SCL Primary Lining Interactions with Concrete Backfilled Temporary Tunnels

Summary

This paper outlines the findings from the design experience of mainly escalator tunnels above a foam concrete backfilled temporary access tunnel, and/or below a partially backfilled pilot tunnel because of the pilot tunnel axis is much higher than the enlarged ones. 2D FLAC numerical analysis with full geological and groundwater conditions was set up to initial equilibrium condition and the planned construction sequence was modelled to assist the understanding of linings’ physical behaviours. Steel fibre reinforced sprayed concrete lining properties have been used in the simulations and the adequacy of lining strength capacities has been checked based on the lining physical behaviours according to Eurocode 2: Design of concrete structures guidelines. The critical physical strength requirements for primary linings are bending moments, axial force/stress, and shear force/stress which will be evaluated in this paper.

The design of SCL primary lining in the past was normally treated as temporary design, possibly also because of the difficulty to install those instruments, therefore lacking of long term monitoring data to understand their behaviours. The advance of numerical analysis technique in modern days has provided an alternative tool to predict SCL primary lining behaviours. The study presented in this paper found that the concrete backfills above the permanent tunnel crown seems to have less long term detrimental impacts to the permanent tunnel linings if the backfilled concrete density is closer to the surrounding soils and the backfills are not finishing with a sharp ending face. However the backfilled concrete area does affect the prediction of the permanent tunnel deflection shape, where the reinforcements or monitoring locations may need to be adjusted accordingly.

For the concrete backfills beneath the permanent tunnel invert lining, special treatment in construction sequence may be required to ease the excessive shearing spots on the permanent tunnel invert lining backed by physical evidences. Backfilled concrete materials (foam concrete, plain concrete, SFR concrete) and the adjacent soils normally have different compressibility, therefore in the long term when the permanent tunnel lining under full loading, excessive shearing spots have developed on the tunnel lining at the edges of the backfills. An over excavation of the backfilled concrete to create a relatively planar surface and replaced by the same materials as the permanent tunnel lining is recommended to alleviate the shearing spots. The other problem is a thicker concrete lining normally attracts more bending moment from the ground loads. To remedy this situation if the bending moment becomes too huge to the lining capacity, a partition layer between the lining and the backfilled concrete may be recommended. The tunnel geometry together with the ground conditions and planned construction sequence will also affect the physical behaviours of the SCL primary lining.