High Performance steel fiber reinforced segment design for a North American combined sewer overflow (CSO) tunnel

The challenge

CSO tunnels are subjected to corrosion due to hydrogen sulphide attacks, especially when built with rebar reinforcement.

In addition, for the Pawtucket project, numerous adits connect to the tunnel along its alignment including one of 1,000ft- (305m-) length excavated by microtunnel.

The design called for specialized precast lining segments to support these breakout areas. 

The solution

Bekaert supplied 4D 80/60BG fibers for the tunnel’s segmental liner, which is predominantly reinforced solely with steel fibres. These will better withstand the corrosive CSO fluids compared to traditional steel reinforcement bars.

In breakout areas, the contractor used hybrid tunnel lining segments with both fibers and bars, which provided the capacity required to carry additional loads when creating the adits. This also avoided disruptive changes to the precast process and optimized logistics in the tunnel.