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 Despite advancements in geotechnical engineering, current standards like Eurocode 7 still rely on this outdated approach, potentially leading to unnecessarily conservative and expensive retrofitting solutions.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.

Debris flows are extremely rapid gravity-driven mass movements of saturated sediment in concentrations between 60% to 80% by volume that move along steep channels, eroding and entraining material, typically terminating in a fan-shaped deposit. Debris flows are responsible for causing innumerable deaths and extensive damage across the world. The mobilisation of rainfall-triggered landslides is the primary cause of such flows. Estimating the debris flow travel distance or runout is essential for managing this hazard.

The conventional approach to debris flow runout analysis idealises the triggering of landslides across the source area into an instantaneous event. When this idealisation is done, a key characteristic of debris flows, their tendency to propagate in surges or waves, is overlooked. This study analyses the effect of accounting for surging on runout estimation by spatially and time-resolving debris flow events. A prime candidate for such a study is the Mocoa Debris Flow of 2017, a tragedy that involved 273 shallow landslides mobilising into a debris flow, resulting in the death of more than 300 people and the devastation of local infrastructure, Sarmiento et al. (2019). First, the landslide inventory was analysed to assess the scale of the disaster. Then, a novel method to spatially resolve events based on stream orders is implemented, after which a runout analysis is performed for different spatially resolved scenarios using a depth-averaged numerical model. Next, the timing and distribution of landslides are assessed based on a four-day storm period using a process-based landslide susceptibility model. This assessment determines the relative volume of each debris flow surge. The surges are then incorporated into a time-resolved runout analysis. The results of both the spatially and time-resolved runout analyses are compared. We find a marginal difference in the estimate of runout based on critical performance criteria, such as area coverage ratio, in favour of the spatially resolved analysis.

This study concludes that incorporating the phenomenon of surging caused by non-simultaneous landslide events does not improve the forensic analysis of the Mocoa Debris Flow runout. One of the main limitations of this study is the absence of data measured during the event to confirm the extent of surging due to non-simultaneous landslides. A possible avenue for future research would be varying the period of rainfall that is considered by reevaluating initial groundwater conditions.

With the continued growth of the offshore sector and plans for new wind farm developments, shallow marine environments are becoming key areas for the renewable energy projects. As a result, the likelihood of encountering glauconitic sands is growing, posing challenges to the offshore wind infrastructure. Glauconite, or glauconitic sand, also known as ‘greensand’, refers to a soil containing peloidal sand-sized grains primarily comprised of mineral glauconite, an iron- and potassium-rich 2:1 interlayer-deficient mica. The tendency of glauconite to transform from sand to fine-grained soil with shearing creates uncertainties in site characterization and negatively impacts pile drivability to targeted depths. This thesis examines shearing behavior of glauconitic sands at soil-pile interface using two samples of Belgian glauconitic sands. With the final goal being the derivation of reliable interface shear strength parameters, the study aims to develop test procedures capable of capturing the transition of glauconitic sands from their natural uncrushed state to a reworked degraded condition, simulating the effects of shear-induced particle breakage during pile installation. To achieve this, a systematic experimental program was completed in two phases. The first part focused on glauconitic sands in their natural state and the second aimed to assess the degraded soil. Comprehensive material characterization, including geotechnical index tests, mineralogical analyses, and soil-steel interface direct shear tests under various conditions, was carried out to support each phase. The findings link the interface shear behavior and strength of glauconitic sands to their intrinsic properties, identify limitations in current experimental procedures, and offer recommendations for future research. New insights are provided into the behavior of Belgian glauconitic sands with the relatively low glauconite content and varying crushability potential. Interface roughness, shear rate, and normal stress effects are considered.

Large-diameter monopiles serve as foundations for offshore wind turbines (OWT), and the diameters are now up to 10 meters. These monopiles exhibit lower embedded length-to-diameter (L/D) ratios compared to the conventional monopile that is widely employed in offshore oil and gas platforms. They undergo rotation when subjected to lateral loading like rigid or semi-rigid bodies. This distinct geometry necessitates a different approach to describe the soil reaction that is induced when a lateral load is applied. Previous research introduced a 1D model incorporating four soil spring components to represent four aspects of soil reaction namely lateral soil reaction, distributed moment, base shear force, and base moment. However, questions have arisen regarding the contributions of base components, specifically the base shear force and base moment, to maintaining monopile stability. In response, a series of monotonic loading tests on monopiles in dry sand is conducted to assess the contributions of base shear force and base moment to monopile stability. Following this assessment of base components, parametric analyses are carried out to investigate the effect of pile diameter (D), the L/D ratio, load eccentricity (e), and sand relative density on monopile responses under cyclic and monotonic lateral loading. In this study, the SANISAND-MS material model is employed within 3D Finite Element (FE) software to model ratcheting during cyclic lateral loading. Finally, an investigation is conducted with the aim of constructing a 0D model to represent base shear force and base displacement responses under both monotonic and cyclic lateral loading.