1,721,148 research outputs found
Modelling free gas overpressure in peat layers
Full text linkThe paper assesses fully coupled hydro-mechanical numerical approaches developed for unsaturated soils to model the effect of free gas overpressure on the response of peat layers. A simple linear model is used for the soil skeleton, however, the global response is non-linear due to changes over time of the compressibility of the solid skeleton over the compressibility of the fluid, and solubility of gas in water. The overpressure generated in foundation peat layers by barometric pressure oscillations is modelled, and the results are compared to literature data. The development of pore overpressure upon unloading is analysed as a function of the soil skeleton compressibility, and the consequences on the average stress acting on the soil skeleton are discussed
Pre-failure behaviour of reconstituted peats in triaxial compression
Full text linkThis paper discusses the results of an experimental programme designed to investigate the deviatoric behaviour of peats. The results are obtained from triaxial experiments carried out on reconstituted peat samples. The interpretation of the experimental results follows a hierarchical approach in an attempt to derive the ingredients that an elastic–plastic model for peats should contain, including the yield locus, the hardening mechanism and the flow rule. The results obtained from stress tests along different loading directions show that purely volumetric hardening is not adequate to describe the deviatoric response of peat and that a deviatoric strain-dependent component should be included. The plastic deformation mechanism also depends on the previous stress history experienced by the sample. Stress and strain path dependence of the interaction mechanisms between the peat matrix and the fibres is discussed as a possible physical reason for the observed behaviour. This work offers a relevant set of data and information to guide the rational development and the calibration of constitutive laws able to model the deviatoric behaviour of peats.Geo-engineerin
The role of the hydraulic resistance of the river bed and the time dependent response of the foundation layers in the assessment of water defences for macrostability and piping
No full textIn the assessment of water defences for macrostability and backward erosion piping the design hydraulic load mostly refers to steady state conditions in equilibrium with the maximum river water level. In this contribution, we show selected results of coupled hydromechanical numerical analyses of a paradigmatic Dutch case, which demonstrate that this assumption leads to high overestimation of the true hydraulic loads at the toe of the water defence embankment. The hydraulic resistance of the bed of the river and the deformability of the foundation layers introduce a decay in the pore pressure time history, which largely reduces the action on the hydraulic protection structure. The finite element model was developed to assist in the assessment of an innovative solution based on passive wells as a measure to reduce the risk for macrostability and piping. It was calibrated on available pore pressure measurements in the foundation of critical sections of the dykes of the river Lek in the Netherlands under the daily tidal action. The model was used to determine the distribution of pore pressure expected in the subsoil of the dykes for the design maximum load. The calibration stage of the model is specifically interesting to the aim of evaluating the reduction of the input pore pressure due to the hydromechanical resistance of the geotechnical system.Accepted author manuscriptGeo-engineerin
Gas exsolution and gas invasion in peat: Towards a comprehensive modelling framework
No full textIncreasing climatic stresses accelerate the degradation of highly organic soils, like peat, by increasing their drying rate above the water table and their decomposition rate under water. Recent experimental studies provide evidence of the consequences of these processes on the hydro-mechanical properties of peat. However, modelling the experimental evidence in a comprehensive framework remains challenging, especially in the case of anaerobic degradation, which is accompanied by gas generation, exsolution and expansion, into an initially saturated matrix of soil. In this research study, experimental results from undrained isotropic unloading on artificially gas-charged peat samples are combined with data from drying tests on the same peat, in an attempt to develop a unified framework encompassing the two desaturation processes. As a first approximation, simple compression laws depending on the average stress acting on the soil skeleton are used to simulate the experimental results. The comparison between experimental data and model simulations suggests the possibility of modelling gas expansion similar to the gas invasion process occurring on drying. The modelling approach, stemming from unsaturated soil mechanics, is meant to offer a possible framework to include the hydro-mechanical consequences of the effects of degradation of peats in the engineering analysis. Accepted author manuscriptGeo-engineerin
Calibration of a simple 1D model for the hydraulic response of regional dykes in the Netherlands
Full text linkA simple numerical model was set up to investigate the hydraulic behaviour of a regional dyke to improve understanding of the response under variable atmospheric conditions. The unsaturated hydraulic properties of the dyke body and the relevant foundation layers were calibrated either on the results of laboratory tests or on a national database, namely the Staringreeks, compiled for typical Dutch soils. The boundary conditions were imposed according to the weather history at the top, and to the pore pressures measured in the field at the bottom of the representative soil column. The results indicate that a simple 1D model is able to accurately reproduce the suction time history in the dyke core, provided the hydraulic conductivity and soil water retention properties are properly calibrated. The optimised hydraulic conductivities are typically two orders of magnitude higher than the saturated hydraulic conductivity from the laboratory tests, but comparable to the ones suggested in the database developed on field data. The work highlights that cautious evaluation of laboratory data is needed for field applications, and that direct information from the field should be used to validate numerical models in the presence of organic soils
