A comprehensive journey on dutch organic clays

Abstract

In the Netherlands, natural gas extraction from the Groningen field has induced seismic activities, notably recording the highest magnitude of ML 3.6 at Huizinge. Despite the lower magnitudes of these induced earthquakes compared to their natural counterparts, their impact is significantly magnified due to the shallow depths at which they occur and the high site amplification over areas with soft soils. In response, the Dutch Research Council (NWO) launched the DeepNL research programme to conduct thorough scientific research into these phenomena. The SOFTTOP project, part of this initiative, aims to fill the knowledge gaps regarding the effects of soft soil deposits on earthquake dynamics and subsequent soil responses. This thesis, nested within the SOFTTOP project, focuses on the cyclic and dynamic behaviours of soft organic clays, which are prevalent in the Netherlands' deltaic subsurface. It starts with a detailed investigation into the monotonic behaviours of these clays, laying the groundwork for further exploration into their cyclic and rate-dependent responses.
Through an extensive experimental programme, a characteristic organic diatomaceous clay from the Netherlands was tested under various stress paths, uncovering limitations in existing models. To address these, an advanced elastoplastic model, JMC-clay, was developed. Starting from a previous formulation for peats, modifications were made to the yield surface shape and the rotational hardening rule, enhancing the models' ability to predict pre-failure behaviours accurately. This foundational work paves the way for studying cyclic behaviour under ‘slow’ testing conditions, deliberately chosen to minimise the effects of non-uniform pore pressure distribution. The study's exploration into the slow cyclic response of Dutch organic clay has unearthed critical insights into the effects of loading frequency, soil composition, initial stress state, and cyclic stress amplitude on the soil's mechanical properties. Among the findings, the soil's cyclic response significantly depends on the cyclic strain amplitude.
A pivotal aspect of this thesis is extending the JMC-clay model to include cyclic behaviour, incorporating bounding surface plasticity for a more accurate and predictive modelling framework of soil behaviour under cyclic loading conditions. This development, however, brought to light challenges in model validation. Simulations indicated that a larger bounding surface, indicative of a higher apparent over-consolidation ratio (OCR), aligns more closely with experimental observations than anticipated. This suggests that contrary to the expected purely elastoplastic response, creep behaviour plays a significant role during consolidation, necessitating adjustments to the model to capture these observations accurately. The JMC-clay model is extended to include an elastoplastic-viscoplastic bounding surface formulation to capture time-dependent soil response. The strain-rate saturation feature of the coupled elastoplastic-viscoplastic framework requires further investigation with experimental data at a higher loading rate.
However, the existing equipment falls short when conducting ‘fast’ testing, designing and constructing a cyclic-dynamic multi-directional shear apparatus for organic soft soils (CYC-DoSS) in-house aimed to fully cover the earthquake frequency spectrum. The development and application of the device, a state-of-the-art earthquake simulator, marks a significant leap in element testing. Designed to overcome the limitations of traditional testing apparatuses, the CYC-DoSS features digitally controlled servo-hydraulic actuators and in-house developed local response sensors, offering a comprehensive suite of measurement capabilities. The inclusion of advanced measurement techniques, such as P-wave and S-wave bender element measurements, accelerometers, and fibre optic pore pressure transducers, alongside custom-developed sensors for capturing detailed deformation patterns, pore pressure responses, and acceleration data, significantly enhances the ability to measure cyclic responses. This is crucial for accurate seismic risk assessment and mitigation strategies and enables a deeper insight into the dynamic behaviours of soft soils under seismic load.
By weaving together experimental research, advanced constitutive modelling, and the deployment of an innovative testing apparatus, this thesis presents a comprehensive approach to unravelling the behaviour of Dutch soft organic clays under seismic conditions. The contributions of this research extend beyond theoretical advancements, offering practical insights and methodologies to enhance the resilience of infrastructure in seismic-prone areas.