Multiple long-term constant rate of strain (CRS) tests were performed on Zegveld peat from the Netherlands. The aim of performing these tests is to check the validity of the Isotach Framework. Different isotachs can be visualized by changing the applied displacement rate during CRS testing. The trajectories of these isotachs are determined to conclude on both the level of parallelism as well as the mutual distance between different isotachs. The results show a high degree of parallelism. The mutual distance between the different isotachs remains largely unchanged. Transient behaviour is observed around a change in strain rate.

The obtained results show a decent decent of parallelism within the studied strain rat ereegime. The mutual distance between the different isotachs remains largely unchanged within this strain rate regime. However, the results show that the trajectory of the isotachs corresponding to the lowest strain rate seem to diverge from those at higher strain rates. In practice, diverging isotachs result in stress-dependency of creep. Furthermore, it is concluded that the mutual distance between different isotachs increases with a decrease in strain rate. Additional CRS tests underline this observation. This increase in distance with lower strain rates in practice results in non-constant creep behaviour on logarithmic time scale, giving rise to tertiary creep. The observations made are important since field strain rates are shown to be a few orders of magnitude lower than those applied in conventional CRS tests.

The performed CRS tests showed transient behaviour around a change in strain rate. The time needed for the soil to fully adjust to the new strain rate increases with decreasing strain rate. This could give rise to invalid parameter determination since the soil’s state has not yet moved to the isotach corresponding to the new strain rate. The obtained results of the step-changed CRS tests are simulated using the NEN-Bjerrum Isotach model, the abc-isotach model, the Soft Soil Creep model and the MIT Elasto-Viscoplastic model of Yuan and Whittle. Overall, a satisfactory fit is found between the models and the actual CRS test data. The MIT Y&W EVP model and the Soft Soil Creep model are capable of accurately simulating the observed transient behaviour around a change in strain rate.

The early 20th century marked a period of significant infrastructural development in Europe, resulting in the construction of numerous bridges and structures that have lasted to the present day. Many of these foundations, often designed as piled rafts but analysed as pile groups, were constructed using simplistic methodologies. These methods neglected the interactions between the raft, soil, and piles. Despite advancements in geotechnical engineering, current standards like Eurocode 7 still rely on this outdated approach, potentially leading to unnecessarily conservative and expensive retrofitting solutions.

Recent research highlights the significant contribution of rafts to the overall bearing capacity of foundations, suggesting that current design standards may underestimate the capacity provided by rafts. This thesis investigates the behaviour of piled raft foundations compared to pile groups and rafts, utilizing both 3D finite element analysis with the SANISAND-MS constitutive model and centrifuge testing. Furthermore, it explores the feasibility of using a viscoelastic model to capture the cyclic behaviour of piled raft foundations. The study focuses on dry sand conditions and quasi-static cyclic loading to address questions regarding load-displacement behaviour, load and moment distribution, internal forces on piles, and the modelling of cyclic loading conditions.

The findings demonstrate the superior performance of piled rafts in terms of stiffness and resistance compared to pile groups and rafts. Specifically, the vertical resistance ratio (β_PR,V ) and combined resistance ratio (ξ_PR,V ) indicate that accounting for the raft nearly doubles the vertical resistance of the piled raft foundation and aligns closely with the combined resistance of the pile group and raft as separate foundations. In lateral loading, the horizontal resistance ratio (β_PR,H) and moment resistance ratio (β_PR,M ) suggest that the piled raft requires more horizontal load and moment to achieve the same displacement and rotation compared to the pile group alone. The raft contributes approximately 45% of the vertical load and 50% of the moment, reducing the axial load on individual piles by around 40%.

Analysis of internal forces reveals that shaft resistance is relatively small. Larger moments are observed in edge piles under axial loading, indicating the deformability of the raft and suggesting a likelihood of failure in the edge piles due to moments. Under lateral loading, the tension piles quickly reach maximum tensile resistance, with moments continuing to increase, highlighting a high probability of failure in the edge piles due to moments.

The study also demonstrates that modelling the elastoplastic cyclic behaviour of piled rafts using a viscoelastic model with frequency-independent viscosity is feasible. While not fully capturing all aspects of foundation behaviour, this approach adequately replicates stiffness and damping characteristics, making it useful for seismic design using a pseudostatic approach.

These findings offer potential for more efficient foundation designs, improved retrofitting strategies for existing structures, and increased understanding of the foundation behaviour of piled raft systems.

Slow-moving landslides are natural phenomena that shape mountainous landscapes over long time scales. These landslides creep at rates ranging from meters to millimeters per year, often unnoticed by people who build houses on them, unaware of the risks. Although slow-moving landslides rarely cause fatalities, they can be triggered into fast-moving landslides by earthquakes, leading to catastrophic outcomes. Therefore, understanding and monitoring these landslides is crucial to mitigate potential hazards, particularly in seismically active regions.

Traditional methods for identifying and monitoring slow-moving landslides rely on fieldwork and in-situ instruments. While these methods are accurate, they are limited in spatial and temporal coverage. Recent advancements in remote sensing, particularly Interferometric Synthetic Aperture Radar (InSAR), offer extensive, high-resolution monitoring capabilities, enabling detailed observations of ground movements. InSAR studies have used Line Of Sight (LOS) velocities to monitor the be- havior of slow-moving landslides. However, these findings do not provide details on the direction of movement, as LOS velocities are always measured along a single dimension. This limitation makes it impossible to determine the direction and type of movement, which are essential for accurate risk as- sessment and developing early warning systems by identifying deformation patterns that may indicate imminent failure.

This thesis aims to investigate the temporal evolution of a slow-moving landslide subjected to an earthquake, focusing on changes in rate and direction, to identify the type of movements. To achieve this, the Multi-Temporal InSAR (MT-InSAR) technique is used, resulting in Persistent Scatterers (PSs) as measuring points to observe changes in the rate and direction of landslide movements post- earthquake. Sentinel-1 data from both ascending and descending orbital configurations were used to obtain LOS velocity time series at the PSs both before and after the earthquake. By combining LOS velocities from both PS orbital configurations, the method decomposed LOS velocities into vertical and horizontal components, providing both changes in velocities and direction, which together enabled the identification of the type of movement.

To test this method, a slow-moving landslide located in Büyükçekmece, Istanbul, was chosen as a case study. This landslide was impacted by a Mw 5.7 earthquake on 26 September 2019. Within the landslide, an urbanized area was selected to analyze both the uniform and spatially variable re- sponses. The uniform results involved averaging the velocities at the PSs, while the individual PSs were analyzed to identify spatially variable movement patterns.

The uniform results indicated three main phases in landslide behavior after the earthquake: an initial translational movement following the earthquake, a roto-translational phase combining rotation and translation, and a new constant state characterized by horizontal spreading. At an individual point level, the results showed that most points transitioned from moving vertically downward to upward, indicating a significant change in underlying mechanisms, highlighting the landslide’s complexity.

The monitored behavior resembles that of an inverse creeping landslide, where after the initial disturbance, the landslide decelerates to a new constant state. Based on the collected real data, it is possible to construct hypothetical risk scenarios with behavior curves characterized by continuous acceleration, which would pose an immediate threat of catastrophic failure. Although this earthquake did not cause the Büyükçekmece landslide to fail, and the landslide decelerated to a constant state, it is crucial to continue monitoring as there are signs of instability and conditions may change.

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.