Five-year monitoring of load losses on prestressed cement-grouted rock anchors

Abstract

The long-term study of the behaviour of prestressed grouted rock anchors constitutes an important aspect of the stability of the structure retained by these structural members. The loads supplied by the anchors must be sufficient so that at all times the stability of the structure is not compromised. The results of an experimental study of nine anchors grouted to various lengths with a cement grout, on site, in a sound rock mass are described. Tension was applied to the anchors which were then blocked at initial loads of about 80% of failure load. During 5 years, the load variations in each anchor were recorded with load cells. It was observed that the load variations versus time make up two phases: phase I, lasting about 6 months, throughout which the losses of load are rapid; and phase II, which is marked by a slower and uniform rate of load loss. In this experimental research, the instantaneous losses of load that occurred during the stressing and the lock-off operation also have been measured. The study shows that it is unnecessary to increase the anchored length to reduce long-term load losses. Key words: grouted rock anchor, prestressed anchor, cement grout, massive rock, load loss, creep, relaxation, long-term behaviour.