Sättning och portryck för en provbank på sulfidjord i Gammelgården : Jämförelse mellan FE-simuleringar och uppmätta resultat

In Gammelgården, north-west of Kalix, is a field test area located with a test-embankment on sulphide soil. The embankment is divided in to two parts with the heights 1.5 and 2.0 m. In this master thesis the FE-software PLAXIS 2D has been used to simulate settlements and changes in pore pressure due to the load from the embankment. The simulations have been performed with plane strain and axisymmetric analysis with the material models Soft soil and Soft soil creep. The results have been compared with calculation from the software Embankco version 1.02, calculations by hand and field measurements of settlements and pore pressures.The embankment is equipped with instruments for measuring horizontal- and vertical displacements and pore water pressure. A number of CRS tests and CRS-creep tests have been performed on core samples from different elevations in the middle of the bank area. Input for the simulations is mainly determined from these, but also from routine tests.In the material models Soft soil and Soft soil creep the stiffness of the soil is described using the parameters modified compression index, λ*, and modified swelling index, κ*, which are defined in isotropic triaxial tests. Three different evaluation methods have been used to determine λ* and κ*, with a large spread in the results. The evaluations were performed with approximated relations to the effective pressure and the oedometer modulus, evaluation of the corresponding normalized stiffness parameters, Cc and Cs, and by back calculations of the parameters using PLAXIS SoilTest to match the output CRS-curve with the actual CRS-curve from the laboratory. Simulations have been performed with all three evaluation-methods to determine the most reliable method based on comparison to the measured settlements and the settlements calculated in Embankco. From these simulations the back-calculations from SoilTest seems to be the most suited evaluation method since the settlements are relatively consistent to the measured ones for the first 133 days. Compared to the calculations in Embankco, the PLAXIS simulations are very well consistent for the first 2 years with the SoilTest¬¬-parameters. The other two evaluation methods show significantly smaller respectively larger settlements compared to the measured ones and the ones calculated with Embankco.The simulations with plane strain analysis were performed for 3 different embankment heights. The two part heights 1.5 and 2.0 m and a third height of 1.76 m which correspond to the load and the surface area of the entire embankment. In the axisymmetric model the embankment is simulated as a circular plate with height and radius corresponding to the load and surface area of the entire embankment. The axisymmetric simulations are performed to resemble a case in 3D, since plane strain analysis is based on the assumption that the embankment is endless in the in-plane direction, which is an approximation for a restricted rectangular embankment. After 133 days the measured settlements are 0.21 m and 0.30 m for embankment part 1.5 m respectively 2.0 m. All of the simulations with Soft soil show similar settlements with the SoilTest parameters. The first 2 years the simulations are more or less equivalent to the calculations in Embankco, there after the simulations show an increased rate of the settlements compared to Embankco. All the simulation in PLAXIS show significantly larger pore pressures than the measured and in Embankco calculated ones. The largest differences are shown in the plane strain models, corresponding to about 20 kPa. In the axisymmetric model the pore pressures are about 50 kPa directly after loading, while the measured are about 35 and 43 kPa, for the embankment heights 1.5 m respectively 2.0 m. Two possible explanations for this could be that the stiffness of the dry crust is larger than simulated or that the over consolidation ratio is larger. The assumptions were tested without any significant differences in the results.The results indicate that further investigation of these differences in pore pressures should be performed. The two embankment heights 1.5 m and 2.0 m should be simulated in an axisymmetric model and in 3D, both of the two separate embankment heights as well as the total bank geometry. Continued monitoring of the settlements and comparison to the simulations should be performed.Validerat; 20160607 (global_studentproject_submitter