Release of the in situ stresses is often the most relevant action in underground projects. Several different field methods are available to measure the in situ state of stress. Some allow the evaluation of the complete state of stress at a given point, while others only supply a single stress component. The paper presents a global methodology for evaluation of the most likely natural stress field from in situ test results. One example deals with the case of an underground powerhouse where high horizontal stresses were determined by overcoring tests, which were later confirmed by flat jack tests performed during the construction of access adits. A second one considers the analysis of a testing programme where overcoring and flat jack tests were both performed during the initial testing programme in different locations. The last one refers to the results of overcoring tests in the vicinity of existing underground caverns, which have to be adequately considered in order to estimate the natural state of stress.
Release of the state of stress is often the most relevant action during the excavation of an underground work, and it can affect the location and orientation of the cavern or tunnel, the design of the support, as well as the construction method used. The large number of factors that influence the in situ stresses in rock masses make of its characterization a difficult task. These factors include lithological and deformability heterogeneities, topography and the existence of nearby excavations, the action of water, the mechanical properties of rocks or even the actions of man. Owing to these factors, the state of stress presents a significant spatial variability and its characterization requires execution of in situ tests using the most appropriate test techniques and a global interpretation model for analysis of the obtained results.