Analysis Of Large-strain Rheological Consolidation Of Ground With Sand Drains

In recent years,an ocean of sea reclamation projects have been carried out,facing the treatment of soft clay ground with a large void ratio,and the most economical and efficient method is vertical drain preloading.Laboratory tests and field observations show that the predicted values based on traditional vertical drain consolidation theory often differ from the actual observed results.This is partly because the former are mostly based on the assumption of small deformation.Soft clay is often highly compressible,and its actual deformation is relatively large,which often exceeds the category of small-strain.Besides,many factors such as geometric nonlinearity,material nonlinearity,and nonlinear flow can also affect the soil consolidation characteristics.Firstly,the rheological consolidation characteristics and resilience rules of the soil were investigated by repeatedly loading and unloading one-dimensional consolidation tests on the saturated remolded samples with large void ratio.Then,Gibson’s large-strain consolidation theory and Barron’s free-strain consolidation theory were combined,and the elastic viscoplasticity of soil deformation,self-weight stress of soil,and non-Darcian seepage were taken into account to rederive a more comprehensive large-strain consolidation equation for vertical drains system.Finally,the finite difference method was employed to deal with the highly nonlinear equations,numerical calculations were performed using Fortran programs and the influence of consolidation model parameters on the consolidation properties of soft clay was initially investigated.The principal works and conclusions of this paper are as follows:1.The repeatedly loading and unloading one-dimensional consolidation tests on the saturated remolded samples with large void ratio show that saturated clay has significant plastic and rheological properties,their rheological laws are influenced by stress history,and resilience laws are related to unloading ratio.The large-strain consolidation model for sand drains ground proposed in this paper is degraded into a one-dimensional unidirectional consolidation model,and the rheological consolidation tests processes are simulated to verify the applicability of the unified hardening(UH)constitutive model considering the time effect.2.The unified hardening(UH)model considering time effect,the non-Darcian flow model described by non-Newtonian exponential,and the classical variable permeability relation equation e-lgk are introduced to establish the large-strain consolidation model for vertical drains considering self-weight stress under free strain condition,and the implicit finite difference method is used for the numerical solution.Parametric analysis shows that increasing the coefficient of secondary consolidation,compression index,initial overconsolidated parameter,non-Newtonian index,and decreasing the permeability index will retard the consolidation rate of sand drains ground,while non-linear seepage does not affect the final settlement.The settlement calculated considering the real distribution of self-weight stress is larger than that assuming uniform distribution,but smaller than that when the effect of self-weight is ignored.3.Based on the above,the effect of the actual variable loading forms in engineering is considered.The data analysis shows that under the linear loading mode,the larger the final load value and the shorter the loading period,the faster the ground consolidation,but the final settlement is not related to the period;while under the cyclic loading,the pore pressure dissipation and ground settlement show periodic oscillation changes,and when the action time is long,the settlement may be larger than the general linear loading.4.A variable well resistance calculation model with exponential decay of the vertical drain permeability coefficient with time and the change of permeability coefficient in the radial direction are introduced to investigate the influence of well resistance and smear effect under non-ideal well consolidation conditions.Parametric analysis shows that when the vertical drain permeability coefficient is small,the drainage performance is hindered,then,the pore pressure dissipation and settlement process of the ground become slower,and the effect of variable well resistance on consolidation properties is more obvious at this time;the larger the smear ratio,the extent of the smear zone,and the extent of the sand drain influence zone,the slower the rate of consolidation and deformation of the ground.5.Due to the rheological properties of the soil,the pore pressure in the previous cases increases abnormally at the beginning of loading,which is closely related to the magnitude of the coefficient of secondary consolidation and initial overconsolidated parameter,and is more pronounced when the load level is small.Also,for the consolidation of the deep soil layer and large load,the settlement calculated by the theory of large-strain consolidation of sand drains is larger than the result of small-strain consolidation.