Estimation of settlements under stacks of stored sawdust at Bio Energy Luleå by the finite element method : Numerical Analysis with PLAXIS 2D

This master thesis focus on predicting settlement with preloading with and without vertical drains with respect to time by using finite element method with PLAXIS 2D. Constructions on soft deposits with high load in term of storing materials are often causing stability and settlements problems because these soft deposits have high compressibility and low shear strength. Preloading is one of the most economical method to stabilize the soft soil and to ensure it does not cause any problem that can pose a danger. Due to preloading the deformation within soil like clay increases because the preloading produces excess pore pressures within the soil which cause the water in the soil to flow toward the surface, until an equilibrium state of stress is reached where excess pore pressures become zero. To estimate and predict the settlements under stacks of stored sawdust at Bio Energy Luleå, we have made analyses with and without vertical drains on PLAXIS 2D. Model layers have been defined with the help of geotechnical investigation. Vertical drains have been used to accelerate the rate of consolidation. The model has been analyzed with preloading to reduce the post-construction settlements under the future load due to the stored sawdust. The preloading has been used with the help of sand embankment on the ground. The results of analyzed the model with PLAXIS 2D program showed that the excess pore water pressures were significantly higher and dissipated slowly for the case without vertical drains compare to the case with vertical drains. The reason is that vertical drains effectively decrease the excess pore pressure and help to accelerate its dissipation during the consolidation period.Also, the rate of settlement during the consolidation with vertical drains is higher than without vertical drains which has been used in the impermeable soil layers due to the low hydraulic conductivity. In both cases with and without vertical drains with preloading, the vertical deformation at the center of an embankment has been observed to be 24cm and 26cm respectively and the total deformation is 39cm and 37cm. These values are close to the field measurements. The overall duration has been reduced around two months with vertical drains because of increased rate of consolidation and the dissipation of excess pore water pressures with applied external load.The results from the PLAXIS 2D program shows to be fairly realistic with compare to the field measurements. On the basis of analysis, it can be concluded that preloading without vertical drains will be an economical method to prevent the post-construction settlements at Bio Energy Luleå. If the time will be highly constraint, vertical drains with preloading can be a feasible method.Validerat; 20130102 (global_studentproject_submitter