Numerical analysis of staged construction Of an embankment on soft soil

Abstract

In Bangladesh, the rapid increase in population and associated development activities have resulted in the reclamation of coastal zones and the utilization of other low-lying soft clay lands for construction. A problem, which is rather common in the fields of Geotechnical Engineering arises when road embankments of moderate to large heights are to be constructed on very soft soils with low shear strength and high compressibility quickly. Due to this reason, the full height of the embankment cannot be built at a time and the staged construction appears to be necessary.

The principal objective of this study was to predict the excess pore pressure and settlement of embankment over soft ground, treated with vertical drain, through numerical analysis of staged construction. To carryout finite element analysis, numerical modeling software PLAXIS 3D was used. The practical demonstration was demonstrated by validating two case studies; the first one was a trial embankment at the KrishnapatnamUltra Mega Power Project in Nellore, Andhra Pradesh, India and the second one was the Second Bangkok International Airport or Suvarnabhumi Airport, about 30 km from the city of Bangkok, Thailand. After the successful validation of the program, detailed finite element modelling of an embankment resting on soft soil was conducted.

The effects of various other parameters namely the effect of installation of vertical drain, the effect of drain spacing, the effect of permeability of soft soil, the effect of cohesion of soft soil, and the effect of thickness of soft soil layer were also investigated. For without vertical drain condition, the developed pore pressure did not reduce substantially, even after sufficient time of consolidation. The rate of settlement for this case was very slow as there were no accelerated drainage paths for the escape of pore water. However, for with vertical drain condition, the reduction of pore pressure was faster as there was an artificial drainage path created for the escape of water, the rate of the settlement was noticeably fast. As the spacing of vertical drain decreased, the rate of settlement increased slightly, due to the accommodation of many numbers of drains. Moreover, the permeability of soil affected the dissipation of excess pore pressure and hence the settlement significantly. Lowering of the soil permeability leads to a lower rate of dissipation of pore water pressure and a lower rate of settlement while the rate of dissipation of excess pore water pressure increased with the rise of cohesion of soil, but the settlement decreased.The reduction of excess pore pressure and the settlement was higher when the thickness of the soft soil layer was higher.