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An experimental study of the rheological behaviour of three natural pyroclastic soils with different depositional processes remixed with water was carried out with the help of a rotational rheometer and inclined plane tests. A homogeneous fluid-like behaviour is obtained only within a very narrow range of concentrations, typically not more than 10%. Below this range the material sedimentates rapidly; above this range it behaves like a solid. In the fluid-like range the typical rheological behaviour of these suspensions is that of a yield stress fluid exhibiting a static yield stress larger than its dynamic yield stress. This effect probably finds its origin in a ‘local’ sedimentation effect: that is, the particles sedimentate just as necessary to form a structure more jammed than the structure during flow. As a result the flow of such materials is usually unstable: they will start to flow beyond a critical stress, but just beyond this value will reach a high shear rate associated with a high flowing velocity. The static and dynamic yield stresses of these materials increase widely from very low to very large values (several orders of magnitude). Inclined-plane tests were shown to provide reasonable although still approximate values for the static and dynamic yield stresses. These results suggest that in the field a small change in solid fraction will cause a slight decrease of the static yield stress, readily inducing a rapid flow that will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of pyroclastic debris flows, which are able to flow over very long distances, even on smooth slopes.

Rheological behaviour of reconstituted pyroclastic debris flow

Scotto di Santolo A;
2012-01-01

Abstract

An experimental study of the rheological behaviour of three natural pyroclastic soils with different depositional processes remixed with water was carried out with the help of a rotational rheometer and inclined plane tests. A homogeneous fluid-like behaviour is obtained only within a very narrow range of concentrations, typically not more than 10%. Below this range the material sedimentates rapidly; above this range it behaves like a solid. In the fluid-like range the typical rheological behaviour of these suspensions is that of a yield stress fluid exhibiting a static yield stress larger than its dynamic yield stress. This effect probably finds its origin in a ‘local’ sedimentation effect: that is, the particles sedimentate just as necessary to form a structure more jammed than the structure during flow. As a result the flow of such materials is usually unstable: they will start to flow beyond a critical stress, but just beyond this value will reach a high shear rate associated with a high flowing velocity. The static and dynamic yield stresses of these materials increase widely from very low to very large values (several orders of magnitude). Inclined-plane tests were shown to provide reasonable although still approximate values for the static and dynamic yield stresses. These results suggest that in the field a small change in solid fraction will cause a slight decrease of the static yield stress, readily inducing a rapid flow that will stop only when the dynamic yield stress is reached, namely on a much smoother slope. This can explain the in situ observed post-failure behaviour of pyroclastic debris flows, which are able to flow over very long distances, even on smooth slopes.
2012
On a re´alise´ une e´tude expe´rimentale sur le comportement rhe´ologique de trois sols pyroclastiques naturels, pre´sentant diffe´rents processus de de´poˆt recycle´s en place avec de l’eau, a` l’aide d’un rhe´ome`tre rotatoire et d’essais sur plan incline´. On obtient un comportement de type fluidique homoge`ne seulement au sein d’une plage de concentrations e´troite, qui ne de´passe pas 10% en moyenne. Au-dessous de cette plage, les matie`res se de´posent rapidement, alors qu’au-dessus elles se comportent comme des solides. Dans la plage fluidique, le comportement rhe´ologique typique de ces suspensions est celui d’un fluide a` limite e´lastique pre´sentant une limite e´lastique statique supe´rieure a` sa limite e´lastique dynamique. Cet effet de´coule probablement d’un effet de se´dimentation « local » – autrement dit, les particules se de´posent tout juste assez pour former une structure plus bloque´e que la structure au cours de l’e´coulement. En conse´quence, l’e´coulement de ces matie`res est ge´ne´ralement instable : elles commencent a` s’e´couler au-dela` d’une contrainte critique, puis, juste avant cette valeur, elles atteignent un taux de cisaillement e´leve´ correspondant a` une vitesse d’e´coulement e´leve´e. Les limites d’e´lasticite ´ statiques et dynamiques de ces matie`res varient e´norme´ment, entre des valeurs extreˆmement basses et des valeurs extreˆmement e´leve´es (plusieurs ordres de grandeur). On de´montre que les essais sur plans incline´s fournissent des valeurs raisonnables, mais encore approximatives pour les limites d’e´lasticite´ statiques et dynamiques. Ces re´sultats indiquent que, sur le terrain, une le´ge`re variation de fractions solides engendre une le´ge`re diminution des limites d’e´lasticite´ statiques, qui entraıˆne facilement un e´coulement rapide, qui ne s’arreˆte que lorsque l’on atteint la limite d’e´lasticite´ dynamique, a` savoir sur un plan incline´ plus re´gulier. Ceci pourrait expliquer le comportement post rupture de ces e´coulements de de´bris pyroclastiques observe´s in situ, qui sont en mesure de s’e´couler sur de tre`s longues distances, meˆme sur des plans incline´s re´guliers.
landslides
laboratory tests
failure
slopes
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12607/5429
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