Tunnel Support Systems

Steel rib sets are the workhorse for poor ground, fault zones, squeezing rock, and rehabilitation of deteriorating tunnels. W-section structural steel (typically W6 through W14, ASTM A992 Grade 50) is rolled to the tunnel arch profile and erected as multi-piece rings on 3 to 6 ft centers, with closer spacing in the worst ground. Crown blocking with timber, grout-filled fabric bags, or shotcrete pillows tightens each set against the rock; lagging spans between sets to transfer ground load to the steel. Steel rib support has been continuously used for over 150 years and remains the design choice when ground loads are large enough to demand high stiffness or when rapid emergency stabilization is the controlling constraint. The trade-off is reduced clearance, mechanical handling weight, and slower advance compared to NATM bolt-and-shotcrete shells.

Lattice girders are lightweight triangulated steel frames, typically formed of three or four longitudinal bars connected by zigzag lacing bars, that combine the rib geometry of a steel set with the shotcrete-integration of NATM construction. The girder is erected in the same arch position as a rib set, then encased in structural shotcrete in successive lifts so the steel becomes integral reinforcement of the shotcrete shell. Lattice girders weigh roughly one-third of an equivalent W-section rib, simplifying handling in confined cross-sections and short work windows, and they shotcrete more easily because the open lattice eliminates the encapsulation shadows that congested rebar grids create. NATM road tunnels under FHWA NHI-10-034 routinely use lattice girders as the primary rib element on moderate ground, with pattern rock bolts and welded wire mesh completing the composite shell.

Pattern rock bolting with welded wire mesh and structural shotcrete is the standard composite shell for good-to-fair rock (RMR 50 plus) and the production approach for most modern hard-rock tunnels. Bolts confine the rock arch immediately behind the face, suspending loose blocks from the intact mass and converting a discontinuous jointed system into a coherent reinforced beam. Welded wire mesh fastened to the bolt heads catches small spalls and reinforces the shotcrete face. Structural shotcrete, typically 4 to 8 inches thick in two or more lifts, forms the continuous reinforced shell across the bolt heads and seals the rock from atmospheric weathering. The system is light, fast, and adaptable to changing ground because additional bolts and shotcrete thickness can be added in real time as conditions evolve.

Pre-support is installed ahead of the advancing face, not behind it, to provide an umbrella that holds weak ground above the crown until primary support can be erected behind the round. Spiling consists of small-diameter rebar dowels driven or grouted at a shallow upward angle into the face crown ahead of excavation. Forepoling uses larger-diameter steel pipes or threaded bars on similar geometry, often grouted in place for greater stiffness. Pipe-roof canopies use 4 to 12 inch diameter steel pipes drilled and grouted from the face on 12 to 24 inch spacing, forming a horizontal canopy 30 to 60 ft long that lets the tunnel advance through soft ground, fault crossings, and shallow-cover portals where stand-up time would otherwise be inadequate. Pre-support is combined with steel rib sets, lattice girders, or bolt-and-shotcrete shells as the primary support behind the advancing face.