Grundvattensänkning vid schakter och byggnadsverk - En jämförelse av olika beräkningsmetoder

Sammanfattning: Many construction projects, entail excavations into water-bearing soils. As a consequence of this a groundwater lowering must be made which can cause major environmental impacts, such as drainage of adjacent rivers or wetlands or subsidence of adjacent buildings. The groundwater that flows into the excavation is also of importance since it determines the volume of water that has to be pumped away during the project. In recent years, Sweco Environment AB has had a need to come up with a strategy that will enable them to more stringently handle projects that deals with shallow excavations into water-bearing soils. The purpose of this thesis is to investigate and compare different calculation methods for inflow and radius of influence when excavating for an underpass. Two cases have been studied, a real case provided by Sweco for an excavation in Lorensborgsgatan and a theoretical case. The excavation for the theoretical case assumed to be located in shallow quaternary deposits. The groundwater flow and the radius of influence was calculated using both analytical and numerical models. Five analytical equations were used to calculate the inflow and one equation was used to calculate the radius of influence. The numerical models were made in SEEP/W and GMS MODFLOW. Sensitivity analyses has been conducted which indicated that the hydraulic conductivity has a major impact on both the results of the analytical and the numerical models. The calculation results from the analytical equations and from the numerical models showed large differences. In the case of Lorensborgsgatan, estimated inflows were between 0,3-6,7 m3/d with a radius of influence varying between 11-110 meters and for the theoretical case, the inflows were estimated between 38-9045 m3/d with a radius of influence varying between 180-1498 meters. As the result shows, the magnitudes for the theoretical case were larger than for Lorensborgsgatan which can be explained by the higher hydraulic conductivity layers present in the theoretical case. A higher order of magnitude increases the need to more accurately determine key input parameters since small differences in parameters leads to large differences in calculation results and thus also a large difference in cost. Since no actual calculated inflows or radiuses were available, no clear method recommendation could be made. However, it appears that GMS works better than SEEP/W in numerical modeling of underpasses, mostly due to the simplicity of the calibration and validation steps in the program, which enabled a higher degree of credibility to the models. Nevertheless, an analytical calculation method should be chosen before a numerical in situations where no further requirements, such as dynamic solutions, needs to be addressed or in cases where the hydraulic parameters indicate less permeable soils. Key words: Groundwater, Modeling, Dewatering, GMS, SEEP/W, MODFLOW, Excavation.