The importance of insitu stress measurements in the prediction of the behavior of underground openings is widely recognized, yet the methods for obtaining reliable values for these stresses have long been the object of controversy. Overcoring methods--besides having generally been limited to holes of tens of meters in length-- have been questioned as to the cause of data scatter, roles of residual stresses, and appropriate scale over which to measure strains. Hydraulic fracturing, the only alternative for deep measurements has gained increased acceptance, yet questions remain particularly in the areas of non-coincidence of the borehole with one of the principal stresses, role of tensile strength in data interpretation, and determination of fracture orientation away from the borehole.
Recently, the Swedish State Power Board has developed a workable method of performing overcoring measurements in holes hundreds of meters in length. With the need to obtain in situ stress values for nuclear waste disposal simulation experiments at the Stripa Mine in central Sweden, we undertook the task of running both the Power Board's Leemah triaxial cell and hydraulic fracturing in the same hole. This paper is a report of the results of the comparative Measurements
The borehole for the stress measurements was located approximately 250m north of the experimental test area at the 348m level of the mine (Figure 1). The borehole and the test facilties are entirely in medium grained granite. The mine had produced iron ore from stratabound layers in leptite (interbedded metasedimentary and metavolcanic rocks of Precambrian age) which had been intruded by the granite. The contact follows the northeast trend of the outcrops in Figure 1, and dips at about forty-five degrees to the southeast. Whereas the iron ore follows the trend of the beds and the contact, the mined out areas are confined roughly to a northeast striking, southeast dipping slab.
The total depth of the stress measurement hole was 381 meters and it was drilled with a 76mm (3 inch) diameter core barrel. Overcoring measurements were carried out in groups of four to five measurements each at roughly one hundred meter intervals in the hole. After completion of most of the drilling, hydraulic fracturing measurements were carried out at approximately 50 meter intervals with additional measurements below 300m at the approximate depth of the test facility.
The Swedish State Power Board stress measurement system is a modification of the Leemah triaxial cell, which measures the complete state of stress from a single borehole. The procedures used in the overcoring by the Swedish State Power Board at Stripa are the same as those described in Hiltscher and others (1979). As with most overcoring techniques, a pilot bore of a few decimeters in length is drilled at the end of the main borehole. A strain cell consisting of three, three-component strain gage rosettes is cemented to the wall of the pilot hole. After a reading of the strain gage outputs, the strain cell is overcored by a drill with a larger diameter bit.