Micromechanics of brittle faulting and cataclastic flow in Alban Hills tuff

Abstract

[1] An understanding of how tuff deforms and fails is of importance in the mechanics of volcanic eruption as well as geotechnical and seismic applications related to the integrity of tuff structures and repositories. Previous rock mechanics studies have focused on the brittle strength. We conducted mechanical tests on nominally dry and water-saturated tuff samples retrieved from the Colli Albani drilling project, in conjunction with systematic microstructural observations on the deformed samples so as to elucidate the micromechanics of brittle failure and inelastic compaction. The phenomenological behavior was observed to be qualitatively similar to that in a porous sedimentary rock. Synthesizing published data, we observe a systematic trend for both uniaxial compressive strength and pore collapse pressure of nonwelded tuff to decrease with increasing porosity. To interpret the compaction behavior in tuff, we extended the cataclastic pore collapse model originally formulated for a porous carbonate rock to a dual porosity medium made up of macropores and micropores or microcracks.

Empirical relations of rock properties of outcrop and core samples from the Northwest German Basin for geothermal drilling

“…It would then be possible to compare the calculated UCS values with the actual UCS values measured with cores of the same wellbore (cf., Vogt et al, 2012). The estimation of rock strength is not only possible with empirical relations as presented in this study but also with micromechanical methods (e.g, Sammis and Ashby, 1986;Zhu et al, 2011), which are powerful tools to understand failure processes in rock. To build a geomechanical model before starting the drilling operation, such micromechanical methods may be a good supplemental option when using data from adjacent wellbores.…”

Inelastic compaction and permeability evolution in volcanic rock

“…Zhu et al, 2010Zhu et al, , 2011, it has also been demonstrated that the local stress field around a pore increases as a function of the incumbent confining pressure (Zhu et al, 2010). A study of fault gouge formation in sandstone (Engelder, 1974) shows that fault-zone fragments are smaller when generated at higher confining pressures, and a similar effect (albeit less pronounced) was noted by Kennedy et al (2012), who investigated fault gouge formation in dacitic dome rock.…”

“…The underlying micromechanical mechanism driving inelastic compaction in volcanic rocks has been shown to be cataclastic pore collapse (Zhu et al, 2011;Heap et al, 2015a;Zhu et al, 2016). Figure 4 illustrates this process by showing images of an intact and a deformed sample.…”

“…In Yakuno basalt, Shimada et al (1989) showed that ductile deformation was associated with distributed microcracking, granulation, and pore occlusion. The underlying physical mechanism -cataclastic pore collapse -is described in detail by Zhu et al (2010) and has since been interpreted in triaxially deformed tuff (Zhu et al, 2011;Alam et al, 2014;Heap et al, 2015b), basalt (Adelinet et al, 2013;Zhu et al, 2016), andesite (Heap et al, 2015a, trachyandesite (Loaiza et al, 2012), and dacite . This micromechanism can be distributed (e.g.…”

“…This micromechanism can be distributed (e.g. Zhu et al, 2011;Heap et al, 2015b) or localised (e.g. Loaiza et al, 2012;Adelinet et al, 2013;Heap et al, 2015a) in the form of bands of compacted pores.…”