Tunnel portal design optimisation in tectonically disturbed and weathered gneissic rock masses

It is often the case that portal areas are subject to slope instability phenomena related, amongst other factors, to the poor-quality ground conditions, where soil or weathered rock material is present. This weathering is more prominent in specific geological settings. In this paper gneissic formations in a setting where weathering is favoured are studied. Information on several gneissic rock mass types and their potential behaviour in slopes is used to propose a set of guidelines regarding slope geometry focusing on benching for different slope heights and angles. Slope stability analyses were conducted for multiple scenarios per rock mass type. It was observed that for moderately weathered rock masses, steeper slopes (<= 3:1) are satisfactory, whereas for weathered types, gentler slopes (similar to 2:1) are suggested, depending on the maximum height of the slope. Through a site-specific study the complexity of gneissic systems and the effect of the weathered zone to stability was highlighted.