Why Aesthetics Needs Early Consideration In Tunnel Design And Excavation – A Case Study From The M8

Road tunnels are more complex than the driving eye would surmise – hidden behind the dark ceilings and colourful wall panels are rock, support bolts, multiple layers of shotcrete, possible water drainage, access passages, fire control systems, electrical systems, and speed cameras. During the design phase of a tunnel project all these components are given significant consideration as they govern the ultimate size of the excavation that is needed. At construction, focus shifts from what will be added to what and how can rock be extracted quickly and safely. The crown and face condition come to the forefront as inputs into the ground support requirements and as short-term prediction of excavation conditions in the following days. Classification of the rock mass is skewed to encapsulate the face, shoulder and crown condition as well as the bolt zone above the crown. Side wall and floor conditions are noted where possible, but visibility is often limited to the current cutting (about 1.5m, with previous cuts already shotcreted) or not possible due to mud and rock waste. Once excavation has progressed, focus shifts to the functionality, endurance and aesthetics the general public will see – the smooth road surface, the painted ceiling with clear road signs, and the architectural panelling on the walls. For the latter two, consideration is required as to how these heavy items will be anchored to the roof and walls. For the M8, the broader face/shoulder/crown rock mass classification was found to be inappropriate to the narrow anchor zone for the architectural panels, which required anchoring specifically at a height 3m above the floor excavation level. This was particularly the case where narrow siltstone beds occurred with the sandstone rock mass and where dykes and shears intersected the tunnels. Fortuitously, compilation mapping including the side walls had been completed for another purpose for the project and was able to be repurposed to assist with panel construction design. Different bolt lengths were required for each rock class in sandstone, shale and dolerite (dykes) with the conditions specific to the zone 3m above the floor level reassessed by experience senior geological staff as the input for the design. A package of works was provided for each wall, indicating the rock class at the anchor zone, both as a map and tabulated. This improved construction sequencing, costing and allowed pre-purchasing of all materials before work commencement. Significant costs would have occurred had the daily mapping not captured the side wall conditions and had compilations been required solely for this purpose.