The under coring method is one of the techniques of stress measurement which is used to determine the two dimensional stress state on a flat surface. In this paper, this method is modified by adding three extra directions for measuring the deformation around the borehole. Actually, there is only one state of stress, but it can be determined with different accuracy by different combinations of measured directions. In the usual UC test, just one determination of the state of stress can be obtained as there are three diagonals and three unknowns, but this modification increases the determinations of stress state to 20, so the average of them would be more reliable. The results of this modified approach were assessed by a laboratory test setup, and also its capability was investigated in a field study. The results are in a good agreement with the available data. In addition, using those extra directions can considerably reduce the errors that can occur in conventional under coring method.
Determination of in situ stress is of great importance in geomechanical engineering, as they are influential information which should be considered in all steps of the process of designing. There are some methods to measure these stresses. Some of them act within a bore-hole, and the others are applied on a flat surface. Among the latter ones Under Coring Method (UC) is an approach which is based on "Stress Relief" technique. This method was proposed by Duvall in 1974 which can be implemented 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 (Fig. 1).