Rock Anchors-State of the Art Part 1: Design
Total Page：16
File Type：pdf, Size：1020Kb
Roc~anciors-s:a:eo':iear: ~ar:: 3esicn'y G. S. LITTLEJOHN", BSc, PhD, MICE, MIStructE, FGS, and D. A. BRUCE", BSc,AMICE, FGS BOND BETWEEN CEMENT GROUT 3. Mechanical interlock. This is similar to e.g. 1.0mm off-set, 40mm pitch, a bond AND STEEL TENDON micro mechanical locking, but on a much length of 65 diameters may be assumed for larger scale, as the shear strength of the the above conditions. Introduction grout is mobilised against major tendon (iii) Galvanised wire provides a poor Little attention has been to this paid irregularities, e.g. ribs, twists. bond, less than half that of comparable of rock anchor aspect design, principally An idealised representation of these plain wire. because engineers usually consider that three major bond components is shown in It may be assumed that 80 cent the fixed anchor length chosen with re- (iv) per Fig. 14. For short embedment lengths the of the maximum stress is developed in a spect to the rock/grout bond ensures adhesive component is most important, but of 70 diameters for the conditions more than adequate tendon embedment length mentioned in and in a length of 54 dia- length. (i) Lu meters for the conditions mentioned in However, as has been demonstrated in (ii). Strand the section dealing with rock/grout bondfi, little standardisation or uniformity of ap- l- R (i) From the available experimental data, Lrt proach is apparent related to the grout/ AL INTERLOCK) the transmission length for small diameter tn ordinary strand is not proportional the tendon bond, and the rather simple design Lu to LZ diameter the assumptions commonly made are in contra- of tendon. Table Vll gives diction to certain experimental observa- values of transmission length for strand tions. working at an initial stress of 70 per cent o ultimate in In this section, the mechanisms of bond ADHESION concrete of strength 34.5-48.3 are discussed and anchor design pro- N/mm'. cedures employed in practice are re- SLIP TABLE VII TRANSMISSION LENGTHS viewed. Bearing in mind the scarcity of — Idealised representation major information pertaining to anchors, data Fig. 14. of FOR SMALL DIAMETER STRAND components of bond abstracted largely from the fields of re- Transmission length inforced and prestressed concrete are also Diameter of are also presented, which relate to the for longer lengths, all three may operate— strand (mm j (mm) (diameters failure initially the j magnitude and distribution of bond. adhesion occurring at proximal end and then moving progres- 9.3 200 (m25) 19-24 sively distally to be replaced by friction The mechanisms of bond and/or mechanical interlock. Frictional and 12.5 330 (~25) 25-28 It is widely accepted that there are three lat- mechanisms: interlocking resistances increase with 18.0 500 (~50) 25-31 eral compression and decrease with lateral 1. Adhesion. This provides the initial tension. the shear strength N.B.—Range of results given in brackets. "bond" before slip, and arises mainly from Clearly, grout and the nature of the tendon surface, both the physical interlocking (i.e. gluing) of the micro-and macroscopically, are major fac- (ii) Tests in concrete of strength 41.4- microscopically rough steel and the sur- tors in determining bond characteristics. 48.3N/mm'ith Dyform compact strand rounding grout (Fig. 13). Molecular attrac- at 70 per cent ultimate show an average Fixed anchor design transmission length of 30-36 diameters. CEMENT GROUT It is common in practice to find embed- According to the results of an FIP ment lengths for bars, wires and strands questionnaire (1974) national specifica- quoted as equivalent to a certain number tions vary considerably for transmission of diameters, as this method ensures a lengths, the most optimistic being those of maximum value of apparent average bond the United Kingdom. It is accepted that stress for each type of tendon. The trans- compact strand e.g. Dyform, has transmis- STEEL BAR OR WIRE TENDON mission length is the length required to sion lengths 25 per cent greater than those for normal 7-wire strand, and that sudden Magnified view of interface transmit the initial prestressing force in a Fig. 13. con- release of load also increases the trans- between grout and steel tendon to the surrounding grout or crete. mission length. (An additional 25 per cent In Britain, the following general recom- is recommended in Rumania). tion is also thought to act. Adhesion is con- mendations may be followed, based on Bar sidered to disappear when slip comparable CP 110, 1972 and information supplied by (i) With regard to permissible bond with the size of the micro indentations on Bridon Wire Ltd (1968). stresses for single plain and deformed bars the steel occurs. Wire in