Hydraulic Impact of Vegetation on the Stability of Shallow Clay Slopes Nature Based Solutions for Mitigation of Erosion in Göta Älv
Typ
Examensarbete för masterexamen
Program
Infrastructure and environmental engineering (MPIEE), MSc
Publicerad
2022
Författare
Margenberg, Maria
Persson, Axel
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Soil erosion is one of the drivers for landslides in natural slopes. In a climate
where both high intensity rain and heavy wind is increasingly common, soil will
erode more rapidly, especially in waterways. Traditional erosion barriers, such as
rock revels or concrete walls, are not always suitable methods when considering
downstream effects and ecological impact. This has led to an increased interest
in combined or fully nature-based solutions (NBS). These methods incorporate
vegetation to limit soil erosion. The body of the vegetation will reduce the speed of
run-off, roots will reinforce the soil and evapotranspiration induces suction in the
unsaturated zone, which increases stability. This project consists of a case study
and a numerical analysis. The case study investigates four NBS constructed by the
Swedish Geotechnical Institute (SGI) in Göta älv. Site visits, soil sampling and
discussions regarding NBS for erosion mitigation were undertaken. These barriers
attempt to reduce wave energy on riverbanks by different designs, which aims to
allow vegetation to take hold. The study found lack of site data and limited time
scope to be a hindrance in evaluating the NBS design. Leaning heavily on state of
the art literature, suggestions are made to improve the design in future projects.
The numerical analyses focused on the hydraulic effects of vegetation in an eroding
slope by creating a Finite Element model of natural clay slopes with varying levels of
suction and slope angles. This is to compare how failure mechanisms are affected by
suction in the topsoil. An exponentially and a parabolic decaying root distribution
is compared in a factor of safety analysis which is then related to the evolution of
shear bands. The differences in factor of safety were found to be small between the
two root distribution geometries, this may be explained by the high water table and
small root depth. The analysis show a clear diffusion of slope failure mechanisms
as shear bands that are clearly defined without suction are scattered into multiple
potential slip surfaces when negative pore pressures are present in the topsoil. These
shear bands merge into one band that is generally shorter and shallower than for the
bare slope, which has a positive effect on the factor of safety. An increase in factor
of safety between 25% and 45%, depending on the amount of suction induced, for
a steep slope, is observed. The increase in stability attributable to suction is less
prominent at gentle slope angles.
Beskrivning
Ämne/nyckelord
sediment transport , landslides , soil suction , slope stability , nature based solutions , NBS , erosion , vegetation , shear bands