To evaluate the stress concentration effects caused by an underground opening in rock, it is necessary to estimate the three-dimensional in situ or pre-excavation stress field. The stress concentrations acting on an excavation influence the deformation of the walls of the excavation. In situ test are therefore necessary to determine the magnitude and orientation of the principal stresses in rock prior to excavation. Models used in laboratory tests do not and generally cannot simulate the geologic factors such as bedding, fractures, joints, faults,, folds, and macroscopic inhomogeneities.
Among the procedures available for determining in situ stresses in rocks, the overcoring method attempts to determine the three-dimensional stress field in one operation. In this method the diametral Changes are measured in the plane of deformation during the overcoring operation. By making simplified assumptions regarding the in situ state of stress, and by applying equations of linear elasticity, the magnitude and directions of the major and minor principal stresses in the Plane of deformation can be determined. Determining the magnitudes and directions of the principal stresses requires three deformation measurements of the diametral Changes of a hole in the rock as well as the determination of the elastic modulus of the rock under consideration. This paper describes the in situ atress measurements Performed at Hartford, Connecticut, along the alignment of a water diversion tunnel. Some of the unique features of this project are:
in situ stress tests were successfully performed to a depth of 155 feet using standard overcoring procedures;
some modifications were made to the standard USBM gage and their advantages and limitations were studied; '
some modifications to the drilling procedures and equipment were made and their effects were evaluated and documented; and
a comparative study was attempted to evaluate the performances of the standard USBM gage, the modified USBM gage, and the Terrametrics three-axis borehole deformation gage.
The main purpose of the investigation was to demonstrate the feasibility of performing in situ stress tests at greater depths than usually attempted in the past. With certain modifications to the borehole deformation gage, the equipment used for performing the field test, and the testing and drilling procedures, tests were successfully performed at a depth of 155 feet. Because of the highly jointed and fractured nature of the rocks at the site, a number of tests could not be carried to completion. There were some instances of instrumentation malfunction. However, at the end of the project it was felt that with minimal modification to the borehole deformation gage and the overcoring procedures, in situ stress tests could be performed at depths greater than 150 feet, possibly up to a depth of 200 feet.
All existing information on the exploratory borings previously drilled along the alignment of the auxiliary conduit was reviewed. The boring logs and the geologic profile along the central line of the conduit were used to establish prospective test locations.
