Riparian ecosystems are crucial for maintaining ecological balance in riverine landscapes, offering diverse habitats, regulating water quality, and preventing soil erosion. However, these ecosystems are vulnerable to slope instability, leading to detrimental effects such as land loss, habitat destruction, and increased sedimentation in water bodies. In the Netherlands, the banks of waterways are typically protected using various materials, some of which emit significant carbon during production. To meet environmental goals such as the Paris Agreement (2022), there is a need for alternative bank protection structures that utilise natural materials.

Root reinforcement, which refers to the ability of plant roots to enhance soil strength and stability, plays a crucial role in assessing slope stability. The presence of roots influences soil strength through hydrological and mechanical effects. Existing methods for quantifying root reinforcement involve mechanical models or time-consuming in-situ measurements using large equipment. Therefore, the corkscrew extraction method has been developed as a quicker, lighter, and simpler approach to measure shear strength in root-reinforced soil. Previous studies have demonstrated the potential of this method for quantifying root reinforcement in field conditions, providing rapid data collection on shear strength at different depths and steep slopes. Throughout the thesis, a corkscrew set-up, inspired from Meijer et al. (2018) was used to assess root reinforcement in riparian environments. Also, it was determined whether this technique is applicable in riparian conditions.

The corkscrew device consists of a garden corkscrew weeder, a tripod with a ratchet winch, a steel cable, a load cell, and a draw wire sensor. The corkscrew is maunally rotated into the soil, and the load and displacement are measured during extraction. The force-displacement curves are analysed to determine rooted soil parameters.

The measurements were conducted at two locations in the Netherlands: the Botanical Garden of the TU Delft in Delft and a testing site in Middenmeer. The Delft location had fields with reed plants (Phragmites australis) and willow trees (Salix fragilis and Salix purpurea), while the Middenmeer site was planted with hawthorns (Crataegus laevigata). Corkscrew extractions produce force-displacement curves, which exhibit different patterns depending on the root content (root area ratio).

The study finds that the corkscrew method is a promising technique for measuring root reinforcement in challenging terrains like riparian areas. It offers advantages in terms of time efficiency, field applicability, and non-destructiveness compared to complex and destructive methods. However, challenges related to root recovery and the limited testing depth need to be addressed through further research.

The thesis also examined root and strength parameters related to root reinforcement. While root biomass provides information about the quantity of roots, it may not accurately quantify root reinforcement. The root area ratio was found to affect soil behaviour and showed correlations with strength parameters for certain selected species. However, other factors such as moisture content, the soil conditions and root diameter could also influence the relationship between root area ratio and shear strength. The force-displacement graphs obtained from corkscrew measurements highlight the significant influence of roots crossing the shear surface on soil behaviour by comparing the pattern of the curves. Also, root breakages are identified as sudden drops in force displacement graphs.

The presence of roots mobilising at higher displacements than the peak strength of bare soil is crucial for slope stability. The combination of species might provide the best reinforcement effect for stability owing to difference in root paterns spatially and with depth.

Given the desire to construct more structures using sustainable building materials, demand for wood and other bio-based building materials has risen dramatically over the decade. While timber itself is a carbon-neutral material and can sometimes even be carbon-negative, reusing wooden structural members made that have been in service for several years can widen the approach to structural design using wood. In the extensive network of rivers and canal systems that the Netherlands has, the banks are often covered with sheet-pile walls and a majority of these are made of timber. Tropical hardwood, especially Azobé (Lophira alata), due to its high biological resistance to decay, is used to make these sheet-piles. However, when exposed to the groundwater table for a long period of time, the wood undergoes decay due to bacteria destroying the cellulose slowly, while the lignin remains constant, and over decades the large cellulose molecules are replaced by water making the walls weaker.

In this study, the characteristic mechanical properties of Azobé sheet-piles that have been in service for 57 years have been found, so that they can be assigned with an appropriate strength class and reused. Destructive, quasi-destructive and non-destructive tests have been performed on the sheet-pile boards to understand the correlation between them and to also ascertain to what level tests on timber specimens that do not affect their usability can be reliable. Since the knowledge of how visual grading can be performed on used, decayed structural timber, especially hardwood specimens is limited, a methodology has been developed in line with NEN-EN 14081-1:2019 along with the definition of a visual decay score. The results from the RPD tests are quantified in terms of the resistographic measure value to identify whether, in tropical hardwood, is there any effect in the drilling direction and whether this value can qualitatively or quantitatively describe the actual mechanical strength of the sheet-pile boards. The stress-wave tests and the four-point bending test are used to calculate the strength and stiffness of the boards, which determine the characteristic values, that are also based on their wet density (at which the tests are conducted). The results obtained are also analyzed for occurring patterns in terms of the location of the board in the sheet-pile wall (top or bottom), testing configuration (E-side or W-side-up) and variation in thickness within the boards due to decay.

The bending test performed on the boards is modelled numerically in multiple iterations with curved-shell, layered-shell and 3D brick elements also varying the respective material models to find which one of them is best suitable to model bending of timber. The load-sharing mechanism observed when grouping multiple timber specimens has been simulated numerically to predict the characteristic load-sharing factor of the sheet-pile wall system.

A study carried out to understand the effects on including vegetation as an additional reinforcement to improve streambank stability.