Il fragipan è un orizzonte sub-superficiale ad elevata densità e limitata permeabilità che impedisce lo sviluppo radicale ed i flussi idrici. Il fragipan è estremamente duro da asciutto, ma la maggior parte dei suoi frammenti si disintegra e sgretola in acqua, un fenomeno chiamato ¿slaking¿. Sebbene gli orizzonti a fragipan siano stati studiati per oltre 70 anni, le teorie riguardanti genesi e sviluppo sono tuttora dibattute. Orizzonti con proprietà fragiche spesso si evolvono su depositi di loess, un materiale clastico delle dimensioni del limo derivante dall'abrasione delle rocce da parte delle coltri glaciali del Quaternario. Al giorno d'oggi il loess ricopre circa il 10 % della superficie terrestre, perciò non si tratta solo di un archivio paleo-climatico fondamentale delle fasi glaciali del Pleistocene, ma anche di un tipo di sedimento prioritario. Molte limitazioni di uso del suolo e problematiche di carattere geotecnico sono state riportate nel corso storia del tempo per la presenza ed inappropriata gestione di depositi di loess e orizzonti a fragipan. I collassi, in particolare, sono stati riscontrati in suoli derivanti da loess con almeno il 30-35 % di argilla e la forma delle particelle di loess, spesso indicata come rassomigliante ad una lama ma sempre in termini generici, è stata ritenuta capace di creare una struttura aperta metastabile. Questo studio è stato condotto sui suoli evolutisi dai sedimenti di loess che coprono il conoide terrazzato del fiume Stura di Lanzo (Nord-Ovest dell'Italia), dove i diversi depositi di loess, posti uno sull'altro, sono stati distinti sulla base delle discontinuità litologiche riscontrate in campo. Sei profili sono stati aperti e descritti e 35 orizzonti sono stati analizzati. I limiti di Atterberg (limite liquido, LL e limite plastico, PL) sono stati determinati, così come l'Indice di Plasticità, PI. L'Area Superficiale Specifica (SSA) è stata ottenuta con il metodo del blu di metilene, il ferro pedogenetico (FeD) è stato estratto con una soluzione di ditionito-citrato-bicarbonato di sodio, la distribuzione delle particelle in classi diametriche è stata determinata con in metodo della pipetta e la Dimensione Frattale (DF) è stata calcolata come un indice di frammentazione. I campioni sono poi stati classificati in base all'intensità dello slaking e le loro particelle di limo sono state osservate al microscopio ottico e valutate usando l'analisi di immagine. La maggior parte degli orizzonti rientra nelle classi tessiturali franco, franco argilloso e franco limoso ed è classificata come limo inorganico scarsamente comprimibile. In generale, mentre i valori più alti di LL e PL corrispondono a orizzonti coi contenuti maggiori di argilla l'Indice di Plasticità è del fortemente anticorrelato al contenuto di limo fine (r=-0.63;P<0.05). Inoltre, per contenuti crescenti di limo fine, le particelle tendono ad essere più allungate (r=0.64;P<0.05), dando probabilmente origine ad assembramenti tipici di particelle dalla forma appiattita. Gli orizzonti a fragipan non sono significativamente differenti da quelli non a fragipan in termini di limo grossolano e fine, DF, LL, PL e FeD, mentre essi mostrano SSA, PI e circolarità (levigatezza delle particelle) significativamente maggiori, insieme ad un colore significativamente più scuro delle particelle.
