The Effect of Matric Suction on Total Stability Evaluations in Clay

Abstract: In this master´s dissertation I investigated if it is possible to increase the total stability for excavations in clay, if suction is taken into account during the calculations. The work has been performed at, and in cooperation with, Sweco Civil AB as well as the Dept. of Construction Sciences at Faculty of Engineering LTH, Lund University. The objective of this master´s dissertation is to assess if it is reasonable to evaluate suction using the finite element method, if it is possible to use this to obtain a higher value of the total stability and if it is reasonable doing so. In order to do so, this work has been divided into three parts. • Two idealized excavations, one sloped and one excavation pit with a reinforced retaining structure was investigated. The objective was to evaluate how suction varies with changes in parameters as, precipitation, evapotranspiration, location of the phreatic level and the appearance of the soil water retention curve. • These two cases are then compared to evaluate how boundary conditions affect the factor of safety for the two excavation pits. • After evaluating the results from the idealized cases, these are applied to an actual case, the excavation for the Target building at the ESS-site. The goal is to see if similar results are obtained for a “real life” case. In order to evaluate this, numerical calculations have been performed in Plaxis 2D. It is based on the finite element method and one of the most commonly used software for geotechnical engineering. To be able to evaluate the effect of suction, every calculation phase where suction was allowed has been compared to an identical phase where the suction effect was ignored. The results clearly indicate that a higher value of the factor of safety is obtained in all but one of the cases when suction is taken into account. Both of the idealized excavation pits show the same pattern: if a higher value of the factor of safety is obtained for one case a higher value is also obtained for the other case. However, an evident difference is visualized; the amplitude for the sloped case is much larger, which means that the factor of safety fluctuates more than the factor of safety for the sheet-piled excavation pit. In addition it is evident that the level of the phreatic surface has a large impact on the total stability, as it increases when the groundwater level is located further from the ground surface.