Numerical Analysis On Secondary Consolidation Of Soft Ground Improved By Prefabricated Vertical Drains

When a soil layer is subjected to a compressive stress,such as during the construction of a structure,it will exhibit a certain amount of compression.This compression is attributed to the following facts:(i)deformation of soil particles,(ii)expulsion of water or air from the void spaces and(ⅲ)relocation of soil particles.When constructing large walls and embankments over soft thick,high compressible,low permeable soils,there is generally a great deal of time associated with consolidation settlement of those soils.Therefore,it is very important to stabilize the existing soft ground before construction and reduce the settlement during the construction period and the post-construction settlement.For soft soil layer,it has been widely recognized that the secondary consolidation cannot be ignored,because the secondary settlement may sometimes dominate the total settlement.Hence,the objective of this study is to numerically analyze the consolidation settlement of soft ground improved by Prefabricated Vertical Drains(PVDs),with the secondary consolidation settlement being considered.To study the secondary consolidation of soft soil improved by PVDs,a series of two-dimensional and three-dimensional finite element analyses using PLAXIS 2D and PLAXIS 3D are performed to investigate the settlement behavior of the embankment constructed on soft ground improved by PVDs.Soft soil model and soft soil creep model are employed for the numerical analysis to simulate the settlement behavior of the embankment underlying soft ground.The following conclusions or observations can be achieved:(1)It is known that PVDs can accelerate the consolidation process.Hence,the spacing effects of PVDs on consolidation are studied,and it is found that the PVDs play an essential role in the primary consolidation by quickly dissipating the excess pore water pressure,but they may not contribute a lot to secondary consolidation.(2)The smear effect was considered in numerical simulations of consolidation,and slightly decreases in the predicted settlement based on the soft soil model or soft soil creep model were observed.Apparently,the slightly reduced settlements attained from numerical simulations were attributed to the decrease of permeability in smear zone caused by the installation of PVDs.(3)Based on numerical analysis,it is found that in the very early stage of consolidation,the soft soil model may lead to a good agreement,but after that,it may underestimate the creep compared to the soft soil creep model.By referring to the field data,it is seen that better settlement prediction results can be obtained by using the soft soil creep model,since the soft soil creep model can better predict the secondary consolidation.(4)For a period of 2 years and 4 months of consolidation,numerical results were compared to the field data.Numerical results disclose that better predictions can be attained by using PLAXIS 3D,compared to PLAXIS 2D.Hence,it is recommended to use PLAXIS 3D for predicting the settlement behavior of embankment.