171,071 research outputs found
La riforma dei servizi pubblici locali. Introduzione
No full textE' l'introduzione al numero monografico "La riforma dei servizi pubblici locali" curata da C. Buratti e G. Muraro. Contiene le linee essenziali di quanto emerso dai singoli saggi
Pre-failure behaviour of reconstituted peats in triaxial compression
No full textThis 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
Modelling free gas overpressure in peat layers
No full textThe 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
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
Experimental determination of the shear strength of peat from standard undrained triaxial tests: correcting for the effects of end restraint
No full textConventional triaxial tests on peats are strongly criticised due to the very high shear strength parameters obtained from standard data elaboration, leading to unrealistic factors of safety when used in geotechnical design and assessment. Various operational approaches have been proposed in the literature to overcome this difficulty; however, they seem to lack consistent mechanical background. Some of the issues related to the shear strength evaluation of peats from triaxial tests come from the non-uniform stress and strain states developing in the samples well before failure is attained, due to end restraint effects. Undrained triaxial compression tests were performed on reconstituted peat to examine the influence of end restraint on the deviatoric stress, excess pore pressure and deviatoric strain response. Samples were tested with standard rough end platens and with modified platens to reduce the friction between the sample and bottom and top caps. Four different initial height-to-diameter ratios were examined, to reduce the consequences of rough end platens on the sample response. The results indicate that end restraint contributes dramatically to overestimating the shear strength of peat, due to the increase in both the calculated deviatoric stress and the measured excess pore pressure at the bottom of the sample. Suggestions are given to quantify the influence of end restraint in the interpretation of standard data, in an attempt to suggest viable procedures to determine more reliable effective and undrained shear strength parameters from standard triaxial tests
Bottom currents on a pelagic carbonate platform. Mounds and sediment drifts in the Jurassic succession of the Sciacca Plateau, Western Sicily
No full textThe stratigraphic succession in the San Vincenzo Gorge (Saccense Domain, western Sicily) documents deposition on a vast pelagic carbonate platform, the Sciacca Plateau, during the Middle and Late Jurassic. This succession caps a peritidal limestone (Inici Formation), which underwent extension during the Western Tethyan Early Jurassic rift phase, and displays a set of unique features, which have never been previously reported on a Tethyan drowned platform. The upper part of the Bositra limestone (late Bajocian-early Oxfordian p.p.) comprises elongate convex-up, mound-shaped bodies, made of thin-shelled bivalve wacke- to grainstone, a few tens of metres across and producing a topographic relief of up to 10 m. Planar beds within the mound cores are seen to thin out laterally with tangential downlaps along sections perpendicular to the mounds' longer axes, and the mounds are in lateral association with concave-up bedsets. Following halt of the Bositra-dominated deposition and demise of mound accretion, the draping units inherited an antiformal geometry. The mounds are interpreted as being part of a sediment drift, produced by bottom currents sweeping the Plateau top, the source areas being sediment-depleted sectors now documented by extremely condensed and hiatus-ridden sections, with parallel-sided beds. Following draping and partial levelling of the submarine relief by the Knobbly limestone (?middle Oxfordian/early Kimmeridgian-late Kimmeridgian), the Coquina limestone is locally a thick (>20 m) ammonite/brachiopod rudstone (Tithonian p.p.). This unit displays evidence for lateral accretion, with large-scale clinoforms dipping up to 12°, and is interpreted as a mud-poor, bioclastic-gravel drift, with the action of bottom currents being apparently linked with a bloom of cephalopods. This is an early-cemented deposit, where clotted, micropeloidal fabrics document the calcification of microbial communities and are followed by growth of early diagenetic fibrous calcite. The description and interpretation of the mounded Bositra limestone and of the clinostratified Tithonian limestone are the main focus of this paper. The San Vincenzo Gorge outcrop displays similarities with pelagic shelves, like the Upper Chalk basin of northern Europe
Developing a constitutive approach for peats from laboratory data
No full textRecent research effort carried out at Delft University of Technology to improve the experimental knowledge and develop a comprehensive modelling approach for fibrous organic soils is summarised. Experimental results and numerical analyses are combined to discuss some contradictory results which have delayed advanced characterisation of peats. Part of the apparent inconsistencies commonly found in the literature is due to the influence of the testing apparatus, including rough platens and membrane restraint, which inhibit homogenous deformation modes and alter the response of the samples compared to the true material behaviour. The consequences of non-homogenous deformation are particularly relevant on peats due to the unique combination of their exceptionally low stiffness and high strength. An elastic–plastic constitutive framework was developed starting from repeatable reconstituted samples of peats, taking care of reducing end restraint to a large extent in the experimental setup. The results suggested that an elastic–plastic model for peats should include a non-associated flow rule and a mixed volumetric–deviatoric hardening law. The role played by different fibres at the laboratory scale is discussed, and the additional reinforcement offered by bigger fibres on the observed behaviour of natural peats is addressed.Geo-engineerin
Numerical investigation of the equipment set-up in triaxial testing of soft soils
No full textElement testing of soft soils is challenging due to the large strains attained in the pre-failure range. Besides the heterogeneity of natural samples, the set-up configuration is the main driving factor for non-homogenous response. Stress, strain and pore pressure non-uniformities induced by the loading system affect the observed behaviour and complicate proper interpretation of the results. Among the difficulties encountered in the interpretation of laboratory data, the unexpected decrease of the stress ratio frequently observed on Dutch organic soft clays on the wet side of critical state is investigated by numerically back-analysing the triaxial test set-up. A 3D finite element simulation using an advanced constitutive model for soft clays developed at TU Delft was performed to clarify the nature of the response. The results indicate that a decrease in the deviatoric stress up to critical state may be interpreted as a true feature of the soil response. However, the response at large strains is very much influenced by the triaxial shear apparatus, in particular, by the rotation of the top cap which triggers geometrical instability. Practical recommendations are given to limit the effects of the set-up configuration on the determination of the undrained shear strength to be used for field applications.Geo-engineerin
Facies and geometries of pelagic deposits in a Jurassic pelagic carbonate platform/basin system - Sabina, Central Apennines (Italy)
No full textImplication of end restraint in triaxial tests on the derivation of stress–dilatancy rule for soils having high compressibility
No full textConstitutive models for soils are developed and validated against laboratory tests assuming these give representative information on the true material behaviour. However, data from standard laboratory tests reflect the sample response rather than the true material behaviour, due to nonuniformities in stresses and strains generated over the experimental test. The work examines the implications of end restraint on the definition of the stress–dilatancy rule of highly compressible soils with a finite element numerical approach. The numerical model replicates a reconstituted peat, typically characterized by a combination of high compressibility and high friction angle, which increases the severity of end restraint effects. Simulated results show that the global measurements from standard triaxial tests with rough end platens would not give the proper stress–dilatancy rule, if they were interpreted as the response of a single soil element at the constitutive level. Both overestimation and underestimation of the true dilatancy compared to the material response can be observed, depending on the deformation mode. To support the validity of the numerical results, experimental findings from drained triaxial tests on reconstituted peat are presented. Practical indications are given on how the standard interpretation of drained triaxial tests data on peats can be improved
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