It is now well accepted that even when stressed to levels significantly below their theoretical elastic (non-linear) limits rocks can accumulate damage that reduces the strength/stiffness of the material, and this effect is cumulative. This behaviour has been observed in many different rock types including volcanic material, but has been mainly observed in the laboratory and applied to engineering applications such as tunnel construction. The effect of such repetitive dyke intrusion causes strain and elevated stresses within the rock mass from both mechanical and thermal processes. If these stresses are high enough these increases would potentially damage the surrounding rock mass. The effect of this weakening in a natural setting, and in particular one applying to volcanic materials is one that is still poorly studied.
Given the likely damage that can occur in a rock or rock mass following repetitive loading from dyke intrusion the question is whether the accumulated damage from multiple events reaches a threshold where this damage is so great that the next dyke causes failure of the rock mass and results in an eruption rather than another dyke intrusion.
As such, there are three main objectives in this project.
- To quantify the mechanical and thermal load experienced by the surrounding rock mass during dyke intrusion
- To analyse through laboratory experiments the amount of damage experienced by volcanic material through repeated dyke intrusion
- Evaluate the role that the progressive mechanical weakening plays in initiating the timing of an eruption.
The project objectives will be achieved through a combination of field, laboratory and Numerical Modelling work.
The first stage of the project will be a thorough investigation of field sites for suitability with likely candidates being The Teno Massife on the Island of Tenerife in the Canary Islands, The island of Pico in the Azores and areas of Iceland. While all of each of these areas have easily accessible field outcrops where field measurements can be undertaken to quantify the amount of strain experienced by the rock mass during dyke intrusions and to acquire samples for testing in the laboratory final decisions on the field area will be made with respect to travel and access permissions closer to the commencement of the project.
The second stage of the project will involve testing the acquired samples using a series of creep and relaxation experiments utilising the CONTROLS stress path triaxial and uniaxial fully automatic test system within the Rock Mechanics Engineering Geology and Geotechnical Laboratory (RMEGG). Using this system, tests can be performed with combined control of axial load and confining pressure (simulating conditions during dyke intrusions) that will allow the entire stress path, including the post load phase to be accurately recorded. Tests will be conducted in both a staged manner, where a sample is taken through the complete sequence of loading and unloading, simulating multiple dyke instructions in a single test, and also where each simulated dyking event is applied separately to the sample with a significant period of time (weeks to months) between each test. This will allow the effect of the time interval between dyke intrusions to be considered, but in addition will also provide general insight into the phenomenon of strength recovery in rocks and whether this only happens to a significant extent when substantial fluid-rock interaction is present.
The third stage will involve incorporating the data obtained from the laboratory testing into numerical models of dyke propagation through the rock mass to assess whether there is a damage threshold, and hence reduced mechanical strength/stiffness that allows dykes to propagate to the surface and ultimately lead to an eruption.
Bruning, T., Karakus, M., Nguyen, G.D. et al. Experimental Study on the Damage Evolution of Brittle Rock Under Triaxial Confinement with Full Circumferential Strain Control. Rock Mech Rock Eng 51, 3321–3341 (2018). https://doi.org/10.1007/s00603-018-1537-7
The three stages of stress relaxation – Observations for the time-dependent behaviour of brittle rocks based on laboratory testing. Engineering Geology, 216, 56-75 (2017). https://doi.org/10.1016/j.enggeo.2016.11.010
Schaefer, L.N., Kennedy, B. M., Kendrick J.E. et al. 54th U.S. Rock Mechanics/Geomechanics Symposium 2020, Online (28 June 2020 – 1 July 2020) Paper Number: ARMA-2020-1876.
Shirani Faradonbeh, R., Taheri, A. & Karakus, M. Failure Behaviour of a Sandstone Subjected to the Systematic Cyclic Loading: Insights from the Double-Criteria Damage-Controlled Test Method. Rock Mech Rock Eng 54, 5555–5575 (2021). https://doi.org/10.1007/s00603-021-02553-5