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Except in the cases where tunnels are started at a portal into the side of a hill or embankment, most utility tunnels are started and finished at the bottom of a vertical shaft. Before determining a shaft design, the minimum size of the shaft must be established. This decision may be driven by the type of tunneling equipment planned or by the size of a structure located at the start or end of a utility tunnel. Shafts can be round, elliptical, square, or rectangular. Both subsurface conditions including soils and groundwater and the tunneling method will dictate construction methods. For example, trench boxes may be suitable for small conventional tunneling through stiff clay above the water table, whereas watertight steel sheeting may be required for the same tunnel setup in sand below the groundwater table. Microtunneling shafts typically require water-tight excavation support regardless of subsurface conditions (except rock) to prevent “blow-back” of the pressurized slurry.
The list of shaft construction methods is long, and includes trench boxes, slide rail systems, soldier piles or steel bibs and wood lagging, liner plates, precast segments, sheet piles, secant piles, drilled shafts, soil mixing, slurry walls, NATM(New Austrian Tunnel Method), ground freezing, caissons, and drill & blast/rock bolt & shotcrete shafts. Depending on the size, soil conditions, and tunneling method, several technologies may be appropriate, but ultimately, price will drive the final selection. In some cases, even if dewatering with a porous support system (e.g. soldier pile and lagging) would be acceptable on a given project, settlement or contaminated groundwater concerns related to dewatering may preclude such a choice. In these cases, steel sheeting or secant piles with a concrete base slab would be required to seal the shaft off completely. For microtunneling launch shafts, water tight excavation support is typically required; however, depending on soil conditions, the recovery shaft can be as simple as a trench box and steel plates.
The best shaft solution starts with deciding if your shaft has to be “water tight” or not. Also, deciding if the shaft can be circular. Circular shafts rely on compression to carry the lateral loads and require much less internal bracing and reinforcement. Sometimes the shaft excavation support can be incorporated into the final structure, a caisson wet well for a pumping station, for instance. Bradshaw has extensive experience with liner plate, steel ring beam and lagging, steel sheeting, soldier pile and lagging, drill & blast/rock bolt & shotcrete, NATM, and caisson shafts. Our experience with custom-designed NATM shafts puts us in the forefront of this technology. NATM shafts utilize steel lattice girders and shotcrete for excavation support. It can be designed to custom shapes or modified in the field to accommodate unknown obstructions. Bradshaw built an intricate NATM elevator shaft down around an existing Metro station at the Pentagon in Arlington, VA. In Atlanta, GA we modified a NATM shaft to incorporate an 84” combined sewer pipeline that was found during excavation.
To answer these questions and any others about your next project, please contact us.
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