Transcript of "Limitations of AS/NZ2566.1 For The Trenchless Technology Industry"
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Limitations of AS/NZ2566.1 For The Trenchless Technology Industry<br />Dr Ian Bateman<br />Director<br />Interflow Pty Ltd <br />
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Outline<br />Use of AS/NZ2566.1<br />Examples where AS/NZ2566.1 is <br /><ul><li>not conservative enough
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too conservative</li></ul>Conclusions and recommendations<br />
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Design in AUS/NZ Trenchless Industry<br />There is no specific design standard<br />We borrow aspects from other standards<br />Fully Deteriorated<br /><ul><li> Borrow from AS/NZ2566.1
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Assumes liner acts like a buried flexible pipe</li></ul>In Tact<br /><ul><li> Borrow from ASTM F1216
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Assume existing pipe acts to enhance the liner strength</li></li></ul><li>Overview of AS/NZ2566.1<br />Scope<br />“This Standard sets out a practice for the structural design of buried flexible pipelines which rely primarily upon side support to resist vertical loads without excessive deformation. The interactive pipe/embedment structure is considered only in the transverse direction. Structural performance is predicted in the long-term for pipes in trenches and embankments but not for jacked or bored lines.”<br />
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Overview of AS/NZ2566.1<br />AS/NZ2566.1 is designed to cover<br /> - installation of a flexible pipe into a trench<br /> - takes into consideration<br /> - pipe characteristics (stiffness and material properties)<br /> - embedment characteristics<br /> - design loads<br /> - prescribes a method of performing a design<br />
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Buckling Condition<br />In most applications the governing equation is<br /> (Applied Loads) x FOS = (St)1/3 x (E’)2/3<br />St = Pipe ring stiffness<br />E’=Modulus of Soil Reactivity<br />
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In Other Words … <br />The applied loads need to be resisted by<br />The ring stiffness of the pipe (liner)<br />The surrounding soil<br />But, the effect of the soil is much more dominant<br />
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AS/NZ2566.1 In Our Trenchless Industry<br />Used successfully for more than a decade<br />Hundreds of thousands of pipes re-lined<br />Industry provides cost effective solutions<br />Installers have effective and practical systems<br />Suppliers are able to produce products for nearly all situations<br />So, what’s the problem?<br />
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Potential Problems<br />As the industry develops …<br />We are faced with ever more challenging situations<br />Suppliers develop more and more sophisticated products<br />Fall outside of the intent of AS/NZ2566.1<br />
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AS/NZ2566.1 – Not Conservative Enough<br />High Modulus Thin Walled Liners<br /><ul><li>AS/NZ2566.1 allows us to determine the RING STIFFNESS of a liner that is needed
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The thickness of the material</li></li></ul><li>High Modulus Thin Walled Liners<br />Example<br /><ul><li>To achieve a desired RING STIFFNESS of 1,000 N/m/m</li></ul>Can achieve desire stiffness using thin, high modulus materials<br />
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High Modulus Thin Walled Liners<br />This is reasonable if the liner of perfectly circular cross section<br />Implicit in AS/NZ2566.1 is that pipes are supplied to site free of defects (then buried)<br />In a trenchless application the pipe (liner) is formed inside a deteriorated host pipe<br />The final shape of the liner is influenced by the shape of the deteriorated host pipe<br />
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High Modulus Thin Walled Liners<br />Liners can contain IMPERFECTIONS<br />x<br />
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Effect of Imperfections On Liner Stiffness<br />Phenomenon is well studied (Moore,I et al)<br />Effect on liner stiffness is a function of<br />Liner Thickness<br />Size of the Imperfection<br />
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Summarising…<br />With a high modulus material, a 150mm liner has almost zero stiffness with a 15mm imperfection<br />The effect is far less severe with traditional materials<br />The effect reduces as the diameter increases<br />Looking at this another way…<br />
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How Deal With This Issue<br />Options<br />Set a minimum liner thickness of (say) 4mm<br />Use an equation to de-rate the actual stiffness and compensate for imperfections of a given size<br />Calculate theoretical thickness and add a constant (say) 2mm<br />
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AS/NZ2566.1 – Too Conservative?<br />Large Diameter Lining<br /><ul><li>By number, >95% of all pipes rehabilitated are at diameters of 1,000mm or below
