ABSTRACT

ABSTRACT:

A noncircular tunnel was excavated in massive granite. The tunnel geometry and orientation relative to the stress field had been selected to minimize the potential for the development of an excavationinduced damage zone. The extent and characteristics of this excavation damage zone was measured using velocity profiling and permeability measurements in radial boreholes. The results showed the distribution of damage was contained in a zone less than 1-m thick adjacent to the excavation boundary and that the damage was not uniformly distributed around the excavation. A damage model was formulated in a discrete element code and calibrated to laboratory properties. The model was then used to analyze the noncircular tunnel. The development of the damage zone in the numerical model was found to be in good agreement with the field measurements.

1 INTRODUCTION

It is well known that the creation of an underground excavation disturbs the stress field. When the stress magnitudes on the boundary of the excavation exceed the rock mass strength yielding occurs and an excavation damage zone (EDZ) can be readily observed (Martin et al. 1997). However, in many situations the stress magnitudes may not exceed the rock mass strength and in such cases it is not obvious if an EDZ has formed or not. Tsang et al. (2005) noted that an EDZ will form around all underground openings and that in crystalline rock the dominant processes are micro and macro fracturing. These processes can lead to significant increases in flow and transport properties. Hence, quantifying the extent and characteristics of the EDZ is important for the nuclear waste industry which relies on the sealing of underground openings to minimize the risk for radionuclide transport. It is well known that the presence of cracks affects the strength and mechanical properties of a rock.

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