(328e) Rheological Investigations of Volcaniclastic Debris Flows

Volcaniclastic debris flows, commonly known as lahars, are hazardous natural phenomena that occur when a mixture of fine and coarse sediments and water flow down slopes after intense and/or prolonged rainfall in response to gravitational attraction. They can be generated both during or soon after explosive eruptions, but they can occur also tens or hundreds of years later.

This is the case of the area surrounding the Somma-Vesuvius and the Phlegrean Fields volcanoes in Campania region (southern Italy), where the slopes of the volcanic edifices and the Apennine reliefs are mantled by loose pyroclastic fallout and current deposits, that can be easily remobilised. Previous studies pointed out how these phenomena highly impacted on ancient settlements and how they can affect population and infrastructures during recent times, too.

In the last five years more than 500 sites among archaeological excavations, stratigraphic trenches, drill cores and outcrops have been reviewed and analysed, paying particular attention to the debris flow deposits related to the sub-Plinian AD 472 eruption of Somma-Vesuvius, characterised by a wide areal distribution. Several samples have been collected and analysed in order to characterise their grain size distribution.

The aim of this work is to acquire new knowledge in order to implement the numerical models generally used for simulating debris flows, and to compare them with the new ones. Moreover, the information acquired will be useful to assess risk and prepare mitigation plans.

For this purpose, the FT4 powder rheometer developed by Freeman Technology Ltd. has been used on several samples with the diameter of the particles less than or equal to 0.710 mm.

The shear tests have been performed with different initial consolidation stresses in order to obtain physical parameters such as the angle of internal friction, that gives an idea of the viscosity of the sample, and the flow function coefficient, that gives information on the flow behaviour. Compressibility tests have been carried out to measure how the density changes as a function of the applied normal stress. Finally, the wall friction properties have been evaluated using the wall friction test to understand how easily a powder flows against the material with which it is in contact. Sandpaper with different grits have been used to simulate different rugosities.

Preliminary results show that the angle of internal friction does not depend on the initial consolidation stresses and on the grainsize fractions. On the contrary, the flowability and the compressibility of the powders are strongly affected by the grainsize distribution and the initial consolidation stresses. Finally, the wall friction angle is affected by both grainsize distribution and the roughness of the wall. Further experiments have to be carried out to obtain additional information.

To verify the interaction between sediment and water, and so to evaluate the effect of the water content on the rheological properties of the samples, a Bohlin rheometer is employed. Furthermore, in collaboration with the British Geological Survey large-scale experiments are conducted at the University of Saint-Luis Potosí (Mexico), and the same samples used are studied to link the rheological measurements to the dynamics observed in the large-scale experiments.