1,721,066 research outputs found
Structure degradation and changes in the mechanical behaviour of a stiff clay due to weathering
No full textThe paper discusses the results of research into the causes of weathering of stiff Pleistocene clays located in the Montemesola Basin (TA, Italy), and on the effects of weathering on the flay's mechanical behaviour, The weathered clays are yellow-brown and overlie the original grey clays in the whole basin. The liquidity indexes of both the yellow and the grey clays, their fabric, as observed by means of scanning electron microscopy, as well as the yellow clay oxidation features suggest that drying has been the main weathering process in the basin. This is confirmed by the results of drying-wetting cycle tests carried out in the laboratory on undisturbed clay samples, which show the drying-wetting cycles to produce changes in the grey clay similar to those present irt situ due to weathering. Results of oedometer, stress path and triaxial tests show that weathering has caused a degradation of the clay bonding and an associated reduction in the size of the clay state boundary surface. Both one-dimensional and isotropic compression data and the shear data indicate that the effects of weathering on the mechanics of the clay may be established according to the decrease in the clay stress sensitivity. Weathering also causes a decrease in the normalised shear stiffness, as observed by means of bender element tests, which is also related to a decrease in the stress sensitivity
A general framework for the mechanical behaviour of clays
No full textIt is suggested that the structure of clays may be distinguished simply as either 'sedimentation' or 'post-sedimentation', depending on whether gross yield in one-dimensional compression in void ratio-vertical effective stress plane occurs on the original sedimentation compression curve or to the right of this curve. A behavioural framework is proposed for clays having either of these structures, the magnitude of the strength sensitivity S-t when consolidated to the gross yield state (in compression) quantifying the effects of structure. The behaviour of natural clay consolidated to stress states pre-gross yield is related to the undisturbed clay sensitivity S-t; the gross yield curve Is generally of arch shape, its size relative to that of the reconstituted clay depending on this sensitivity, Clays having S-t >1 consolidated to stresses beyond gross yield exhibit post-gross yield behaviour in which values of sensitivity reduce with increasing post-yield strain. It is found that following normalization for composition (using the stress ratio at critical state M), volume (using p(e)*) and structure (using the sensitivity at gross yield S-t), many different clays have the same gross yield curve, A basically frictional linear flow rule is found to operate from small or medium to large strains for both natural and reconstituted clays
The influence of structure on the pre-failure behaviour of a natural claY
No full textResults are presented of an extensive laboratory and field investigation of a Pleistocene stiff clay in its natural and reconstituted states, including constant rate of strain oedometer and triaxial tests over a wide stress range, scanning electron microscopy and chemical micro-analysis. The relationships between the structure of the clay, its geological history and its mechanical response are demonstrated. The structure of the clay is strongly influenced by bonding, apparently largely due to a film of amorphous calcite which developed in situ at the end of normal consolidation. The effects of structure on the stiffness of the clay are identified, with sensitivity as the parameter quantifying the mechanical effects of structure. A frictional flow rule is found generally to be applicable over a wide strain range, including yield, even when strong bonding is present, although its form is influenced by structure. A new normalization shows that the gross yield surface is isotropically hardening in terms of both volume and structural change
Mechanics of a tectonized soil slope: influence of boundary conditions and rainfall
No full textThe Vadoncello landslide was mobilized in December 1993 and is still active. It involves highly tectonized soils and is the reactivation of a landslide dragged by a larger landslide at the toe of the slope soon after the 1980 Irpinia (Southern Italy) earthquake. Investigations and monitoring of the Vadoncello landslide were carried out, between 1994 and 1996, within an EC funded research project. The slope has been found to be formed of chaotic successions of soil and rock strata which have been grouped into soil complexes. The soil mechanical properties are shown to be very poor, the deep soils being prone to large plastic straining even due to relatively small loading changes. The soil displacements show that a shallow fast rotational sliding has occurred at the top of the slope and a shallow earthflow has developed downslope, both lying above deeper soils involved in a mechanism of slow and long-lasting irrecoverable movements. These slow deep movements are considered to be consequent to the plastic flow of the clayey soils. They can be activated by the effects of seasonal rainfall, of low-medium intensity seismic events and by the effects of the morphological changes resulting from the slow movements themselves. The landslide reactivation in 1993 is seen to have been the combination effect of a low return-period rainfall event and the slow movements active at depth in the slope.Published165-185JCR Journalope
La Geotecnica per lo sviluppo sostenibile del territorio e per la tutela dell'ambiente
No full textI fenomeni franosi sono ancora oggi causa di perdita di vite umane e di ingenti danni economici, dunque fonte di alto rischio per la società civile, e l'Italia è il paese a più alto indice di franosità in Europa.
Un deciso avanzamento nel rendere sostenibili la gestione e la mitigazione del rischio da frana rientra oggi tra gli obiettivi delle più recenti agende sovranazionali per lo sviluppo sostenibile a scala globale.
