1,720,973 research outputs found
Laboratory Evaluation of Geosynthetic Interface Friction under Low Stress
Full text linkIn landfill cover, geosynthetic packages are often used to fulfil different and simultaneous functions: drainage, waterproofing, separation, reinforcement, and soil protection. In this regard, various types of geosynthetics are combined in succession to allow for water and biogas drainage and to waterproof, reinforce, and provide protection from erosion over the useful lifetime, ranging over many decades if we consider the long phases of disposal, closure, and quiescence of the landfill itself. The creation of the composite cover barrier requires the evaluation of various interfaces’ frictional strength under low contact stresses, both in static and seismic cases. The main purpose of this study is to summarize the results of past laboratory tests carried out on different geosynthetic–geosynthetic and geosynthetic–soil–geosynthetic interfaces using experimental instrumentation developed at the geotechnical laboratory of the University of Padua, which allows for the characterization of the interface geosynthetic friction at low contact stresses. The main aspects highlighted are the kinematic mode of failure, the wearing of the contact surfaces, the presence or absence of interstitial fluid, and, finally, the density level of the granular soil in contact with the geosynthetics
Development of geosynthetic interface strength in landfill capping under seismic conditions.
No full textThe aim of this research is to carry out an analysis of the mobilisation of interface strength, in the geosynthetic liner system of a landfill capping, during seismic activity, since geosynthetics, widely-used for waterproofing, drainage and reinforcement, affect stability. Contact surfaces between geosynthetics and between geosynthetic and soil may become potential surfaces of slippage due to low available friction. The problem has been examined by means of a numerical Flac 2D model which, according to some simplifying hypotheses, could highlight the evolution of interface strength in the dynamic field. The analyses have shown that the seismic displacements may be large enough to mobilize dynamic friction at the level of the geosynthetic interface and thus that a dynamic characterisation of the interfaces is required in order to accurately predict stress in the liner and displacement of the capping
Compressibility of municipal solid waste and its implications
No full textThis paper deals with the compressibility of municipal solid waste (MSW) with the purpose of highlighting the engineering implications related to the waste subsidence during disposal, closure and quiescence. A parametric analysis of settlement is carried out for a 40 m high MSW column, by considering two biological degradation models, EPA and SWANA, known in the technical literature. The biodegradation models are implemented inside a one-dimensional numerical model of waste column, increasing in height with a prefixed law. The model takes into account the mass loss resulting from the biodegradation process and the fluid transport along the vertical direction, from the most saturated areas towards the less saturated. The effect of pore pressure, resulting from both the production of biogas or from the raising of the leachate level, is neglected in the hypothesis that the plant remains efficient in time. It is assumed that the leachate is continuously recirculated from the top to the base of the waste column. The analyses have the purpose of investigating the magnitude of the three components of settlement, immediate, creep and biodegradation, at the end of the disposal phase, after the construction of the coverage barrier and, finally, after thirty years from the closure. Bio-mechanical parameters are in part adapted from the technical literature [GOURC et al., 2010] and in part obtained from large diameter oedometric tests carried out at the ICEA Department of the University of Padua [COSSU et al., 2015]. In the light of engineering applications, the main results achieved are: - The two biodegradation models, EPA and SWANA, do not have a decisive influence on the waste settlements for disposal rates in the range of the usual practice; however, as showed by the data of Tab. VII, the choice of the model may be helpful in order to calculate the mean degree of maturation of biodegradable matter. - The filling coefficient, that is the ratio between the reached MSW height and the planned one, is strongly dependent on the disposal rate. The engineering goal is of assessing the optimal disposal rate to maximize the landfill volume with the lower operating costs. - The differential settlements due to creep and biodegradation can induce failure in the covering barrier. Such settlements are accentuated by a fast disposal rate; a slower rate minimizes this hazard because part of the settlement occurs during the disposal itself. For slower disposal rate, the creep settlement can become more crucial than the biological one. - A careful assessment of the creep parameter is recommended, also in the light of the European policies on MSW, according to which the organic component must be minimized and stabilized by pre-Treatment. - The immediate settlement of MSW and the relative law of evolution represent a very critical point for the subsidence problems occurring during disposal and closing; in this respect, it should be taken into greater consideration the extra-situ pretreatments and those in-situ (compaction, preloading), in order to stabilize the medium. As a final practical indication, it is suggested to run numerical simulations of the landfilling program in order to ascertain the final landfill profile and the coverage stability; on the basis of such analyses it will be possible to redefine the cultivation plan to minimize the costs of management and postclosure. Concluding, it is clear how geotechnics can bring a basic contribution in the design of MSW landfills; anyway, the need of a multidisciplinary approach is also evident, in order to avoid drawbacks in a complex field not yet fully consolidated
The characterisation of geosynthetic interface friction by means of the inclined plane test.
