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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 April 1976 issue and is the second part of a paper published in GE’s March 1976 issue.

Part 1 and part 2 of this series were also split into two parts and were published in GE’s May, July, September and November 1975 issues


Shot term acceptance tests on all production anchors highlight potential difficulties pertaining to service behaviour and provide measured safety factors related to the design working load. These tests are associated with the initial stressing operations and normally include quality control observations over a period of up to 24 hours.

As a first priority, the testing procedure must yield a measured safety factor as determined by overloading for a short period. Such overloads, however, must be compatible with the allowable stresses and safety factors permitted in the country concerned. The relevant details are discussed in Part 1—Design (Table XV), and these suggest an encouraging trend towards standard safety factors throughout the world at the present time.

To check the measured performance against that predicted by calculation, it is essential that a load-extension graph be plotted for each anchor, in the manner discussed in Part 3—Stressing.

In addition, an attempt should be made on either preliminary test anchors or on early production anchors to obtain an indication of fixed anchor movement, since this information allows the analyst to assess a component of permanent displacement which in turn permits a reasonable estimate of the degree of debonding, if any.

Finally, it is necessary to ensure that the service load locked-off after stressing is stable. The alternative methods employed in practice are monitoring loss of prestress with time, and monitoring creep displacement of the anchor with time. 

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