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Airmartech Australia Pty Ltd ( ACN 057 067 955 )
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DESIGN GUIDELINES
Geological surveys and soil analysis reports are useful and informative to establish holding capacities. The wide range of soil conditions complicates the task of specifying anchors, without prior testing. It is recommended site proof testing be carried out before commencement of work. Key System Design Requirements:
All wire tendons contain properties of non-permanent and permanent elastic deformation, when considering options for system specification, this should be taken into account.  Round Strand Wire Tendon When this type of tendon is put under load, an initial permanent constructional stretch will occur. The permanent constructional stretch is caused by the wires adjusting themselves into their proper positions in the strands and by the strands seating themselves in the tendon and compressing the core. If this tendon is taken to approximately 50% of its Minimum Failure Load (MFL) non-permanent elastic deformation will occur. When unloaded the tendon will return to its original unstressed length. Loads above 50% of the MFL will normally result in permanent deformation of the tendon. Compacted Strand Wire Tendon Due to its dense construction, little if any, initial permanent construction stretch will occur. As the modulus of elasticity of this type of tendon is much higher than conventional round strand tendons, the non-permanent and permanent elastic deformation is significantly reduced, by as much as 30%. Proof Loading When proof loading an anchor system, non-permanent and permanent elastic deformation is always responsible for a small percentage of the final extension measurement. PERFORMANCE Load capacities of anchor systems depend at all times on the quality and consistency of the material into which they are placed. As natural ground throughout Australia is particularly varied, it is essential to ascertain the qualities of the ground into which the anchors will be driven through testing procedures. The ultimate performance of the anchor system is based upon a number of key factors:
In many cases, it will be necessary to cut core holes in concrete, brick or steel work, so that the relevant anchor type can be inserted through the remaining structure. Care should also be taken to ensure that the load plate is adequately spread over the structure to avoid point loading imposed by the anchor system. The table below shows anchor types and core hole sizes for both circular and rectangular installations.  SLIP PLANES With unsupported soil structures such as steep embankments, cuttings and deep trenches, there is a potential risk of slip or collapse. The failure surface between the slipped material and the stable material is referred to as the slip plane. When designing an earth retaining structure to prevent such failures, the specifier must determine the slip plane position based on analysis of site and soil investigations and soil properties data etc. The anchor system installation depth and required holding capacity can then be specified. For an anchor system to work effectively, it must be driven far enough beyond the predicted slip plane to allow the frustum cone to work independently in stable soil. Then, if a slip in the soil were to develop there would be no reduction in the frustum cone size and load achieved. Some guidelines for the minimum installation depth required to allow the anchor to generate its frustum cone and work independently of the slip plane are provided in the diagram and table below. (These guidelines are based on extensive field experience and are not calculated values.) The guidelines also allow some approximate tendon lengths to be calculated for budgeting purposes. If the installed depth (relative to the slip plane) selected disagrees with these guidelines (i.e. is less than or significantly greater than the minimum depth), careful reconsideration of the suggested depths is required to ensure the frustum cone will not be adversely affected by the slip plane.  LIFE EXPECTANCY It is important to establish from the design engineer exactly how long the anchor is intended to be in use. This linked with information about corrosive properties present in the soil will indicate the correct anchor materials to use. An indication of life expectancy is also provided by the anchor system’s application. For instance, a temporary sheet pile retaining wall will normally only require un-galvanized materials. An exception to this would be if the soil was very corrosive. SOIL / AIR INTERFACE The soil/air interface is the area immediately above and below the natural ground level where most corrosion is likely to take place. It is a region greatly affected by natural sunlight, consistent damp, still air and the action of microbiological bacteria. Increased corrosion can also occur if this area is exposed to agricultural fertilizers, animal excrement and any man made chemicals with excessive pH levels. Further down in the ground the tendon is surrounded by compacted soil, which reduces its vulnerability to this elements and the chance of corrosion. The five main factors contributing to corrosion are:
 BURIED SERVICES Locations of any buried services such as gas, electrical, water and telecommunications on the proposed site should be clearly identified so that anchor installations do not interfere with them. If documented details are not available then an examination of the site will often reveal the location of inspection hatches and valve covers. This will give an indication of the depth of services and their approximate tracks. Written assurance from the Local Utility Authorities confirming the location of buried services is also advisable. Only when all buried services and their exact location is known can suggestions be given on how they may be avoided. Altering the angle of installation, position and depth can sometimes provide an immediate and effective solution to this type of problem. |
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