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By dollar, ~60 -70% </li></li></ul><li>Returning to Our Design Equation…<br />(Applied Loads) x FOS = (St)1/3 x (E’)2/3<br />St = Pipe ring stiffness<br />E’=Modulus of Soil Reactivity<br />The industry’s default approach has been<br /> - Use values of E’ of between 2 and 5 MPa<br /> - Design a liner with sufficient Stiffness<br />
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Example<br />Stiffness required to re-line a pipe 5m below surface assuming constant E’=4<br />** Long Term Stiffness<br />
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A Perspective On Pipe Stiffness<br /><ul><li>Most large diameter plastic sewer and stormwater pipes will have LT Stiffness of less than 3,300N/m/m
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Flexible pipes with a long term stiffness of >8,000N/m/m do not exist
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Furthermore the reason for the stiffness is due to installation damage not deflection over time
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Flexible plastic pipes are commonly made up to 2,400mm diameter (LT stiffness~1,500 N/m/m)</li></li></ul><li>Using a Typical Liner Material…<br />~10% Diameter Loss<br />3000mm<br />2700mm<br />2400mm<br />2100mm<br />1800mm<br />1500mm<br />1200mm<br />900mm<br />600mm<br />300mm<br />At large diameters a solution would be not be possible and/or would be very expensive<br />
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With E’=14 instead of E’=4<br />E’=4<br />E’=14<br />Changing the value of E’ has a major affect on what is possible<br />
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…. Even more dramatic effect on required stiffness<br />E’=4<br />E’=14<br />
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How Is E’ Determined<br />Selecting a realistic value of E’ has a huge bearing on the solution (and economics)<br />AS/NZ2566.1 provides the following table<br />But there are other methods<br />
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AS/NZ2566.1 and E’<br />Suggests a range of values between 1 and 20<br />Suggests the values are conservative<br />Suggests with cover heights greater than 10m higher values should be used<br />Shows that the value increases as greater compaction occurs<br />BUT, Trenchless Industry tends to use values of between 2 and 5<br />WHY?<br />
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Selection of E’<br />Estimating E’ is difficult and time consuming<br />We often do not know what occurred during initial pipe construction<br />We don’t know what has happened to the soil during its lifetime<br />Cannot ensure 100% uniform support of the liner by the host pipe and/or soil<br />The cost involved in estimating the actual E’ outweighs the cost of installing a stiffer liner – in smaller diameter pipes.<br />
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Estimating E’<br />… but in large diameter pipelines this is probably not true.<br />Estimating an appropriate value for E’ will have a significant bearing on the overall economics<br />Not understanding the condition of the soil in large diameter pipelines can lead to serious consequences<br />Above a certain diameter it is worth determining a realistic value of E’<br />
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Silo Reduction Factors<br />AS/NZ2566.1 allows the use of silo reduction factors when the depth of cover exceeds 10 times the diameter<br />For small diameter pipes this seems reasonable<br />At large diameters this becomes very conservative<br />Silo effects actually occur at much lower cover heights<br />ALSO, E’ has been shown to be related to depth<br />
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AS/NZ256.1 For Large Diameters<br />Using a constant AND/OR low values of E’<br />Not applying silo reduction factors to soil loads until 10 x D<br /> VERY CONSERVATIVE<br /> EXPENSIVE<br />NOT POSSIBLE<br />
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How To Deal With This Issue<br />Suggestions<br />Continue to apply the current approach up to a diameter that provides cost effective outcomes<br />Above this diameter, establish more information about the condition of the soil (E’)<br />Allow silo reduction factors below 10 times D<br />
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Alternatively…<br />If we don’t then we will have to …<br />Ensure that large diameter pipelines are rehabilitated before they reach the fully deteriorated condition<br />Use a different design method at large diameters (not AS/NZ2566.1)<br />
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Conclusions<br />The design approach borrowed from AS/NZ2566.1 has served the industry very well<br />As products and the industry have evolved some limitations of this approach have arisen<br />As these situations present themselves specifications should be enhanced with specific guidelines<br />
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