ROCK ANCHORS-STATE OF THE ART. PART 1: DESIGN

This article, part 1 of a 3 part series, describes current practices in the design of rock anchors. Present design criteria are classified into two broad groups: (1) procedures based on the classical theory of elasticity, and (2) methods based on trial and error. The main design concepts are reviewed with respect to the following 4 failure modes: (a) failure within the rock mass; (b) failure of the rock/grout bond; (c) failure of the grout/tendon bond; and (d) failure of the steel tendon or top anchorage. In considering methods currently used in practice to estimate the anchor depth required to ensure that the working load will be resisted safely without failure occurring in the rock mass, the methods described apply to anchors which have been constructed in a vertical or steeply inclined downwards direction. It is found that no experimental or practical evidence and only very little theoretical data substantiate the methods currently used to calculate the ultimate resistance to pull-out of individual or groups of anchors. In considering the bond between cement grout and rock, design criteria are reviewed relating to the magnitude and distribution of bond, fixed anchor dimensions, and factors of safety. For comparison, the results of relevant theoretical and experimental investigations are presented. It is found that the distribution of the bond mobilized at the rock/grout interface is unlikely to be uniform unless the rock is "soft". It appears that non-uniformity applies to most rocks where E-grout/E-rock is less than 10. In the case of high capacity anchors evidence exists that partial debonding in the fixed anchor occurs, and that debonding progresses towards the end of the anchor as the load is increased. Information is scarce concerning the conditions where debonding is serious. It is recommended that the following geotechnical properties should be evaluated during the site investigation stage, in addition to the conventional descriptions of lithology and petrography: quantitative data on the nature, orientation, frequency and roughness of the major rock mass discontinuities; shear strength of these discontinuities; and compressive and shear strength of the rock meateral. Also, weatherability and durability should be assessed, and the ground water regime is of prime importance. (The discussion of design with respect to failure modes c and d is found in the next issue of Ground Engineering.)

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  • Corporate Authors:

    Foundation Publications Limited

    7 Ongar Road
    Brentwood CM15 9AU, Essex,   England 
  • Authors:
    • Littlejohn, G S
    • Bruce, D A
  • Publication Date: 1975-5

Media Info

  • Features: Figures; Tables;
  • Pagination: p. 25-32
  • Serial:
    • GROUND ENGINEERING
    • Volume: 8
    • Issue Number: 3
    • Publisher: EMAP CONSTRUCT LIMITED
    • ISSN: 0017-4653

Subject/Index Terms

Filing Info

  • Accession Number: 00098999
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Oct 18 1975 12:00AM