Apendix IV. Estimations

number of joints in the tunnel lining profile, and also to simplify as far as possible the connection detail.

As can be seen from Figure 77, the top heading, bench – invert exca-vation sequence, with or without the initial pilot drive, offers the best solution to this problem. Of particular concern with the single and double sidewall drift methods is the complexity of the connection detail between sidewall and main tunnel lining. These joints contain lattice girders from the main and temporary sidewall, in addition to consider-able connection reinforcement to provide the required structural per-formance of the complete profile, all exacerbating the water ingress problem. The typical location and number of such joints are indicated in Figure 77 by the black circles.

With respect to the SPTL method, where the top heading, bench – invert construction sequence is not possible due to ground stability, and a need to sub-divide the face further is required, the pilot enlargement method (Figure 77) may be regarded in favour of the sidewall drift methods.

Figure 77: Joint types for sprayed concrete lined tunnel excavation methods (only joints in final lining indicated)

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9.8 SPTL two layer method – second layer construction joints

The second layer of the SPTL two layer method is formed of sprayed concrete as discussed earlier. The system should have no longitudinal joints apart from the connection to the invert concrete slab. The bay lengths for the sprayed concrete lining should be approximately 4 to 5 m long as this is the typical lateral extent of a spraying manipulator (Figure 78). The circumferential joints at the bay ends should be staggered by a minimum of 0.5 m in relation to the construction joints of the first layer to reduce the potential water path to the inside tunnel surface.

As one of the primary functions of the second layer is to produce a watertight structure, further security can be achieved by the installation of waterbars or joint sealing systems at the joint interfaces.

Figure 78: SPTL two layer method: 2nd layer sprayed concrete

For sprayed concrete linings, a robust system is required. An economi-cal solution is the Masterflex^® 900 – Fuko system that consists of a PVC injection tube securely fixed to a semi-circular rebate formed at the joint surface during the construction of the previous bay by means of a leading edge shutter. The Fuko tube is perforated along its length, with four neoprene strip pads covering the perforations over the entire length of the tube. These PVC pads act as one-way valves. The entire tube and pads are bound by an open webbed nylon mesh. The Fuko injection tube can be injected with either MEYCO^® MP308 injection resin or Rheocem^® microfine cement should water ingress occur during the operational life of the tunnel. It is advisable that the first injection proc-ess occurs one year after completion of the tunnel so that any potential

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water paths that may establish can be identified and the respective joints targeted for treatment, rather than the unnecessary blanket injec-tion of all joints.

The Fuko system enables re-injections to be performed during the life of the structure if required. The injection tubes should be positioned at a minimum of 50 mm from the intrados face of the tunnel lining.

9.9 SPTL two layer method – first and second layer bond

In order to provide a monolithic structure, the bond between the first and second layers must be frictionally tight and form fitting to permit the transfer of shear forces across the bond. Shear reinforcement between the two layers should be avoided as it will aid the development of water paths to the inside surface of the tunnel and a consequential reduction in durability. The shear and tensile bond between layers can be ensured by the surface roughness of the first layer to provide an effective, inter-locking surface as indicated in Figure 79.

Figure 79: SPTL two layer structure indicating shear forces acting along bond interface

Additionally, the lining surface to receive the new layer should be suitably prepared following the precautions listed below.

< Removal of any damaged and disintegrated sections of the first

layer

< Cleaning by high pressure air and water jetting to remove dust, soot

and curing membrane applied to first layer. This is most effectively achieved using the sprayed concrete equipment

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< Removal of grease or oil using a detergent

< Surfaces should be damp, not saturated, before installation of

sec-ond layer.

Bond strength is also enhanced by a concrete mix design that promotes a reduction in drying and early thermal shrinkage by lowering the heat of hydration, and also by the necessary process of concrete curing. Once this bond has been achieved, the monolithic behaviour of the first layer to the second layer can occur.

To aid this structural requirement UGC International has developed the concrete admixture MEYCO^® TCC735. This product ensures effi-cient homogeneous cement hydration from the moment of spraying, and throughout the sprayed or cast in situ layer. This internal curing action substantially reduces initial shrinkage, increases both strength and density, resulting in enhanced bonding characteristics to previous layers. A crucial benefit to the working cycle, is that the introduction of MEYCO^® TCC735 to the mix design eliminates the need for external curing agents to be both applied, and removed, prior to the installation of the next layer.

9.10 Surface finish

Depending on the intended role of the tunnel structure, several surface finishes can be provided with the SPTL method varying from a sprayed concrete finish, to float finished surface.

9.10.1 Screed and float finish

Sprayed concrete linings can be surface finished by screeding and hand floating to produce a surface finish similar in quality to that of a cast in situ lining. This process is performed on a sprayed mortar layer applied to the final structural sprayed concrete layer, and is typically 25 mm thick. Polymer monofilament may be included in the mix design to control surface crazing produced by thermal and surface drying effects, such as with Emaco^® S88C sprayable mortar. The screeding process is relatively simple to perform using 25 mm diameter screed rails bent to the finished profile of the tunnel, and if required, further improvement to the surface finish can be attained by hand float work.

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In the case of highway tunnels as shown in Figure 80, the required sur-face reflectance for the tunnel sidewalls up to a height of 4 m from road surface should have a smooth finish, have a high reflectance and be an average light colour. Above this zone, the crown sections of the tunnel are dark coloured and of low reflectance.

This reflectance and colour coding provides the following benefits:

< Avoids the claustrophobic effect of a reflective tube, and allows a

more visual rectangular appearance, giving width to the tunnel

< A reduction in the power consumption for ventilation and luminaires

< Obscures services and hardware in the crown of the tunnel

< Provides a limit for the cleaning machine and hides a soiled

appear-ance where the surface is not cleaned

< Aides the distribution of light onto the road surface

To achieve these surface finishes, it is recommended that the 4 m wall sections are initially screeded to the correct profile and finished by hand float work. To provide the high reflectance and light colour require-ment, the application of a pigmented cementitious fairing coat such as Masterseal^® 333, or an epoxy coating such as Mastertop^® 1211 to the colour required is proposed. For the crown sections, the surface can be left as screeded and a similar painted treatment as above can be applied with a black pigment.

Figure 80: Surface reflectance requirements for a dual lane highway tunnel (UK)

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