No full textThe paper focuses on the evaluation of the shear strength in conditions of low normal stress of various
geosynthetic-geosynthetic interfaces, which are typical of landfill cover systems, by means of the inclined plane
test, with the aim of studying the friction mobilisation in relation to various kinematic behaviours. The results of
three different methods to evaluate the angle of friction were analysed, together with the sensitivity of the interfaces
in relation to the wear effect and the influence of the state of hydration. The results showed very
different responses of the interfaces to the shear stress, which involved three main types of sliding mechanisms,
referred to as sudden, gradual and uneven sliding. Another outcome observed was that the shear strength of
geosynthetic-geosynthetic interfaces cannot always be properly characterised following the procedure proposed
by the European standard for soil-geosynthetic interfaces (EN ISO 12957–2), since the actual mobilised kinematic
behaviour should be taken into consideration. In this regard, the paper provides some hints on the choice of the
more representative parameter of friction for each type of sliding. A particular focus was given to the case of
gradual sliding interfaces, for which the static friction is difficult to detect due to the very slow movements; for
practical purposes, the design friction of these interfaces should be evaluated by using an adequate safety factor
with respect to the friction evaluated at 1 mm of displacement
Geosynthetic Interface Friction at Low Normal Stress: Two Approaches with Increasing Shear Loading
Full text linkThe evaluation of geosynthetic interface friction is a key parameter for the stability of coupled geosynthetics, as in landfill capping liner. At the present time, few types of tests are suitable for measuring the interface friction at low normal stress: one of these is the inclined plane, usually carried out under a vertical stress of 5 kPa. This type of test is not without critical aspects, mainly due to the nonuniform normal stress state induced by the inclination of the plane, but, on the other hand, the most widespread direct shear test generally cannot be performed at such low values of normal stress. After a short discussion on the pros and cons of these two types of test, the paper presents a comparison of the interface friction angles obtained, for three interfaces, by means of an inclined plane and an unconventional direct shear apparatus, under the same low normal stress. The peculiarity of this latter device is of ensuring a gradual increase of the mobilized strength, in a way similar to what occurs during the inclined plane test. The good correspondence of the results of the two types of tests confirmed the validity of both the test approaches
Correlation between compressibility and degradation for a PTMB waste
No full textSettlement of Municipal Solid Waste (MSW) in landfill is a geotechnical key concern since it could lead to dramatic failure of cap cover systems. On the other hand recent research developments demonstrate that it is possible to propose robust relationship between secondary settlement and kinetics of biodegradation. It is a matter of fact that settlement could be divided into two quasi-independent fractions, a classical time-dependent mechanical contribution, similar to the creep phenomenon for cohesive soils, and a biodegradation contribution. There is a practical interest in this finding: settlement monitored at the surface of the landfill cap cover could be considered as a relevant indicator of the biodegradation level. This is specifically interesting in the case of a landfill bioreactive cell, where the influence of leachate injection in this way could be assessed. The present proposal includes two parts: a presentation of the bio-mechanical relationship through the relation between biogas production and compressibility, and an application to mono-directional (oedometer) tests on MSW
Numerical analysis of a geo-tube subjected to seismic soil liquefaction
No full textGeo-tubes are geosynthetic tubular containers, of various length and diameter,
which are filled by pumping a fluid mixture of water and soil: the permeability of the
geosynthetic allows water to drain rapidly so that at the end of the installation a solid nucleus is
obtained. Considering that geo-tubes have been widely used as submerged elements, for example
for shoreline protection, and that the filling material is a low density loose soil, hydraulically
deposited, they may be subjected to liquefaction during seismic events. In order to investigate
the behavior of a single submerged geo-tube when subjected to soil liquefaction, a numerical
analysis is presented. The El Centro earthquake was considered and, under this solicitation, a
rapid buildup of the pore pressure in the filling soil was observed. Consequently, since the first
seconds of the seismic event, a decrease of the effective vertical stress took place up to values
close to zero, showing the capability of the model to replicate a condition of soil liquefaction. At
this stage, a general increase of the tensile force in the geosynthetic was noted, up to values that
are significantly higher than those experienced in the static phase
Resistenza alla riattivazione dell’argilla Flyschoide di Montona.
No full textL'articolo presenta i risultati di uno studio condotto su argille flyschioidi atto ad evidenziare gli effetti della struttura e della storia tensionale sulle caratteristiche di resistenza al taglio. Il terreno argilloso naturale analizzato è stato coinvolto nella frana avvenuta nell'autunno del 1998 in seguito ad un evento meteorologico molto intenso che ha interessato la frazione di Montona nelle vicinanze di Cormons (UD), centro situato nell'area del Collio Orientale del Friuli - Venezia Giulia (Italy).
I campioni indisturbati di terreno sono stati prelevati in prossimità della superficie di rottura, raggiunta attraverso gli scavi eseguiti durante i lavori di posa in opera dei sistemi di drenaggio e di sistemazione del versante, ed analizzati presso il Laboratorio di Geotecnica dell'Università degli Studi di Udine (Italy).
Le principali proprietà indici sono tipiche dei terreni argillosi inorganici (CL secondo la classificazione USCS) e presentano un limite liquido WL = 34%, un indice di plasticità IP = 14, la frazione fine risulta essere del 70%, la frazione argillosa C.F. = 41.4 % con un indice di attività IA = 0.37.
Le prove di taglio diretto ed edometriche condotte su campioni prelevati in sito hanno permesso di misurare la resistenza al taglio di picco, la storia tensionale e le caratteristiche di compressibilità delle argille flyschoidi allo stato naturale mentre la resistenza residua è stata valutata mediante l'utilizzo dell'apparecchio di taglio torsionale tipo Bromhead.
Sono state eseguite inoltre altre prove edometriche su campioni ricostituiti con un contenuto d'acqua pari a 1.5 volte il valore del limite liquido al fine di confrontare le caratteristiche di compressibilità del terreno allo stato naturale con quelle dello stato intrinseco come suggerito da Burland (1990) ed i risultati sperimentali mostrano che i valori dell'indice dei vuoti Iv giacciono al di sotto della ICL.
Successivamente sono stati analizzati gli effetti della storia tensionale imponendo differenti rapporti di sovraconsolidazione a tre serie di campioni ricostituiti con valori di OCR compresi fra 1 e 5.
La resistenza al taglio è stata inoltre normalizzata introducendo la *ve, che rappresenta la tensione equivalente misurata sulla ICL e corrispondente all'indice dei vuoti del terreno, ed i risultati evidenziano chiaramente l'influenza della storia tensionale sulla pendenza degli inviluppi di rottura normalizzati
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