PhD application in Geomechanics at University of Lyon

Context:

Volcanic edifices can lose a huge amount of mass (several cubic kilometers) when their flanks collapse (Ui et al., 2000). Such events typically generate fast-moving debris avalanches (debris flows) of disaggregated rock materials, which can occur suddenly as they are not necessarily initiated by an eruption. Thus, catastrophic collapses of steep volcano flanks threaten many populations (e.g. Mt. Rainier, USA), especially in developing countries. Among the factors that promote such instabilities, the central role of hydrothermal alteration in weakening volcanic edifices is emphasised (Ui et al., 2000; Zimbelman et al., 2005). Hydrothermal alteration results from reactions between volcanic rocks and corrosive hot fluids (gas and water) within a volcano. These reactions lead to the formation/deposition of a suite of secondary minerals that differ from the primary minerals. Recent geotechnical studies (e.g., Heap et al., 2015; Wyering et al., 2015; Mayer et al., 2016) suggest that hydrothermally alteration induces profound changes in the physical and mechanical properties of the rocks, which in turn may reduce the overall strength of the volcanic edifice (Reid et al., 2001; Detienne, 2016).

Objectives:

Describing the effect of hydrothermal alteration on rock physical and mechanical properties is of prime interest in the development of models that can be used to inform volcano stability assessments (Reid et al., 2001). While geotechnical tests produce valuable measurements, the approach remains largely phenomenological. The relation between the macroscopic mechanical behaviour of an altered rock and that of the hydrothermal minerals it contains should be better understood. The latter will strengthen our capacity to build constitutive models that can predict the mechanical behaviour of hydrothermally-altered rocks with known mineralogical composition. This requires a combination of detailed mineralogical characterisations, mechanical strength measurements (at mesoscale and mineral level) and material modelling. Thus, the main objectives of the proposed research are:

(i)   to describe the mechanical characteristics of hydrothermal-alteration minerals (and mineral assemblages);

(ii)  to establish a relationship between these estimates and the macroscopic mechanical behaviour of the rock;

(iii) to model the effect of alteration on volcano flank collapse hazard.

Method:

A selection of rocks, representative of a hydrothermally-altered environment involved in volcano flank collapses, will be done. The

hydrothermally-altered volcanic rocks will be studied by the following steps.

Step 1 – Structure and microscale mechanical properties

Step 2 – Failure modes

Step 3 – Macroscale mechanical properties

Step 4 – Numerical modelling

Application:

The deadline for the CSC scholarships application is before the 30th of March 2021. Applications should be submitted by emailing a CV, academic grades (detail of marks), scientific/academic references, and recommendation letter to the supervisors Benoît PARDOEN and Antonin FABBRI at the email address benoit.pardoen@entpe.fr . Any additional document relevant for the application can also be transmitted. The scholarship of the successful applicant is due to start on the 1st October 2021.

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