In recent years there has been a trend towards reinforcing tunnel segments using steel and polypropylene fibres rather than conventional steel rebar cages. This has been for a number of reasons, not least a very effective marketing campaign by fibre manufacturers and a perception that fibres are more cost-effective than standard rebar reinforcement. Little has been published to counter-balance this notion and to address the added benefits of conventional rebar in its ability to target the reinforcement where it is most effective, rather than a ‘scattergun’ approach to reinforcement design.
The tunnel lining is the key product of the tunnelling process and usually determines the durability, serviceability and longevity of the entire installation whatever its function. Some forms of tunnel lining have exhibited phenomenal performance in these respects, such as brick and cast iron, but have gone out of favour owing to their cost and time to install, often requiring labour intensive methods.
Other, non-conventional methods, such as steel fibre reinforced concrete and shotcrete, although showing much promise, have yet to prove their long-term effectiveness.
Reinforced pre-cast concrete using wrought iron bars was first used in the latter half of the 18th Century but found wider acceptance during the early part of the 20th Century with the invention of Portland cement and a wider availability of good quality mild and high-yield rebar steel. Pre-cast concrete segments for tunnelling were first developed for the London Underground in the mid-1930s and both plain and steel reinforced segmental linings have been in continual use worldwide ever since. Steel rebar reinforcement, still accounts for almost all pre-cast, cast-in-situ and sprayed reinforced concrete, including the tunnelling and shaftsinking industries, largely because of its worldwide acceptance through international standards and codes of practice, which when properly implemented ensure the delivery of a product with proven structural performance, longevity and durability.
Myths and hearsay
It is curious that some of the most successful techniques available to the tunneller become branded as somehow risky or potentially problematic or costly. Usually based on isolated adverse experiences, these invariably can be identified as poor design or implementation rather than an inherent fault in that technique. Unfortunately, steel rebar reinforced tunnel segments are increasingly subjected to these irrational and unsubstantiated arguments.
For example, the often quoted susceptibility to corrosion damage by aggressive groundwater, which when this has occurred has been entirely attributed to insufficient cover to steel or poor concrete mixing and casting. If such susceptibility were a serious or even remotely significant problem then many other important structures such as, bridge piers, offshore installations, immersed tunnels etc. would also be suspect.
Fire resistance is also referred to being a significant risk, but as was seen in the severe fire in the Channel Tunnel, although the concrete had been almost completely destroyed, the rebar cages remained largely intact and supported the tunnel until they were incorporated in the remedial shotcrete lining.
Had there been no rebar reinforcement in the segments the implications for the Channel Tunnel could have been dire. The myth that concrete spalling could be attributed to expansion of the steel under such a fire load has never been substantiated, as the damage to the concrete is caused primarily by the vaporisation of water in its matrix. These days, polypropylene fibres are usually incorporated in the concrete mix as fire protection and are routinely used with rebar cages and serve a dual purpose of reinforcing the edges and corners of the segment against accidental damage.
Robustness and durability are also often questioned but experience around the world, from the recently completed Deep Tunnel Sewerage System in Singapore, to the Airside road tunnel at London’s Heathrow airport, Madrid’s Metrosur and Dublin’s Port Access Road tunnel, clearly demonstrate the continuing preference for conventional rebar cage segments in key high-profile infrastructure projects.
These examples also illustrate situations where segments are subjected to high loadings due to handling and transportation, TBM thrusts, ground loadings and aggressive chemical environments where the need to maintain a high aspect ratio of lining thickness to tunnel diameter were imposed by the economic considerations of the schemes.
Benefits of steel segment cages
The main benefits of steel rebar cage reinforced segments can be summarised as follows:
Since the placement of conventional steel reinforcement can be targeted to the most effective positions in the segment to counteract imposed loadings, weight-for-weight it is many times more effective in increasing the structural capacity of the lining than non-conventional steel fibre designs.
