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.
Highlights
Several studies of non-volcanic slopes using numerical models and geomechanical material properties have been adapted for the investigation of volcanic collapses, slope stability, and ground deformation
We evaluated the mechanical properties of rocks at Pacaya Volcano (Guatemala) using a variety of laboratory experiments, including room temperature, high temperature, and cyclicallyloaded uniaxial compressive strength tests (UCS) on as-collected and thermally treated rock samples
Uniaxial Compressive Strength Results Here we present the results of 64 UCS experiments and the effects of (1) strain rate, (2) porosity, (3) cyclic loading, (4) thermal stressing, and (5) temperature on the strength of the tested rocks
Summary
Several studies of non-volcanic slopes using numerical models and geomechanical material properties have been adapted for the investigation of volcanic collapses, slope stability, and ground deformation. Recently have laboratory studies begun to systematically investigate the microstructural, physical, and mechanical properties of volcanic rocks, a material infamously known for its formation under disequilibrium conditions, thereby rich in heterogeneities at all scales These include experiments on a range of material under relevant volcanic conditions such as thermal stressing (Vinciguerra et al, 2005; Kendrick et al, 2013a; Heap et al, 2014a), cyclic inflation-deflation cycles by intrusions (Heap et al, 2009, 2010; Kendrick et al, 2013a), fragmentation (Spieler et al, 2004; Kueppers et al, 2006; Scheu et al, 2008), and flow or fracture at high temperatures and/or pressures (Balme et al, 2004; Rocchi et al, 2004; Smith et al, 2005; Lavallée et al, 2007, 2008; Benson et al, 2008; Cordonnier et al, 2009; Loaiza et al, 2012; Kendrick et al, 2013b)
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