18. Renovation technology for large scale station widening work

Facing the growing need in large city undergrounds to reduce congestion and improve functionality there is increasing recourse to station extension work where part of an existing underground tunnel is opened and connected to a new tunnel to widen the area. However, large scale extension work involves significant reinforcement of the existing tunnel.

A new working method was therefore developed, which combines vertical and horizontal anchors crossing over, overlapping onto the existing tunnel (on the ground side) with a projected reinforced beam (Figure 1).

The reinforcement beam supporting the structure in the track direction serves to consolidate the existing and new tunnels into a monolithic structure ensuring transverse resistance, and which prevents any increase in sectional force in the existing tunnel. Full scale loading tests showed that the connecting section did not break, and the new and existing structure behaved as a monolithic block validating the fact that this construction method possesses the desired flexural capacity required for applying the proposed design method into practice (figure 2).

Trial calculations for a standard two tier, double span tunnel showed that compared to the existing method, less reinforcement work is required on the existing tunnel to carry out the extension work, which should enable a cost reduction of approximately 10%.