In addition, the upper limit to steel rebar content is several times greater enabling extra section capacity to be achieved if required, whilst maintaining a minimal lining thickness. This flexibility in design to cope with extreme loading conditions is seen as the key to the future of the product as tunnelling becomes ever more challenging.
Manufacturing techniques
Segment cages are a precision-made factory product that requires a high degree of skill in design and fabrication in order to meet the stringent specification and tolerance requirements for modern segment manufacture and performance. The reinforcement normally comprises deformed high-yield rebar that has been accurately cut and pre-bent for assembly in precision built jigs by MAG welding to an approved standard such as CARES and strict quality inspection such as ISO 9001.
This process can be labour-intensive but modern automated and semi-automated processes can be employed for a large portion of the work, in particular cutting, bending and radiusing. The modern manufacturing approach increasingly utilises machine welded wire fabric that can be rapidly mass-produced to a degree of accuracy, quality and cost-effectiveness that cannot be matched by manual assembly. Welded wire fabric can be produced in a range of mat size, bar size and bar spacing combinations to suit virtually all designer requirements and accurately radiused to suit the intrados and extrados of the segment.
When assembled on the jig it is a simple matter to weld separation frames to form the basic cage geometry followed by additional reinforcement, particularly around the peripheral sides to suit requirements for thrust jack loadings, bolt and erector sockets. An added advantage of pre-fabrication is that the reduced amount of welding needed to complete a complex cage minimises heat distortion that would otherwise require additional allowances in the cage design and rejection/re-welding following routine inspection. For simpler segment cages machine-fabricated ‘deckchair’ spacers can be used between the welded wire fabric layers.
This prefabricated element approach offers significant cost savings in terms of speed of assembly and reduction of labour and since the production of these units can be easily integrated into the normal product runs for conventional civil engineering structures, tunnel segments can benefit from the bulk production savings inherent in this product.
For larger projects, final assembly of the segment cages is often undertaken by the supplier in temporary facilities immediately adjacent to or within the casting yard. This offers a number of advantages including:
Looking forward
Increasingly, contractors are turning to suppliers for cost saving solutions and the steel fabrication sector is no exception. To meet this demand specialist engineers and designers, both in-house and freelance are employed to produce detailed fabrication designs, often in association with the segment manufacturers. Involvement by the preferred manufacturer, who can provide a full design/supply service, will likely be increasingly sought to meet a performance specification set by the client’s engineer rather than commissioning a detailed engineer’s design.
This approach, together with the use of machine-welded wire fabric and other pre-fabricated elements will allow increased standardisation of design whilst retaining a bespoke capability for fine adjustments to suit the project requirements, thereby reducing overall costs. To a large extent some of the major segment manufacturers already supply their own ranges of standardised segmental ring designs that have established proven capability. However, tunnel specification and TBM performance are continually evolving and the segment manufacturing industry must evolve in step with them.
Special steel coatings such as epoxy paint have received little attention owing to the problems of ‘pinholing’ of the finish during curing or difficulty in handling and damage. However, the advantages in terms of reduced cover to steel requirements and durability may see resurgence in this technology as tunnelling is increasingly faced with more extreme environments.
Galvanising, though possible, is not seen as a practical option owing to the cost and difficulty of the process, particularly for large segment cages.
The quest for improved efficiency and speed of full-face TBM driven tunnels that rely on thrust reaction from the built segment rings requires increasing ram pressures on the ring that temporarily creates more severe stress locally in the lining than occurs from ground and hydrostatic loadings. In order to meet these demands increased sophistication of segment reinforcement design will be needed to maintain reasonable lining thickness and weight.
In his British Tunnelling Society 2000 Harding Lecture, John King envisaged the appearance of new, stronger and lighter-weight materials to replace the existing segmental lining technology, but none have yet appeared. So, for the foreseeable future the emphasis must be to maximise the efficiency of existing proven technology.
