The use of segmental linings for tunneling works is increasingly being preferred to other forms of tunneling, and the diameter of these tunnels is also increasing due to advances in TBM capability. With mechanization, tunneling costs are reducing, resulting in larger segmentally lined tunnels, now in particular for roads. Road tunnels generally have a much lower cover to diameter ratio than metro tunnels. Many of the current design approaches have been developed for smaller diameter service or metro tunnels, and these methods are being scaled up without, in all cases, the consideration of what design approaches are scalable such as the aspects of segmental lining detailing that require special consideration at larger diameters. In the English-speaking world of tunnel design, the most commonly used design approach is due to Muir Wood (1975) and Curtis (1976). Curtis's approach assumes the combination of uniform and biaxial stress state, but ignores the gradient of a gravitational stress field: the stresses, displacements, thrusts and moments are identical at crown and invert. This simplifying assumption can introduce significant errors for large road tunnels at low cover. Hartmann (1970 and 1985) published a solution which accounts for the gravitational stress field, and this paper demonstrates this effect on lining actions and compares them to FE solutions. The authors have been involved design reviews of the 3 Brisbane Road tunnels, and in the verification of the 14m OD 3-lane road tunnel In Auckland NZ. These designs are used to demonstrate some issues with the adoption of traditional analysis methods, and some design recommendations are made.
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