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Technical paper: Rock anchors – state of the art – Part 2: Construction

G S Littlejohn, BSc, PhD, MICE, MIStructE, FGS, and D A Bruce, BSc, AMICE, FGS. This paper was first published in GE’s September 1975 edition.

Irrespective of the care and conservatism applied to the designs of an anchor system, thoughtless or careless constructional procedures can cause rock anchors to fail at very low loads. The majority of failures seem to be related to the grouting stage although some bond failures have clearly been due to poor tendon preparation. On a few occasions the drilling and flushing techniques may have been incorrect. Fortunately, failures have not occurred too often and these have usually been highlighted at the stressing and testing stage.

It is significant that although the technology of drilling and grouting can be highly complex, site techniques on the whole are left to skilled and experienced specialists, and close on-site inspection by supervising engineers has been relatively uncommon to date. Thus, rock anchoring after 40 years is still regarded as an art. Whilst it is appreciated that the highly variable ground conditions encountered in practice, giving rise to a large number of construction techniques, add to the mystique of anchoring, nevertheless it seems that the time is overdue for certain guidelines on construction practice to be presented for consideration by civil engineers.

The second part of this review discusses anchor construction techniques related to drilling, flushing, water testing, tendon preparation and installation, grouting and finally corrosion protection. Since anchor construction is sensitive to poor workmanship emphasis is placed on quality control and close on-site supervision.

Aspects of anchor stressing and testing will be reviewed in the third and concluding part of this series of articles. 

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