Modell för lerans odränerade skjuvhållfasthet i anslutning till Göta älv

A common issue in stability analyses preformed in connection to Göta River and otherwatercourses is how the undrained shear strength of the clay should be modeled in transitionbetween land and river. This issue is in Sweden relatively unexplored and only a fewguidelines and publications are available regarding the subject. Furthermore, geotechnicalfield investigations are relatively expensive when preformed in the water areas which resultsin a limited surveyed data. The consequence of these aforementioned factors frequentlycreates underestimated, undrained shear strength. This will eventually result in a misleadingoverall safety factor in the analysis.The aim of the thesis has been to suggest a model of how the undrained shear strength shouldbe modeled in the labeled transition zone, based on a large amount of geotechnical surveysand the geological history. Geotechnical investigations have been studied in five sections inthe southern region of the Göta River. These studies include measured; undrained shearstrength, consolidation conditions and basic material properties of the clay. The results ofthese studies show that the shallow sediments in the river bed, has significantly lowerundrained shear strength than the corresponding level at the shore. The clay layer which hasthis lower shear strength corresponds to the actual water depth. In deeper areas of the riverbed the shear strength is similar from land to the river area. The undrained shear strengthseems to have a parable increment in this clay layer with lower shear strength.Several empirical calculations have been performed and compared to the geotechnical surveysto examine how well the undrained shear strength can be estimated, primarily the area of theriver bed with lower strength. It has been shown that the empirical estimation requires a lot ofknowledge, in particular consolidation conditions and stress distribution as a result of loadingand unloading.Furthermore, a simple stress analysis has been performed in Plaxis to illustrate the effectivestress distribution as well as the stress dispersion as a result of loading and unloading in anhomogeneous clay. The result shows in particular that the stress dispersion effects arisingcollaterally at both loading and unloading.A generalized stability analysis was executed to reveal how the different ways to model theundrained shear strength affects overall safety factor. This analysis resulted in a significantlyhigher overall safety factor, when the undrained shear strength was modeled according to themeasured results than how it has been modeled in many previously investigations.It may finally be noted that the uncertainties regarding how the undrained shear strengthshould be modeled in many cases has resulted in a conservative way to model the undrainedshear strength. in which the consequence has been an underestimated overall safety factor.Validerat; 20121203 (global_studentproject_submitter