R OCK SUPPORT INSTALLATION IN TUNNELS

The support design in Norwegian tunnels is based on the geological investigation and the classification of the rock mass quality, as performed for every blast round. The rock support is installed at two main stages: temporary rock support and permanent rock support. The temporary (initial) rock support is applied at the tunnel face to provide safe working conditions during the construction period, and will thus delay the progress of tunnelling. The permanent support is installed to meet the requirements on the long-term durability of the tunnel. The temporary rock support is the contractor’s responsibility whereas the permanent support is the responsibility of the client. However, an important aspect of NMT is that the temporary support will later be included as apart of the permanent support. The temporary support thus has to meet the quality requirements and standards as set by the client.

Therefore, the support design at the tunnel face is usually proposed by the contractor and decided by the contractor and client in agreement (Kveldsvik and Aas 1998, Norwegian Tunnelling Society 2004).

The most commonly used method includes rock bolts, shotcrete and cast concrete using steel shuttering. The length and spacing between rock bolts and the thickness of the shotcrete can be assessed according to the Q-system described in Section 3 (Barton et al. 1992). A revised support system based on the Q-system prepared by the owner is commonly used for determining the support design for road tunnels in Norway (Norwegian Public Roads Administration 2010).

During excavation work, special care is taken to optimise the tunnel contour in blasted tunnels, as this is important with regards to tunnel stability and will minimise the need for

rock support. An optimal tunnel contour will be to meet the tunnel design, meaning low overbreak and low underbreak. Overbreak is when the blasted profile is larger than the designed profile, while underbreak is the opposite. Underbreak has to be removed at a later stage, while overbreak will require extra shotcrete (Maerz et al. 1996).

3.2.1 Shotcrete volume and thickness

Application of sprayed concrete for rock support was introduced in Norway in the 1960’s, but did not gain any widespread popularity until 1980. Today, the shotcrete used in Norwegian tunnels is fibre reinforced and performed in accordance with the wet-mix method. The new alkali free accelerators offer improvements of the application method, by increasing the final strength and allowing for 30 cm of shotcrete thickness to be sprayed in one application (Norwegian Tunnelling Society 2004). A project initiated by the Norwegian Public Roads Administration found that a governing parameter for the durability of sprayed shotcrete seemed to be related to the applied thickness. Therefore a minimum thickness of applied shotcrete has since been required in Norwegian road tunnels. At present the requirement states that the mean thickness of applied shotcrete should be minimum 8 cm, and the measured minimum thickness should be at least 50% of the applied mean thickness (Norwegian Public Roads Administration 2007).

The application of shotcrete has to be performed according to the guidelines as established by the Norwegian Concrete Association in Publication No. 7 (2003). The publication states that the application of shotcrete must be conducted so that cavities, fractures and joints are filled first, before a uniformly distributed covering layer is applied. The owner can specify a minimum thickness of shotcrete for the permanent support, which the contractor then has to fulfil. This means that the mean thickness of the applied shotcrete has to at least equal the specified thickness. The volume needed to fulfil the requirements concerning shotcrete thickness is calculated from the theoretical area of the tunnel profile and the specified thickness. This volume is multiplied with a rebound factor, due to rebound of the shotcrete when hitting the rock surface, and an estimated roughness factor accounting for the roughness of the rock surface. The roughness factor needs to take two conditions into consideration: the uneven application and distribution of the shotcrete, as hollows and cavities are filled first; and the rough nature of the blasted profile leading to the fact that the true surface area will be larger than the theoretical area. Today there is no standardised method of determining the roughness factor. The Norwegian Concrete Association therefore

refers to primarily guessing a factor for controlling the applied shotcrete thickness after appliance, and later adjusting the factor according to the results of the control. The roughness factor will also need to be adjusted according to varying rock mass conditions, and the specified target thickness. A thin layer of shotcrete will require a higher roughness factor than a thick layer.

The applied thickness is one of the most important parameters governing the quality of the shotcrete, and inspections are therefore required to control that the actual applied shotcrete is in accordance with the ordered thickness. The guidelines issued by the Norwegian Concrete Association (2003) state that the thickness control can be performed by drilling holes through hardened shotcrete according to a predefined grid, e.g. 2 x 2 m. The holes are then drilled 2 m apart in the tunnel profile, at random locations including both underbreak and overbreak. At least 10 control points have to be drilled. One can also choose to measure the shotcrete thickness in the holes drilled for rock bolts. Control of shotcrete thickness is performed for every 250 m³ of excavated rock (Norwegian Public Roads Administration 2007).