*Professor, Mining Engg Department, National Institute of Technology, Rourkela
**Scientist & Head, Rock Fracture Mechanics Division, National Institute of Rock Mechanices, Kolar Gold Fields
Online published on 15 May, 2014.
Design of any excavation in the ground demands clear perceptive of the stress regime surrounding the infrastructure projects including deep tunnels, power house caverns, transformer hall etc for hydro power generation. Design of stable structures for projects located in complicated geological settings is a challenge to the geotechnical engineers for selecting reliable inputs to any design criteria such as in situ stresses for numerical modeling and its implications on design. In the present paper, an acoustic emission trend in the Kaiser effect of rocks belonging to infrastructure projects was studied and possibility of its application in the estimation of in-situ rock mass stress was explored. Several methods have been proposed for estimating in-situ stress in the laboratory from rock core samples collected from the boreholes. Among them, Acoustic Emission (AE) method based on Kaiser Effect is the simplest. Conventionally, in-situ stress is estimated by plotting cumulative events versus stress. The change in the slope of the curve is an indicator of the Kaiser stress. In the present study, bore hole drilled samples from five sites were investigated in the laboratory by AE method for which the in-situ stress was already measured by hydrofracturing method. A plot of cumulative events versus stress indicated three types of trend and is named as Type 1, 2 & 3. It is easy to identify the Kaiser stress if the trend is of Type 1. Type 2 & 3 trends do not show any perceptible change in the slope of the curve and it is impossible to identify the Kaiser stress. Since most of the samples showed Type 2 & Type 3 trend, to estimate the Kaiser stress, other AE parameters namely ring down count, energy and absolute energy of the events were used. Among these parameters, absolute energy plot showed more significant change in the slope, and hence preferred for identification of the Kaiser stress. The Kaiser stress method shows potential for application to other infrastructure projects and formulation of further strategy for input of various rock parameters in geotechnical design of stable structures.