Fragipan is a subsurface soil horizon with a very high density and a limited permeability that inhibits root growth and water movements. The fragipan is extremely hard when air-dry, but most of its fragments disintegrate and crumble in water, a phenomenon called slaking. Although fragipan horizons have been studied for more than 70 years, the theories behind their genesis and development are still debated today. Horizons with fragic features often evolve on loess deposits, a silt-sized clastic material derived by glacial abrasion of rock outcrops by the Quaternary ice-sheets. Today loess covers as much as 10% of Earth surface, and therefore it is not only a fundamental paleoclimatic record of the glacial phases of Pleistocene but also a major sediment type. Many land use limitations and geotechnical problems have been reported along time because of the presence and inappropriate management of both loess deposits and fragipan horizons. Collapse in particular was recorded in loess-derived soils with clay contents below 30-35 % and the shape of loess particles, often addressed as resembling a blade but just in general terms, was ascribed as able to create an open metastable structure. This study was carried out on the soils developed on loess deposits covering the Stura di Lanzo River terraced alluvional fan (North-Western Italy), where the different loess deposition, stacked one on top of each other, could be distinguished by field-recognized lithological discontinuities. Six profiles were opened and described, and 35 soil horizons were analyzed. The Atterberg limits (i.e. liquid limit, LL and plastic limit, PL) were determined, as well as the Plasticity Index, PI. The Specific Surface Area (SSA) was obtained through the methylene-blue spot method, the pedogenic iron (FeD) was extracted with a Na-dithionite-citrate-bicarbonate solution, the particle size distribution was determined by the pipette method and the Fractal Dimension (DF) was computed as an index of fragmentation. The samples were also ranked by slaking intensity and their silt-sized particles were observed under optical microscope and evaluated using image analysis. Most of the horizons belong to the loam, clay loam and silt loam textural classes and are classified as inorganic silts of low compressibility. In general, while the highest values in LL and PL correspond to horizons with the highest clay contents, the Plasticity Index is strongly negatively correlated to fine silt content (r=-0.63;P<0.05). Furthermore, with increasing fine silt contents the particles tend to be more elongated (r=0.64;P<0.05), probably giving rise to a particular stacking of flat-shaped grains. Fragipan horizons are not significantly different from non-fragipan ones in terms of coarse and fine silt content, DF, LL, PL and FeD, while they show significantly higher SSA, PI and circularity (smoothness of the particles) together with a significantly darker color of the silt grains.
Geotechnical and pedological characterization of loess-derived fragipan horizons in North-Western Italy with integration of image processing techniques
NEGRI, SARA
2017/2018
Abstract
Fragipan is a subsurface soil horizon with a very high density and a limited permeability that inhibits root growth and water movements. The fragipan is extremely hard when air-dry, but most of its fragments disintegrate and crumble in water, a phenomenon called slaking. Although fragipan horizons have been studied for more than 70 years, the theories behind their genesis and development are still debated today. Horizons with fragic features often evolve on loess deposits, a silt-sized clastic material derived by glacial abrasion of rock outcrops by the Quaternary ice-sheets. Today loess covers as much as 10% of Earth surface, and therefore it is not only a fundamental paleoclimatic record of the glacial phases of Pleistocene but also a major sediment type. Many land use limitations and geotechnical problems have been reported along time because of the presence and inappropriate management of both loess deposits and fragipan horizons. Collapse in particular was recorded in loess-derived soils with clay contents below 30-35 % and the shape of loess particles, often addressed as resembling a blade but just in general terms, was ascribed as able to create an open metastable structure. This study was carried out on the soils developed on loess deposits covering the Stura di Lanzo River terraced alluvional fan (North-Western Italy), where the different loess deposition, stacked one on top of each other, could be distinguished by field-recognized lithological discontinuities. Six profiles were opened and described, and 35 soil horizons were analyzed. The Atterberg limits (i.e. liquid limit, LL and plastic limit, PL) were determined, as well as the Plasticity Index, PI. The Specific Surface Area (SSA) was obtained through the methylene-blue spot method, the pedogenic iron (FeD) was extracted with a Na-dithionite-citrate-bicarbonate solution, the particle size distribution was determined by the pipette method and the Fractal Dimension (DF) was computed as an index of fragmentation. The samples were also ranked by slaking intensity and their silt-sized particles were observed under optical microscope and evaluated using image analysis. Most of the horizons belong to the loam, clay loam and silt loam textural classes and are classified as inorganic silts of low compressibility. In general, while the highest values in LL and PL correspond to horizons with the highest clay contents, the Plasticity Index is strongly negatively correlated to fine silt content (r=-0.63;P<0.05). Furthermore, with increasing fine silt contents the particles tend to be more elongated (r=0.64;P<0.05), probably giving rise to a particular stacking of flat-shaped grains. Fragipan horizons are not significantly different from non-fragipan ones in terms of coarse and fine silt content, DF, LL, PL and FeD, while they show significantly higher SSA, PI and circularity (smoothness of the particles) together with a significantly darker color of the silt grains.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14240/47672