Influence of high stress conditions in hard rock bolting

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School of Engineering |
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The LAGUNA-LBNO project is a pan-European project which consists of the excavation of a certain amount of large-size underground facilities aimed for nucleon decay research purposes. Including several neutrino-physics experiments, the last updates of the project still consider the lowest levels of the Pyhäsalmi Mine (central Finland) for hosting the Liquid Argon (LAr / Glacier) and Liquid Scintillator (LSc / Lena) experiments. At that depth, and taking into account the rock mass characteristics, the rock failure mechanism changes compared the blocky shallow rock masses, therefore, the layout and size of the bolt system must be revised from the traditional methods of assessments. This final project tries to understand the response of the bolts installed in massive brittle rock masses under high stress levels. In this sense, using the Finite Element Method (FEM), the aim is focused to study the behaviour of a representative bolt system in one of the caverns designed for LAGUNA project, as well as how the development of the excavation affect the bolt response. In addition, considering a weaker geological structure, how it could influence the behaviour of the bolt system from the homogenous case. Therefore, two kind of environment were created; one with one entire homogeneous rock mass and another one adding a sub-vertical layer of weaker material in the host rock. In the models the load taken by the bolts never exceeded the 30% of the assumed capacity for the steel rebar, and the pick-up length of those was in average of 8m. The most critical point during the excavation was the achievement of the dome, where a big redistribution of displacements occurs. On the other hand, the excavation of the bench lightly compensates the displacements after the excavation of the dome. By introducing the weaker material into the models, the bolt response showed a tension peak where the footwall interface of the weaker body intercepts the bolt, which is big enough to break both grout and steel. The shear stiffness of the joints is the parameter which strongly influences such behaviour. As a conclusion, the bolts installed in such conditions of stress showed no problem when there are no interfaces in the host rock. In addition, the achievement of the dome was the most critical point in the excavation sequence. Anyway, the near-field failure of the rock mass, related with the plastic areas, does not affect the integrity of the bolts, but the creation of new far-field joints could strongly influence the integrity of the bolt system.
Rinne, Mikael
Thesis advisor
Nuijten, Guido
Ström, Jesse
high stress, large caverns, hard rock, bolting, FEM, brittle rock mass
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