First approach to the rheological behaviour of syn-eruptive and post-eruptive lahars.

Lahars are two-phase gravity flows which behavior strongly depends on the interaction between the solid phase (large clasts) and the liquid phase (interstitial fluid or matrix). In particular, the rheological behavior of the interstitial fluid is the parameter with the greatest influence on the lahar movement, since the interstitial fluid controls the transportion and sedimentation of the solid phase. The characteristics that define the rheological behavior of the interstitial fluid are particle volumetric concentration, size distribution, and sediment composition. Because of these, here we present the rheological characterization of syn-eruptive and post-eruptive lahars occurred at Popocatepetl volcano, Mexico.

The interstitial fluid characterization was carried out by a strain rate dependence test using a rotational viscometer with a coaxial cylinder geometry. We studied volcanic sediments with different densities and compositions (pumice and lava fragments) sampled from syn- and post-eruptive lahar deposits (at room temperature). In addition, we studied the range of the volumetric sediment concentration that defines the flowage based on the geotechnical characteristics of sediments.

The results suggest a Bingham-type behaviour model with yield strength values between 10^-2 and 10⁰ Pa, for shear rate conditions between 50 and 100 s^-1. In addition, an inverse relationship of apparent viscosity throw values between 10^-2 and 10^-1 Pa s at  the same shear rates, suggesting a thinning behaviour. The data suggest exponential relationships between volumetric sediment concentration and yield strength or apparent viscosity. The fitting coefficients describe volcanic sediments, unlike those found in the literature and often used in lahar simulations for hazard modelling, which are derived from the study of soil materials and mine tailings.

The interstitial fluid of syn-eruptive lahars with monolithologic composition (pumice fragments) has yield strength and apparent viscosity values higher than those of syn-eruptive and post-eruptive lahars with heterolithologic composition (pumice and lava fragments). In addition, the dominant clay-sized interstitial fluid has apparent viscosity values between two and three orders of magnitude higher than the dominant silt-sized interstitial fluid.

Finally, the rheological behavior of the studied lahar samples will contribute to solving  the constitutive equations that describe their movement under a viscoelastic flow behavior for the studied shear rates. Furthermore, this information could be implemented in numerical simulations with similar volcanic sediment characteristics and deformation conditions to those studied, with the aim of estimating the inundation area that would be affected by the occurrence of a similar event.