Stresses in sprayed concrete tunnel junctions

The low mobilisation cost and flexibility of geometry that sprayed concrete allows means that sprayed concrete linings (SCL) are now almost always used for short lengths of tunnel, tunnels of varying geometry and tunnel junctions. While considerable effort has been spent investigating the deformations of the ground during tunnelling of straight, constant diameter tunnels, comparatively little work has been done on the stresses in SCL tunnels or junctions. The structural behaviour of an SCL tunnel junction in terms of stresses is not well understood because design models are usually simplistic and field data are scarce. This research addressed these gaps in knowledge through literature review, field monitoring and a suite of numerical experiments. Study of the behaviour of pressure cells revealed a new type of temperature sensitivity, the ground reaction temperature sensitivity, which constitutes a real radial stress as the SCL ring expands and contacts against the ground with change in temperature. The field data showed that stresses on an SCL in soft ground were dependent on ring closure and if this occurred close to the face the stresses would approach a long-term maximum value well below hydrostatic overburden pressure once the face had advanced approximately two tunnel diameters away. The literature review indicated that if more ground deformation were allowed, the stresses would continue increasing over a long period of time, possibly approaching hydrostatic overburden pressure in the long-term. The numerical experiments showed that the axial stress concentration factor at the junction was not dependent on nonlinearity, anisotropy, stiffness and plasticity of the ground, or its in situ stress distribution. However, the bending stress concentration factor was dependent on the ground stiffness and to a lesser extent the undrained shear strength of the ground.