In situ stress determination has always been a difficult problem to solve in rock engineering projects. Several methods have been developed for this purpose with advantages and drawbacks for each. Undercoring technique, which is a method classified under stress relief technique, has not been used extensively due to some of its limitations. However, in certain situations it has its own merits and can be used easily with very low cost. The present paper describes a laboratory investigation in which a concrete block is stressed with a material testing system (MTS) and undercoring technique has been utilized to determine the stresses and to compare the theoretical calculations with the actual applied stresses. In this research, a sensitivity analysis is performed and the analytical formulations are also developed for probable deviations in performing the test compared with the theoretical procedure and the sensitivity of the obtained results to this deviation is determined.
Knowledge about in situ stress field in most of rock engineering projects is critical information which can have considerable influence on a safe and economical design. Therefore a lot of research has been spent on stress measurement methods one of which is known as "Under Coring Method". This is based on the "Stress Relief" technique and is performed at surface of the rock mass under study. For the first time, Duvall in 1974 proposed this method which can be used to determine 2D stress field on a flat surface. At the location of interest, three sets of pins are located around a circle with 10 inch diameter along three 60 degree diagonals and the distance between pins on one diagonal is measured with one micron accuracy (Figure 1). At the next stage a 6 inch hole is drilled at the center of this arrangement.