Rheological behaviour of reconstituted pyroclastic debris flow

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.