In this paper we evaluate the possibility of the acoustic emission (AE) technique to measure in situ vertical stresses around an excavated drift using the Kaiser effect, which is characterized by an increase in AE when escalating stress exceeds the previous maximum stress. Rock cores were obtained from horizontal boreholes drilled into the wall of a drift excavated in soft sedimentary rock. This allowed the vertical stress variation with distance from the drift wall to be determined up to a depth of 5 m. The vertical stresses determined using rock cores varied with distance and reached a maximum of 4 – 5 MPa at a distance of 2 – 3 m from the drift, when the highest concentration of stress was expected. The vertical stress variation was consistent with calculations by the three dimensional FEM program, ANSYS. Furthermore, the stress measured by the over coring method just about agreed with the stress determined by the AE and DRA methods. The AE and DRA methods described in this paper should be applicable to in situ stress measurement with reasonable accuracy even in soft sedimentary rock. A delay time of up to 101 days did not affect the determination of in situ stress in soft sedimentary rock.
The reliable evaluation of in situ stress is important in the analysis and design of underground excavations, particularly for evaluating the stability of underground structures to prevent failure or collapse of underground openings. Although a number of techniques have been proposed and developed to determine in situ stress, the task is not easy and all methods suffer from deficiencies and limitations. The main deficiencies of established techniques, such as the over coring and hydraulic fracturing methods, is that they are usually expensive and time consuming.