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Journal Articles Geotechnique Year : 2012

Rheological behaviour of reconstituted pyroclastic debris flow

Philippe Coussot
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A. Evangelista
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A. M. Pellegrino
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A. Scotto Di Santolo
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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.
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hal-01157376 , version 1 (27-05-2015)

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Philippe Coussot, A. Evangelista, A. M. Pellegrino, A. Scotto Di Santolo. Rheological behaviour of reconstituted pyroclastic debris flow. Geotechnique, 2012, 62 (1), pp. 19-27. ⟨10.1680/geot.10.P.005⟩. ⟨hal-01157376⟩
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