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Technical paper: Rock anchors - state of the art Part 3 - stressing and testing

By G S Littlejohn and D A Bruce, University of Aberdeen

This paper was first published in GE’s March 1976 issue.

The first of this three-part series dealt with the design of rock anchors. Because of length considerations it was divided into two sections which appeared in the May 1975 and July 1975 issues. For the same reason the second part, on construction aspects, was also split, with the first section published in September 1975 covering drilling, water testing and waterproofing and tendon work. The second section, in November 1975 was devoted to grouts and groutrng, corrosion and corrosion protection. The third and final part, on stressing and testing, will again appear in two sections, this first dealing with stressing and precontract component testing. The second section, appeared in April 1976, and included details on acceptance testing and long-term monitoring of anchors and the overall anchor/rock/ structure system. 

Prestressing an anchor automatically tests the installation, confirms to a certain degree design safety factors, and ensures satisfactory service performance. This is equally true for prestressed anchors and those subsequently intended to act as “passive” untensioned members, and in both cases an initial stress history often enhances subsequent behaviour.

In addition, acceptance criteria based on standardised tests gauge the suitability and effectiveness of the installed anchor with respect to the intended application. Possible errors made in either the design or construction stages will be pinpointed immediately and potentially dangerous and expensive consequences avoided.

Incorporating these important precepts, this third part of the rock anchor review describes anchor stressing techniques, the monitoring and presentation of data, and provides guidance on the interpretation of stressing results. This basic information is intrinsic to anchor testing.

The authors believe that a standard approach to the testing and analysis of anchor behaviour should be established, relating to both short and long-term behaviour. 

Accordingly, the following basic types of test and quality control are recommended for consideration, and are described in detail:

  1. precontract component testing,
  2. acceptance testing of production anchors,
  3. long term monitoring of selected production anchors,
  4. special test anchors, and
  5. monitoring of the overall anchor/rock/ structure system.

A final section deals with aspects of long-term service performance, and reviews the relatively small number of case studies published to date. These highlight various parameters and phenomena which influence anchor behaviour in the long term. 

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