La presente relazione documenta come il recente Sendai Framework delle Nazioni Unite indirizzi ad approfondire la conoscenza dei processi che generano i fenomeni franosi, attraverso analisi multidisciplinari che contemplino la complessità dei pendii che ne sono sede, per accrescere l'efficacia e la sostenibilità delle soluzioni di mitigazione, evidenziando come la fase di analisi del meccanismo di frana sia una parte fondamentale della progettazione della soluzione di mitigazione. Dunque, la relazione espone diversi aspetti delle metodologie che la letteratura propone per conseguire interpretazioni su base geo-idro-meccanica (GHM) dei processi franosi, evidenziando l'importanza della sinergia tra le analisi geologico-geomorfologiche e la modellazione ido-meccanica dei processi di pendio, per il successo delle interpretazioni del dissesto, l'identificazione delle cause e la previsione della sue evoluzione nel tempo. Inoltre, la relazione documenta, con molteplici riferimenti alla letteratura, anche sintetizzando a titolo d'esempio alcuni casi di studio, come applicare con successo le metodologie di analisi GHM dei meccanismi di frana. In particolare, la trattazione mostra come tali analisi siano premessa di valutazioni sia della pericolosità, sia della vulnerabilità da frana. Poiché dette metodologie sono invalse a scala di pendio, ma difficilmente implementabili su area vasta secondo un approccio bottomup alla valutazione del rischio, la relazione propone come dette analisi possano essere integrate nel processo di gestione del rischio da frana su area vasta, secondo una approccio multiscalare. Infine, la relazione propone un progetto di ricerca, da condursi in termini multi-disciplinari tramite la collaborazione sistematica dei ricercatori impegnati nell'analisi dei processi franosi su base GHM, finalizzato alla caratterizzazione geo-idro-meccanica delle frane, cui far corrispondere le classi di strategia di mitigazione più sostenibili. Tale proposta è avanzata nella prospettiva che solo la classificazione GHM della franosità potrà fornire al mondo delle istituzioni e delle professioni linee guida utili all'attuazione diffusa di strategie di mitigazione del rischio da frana che siano sostenibili
Microstructural Changes in Clays Generated by Compression Explored by Means of SEM and Image Processing
Full text linkAbstractA study on the microstructure of an illitic marine clay is carried out through a thorough investigation of the clay origin, composition and current microstructure. The clay fabric is investigated by means of scanning electron microscopy (SEM) and statistically analysed by means of image processing. The nature and strength of the clay bonding are probed by means of direct chemical micro-analyses and on purpose strain paths. The same investigation is carried out both on the natural and on the reconstituted clay, at the initial state and after one-dimensional compression to medium and large pressures
One-dimensional compression of a natural clay: structural changes and mechanical effects
No full textPart of a research programme concerning the influence of structure on the mechanics of clays is presented. The material is a stiff overconsolidated natural clay, strongly bonded by diagenetic processes. Its one-dimensional compression and consequential structural changes are compared with those of the same clay when reconstituted. The structural changes are evaluated using scanning electron microscopy. The results of oedometer tests are discussed, some carried out in a purpose-built constant-rate-of-strain oedometer which enables the behaviour of the natural clay to be studied well beyond yield. The variations in swelling index, stiffness and coefficients of consolidation and permeability are compared pre- and post-yield for the two clays. Both clays are found to have significantly different states and fabrics at all stages of compression. Post-yield the structural changes in the natural clay are initially a destruction of the original fabric, and then a rearrangement of the clay particles into a systematic packing of honeycomb and perfectly oriented fabrics, in contrast to the reconstituted clay in which a random fabric develops. Despite the very different structures of the two clays, the trends of compression and consolidation behaviour are similar
Mechanics of a tectonized soil slope: influence of boundary conditions and rainfall
No full textThe Vadoncello landslide was mobilized in December 1993 and is still active. It involves highly tectonized soils and is the reactivation of a landslide dragged by a larger landslide at the toe of the slope soon after the 1980 Irpinia (Southern Italy) earthquake. Investigations and monitoring of the Vadoncello landslide were carried out, between 1994 and 1996, within an EC funded research project. The slope has been found to be formed of chaotic successions of soil and rock strata which have been grouped into soil complexes. The soil mechanical properties are shown to be very poor, the deep soils being prone to large plastic straining even due to relatively small loading changes. The soil displacements show that a shallow fast rotational sliding has occurred at the top of the slope and a shallow earthflow has developed downslope, both lying above deeper soils involved in a mechanism of slow and long-lasting irrecoverable movements. These slow deep movements are considered to be consequent to the plastic flow of the clayey soils. They can be activated by the effects of seasonal rainfall, of low-medium intensity seismic events and by the effects of the morphological changes resulting from the slow movements themselves. The landslide reactivation in 1993 is seen to have been the combination effect of a low return-period rainfall event and the slow movements active at depth in the slope.Published165-185JCR Journalope
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