Time-dependent mechanical behaviour of rocks around underground openings: Implications of long-term stability due to delay fracture or ongoing deformation (self-healing)

OVERVIEW
This project will be of interest to someone looking for a multidisciplinary project involving fieldwork, laboratory analysis and numerical modelling in the broad areas of rock mechanics/engineering and tunnelling, engineering geology and geomechanical modelling. The research will place the successful candidate in an ideal position to gain future employment in industry or academia.

SCIENTIFIC BACKGROUND
Different rocks and rock masses respond in different ways over time. The main factor that controls their behaviour is geology. The mineralogical content and the geological structure impact the mechanical behaviour of rocks. The stress regime and the environmental conditions also influence the rock materials’ behaviour. The main separation between rock types according to their stress-strain reaction is into either brittle or ductile materials. Brittle materials have the tendency to abruptly fail as the stress approaches their short-term strength and as such they absorb less energy whereas ductile materials can sustain an applied stress state through more deformation. When ductile materials (i.e. rock salt or potash) are subjected to a constant differential stress below their nominal yield strength they can behave as visco-elastic materials and further deform as time elapses. In contrast, brittle materials (i.e. granite or limestone) under similar stress conditions may only exhibit micro crack damage with progressive crack propagation with time that results in the eventual interaction of the previously isolated microcracks which leads to sudden failure. Being able to predict how the material will behave over time can be of paramount importance especially in underground excavations. There is not a simple model that can be utilized to describe all time-dependent phenomena for all rock types. Even in the case that such a model did exist, the use of it most likely would be found impractical and complex. Most sophisticated theories describing time-dependent phenomena require numerous input parameters. The need to create simple practical tools and models that can find use not only in the field of research but also in industry is of utmost importance. However, such an observation should not be an impediment in considering time in future analyses. One could still use existing models that can fairly simulate time-dependent behaviour of different rock types. However, careful attention should be paid in selecting the appropriate time-dependent constitutive model and input parameters and it is suggested to be derived from laboratory tests. Although noticeable research has been made for assessing the presence and extend of excavation damaged zones in tunnelling over time there is limited knowledge on borehole stabilities in deep environments.

AIMS AND OBJECTIVES
The main aim of the project will be to assess the time-dependent mechanical properties of a variety of rock materials (i.e. creep) as this will ultimately control the long-term behaviour. The objectives include:-
• Gaining understanding of how the time-dependent mechanisms can affect the behaviour on a range of time-scales.
• Improving understanding of the mechanical properties of a range of rock samples as a function of the stress conditions and time.
• Developing constitutive relationships to describe the mechanical behaviour of rocks.
• Assessing the implications of these results for issues such as closure of fractures, long-term stability of underground openings and abandonment.
These objectives will be met with the help of state-of-the art laboratory facilities and numerical modelling software.