects and geotechnical engineering.The current research work was mainly carried out in the calculation of the permeability coefficient,the identification of the sediment transport mode,the critical conditions of sediment initiation,and the sediment transport law.Summarizing the existing research results,it is found that the existing theoretical and experimental studies are mostly concerned with macroscopic seepage stability of soil,and there are not many studies on the stability of microscopic particles,and the research on the critical condition and transport law of seepage is mainly focused on one-dimensional seepage effect in the vertical direction,it is necessary to carry out relevant research to improve the existing research results.This article mainly uses the method of theoretical analysis and model test to carry out research work.Using the theoretical method,the calculation method of permeability coefficient based on the improved Hagen-Poiseuille law was deduced.The model test was conducted and the similar test results were collected and verified.Using the theoretical analysis method,the critical hydraulic condition of sediment movement under two-dimensional seepage is derived and verified by the test results.The sediment transportation model under one-dimensional seepage was proposed by using the theoretical analysis method.The calculation method of sediment transport rate was given.The method of model test was used to study the sediment transport under the effect of two-dimensional seepage,and the relationship between sediment transport process and soil deformation was discussed.The main conclusions are as follows:(1)Based on the principle of resistance equivalence,the modified Hagen-Poiseuille law can be used to calculate the permeability coefficient of cohesiveless sands,and the average diameter method is recommended for the diameter of the tube.The modified calculation method is more adaptable than traditional Hagen-Poiseuille law calculation method,and can more accurately express the influence of soil pores on the permeability coefficient;the revised calculation method has higher accuracy than the classical empirical method.At the same time it can be used to calculate the soil permeability coefficient under different temperature conditions.Through the discussion of the parameters,it is found that the calculation method is more sensitive to the soil parameters.Subtle differences in gradation and porosity can lead to larger difference in calculations of permeability coefficients.The calculation formula does not take into account the influence of soil non-uniformity.It is not recommended that the soil with non-uniform coefficient greater than 5 to use this formula.The soil permeability coefficient is also related to the experimental conditions.The unsaturated soil of the experimental soil and the internal stability of the soil can both cause deviations in the results.(2)Based on the force law of a single particle of soil,the calculation method of the critical hydraulic gradient of the soil under two-dimensional seepage is deduced.In the derivation,the influence of the seepage direction and the relative position of the soil particles was fully considered.Qualitative analysis and experimental verification show that the formula proposed in this paper is reasonable,and the accuracy of calculation is obviously better than the calculation method proposed by Terzaghi.By analyzing the parameters such as internal instability,soil grading,and void ratio,it is found that the critical hydraulic gradient for the internal instability soil is smaller.The more uniform the thickness of soil particles is,the smaller the critical hydraulic gradient is;and the larger the void ratio is,the smaller the critical hydraulic gradient is.Based on the calculation formula and the distribution law of exposure degree,the starting probability of soil particles can be calculated.This variable is a very critical parameter in model derivation of seepage and sediment transport.(3)Based on the basic theory of sediment movement,the movement law of sediment particles in the seepage field is deduced from the force law of single-grain sediment.Combining with the movement speed of soil particles,the calculation method for the seepage and sediment transport rate was proposed by deriving the starting and moving probability of soil particles.The cumulative sediment load of soil can be used to derive the change of soil porosity and gradation,and then the calculation method of seepage coefficient after seepage and sediment transport is proposed.The coupling calculation of soil seepage and sediment transport field is realized.The cumulative sediment transport,soil porosity,gradation and seepage field after sediment transport can be obtained by model calculation.Experimental verifications show that the calculation method in this paper is reasonable.Through the analysis of the parameters,the effects of porosity,hydraulic gradient,soil gradation,and the number of movable particles on the sediment transport process were discussed.In the early stage of sediment transport,porosity is the main factor affecting sediment transport,while in the later stage,the number of movable particles is the main factor;the greater the hydraulic gradient,the greater the sediment transport strength of the soil.And if the hydraulic gradient is lower than the critical hydraulic gradient,the amount of sediment discharged is significantly reduced.;when the other conditions are constant,the sand transport rate is smaller when the movable particles are coarser;the smaller the amount of mobile particles is,the smaller the final amount of sediment is,and the faster the sand is discharged in the early stage.(4)Based on two dimensional sand tank test,sediment transport and soil slope deformation under two-dimensional seepage are studied.The experimental results show that the law of soil transport is time-dependent with the time,however,a and b values in the fitting parameters need more experimental data to be determined.The accumulated sediment volume is affected by the head,and the higher the head,the greater the accumulated sand production.For different grades of soil,the law of sand transport is also different.Under the same water head conditions,the internal flow velocity of the coarser particles and less fine-grained soil is larger,but the movable particles are reduced.Therefore,the relationship between the size of sediment discharge is more complicated.Through measuring the soil gradation after seepage and sediment transport,it was found that seepage and sediment transport led to coarsening of the soil.The higher the water head,the higher the coarsening of the soil.The coarsening positions of different graded soils also differ.Seepage and sediment transport lead to inhomogeneities in the internal gradation of the soil,which is related to the porosity and head of the soil.In this paper,the slope of the soil shows different degrees of deformation under seepage.The higher the head,the more obvious the deformation of the soil is.Through the analysis of the landslide soil mass,the deformation position of the soil slope is consistent with the interface of the seepage and erosion,and the damage mechanism of the soil slope is different for different gradation soils.For the soil with more fine particles,the deformation is mainly due to the effect of seepage force,and the deformation mass is a uniform mixture of coarse and fine sand;for the soil in which coarse particles account for more,the deformation is mainly due to seepage and sediment transport.