Consequences of drying on the hydro-mechanical response of fibrous peats upon compression
Full text linkPeats are encountered in waterlogged deltaic areas, where degradation is delayed by favourable environmental conditions. The recent increase in frequency and severity of droughts is expected to accelerate peat degradation, in turn increasing subsidence and flood risk, urging better understanding of the response of peats to drying events. To this aim, compression tests on natural and reconstituted peat samples were performed, supported by X-ray micro-computed tomography. The peat fabric
was found to be the key factor in the response to drying, with fibres playing the most significant role. Drying in peats starts affecting the macro-fabric, with an irreversible reduction in volume and disruption of the fibrous network occurring under saturated conditions until a threshold void ratio is reached, below which desaturation occurs of the intra-fibres and intra-peds pores. The first drying stage dramatically decreases the compressibility, while the hydraulic conductivity is hardly affected due to the enlargement of macropores. Secondary compressibility is affected by the peat fabric besides the organic content. The total organic content does not change substantially during drying; hence, it is not the best proxy to describe the consequences of drying on the response of fibrous peats. The fibre content can be better used to serve the aim
Determination of water retention properties of silty sands by means of combined commercial techniques
Full text linkA recent increase in frequency and severity of exceptional climatic events is of concern for the stability of natural and artificial slopes. These undergo continuous evaporation and infiltration cycles, which change the suction distribution and trigger shrinkage, swelling, cracking, and surfi-cial erosion, overall decreasing the soil strength. To assess the impact of these climatic stresses, the determination of water retention properties is a priority. Although advanced techniques have been proposed in the last few decades to this end, simpler commercially available techniques allow col-lecting data for a larger number of samples in a shorter time, thus enabling a basic description of the water retention properties for a larger database of soils. Data on two silty sands, coming from very different climatic environments in Europe, were collected with a combination of two simple commercial devices, and the results were modelled with a van Genuchten’s law. The fitted pa-rameters were found to correlate well with the amount of fines, irrespective of the different origin and composition of the two soils. Eventually, the limitation of the approach is discussed based on the results of cyclic drying–wetting tests.</p
Managed entry agreements and high cost medicines (European perspective)
No full textThis chapter provides an overview on Managed Entry Agreements (MEA). Managed Entry Agreements are contracts between the industry and payers under which a drug gets access to the market. The first two sessions illustrate the rationale and taxonomy of MEA, that could be designed as financial-based or outcome-based agreement. The third one describes how these agreements have been implemented in Europe. The fourth section illustrates the results of the few studies that have investigated the impact of MEA and some thoughts on future perspectives for these contracts
Key drivers of innovativeness appraisal for medicines: The Italian experience after the adoption of the new ranking system
No full textObjective In 2017, the Italian Medicines Agency (Agenzia Italiana del Farmaco, AIFA) introduced a standardised process to appraise innovativeness of medicines. Innovative medicines are provided speeder market access and dedicated funds. Innovativeness criteria are: unmet therapeutic need, added therapeutic value and quality of the evidence (Grading of Recommendations Assessment, Development and Evaluation method). We investigated the role played by these three criteria on the final decision aimed to understand how the new Italian innovativeness appraisal framework was implemented. Design A desk research gathered AIFA's appraisal reports on innovativeness and data analyses were conducted. No patients were directly involved in this study. Setting and participants We scrutinised all 77 appraisal reports available on AIFA's website (2017-2020). Primary and secondary outcome measures The impact of the three domains on final decision was investigated through a series of univariate analyses. Results Among 77 appraisal reports on innovativeness available, 49 (64%) and 28 (36%) were for oncology and non-oncology medicines, respectively. The appraisals were equally distributed among 'fully innovative' (36%), 'conditionally innovative' (30%) and 'not innovative' (34%). Added therapeutic value was the most important driver on innovativeness decision, followed by quality of the evidence. Drugs for rare diseases and with paediatric/mixed indications were appraised 'innovative' by a larger proportion, but no statistical significance was found. Conclusions Despite some limitations, including the moderate number of appraisals, this paper provides an insight into the determinants of innovativeness appraisals for medicines in Italy and the accuracy of the appraisal process. This has important implications in terms of transparency and accountability in the prioritisation process applied to innovative medicines
- …
