Topias Siren

Modelling Fracture Propagation in Anisotropic Rock Mass

Anisotropic rock mass is often encountered in rock engineering, and cannot be simplified as an isotropic problem in numerical models. A good understanding of rock fracturing processes and the ability to predict fracture initiation and propagation in anisotropic rock masses are required for many rock engineering problems. This paper describes the development of the anisotropic function in FRACOD—a specialized fracture propagation modelling software—and its recent applications to rock engineering issues. Rock anisotropy includes strength anisotropy and modulus anisotropy. The level of complexity in developing the anisotropic function for strength anisotropy and modulus anisotropy in FRACOD is significantly different. The strength anisotropy function alone does not require any alteration in the way that FRACOD calculates rock stress and displacement, and therefore is relatively straightforward. The modulus anisotropy function, on the other hand, requires modification of the fundamental equations of stress and displacement in FRACOD, a boundary element code, and hence is more complex and difficult. In actual rock engineering, the strength anisotropy is often considered to be more pronounced and important than the modulus anisotropy, and dominates the stability and failure pattern of the rock mass. The modulus anisotropy will not be considered in this study. This paper discusses work related to the development of the strength anisotropy in FRACOD. The anisotropy function has been tested using numerical examples. The predicted failure surfaces are mostly along the weakest planes. Predictive modelling of the Posiva’s Olkiluoto Spalling Experiment was made. The model suggests that spalling is very sensitive to the direction of anisotropy. Recent observations from the in situ experiment showed that shear fractures rather than tensile fractures occur in the holes. According to the simulation, the maximum tensile stress is well below the tensile strength, but the maximum shear stress is probably enough to displace mica contact.

Elastoplastic Modelling of an In Situ Concrete Spalling Experiment using the Ottosen Failure Criterion

An in situ concrete spalling experiment will be carried out in the ONKALO rock characterization facility. The purpose is to establish the failure strength of a thin concrete liner on prestressed rock surface, when the stress states in both rock and concrete are increased by heating. A cylindrical hole 1.5 m in diameter and 7.2 m in depth is reinforced with a 40 mm thin concrete liner from level −3 m down. Eight 6 m long 4 kW electrical heaters are installed around the hole 1 m away. The experiment setup is described and results from predictive numerical modelling are shown. Elastoplastic modelling using the Ottosen failure criterion predicts damage initiation on week 5 and the concrete ultimate strain limit of 0.0035 is exceeded on week 10. The support pressure generated by the liner is 3.2 MPa and the tangential stress of rock is reduced by −33%. In 2D fracture mechanical simulations, the support pressure is 3 MPa and small localized damage occurs after week 3 and damage process slowly continues during week 9 of the heating period. In conclusion, external heating is a potent way of inducing damage and thin concrete liner significantly reduces the amount of damage.