When a tunnel is excavated in all but the most competent of ground conditions it is an inevitable consequence that some form of mining support will be required if the tunnel is to retain adequate stability and/or maintain sufficient dimensions to facilitate its use in the intended manner.
The form and function of the support will vary according to a wide range of factors apart from just geotechnical considerations and it would seem that for every different tunnel there is a different lining solution
Tunnel linings: main types. Tunnel linings are grouped into three main forms some or all of which may be used in the construction of a tunnel:
1. Temporary mining ground support
2. Primary lining
3. Secondary lining
Temporary mining ground support. In rock tunnels where the ground has insufficient stand-up time to allow the construction of the primary lining some distance behind the face, then some form of temporary ground support applied at the- tunnel face is required eg rock bolts, shotcrete and steel sets tunnel support.
Such support is not required in soft ground in conjunction with a shield driven tunnel as temporary ground support is provided by the body of the shield itself.
Primary lining. A primary lining is the main structural component of the tunnel support system which is required to sustain the loads and deformations that the ground may induce during the tunnel's intended working life. A further function performed by the primary lining is the control of water egress (exit) and ingress (entrance).
Secondary lining. Various tunnels require smooth bore profiles for their intended use, eg sewer and water tunnels or aesthetic finishes for public usage, eg highway and pedestrian tunnels. Erosion and corrosion protection for the primary lining and further waterproofing may also be required, all of which are provided by secondary linings.
The terms temporary, primary and secondary linings are not always defined on a consistent basis in various texts
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