concrete, Table Vill illustrates the 2. Friction. This component depends on (i) For a bright or rusted, plain or inden- values stipulated by the British Code for confining pressure, the surface charac- the ted wire with a small off-set crimp e.g. different grades of concrete. These values and the amount of teristics of the steel, 0.3mm off-set, 40mm pitch, a transmission are applied to neat cement grouts on occa- slip, but is largely independent of the mag- length of 100 diameters may be assumed sions. nitude of the tendon stress. The pheno- when the cube strength of the concrete or (ii) For a group of bars, the effective mena of dilatancy and wedge action also grout at transfer is not less than 35N/mm'. perimeter of the individual bars is multi- contribute to this frictional resistance as (ii) For a wire of a considerable crimp plied by the reduction factors overleaf. radial strains are mobilised where the lofrg- itudinal strain changes. TABLE VIII—ULTIMATE ANCHORAGE BOND STRESSES s Geotechnics Research Group, Engineering Dept. Characteristic strength of concrete N/mms) Marischal College, University of Aberdeen (fr„, Ii This is the second section of this article on rock Type of anchor design; the first which appeared in our 20 25 30 Msy issue, pp 25-32, gave an introduction to the bar subject and dealt with uplift capacity of. the Maximum bond stress, rock anchor system, and the bond between N/mm'.4 cement grout and rock. These two articles re- present the first of a three-part series that pro- Plain 1.2 1.9 vides a state-of-the-art review of rock anchors, covering design, construction, and testing and ~fermed 1.7 1.9 2.2 2.6 stressing. July, 1975 41 No. of barsin group Reduction factor stand about 156-178kN with 0.6m embed- and wires with grout. The value of the bond 2 0.8 ment. Since the capacity of such strand (up to 0.88N/mm') for 15.2mm strand is 3 0.6 is usually in the range 178-270kN, Golder slightly higher overall than that for 12.7mm 4 0.4 Brawner Assocs. conclude that no strand strand (up to 0.72N/mm'), and in both of a rock anchor logically needs an embed- cases there is a trend towards a reduction It is important to note that no information ment length in excess of 1.5m. However, of the bond with an increase in number of is provided in the Code on group geometry for other reasons, a length of 3m is usually strands. e.g. minimum spacing, where the reduction considered the minimum acceptable. (ii) The actual safety factor against fail- factors should be employed. In addition no Data abstracted from papers describing ure of the grout/tendon bond is usually guidance of any kind is given for groups rock anchor contracts is presented in well in excess of 2. of strands or wires. Tables IX, X and XI for bar, wire and (iii) The average bond developed by With reference to minimum embedment strand, respectively. In all the calculations, bars, especially deformed types, is on aver- lengths, Morris and Garret (1956) have except where otherwise noted, the bond is age significantly higher than that developed calculated from stressing tests on 5mm dia- assumed uniform over the whole tendon by strands or wires. Also the presence of meter wires that the minimum necessary embedment zone, which is taken as equal deformities increases the bond magnitude embedment is just over 1m. Golder Braw- to the length of the fixed anchor. by up to 2 times with respect to plain bars. ner Assocs. (1973) found that although the Bearing in mind the relatively small num- grout/strand bond is higher than expected ber of values, comments are limited to the Distribution of bond from tests on single wires due to "spiral following: Much of the work to investigate the dis- interlock", the value drops rapidly if the (i) There would appear to be a greater tribution of bond along grout/steel inter- embedment length is less than 0.6m. Re- degree of uniformity on values chosen for faces has been carried out in the United sults from Freyssinet anchors with spacers the working bond between strand and States in connection with prestressed and- have shown that each strand can with- grout, than for the bond developed by bars reinforced concrete. Gilkey, Chamberlin TABLE IX—GROUT/BAR BOND VALUES WHICH HAVE BEEN EMPLOYED OR RECOMMENDED IN PRACTICE Working Test Ultimate Embedment Load load bond bond Bar tendon (m) (kN) (N/mmt) (N/mmt) (N/mmr) Remarks Source Plain 1.2-1.9 Design criteria: bond dependent Britain —CP110(1972) Deformed 1.7-2.6 on concrete Square twist 5.25 Design criteria Britain —Roberts (1970) Ribbed 7.0 Plain 1.38 Short embedment test Canada —Brown (1970) Plain and threaded end 2.62 Bond dependent on embedment and grout tensile stress Deformed bar 30d Design criteria: "solid" rock Canada —Ontario Hydro (1972) Deformed bar 40d + Design criteria: "seamy" rock 20 No.