Rheological behaviour of reconstituted pyroclastic debris flow

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

The Sphere Drag Rheometer: A New Instrument for Analysing Mud and Debris Flow Materials

“…In order to ensure the stability of the measurements, each value of velocity was imposed for 30 seconds. This time was in agreement with the experimental results carried out with conventional rheometer on the same pyroclastic debris flow materials as described in [5]. …”

“…The considered conversion theory is applicable only in the treatment of Yield Stress Fluid. As observed in [5] the soil-mixtures considered in the present study are Yield stress Fluid and their behavior is well described using the Herschel & Bulkley model (Eq. (2)).…”

“…hair gel and tomato sauce). After, the calibration was carried out using a representative soil-water mixture, composed by water and kaolin, in order to approach the feasibility of the experimental apparatus with particles-fluid suspensions close to debris flow material mixtures tested in traditional rheometer before [5]. The kaolin-water mixture was reconstituted according to the total solid volumetric concentration Φ T , defined as the ratio of the volume of solids to the total volume (water plus solids), as follow:…”

“…Numerical and physical modeling help in the hazard zones delineating and in the structural design and testing [4]. In order to understand and simulate the flow and the deposition process, an appropriate rheological model is a very useful physical concept for mud, debris and hyper-concentrated flows [5]. The simplest method is to assimilate the flowing mass to a continuous viscous fluid and refers to rheological models derived from the resistance formula of Newtonian and Non-Newtonian fluids [6].…”

“…expressing the shear rate-shear stress relation, is required. Nowadays, efficient rheometrical apparatuses for the determination of the rheological parameters of mixtures and fluids containing fine particles exist and the experimental methods related to them are widely consolidated [5]- [7]- [8]- [9]- [10]. For mixtures and fluid containing large particle, the study of the influence of the grain size is more complicated.…”