Study On DEM-based Numerical Methods For Seepage-induced Fine Particle Migration In Gap-graded Soils

Gap-graded soils are often encountered in natural soils and earthen structures,such as sandy gravel alluvium,filter-base soil systems in embankment dams and gravel packing for sand control in oil&gas production.The gap-graded soils are susceptive to the migration of fine particles through the coarse skeleton under the action of seepage flow.The seepage-induced fine particle migration will lead to the erosion of fine particles from the coarse skeleton,the formation of preferential seepage channels in gap-graded soils and the reduction in soil strength,which may cause severe engineering disasters,such as piping failure of embankment foundations,erosion of impervious soils and failure of gravel packing.The mechanism of seepage-induced fine particle migration in gap-graded soils involves the particle-particle interactions at the particle-scale and particle-fluid coupling at the pore-scale.The above microscopic mechanism can be hardly understood using the laboratory tests and numerical simulations at the macro-scale.In the Discrete Element Method(DEM),the mechanical behaviour of soils is modelled at the particle-scale.The DEM-based particle-fluid coupling methods provide promising tools for the investigation of the microscopic mechanism of seepage-induced fine particle migration in gap-graded soils.The determination of microscopic parameters of soil particles is a crucial issue for the application of DEM in geotechnical engineering.However,the coupled effects of microscopic parameters are always under-estimated in the existing methods for the calibration of microscopic parameters in DEM.It results in the needs of repeated trial-and-error in the calibration procedure and,therefore,makes the calibration procedure time-consuming and the results have a relatively low accuracy.On the other hand,the present DEM-based particle-fluid coupling methods cannot reconcile the resolved scale of the pore fluid flow and the computational burden.This limits the application of these methods to the modelling of fine particle migration.In this study,the coupled effects of microscopic parameters are considered in the calibration of microscopic parameters based on laboratory triaxial tests and DEM simulations.Using the DEM and Computational Fluid Dynamics(CFD),novel CFD-DEM coupling methods are proposed to reconcile the resolved scale of the pore fluid flow and the computational burden in the modelling of seepage-induced fine particle migration.Then,the proposed calibration method and the novel CFD-DEM coupling methods are applied to the modelling of piping erosion and gravel packing.Based on the above works,the following conclusions can be drawn:(1)A calibration method,in which the coupled effects of microscopic parameters are considered,is proposed for the determination of microscopic parameters of sand particles in DEM simulations using Hertz-Mindlin contact model with a rolling model.The proposed calibration method has the advantage that repeated trial-and-error,which is inevitable in previous calibration method,is avoided during the calibration procedure.The microscopic parameters of Fujian quartz sand are determined using the proposed calibration method.(2)A novel DEM-based particle-fluid coupling method,i.e.,semi-resolved CFD-DEM method,is developed to reconcile the resolved scale of the pore fluid flow and the computational burden.In the semi-resolved CFD-DEM method,the pore fluid flow through the coarse skeleton is resolved using the Fictitious Domain method;while the coupling of fine particles and pore fluid flow is treated at the scale of locally averaging based on the Locally Averaging theory.The semi-resolved CFD-DEM method are applicable to the modelling of seepage-induced fine particle migration in gap-graded soils with a coarse-fine particle size ratio larger than ten.(3)Through the modelling of piping erosion of a gap-graded soil packed in a cylinder,it is demonstrated that the semi-resolved CFD-DEM method has the capability in reconciling the resolved scale of the pore fluid flow and the computational burden in the simulation of seepage-induced fine particle migration in gap-graded soils,which is impossible in either the fully-resolved or the un-resolved CFD-DEM method.With the aid of the semi-resolved CFD-DEM method,the development of the pore fluid flow through the coarse skeleton is understood at the pore-scale during the piping in the gap-graded soil packed in a cylinder.That is,preferential flow channels are formed only in the upper part of the soil column that adj acent to the lateral wall of the cylinder at the local piping stage.At the violent piping stage,preferential flow channels become coalescent throughout the soil column and extends to the interior of the soil column.(4)A framework for the combination of an arbitrary weighting function with the semi-resolved CFD-DEM method is proposed to eliminate the limitation on mesh refinement and the limitation on coarse-fine particle size ratio(i.e.,the size ration is needed to be larger than ten)in the semi-resolved CFD-DEM method.An arbitrary-resolved CFD-DEM method is developed through combining the semi-resolved CFD-DEM method with the Gaussian-based weighting function with the aid of the proposed framework.Through the modelling of migration of fine glass beads through a coarse skeleton and relevant laboratory tests,the arbitrary-resolved CFD-DEM method is confirmed to have the capability in eliminating the limitation on mesh refinement.Based on the simulations of filter-based soil systems with coarse-fine particle size ratios smaller than ten and relevant laboratory tests,the arbitrary-resolved CFD-DEM method is demonstrated to be suitable for the modelling of seepage-induced fine particle migration in gap-graded soils with relatively small coarse-fine particle size ratios.(5)Gravel packing for sand control in oil&gas production is simulated through using the arbitrary-resolved CFD-DEM method.In the numerical simulations,the concentration of the produced sand in the outflow increases exponentially while the permeability of the gravel packing firstly increases and then decreases as the increasing of the Gravel-Sand size Ratio.The above results are consistent with the experimental observations in the previous researches.The particle-scale phenomenon of the blockage of reservoir sand in the gravel packing is investigated using the numerical model.The results show that,the blockage intensity of sand particles in the gravel packing has an overall decreasing trend while relatively dense and relatively loose distributions of sand particles appear alternately as the increasing of the distance of blocked sand particles from the Gravel-Sand Interface.The pore-scale mechanism of the contamination of the gravel packing permeability induced by the blockage of sand particles is understood with the aid of the numerical model.That is,As the blockage of sand particles in the pores of the gravel packing,the local velocity of fluid flow in the pores decreases while the local pressure and pressure gradient increase,which indicates the contamination of the gravel packing permeability.The influences of the blockage of sand particles on the local velocity and pressure fields increase,which leads to a more serious contamination of the gravel packing permeability,as the distance of the blocked pores from the Gravel-Sand Interface decreases.