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Quantification of vegetation effects on the hydro-mechanical properties of an unsaturated soil prone to rainfall-induced shallow landslides in Ryo, Western Norway

Rainfall-induced shallow landslides are amongst the most common gravitational mass movements on slopes in steep terrain (Masi et al., 2021). Vegetation can contribute to slope stability through hydrological reinforcement in the unsaturated domain and mechanical reinforcement via root cohesion, improving resilience against shallow landslide triggering.

Published 06.05.2024

A hydrological monitoring program is installed in a mixed deciduous forest and a hayfield at the Ryo case study site in Western Norway. A site-specific characterization of the mechanical properties of roots and rooted soils is also conducted, consisting of root tensile tests in the lab and field pull-out tests. The data obtained from field monitoring and laboratory testing will be used to feed the model SlideforMap, a probabilistic finite slope approach for modeling shallow landslide probability in forested areas (van Zadelhoff et al., 2022).

The assignment

The primary purpose of this research is to quantify:

  • the additional root cohesion on soil shear strength, to be used as input parameters for the probabilistic modeling of shallow landslide triggering in the Ryo case study site
  • the field hydraulic behavior of rooted soils and possibly the soil water retention curve (SWRC), with hysteresis and wetting-drying cycles, from volumetric water content (VWC) and soil suction monitored at different depths in the vegetated area

Potential MSc research project should include at least one of the following tasks:

Objective Task
Determination of the root tensile strength power law as a function of the root diameter for two different tree species and comparison with values from existing literature. Interpretation of both laboratory root tensile tests and field pull-out tests. The task will include:
-    Data collection;
-    Data analysis;
-    Review of existing relevant literature;
-    Determination of power law function
     and comparison with existing literature;
-    Determination of additional root cohesion
     to use as input for numerical models.
Determination of the field soil water retention curve (SWRC) of unsaturated soil vegetated respectively with grass (hayfield) and with trees (forest field) The task will include:
-   Data collection of VWC and soil suction
     values monitored at two different depths;
-   Extraction of wetting and drying cycles
     from the monitored data, grouping of
     the cycles, determination of hysteresis
     curves and trends of the SWRC;
-   Determination of best-fitting curves for
     the monitored data and comparison
     with relevant literature studies.
Modeling the effects of climate change and change of forest cover on the probability of triggering shallow landslide in the Ryo case study Tasks will include:
-   Definition of climate change scenarios
     and forest cover/land-use change
     scenarios for the modeling;
-   Collection of input parameters for
     the SlideforMap model for different
     climate change scenarios and
     forest cover scenarios;
-   Modelling of the different scenarios
-   Interpretation of the results

The proposed research project is desk‐based. In-person presence at the NGI office in Oslo (Sandakerveien, 140) is desired, but remote work for short periods can be discussed.

During this project, the student will learn to:

  • Determine conventional soil geotechnical parameters, accounting for the effects of vegetation on hydro-mechanical properties of partially saturated soils.
  • Interpret root tensile and pull-out tests to quantify the additional root cohesion to soil shear strength.
  • Understand the behavior of partially saturated soils vegetated with tree roots and grassroots via VWC and soil suction change due to seasonal changes and climate drivers (e.g., rainfall, temperature, relative humidity, etc.)
  • Interpret field soil water retention curves, including the presence of hysteresis and wetting-drying cycles.
  • Use the software SlideforMap to model shallow landslide probability in a forested area.

Background

The student should have a general engineering background, an understanding of soil mechanics, ideally unsaturated soil mechanics, and a curiosity to study composite soils like rooted soils. The student will have support to develop expertise in the following areas:

  • Mathematics, statistics, laboratory testing, field testing, and field monitoring
  • Interpretation of soil hydro-mechanical properties of unsaturated soils
  • Data analysis using computational tools like Python and Excel.

NGI supervisor(s):

  • Vittoria Capobianco
  • Amanda Johansen DiBiagio 

References

  • Masi, E. B., Segoni, S., & Tofani, V. (2021). Root Reinforcement in Slope Stability Models: A Review. Geosciences, 11(5), 212. https://doi.org/10.3390/geosciences11050212
  • van Zadelhoff, F. B., Albaba, A., Cohen, D., Phillips, C., Schaefli, B., Dorren, L., & Schwarz, M. (2022). Introducing SlideforMAP: a probabilistic finite slope approach for modeling shallow-landslide probability in forested situations. Natural Hazards and Earth System Sciences, 22(8), 2611–2635. https://doi.org/10.5194/nhess-22-2611-2022