Evaluation of Crack Extension with Stress Relief in Rock Mass surrounding Underground Cavern by means of the Ratio of Crack Opening Dependency.

Evaluation of crack extension in the stress relief process is successfully performed, using a new index called the Ratio of Crack Opening Dependency (RCOD), that is theoretically in inverse proportion to the normalized Young's modulus of rock mass, and by applying the fracture mechanics. Cracks with stress relieving are classified into the primary crack of microscopic, which plays an important role for determining the effective Young's modulus in the first stage of stress relief, and the secondary crack of macroscopic, the excessive propagation of which results in forming of plate structure within rock mass and subsequent the strength failure and strain softening of rock mass. The present paper focuses on the evaluation of the secondary crack extension by means of RCOD. Theory and practice of quantitative crack evaluation is demonstrated, along with the analysis of the field data obtained in the construction of two underground power houses. Subsequently the application of fracture mechanics is discussed to forecast the secondary crack extension, along with case examples. The increase of rock strain with the secondary crack extension is successfully forecasted, using the intensity factor and the normalized compliance of the homogenized multi-crack model in an alternative arrangement.