Geomechanical rock properties of a basaltic volcano

Abstract

In volcanic regions, reliable estimates of mechanical properties for specific volcanic events such as cyclic inflation-deflation cycles by magmatic intrusions, thermal stressing, and high temperatures are crucial for building accurate models of volcanic phenomena. This study focuses on the challenge of characterizing volcanic materials for the numerical analyses of such events. To do this, we evaluated the physical (porosity, permeability) and mechanical (strength) properties of basaltic rocks at Pacaya Volcano (Guatemala) through a variety of laboratory experiments, including: room temperature, high temperature (935 • C), and cyclically-loaded uniaxial compressive strength tests on as-collected and thermally-treated rock samples. Knowledge of the material response to such varied stressing conditions is necessary to analyze potential hazards at Pacaya, whose persistent activity has led to 13 evacuations of towns near the volcano since 1987. The rocks show a non-linear relationship between permeability and porosity, which relates to the importance of the crack network connecting the vesicles in these rocks. Here we show that strength not only decreases with porosity and permeability, but also with prolonged stressing (i.e., at lower strain rates) and upon cooling. Complimentary tests in which cyclic episodes of thermal or load stressing showed no systematic weakening of the material on the scale of our experiments. Most importantly, we show the extremely heterogeneous nature of volcanic edifices that arise from differences in porosity and permeability of the local lithologies, the limited lateral extent of lava flows, and the scars of previous collapse events. Input of these process-specific rock behaviors into slope stability and deformation models can change the resultant hazard analysis. We anticipate that an increased parameterization of rock properties will improve mitigation power.

Cyclic and Fatigue Behaviour of Rock Materials: Review, Interpretation and Research Perspectives

“…2. Strain accumulates slowly with the number of cycles before peak (Heap et al 2010;Schaefer et al 2015) but in a much lower amount than previously described for constant amplitude cycles. However, cycles show a hysteresis, increasing as the maximum stress applied rises up (Martin and Chandler 1994;Heap et al 2010;Yang et al 2015).…”

“…Some authors report a lower strength after cycles of unloading/reloading (Kendrick et al 2013;Meng et al 2016), while some others (Yang et al 2015) present example of increasing strength. A particular case where a sample failed for a load lower than the previous applied maximum stress during the same test is even reported in Schaefer et al (2015). The effect of these cycles on the final strength depends on the type of rock, the confining pressure and the number of cycles applied.…”

“…In these studies, such kind of low-cycle test is used to correlate damage with variations in cohesion and mobilised friction. Some other authors investigate the evolution of the Young modulus and Poisson's ratio as representative of damage (Heap et al 2010;Trippetta et al 2013;Schaefer et al 2015;Yang et al 2015) or assess the stress history or a rock sample (Lavrov 2001). Finally, ramp signals are defined by a constant amplitude but an increasing mean stress, as represented in Fig.…”

“…The number of cycles applied is mostly low and varies between 5 and 50 (Eberhardt et al 1999;Heap et al 2010;Trippetta et al 2013;Schaefer et al 2015;Yang et al 2015). The increasing stress from one cycle to the other approaches a monotonic loading, although some differences are noted, especially a slight decrease in strength (Yang et al 2015).…”

Rock Magnetic, Petrography, and Geochemistry Studies of Lava at the Ijen Volcanic Complex (IVC), Banyuwangi, East Java, Indonesia

“…The faster the magma cooling, the smaller the size of the mineral formed. The percentage of porosity is strongly influenced by stress [51]. Little stress in the lava flow causes the porosity of igneous rock to increase.…”