Solid loads on cut and cover tunnels under high fills

In recent years precast concrete arches have been increasingly popular for use in applications such as drainage culverts, conveyor tunnels, rail and road tunnels and alternatives to short span bridges. The design of these structures for fill heights greater than about 10 metres over the crown presents potential problems that require experience and careful analysis to resolve. These include:

• High axial loads and shear forces that may be sensitive to the stiffness parameters of the foundation and backfill materials.
• Design of foundation structures for high vertical loads.
• Longitudinal differential settlement.
• Longitudinal loads on the structure due to steep fill slopes, inclined structural elements and longitudinal spreading of the foundations.

In this paper examples are given of structures that have exhibited these problems and design methods are presented that will ensure that problems will be avoided. Examples are presented of successful projects in Australia and overseas, with fill heights of up to 50 metres over the structure.