ABSTRACT: An acoustic emission (AE) technique has been applied as a diagnostic method for grouted rock anchors subjected to uplift loads, in order to characterize their major failure mechanisms. The laboratory tests with limestone consisted of three AE generating mechanisms that included debonding, crack propagation and non-elastic deformation. Signals resulted from extraction of model bars grouted in limestone NX core samples were employed to classify AE time histories. These observations were then extended to bar extraction from 300 x 300 x 100 mm blocks of the same rock. Debonding dominated behaviour and there was an almost complete lack of crack propagation.
1 INTRODUCTION Grouted and expoxied rock anchors, generally subjected to tensile loads, are increasingly used as structural elements to ensure stability of rock masses. Moreover, current construction techniques and the development of new products (high-performance grouts and expoxys, high-strength tendons, enhanced injection techniques and drilling equipment) continually expand the maximum strength limits for grouted rock anchors. Accordingly, grouted rock anchors with an ultimate load capacity of more than 12.5 MN have been used to increase the height of dams (Natau and Wullschäger). Design of grouted rock anchors is assumed to be well-known (Littlejohn and Bruce, 1975a, b; Ballivy et Martin, 1984; Benmokrane, 1990). For instance, the bonding stresses are considered distributed exponentially for anchors grouted in hard rocks (Hollingshead, 1971; Farmer, 1975; Benmokrane, 1990) while the distribution is considered uniform in the case fo soft ones (Coates and Yu, 1971; Hobst and Jajic, 1983). To date, three approaches have been used to increase the design loads for grouted anchors: 1) analytical models; 2) finite element models and 3) instrumentation (strain gauges). Analytical and finite element models appeared to be very useful in determining ideal grouted anchor behaviour (Coates and Yu, 1970; Hollingshead, 1971; Yap and Rodger, 1984; Benmokrane, 1990). Unfortunately, in real applications, however, grouts and rocks are characterized by many structural defects that can lead to premature failure are unaccounted for in typical finite element analyses. Until now, grouted rock anchors could be tested only by destructive tests using hydraulic jacks. Usually, these tests show four elementary failure modes: a.bond failure (debonding) at the rock/grout interface; b.bond failure at the tendon/grout interface; c. failure of the rock mass; and.d. failure of the steel tendon. Their "in situ" applications, however, have not gained popularity among civil and mining engineers. Using an acoustic emission technique, Kobayashi et al., (1980) established a correlation between acoustic activity and breakdown of the bond between reinforcing bars and concrete. They concluded that acoustic emission can help in understanding and identifying major failure modes.
2 METHOD OF INVESTIGATION For most materials, it is generally accepted (Dunegan and Green, 1972; Rotem, 1986) that an acoustic source generates a specific signal in the time domain. While each time history can be characterized by five features (amplitude, number of counts, energy, duration, and rise time (Fig. I)). Different failure mechanism will produce different wave forms which in turn will produce different time histories.