Plaxis 2D Comparison of Mohr-Coulomb and Soft Soil Material Models

Soft soils such as clays, clayey silts and peats demonstrate a high degree of compressibility compared to other soils. In oedometer testing, normally consolidated clays behave up to ten times softer than normally consolidated sands. The task of this master’s thesis has been to investigate the primary consolidation behaviour of soft clay by comparing the results obtained from finite element analysis computations in Plaxis 2D with analytical calculations and survey measurements. Two different material models were utilized during the finite element computations, comparing the performance of the more advanced constitutive Soft Soil material model against the Mohr-Coulomb material model commonly used today. Two case studies provided by Subsea 7, a global contractor for the offshore oil and gas industry, have been investigated with regards to primary consolidation settlements. The case studies comprise rock berms installed on soft clay in the North Sea. A common feature for both case studies were that after installation of the rock berm, parts of the subsoil experienced virgin compression. Survey measurements were obtained for 240 days of consolidation for case study 1, and 31 days of consolidation for case study 2. Results indicate that performing design with the Mohr-Coulomb material model is inadequate when trying to predict the primary consolidation behaviour of soft clay. The most accurate results, when compared to actual survey measurements, were achieved when computing with the Soft Soil material model in Plaxis 2D. For case study 2, the computational results obtained with the Soft Soil model correlated accurately with actual survey measurements of 5 cm, in contrary to the Mohr-Coulomb model, which only predicted a 1 cm consolidation settlement for the same time period. Accurate results were not obtained for case study 1, most likely due to poor quality of the soil parameter data. However, the Soft Soil model still performed better when compared with the Mohr-Coulomb model and analytical calculations. The result from the Soft Soil computation indicated 6 cm of settlement whereas the survey measurement suggested a settlement in between 8 to 12 cm. The study performed in this master’s thesis conclude that projects including soft clay subjected to virgin compression should benefit from the use of the Soft Soil model during Plaxis consolidation computations rather than with the Mohr-Coulomb model.Validerat; 20130930 (global_studentproject_submitter