INSIGHT | WATERPROOFING


ACCESSORIES AND YOU


SEGMENT For use of accessories with segment lining, Christoph Eberle, Technical Director, Mott MacDonald, discusses assurance in the context of Load and Resistance Factor Design (LRFD)


Designing segmental tunnel linings is fundamentally a process where the client’s requirements and the conditions in which the segments have to operate are assessed and a robust, compliant solution is developed. Client requirements range from elementary asks, such as demonstrating structural robustness, to specific aspects of performance, such as a maximum permitted leakage rate. Structural design is normally undertaken within the


Below:


BTS Specification for Tunnelling, 4th Edition, to be published Dec 2023 PHOTO CREDIT: BTS


framework set by design codes. Modern design codes such as the ACI 318, the AASHTO and the Eurocodes are based on Limit State (LS) Design where the (factored) loads need to be less than the (factored) resistance. This approach, often abbreviated as Load and Resistance Factor Design (LRFD), typically applies for the design of concrete and steel structural elements of our tunnels in Ultimate Limit State (ULS) and Serviceability Limit State (SLS). With this approach we can confidently demonstrate compliance of our designs with standard rules applied by the wider construction industry and, in turn, due professional skill and care. The situation is slightly


different for some performance requirements such as leakage rates which should be zero if our (safe and robust) design would be working exactly as designed. In the real world zero leakage is however only attainable with exceedingly robust and redundant designs and construction methods, and would be prohibitively expensive. Staying with


the example of segment gaskets, this mismatch between


16 | December 2023


design and reality is often caused by imperfections of the single shell tunnel lining such as cracks, or gasket damage, or leakage around grout ports. A limited number of defects are typically not detrimental to the usefulness of the built tunnel for the asset owner, and in consequence are considered to be acceptable for a non safety-critical serviceability check. These considerations are often reflected through the selection of tunnel class by the client (following BTS Specification for Tunnelling 3rd


edition, Table 16) according to its intended use, which


includes a definition of the acceptable leakage always greater than zero. The code-based design approaches typically presume


that the used materials and products are regulated. Especially in segment design this is, however, not always the case – accessories such as dowels, gaskets, bicones, guiding rods, and bolt sockets are unregulated and not standardised.


THE PROBLEM As tunnel designers we fulfil a design integration role in addition to our ‘first principles’ structural design. Every structural element we use, be it concrete, steel, wood or masonry, comes with a large package of associated material and test standards before it is deemed compliant with the requirements of the structural codes (ACI/AASHTO/EN Eurocodes). This set of standards ensures that the material performs as required in all design situations captured by the underlying code. Such comprehensive rules do not exist for many


segment accessories. While some accessories such as gaskets are backed up by detailed specific guidance (STUVA Recommendations for Gasket Frames, 2019) this is not the case for other accessories, which are typically characterised by supplier provided, product specific, data sheets. At this point, it is useful to consider an analogy to


the design of fixings. The manufacturer of a fixing will define and certify the specific performance of their product, such as tensile and shear strength, strain at failure, and specific material composition. This definition is backed up by an ISO 9001 certified assurance chain inclusive of the raw materials used in the manufacturing process.


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